Furniture Construction

' RUDOLPH WlLLARQ NORTH CAROLINA STATE UNIVERSITY

FURNITURE CONSTRUCTION

by Rudolph Wi11 ard

Furniture Manufacturing and Management Curriculum North Carolina State University

Fourth Edition

Under the Sponsorship of:

The Foscue Furniture Fund The Southern Furniture Manufacturers Association and the School of Engineering North Carolina State University

De pa rtment of I ndu str ia1 En gineer ing North Carolina State University Raleigh, North Carolina

All rights reserved. This book, or any part thereof, may not be reproduced in any form without written permission of the Head of the Department of Industrial Engineering, North Carolina State University.

Printed in United States of America FOREWORD

The School of Engineering at North Carolina State University offers a four-year curricul um in Furniture Manufacturing and Management within the Industrial Engineering Department. The purpose of this program is to provide young men and women with academic prepartation for a career in the furniture industry.

The preparation and publication of this text has been made possible through support from the Foscue Furniture Fund.

We acknowledge the help and encouragement from the Furniture Founda- tion, Inc., and its president, Dr. Henry A. Foscue, who has been responsible for the growth and many improvements in the Furniture Manufacturing and Management Curriculum. L. K. Monteith Dean of Engineering North Carol ina State University i FURNITURE CONSTRUCTION

Preface to the Fourth Edition

The fourth edition of Furniture Construction is the first one pub- 1 ished since Rudolph Willard's death in 1981. This book and its com- panion volume, Production Woodworking Equipment, are testimony to Rudy's great contribution to the development of the Furniture Manu- facturing and Management Curriculum. We will always cherish his memory.

Edward L. Clark and I have revised many details of the text to reflect current practice in manufacturing, but we trust that the book retains the unique fl avor Rudy gave it original ly.

All comments, suggestions, and criticisms of the book are appre- ciated.

Anco L. Prak James T. Ryan Professor of Industrial Engineering in charge of Furniture Manufacturing and Management North Carolina State University

Raleigh, NC July 1982

TABLE OF CONTENTS

Page

Chapter 1 PRODUCT ENGINEERING...... 1 Chapter 2 MECHANICS OF FURNITURE CONSTRUCTION ...... 9 Chapter 3 CHARACTERISTICS OF WOOD AND LUMBER ...... 29 Chapter 4 CHARACTERISTICS OF VENEER AND PLYWOOD ...... 41 Chapter 5 PLASTICS IN FURNITURE CONSTRUCTION ...... 63 Chapter 6 INTERNAL STRESSES ...... 79 Chapter 7 WOOD JOINTS...... 89 Chapter 8 MECHANICAL FASTENERS ...... 111 Chapter 9 MINIMIZING COSTS ...... 129 Chapter 10 PERMANENT SPECIFICATIONS ...... 147 Chapter 11 CASEGOODS ...... 179 Chapter 12 CASEGOODS .RIGIDITY ...... 199 Chapter 13 DRAWERS ...... 215 Chapter 14 CUPBOARDS AND DOORS ...... 231 Chapter 15 CHAIRS ...... 253 Chapter 16 UPHOLSTERY FRAMES ...... 287 Chapter 17 BEDS AND TABLES ...... 293 Chapter 18 PACKAGING...... 307

CHAPTER 1

PRODUCT ENGINEERING

In examining the broad field of product engineering, one must look at the steps normally followed in introducing a new suite or new item into a line of furniture. Each step essentially involves only one functional area of the organization. An outline of this procedure is given below:

Steps Functional Area

1. Recognize a need Sales 2. Determine general type and price range Sales 3. Design for appearance Design 4...... Approve design for marketability Sales 5. Work out details of construction Product Engineering 6. Prepare preliminary cost estimate Cost Accounting 7. Compare cost estimate with desired price range and modify design if necessary Sales 8. Rework construction details for modifications in design Product Engineering 9. Build samples according to the detailed construction drawing Shop 10. Check samples for marketability and modify if necessary Sales

------______c______------11. Determine price, show samples at market, and take orders Sales 12. Prepare final drawings, bills of materials, and machining sequences Product Engineering 13. Make route tickets, jigs, and fixtures Product Engineering 14. Issue manufacturing order for first lot Production Management

The first four steps answer the question, "What will sell?" The next six steps are a feasibility study to determine how to make a saleable item at a profit. The final steps involve the mechanics of getting the furniture sold and into production.

For the purpose of clarity, the above procedure shows a sharp separation of the three main functions of sales, design, and product engineering. Under actual circumstances, there is a great deal of overlapping and cooperation required to introduce new items to the buying public efficiently . The sales management and design force combine a knowledge of the market place and artistic talent to develop pieces of furniture that people will want and be willing to pay for. Product engineering is g ven the difficirlt job of working out the details of exactly how the piece should be constructed in order to achieve:

a, Satisfactory performance in the final user s home

1 b. Economical manufacture with machinery, equipment, methods, and personnel available in the plant.

The ultimate objective is a piece of furniture with satisfactory appearance, comfort, durability, functional utility, quality, and satisfaction that can be sold at a maximum profit.

This book does not deal with artistic appearance design or the merchandising function except as incidentals. It does deal with the product engineering functions, and good product engineering should involve the following:

1. Knowledge of problems and difficulties encountered by the retail dealer and by the ultimate customer with furniture in use in the home and the relative frequency and importance of the different types of problems.

2. Knowledge of stresses, changes in temperature and humidity, and other conditions which the product will be subjected to in getting it from the factory to the consumer's home and in normal use in the home.

3. Knowledge of the principles of physics and strength of materials which permit analysis of forces imposed on the furniture in use and its ability to resist those forces.

4. Characteristics of available materials which affect construction, processing, and final trouble-free performance.

5. Ways and means to eliminate or at least minimize customer p rob1ems.

6. Knowledge of equipment, processes, and personnel in the plant which is to produce the furniture.

7. Creative imagination as to alternatives in construction and processing. If two or more alternatives will achieve the objective of trouble-free performance, they should be compared on the basis of cost and the most economical alternative selected.

8. Awareness of new materials, inventions, machines, etc.

These points will be discussed in more detail later.

Product engineering does not deal primarily with appearance, a5 this is in the area of design. However, minor changes in appearance sometimes make major changes in cost. In such cases, the product engineer should suggest changes to design and sales. For example, most flush joints cost more than joints which are nearly flush. In many instances, the general appearance of a nearly flush joint is as acceptable as a truly flush joint.

2 Comfort is an important feature in chairs, sofas, and other sitting furniture; it should also be considered in tables and desks at which people sit. It is important in bedding items such as springs, mattresses, and pillows but these are outside the scope of this book. Responsibility for comfort is divided between the designer and the product engineer.

Although the finish on a piece of furniture is of prime importance in customer satisfaction, it has little connection with construction. In the choice among construction alternatives, it is rare that the choice of alternatives will affect the finish. On exposed surfaces of furniture, choice of species of wood is generally governed by considerations of sales and marketing rather than product engineering. An exception might be the choice of a hard species like maple instead of a soft species like basswood. The two species are much alike in appearance, but maple resists bruises and dents much better than basswood. But in most cases, choice of species for exposed parts is determined by the appearance desired.

The importance of good product engineering should not be affected by the quality and price range of the furniture. It should be equally important in low, medium, and high price ranges.

Since one of the main objectives of product engineering is to plan a product which will give satisfaction to the customer, it is important to guard against customer problems and dissatisfaction. However, it is difficult to guard against problems in general ; effective prevention implies knowledge of specifically what problems to guard against. This sounds sensible and simple, but in practice, it is not so easy. Problems can be placed into the following categories:

a. Those which your customers have had and reported to you. b. Those which your customers may have had but did not report to you. c. Those which customers of other factories have had and reported.

Problems Reported by Your Customers

This is the easiest category to pin down. An analysis of your records of complaints, returns, and allowances will develop information on what the various problems were, and for each problem, how frequent and serious it was. Or is it true? It all depends upon how the records have been kept. It might be worthwhile to modify your procedure on complaints so that the records would be more meaningful in the future for the purpose of determining what causes each problem and how far you should go in guarding against the different kinds of problems.

Problems Not Reported by Your Customers

Problems in this category happen, but you seldom hear about them. Examples are:

1. Transit damage which the customer adjustc directly with the railroad or trucker.

3 2. Consumers' complaints which the retailer fixes at small expense and about which he simply does not bother to lodge a complaint with the factory.

3. Problems which the retailer has with furniture from all of his factory sources and which he just assumes nobody can do much about.

4. Complaints from housewives for which the retailers feel there is no legitimate cause and on which he makes an adjustment solely as a matter of store policy.

Within this category, an individual is likely to be guided by his own experience in anticipating potential problem areas. The following list, from the writer's experience, illustrates what is meant. In no case was a complaint lodged with the retailer, and consequently, the manufacturer never heard about it.

A solid maple chest has a deep storage drawer that sticks shut all summer every year but runs fine in winter when the house is heated.

The drawer bottoms in a dresser are nailed up into the backs instead of set into a groove in the drawer back ("boxed in"). Periodically the nails pull out, and the drawer needs to be re-nai 1 ed.

Torn grain on the inside face of a drawer side occasionally snags ladies' stockings.

Many homes have at least one dining chair which has become loose in the joints.

The surface finish on tops goes bad sooner than seems reasonable. Sometimes it looks like checking of finish, sometimes raised filler, sometimes loose cut veneer, etc.

Cracks or checks develop in wide pieces of wood. Sometimes a glue joint lets go, sometimes the wood itself lets go, sometimes fine hair checks develop; sometimes a joint in the face veneer opens enough to crack the finish, etc.

The cushions on a sofa were originally comfortable but now each spring can be felt by anyone sitting on the cushions.

The seat of an upholstered piece was stable when purchased but gradually developed a sideways "shimmy" indicating loosening or breakage of top tying of springs.

Problems in this category are often those that do not exist or are not apparent when the furniture is new but which develop over a period of time

4 and develop more quickly or become worse than it seems they ought to. A manufacturer very seldom gets a direct complaint on this kind of problem. In many cases, however, proper construction could eliminate some of these kinds of problems or at least prolong the time it takes to appear into a reasonable service 1 ife.

Problems of Customers of Other Factories

Customers of other factories have problems, but normally little is heard about them. However, there is nothing to prevent factories from exchanging information on complaints except that it is seldom actually done. While the experience of other factories might be useful as far as avoiding problems is concerned, most plants are usually too involved with solving their own problems to be interested in the problems of others.

One way a company might get information on the problems experienced by other factories would be to get its regular retail stores to furnish it copies of their complete records on customer problems. In this way, a company would get data that included information on all the factories from which its retailers buy. If retailers knew that the purpose behind this was to help produce more trouble-free furniture, many of them would probably be glad to provide manufacturers with copies of their records.

Check List of Problems

Studies have shown that poor product engineering is the cause of only a small percentage of all complaints, whereas poor quality control is the cause of most customer complaints. This serves only to emphasize the importance of quality control and not to condone poor product engineering. Below is a check list of problems which can be attributed to poor product engineering.

1. Strength not adequate in use

2. Rigidity not adequate for use

3. Improper design of carton, crate, or packing

4. Drawers or doors tight or stuck or otherwise faulty in operation

5. Drawer guides loose or split

6. Drawers not lined up

7. Spl its or checks

8. Warpage

9. Open joints

10. Loose joints

5 11. Poorly fitting parts

12. Moldings coming loose

13. Shelves sagging

14. Bed rails fit loosely to bed ends

15. Loose cases, chairs, and frames

16. Upholstered spring fasteners weak

17. Noisy springs

18. KD (knock down) units do not assemble properly

19. Finish printed from packing

The first three items are evidently poor product engineering. As for the remaining items, it is very likely that poor quality control is the cause of most of these complaints, but any one of them could result from poor product engineering. Examples of this are as follows:

Loose or open joints not having enough inherent strength to resist forces which can be expected in normal use.

Splits or checks in solid wood caused by failure to use floating construction where it is needed to permit shrink and swell with weather changes.

Warpage caused by failure to adequately brace against warpage.

Unaligned drawers caused by too much side play between members that the drawer guides.

It may be impossible to make a complete list of all the things that can create problems for a customer, but this is no excuse for inaction. Product engineering should make and keep up to date as complete a list of complaints and problems as possible. It will prove an invaluable tool for evaluating and "debugging" new constructions as they develop.

Some problems occur rarely, or else they cause only minor expense or irrita- tion. A construction for the complete elimination of these types of problems would be too expensive. When it is possible, one should compare the cost of solving a problem with the cost of the problem itself and then decide whether to adopt the solution or put up with the problem. Very often such a decision will be based more on judgment than on quantitative mea- sures, however, this should not keep one from reaching a decision one way or the other.

6 The Future and Product Engi neeri ng

A discussion of the function of product engineering would not be complete without a look into the future. The furniture industry is on the threshold of technological breakthroughs that will require a great deal more from the product engineer. The introduction of substitute materials, such as plastics, metals, glass, synthetic fibers, etc., is proceeding at a pace which is bewildering to furniture makers of a generation ago. Printing and em- bossing are turning low-cost woods into furniture parts which defy detection when compared with scarce and expensive hardwoods. Plastic parts in structural, as well as decorative functions, are giving the public an appearance in mass-produced furniture which heretofore was reserved for only the highest priced, handcrafted pieces. New machinery is being developed to reduce the high labor cost which has plagued the industry for many years. Computer technology is now available to run woodworking machines and to assist in establishing better manufacturing controls.

These innovations are just a few of the new ideas which will concern the product engineer. He occupies an important position between marketing and production and plays an important role in getting new products from the designer's mind to the consumer's home.

7 8 CHAPTER 2

MECHANICS OF FURNITURE CONSTRUCTION

Good product engineering involves knowing to what conditions the product will be subjected in normal use and constructing the product to meet all these conditions with a minimum amount of trouble.

For furniture, the term --in use needs to be interpreted as including the f ol 1 owi ng situat i ons :

1. Going through the factory

2. Transportation from factory to retailer to home

3. In the home

4. Moving from one home to another.

The conditions and environment in each of these four situations include:

Weather Temperature Humidity External forces acting on the furniture Internal forces acting on the furniture

The chief effect of weather is to set up internal forces in the furniture due to the fact that wood shrinks or swells with changes in weather humidity. High or low weather temperatures of themselves seldom affect furniture in ways which can be counteracted by construction, but weather temperatures affect humidity, and humidity changes do require precautions in construction.

In the home, there are other conditions, such as:

Direct sunlight on the furniture Spilling of food acids, liquor, and nail polish Cigarette burns.

These conditions do affect the finish but seldom involve construction except to make the use of plastic laminates on tops of furniture preferable to wood or veneer.

To summarize, most of the conditions to which furniture is subjected that need the consideration of the product engineer involve external forces or internal forces.

This chapter will discuss external forces. Internal forces will be discussed later, after a study of the characteristics of wood.

9 In the designing of buildings, bridges, and similar structures, it has been customary for years to make a stress analysis and to calculate the sizes of beams and columns so that they have the required strength to withstand the stresses caused by weight, wind, and other external forces with a factor of safety. It is not customary to do this in furniture construction. In most furniture parts, if the part is well proportioned, it has more than the required strength. There are a few exceptions, such as back posts on chairs, extra sl im Scandinavian modern legs, and del icate mi rror frames. But in such cases, the customary procedure is to rely on experience or on trial and error.

In spite of the lack of basic data for accurate theoretical calculations of strength in furniture, a knowledge of a few principles of physics, mechanics and strength of material is helpful.

Leverage

The lever has been known for years. In Figure I, the fulcrum is fixed, the distance FA is five times the distance FB. If a downward force of 100 pounds is applied at A, the lever will support a load at B five times as large or 500 pounds. The lever need not be a straight line. Figure I1 shows the action of a lever which is not a straight line with the fulcrum provided by a pin through a hole at point F. A crowbar is a tool acting like Figure I, a claw hammer pulling a nail acts like Figure 11.

500 Ib

I w F

BvAi100 AIb fulcrum Figure I. Example of a simple lever.

lftl I

I I I 5ft t IO0 Ib

Figure 11. Example- of a lever which is not a straight line. 10 A furniture construction example of the action of a straight lever is shown in Figure 111. A drawer guide construction for a chest of drawers is shown in (A). The guide consists of two parts. Part R is attached to the dust frame, which is solidly anchored to the body of the chest. Part S is attached to the drawer underneath the drawer bottom. Lengthwise, parts S run the full depth of the drawer front to back (20" Figure III(B)) . Lengthwise, part R runs the full depth from front to back of the dust frame except for clearance at the front so that the drawer front will not hit part R when the drawer is closed. As the drawer is pulled out of the chest, parts S slide longitudinally along part R. Sidewise motion of the drawer is controlled by the lips on parts S riding against the stem of the T-shaped part R. When the drawer is pulled quite far out of the chest, as in Figure III(C), the front of the drawer D will tend to drop. The drawer is in contact with the dust frame at E, so a drop at D will cause a rise at F. The amount of rise at F is limited by the lips on parts S rising up until they strike the lips on part R. When F can rise no higher, D can fall no lower. In this construction, parts R and S act not only as drawer guides but also as a drawer tilt. Sidewise motion and tilting motion are both controlled.

A B C

Figure 111: Drawer guide construction illustrating lever action in opened drawer.

Referring to Figure III(B), if the drawer is pulled halfway out, ED = EF = 10". Point E is a fixed fulcrum, so if a 10-pound force is applied downward at the drawer front, an upward force of 10 pounds will be exerted at F due to lever action. Suppose the drawer is pulled nearly out of the case as in Figure III(C) and the distance ED is 19 inches and EF is one inch. The leverage ED/EF = 19/1, and a 10-pound downward force at 0 would exert a 190- pound upward force at F. This force would be resisted by the lips on parts S in contact with the lips on part R. In some cases, this force would be great enough to split off the lips on one or the other of these parts unless the parts were constructed to have adequate strength to resist the splitting force.

Another leverage situation also exists in the above example, Fig. III(C). If the guides are strong enough, point F can be considered the fixed fulcrum, and a downward force of 10 pounds at point D would result in a downward force at point E of x 10 lbs. or 200 pounds if EF is 1 inch. In practice, this is seldom serious because dust frames usually have much more than the strength required to resist the force. But the leverage is still there.

11 A furniture construction example of a lever which does not form a straight line is shown in Figure IV. This shows the joint between the side seat rail and the back post of a dining chair. Generally this joint is made with two dowels which are glued in the holes into which they fit.

c * ?7,7,-!7.?3 *= b GLiL2q,e %

Figure IV: Diagram of a chair joint where side seat rail joins back post.

If there were no glue on the dowels, a downward force at point A would tend to rotate the side seat rail around point B which would tend to open up the joint at point C. This could be done only by pulling the dowels straight out of the holes. This action is similar to the lever in Figure I1 or the action of a claw hammer in pulling a nail. Assume that there is no glue on dowel E but that dowel D is glued. A downward force of 100 pounds at point A would result in a much larger force acting at point D due to leverage action trying to pull the dowel out of the hole. The amount of the force at D would be -AB x 100 lbs. = 15 x 100 = 920 lbs. If we assume no glue BD -8 on dowel D but dowel E glued, the force trying to pull out dowel E would be -AB x 100 lbs. = 15 x 100 = 2,400 lbs. With both dowels glued, the BE T force trying to pull out the dowels would be divided between both dowels. If the side seat rail was only one-half inch wider (2 3p"' instead of 2 1/4"), BE could be 1 1 /8 'I instead of 5/8 '' and the force on dowel E would be 15 x 100 = 1340 lbs. rather than 2,400 lbs. figured previously; 11/8 the force on dowel D would be 15 x 100 = 710 lbs. instead of 920 lbs. 21/a It is sometimes startling to see how much leverage forces can be reduced by a small change in some of the dimensions involved in the lever action. If strength trouble is anticipated as a result of leverage action from external forces, one should check to see whether a change in dimensions, permissible from the viewpoint of styling, can substantially reduce the size of the force at a critical point.

12 Rectangul ar Beams - Strength

Some furniture parts resemble a beam or a floor joist in a building in that they are horizontal, are supported only at each end, and support a load or resist a force applied somewhere between the support points. On account of this similarity, it may be helpful to discuss beams. Figure V shows a beam of rectangular cross section which may be assumed as 2" wide and 6" deep.

Figure V: Load applied to a simple beam.

This beam is supported at two points, one near each end. The distance L between support points is called the span. The vertical dimension of the cross section d is called the depth. The horizontal dimension of the cross section b is called the width. Assume a downward force or load applied to the beam at a point halfway between the supports, 1/2" L from each support. This force tends to bend the beam into a curve. The lower edge would become convex, and the upper edge, concave. This imposes a tension force in the bottom section of the beam and a compression force in the top section. The tension and compression forces are resisted by the wood which has a certain maximum strength in tension and a different maximum strength in compression. The maximum strengths in both compression and tension are fairly well defined within a range which has been found by experiments. Different species of wood have different maximum strengths, but various pieces of the same species will be more or less uniform. If the load is

13 increased, the internal forces increase. When either the tension force or compression force exceeds the maximum strength of the wood, failure occurs, and the beam breaks. The formula for the maximum allowable load, or force P, without breaking the beam is: b d2 P=k 7- where k is a constant which depends upon the maximum strength of the species of wooa, moisture content of the wood, factor of safety, and cross-sectional shape of the beam (in this case, a rectangle).

From the formula, it can be seen that the maximum allowable load varies as foll ows:

a. in direct proportion to width b

b. as the square of the depth d c. inversely as the span L. If it is desired to have the beam carry more load, this can be accomplished by increasing width b or depth d or by reducing span L. But doubling width b only doubles the allowable load, whereas doubling depth d permits four times the load.

Two examples will illustrate this principle. Assume that span L is fixed and that the beam is 2" x 6". If the force is applied at right angles to the 6" dimension, then b = 6, d = 2.

Pi = ki(6) (2)2 = 24 kl

If the force is applied to the edge (at right ang es to the 2" dimension), then b = 2, d = 6

P2 = kl(2) (6)2 = 72 kl

The same beam will carry 72/24 or three times as much load applied on the edge as it would if applied on the side or flat.

Assume the same 2" x 6" beam, lying on its edge with span L fixed. It is desired to replace it with a beam that will carry twice the load. One alternate is to increase the width; another alternate is to increase the depth. The above equation modified for double strength by increasing width would be:

kl(b) (6)2 = 72 kl (2)

Solving for b, b = 4, and the beam dimensions would be 4" x 6".

14 The same equation modified for double strength by increasing the depth would be:

kl (2) (d)2 = 72 kl (2)

Solving for d, d = 8 /2, and the beam would be 2" x 8 1 /2 'I. This gives a cross section of 17 square inches for the deeper beam compared to 24 square inches for the wider beam. It is evident that a given increment of strength can be obtained with less extra material by increasing the depth instead of increasing the width.

For maximum economy of material in a beam to support a given load, make the beam narrow and deep. However, this must not be carried too far. If the beam is too narrow, it will twist away from vertical under the load at the center. Under such a condition, the formula does not apply. The beam would not support as much load as the formula says it should. If there is much twist, the beam becomes much weaker.

Rectangul ar Beams - Rigi di ty Sometimes a furniture part is strong enough to withstand the load or force without breaking, but it is not rigid enough to be satisfactory. Referring to the beam in Figure VI, as the load P is increased, the beam will bend down as shown by the dotted lines. In wood, the deflection can become quite P

------__ _--__-----

Figure VI: Beam deflected by loadd b P. large before the beam actually breaks. A good measure of rigidity is the maximum deflection. With a beam loaded as in Figure VI, the maximum deflection occurs directly under the load in the middle of the span. Maximum deflection is shown as C in Figure VI. It is the greatest distance between the position of a beam under load and the position of the beam without load. The formula for the maximum allowable load or force P without exceeding a specified maximum deflection c is:

b d3 P=m- L3 c

15 where m is a constant which depends on the modulus of elasticity of the species of wood, moisture content of the wood, and cross-sectional shape of the beam (in most cases, a rectangle).

By comparing the above rigidity formula with the strength formula, it will be noted that in both cases the maximum allowable load varies in direct proportion to the width of the beam. For strength, the load varies as the square of the depth of the beam; for rigidity, it varies as the cube of the depth. This means that, if the depth of a beam were doubled, it would have four times the strength, eight times the rigidity. For strength, the load varies inversely as the cube of the span. This means that, if the span were cut in half, the beam would have the strength to carry twice the load in the middle of the new span, but it would have the rigidity to carry eight times the load.

To summarize, the maximum safe load varies as the following factors change:

With Regard to Strength With Regard to Rigidity

Directly proportional to width Directly proportional to width

Directly proportional to square of Di rectly proportional to third depth power of depth

Inversely proportional to span Inversely proportional to third power of span

Columns and Cantilever Beams

Many furniture legs and posts are vertical members and carry a load. At first thought, they resemble columns or studs in a building. But experience shows that such legs never break from a straight vertical load. If they break, it is most likely to be caused by pushing the piece of furniture across a horizontal floor and hitting an obstruction on the floor; or else they break from a sidewise blow, generally near the unsupported bottom end of the leg. As with strength, rigidity of vertical legs is not needed against a straight vertical load but against a horizontal force, such as a person bumping against the edge of a table. Thus experience shows that for strength and rigidity, these vertical posts act not like columns, but like canti1 ever beams. A1 though canti 1 ever beams are general ly horizontal and furniture legs vertical, if the weight of the beam is neglected, the strength and rigidity formul as for canti 1 ever beams would apply whether the beam was horizontal or not. Figure VII(A) shows a cantilever beam in customary horizontal position. Figure VII(B) shows the same beam in vertical position like a furniture post.

16 b 1p L-

Figure VII: Cantilever beam (A) is illustrated in furniture post (B).

The load P is assumed concentrated at the free end of the beam. L is the effective span, which s the distance along the beam from the support point to the free end of the beam. Furniture posts are customarily square so that the width of the beam s the same as the depth of the beam (shown as d in Figure VII). The formula for maximum allowable force P without breaking the beam is:

where k is a constant which depends upon the maximum strength of the speciesof wood, moisture content of the wood, factor of safety, and cross- sectional shape of the beam (in this case, square) . The formula for maximum allowable force P without exceeding a maximum deflection c is:

where m is a constant which depends on the modulus of elasticity of the specie? of wood, moisture content of the wood, and cross-sectional shape of the beam (in this case, square).

Comparing these formulas, it will be seen that the strength of a square post varies with the third power of its width; the rigidity varies with the fourth power of its width. The strength varies Inversely with the unsupported length; the rigidity varies inversely with the third power of the unsupported length.

17 Generally the unsupported length of a post is dictated by considerations of appearance and can be changed very little. But if either strength or rigidity is doubtful, a small change in thickness will have a large effect. A 10% increase would give 47% increased rigidity (l.lO)'+ = 1.47.

The same formulas would apply to a beam or post with a circular, instead of square, cross-section except that the constants -k and -m would have different Val ues.

Shear

It is quite rare that a furniture part fails due to a pure shearing force, but it is well to understand what is meant by shearing strength and by shearing forces. In Figure IV, if a downward force were applied to the back end of the side seat rail close to point C, it would try to push this end of the seat rail down. This force would be resisted by the two dowels. The rail could not move down without shearing off the two dowels. Most materials have a fairly definite strength against shearing which can be tested and expressed in pounds per square inch. The shearing strength of one material will be different from another material. But in any given material, the shearing strength in pounds per square inch is fairly constant. Hence, if it is desired to double the force which can be resisted, double the square inches of material being subjected to the shearing force. With 3/8" diameter dowels in Figure IV, each dowel would have 0.111 square inches resisting the shearing force. Given a shearing strength of the wood in the dowels as 10,000 lbs. per square inch, each dowel should stand a force of .111 x 10,000 = 1110 lbs. before breaking due to shear. If the diameter were doubled, the shearing force it could stand would be, not just doubled, but quadrupled.

It was pointed out that leverage action in Figure IV would try to pull the dowel out of the hole if a force were appl ied downward at point A. The force trying to pull the dowel out would be resisted by glue fastening the dowel to the back post and to the side seat rail. If the dowels are 1 /2 'I long and half the length goes into the back post, the surface which is glued would be the surface of a cylinder 3/8'' in diameter and 3/4" long. This would be (3/8)(~)(3/4) = 0.84 square inches. If the glue had a shear strength of 800 pounds per square inch, the joint could resist a force trying to pull out one dowel 0.84 x 800 = 670 lbs. This is because it would be necessary for the glue to fail in shear before double the number of square inches of glue line and thus double the total force which could be resisted.

Elast icity

Most of the principles of physics just discussed assume that the parts are rigid bodies. But nearly all structural materials are elastic instead of rigid, and this applies to wood. In an elastic material, as a force is applied to it, the material deforms. When the force is removed, the material returns to its original shape provided that the amount of deformation has not been too great. In metals or woods, this point is called the elastic limit. At a point below the elastic limit is the proportional

18 limit, a value usually of more interest when working with wood. Up to the proportional limit, as force increases, deflection increases and in strict proportion. Figure VI11 reproduces the chart drawn by a testing machine when applying side grain compression load to a piece of wood. The machine itself measured and drew the chart for both load and deflection. It will be noticed that from 0 load to about 1,500 pounds, the deflection was not proportional to the load and the chart shows a curve. From about a 1,500-pound to 12,000-pound load, the deflection was strictly proportional to the load, and the chart was a straight line. By drawing the dotted straight line, it becomes apparent that proportional limit for this particular test sample was at approximately 12,000 pounds because that is the point where the chart starts to deviate from a straight line. For loads higher than this, the deflection increases faster for successive uniform increases of load, such as 1,000-pound load increment.

Load (Pounds)

Deflection = Thousandth of an inch

Figure VIII: Graph of load vs. deflection for wood member in compression.

The curve near zero load (instead of a straight line) is characteristic of wood but not of most metals. It is generally explained by the fact that it takes a certain amount of motion of the metal platen before it makes 100% contact with the wood due to the difficulty of machining wood samples to high precision. Customary practice is to draw the straight dotted line for determining the proportional limit and extend it downward to the deflection axis (X-axis). It is assumed that, where the straight line intersects the deflection axis, there should be zero deflection. The true stress-strain diagram would start at 0-0 and would be a straight line up to the propor- ticrnal limit. The initial curve near zero is due to inaccuracies inherent in the test method. In Figure VIII, the straight line intersects the deflection axis at a deflection reading of 2/10,000 of an inch, hence all other deflection readings on the chart are assumed to be 2/10,000 of an inch too high. This is to say that each deflection reading between zero and the load at the proportional limit must be reduced by 2/10,000 of an inch from the value given on the straight line.

19 Further study of the subject of load versus deflection will be reserved for subsequent courses. But at this point, it should be emphasized that a sample of wood or of a wood construction seldom breaks or shows complete failure with loads at or below the proportional limit unless the material is defective. Final failure is usually at loads much higher than the proportional limit.

Assemblies - Strenath and Riaiditv Most of the discussion so far in this chapter has applied to individual parts of a piece of furniture or to the joint between two individual parts. There are some other mechanical considerations which apply primarily to the assembled piece of furniture as a whole or to sub-assemblies of several individual parts.

The strength of an assembly is generally determined by the strength of individual parts and of the joints between two parts. If all individual parts and individual joints are strong enough to resist all forces imposed by normal service, the whole assembled piece of furniture will resist those forces.

But with rigidity, as distinguished from strength, it is a different story. All individual parts may be sufficiently rigid, but the assembly may not be.

Rectancll e Versus Trianal e

Consider the situation illustrated in Figure IX. Assume some pieces of wood measuring 12" x 3" x 1" are joined together at their ends by hinges. Assume further that the bottom horizontal piece is held firm and immovable as if nailed to the floor. If a horizontal force (F) is applied as shown in Figure IX, a very small force will distort the four-sided structure to some position like that shown with dotted lines in Figure IX(A). The structure itself offers no resistance to this force, and distortion is easy from a rectangular shape to a parallelogram without right angles. But with the

-F

A B Figure IX: Nonrigid (A) and rigid structures (5).

20 three-sided structure, shown in Figure IX(B), the situation is different. The structure itself resists being deformed. Even though the joints are free-swinging hinges, the triangular structure will retain its true shape until the force is great enough to distort the individual parts making up the structure or to tear out the hinges.

Structures with five, six, or more sides are like the rectangle in that they distort easily. The triangle is the only inherently stable polygon in plane geometry, and this applies to assembled structures. This principle can be illustrated with a crate. In Figure X(A), the only resistance to distortion of one face of the crate away from a right-angled structure by a force F is the nailing where one piece is fastened to another. The face of the crate is not very rigid. Now if an angle brace is nailed on as in Figure X(B), the four-sided structure in A has been converted into two triangles KLM and KNM. Both these triangles are inherently stable, and the crate front will be extremely rigid against distortion. This is accomplished with the addition of very little material. In fact, if strength permits, the width of the pieces in the four sides can be reduced enough to offset the extra material in the angle brace, and the crate front with angle brace will be much more rigid than the one without the brace but with the wider boards.

The inherently stable triangle is often used without realizing it. In the crate front of Figure X(A), suppose that instead of applying an angle brace like Figure X(B), the whole front was covered with a rectangular plywood panel securely nailed around all its four edges to the four members of the crate frame. Imaginary lines could be drawn on this panel in such a way as to create a large number of triangles. The crate front would be extremely rigid against distortion even though the plywood panel was very thin.

Y X

AVERAGE 0

LXCLtD S IUCHES NAILED OCHLD Ens

Figure X: Increased rigidity obtained in the side of a crate by providing an angle brace.

21 In a triangular structure, distortion is resisted, not only by the joints, but also by the strength and rigidity of the individual parts. In a rectangular structure, the joints alone have to furnish all the resistance against distortion. It is often cheaper to prevent distortion of a rectangular structure by creating one or more triangles than it is to get joints strong enough and rigid enough to prevent the distortion without the triangles.

Balance of Forces

Assume you have a material object small enough to be considered as a point in space. If this point is not in motion but is at rest, each force acting on the point will be balanced by an equal but opposite force. Were this not so, the point would move. A simple illustration would be a small weight hung by a string. Figure XI shows this situation.

Figure XI: Equal and opposite forces acting on a weight.

If A is a one-pound weight, there will be a one-pound force AB exerted by gravity trying to move the weight down. This will be resisted by a one- pound force AC exerted by the string. The two forces are equal and opposite, and the weight will not move. If the weight is increased, the force exerted by the string will increase by the same amount, and there will still be no motion of the weight. This can continue until the weight becomes great enough to break the string.

Vectors

In describing a distance, it can be completely specified by a number and measuring unit such as 20 feet. But in describing a force, it is necessary to specify the direction of a force in addition to the number and measuring unit. The expression ''a ten-pound force" does not give a complete description until the direction of the force is specified. Where several forces are acting upon a given point, it is convenient to analyze these forces by using vectors. Each vector is an arrow in which the length of the arrow represents the direction of the force, and the arrow head represents which way the force is acting along the direction of the arrow. In Figure XI, the arrows AB and AC were vectors representing the two forces acting on the point A.

22 For purposes of analysis, it is found that two forces acting upon a point in different directions give the same result as one force which is the result of a geometrical addition of the vectors representing the two forces. This one force is called the resultant of the other two. Figure XI1 illustrates several cases. The dotted lines are the actual forces, and the solid lines are the resultant forces.

Figure XII: Resultant forces.

The vector representing the resultant of two forces is the diagonal of a parallelogram formed by using the vectors of the two forces as adjacent sides of the parallelogram. By extending this same principle, the action of any two or more forces which act upon a given point can be combined into a single resultant. If the forces are not all acting in the same plane, the problem of determining the resultant gets much more complicated.

It is often more convenient to use a triangle instead of a parallelogram. Referring to Figure XIII, ifAB and AD are the two forces acting on point A, AC is the resultant of these two forces (using the parallelogram method). But since the opposite sides of a parallelogram are equal and parallel, the construction of the triangle ADC gives exactly the same resultant AC as does the construction of the parallelogram ABCD.

B C C

A D A D

Figure XIII: Force triangle.

23 The process of getting one resultant of several forces can be reversed. The result of one force acting on a point will be the same as that of two or more components of that force acting in different directions. In Figure XI1 ifthe solid line represents a single force acting on the point, its action will be the same as that of two component forces represented by the two dotted lines.

An example of the application of resolving a single force into components is illustrated in Figure XIV. A popular modern or contemporary style chest of drawers with short legs attached under the case is shown in Figure XIV(A). These legs are splayed out at an angle for appearance purposes. If the chest has four such legs and weighs 120 pounds, each leg will support 30 pounds of weight. Figure XIV(B) shows the leg. EF is a vector representing a 30-pound force acting vertically downward. At the other end of the leg, where it is in contact with the floor, GH is a vector representing this same downward force which is transmitted through the leg. Since the point G is at rest, there must be an equal and opposite force acting on it. GJ is a vector representing this force exerted by the solid floor. The situation is similar to the string and weight shown in Figure XI.

MG MM G MM G M

M M

A. e C D

Figure XIV: Forces acting on furniture are related to the design.

The vector GJ can be resolved into two components. One, KJ, is parallel to EG. This force would try to compress the leg. But the leg will have more than adequate strength to resist the force so no harm results. The other vector, GK, is at right angles to EG and represents a force trying to pull the leg loose from its fastening to the case. Assume that the leg is fastened to the case with a single dowel at L. Here we have a lever with fulcrum at E and lever arms of GE and EL. The situation is similar to Figure 11. If the distance GE is five times the distance EL, there will be a force 5 x GK trying to pull the dowel out of the hole.

24 Not only does the relationship of distances EG and EL affect the size of the force trying to pull the dowel, but the angle between the leg EG and the floor MM affects the size of the force. Figure XIV(C) shows the leg EG at right angles to the floor MM. When vector GJ is resolved into components, the component GK at right angles to EG turns out to be zero. This means that there is no force trying to pull the dowel. Figure XIV(D) shows the leg at a sharper angle to the floor than Figure XIV(B). The vector GK at right angles to EG is much larger in Figure XIV(D) than in Figure XIV(B). This means that, with a sharper angle between the leg and the floor, the force trying to pull the dowel is greater . The size of the forces can generally be determined accurately enough for practical purposes of furniture construction by a careful geometric drawing of the vectors to scale at the proper angles. But if it is desired to calculate the size of the forces, it can be done by trigonometry. From reference to Figure XIV, it will be noticed that JKG is a right triangle in which JKG is the right angle. This means that GK/GJ = sin KJG = cos KGJ. GJ is known, one of the angles can be measured, so

GK = GJ sin KJG = GJ cos KGJ

This permits calculation of the size of the unknown force GK.

Torque and Moment

In "Balance of Forces," it was stated that, if a point is not in motion but is at rest, each force acting on the point must be balanced by an equal but opposite force. Figure XI showed an example of a point under balanced forces. Figure XV represents an object which cannot be treated as a point. It is acted upon by two equal but opposite forces, CD and EF. It does not stay at rest but rotates in the direction of the arrow until its position is as in Figure XV(B) where CD and EF both fall along one straight line, DF. The rotative effect of a force is determined, not only by the size and direction of the force, but also by its "moment arm." This is illus-strated in Figure XVI by a bar which is attached at one end to a fixed pin P, but the bar can swing around the pin. Figure XVI(A) shows a force DE applied at point D in a torque (rotating effect). The magnitude of the torque is calculated by taking the product of force DI and the length of the moment arm, DE. Torque is often measured in pound feet. If DE were a two pound force and DP were three feet, the torque would be 2 x 3 = 6 lb. ft. If GH represents a force equal to DE but applied at point G where distance DP is twice GP, the force would be the same but the torque only half as large. Similarly, if force DE were only half as great but still applied at D, the torque would be only half as great.

25 -F D

A B

Figure XV: Forces act as a couple producing rotation.

Moment -arm is -not defined as the distance between point of appl ication of the force and center of rotation. Figure XVI(B) illustrates this. Draw a straight line through the point of application of the force D with the line coinciding with the direction of the force DE. From the center of rotation P, draw a line at right angles to this other line intersecting it at J.

A 6 C

Figure XVI: Forces providing rotation through a moment arm.

Going back now to a point at rest, the set of forces acting on the point must be balanced by a force equal to and opposite the resultant. For a body this is also true, but it is not enough. The second requirement is that each torque acting on the body must be balanced by an equal and opposite torque .

26 In Figure XIV(B), we discussed the size of the force trying to pull the dowel and used the concept of a lever in the analysis. The same result would come out if we used the concept of balance of torques. Considering point E as the center of rotation, there is a torque trying to rotate the 1 eg clockwise around point E. The size of the torque is force component GK multiplied by moment arm GE (note that GK was at right angles to EG). If the leg is to resist this rotation, there must be an equal torque in a counter-clockwise direction. This torque is furnished by the vertical force with which the glued dowel resists pulling out, multiplied by the horizontal moment arm LE (vertical and horizontal are at right angles). If the turning moment of force GK becomes too great for the strength of the dowel joint, the joint will fail just as the string would break in the example of Figure XI if the weight exceeded the strength of the string. Remember that with torques, the size of the moment arm is as important as the size of the force.

27 28 CHAPTER 3

CHARACTERISTICS OF WOOD AND LUMBER

In order to intelligently plan construction of an article to give satisfactory service, it is necessary to know something about the conditions to which it will be subjected in normal use. This includes knowledge of forces to which it will be subjected (both external forces and internal forces). To successfully resist these forces, it is necessary to know something about the strength and other characteristics of the materials to be used in the article. Since one of the main materials used for furniture is wood, this chapter will discuss some characteristics of wood which affect furniture construction. Students should have had courses in wood technology in addition to furniture construction, so this chapter will not attempt an extensive discussion of the characteristics of wood but will be more of a summary and review of those which most directly affect furniture construction. More complete details can be found in the many publications in the field of wood technology. Especially helpful for reference is the --Wood Handb0ok.l

Grain

Wood, as used in furniture, is often in the form of lumber or veneer. Both are cut from logs which are the trunks of trees. The wood fibers, which are somewhat like long slender tubes, grow in the tree with their length parallel to the trunk of the tree. Around knots, the direction of the fibers is distorted, and there are other distortions, but the general trend is for the fibers to be parallel with the tree trunk. When the term ---with the grain is used, it means parallel to the wood fibers; when the term across --the grain is used, it means at some angle to the wood fibers, generally at about right angles to them. Due to the anatomy of wood , many of the characteristics of wood are different with the grain than across the grain. Examples are strength, machineabil ity, and the holding power of glue.

Strenqt h

The strength of wood varies, depending on whether it is stressed in tension, compression or shear. Due to grain, the strength of wood while under tension is much greater if the force is with the grain than if the force is across the grain. Strength values for compression and shear are also much different with the grain than across the grain.

U.S. Department of Agriculture, Forest Service, Wood Handbook, by Forest Products Laboratory, Agriculture Handbook No. /2 , Rev. ed. (Washington, DC: GPO, 1974). For sale by the Superintendent of Documents, Washington, DC 20402.

29 The strength of wood also depends on the species of wood. In general, the heavier species are stronger than the lighter ones. For instance, hickory, which is very heavy, is much stronger than basswood, which is very light. Much experimental work has been done to determine the strength of various species, and fairly complete published data are available in the Wood Hand- book and elsewhere.

The moisture content of wood also affects its strength. But in furniture construction, it is customary to use kiln dried wood in a fairly narrow range of moisture content, usually from 4 to 12%. There is very little change of strength from top to bottom of this narrow range.

Strength values for wood are generally determined and published in terms of pounds per square inch.

Rigidity

Like steel and many other materials, wood is not rigid but flexible or elastic. As load is applied, the wood yields; the more the load, the more it yields. With steel, if the load does not exceed the elastic limit of the material, it will return to the original position when the load is removed. Wood acts in the same way except that the term proportional limit is used for wood rather than elastic limit as for steel, but it means about the same thing. As with steel, wood loaded beyond the proportional limit (elastic limit) will not return to the original position but will be permanently deformed.

As with strength, the amount which wood yields depends on whether it is in tension or compression or shear. It also depends on direction of grain, species, and moisture content. But within the range of moisture content of wood used in furniture, there is very little difference from top to bottom of the range.

Effect of Temperature

Many materials expand appreciably with increased temperatures. Within the range of temperature to which furniture is subjected, temperature has no effect on the size of a piece of wood of any species either with or across the grain. Theoretically this is not true, but for practical purposes of furniture construction, the dimensional change is so small it can be disregarded.

Effect of Moisture

Wood is hygroscopic. When exposed to damp air, it will pick up moisture; when exposed to dry air, it will give off moisture. At any given temperature, there is a certain definite moisture content which a piece of wood will finally reach for any particular relative humidity of the surrounding air (if this humidity is kept constant). This moisture content of wood

30 is called "equi 1 ibri urn moisture content'' and is abbreviated as Ilemc." The value of emc for a certain temperature and relative humidity of the air is the same for all species of wood, and it is not affected by the previous moisture content of a piece of wood.

The moisture content of wood is specified in percent. It is defined as the ratio resulting from dividing the weight of the water in the piece of wood by the weight of the wood substance if bone dry times 100. Weight of water Percent Moisture Content = Weight of bone dry wood x 100

In green lumber, sometimes there is more weight of water than dry wood so that is is possible to have a green board with a moisture content higher than 100%. This may seem impossible, but it is true because of the way percent moisture content is defined.

Table I shows the relationship of temperature and humidity of air related to emc of wood.

Relative Humidity 30" F Temp. 50' F Temp. 70" F Temp. 90" F Temp, 0 (% (%1 (%1 (%1 90 X 21 21 20 80 16 16 16 15 70 17 13 13 12 60 11 11 11 10 50 9 9 9 9 40 8 8 8 8 30 6 6 6 6 20 4 5 4 4 10 2 3 2 2

Table I: Relation between emc of wood and relative humidity of air surrounding it (data taken from Table 3-4 of Wood Handbook).

Shrink and Swell

In furniture construction, the effect of temperature can be disregarded. Once the lumber has been kiln dried, the range of emc is rather small at temperatures and humidities to which furniture is subjected. It would be pcjssible to disregard moisture content of the wood as well as temperature except for one thing -- as the moisture content of wood increases, the wood swells; with decreased moisture content, the wood shrinks. Even with small changes of moisture content (which are within the range expected of furniture), the amount of change in dimension is great enough to cause serious trouble unless precautions are taken in designing the construction to allow for the shrink and swell.

31 This shrinkage occurs when the wood is dried from its original green condition and is one of the big problems in drying lumber. But the original shrinkage is not the end of the story. As long as the piece of wood lasts, it will continue to shrink or swell whenever its moisture content changes. And its moisture content will continue to change whenever there are humidity changes in the air to which it is exposed. There is a theory that as time goes on the amount of shrink or swell is somewhat less than at first. Some antique pieces of furniture seem to confirm this, but there is no solid proof of it. The safe procedure in working out furniture construction is to assume that the amount of shrink or swell will always be the same as at first. It is at least known that the dimensional changes do not increase as time goes on.

With shrink and swell, the peculiarities of grain enter the picture again. The amount of shrink or swell with the grain is so small that it can be disregarded in practical furniture construction. It can be assumed that the length of a piece of wood with the grain will not be affected by changes of moisture content within th-rniture range.

But across the grain, it is different. In fact, it is found that there are different kinds of "across the grain" so that this term must be broken down and redefined. Experiments have shown that any given piece of wood has two different rates of shrink or swell. One is called "tangential"; the other is called "radial." These are illustrated in Figure I below. If a line is drawn from the heart of a log to the outside where the bark is, the line will be approximately a radius of the circle represented by the end of the 1 og.

Plain or flat sawn

Bastard sawn 'awn

Figure I: Board sawn from position A will have tangential shrinkage in width; board from B will have radial shrinkage; board from C will have both.

32 Shrinkage or swelling measured along such a radius is called "radial .Ii It is found experimentally that it makes very little difference which radius is drawn. Amount of shrinkage in any one log per unit length of the radius is fairly uni form throughout.

If a line is drawn tangent to the outside circle of the log, shrinkage or swelling along such a line is called "tangential." The tangent can be drawn to any of the circles formed by the growth rings of the tree instead of being drawn tangent to the outside of the log; the result will be the same, and shrinkage along this line will also be called "tangential." The amount of tangential shrinkage of any one log is about the same for any tangent.

As a board is dried from the green condition, it is found that very little shrinkage occurs until the board dries down to "fiber saturation point." This is approximately 30% moisture content regardless of species. It is further found that, below fiber saturation point, the shrinkage is approximately in proportion to the loss of moisture content. This means that, if a certain species shows 10% tangential shrinkage from green to bone dry, there will be no shrinkage from green to 30% moisture content. There will be 10% shrinkage from 30% to 0% moisture content. Since this is a straight line relationship, it means that a one point reduction in moisture content anywhere between 30% and 0% will result in 1/3 of 1% tangential shrinkage. Also, a one point increase in moisture content will result in 1/3 of 1% tangential swell. Percent shrink or swell is figured on the original green width. If a board is 8 inches wide when green and 7.2 inches wide when bone dry, it has shrunk 8 - 7.2 = 0.8 inches, and 0.8/8 = 10% shrinkage.

Table I1 gives the amount of shrinkage for several species commonly used in furniture. Data are from experimental results pub1 ished in Wood Handbook. Using these data, it is possible to calculate the amount of shrink or swell to be expected in a furniture part provided moisture content of the part is known when it is machined and provided the maximum and minimum humidities to which the furniture is likely to be exposed are known. Get emc for these humidities from Table I, calculate the number of points change in moisture content from that at time of machining, and calculate percent shrink or swell for the species as listed in Table 11.

In calculating the amount of shrink or swell of a furniture part, using Table 11, the question arises whether to use tangential or radial shrinkage figures. This requires further explanation. There are several methods of sawing lumber. Refer back to Figure I. Board A is sawn from the log close to the bark and as nearly parallel to the outside of the log as may be. This kind of a board is called "plain sawn" or "flat sawn." The width of this board will be affected by tangential shrinkage, the thickness, by radial shrinkage. Board B is sawn from the log with its faces parallel to a radi\;s of the log. This kind of a board is called "quarter sawn." The width of this board will be affected by radial shrinkage; the thickness, by tangential shrinkage. Board C is sawn from the log so that it is neither strictly "plain sawn" like A nor "quarter sawn" like B, but it is a compromise between them. It is called ''bastard sawn."

33 Total Shrinkage to Shrinkage per 1 0% Moisture Content' Point Change Moisture Conten

Radial Tangential Radial Tangential

Beech 5.5% 11.9% 0.1 8% 0.40%

Ye1 1 ow Bi rch 7.3% 9.5% 0.24% 0.32%

Cherry 3.7% 7.1% 0.12% 0.24%

Hackberry 4.8% 8.9% 0.1 6% 0.30%

Red Lauan 3.3% 8.0% 0.11% 0.27%

Ma hogany 3.7% 5.1% 0.12% 0.17%

Hard Map1 e 4.8% 9.9% 0.16% 0.33%

Red Oak 4.7% 11.3% 0.16% 0.38%

White Oak 5.6% 10.5% 0.19% 0.35%

Sweet g um 5.3% 10.2% 0.17% 0.34%

Sycamore 5.0% 8.4% 0.17% 0.28%

Tupelo 5.1% 8.7% 0.17% 0.29%

Walnut 5.5 7.8% 0.18% 0.26%

Pop1 ar 4.6% 8.2% 0.15% 0.27% 'Assumes no shrinkage from green to 30% moisture content

Table 11: Shrinkage values of wood based on its dimensions when green. (Data from Wood Handbook, Table 3-5)

When a carload of purchased lumber arrives at a furniture factory, no one knows from what position in the log any board was sawn. But the growth rings of the log show on the end of every board, and from the growth rings, the story can be told. If the growth rings are practically at right angles to the faces of the board, the board is strictly quarter sawn. If they are practically parallel to the faces of the board, the board is strictly plain sawn. If in between, the board is bastard sawn.

34 The commercial grading rules for lumber do not draw the lines as tightly as indicated in this discussion. Look at one end of a board; if the angle between the growth rings and the faces of the board is greater than 45", the board is commercially considered quarter sawn. Otherwise, it is considered plain sawn. Many times a batch of plain sawn lumber may contain some quarter sawn, but lumber sold as quarter sawn is not supposed to contain anything except commercially defined quarter sawn.

Many small sawmills will cut a log into boards without turning the log. This is called "live sawing'' and results in lumber which is a mixture of plain, quartered, and bastard sawn. For many uses, this makes no difference. Most large mills with mechanized saw carriages and log handling do turn the log. Most of the defects in logs are near the center. To get the highest grade of lumber, the big mills cut in from one side of the log until they start hitting defects, then turn and cut in from another side and so on until they have cut in from all four sides. Thus the high-grade lumber (#1 common and better) is likely to be nearly strictly plain sawn. The center of the log's being defective is likely to produce #2 common and worse. No one cares too much how this is sawn, so a big mill is likely to saw the center live sawn with a resultant mix of plain sawn lumber, quartered, and bastard sawn in the low grades. If nearly uniform true plain sawn lumber is important, buy high grades from a big mill. If nearly uniform quarter sawn lumber is important, buy quarter sawn on national grading rules. If neither is important, just buy lumber.

-Warp In general, when boards are cut from a log in the sawmill, the faces of the boards will be flat and the edges will be fairly straight. But as the boards lose moisture content in air drying or kiln drying, various forms of crookedness develop due to piling the lumber so it does not lie flat or due to shrinkage of the wood as it dries (see Figure I1 on the following page). As discussed in Chapter 6 of Production Woodworking Equipment,l these various forms of crookedness can be machined out of the lumber so that the blanks for furniture parts, as they leave the rough mill, are flat and straight. If the stock is machined at 8% moisture content and the furniture parts always stay at the same moisture content, there would be no problems with warpage to avoid in furniture construction. But the moisture content of furniture parts does vary with changes in atmospheric humidity. This causes shrink or swell and brings into play the same forces which caused the lumber to warp in the original drying process. Ordinarily, there is very little trouble with "crook" or "bow" in furniture parts if the stock has been originally machined flat. But this is not true of "cup" and "twist," which do cause trouble.

Rudolph Willard, Production Woodworking Equipment, 4th ed. (Raleigh, NC 27650: N.C. State University, 1980)

35 TWIST (Of? SPIRAL WARP)

Figure 11: Illustration of different forms of crookedness or wrap which may occur in lumber as it dries.

36 cup Furniture parts can develop cup from two different causes. The first cause is a moisture content either greater or lesser than the moisture content at which the stock was machined flat. Figure I11 illustrates this. Figure III(A) shows the end of a strictly plain sawn board. If this board picks up moisture, it will swell in width. But both the top face and bottom face of the board have nearly perfect alignment with the growth rings so that both faces will swell the same amount (the tangential shrink and swell rate of the species involved). Although the board is now wider, it will stay flat.

Figure 111: Shrink and swell causes lumber to "cup" when grain is not uniform.

37 A similar situation exists in Figure III(B) except that both faces are nearly perfect quarter sawn and both will swell the same amount (dependent upon the radial shrink and swell rate of the species involved). (B) will swell less than (A), but both will stay flat.

In Figure III(C), the top face of the board is nearly perfectly plain sawn, but the bottom face approaches quarter sawn. If this board picks up moisture, the top face will swell more than the bottom face, and the board will cup as shown in Figure III(D). If the board lost moisture instead of picking up, the top face would shrink more than the bottom face, and the board would cup in the opposite direction. This illustrates that, if the two faces of a board have about the same grain characteristic (tangential, radial, or mixed), the board will stay flat at various moisture contents; if the two faces have different grain characteristics, it will cup as moisture content changes.

The second cause of cupping affects boards of all sorts of grain characteristics. If the top face of the board is exposed to high humidity but the bottom face is not, the top face will pick up moisture and swell but the bottom face will not swell. This will cause the board to cup as in Figure III(D). This effect can often be seen in a furnit ure factory where wide tops are bulk piled on shop trucks. The top face of the upper piece on the pile is exposed to the air; the bottom face is not because it lies tight on the pile. If the weather is humid, the upper piece on the pile will cup with the convex side up. If the piece is turned over, it will flatten by the next day or maybe even reverse its cup. If it is left alone and the humidity decreases, the piece will flatten again.

Another way this effect shows up in a furniture factory is on a finished chest of drawers. The chest top has several coats of finish on its upper surface but no finish on its under surface (inside the case). If the weather changes, the unfinished under surface will change moisture content much faster than the finished upper surface,and the chest top will cup unless it is anchored to the case with fastenings strong enough to resist the forces trying to cup the top.

There is a widespread belief that a finish seals the wood against transfer of moisture to and from the air. This is not true. A finish does slow down the transfer very much, but it does not prevent it. If the finished wood is exposed to humid air long enough, it will eventually reach the same equilib- rium moisture content as an unfinished piece.

Twi st

Twist is not as easy to explain as cup. But if the wood structure of a board is such that it twists in the original drying, it is very likely that it will continue to twist with changes in moisture content even if it has been machined flat at a certain moisture content, say 8%. Tendency to twist is one of the characteristics of certain species. White pine, basswood and chestnut have very little tendency to twist; sap gum and black gum have a much greater tendency to twist. Most other species fall between these two extremes.

38 Since shrink, swell, and warp are natural characteristics of wood when exposed to varying humidities of air and since furniture is so exposed, it is important to work out construction of furniture so that no harm results to the finished piece of furniture due to the tendency of its various parts to shrink, swell, or warp. Sometimes this is quite a challenge.

Gluing

In furniture construction, gluing is much used to fasten different pieces of wood together, so it will be well to discuss here some of the characteristics of wood and glue which affect furniture construction.

There are many different kinds and types of glue. But it is very seldom that the details of furniture construction are dictated by the use of a particular glue; generally, the construction details are decided, and then a suitable glue is selected. A particular glue often dictates manufacturing process and equipment, but rarely dictates construction.

When properly and carefully used, almost any kind of glue has as much strength as the wood itself where two pieces of wood are glued side-grain- to-side-grain. In an edge-glued core or solid lumber panel, .forces may be applied which try to shear the glue joint or to pull it apart by tension, and the wood itself is more likely to fail than the glue joint, provided the joint is properly made. Different species of wood have different strengths, so in testing glue for shear strength, it is customary to shear a joint made between two pieces of a high-strength species (generally hard maple). If the wood fails, the glue is stronger than the wood, and it would naturally be stronger than a lower-strength species. If the glue is stronger than a strong wood, it is not very important how much stronger because the wood will fail before the glue.

In order to get some idea of glue strength, Table 111 gives wood strength for a few species commonly used in furniture. These data, taken from the Wood Handbook Table 4-2, are the results of laboratory tests.

S2 P Shear Strength Tensile Strength

Hard Map1 e1 2330 --- Yell ow Bi rch 1880 920 Red Oak 2000 840 White Oak 2000 800 Sweet Gum 1600 760 Tupelo 1590 700 Pop1 ar 1190 540 Walnut 1370 690

Ta ble II I : Strength properties of sel ected woods

Small clear specimens, M.C. of 12%. All figures are maximum strength in pounds per square inch. Column "S" = shear parallel to grain (way glue joints usually tested) Column "T" = tensile strength perpendicular to grain (as if glue joint were being pulled apart instead of sheared apart).

39 From Table 111, it would appear that in laboratory tests more glue joints have a shear strength somewhat above 2000 pounds per square inch and a tensile strength about half that (the strengths of the strongest species).

What has just been said applies to glue joints that have been very carefully made with laboratory procedures. In production the procedures are not as precise or carefully controlled, and glue joints frequently fail before wood failure, which indicates a joint which is weaker than the wood, not stronger. Nobody knows how much discount should be applied to laboratory tests of glue joint strength in order to get a figure representative of production shop practice. This is partly because gluing practice varies so widely from one shop to another.

When gluing side-grain-to-side-grain, the strength of a glue joint seems to be affected very little whether a tangential face is glued to another tangential, or radial-to-radial, or radial-to-tangential. The important thing is whether side grain is glued to side grain. This is the common situation in edge gluing and also in laminating as in plywood.

If an attempt is made to glue end-grain-to-end-grain, the strength of the resulting glue joint is so poor as to be nearly worthless. Tests seem to indicate that the strength of this kind of joint has not more than about one fourth the tensile strength of a side-grain joint. This would correspond to about one-eighth of the shear strength of a side-grain joint.

Fortunately, there are very few situations in furniture construction where end-to-end gluing would be of any worthwhile advantage even if the glue joint were strong. In rare cases where the ends of two pieces meet and should be securely fastened, a machined joint, such as a fingerjoint, is used instead of simply butting the ends together and gluing.

If an attempt is made to glue end-grain-to-side-grain, the strength of the resulting glue joint is poor. Very little quantitative data is available, but the strength is probably not much better than gluing end-grain-to-end- grain. In furniture construction two parts often meet end-grain-to-side- grain and must be fastened together. But in such cases, a glued butt joint is almost never used. Instead, some sort of machined joint is common; examples would be mortise and tenon, dowel, dovetail, and box corner. This situation will be discussed further in the chapter on joints.

Sometimes parts come together at angles between 0" and 90". In such cases, the strength of a glue joint would be somewhere between that of a side- to-side joint and that of an end-to-side joint. The more closely the joi.nt approached true side-to-side gluing, the stronger the joint would be. No reliable data are available for calculating the strength of angle joints. In practice, if the deviation from true side-to-side gluing is less than about 15", a glue joint is often used; if more deviation than that, a machined joint is recommended.

40 CHAPTER 4

CHARACTERISTICS OF VENEER AND PLYWOOD

Veneer may be defined as thin layers or sheets of wood. It is produced on a 1 athe or a sl icer and is commonly referred to as "rotary" or l'sl iced" veneer, according to the manner of cutting. It may be used as a single ply or as a combination of plies bonded together to form plywood, or it may be glued to lumber or other core materials to form veneered products. Single- ply veneer is seldom used in furniture construction, while plywood is used quite extensively. This chapter will discuss the characteristics of both veneer and plywood.

Years ago, most furniture plywood was fabricated with a vegetable glue that had poor moisture resistance. In damp climates, many household furniture users had trouble with veneer delamination due to loss of "holding power" of the glue. As a result, many people came to believe that veneered furniture was not durable and was somehow inferior to solid furniture. Much of this prejudice still exists in spite of the fact that today most furniture plywood is fabricated with synthetic resin glues which are water resistant and highly resistant to the delamination caused by humid climates. Recent ad- vances in adhesive technology have helped to make plywood more durable and trouble free than solid wood for applications in furniture.

The discussion of plywood and how it affects furniture construction will be considered under six headings:

Mechanical Characteristics Cores and Crossbanding Faces Edges Appearance and Adverti sing Cost Comparisons.

Mechanical Characteristics

The principal mechanical characteristics of plywood which affect furniture construction are shrink, swell , and warp.

Shrink and Swell

One of the most important differences between a plywood panel and a panel made of solid lumber arises from the fact that the solid panel will shrink and swell significantly across the grain with changes in moisture content, whereas the plywood panel will not. For purposes of furniture construction, it can be assumed that changes in moisture content do not affect the 1 ength or width of a plywood panel. However, plywood panels will change in thickness according to moisture content. This seldom affects furniture construction. It is the dimensional stability with respect to length and width that is the important consideration.

41 To illustrate this point, consider the following example: A 13/16" thick, 5-ply panel contains a lumber core 5/8" thick, two cross bands l/16" thick, and two faces l/28" thick. The grain of the core runs with the length of the lumber. Applied on each side of the lumber core are the two crossband veneers with grain at right angles to the grain of the core. Applied on the crossbands are the two face veneers with the grain at right angles to the grain of the crossbands. This makes the grain of the faces parallel to the grain of the core. If the moisture content of the lumber core increases, it will tend to swell across the grain. However, the core is glued to the crossbands, and "across grain" of the core is "with grain'' of the crossbands. Consequently, the two crossbands glued to the core tend to keep the core from swelling. These opposing tendencies exert a force trying to shear the glue bond. Each square inch of glue line has a shearing strength of between 1,000 and 2,000 pounds; therefore, the total strength of the glued surface between core and crossbands is great enough to resist the swelling force of the core. If the crossbands were extremely thin, the swelling force of the core could pull the crossbands apart by tension failure of the wood in the crossbands, but this does not occur with the crossband thicknesses normal ly found in furniture plywood construction.

Warp

Warpage is caused by shrinking and swelling. If wood is kept at a constant moisture content, a panel that is machined flat will remain flat. However, this is never the case. The study of the basic causes of warpage falls into the field of wood technology. In furniture construction, it is more important to know what to do to minimize troubles caused by warping.

1. Balanced Construction

The best way to reduce warpage in plywood is to use a "balanced" construction when making panels. This is done by arranging the plies in pairs about a core of lumber, veneer, particleboard, or other material so that for each ply there is an opposite, similar and parallel ply. Matching the plies involves the consideration of (1) thickness, (2) species, (3) moisture content, and (4) grain direction.

The use of an odd number of plies permits an arrangement that gives a substantially balanced effect; that is, when three plies are glued together with the grain of the outer two plies at ri.ght angles to the grain of the center ply, the stresses are balanced, and the panel tends to remain flat with changes in moisture content. With five, seven, or some other uneven number of plies, the forces may be similarly balanced. If only two plies are glued together with the grain of one at right angles to the grain of the other, each ply tends to distort the other when moisture content changes occur, and cupping usually results. Similar results are likely when any even number of plies are used unless the two center plies are parallel and act essentially as one ply.

The use of balanced construction is highly important for thin panels (3/8" or less) that must remain flat. However, thin panels rarely stay as flat as thick panels. Being thin, they are more flexible, and it is easier to pull thin panels flat when attaching them to a frame. Thus, warpage is not

42 really a serious problem. For thick panels (3/4" or more), the core can have a direct effect on the degree of flatness of a plywood panel.

2. Critical Furniture Parts

Certain parts of furniture can tolerate considerable warpage in the plywood from which they are made. A thin panel is quite flexible. If it is recessed into a frame or nailed to a flat structure, the panel can be badly warped without doing much harm. Framing it in or fastening it down straightens out the warp. Examples of this are drawer bottoms, dust panels, and back panels. Thick plywood is more rigid. However, it does not take a very large force to straighten it. It can be straightened if it is to be fastened to a structure by screws or other fastenings which will pull the warp out of it. An example is a case top which is screwed to a case.

A plywood door is an example of a part which requires flat plywood. It is attached to a case by hinges on one edge, and there is no way to pull the warp out of it. If the panel is crooked, the door will be crooked and will not fit properly along all four edges. The drop leaf on a table is similarly attached, but it does not need to be as flat as a door because it does not fit up to anything except along the one edge which is hinged.

Curved Plywood

Curved plywood is used for drawer fronts, chair backs, and other appli- 'cations. It differs from ordinary plywood in its shape and in the manufacturing method. Also, the core cannot be made of particleboard or fiberboard. Bandsawn lumber cores are not used any more, so all curved plywood today is "all veneer" construction.

The manufacturing method most common today is high-frequency pressing. Wood molds are covered with aluminum electrodes, and these are connected to a high-frequency generator. This method produces a panel every two minutes or so, even though the press will hold only one panel at a time.

Cores and Crossband

While there are many kinds of core material, the most widely used are lumber, veneer, particleboard, and medium-density fiberboard. Each will be discussed.

Lumber Core

Given proper drying of core lumber and balanced construction of plywood, the species of core lumber can have a significant effect on plywood warpage. Species such as basswood and chestnut are among the best for staying flat, while gum is one of the worst. Poplar, maple, and birch are all fairly stable core species.

Another important factor is width of the strips which are edge glued to make the core. Many furniture plants set a limit for the maximum width of any strip to be used for a core. Typical limits are 2 /2" for gum and 4" for

43 poplar. With species given to warpage, the maximum width should be narrower than with species that are not given to warpage.

Quarter sawn lumber does not warp as badly as plain sawn. A quartered gum core might stay about as flat as a plain sawn poplar core. Most sawmills do not quarter saw many species except gum, tupelo, sycamore and oak. Proper processing is also important but is not a matter of construction. It is more appropriately discussed as a topic in Furniture Manufacturing -Pro- cesses .1

Veneer Core

For flat plywood on veneer core, the species of the core veneer is important. Species prone to warp in lumber are also prone to warp in veneer. It is not customary to clip and splice narrow strips of veneer in order to reduce warpage. Quartered veneer is almost never used to get a flat core. Veneer core panels are likely to give more trouble from warpage than panels with lumber or particleboard cores. The use of veneer core for a 5-ply panel results in a thin panel with a close proximity of face to back. Therefore, trouble develops in veneer core panels because they are thin, not necessarily because they have veneer cores.

Particleboard Core

There are two basic types of particleboard: extruded board and platen board. Panels made by the extrusion process generally are produced in cap- tive board manufacturing departments by the furniture manufacturers for their own consumption.

Platen board is primarily made by commercial companies. Modern platen board plants tend to make large panels of up to 200 square feet which are cut to the sizes specified by the furniture manufacturer before being shipped there.

Extruded board is usually made from rough end waste that is hogged into splinters and mixed with urea formaldehyde glue. This mixture is fed into the extruder, which has a reciprocating ram that moves between two hot platens.

Fi gure I : Extruded particleboard machi ne

1A.L. Prak and T.W. Myers, Furniture Manufacturing Processes, 3rd ed. (Raleigh, NC 27650: N.C. State University, 1981)

44 Figure 11: Extruded particleboard.

Platen board is made most often from green wood waste, such as sawmill slabs. The particles cut from this waste are mostly flakes; and these are dried, mixed wit h urea formal dehyde gl ue, and pressed between heated platens.

Figure 111: Platen board manufacturing process.

Figure IV: Platen-type particleboard.

45 The major particleboard plants today are making a multi-layer platen board with three-stage matforming. The surface of the board is finely textured and suitable for printing as well as 3-ply veneering.

The extruded board has a rough surface and can only be used in 5-ply panels. The cross band serves as an insulation layer and prevents "telegraphing." Telegraphing is the uneven surface of the lower layers showing through the face veneer. This is especially objectionable when a glossy finish is used. Besides the surface of the panel, there is an even more important reason for 5-ply construction. Extruded board is very weak in the extrusion direction, and the cross bands serve to make this material strong enough for furniture applications. It should also be noted that extruded board is very unstable in the extrusion direction. If it were applied, or example, as carpet under1 ayment, the panel s would buck1 e severely.

Platen-type board is very stable in both length and w dth and may be used in 3-ply as well as 5-ply constructions. When solid lumber bands are used, most companies prefer a 5-ply construction to mask the joint between the board and the lumber band.

Particleboard does not have the rigidity of solid lumber in the grain direction. In book shelves, for example, particleboard shelves will sag about three times as much as solid lumber shelves. For this reason, long unsupported conference table tops are made with lumber core panels.

Medi um-Densi ty Fiberboard

Medium-density fiberboard (called MDF) is a composite board formed by hot pressing a mat of resin-coated wood fibers. These thin fibers (a few thousandths of an inch thick) result in a product that has excellent surface smoothness, a tight edge and uniform density. Most MDF comes in the density range of 44-50 pounds, and it can be engineered to precise specifications.

The fine fibers in MDF result in a smooth homogeneous edge that can be shaped, sanded, stained, and grained to conform with traditional or contemporary sty1 es. Surface treatments may include wood veneers without crossbands, plastic laminates, vinyl, paper and foil films, as well as liquid finishing systems, such as printing or painting.

The principal reason for using particleboard and MDF is the substantial economy achieved. A particleboard core can be produced for less than one third of the cost of a lumber core.

Crossbanding and Outer Veneers

Separate from the core, the crossbanding and the outer layers of veneer can have an effect on warpage in a plywood panel. Often the grain in a piece of veneer does not run exactly parallel to the edge of the veneer when it is clipped to size.

46 Figure IV(A) shows a piece of veneer with the grain parallel to the edge of the veneer, whereas Figure IV(B) shows a piece of veneer with the grain not parallel to the edge. Since shrink and swell are at right angles to the grain, the arrows show the direction of shrink and swell. If a three-ply panel with thin, flexible veneer core has a face and a back both like Figure IV(A), the face and back will try to shrink the same amount in the same direction, and the pull of the face will be exactly balanced by the pull of the back. As a result, the panel will stay flat. The same would be true if both the face and the back were like Figure IV(B) if the veneers were laid so that the direction of shrink were the same on both face and back veneers. But if the face were like Figure IV(A) and the back like Figure IV(B), the direction of shrink of the face would be different from that of the back, and the panel would warp when shrink or swell occurred. In practice, it is difficult to insure that the grain of a piece of veneer is strictly parallel to the clipped edge; however, a close approximation is better than a wide variance. Species like gum (with spiral grain) are more likely to give trouble than pop1 ar, which has straight grain.

A -Direction of grain. Figure IV: Different grain orientation in plywood.

Crossbanding is the veneer used in the construction of plywood with five or more plies. In a 5-ply construction, it is placed at right angles between the core and faces. Although crossbands have a definite effect on the quality and stability of a panel, they are typically sliced from the less expensive species of wood, such as poplar. Imperfections in crossbands, such as marked differences in the texture of the wood or irregularities in the surface, can easily be seen in panels with thin face veneers. Therefore, it is important that veneer used for crossbanding have high strength in all directions, uniform thickness, smoothness, and good bonding properties.

Synthetic materials have been developed that exhibit all the properties of good crossbanding while minimizing the common veneer problems of checking, splitting, and telegraphing. These are wood products made from wood-

47 cellulose material that is compatible with most woods and woodworking processes. This material is commonly referred to as fiber crossbanding and is being used in crossbanding, facing, and edge banding applications.

Fiber crossbanding comes in rolls which permit easier storage and a greater degree of automation than conventional veneer crossbanding. Fiber crossbanding can be applied to particleboard with dry phenolic glue for crossbanding and backing on table tops, with liquid urea formaldehyde adhesive for crossbanding in curved, laminated seats and backs, and with hot melt adhesive for edge banding. As a backing material, fiber crossbanding accepts lacquering, painting, or grain printing. Fiber crossbanding exhibits good heat resistance, and it is not adversely affected by moisture.

Three-Ply vs. Five-Ply Construct ion

The core of a 3-ply panel must be very smooth and free of defects because any imperfections will show through (telegraph) the thin face veneers. These cores are commonly made of high-quality, fine-surface particleboard or medium-densi ty fiberboard, see Figure V(A). Three-ply panels are usually edge banded with veneer or plastic edge bands to simulate a solid wood panel. On less expensive furniture, the smooth homogeneous edge of a medium-density fiberboard core may be stained and grained rather than edge banded.

Figure V(A): 3-Ply panel construction with a veneer core.

Figure V(6): 5-ply panel with lumber-banded particleboard core, veneer crossbands, and veneer faces.

48 Five-ply panels (see Figure V(B)) may be veneer edge banded, but they are more often lumber banded for use on furniture styles that require a shaped edge. The lumber banded panel is profile shaped to give the appearance of a solid wood top. Crossbands are necessary because the joint between the lumber band and the particleboard core would telegraph through the thin face veneer if only three plies were used. Edge treatments will be covered in more detail later in this chapter.

Faces

The major appearance effect of a panel has to do with its face. The grain on the face of a board is not the same as the grain on the face of any other board. Consequently, if a panel is made of edge-glued lumber, each individual strip will have its own individual grain unlike that of any other strip. It is impossible to make a panel out of lumber and have "matched- grain" appearance where the grain of each strip in the panel looks like the grain of every other strip in the panel. With face veneers, "matched-grain" appearance is possible. If the design of a piece of furniture requires a matched-grain effect, plywood must be used instead of solid lumber.

The matched-grain effect is possible with veneer because each sheet of veneer is thin (l/28" thickness is fairly standard) and the grain of a log does not change much in 1/28". When face veneer is cut from a log, all the sheets of veneer are kept together, and they are kept in the same order as they are sliced off the log. This means that each sheet in the flitch grew in the tree right next to the sheet immediately below it so that there is little difference in the grain of these two sheets. When a furniture factory buys face veneer, it buys specific flitches which are identified by a number. The veneer salesman has sample sheets of each flitch, so the buyer can decide which flitches meet the character of grain and color he desires.

Species of Wood

Grain appearance and color of veneer are affected by the species of wood from which the veneer is cut. Among our domestic hardwoods, oak has a coarse, porous grain, which is very apparent,while the grain of poplar is so faint that it is hardly noticeable. The color of sapwood is generally different from that of heartwood. In walnut and cherry, the sapwood is very light, and the heartwood is dark. In sycamore, there is not much difference in color between sapwood and heartwood. The furniture designer usually de- cides on the species of face veneer to be used in order to achieve the color and grain appearance he wants for each particular design.

Method of Cutting Veneer--

The grain pattern of veneer is affected by the method of veneer cutting. Any log of a porous species of wood can produce veneer with quite different grain effects depending upon the particular method used to cut it. The three most commonly used methods of cutting veneer are the following:

Rotary cut Flat sliced Quarter sl iced.

49 In rotary cutting, the log is mounted between the spur centers of a lathe and rotated against the veneer knife. The cutting is accomplished by re- volving the log against the knife with the veneer's being peeled off in a continuous sheet very much like unrolling paper. As the veneer comes from the lathe, it is either cut off into convenient widths for handling or reeled up behind the lathe and clipped to width later. Rotary-cut veneer is usually flat-grained as this method of cutting makes the face of the veneer approximately tangent to the growth rings in the log. In a species such as Douglas fir, where growth rings are quite noticeable, rotary cutting develops a veneer with an extremely "violent and wild" grain. It is difficult to get a grain-matched face from rotary-cut veneer; therefore, it is never used when grain-matched face is desired. However, rotary-cut veneer is used almost exclusively for crossbanding and panel backs. The quality of the veneer produced on a lathe depends on the quality of the logs used, the skill of the operator, the condition and adjustment of the lathe, and the proper conditioning of the wood for cutting. Figure VI is a schematic diagram showing how rotary veneer is cut, and Figure VI1 shows a piece of rotary-cut wal nut.

Figure VI: Schematic diagram of rotaryxut veneer operation.

Figure VII : Rotary-cut wal nut veneer.

50 In flat slicing, the log is generally sawn in half and debarked. The half log is mounted on a table which moves the log up and down in a vertical stroke motion. The veneer knife remains stationary in the vertical direction but is advanced toward the log in increments of thickness which are synchronized with strokes of the log so that on the down stroke the knife slices a sheet of veneer off the log. After slicing, the sheets are stacked just as they grew in the tree. Figure VI11 is a schematic diagram showing flat slicing. The grain appearance of a sheet of flat-sliced veneer is much like that of a plain sawn board. Figure IX shows a piece of flat-sliced walnut.

Figure VIII: Schematic diagram of flat-slice veneer operation.

Figure IX: F1 at-sl iced walnut veneer.

51 In quarter slicing, the veneer cutting machine is the same as in flat slicing, but the log is prepared differently. It is sawn into quarters and slabbed. A quarter log is mounted on the slicing machine, and the slicing proceeds as diagrammed in Figure X. Quarter slicing results in a grain appearance that is primarily a series of parallel straight lines. Figure XI shows a piece of quarter-sl iced walnut. ~

Figure X: Schematic diagram of quarter-slice veneer operation.

Figure XI : Quarter-sl iced walnut veneer.

52 Unusual Tree Growths

The appearance of face veneer is affected by the way in which a tree grows. All previous discussion has applied to veneer cut from a log which was the trunk of an ordinary tree. In some species, an occasional tree will have a peculiar grain structure different from normal straight grain. When such a tree is cut into veneer, it shows different grain figures such as curly maple or birds-eye maple. Figure XI1 shows a piece of walnut veneer with curly grain similar to curly maple.

Figure XII: Walnut veneer cut to show a curly grain.

Figure XIII: Mahogany veneer with a ribbon stripe.

53 Apart from unusual grain growth in a tree, certain parts of the tree have different grain figures from the trunk of the tree. Figures XIV, XV, and XVI show veneer cut from stump, crotch and burl, respectively. Burls occasionally occur as a growth lump on the trunk of a tree. They are comparatively rare, and burl veneer is consequently expensive.

Figure XIV: Veneer cut from stump walnut.

Figure XV: Veneer cut from crotch walnut.

54 Figure XVI: Veneer cut from a burl.

Matching

Appearance of a veneered panel can be affected by the method of matching the veneer. Three of the more common methods of matching are the following:

S1 ip match Book match Geometric match.

Figure IX showed a piece of flat-sliced walnut. If four pieces of this veneer are to be matched side-by-side, there are two ways of doing it. If the pieces are simply slipped sideways without turning any piece over, the result is a slip match as shown in Figure XVII (on the following page) . If one piece is placed on a table, the next piece turned over, the third piece is not turned over, and the fourth piece is turned over, the result is a book match as shown in Figure XVIII (on the following page). It will be noted that the book match is symmetrical with respect to a vertical center 1 ine, whereas , the slip match is not. Symmetry is often desired on the front of a case or a bed panel; however, it is often not important on tops. After the staining and finishing, the appearance of color usually depends upon the angle at which light strikes the pores of the wood when the pores are not parallel to the face of the veneer. For this reason, book matches sometimes result in panels which give the appearance of alternate pieces of veneer being darker than the ones in between. Slip matches seldom produce this effect. The effect is more pronounced on book matches with stump, crotch, or burl than it is with flat or quarter-sliced veneer.

55 Fi gure XVII : S1 ip-match veneer.

Figure XVI I I : Book-match veneer.

56 Geometric matches are frequently four-piece matches, but an infinite number of possibilities exist. Veneers can be specifically selected for figure and jointed together within a panel to produce special patterns. Effects obtainable by matching are almost unlimited and depend only on imagination and ingenuity. As previously stated, matching is not feasible with solid 1 umber.

Surface Fini sh

Plywood fabricated with face veneer of hard species like maple resists dents better than soft species like poplar. But in practice, there is usually very little choice; the species of the face is generally dictated by the appearance wanted for design and sales considerations. The type of finish applied to plywood with a wood veneer face has an important bearing on the resistance of the finished surface to wear and tear. For example, a good heat conversion synthetic varnish will resist wear and scratching better than lacquer. But if the face veneer itself is soft, a tough finish does very little to improve resistance to dents.

P1 astic Lami nates

Where hardness and wear resistance of face is desired, plywood can be faced with a high pressure plastic laminate rather than a wood veneer. Laminated surfaces are desirable because of their good appearance, durability, resistance to stain and heat, and uniformity in matching. Wood grain can be photographically reproduced on the top of these laminates, and it is very difficult to tell them from actual wood veneer at a casual glance. Such faces are particularly popular for table tops which are subjected to more abuse in service than other furniture surfaces.

A typical plastic laminate is composed of several layers of phenolic resin impregnated kraft paper filler stock, a layer of melamine resin impregnated printed pattern paper, and a transparent overlay sheet containing melamine, all of which are assembled in a press with pressures of approximately 1,500 P.S.I. at temperatures exceeding 250°F. The backs of the sheets are generally sanded to permit easier bonding.

Plastic laminates are bought pre-finished, and the finish is highly resistant to wear and scratches. They are 1/16" or l/32" thick and come in a wide range of standard sizes, colors, and patterns. High pressure laminates are bonded to a co,re material such as particleboard or plywood. The surface is tough enough to use over lumber-banded particleboard; however, most manufacturers use high-quality flakeboard as a core material because the lower- priced extruded board may telegraph through the surface of a laminated sheet.

Frequently a special back is used to more nearly balance the panel construction with the plastic laminate face. The use of plastic laminates on both faces of a panel is rather costly, although it gives a perfectly balanced construction. Inexpensive backing sheets were developed to achieve the effect of a balanced construction at much lower cost. One manufacturer of high quality, contemporary, rimless, unsupported conference tables has solved this problem by using reject laminate sheets for backing. He runs

57 the face of the reject through a wide belt sander and glues it, face- to-bottom, to a lumber core. In this manner, he achieves the closest thing to a perfectly balanced construction in his table tops.

Carrying the above experience in plastic back to wood veneer, consider the case of a chest top where a walnut face is desired for appearance. It would be necessary to use a walnut back in order to obtain a perfectly balanced construction. But most 5-ply lumber core tops use a lower-priced veneer like gum or poplar for the back veneer. This violates the principle of a balanced panel because face veneer is walnut and back veneer is gum. Furthermore, there may be a difference in the actual thickness. For a furniture part such as a chest top which can be screwed down to the case on all edges, the fastening can generally pull the warp out of the panel, and the slight unbalance in plywood construction gives no serious trouble.

Edges

On many furniture parts for which plywood is used, the edges do not show so that the appearance of the edge of a panel is not important. Examples of this are dust bottom panels and end panels. On some parts, the edges show only when a drawer or door is opened. Examples are drawer fronts and doors. While the appearance of such edges cannot be completely disregarded, there is no need to be as particular about edge appearance as with tops and bed panels where edges are exposed all of the time. For parts with exposed edges, the edge appearance must be good, and this frequently requires special features in the plywood construction. The specific type of construction depends upon the kind of core used.

With lumber core plywood, no special construction is needed for most medium and lower quality furniture. For high-quality furniture tops, it is frequently desirable to band three edges of the core with bands as shown in Figure XIX below. This is done to the core before the plywood is fabricated.

Figure XIX: Diagram of an edge-banded core.

58 When the core is banded with lumber as shown in Figure XIX, only a small end-grain surface will be shown on the edge. Because it absorbs stain better than side grain, end grain finishes darker. Therefore, it is desirable to minimize the amount of end grain shown by selecting a modest width for the front band.

Sometimes the edges of a top are not to be shaped, but are to be flat as in a design calling for "plank top" effect. In such cases, the edges of the lumber-core plywood top can be veneered with the same species as the face veneer, or one of the vinyl veneers, in which case it is an "edge-veneered" construction. This is done after the plywood is fabricated. Edge veneer can be used with a band saw pattern on the edge of a top provided the bandsawing has no short radius curves. An example of this is the top of a case with a serpentine front such as some Hepplewhite designs. A top with walnut edge veneering can be sold as ''genuine walnut."

With veneer core, the edge appearance is not as good as with lumber core. Since the grain of the adjacent plies is at right angles, it is impossible to avoid end grain on any edge. Veneer core plywood can be shaped on the edges, but it is expensive to sand the shaper cut smooth enough for a finish. In addition, the stain soaking into the end grain gives light and dark stripes after finishing. Only the cheapest furniture can tolerate the edge appearance of veneer core plywood on exposed surfaces, although it can be seen on the edges of drawer fronts in some medium priced furniture. Veneer core can be edge veneered in the same way as particleboard core.

In office tables and desks, the plywood tops are frequently edged with a molding. Figure XX shows one such construction. It could be used on household furniture but usually is not. For edges banded with a molding, the core can be lumber, veneer, or particleboard.

\\\\\\\\\\\\\\\\\\\\\\\\\\\ I

Figure XX: Molding used to edge plywood top.

59 Amearance and Adverti sinq

The use of plywood panels instead of panels made of edge-glued lumber is generally dictated by considerations of either appearance, advertising or cost. In addition, plywood exhibits less shrinkage and greater splitting resistance than solid wood; and it can be produced in larger sizes with increased utilization and more efficient use than expensive woods. For thin panels (l/4" and under), solid lumber would split easily, but plywood will not.

Some customers attach a great deal of importance to "solid" furniture, especially in prestige woods, such as walnut, mahogany or cherry. In order for a piece of furniture to be legitimately represented as %olid" walnut, all exposed surfaces must be walnut and must be lumber, not veneer. This means that walnut-faced plywood cannot legitimately be used on any exposed surface if the furniture is to be sold as "solid walnut." If a piece of furniture has all exposed surfaces made of walnut, it can be sold as "genuine walnut'' even though some of the surfaces are walnut veneer.

With respect to the advertising of furniture, the Federal Trade Commission in 1963 promulgated a set of Trade Practice Rules for the Household Furni- ture Industry. The general rule states that ''members of the industry shall not distribute any industry product under any representation or circumstance (incl uding fai1 ure to adequately disclose re1evant facts) which has the capacity and tendency or effect of misleading or deceiving purchasers or prospective purchasers with respect to its utility, composition, construction, durability, design, quality, quantity or number of pieces , model, origin, manufacture, price, grade, or in any other respect." The rule on wood and wood imitations further states that "members of the industry shall not use any deceit or indirect representation or sales method which is false, likely to mislead because of telling a half-truth, or likely to de- ceive by failure to adequately present facts concerning the composition of plywood having the appearance of solid wood, of simulated finishes on wood, or wood imitations." All this means is that it is a federal offense to represent plywood and plastic as solid wood.

Cost Comparisons

When appearance and sales considerations permit a choice between solid wood and plywood for a furniture part, the choice is usually made by a cost comparison. The figures used for such a comparison should be calculated accurately and compared on an equal basis. Experience has shown that comparisons between solid wood and plywood usually follow results similar to those presented bel ow.

If the exposed surface can be an inexpensive wood such as gum, a veneer-core plywood panel will probably be less expensive than solid wood edge glued. Both will be considerably less expensive than lumber-core plywood. This applies to thicknesses of 3/4 inch or more. On thinner panels, the cost advantage for veneer core becomes greater.

60 If the exposed surface must be higher-priced wood like walnut, any kind of plywood will be less expensive than solid wood.

In any kind of exposed wood surface (low or high priced), plywood with a particleboard core will probably be considerably less expensive than any other plywood or solid wood provided no cost must be added for edge appearance. This applies to thicknesses greater than 3/4 inch. Veneer core is probably cheaper for thin panels.

If the part is narrow (two inches or less), solid will probably be the least expensive unless a high-priced species is required.

Acknowl edgments

Acknowledgment is made to the following organizations through whose courtesy and by whose permission information and illustrations in this chapter have been reproduced.

American Walnut Manufacturers' Association, Chicago, I11 inois.

Chester B. Stem , Inc., New Albany, Indiana.

Nati onal Parti cl eboa rd Associ ati on, Wash ingt on, DC.

61 62 CHAPTER 5

PLASTICS IN FURNITURE CONSTRUCTION

Plastics have been used in the manufacture of furniture for many years. Some of the well-known applications are as follows:

1. High pressure laminates. This is a paper-based product using melamine and/or phenolic resins (see Chapter 4).

2. Flexible polyurethane foam. Polyurethane foam is the number one cushioning material in furniture.

3. Polyvinyl chloride upholstery covers. "Vinyl ,'I as this product is commonly called, is the most important cover material for institutional upholstered furniture. It is also used in household pieces and, of course, in auto- motive seating.

4. Polyester imitation marble and slate. The high cost of marble and slate has left a significant portion of the market to the less expensive imitation product.

5. Fiberg1 ass reinforced polyester chair she1 1 s. When reinforced with fiberglass, polyester is an excellent material for making durable chair shells.

6. Nylon glides, casters and drawer guides. For several hardware appl ications, nylon has very good strength and friction properties.

7. The nature of several finishing materials is such that the furniture finish itself may be classified as a plastic.

The application of plastics in places where wood used to be the principal material will be covered in this chapter.

The commercially important appl ications are the foll owing:

"Vinyl veneer"

Decorative over1ays in polystyrene polyurethane polyester

Structural components with wood grain in polystyrene ABS polyurethane polyester

Upholstery frames in polyurethane.

63 Vinyl Veneer

Vinyl veneer consists of a film printed with a wood grain or other decorative pattern which is embedded in a tough, clear, flexible vinyl film. The film may have a smooth or embossed texture. Vinyl veneer is often pre- coated with a heat or solvent-activated adhesive. The vinyl film is sufficiently flexible to be cut to shape and applied to flat, contoured, or embossed surfaces with radii as small as 1/32" and to be used for application on curved parts. Vinyl veneers can withstand punishment in the form of spills and stains, and they are impervious to fading, chipping, scratching and corroding. The resistance to peeling depends upon the adhesive and the substrate and not on the vinyl which in itself will not delaminate or flake. In addition, vinyl veneer exhibits a remarkable resistance to abrasion, plus easy repairability in the case of abusive gouging. The main problem experienced with vinyl veneer is with respect to its heat resistance; panels covered with vinyl do not pass standard cigarette burn tests. This limits the material's applicability on furniture tops.

The film and pre-coated adhesive is approximately 0.008" thick and can be permanently bonded to almost any smooth surface including wood, plywood, smooth surface particleboard, hardboard, aluminum, and steel. When flat panels are covered with vinyl veneer, a contact roll may be used in bonding the vinyl veneer and the panel together. This way, a continuous process is possible.

Other applications of interest in furniture construction are as follows:

1. Wrapping Technique A typical example would be a molded table leg with four slightly rounded corners and a groove. The wood is fed onto a piece of vinyl veneer. A conveyor belt carries both under a pressure roll assuring contact. Then the vinyl is folded onto one of the vertical sides, and again a pressure roll makes the bond permanent. The process is repeated until all sides are covered. The edges are pressed into a groove and locked in with a small vinyl molding. (See Figure I.)

Figure I: Wrapping technique for applying vinyl veneer to table leg.

64 2. Vacuum Forming In vacuum forming, the material has to stretch, therefore, preheating is necessary. When the vinyl veneer is quite soft and flexible, it is pressed onto a wood part. The air pressure is sufficient to bring the vinyl in contact with the surface. In order to be able to draw a vacuum underneath, the vinyl has to have dimensions several inches in excess of the wood part dimensions. After the vinyl veneer is applied, the excess can be trimmed off, (See Figure I1 below.)

Figure 11: Typical drawer front covered with vinyl veneer by vacuum forming process

3. Folding Construction The application of vinyl veneer to flat boards (usually particleboard) is done by means of contact rolls. The vinyl covered panel is subsequently machined to receive grooves and then can be folded into many shapes. (See Chapter 11, page 195.)

Decorative Over1 ays

Decorative overlays are applied to the fronts of cases, on doors and drawer fronts, bed panels, legs, and the rims of tables. Their purpose is to add a visual effect to the furniture which would be costly to achieve in wood. In a real sense, these overlays are intended to give low and medium-priced furniture an expensive look. The three materials used principally for these over1ays are polystyrene, polyurethane and polyester.

65 Structural Parts

If a relatively plain drawer front is to have a small carving in the center, a practical solution is to use an overlay. This has the advantage that there is no difference in dovetailing and drawer assembly. If, however, the overlay covers the whole drawer front, it may be more advantageous to make the entire part in plastic. The main difference in moving from an overlay to a drawer front is that the larger part must have stiffness, machinability, and so on to function properly. An application where overlays are virtually unknown is the ornate insert in the back of a dining room chair. The appearance may be desired on both sides, and also the structure is often so open that there is no alternative but to make the whole thing out of plastic. To examine the proper use of plastics in these various parts requires a knowledge of the properties and limitations of each of these materials.

Polystyrene

Polystyrene is heavy in comparison to wood. A case with solid polystyrene drawer fronts is noticeably heavier than one with wood drawer fronts. How- ever, polystyrene overlays can be molded with a substantial core, reducing the weight significantly as shown in Figure 111.

Polystyrene Door using poly- Over1ay styrene overlay Mold Core

Figure II I : Schemati c diagram of cored polystyrene over1ay and its application on a door.

66 Polystyrene feels and sounds like glass and can be readily distinguished from wood by tapping on it. Wood grain can be reproduced quite well in polystyrene, but the guality of grain is better for the coarse structured woods. Oak is a popular choice.

Polystyrene is fairly strong. Its strength is sufficient for all overlays and for many parts. Medium-impact polystyrene is satisfactory for drawer fronts but not for long furniture legs. High-impact polystyrene has slightly better properties; but ifhigh performance is required, the choice will often fall on ABS (Acrylonitrile Butadiene Styrene), which is very similar to polystyrene except for the greater strength and higher price.

You can nail and/or screw polystyrene when the proper precautions are taken. This usually means that screw bosses are molded into the parts. (See Figure IV)

Figure IV: Location of screw bosses on cross section 0 plastic drawer front.

Nailing has to be done with a jig to hit the right place. Thin and open overlays are more often nailed through the face with small finishing nails as shown in Figure V be ow.

Figure V: Nailed overlay in cross section of plastic drawer front.

Polystyrene can be machined with woodworking tools; machining must be done with a slow cutterhead speed and a fast rate of feed to prevent it from melting. While most features are molded in, the necessary absence of undercuts occasionally causes machining to be required. For example, notice the French dovetail in the drawer front shown in Figure VI(9). The molding process limits the shape of the groove to that shown in Figure IV(A).

67 But a carbide-tipped router bit can machine the groove to that shown in 6. Groove A is molded in the drawer front to save wear on the router and to save materials as well.

A B

Figure VI: Diagram showing process of machining French dovetail into polystyrene drawer front.

While polystyrene can be nailed, screwed, machined, etc., it is difficult to glue. Even though laboratory experiments have produced satisfactory glue bonds between wood and polystyrene, in actual production, there have been very serious problems. As a result, many engineers have rejected these bonds in favor of constructions similar to that shown in Figure VII.

egbolt molded into ABS furniture leg

.LM eta1 bracket

v. Figure VII: Example of one method of attaching polystyrene to wood.

68 Polystyrene is the cheapest of these plastics. The material is bought in the form of beads or pellets. The price depends on the method of shipment; bulk cars offer a lower cost.

An injection molding machine converts polystyrene into furniture parts. A typical machine has a shot capacity of 60 ounces and a clamping force of 450 tons. The shot capacity gives the maximum weight of the product, and the clamping force is the available pressure to keep the mold shut.

GEAR DRIVE COOLED MOLD \ \

NOZZLZ VALVE

Figure VIII: Schematic diagram of screw-type injection molding machine.

The machine requires only one operator, who removes the part each time the machine opens. The longest portion of the machine cycle is the cooling of the mold with the part in it. The polystyrene is introduced in melted form and solidifies by cooling. A cycle time of one minute is typical for a drawer front,

The low materials cost and the relatively low machine and labor costs per part are offset to some extent by the high tool cost. A mold has to have good surface detail and should be able to be cooled quickly. Beryl 1 ium copper is the common material used for molds. To make a mold, first a wood master is produced (a little oversized because of shrinkage of the part after de-molding). From this wood pattern, a rubber mold is made. Into the rubber mold a ceramic copy is cast. The ceramic copy is used for casting the beryllium copper. Considerable hand tooling must fol ow. The nature of the molding process does not allow wood grain on edges, and undercuts are not permitted. (See Figure IX on the following page.) A single mold will cost thousands of dollars. Use of th s process is feasible only for very large production where the initial mold costs can be amortized over many thousands of parts.

69 Ceramic Copy

Rubber Mold

Beryllium Copper Mold

Ceramic Copy

Figure IX: Initial steps in making mold for injection molding machine (1. wood master; 2. rubber mold; 3. ceramic copy; 4. beryl 1 ium copper mold) .

Polvurethane

Polyurethane foam can be manufactured at many levels of density. It is common to employ densities somewhat like wood, generally 18 to 26 pounds per cubic foot. Consequently, it feels and sounds very much like wood.

It is an excellent material for the reproduction of wood grain. The grain structure is very clear and crisp; and even for an expert, it is often impossible to tell that the finished part is not wood. The strength of polyurethane depends upon its density. For structural parts, it is often necessary to embed wood or metal reinforcements in the material.

70 Polyurethane has the highest materials cost. The chemicals are bought in liquid form, and the volume and shipping method influence the price. When the chemicals are mixed, polyurethane is formed at the same time as bubbles blow up the liquid mass. Both reactions are very fast, and to produce a consistent product, very thorough mixing of exact quantities must take place in a very short length of time. This is achieved by a dispensing unit and a mixing head as shown in Figure X.

(9 indicates metered pump

Dispensing Unit Chemical Tanks

Figure X: Dispensing unit and mixing head for polyurethane.

All of the ingredients are pumped by metered pumps to the mixing head; exact proportions of the material start flowing into this chamber. The cream-like mixture is poured into an open mold. The mold is shut, and the polyurethane expands to fill the mold completely. While the polyurethane cures inside the mold, another mold (of different shape and/or size) can be filled. The curing time of polyurethane is about 20 minutes, and it takes about 30 molds to keep one dispensing machine operating full time.

71 Figure XI: Photograph of polyurethane mold pour.

A dispensing machine costs only about one tenth as much as an injection molding machine, but the process requires much more labor. Mold preparation and de-molding bring the labor per part to perhaps six times that of polystyrene. Polyurethane parts are produced in rubber molds which are made directly from a wood master. Because the molds are flexible, small undercuts are possible, and the edges receive a grain pattern as good as the main surface. The rubber mold must be supported in a sturdy wooden box because otherwise the foaming pressure would tend to result in bulging shapes.

1 support mold

Figure XII: Diagram of rubber mold.

(Note: Grai n pattern is exaggerated in drawing. )

72 A most regrettable disadvantage of polyurethane is the short life of these molds. Under ordinary production conditions, they produce perhaps 120 parts on the average before they must be replaced. For small runs, the low cost of a mold makes the polyurethane process feasible where polystyrene is not. But where very large numbers are required, the mold cost in polystyrene decreases, whereas it is a fixed expense for each polyurethane part.

Bearing in mind mold costs, some companies have designed suites that utilize one or two of the same size overlays on drawer fronts of all sizes, on doors, and on bed panels. This brings the quantity to high levels. How- ever, this will not work with structural parts. A complete line of bedroom furniture generally requires five to eight sizes of drawer fronts alone.

Polyester

Polyester is, in many ways, similar to polyurethane. The liquid chemicals are mixed to produce the plastic. Rubber molds are used, so excellent grain detail and small undercuts are possible.

Polyester is used as a solid material; therefore, it is heavier than foamed polyurethane. To bring the cost down, it is extended with shell flour or clay. The type and the amount of filler used determine, to a large extent, the properties. The sound and feel of polyester put the material between polystyrene and polyurethane. It is not as glassy as polystyrene but less wood-1 ike than polyurethane.

Polyester is more rubber-like than polystyrene or polyurethane. When heated, it becomes pliable; this allows, for instance, the application of a straight molding to a curved surface.

It is dimensionally far less stable. This can be prevented to a certain extent by including wooden reinforcements in the product, for instance, dowel rods. Overlays can be held in place by the substrate to which they are attached, but structural parts require some form of reinforcement. The best known reinforcement is, of course, fiberglass.

Manufacturing polyester parts is a casting-like process. Unlike polyurethane, there is no foaming and, therefore, no pressure. The sturdy wooden box holding the shape of the rubber mold is not needed for polyester. A sheet of plywood under the mold will often be enough. (However, to save money on the rubber molds, they are often made with rather thin walls; so some support is needed, although it need not be very strong.) The ingredients are mixed and simply poured into the mold until it is full. The polyester part will solidify in about 15 minutes. A good production rate can be reached by filling a number of molds in rotation similar to the polyurethane process.

Speci a1 Devel opment s

Recently, several new ideas have been introduced that hold a great deal of promise for furniture applications of plastlcs.

73 It is possible to mix a blowing agent with polystyrene beads to create a foamed polystyrene. In one process, the polystyrene is introduced into the mold after which the liquid expands due to the foaming action. The mold has a moving section allowing the volume to increase. The net result is a part that has much more of a wood feel and sound. The weight is reduced by perhaps 25 percent. The problem is that cool ing the product takes a 1 onger time to prevent the part from bulging rather badly when removed from the injection molding machine. Thus, a lower materials cost requires a longer machine cycle. The product quality is undoubtedly better, as it approaches wood in all its properties.

A recent development in polyurethane is the realization of a non-uniform product with a high density skin, giving good strength, and a low-density center, which keeps weight and materials cost down. This process is used in making curved upholstery frames that are difficult to machine from solid wood.

Another interesting development is sonic welding of polystyrene. To build a mold for a dining room chair would be extremely difficult and expensive. But to build five molds for more or less flat sub-assemblies is quite feasible, both technically and economically. The joints have been the bottle- neck, requiring metal inserts and so on. It appears that ultrasonic vibra- tions may be used to melt the surface of the plastic and fuse it to the other part. The apparatus for this technique must work from an inside position as some surface marring may occur.

Finishing

In the early days, many furniture companies had a great deal of trouble with the finishing of polystyrene parts. A finishing system had to be designed to make two dissimilar materials, wood and plastic, come out with an identical appearance. The finishing materials suppl iers have succeeded quite well in achieving this effect. It would proceed beyond the scope of this text to go into details, but a few points should be considered.

Most common furniture finishing systems will attack polystyrene chemically. This results in destruction of the surface detail (see Figure XIII).

B Poly styrene surf ace 1 acquered

Figure XIII: Destruction of surface detail by finishing materials shown in cross section of plastic panel.

74 What is needed here is a barrier coat to prevent the solvents in the finishing material from attacking the plastic.

/Finish

Ba r r ie r coat

Figure XIV: Steps in applying barrier coat to plastic panel.

A barrier coat inhibits the destructive action of finishing material, but it tends to round off the edges and causes the grain effect to look more arti- ficial. While very good wood-look finishes are possible on polystyrene, these systems tend to be incompatible with wood itself and, therefore, are limited to all-plastic items, such as mirror frames.

Conventional furniture finishing systems are compatible with polyurethane and polyester. The wood grain detail on these plastics stands out with amazing clarity and is easily blended to match the real wood components.

Applications

The product engineer and designer must decide where to use plastics, how to use them, and which type to use. The factors influencing his decision are the furniture design itself, the required quality level of appearance and performance, and the anticipated volume and associated cost calculations. In high priced household furniture, the single most important factor is appearance. In institutional furniture, it is performance. In the low and low-medium price brackets, low cost is more important.

In many instances, a material or method will make possible the development of special design features. For example, the vinyl veneer groove-fold technique discussed in Chapter 11 has resulted in the design of several lines of furniture. And it was only after molded plastics became available that designers of commercial furniture started using very complex ornaments. When a designer specifies a particular plastic with certain properties, the product engineer is relieved of the responsibility of selecting a plastic

75 material; however, it is often the product engineer who first suggests a plastic with specific characteristics to the designer for use in a particular application.

The Choice Between a Wood or Plastic Overlay and a Plastic Structural Part

Iftwo identical appearing pieces of furniture are offered to the average retail customer and the price is the same, the customer will usually select an all-wood piece rather than one made with a combination of wood and plastic. This decision is based more on tradition than logic, but it is a fact of life in furniture merchandising. However, the furniture business is very style oriented. Styles change constantly to meet the customers' demand for furnishings which reflect their lifestyles and are different from those of the neighbors and friends. Customers will buy furniture using plastic parts if the design and appearance satisfy their individual tastes and if they see the purchase as a good value.

Wood is a less expensive raw material than the petrochemically based plastics. But raw material cost is only one factor in deciding whether to use plastic or wood.

This chapter has shown that certain plastics have characteristics which have greatly expanded the field of furniture design while giving the customer better and more serviceable products. Elaborate carvings and finely detailed miter work, when done in solid wood, are very labor intensive and requi re highly ski 11ed craftsmen. The average furniture customer could not afford these elaborate designs before the introduction of the molding processes for polystyrene, polyurethane and polyester. Popular-priced furniture can now offer the homeowner a "look" that was once reserved for only the wealthy customer.

P1 astics are very versati 1 e materials. Many contemporary or modern furniture designs feature the plastic look with no attempt at making it resemble wood. The fiberglass reinforced polyester chair shells are a good example of plastic sold as plastic. Most ,furniture buyers, however, prefer the "wood 1 ook ,'I so furniture designers and product engineers must blend the various materials to achieve an attractive combination. For example, one dresser might contain solid wood parts, face veneers, printed composite board panels, plastic overlays, plastic legs, vinyl-veneered drawer sides and a melamine top. There is no single solution, and each company must evaluate each new style in terms of probable customer acceptance and sales volume, comparative costs, factory capability, etc.

Appl icati ons Where Strength Is Important Wood-grained components that have to bear a load must be designed to stand up under it. As a result, only the better grades are considered in each material used for a structural component. This is nothing new. A furniture manufacturer would not make a Queen Anne leg from basswood or pine, but would choose mahogany or beech, or another strong species.

76 In polystyrene we can use the h gh impact variety, about 15 percent more expensive; or ABS, which doubles the cost of materials, can be used. For polyurethane, one must choose a h gher density. At 45 pcf, the material is 50 percent more expensive than at 30 pcf. In polyester, it is possible to improve strength by choosing the right kind of reinforcement, such as fiberglass.

This does not mean, however, that the right kind of plastics may be found for every job. The example of the Queen Anne leg is a case in point. If long and thin, it may not be possible to make the legs out of solid ABS and give the case sufficient rigidity. Molded ABS has only about half the stiffness of hard maple.

In such cases, a solution can almost always be found by using metal or wood as a reinforcement. A steel tube would be molded inside the Queen Anne leg. A door in polyester is given a sturdy wood frame and so on. Pound for pound, plastics are more expensive than wood. In many applications, the use of a lesser amount of plastic saves enough money to pay for the wood insert. Each one of these situations must be examined on its own merits.

An additional application of plastics in furniture is the upholstery frame. Polyurethene is used extensively in the manufacture of frames for upholstered furniture. See Chapter 16 for a discussion of plastics in upholstered frames.

As a form of summary, Table I is a chart comparing the four basic plastics used in furniture construction in terms of their individual properties and specific applications.

77 HIGH IMPACT GENERAL- RIGID FILLED I TEM POLYSTYRENE PURPOSE ABS POLYURETHANE POLYESTER RESIN

MANUFACTURING Injection Injection Dispensing in Dispensing in TECHNIOUES Mol dinu Mol dinu closed mold closed mold SPEC. GRAVITY 1 -04-1 -1 0 1.02-1 e07 0.3-0.4 0.9-1 .o DENSITY 65-70 PCF 63-68 PCF 18-24 PCF 56-60 PCF TYPE Beryl 1 ium Beryl 1 ium Silastic rubber Silastic rubber MOLD copper copper /wood frame /wood frame MOLD LIFE (IN PARTS) 200,000 + 200,000 + 100 - 250 250 - 1,000 LABOR COST Lowest Lowest Highest High COST OF PART WHEN MAKING 500 High Highest Acceptable Lowest COST OF PART Acceptable but Acceptable but WHEN MAKING higher than all higher than all 50,000 Lowest Low others others WOOD GRAIN APPEARANCE Fair Fair Excel 1ent Very good FEEL AND G1 assy , G1 assy , Very much Re a son ab 1y SOUND plastic plastic wood-1 ike wood-1 ike FIN1SHI NG Good Good Very good Very good Good, but de- Excellent if IMPACT Good Better pendent on skin reinforced RESISTANCE & spec. gravity MACHINABILITY Fair Fair Good Depends on filler Good to very good NAILAB1 LITY Good Good Fair to Good depends on filler HIGH TEMP. RESISTANCE Good Better Excel 1 ent Depends on fil ler DIMENSIONAL Dependent on STAB I L ITY Good Better Best rein f o rcemen t USE AS OVERLAY Excellent Excel 1 ent Excel 1ent Excel 1 ent USE AS Fair, needs MIRROR FRAMES Excel 1 ent Excel 1 ent Good reinforcement Good to Very good Good if Good if USE AS LEGS excel 1ent to excel 1 ent reinforced reinforced Best in self- (Expansion skinning foam. USE AS UPHOL- Not cast ABS Low density, tack STERY FRAMES practical used) strips req'd Few appl ications

Figure XV: Properties and applications of the four basic plastics used in furniture construction.

78 CHAPTER 6

INTERNAL STRESSES

In Chapter 2, it was stated that the stresses set up in furniture during use could be separated into external and internal stresses. Chapter 2 discussed external stresses--internal stresses will now be discussed. A1 1 internal stresses are caused by the tendency of wood to shrink or swell with changes in humidity. For purposes of furniture construction, warpage will be considered apart from shrink and swell, even though the basic cause of warpage is shrinking and swelling.

Shrink and Swell

Chapter 3 told how to calculate the amount of shrink or swell for various changes in humidity; however, this procedure assumed that the piece of wood was free to move. It is an entirely different situation when one has to consider the effects of shrink and swell on an assembled piece of furniture in which each individual wood component shrinks and swells. It should be realized that this can have an important bearing on how the parts are put together. Two general policies to remember are:

1. Make the construction strong enough to resist the forces which wood exerts in shrinking or swelling.

2. Make the construction such that a part may move by shrinking and swelling without harming the strength or rigidity of the assembled structure. This is accomplished by using a I' f 1 oat in g con struct i on. I'

Wood can exert a great force when it tries to shrink or swell; it takes considerable strength in a structure to prevent it from swelling. In the case of shrinkage, the forces are large enough to actually split the wood if it is not permitted to move.

Chapter 4 stated that plywood did not shrink or swell much in length or width. This is because the large area of glue bond gives sufficient strength to contai'n the forces of shrink and swell such that no movement actually occurs even though each individual layer of wood in the panel will try to shrink or swell. Plywood is one of the best known and most widely used constructions where the strength of the structure prevents motion due to shrink or swell. However, it is generally safer to use some sort of "floating construction" when two wood parts must be fastened together and where the grain of the parts is in such direction that shrink or swell would cause one part to move in relation to the other. On the other hand, it is not wise to use a floating construction where a tight joint would cause no t roubl e.

Figure I shows a simple bookcase wi.thout any shelves. If all parts are of soljd wood, tight joints may be used between ends and top, and between ends and bottom. If moisture content increases, the ends will tend to swell in

79 width. The top and bottom will also swell the same amount. Since there is nothing in the construction to prevent this swelling, the parts will move and will all be wider at the higher moisture content. There is no motion of the bottom or top relative to the ends because all swell the same amount. Likewise, if all parts are plywood, tight joints may be used because there will be no shrink or swell in the width of any parts and consequently no motion of any part relative to a part attached to it.

, ------I

Arrows indicate direction of grain.

Figure I: Diagram of a simple bookcase.

If the parts are solid except for a thin plywood panel nailed to the back of the bookcase, tight joints may still be used as the plywood back panel will not shrink or swell. Although the width of the solid parts will swell, changes in width do not affect the joint between back panel and case. A change in length would affect this joint, but solid wood does not swell lengthwise with the grain. But if all parts were solid except the top which was plywood, tight joints could be used only between ends and bottom. The joints between ends and top would require a "floating construction" because width of the solid ends would swell while width of the plywood top would not. Consequently, there would be motion of the ends relative to the top.

Figure 11 shows a conventional poster bed in which the two prongs of the head panel are mortised into the head post. If the head panel were plywood, there would be no shrink or swell in width; there is no shrink or swell in the length of the post. Tight joints could be used, and there would be no problem. Suppose, however, that the head panel is solid maple (very popular in Colonial poster beds). If moisture content increases, the panel will

80 swell. Above point A, there is nothing to restrain movement and the part is not stressed. But between points A and 6, the post will not swell so that the distance between mortises remains constant. As the panel swells, it must partially shear off the prongs which go into the mortises in order to move. Experience has shown that the swelling force is not great enough to shear the prongs so the force is contained with no motion and no trouble. On the other hand, suppose that the moisture content decreases. As the panel shrinks between A and 6, the mortises in the post will resist this movement. In many cases, the panel will split as shown by the dotted lines at 0, and the bed will become a reject.

Figure 11: Poster bed showing construction of head panel and head post.

One way to avoid this type of trouble is shown in Figure 111. The B prongs on the panel are made narrower than the vertical size of the mortises while the shoulders on the panel are made to cover the cracks which would show between prongs and mortises. In assembling the headboard, joints A and C are securely glued. Joint B is not glued at all so that the bottom edge of the panel at B is free to move up and down with shrink or swell. This is one method to achieve a "floating construction'' at point 6. The amount of

81 clearance at B between prong and mortise can be calculated as follows (see Chapter 3) :

Assume 8% moisture content when machined. Assume 2% moisture content minimum in service. Assume 14% moisture content maximum in service.

Concerning ourselves only with the amount of shrinkage, a change in moisture content of 6 points will give approximately 2% shrink. If the distance between mortises is 10 inches, 2% of 10 = 0.2 inches or about 3/16". This is the clearance needed for the assumed conditions.

POST

Figure 111: Construction modified to permit movement in joint at B in Figure 11.

Examples of Floating Construction

The basic principle in floating construction is to design so that movement is permitted in the direction needed for shrink or swell but not in other directions. Other than this, the product engineer is free to use his own ingenuity to work out the details. A few examples are given, but many others are possible.

In the construction illustrated in Figure IV, a cleat can be glued and screwed to the plywood top. Then the cleat can be spot glued and screwed to the solid end at the front. The round head screw with a washer in a routed slot instead of a bored hole permits motion between the cleat and the end toward the back.

82 1 I\ !! II II II II e---.- - -\-I------.---&I--- @ ;; ,I ( ,; (. f ' SPO$GLUE CLEAT NO GLUE TO ErJV TO KEEP PkRTS FLUSH AT FRONT.

Figure IV: I1lustration of a possible floating construction for bookcase in Figure I.

In Figure V the solid case end is jointed to a plywood top using dovetails machined to provide a slip fit. Spot gluing of the dovetail joint at the front end holds the two parts flush while still permitting the solid case end to move along the joint as it shrinks and swells.

I NO CLULE SPOT GLUE DOVETA)L AT FRONT

Figure V: Dovetail joint permits movement along the joint.

83 An oversize hole for the screw may be bored as shown in Figure VI. This arrangement permits two pieces to slip in any direction but not to pull apart. Figure VI1 illustrates the use of a metal clip to hold the top firmly against the end. Lateral movement is permitted as the metal clip slides in the recess machined in the case end.

Figut e VI: Use of oversize hole to permit movement.

Figure VII: Use of metal clip to permit lateral movement.

84 ExamDles of What Not to Do

Frequently a designer will want an end-banded effect on the tops of Early American style pieces. The construction illustrated in Figure VI11 with the tenons glued into the grooves seems to be an easy solution to the problem. Experience has shown that, when the center tries to shrink or swell, the forces are simply too great for the strength of the glue bond resulting in separation. If the center were plywood instead of solid, such a construction would cause no trouble.

Figure VIII: End banding on case top. Another top construction that looks attractive, but causes problems, is a solid center with mitered bands on all four edges as shown in Figure IX. Any swelling in the center panel would almost certainly open up miters regardless of how they are fastened. A plywood center would solve the problem, but so would a face veneer pattern, which gets the appearance without using any lumber bands.

Figure IX: Mitered bands around a case top.

85 Figure X (see following page) shows another mitered frame construction that designers like for mirror frames and table tops that have either a marble center or a leather-covered plywood center. Difficulties can occur with this construction if the rails of the frame are wide and made of solid wood. Figure X(A) shows a tight miter joint with the moisture content the same as at the time of machining. As the moisture content increases, each rail increases in width so that JG is greater than DE and JH greater than DF. Un- less the strength of the miter joint is sufficient to withstand the forces of swelling, the miter will open up at the toe as in Figure X(B). As the moisture content decreases, each rail decreases in width so that KL is less than DE and KM less than DF. This would tend to cause the miter to open up at the heel as in Figure X(C). If the rails are narrow, this problem seldom develops because the forces of shrink or swell are small and can be contained by the strength of the joint and the natural elasticity of the wood. From this, one wonders: "HOW wide is too wide for a rail?" The fact is that each factory pretty much goes by its own exerience. If problems develop, one possible remedy is to use quarter sawn lumber instead of plain sawn because the width shrinkage is only about half as great, being radial instead of tangential shrinkage (see Chapter 3).

While many other illustrations are possible, suffice it to say that solid wood shrinks and swells. The basic principles to keep in mind are either construct to permit motion without damage to the structure, or else have a structure strong enough to prohibit motion without damage to the structure.

Warp

Even though the forces of shrink and swell are great, it takes very little force to flatten a warped part provided there is something flat to which it can be fastened. Thin plywood, such as is used for dust bottoms or drawer bottoms, flattens easily when the warped panel is put into grooves in dust frames or drawer front, sides, and back. Thicker stock, such as that used for tops, can be pulled flat by wood screws spaced 10 inches apart when attaching the top to a case. Where there is no flat structure to which the part can be attached, precautions have to be taken with the part itself (see Chapters 2 and 3). Examples are doors and table drop leaves.

How Far Should You Go?

In view of the principles just discussed about shrink and swell, the question arises whether or not two wood parts with adverse grain relationships should ever be fastened together without floating construction. It seems as though they should not, but many furniture manufacturers have gotten by with it for years without getting into trouble. Very few experimental data are available in this area; however, & fairly safe rule of thumb does exist. The rule states that, if the relative motion between parts is 1/32" or less, it will probably be all right to use a tight joint; and if the motion is 1/8" or more, it probably will not be all right. In between these limits, it is questionable which type of joint should be used. Like most arbitrary rules, there are many exceptions in specific situations. The only reliable guide is experience.

86 H i

M K c

Figure X: Miter joint opens as wood swells (B) or shrinks (C) from the original dimensions (A).

87 Most areas in the United States experience 2 percent shrink or swell. Maxi- mum swell occurs in humid late summer; maximum shrink occurs in mid-winter when homes are heated. If the dimensions across grain of all wood parts are 1 1/2" or less, there will probably be no problems in using tight joints, as 2 percent of 1 1/2" is approximately 1/32". The reason why movement of less than l/32" does not create problems is that wood has a certain amount of compressibility and flexibility that enables it to absorb this amount of movement. As an example, ifa 3/4 x 3/4 tenon on a front rail is fitted into a 3/4 x 3/4 square mortise in a post, the tight joint would probably be all right regardless of the direction of the grain. This conclusion agrees with experience.

88 CHAPTER 7

WOOD JOINTS

Almost all items of furniture consist of several individual wood parts which are assembled to form the complete bed, table, chest, or chair. Where one part joins another, there is usually some kind of joint. Proper joints are an important part of furniture construction.

Most wood joints involve gluing, so the reader is advised to review the characteristics of glue before studying wood joints.

Two Functions of Joints

In general, joints have two functions:

1. They can hold the parts together and resist forces trying to separate the parts.

2. They can accurately position the parts with respect to each other.

Edge gluing is an example of the first function (holding parts together). Two flat edges are glued together and held under pressure until the glue sets. Thereafter, the two pieces are held together by the glue which is as strong as the wood if the gluing has been done properly. This flat edge joint does not perform the second function of accurately positioning the two pieces with respect to each other. But it would be possible to bore the edges for two or three dowels before gluing together. In this case, the dowels would permit the two pieces to come together in only -one position. In other words, the dowels would accurately position the two pieces with respect to each other; the edge-glued joint would still furnish most of the strength of the joint.

Two Approaches for Strong Joint

There are two different approaches to getting adequate strength and rigidity in a joint:

1. Have enough square inches of glue line to resist the forces invol ved

2. Design the joint so that the wood structure itself resists the forces and the glue simpiy holds the parts in position relative to each other.

Frequently, it is possible to combine both approaches.

Edge gluing is an example of the first approach. The strength of the joint depends upon glue alone. The lock corner in Figure I is an example of the second approach. Parts A and B have to be slipped together as shown by

89 arrows C-C. Once the top and bottom edges are flush, any force, such as D or E, trying to pull the corner apart will be resisted by wood structure in the joint even if the joint has not been glued. In practice, the joint would be glued for extra strength, and also the glue would resist motion in the direction of arrows C-C once the glue is set. The lock corner also illustrates a joint which gives accurate positioning in two directions. The joint does not accurately position the parts in the direct of arrows C-C. The operator must get the edges flush, the joint will not do this for him. But in the directions D and E, the joint insures accurate positioning.

I A

0 -D

Figure I: Illustration of a lock corner.

Low-Cost Machining and Assembly

From the viewpoint of the production shop, an important feature of a joint is that it should be easy and fast to machine and assemble in order to hold cost to a minimum. Figure I1 on the following page illustrates this principle.

90 Figure 11: Tenons requiring different machining operations.

Figure II(A) shows a rail /4 'I x 2" with a tenon 3 /4 I' x 3 /4 I' and 1 I' long. This tenon is to fit into a square mortise 3/4" x 3/4" cut with a hollow chisel mortiser. The area of glue surface is 3/4" x 1" x 4" = 3 square inches. Figure II(B) shows the same rail with a tenon 1/2" x 1" x 1". The area of glue surface is the same--3 square inches. The length of the tenon is the same. The strength of the two joints should be about equal in resisting a force which tries to pull the tenon out of the mortise. But in construction II(A) , one stroke of a hollow chisel mortiser with 3/4 square chisel will cut the mortise, and one pass over a double end tenoner will cut the tenon. In construction II(B) , it will take two strokes of the mortiser (with l/2" square chisel) to cut the mortise, and it will take two passes over the tenoner to cut the tenon. Thus, construction II(B) requires twice the number of machine operations as construction II(B) . There will be no significant difference in assembly time. Referring back to Figure I, the lock corner joint looks complicated to machine. But one pass over a double end tenoner will cut the joint on both ends of either part A or part B. This is fast and cheap machining. (The reader should figure out how to set up a tenoner to do this job in one pass.) Assembly is slower than with a joint which can be clamped unless there is production enough to justify a bulldozer type clamp to slip the parts together, in which case assembly is about as fast as any other type of clamped joint. Machining this lock corner is considerably faster than machining a dovetailed corner joint.

91 Either of the mortise and tenon joints in Figure I1 will assemble faster than Figure 111, where the mortise is cut with a router, because in Figure 11, the joint gives precision location of the parts with respect to each other. But in Figure 111, the position of the tenoned rail can vary 1/16" to l/8" and if precise location is necessary, a gage must be used in assembly.

ONE PASS TENOW TWO PASS TENON

Figure 111: Router cut mortise with one pass or two pass tenons.

The lock tenon joint, Figure IV, is an excellent joint for strength because wood structure resists motion in any direction even with no glue in the joint. It machines as fast as Figure II(A), but it assembles more slowly. This is so because the holes for the lock dowel cannot be pre-bored as they would not be located accurately enough to line up perfectly when assembled. In assembly, glue is applied in the mortise, and the two rails are pulled firmly together in a clamp. While in the clamp, the dowel hole is bored with a portable drill. Glue is then applied in the dowel hole, and the dowel is driven in. Clamp pressure can now be released without fear of the joint's opening up while the glue is setting. In spite of the slow assembly, this joint is sometimes justifiable when extra strength is required.

92 Figure IV: Lock tenon joint.

Wiping Action of Joints

In many joints, there is a tendency to wipe the glue away from where it should be as the two parts are put together. This is true of all mortise and tenon joints and of dowel joints. The male member of the joint acts like a plunger and forces much of the glue down into the space at the bottom of the hole. This wiping action does not occur in edge gluing. Neither would it occur in a glued and screwed joint like Figure V. Figure V(A) shows a joint which does not give precision location; Figure V(B) shows one which does. The joint in Figure V(6) is often called a "half-lap" joint. Another joint that avoids wiping action is the so-called "finger joint" as shown in Figure VI. The taper of the fingers prevents wiping as the pieces go together. If the taper is a thin angle, the joint will not back away when the clamp is released. The action is much like a taper shank drill in a drill press spindle or tapered centers in a hand lathe spindle. Another advantage is that if the taper is a thin angle, the wood is glued almost true side-grain-to-side-grain; this gives the best glue bond strength. Very 1 ittle of the joint glues end-grain-to-end-grain or side-grain-to-end-grain, both of which give poor glue bond strength.

93 ED6P FAC€ EDGE FACE

-. ?; A B

BOTTOM

Figure V: Lap and half-lap, glued and screwed joints.

iWGE

Figure VI: The taper in the finger joint prevents wiping of glue in assembly.

94 Thickness of Glue Line

In joints so constructed that the strength of the joint depends on the glue bond, there is another factor to be considered. In gluing plywood, it is possible to regulate the total thrust of the press ram so that the pressure on the glue line closely matches the number of pounds per square inch which has been found best for the particular glue, species of wood and moisture content. As pressure on the glue line is increased, the thickness of the glue line is decreased. Enough pressure must be applied to assure contact over the entire surface. With male and female joints, such as dowel or mortise and tenon, there is no way to adjust or regulate pressure on the glue line. The thickness of the glue line is affected only by the machining of the joint. If a 3/8" dowel is inserted into a 25/64" hole, the clearance on one side of the dowel can be 0" and on the other side 1 /64 'I. Ideally, the dowel would be exactly centered in the hole with 1/128" clearance all around it. While the machining of the joint can be planned for any thickness of glue line desired, variations from planned dimensions will alter the thickness of the glue line. Extreme accuracy of machining is difficult to maintain in a production shop. The result is that the strength of the glue bond in joints frequently falls far below the theoretical glue strength.

Di rection of Grain

The direction of grain in the two members of a joint can make a big difference in the strength of the glue bond in a joint. The strongest glue bond is obtained where side grain is glued to side grain as in edge gluing. End- grain-to-side-grain gives a comparatively weak glue bond. Figure VI1 illustrates this. The glue bond in A is strong; in B it is weaker (approximately 25% as strong).

Figure VII: Glue joints showing grain directions.

95 There is another situation where side grain is glued to side grain in different ways. Figure VI11 illustrates this. Both A and B give a strong glue bond when first glued. But if the joint is exposed to alternating low and high humidities, the two parts which are jointed try to shrink and swell. In A, both parts try to move together so that very little force is exerted trying to break the glue bond.

Figure VIII: Two types of side-grain glue joints.

In B, one part tends to change width while the other part retains a constant length. As a consequence, large forces are set up trying to break the glue bond. Over a long period of time under varying conditions of humidity, the strength of B weakens; that of A does not change much.

With a mortise and tenon joint, the glue bond is partly side grain, partly end grain as shown in Figure IX. In Figure IX(A), with the tenon 3/4" x 3/4" x 1" the area of glue line is (3/4")(1!)(4) = 3 square inches. In B, with the tenon 1/4" x 1 1/4" x l", the area of the glue line is (1/4")(1")(2) + ( 1/4")(1")(2) = 3 square inches.

The area of glue ine is the same in both A and B, but if the joints are accurately machined so as to get proper fit, B will have a stronger glue bond than A. This is because only half the glue line in A is side-grain-to-side- grain whereas B has 2 1/2 i 3 = 83% of the glue line side-grain - to-side- grain.

96 /-- / / /

GRAfN B GRAIN I E3

/ A 0 B

Figure IX: Redesigned mortise and tenon joint to increase strength of glue bond.

In a dowel joint, Figure X illustrates the situation, In Part D, the glue bond between D and the dowel is 100% side-grain-to-side-grain. In part C, the side grain of the dowel is glued to side grain of the part only along a small part of the cylindrical wall. Most of that cylinder acts approximately as end grain in terms of the glue bond formed.

I"Lu

Figure X: Orientation of grain in a dowel joint.

97 As a consequence, the glue bond between the dowel and part D is stronger than the bond between the dowel and part C (provided half the length of the dowel is inserted into each part). The weakness of dowel joints in components like part C in Figure X is due to some extent to the wiping action and subsequent Itsponge" action of the end grain-like surface. The strength of the joint can be improved by applying glue, both in the hole and on the dowel, and by selecting a hole dimension a few thousandths wider than the actual dowel diameter.

GRAIN cc

Figure XI: Mitered joint with dowels.

In a mitered joint which is doweled together, both parts present mixed grain to the dowel, and the strength of the bond would be the same in both parts (see Figure XI).

98 Durabilitv with Humidity Chanaes

Many joints are strong and tight when made, but as time goes on and humidity changes cause the wood to gain and lose moisture content, the strength of the joint decreases, The exact effect of the different factors is not known, but the U.S. Forest Produfts Laboratory ran a series of tests which were reported by Selbo and Olson. The experiments covered seven different types of joints and ten different glues. Sample joints were made with hard maple at 6% moisture content. Some of each were tested to failure seven days after gluing to establish control figures of strength. The others were subjected to low humidity for four weeks, then high humidity for four weeks. Batches of samples were again tested for strength after one, two, and three years. Strength figures were then compared to the control figures obtained from the sample not exposed to the humidity cycles.

The dry part of the humidity cycles was constant at 30% relative humidity, which corresponds to about 6% moisture content in the wood. Three different humidities were used for the wet part of the cycle.

a. 65% corresponds to about 11% moisture content in the wood b. 80% corresponds to about 16% moisture content c. 90% corresponds to about 20% moisture content.

Cycle c. probably represents conditions in some tropical climates but not found in the United States. Cycle b. represents extreme conditions in the U.S. such as along the Gulf Coast. Cycle a. represents average conditions in most of the U.S. except that moisture content of wood in the winter in steam-heated rooms in the north goes considerably below 6%, probably as low as 2%.

While some of the evidence seemed conflicting, certain generalizations are warranted.

1. The side-grain-to-side-grain joints (such as used in edge gluing) showed no deterioration of strength; dowel or mortise and tenon joints showed considerable strength loss. This result appears reasonable because in edge gluing both parts tend to shrink and swell together so that no particular stress is set up in the joint. With dowel, or with mortise and tenon, the length with the grain of the male member went into side grain of the matching part at right angles so that one member of the joint would swell more in one direction than the other, This would put stress on the glue bond. Table I shown on page 100 gives retained strength after exposure as a percent of control strength before exposure. The maximum figure applies to the type of glue with minimum strength loss; the minimum figure

M. b. Selbo and W. F. Olsen, "Durability of Woodworking Glues in Different Types of Assembly Joint," Journal of the Forest Products Research Society, 3, No. 5 (1953), 50-60.

99 'EAR(SI MILD CYCLE MEDIUM CYCLE SEVERE CYCLE Min. Max. Avg. Min. Max. Avg. Min. Max. Avg.

DOWEL JOINTS

6 3% 104% 89% 17% 80% 48% 16% 82% 42% 72% 98% 87% 9% 66% 37% 5% 66% 31 % 5 7% 106% 84% 6% 69% 35% 3% 64% 27% MORTISE AND TENON JOINTS

7 8% 94% 86% 57% 95% 72% 29% 7 5% 54% 71 % 99% 86% 40% 84% 58% 26% 59% 44% 59% 97% 80% 34% 73% 49% 18% 64% 39%

SIDE-GRAIN-TO-SIDE-GRAIN JOINTS

94% 119% 106% 53% 148% 105%* 94% 122% 109% Inadequate data 25% 135% 106%* 103% 136% 111% 0% 131% 96%*

'Excludes two glues that failed completely on the three-year test.

Table I: Relative strength values of three types of glue joints after humidity cycling (ten different glues were tested.)2

applies to the glue with the maximum strength loss; other glues lie in between. In edge gluing two glues, polyvinyl and non-crazing urea failed completely by the end of the three-year period of the severe cycle.

Under any of the tests, the edge-glued joint stood up well enough that there isano need to worry about durability under humidity changes except that polyvinyl or non- crazing urea should not be used for severe high humidities. The dowel joint retained 84% of its original strength on the average with the mild cycle, 35% with the medium cycle and 27% with the severe cycle. This is a big loss of strength and should be considered by the product engineer in working out joint construction. The mortise and tenon joint gave test results about the same as the dowel joint for the mild cycle but tested somewhat higher than the dowel joint for the medium and severe cycles. But it still lost a lot of strength, enough so that its limitations should be considered in designing joints.

Selbo and Olsen.

100 SIDE-GRAIN MORT I S E TO AND GLUE SIDE-GRAIN DOWEL TENON

Casein 82 97% 6 2% 7 3%

Phenol -resorci no1 G1 81% 90% 81%

Urea, 02 82% 7 8% 85%

Animal, A3 100% 77% 85%

Acid-phenol I 71 % 91 % 87%

Polyvinyl -resin 32 91 % 98% 90%

Animal, A4 94% 7 0% 94%

Resorcinol F 71 % 100% 94%

Urea, D13 96% 7 9% 96%

Urea, D12 78% 83% 100%

Ta bl e II : Re1 ati ve strengths of glues before humidity cycl es .3

2. The various glues performed quite differently. As mentioned, the polyvinyl failed almost completely for edge gluing with the severe cycle. However, it was among the best glues in the dowel joint and average or better than average for mortise and tenon. For all round retention of strength, the resorcinol and phenol-resorcinol glues were the best. Animal and casein glues were among the poorest except that both did very well on the edge glued joint. The urea glues (three kinds) were average except that one of them failed completely on the severe cycle for the edge glued joint.

3. In addition to the deterioration which occurred in humidity cycling, strength of the control samples before exposure to humidity cycles is of interest. Table II gives relative strengths of the different glues as shown by control sample tests. Most glues showed 80% or more as compared to 100% for the strongest glue.

Selbo and 01sen.

101 Compressed Dowel s or Tenons

It is known by experience that if dry wood is slightly compressed perpendicular to the grain, it springs back nearly to the original size when it picks up moisture. An old-time cabinetmaker's trick, in the case of a small dent in a flat surface, is to wet the dent, wait for it to swell back, and then sand the surface smooth.

This principle has been used to make compressed dowels. If the dowel is to be used in a 7/16" hole, the dry wood is machined to a diameter slightly larger than 7/16". It is then compressed by rolls or dies to a diameter slightly smaller than 7/16". In use, glue is applied in the hole; the dowel, being a loose fit, can be inserted by hand. Within a few minutes, the water in the glue swells the dowel tight against the hole. This is a relatively foolproof way of getting wood-to-wood contact between dowel and hole in spite of small variations in the diameter of the hole or of the dowel. To work successfully, the dowels must be dry when compressed and must be kept dry until used. Also, there must be only a relatively small amount of compression. Too much compression crushes the wood structure so much that it will not spring back when wet. The exact limits for the amount of compression to yield best results are not known.

Some factories have used compressed dowels in production and are pleased with the results; some have had a bad experience with compressed dowels. Neither group seems to know just why the results were good or bad. In theory, it should be a good technique. It is probably worth trying out, but the results should be watched closely, at least until more is known about the factors that cause success or failure.

The compression technique should apply just as well to tenons as to dowels, whether the tenons be square, rectangular, or round-chucked tenons. In one experiment, where ye1 1 ow pop1 ar tenons were compressed, the amount of springback was quite variable. Whereas the species for dowels can be chosen freely (usually maple or birch), tenons are by necessity of the same material as the rail. Cost and straightness of grain are prime considerations in the choice of species.

102 Crushinq PerDendicular to the Grain

In many instances, the forces tending to deform a furniture joint do not pull the joint apart. A simple analysis of the stresses which occur can be made following the principle of the free body diagram used in mechanics. Figure XI1 illustrates a common situation. A horizontal stretcher is mortised into a vertical post. A force is applied at F. Substituting a back post for the vertical post, a side stretcher for the horizontal stretcher, and a seated person tilting the chair onto its back legs for the force at F, the result is analogous to Figure XII. The action of force F would not try to pull the stretcher out of the mortise but would try to rotate the stretcher around point A (or maybe around point B). If distance AF were 10 times distance AC, a force of 100 pounds at F would exert a downward force at A and an upward force at B. Both would be about 1,000 pounds. The downward force at A would act to crush the wood structure on the bottom face of the stretcher at A. It would also tend to crush the wood structure of the post along the bottom of the mortise at A. The result would be deformation of the wood structure as illustrated in Figure XII(B).

I A

Figure XII: Horizontal stretcher mortised into post before (A) and after (B) deformation.

103 If post and stretcher are of the same species and have equal moisture content, the stretcher will crush before the post will. The post in Figure XI1 has the force applied parallel to the grain; the stretcher has forces applied perpendicular to the grain. Table 111 (data from Wood Handbook) gives fiber stress at proportional limit for a few species commonly used in furniture.

FORCE APPLIED FORCE APPLIED SPECIES PARALLEL TO GRAIN PERPENDICULAR TO GRAIN (#/Sq. in.) (#/Sq. in.) Yellow birch 6130 1190

Black cherry 5960 850

Pecan 5180 2130

Red maple (soft) 4650 1240

Sugar maple (hard) 5390 1810

Red oak (northern) 4580 1250

Red oak (southern) 3910 1080

Sweet gum 3670 660

Walnut 5780 1250

Ye1 1ow pop1 ar 3730 560

Table 111: Fiber stress at proportional limit for compression at 12% moisture content.

For an unglued joint of the type illustrated in Figure XII, the concen- tration of stress (lbs. per square inch) is further magnified because the contact at A between the stretcher and the post at the start of the rotating action of the stretcher is confined to the front edge of the post (and the line on the stretcher in contact with this edge). The actual area resisting the force at A is very small at first, and only as the wood is compressed does the area enlarge. The same condition is true at B. The 1,000-pound forces assumed as acting at A (and B) would, therefore, be concentrated on an area much smaller than the area defined by the distance 1/2 AC times the width of the stretcher tenon. If there is movement relative to the two parts, deformation can always be expected; crushing and permanent deformation occurs when the stresses exceed the proportional (elastic) limit of the wood with a loose joint as the result. Addition of glue to the mortise and tenon joint will resist relative movement of the two parts. Since the glue joint will be stressed in shear, the small effective surface in the joint may not withstand the forces applied. Once the glue bond is broken, the joint will resume a behavior much like an unglued joint.

104 It should be noted that the analysis of forces in a tightly fitting glue joint becomes a very complex problem. Wood will deform (compress and bend) when stressed, and therefore, the assumptions used in the free body diagram do not apply strictly. An intuitive interpretation of what takes place and what the results will be is still possible.

It is logical to reason that if the stretcher is made wide, there will be more square inches on the top and bottom faces inside the mortise and the resistance to crushing will be increased. If the stretcher goes further in- to the post, the distance AC will be greater. This would tend to increase the area to resist crushing and reduce the leverage. Thus if AC is twice as long, the crushing force exerted at A by a fixed force F, at distance AF, will be cut in half.

If AC is shortened, the face of the stretcher will crush before the stretcher breaks off at A from bending, or the tenon will pull out of the mortise. If the joint is glued, the extra side-grain-to-side-grain glued surface will help offset the easier crushing by resisting motion of the stretcher in the mortise. If AC is long enough so that the square stretcher breaks before it is crushed, the double depth and half width will resist breaking more than the square stretcher.

Substitution of a species of wood with greater resistance to crushing may be a better solution than changing dimensions. Substitution of hard maple for a sweetgum stretcher will provide three times the resistance to crushing (see Table 111). Even with a hard maple stretcher, the sweetgum post at 3670#/square inch will resist crushing parallel to the grain better than the stretcher perpendicular to the grain.

The above example is not complicated by having shoulders on the stretcher. With shoulders, there would be different fulcrum points and different moments caused by resistance to force F. Each joint must be analyzed by itself, and no generalizations are valid for all joints unless it is the Sen- eralization that wood is relatively weak in compression perpendicular to the grain. The possibility should be faced that the weakest feature of a particular joint might turn out to be the resistance to crushing of one part perpendicular to the grain.

Breaking Versus Splitting

In joint construction, it is wise to keep in mind that wood is much stronger against breaking across the grain than it is against splitting with the grain. An example would be the dust frame for a chest. Figure XI11 illustrates a construction which is widely used and is low in cost. The four rails are run on the moulder with a groove in one edge. The groove receives the thin dust panel, and the same groove in front and back rails receives tenons cut on both ends of the end rails, thus giving tenon and groove construction for the frame. Once the dust frame is assembled into the dresser case, there is very little in the way of forces which it needs to resist. But as the dust frame itself is handled, it is likely to be twisted considerably. Any twisting acts either to break off the tewn or to split the

105 Figure XIII: Dust frame. groove in the front and back rails. Many shops will run all rails 3/4" thick with the groove lb" wide. This means the tenon will be 1/4" thick and the wood on either side of the groove in the front rail will be lpt'I thick. If the frame is twisted enough to break, the wood beside the groove will invariably split before the tenon breaks across the grain. If the groove is 3/16" wide instead of 1/4", it will leave more wood beside the groove, and the frame will be stronger. Of course, this cannot be carried to extremes. A tenon only 1 /16 'I thick will undoubtedly break off before the groove splits out the 11/32" thickness of wood beside it. These strengths are seldom calculated, but experience can be a good guide on how far to go.

106 Wood Enough for a Good Joint

Sometimes there is not enough wood at the location of a joint to make that joint as strong or rigid as desired. One such instance is a delicate mitered mirror frame. Figure XIV shows an example full size.

ci LASS

I I 1I A

Figure XIV: Mitered mirror frame held with dowels (A) or a 3/16" spline (B).

107 One of the favorite ways to hold miters together is with two dowels. But as can be seen from the dotted lines in Figure XIV(A) , two dowels would have to be of small diameter, short length, and quite close together. An alternate design is a 3/16" spline as in Figure XIV(B). The grain of the spline should run perpendicular to the miter. If parallel to the miter, the spline will split too easily. Sand the spline flush with the outside edge of the frame at its corners. This is another case of greater strength against breaking across grain than against splitting with the grain.

Another situation is the mortise and tenon joint between front rails and the solid slab end of a chest of drawers. Figure XV (drawn full size) illustrates this problem. The thickest case end possible out of 4/4 lumber would be 13/16". Good practice requires a mortise no closer than l/g" to the outside face. The hollow chisel mortise is usually l/8" deeper than the length of the tenon. This will give a maximum tenon length of 9/16" which may be too short for a rigid case. Many shops make such a joint, but the design invites trouble with loose cases. If the front rail runs into a 1 3/4" x 1 3/4" post instead of a 13/16" slab end, there will be no problem in designing a tenon long enough to provide a rigid joint.

Figure XV: Front rail jointed to case end with mortise and tenon joint.

Many other examples could be given, but the important point to remember is to make full-size drawings of all the views of questionable joints so as to see what can be done with the wood available.

Holding Until Glue Sets

In order to get a good glue bond, it is important to hold the two parts in contact with no movement relative to each other until the glue sets. In some joints, such as in edge gluing, there is nothing about the machining of

108 the joint which tends to hold the parts together. In such cases the assembly is frequently left in clamps until the glue sets. Time of set can be reduced by using a heat-setting glue and applying heat either by contact heat or by high-frequency electric field. Another solution is to nail or screw the parts together in such a way that the nails or screws maintain contact without clamps until the glue sets. Figure V is an example of this p rocedure.

In some joints, such as dowel or mortise and tenon, the joint itself will tend to hold the parts in contact once they are driven or clamped tightly together. This is especially true if the joints are machined for a tight fit. Chairs are frequently removed from the final assembly clamp as soon as corner blocks have been applied in the clamp. The dowels generally hold tight enough to prevent the parts' pulling away from each other before the glue sets. Of course, it helps if the assembled piece is not handled until the glue sets instead of subjecting it to handling and the stresses of fitting or cleaning operations as soon as it comes out of the clamp. If parts are crooked or slightly mismachined, an air clamp will often drive the assembly up tight; but it will be under stress, and as soon as the clamp pressure is released, the parts may spring back out of contact. To reduce this trouble, many factories lock-nail mortise and tenon joints where possible while the piece is in the case clamp.

TOP VIEW

CASE END FRONT RAIL B

FRONT VIEW

Figure XVI: Lock-nailing mortise and tenon joints.

In lock-nailing, there is no force pulling the nail out. Forces tend to shear the nail. This gives a nail much more strength than straight nailing or toe nailing.

109 110 CHAPTER 8

MECHANICAL FASTENERS

Wooden parts may be bonded together by typical woodworking joints that combine mechanical means with adhesives for rigidity. Mortise and tenon, lock- miter, and dowel joints are examples of this technique. However, another common way to hold two wooden parts together is through the use of a mechanical fastener. Joints of this type are not always as rigid as glued joints, but mechanical fasteners are easy to use, can be driven mechanically, and provide adequate strength when properly used. The usual fasteners used to join pieces of wood include nails, staples, screws, and corrugated fasteners.

Nails

There are a few basic differences between nailed joints and glued machined joints. Where two parts are joined together with nails and without a machined joint, the nails hold the parts together but do not give precision location of one part in relation to the other; many machined joints do give precision location. With a nailed joint, external forces can loosen the joint considerably without causing it to actually fall apart. With a glued joint, any motion of the parts due to external forces can break the glue bond and completely destroy whatever strength the glue contributed to the joint. In other words, a glue joint is tight up to the point of complete failure; a nailed joint loosens gradually until it finally fails completely . Joint Strenath

Nailed joints have different strengths according to the direction of the forces acting on the joint. The resistance of nails can be considered in two categories: &~ a. Against withdrawal . b. Against lateral displacement.

Figure I illustrates these actions. Force A tends to withdraw the nail. Force B would tend to rotate the part around fulcrum point C and would result in an upward force similar to A which would also tend to withdraw the nail. Force D or force E would try to cause lateral displacement of the two parts. If there were two nails (dotted circles, Figure I), a force H would set up a moment which would be resisted by the two nails, and the action of the two nails would be resistance to lateral displacement. In general, strength of a nail against lateral displacement is better than against withdrawal, so if two parts are to be nailed together, it is well to work out the construction so that forces act to cause lateral displacement instead of withdrawal.

The resistance of a nail against withdrawal varies in direct proportion to the diameter of the nail and to the depth which it is imbedded in the second wood part (such as G, Figure I). It also varies directly as the 2 1/2 power

111 of the specific gravity of the wood. The resistance to withdrawal is greater if the nail goes into side grain than if it goes into end grain. Strength into end grain is only about one half to three quarters as much as into side grain. Figure I shows the nail going into the side grain of part G, which is the part that counts; the nail head gives extra strength against withdrawal through part F. Withdrawal strength can be greatly affected by splits around the nail, by the surface roughness of the nail, by the type of nail point, and by changes in moisture content after nailing. Under adverse combinations of these factors, the resistance to withdrawal may fall to as low as one-fourth of normal; under favorable conditions, it may rise to a little above normal. With such a wide range of possible strengths, calculations are not very dependable.

I I

Figure I: Forces acting on a nailed joint.

112 If nails are driven parallel to each other but at an angle other than 90°, resistance to withdrawal is about the same as at a 90" angle. But if the nails are not parallel, resistance to withdrawal is increased. Figure II(A) and II(B) have about the same strength, while II(C) has the greatest strength.

A B C

Figure 11: Method of nailing to increase resistance to withdrawal.

Strength of nails against withdrawing can be much increased by driving nails clear through the second piece and clinching the point ends. In furniture factories, this 'is usually done only for skids and crates. When clinching, bending across grain gives about 20% more strength than bending with the grain.

The strength of a nail against lateral displacement varies with about one ana one-half power of its diameter. it also depends on species; the harder species, such as oak, birch, or hard maple, have about twice the strength of soft species, such as poplar, basswood, or spruce. The depth of penetration makes no appreciable difference provided that it is more than 10 times the diameter of the nail. If the nail is parallel to the grain of either piece, strength is only about two-thirds what it would be if the nail were perpendicular to the grain of both pieces.

113 There are many types of nails available to meet almost any application. The four main types are brads, finish nails, round-head nails, and T-nails. They usually come cohered in strips for easy loading and automatic driving with electric or pneumatic tools. Brads are good for face nailing, since they can be countersunk automatically and leave only a tiny hole. They are also used in "pinning" operations to lock wood joints. Finish nails pick up where brads leave off, being longer, being heavier, and having a little more bearing surface beneath the head. They are used extensively in cabinet work and can be countersunk automatically. Round-heads are the same nails that have always been driven by a hammer, and T-nails were the first substitute for round-head nails. Both have lost out to the staple for production work, however. Figure I11 shows the four basic types of nails used in furniture construction.

Brads Finish nails

Round-head nails T-nails

Figure 111: Nails used in furniture construction.

114 While nailed joints are often used for furniture crates and packing crates, they are seldom used for furniture itself unless in conjunction with glue. Back panels and mirror backs are frequently nailed on, and lock nails are often used at the case clamp to keep tenons from backing out of mortises when the clamp is released. However, recent developments in portable staple guns and staples have resulted in the stapl ing gun's rep1 acing nail ing operations. For most applications, there are a stapling gun and a staple that will give as much strength as hand nailing with a substantial savings in labor time. The basic decision to be made, when deciding whether to use heavy staples or nails, is the lateral load stress to which the fastener will be subjected. A nail is the correct selection when substantial side strain conditions exist, as nails withstand lateral shearing better than staples do.

Automatic Nailina

Although automatic nailing machines have been around for some time, a discussion of mechanical fasteners would not be complete without some mention of them. These machines generally make their own nails from wire and drive them instantly and effortlessly. At the same time, they offer precision control over many types of nailing operations. One of the latest and fastest-growing applications of automatic nailing is in attaching moldings and overlays of wood or plastic.

Adhesive technology has not yet come up with a glue that will satisfactorily bond plastic to wood under ordinary furniture production conditions. The common glues used to assemble wood parts simply will not work with plastics. As a result, manufacturers have turned to automatic nailing machines for fastening plastic overlays to wood. In particular, they are using machines to drive nails from the backside, thereby eliminating the need to fill nail indentations in moldings and overlays. The heat from a nail driven through wood into a polystyrene part is such that it causes the thermo-plastic to melt and re-form around the nail. The bond thus formed is virtually impossible to break, making it the best means of fastening plastic to wood yet devel oped.

Stapl es

Staples usually provide more holding power than tacks and small nails because they have two legs for holding instead of just one. Staple legs follow the cut of the point on each leg, and they will diverge, toe in or flare out, depending upon the type of point used. This action is what gives staples their excellent holding power. The following is a description of several types of points used on staples:

The divergent point is a saw-tooth cut, with the direction of the cut on each ieg being in opposite directions. When driven, each leg follows the cut of its point, resulting in a twisting or diverging action. This is a good point for shorter staples driven into soft woods. It drives and holds well. However, it is not recommended for harder woods or in long lengths, because the diverging action of the legs becomes extreme and impairs good driving of the staple.

115 The chisel point, as the name implies, has chisel cuts on both legs from the outside in, resulting in a toe-in action when driven. Although it does not provide as much holding power in soft wood as does the divergent point, it is a good, all-purpose point. The chisel point staple is the one used with a clinching anvil, as on a carton stapler.

Modified points are used for hard species of wood. With species like hickory, ash, or oak, a divergent point simply cannot be driven, and a chisel point might toe in so severely that the legs would come together, resulting in poor holding power. The points are modified to produce a slight divergence and toe out when driven, thereby giving a good joint. Fastener engineers are constantly designing special points to solve other staple driving problems that arise.

Step points enable a staple to pass through two pieces of wood and then clinch the points back securely locking the two pieces together. They work well when driven against a metal back-up plate. Other types of points tend to give inconsistent results in similar applications.

Divergent-chisel points are used on long, narrow-crown staples up to 2 inches in length. The legs twist in opposite directions and toe in slightly. This action gives these stapl es remarkably strong hol ding power .

It should be remembered that staple legs follow the cut of the point when driven. The harder the wood and the longer the staple, the greater the action of the staple legs. Thus, it is important to select the right point for a particular operation. The leg action for some of the common staples used in furniture construction is shown in Figure IV on the following page.

Staples are manufactured largely from tinned or lacquer-coated wire of a prescribed tensile strength so that they will drive easily, cohere properly and have a good appearance. Stiffer staples made of high carbon wire are available for extremely tough or hard materials. Additional holding power is achieved with the addition of a chemical coating to the staple legs. To- day, finishing technology enables staple crowns to be colored to match fabrics into which they are driven, to blend in with wood paneling, and so on. Finally, galvanized wire is used to obtain rust-resistant qualities in staples for use where rustproofing is required.

116 II] Divergent point

Step point

Chisel point n

Modified point Divergent-chisel point

Figure IV: Staples used in furniture construction.

Sc rew s

While much more expensive than nailed or stapled joints, screwed j ints r ! more dependable and generally give greater strength. The best strength is obtained by boring the first piece to the shank diameter of the screw and boring the second piece with a lead hole about 90% of the root diameter of the thread if hardwood and 70% if softwood. The lead hole can be omitted, but strength is better with it, presumably because there is less destructive distortion and tearing of the wood fibers. To avoid distortion, screws should be turned in, not started with a hammer.

117 Joint Strength Strength against withdrawal varies directly with the length of the threaded portion of the screw imbedded in the second part, directly with the diameter of the screw and directly with the square of the density or specific gravity of the wood. In hard species, long thin screws can go deep enough into the wood so that resistance to stripping the thread is greater than the tensile strength of the screws. When this takes place, additional depth of penetration adds no strength as the screw will break before stripping. In such cases, trouble is sometimes encountered in twisting off the screw during driving, especially with brass screws. If the screw thread runs into the wood parallel to the grain instead of perpendicular, strength is only about three-fourths but is more erratic and subject to wider variations.

The strength of a screw against lateral displacement varies approximately as the square of its diameter. Length of penetration makes no difference provided it is at least seven times the diameter of the screw. While a screw generally has more strength against 1 ateral displ acement than a nail, neither one is dependable to hold two parts rigid and free from all motion. Thus, if complete rigidity is wanted, glue should be used in addition to the screw. The screw pulls the two surfaces close together with a resulting good glue bond.

Types of Screws’

There are basically four types of wood screws: standard, twinfast, self- drilling and specialized screws for particleboard. The standard wood screw has a continuous helical thread around a slightly tapered shank. The maximum thread diameter is the same as the diameter of the plain-shank section under the head. The point of the standard screw is slightly blunt and eccent ric . The twinfast wood screw is an improvement over the standard wood screw. It has parallel twin threads which permit faster turning of the screw into wood without a decrease in holding power. A maximum thread diameter greater than the plain-shank diameter gives the twinfast screw increased holding power in wood. Its use results in better wood assemblies and fewer rejections and allows faster driving and tighter fastening. Even with these improvements, the cost of this screw is the same as that of a conventional screw of the same size.

Self-drilling wood screws have cylindrical shanks and centered points. In addition, an off-center slot is milled in the shank at the point. The slot cuts mating threads in wood when the screw is turned and provides room for wood chips. The screw is hardened to retain a sharp cutting edge, and the gimlet point assures fast starting and helps the screw pull itself into the wood. This improved screw offers the same or slightly greater withdrawal resistance than a common wood screw. It is not necessary to drill a pilot hole for this type of screw.

E. George Stern, Wood Screws for Building Construction and Wood Product Assembly, (61 acksburg, VA.: Virginia Polytechnic Institute), pp.6-9.

118 Most wood screws are available with slotted or recessed heads. Recessed "Phillips" screw heads offer an advantage over slotted heads in that a screwdriver is less likely to slip and mar the screw head or the material through which the screw is driven. The screwdriver is centered automatically and stays aligned. Thus, assembly time can be reduced and workmanship improved by using a screw with a recessed head.

The specialized particleboard screws have wider flanges to "bite" deeper into the walls of the hole. They are somewhat like sheet metal screws. In fact, many manufacturers use sheet metal screws for screwing into particleboard. Ordinary wood screws have poor holding power when used in particleboard or fiberboard.

It is expected that improved wood screws like the twinfast and self-drilling screws just discussed will eventually replace the standard wood screw in furniture assembly operations. Figure V shows the three basic types of screws.

Self-drilling Conventional wood screw wood screw Twinfast wood screw

Figure V: Three types of wood screws used in furniture construction.

119 ' C1 amp Nail

The clamp nail is an ingenious metal fastener which acts like a clamp in drawing two parts tightly together and a spline in holding them together. Figure VI shows a clamp nail approximately full size, except that the flare of the two flanges of the I-beam section at end A is exaggerated. In practice, the flare is only about l/32'' on either side of the side. I

Figure VI: Clamp nail.

Figure VI1 shows a mitered joint fastened with a clamp nail . Grooves AA are cut to receive the clamp nail. The depth of these grooves is the same as the thickness of the web of the clamp nail, which is less than the width of the top and bottom faces if considered as an I-beam. In assembling the joint, the flared end (A in Figure VI) of the clamp nail is driven into the grooves. The flare creates a squeezing action which tends to pull the two parts tightly together. Glue can be applied on the matching faces of the wood parts to give strength.

Figure VII: Clamp nail used to fasten mitered joint.

120 Corrugated Fastener

The corrugated fastener, like the clamp nail, draws two parts tightly together and then holds them, but unlike the clamp nail, it requires no machining. It is driven into solid wood like a staple. Figure VI11 shows a corrugated fastener approximately full size except that the tapered draw of the corrugations is exaggerated.

Figure VIII: Corrugated fastener.

F,gure IX shows a m ered joint fastened with corrugateL fasteners. To hold the miter tight, experience has shown that fasteners need to be used on both faces at A and B. If used only on face A, the fastener pulls the joint tight at A but, in doing so, opens up a crack on face B. Whereas the clamp nail can be hidden so it does not show, the corrugated fastener remains exposed. As a consequence, the corrugated fastener is not used very often in furniture, although it is used considerably for wooden boxes.

Figure IX: Corrugated fasteners used on a mitered joint.

121 Mi scell aneous Fasteners

There are many ingenous special fasteners on the market. Some are primarily to permit ''knock down" construction for shipping; some are for "floating constructions" to avoid problems with shrink and swell; and some apply only to special pieces of furniture, such as tables, beds, mirrors, etc. A few of these will be discussed in the remainder of this chapter.

Connecting Fittings

The Europeans have devel oped some unique fasteners for their casegoods furniture. Examples are pictured in Figures X and XI. The great bulk of case goods sold in Europe consists of modular units made up of shelves, doors, and drawers, most of which are fabricated to permit knock down and re- assembly. This type of construction requires fasteners that are sturdy yet easy to operate. Figure X shows an angle dowel and two casings used to fasten a miter joint, while Figure XI shows a socket, threaded dowel , and casing used to join a top or bottom panel to an end panel.

Figure X: Connecting fitting for a Figure XI: Connecting fitting for mitered joint. end panels to tops or bottoms.

122 The principal difference between the connecting fitting in Figure XI1 and that in Figure XI is in the design of the metal casing. In Figure XI1 the casing remains stationary, and the cam-shaped key is rotated in the casing with a screwdriver until secure in the eye of the threaded dowel. The casing in Figure XI is rotated in the wood itself until tight against the head of the metal dowel. The advantage of the fitting in Figure XI1 over Figure XI lies in the fact that its casing serves as a bearing and does not move after it is inserted in a panel. This eliminates the abrasive action of the casing in Figure XI, which has a tendency to enlarge the pre-bored hole, thus reducing the effectiveness of the joint.

Figure XII: Connecting fitting for joining end panels to tops, bottoms and she1 ves.

Figure XI11 is a schematic diagram of another type of connecting fitting. This particular one is made out of plastic and is designed to be used with various configurations of top and bottom panels, end panels, and shelves. The two main parts of the fastener are mounted in separate panels, and a joint is made by tightening the spiral-flanged screw in the casing against the foot of the dowel.

Figure XIII: Alternate form of connecting fitting for joining end panels to tops, bottoms, and shelves.

123 The ultimate use of connecting fittings like those just shown was found in the Ums-Pastoe made-to-measure system of modular wall units where a number of horizontal and vertical parts can be joined together by a patented corner fitting into a case or ''element." The unique assembly feature in all Pastoe combination units is the corner bar fitted with metal pins and dowels which slides into two or more adjoining horizontal or vertical parts and joins them securely (see Figure XIV). This particular corner construction enables one to expand vertically as well as horizontally into larger units covering an entire wall if desired.

Figure XIV: Connecting fitting for Ums-Pastoe System.

Fasteners like those in Figures XI, XII, and XIII are used extensively in knock-down furniture in Europe. A single fitting is generally used with dowel pins on either side to fasten the side panels to tops, bottoms, and shelves in bookcases and other small cases, whereas two fittings are used for larger constructions like chests and wardrobes. As the trend in KD furniture continues to grow in this country, more attention will undoubtedly be focused on fasteners of this type.

124 P1 asti c Miter Dowel

A solution to the problem of obtaining satisfactory miter joints for case goods with chip core, plywood, or solid wood panels is the miter dowel. Figure XV is a photograph of a plastic miter dowel , and Figure XVI shows it being used in a case panel joint. Miter dowels can also be used for frame joints in mirrors or door frames with no danger of breakthrough, as with conventional dowels at right angles to the face of the miter cut. The cup shapes on the sides of the dowel, in conjunction with virtually any furniture adhesive, serve to form a strong mechanical bond in the wood that is difficult to pull out. Machining for mitered dowels is similar to that used for straight dowels in lap joints or butt joint doweling, requiring only a boring machine and a miter saw. The plastic material in the miter dowels is not affected by moisture changes and will not shrink or swell to break the joint or loosen the bond.

Figure XV: Plastic miter dowel. Figure XVI: Plastic miter dowels in a case panel joint.

Table and Shelf Pins

Table and shelf pins (Figure XVII and Figure XVIII, respectively) are used to hold table leaves and case shelves in place. Made from plastic, they will not shrink, swell, check, or rust. While finishing materials will not adhere to these pins, they will not scratch or mar any finish with which they come in contact. No glue is required as they are designed to adjust automatically to holes slightly over or under the specified diameter.

Figure XVII: Table pins.

125 c-$4 OR THICKER

Figure XVIII: Shelf pin.

Glass and Panel Retaining Buttons

Glass and panel retaining buttons are designed to hold glass plates or panels to wood frames. They are usually styled for flat, oval-head, and round-head screws. Some buttons are made for removable panels and glass set flush to a frame, while others are designed for recessed glass or panels. Figure XIX illustrates a semi-permanent, glass and panel retaining button for a recessed frame.

Figure XIX: Retaining buttons.

126 Shipping Stops

Figure XX shows a shipping stop used to fasten drawers and doors closed during transit. It is a plastic-coated steel band, especially designed and fabricated to prevent damage to furniture finishes. It is not supposed to scratch even the finest finish while it keeps drawers and doors from coming open after packing.

Figure XX: Shipping stop.

Leg Fasteners

The T-nut and corner leg bracket, Figures XXI and XXII, are used to attach legs to cases and tables. Both go into a large percentage of the knock-down furniture produced in this country. T-nuts find extensive usage as leg fasteners in contemporary furniture while almost every furniture manufacturer uses corner leg brackets like the one shown in Figure XXII to fasten legs to dining tables. Fasteners of this type are extremely common; all you have to do is look under any large table to find one.

Figure XXI: T-Nut.

127 Figure XXII: Leg plate.

These are but a few examples of the many types of fasteners available to the product engineer. The number of different fasteners is almost limitless, as new types are being developed all the time. It remains the product engineer's responsibility to seek out and find the correct fastener for his specific application. It is, therefore, important for him to keep up with and know what is happening in fastener technology so he can produce the best product possible.

Acknowl edgments

Acknowledgment is made to the following organizations through whose courtesy and by whose permission illustrations in this chapter have been reproduced:

Duo-Fast Fastener Corporation, Frank1 in Park, I1linois. Paul Hettich & Company, Kirchl engern, Germany . Ronthor Reiss Corporation, New York, New York.

Selby Furniture Hardware Company, New York, New York.

Ums-Pastoe NV, Utrecht , Holl and.

128 CHAPTER 9

MINIMIZING COST

As was previously stated, good product engineering places primary emphasis on developing a product to meet customer requirements. However, it is also important that this be accomplished at minimum cost. To achieve minimum cost, the product engineer should continually search for alternative constructions. This process can be divided into two steps. The first step is called "brainstorming," or a "think tank," when one or more individuals 1 ist a1 1 possible alternatives regardless of merit. The second step involves evaluating the alternatives. Those which will not yield a good product or a satisfactory improvement in a product are eliminated. The remaining alternatives are then compared on the basis of cost to determine the least costly solution.

In comparing alternatives for cost, it should be remembered that total cost consists of three elements:

1. Material.

2. Labor.

3. Overhead.

It is very seldom that the choice of one alternative over another will affect overhead cost; therefore, it is generally safe to make a selection on the basis of material plus labor. In doing so, it is necessary to consider all departments in the plant, not just one. Alternatives usually differ in cost with regard to material purchase, material utilization, labor requirements, machining, and assembly. They occasionally differ in cost in finishing but not very often. Packing and shipping is a different situation and will be discussed separately in the last chapter.

In order to intelligently minimize material and labor costs, the product engineer must be familiar with each process used in his plant and with any additional processes that could possibly be introduced into the plant. Occasionally the selection of an alternative in product construction may affect the plant layout, although it is more likely to affect processing. However, any effect on 1 ayout cannot be completely disregarded.

Minimi zing cost wi11 be discussed under the followi ng headings : P u r c ha sed Mate r ia 1 Material Uti1ization Labor Savings Allowances for Manufacturing Variation Fool proofi ng.

129 Purchased Material

Frequently the product engineer has a choice of several materials, any one of which would be satisfactory for a certain furniture part or class of parts. He must somehow make a decision as to which material he wants to use. This is often done by an economic analysis of the available alternatives. Rather than give a long discussion of generalities, a few specific examples will be given to illustrate what is meant.

Solid or Plvwood

If a suite of furniture is not to be sold as "solid" maple, there is generally a choice of alternatives for the wide, flat exposed parts like bed panels and tops. They can be edge-glued from lumber, or they can be plywood. If plywood, they can have cores of lumber, lumber-banded particleboard, medium-density fiberboard, or veneer. The face can be maple or (cheaper) gum veneer, vinyl , low-pressure laminate, high-pressure laminate. Then some veneer versions can be made 5-ply or 3-ply. The combinations of alternatives that are commercially attractive for some plants number 16.

Things such as price range, appearance and construction have a great deal of bearing on what type of material can be used. Frequently the choice of core is limited if the edges of a part will show (as on a table top). If the edges of a part do not show in an assembled piece of furniture, the appearance of the edge of the part is not important, so the core material can be the least expensive available, probably particleboard for thicknesses greater than 3/4" or veneer if less than 5/8". Most core materials, with the exception of lumber, do not have a good appearance on the edges after finishing. If edges show, one possibility is to band the core with lumber or band the panel with veneer. The choice of core may also be influenced by the manner in which the part is to be attached to an assembly. If screws are to be used, consideration should be given to the screw holding power of the various types of cores. Paper honeycomb has no screw holding power. Some types of particleboard have poor holding power for screws, while other types are fairly good. Lumber and veneer cores each have good screw holding power.

A1 though the above discussion concentrated primarily on core materials, the same sort of process can be carried out for crossbands and faces. After the obviously unacceptable alternatives have been rejected, the task of selecting the appropriate alternative becomes more difficult. One particular alternative may be less expensive or more expensive than another, depending upon the specific application, thus each situation should be decided by a cost comparison. This procedure may appear to be time consuming, but it is the key to minimizing costs.

Finished Thickness

If a part is to be made of lumber, the finished thickness should be made to come out of some standard lumber thickness. Many designers like to specify part dimensions slightly larger than standard size, such as calling for a 7 /8 'I-thi ck f font rai1 . With a minimum dressing a1 1 owance of 3 /16 'I from rough lumber to finished part, 4/4 rough lumber will not finish thicker than 13/1611.

130 This means that it would require 5/4 lumber to get 7/8" finished thickness for the front rail when 5/4 lumber can be used to produce parts 1" or 1 1/6" thick. It would be better to use 4/4 lumber and finish the rail l3/16" thick, provided it would not harm the appearance or sales acceptance of the product. It would certainly reduce cost to use 4/4 instead of 5/4 lumber.

Where thickness does not affect appearance, one should use the thinnest standard rough lumber that will produce the parts with adequate strength, rigidity, and other desired mechanical properties. The same principle applies to plywood; use the thinnest piece that will give satisfactory performance because, as a rule, plywood is less expensive if it has a veneer core. With lumber-core plywood, there are little, if any, savings to be derived from planing down a 4/4 lumber core to get a thinner plywood panel. With particleboard core, it is advisable to obtain quotations on the various thicknesses which are available. Some mills quote the same price for 1/2" as for 3/4", others list different prices.

If a plant has a band resaw, there exists an opportunity to reduce costs. It is possible that drawer sides out of resawn 5/4 or 6/4 lumber may be less expensive than out of 5/8 lumber. There are other such possibilities for thin parts made from lumber if only the product engineer would put his imagination to work.

Thin Parts

Veneer-core plywood often costs less than lumber for thin parts less than 1/2'' thick. Examples are 3/8" or 7/16" drawer sides and backs. One drawback of plywood drawer parts lies in the fact that the top edge shows alternate stripes of end and side grains. Some plants seek to avoid this problem by using straight grain laminate where the grain of all layers of veneer runs in a lengthwise direction. It makes the edge look much better than plywood, but not as good as lumber. The straight grain laminate will shrink and swell in width the same as lumber, whereas regular plywood will not. However, plywood is more likely to warp and show a crooked top edge than straight grain laminate or lumber. The least cost drawer side material today is vinyl coated fiberboard or particleboard. Specialized plants wrap the vinyl around the material with the ends meeting in the groove. Thus in the finished drawer, there is no seam visible. The vinyl acts as a hinge in folding the drawer which reduces labor cost in drawer assembly (see also Chapter 4). Hardboard is yet another possibility or thin parts. It is a low-priced material and has excellent mechanical properties. It can be veneered to make thin 3-ply panels, or it can be pr nted.

Material Utilization

Lumber

Good lumber utilization is primarily a problem of processing and plant operation rather than a problem of product engineering. But there are things the product engineer can do to help or hurt utilization, and lowered utilization means increased cost.

131 Casegoods generally have quite a few short interior parts, such as drawer guides and dust frame ends. Frequently they may be sound grade and any species. On most cost estimates, these parts are figured at the overall average cost. In actual practice, however, most shops accumulate more shorts than they know what to do with at the cut-off saw and the salvage saws. In such cases, the increased width of a drawer guide would show extra cost on the cost estimate but would really not increase the actual cost because they would be made out of shorts which would otherwise be thrown away. This illustration means that if one can work out a construction to use shorter sticks, he can reduce the actual cost regardless of what the estimate says. One extreme example was a casegoods factory making solid maple, modern-style bedroom suites with solid slab case ends. In many plants, the grain of the solid ends would be run vertically which would require long sticks. This factory ran the grain horizontally and converted all the footage in case ends to short sticks instead of long. There was no reduction in footage, but there was a large reduction in actual cost because it improved lumber utilization.

When thinking of lumber waste, many people concentrate on the cut-off saw and disregard the ripsaw. In most plants, there is more waste at the ripsaw than at the cut-off saw. At the ripsaw, it is fairly easy to get waste down to a minimum when ripping a random-width product which will later be edge glued, but when the product is to be a specific width and not edge glued, it is harder to avoid large waste. In such a situation, the ideal ripping program for minimum waste is to rip two products at the same time, both of which come out of the same length of stock at the cut-off saw. One product is fixed width, the other is random width. The procedure is to rip all the fixed width cuttings first, then rip the remainder into random widths for the second product. This eliminates most of the inherent waste in ripping f ixed-width product out of random-width stock. Whenever the product engi - neer can match furniture parts this way, maximum utilization is possible, hence minimum cost.

One modification of this procedure is to rip only for the fixed-width product, take what one-piece product the stock will make, edge glue the rest, and re-rip after gluing to get the required fixed width. Lumber utilization is about as good, but extra labor cost is incurred for gluing and re- ripping. One further disadvantage is the problem of getting the glued parts together with the solid parts a few hours or days after the solid parts have been ripped. One advantage is that parts need not be matched in pairs corn- ing out of the same cut-off length.

Another possibility is to lay out construction so that several parts can be produced from the same cut-off length but have different widths. If all widths are ripped simultaneously, waste will be lower than if only one width is ripped. Some plants get better utilization by ripping the various widths into board lengths from the uncut board by running lumber through a gang ripsaw first and cutting for length later. Still another possibility is to work out construction so that an exposed part and a hidden part are identical through the moulder except for grade. An example would be front rails and back rails for case dust frames. When the moulder offbearer inspects, any piece clear enough and straight enough for a front rail is put on one truck; the others are put on a different truck for back rails. With such a

132 procedure, it is often possible to disregard defects at the cut-off saws and ripsaws and thus increase utilization. An alternative to identical front and back rails would be back rails made out of a cheaper species or a lower lumber grade. Still another alternative would be to make back rails narrower and use less footage.

If lumber could be bought in specified widths and if the product engineer could design the width of some parts to fit the lumber width, ripsaw waste could be eliminated on those parts (except for saw kerf). Pine, Douglas fir and other species which are chiefly used for building constructions can be 6ought to specified widths such as 1" x 4", 1" x 6", and 1" x 8". In some instances, furniture parts can be made of these species either because the desired appearance is pine or because they are hidden. If a back rail finished 1 1/2" wide through the moulder, it should be 1 3/4" wide leaving the ripsaw. Ordinary 1" x 6" building lumber is actually 5 l/zl1 wide. Three rails could be ripped from 1" x 6" as follows, and there is no ripping waste except saw kerf and lumber defects.

3 strips ripped 1 3/4" wide 2 saw kerf /8 I' wide

Veneer

Utilization of shorts can be as important in face veneer as in lumber. But it is harder to work out because the effectiveness of a design often depends on the grain of the face veneer running in a certain direction. Some plants make their own drawer bottoms and use face veneer shorts in them by running the grain in the narrow direction of the drawer bottom. Or perhaps the design can stand horizontal grain on the end panels, permitting the use of shorts.

In clipping face veneer to width, most factories clip the individual pieces an even fraction of the final width desired. If final width is ZO", veneer will be clipped 6 2/3" wide for a three-piece face, 5" for four piece, 4" for five piece, etc. This permits either slip match or book match. It also permits book match symmetrical to a center line if the number of pieces is even, such as four or six. But frequently a particular part, such as a bed bed panel, case top, or case front, looks just as well with unsymmetrical slip match and with individual pieces of the face not necessarily the same width. In such a situation, each flitch of veneer can be clipped and jointed to the widest width it will make. Individual pieces are edge spliced wider than needed and then clipped to the required width. The process is basically the same as when random-width lumber-core strips are matched to width at the matching saw. It is surprising how much veneer waste this procedure saves compared to clipping for pt , l/5, etc. of the required width. This procedure is especially adaptable to face veneer with straight-line grain such as rift-cut oak or quartered walnut.

Plywood

The general practice when buying furniture plywood is to order the exact finished size needed from the plywood mill. On the other hand, building

133 plywood is generally made in stock-size sheets, such as 4' x 8'. If pine is a satisfactory species for a certain furniture part, the price per square foot will often be less than furniture plywood. But in cutting small sizes out of a stock-size sheet, there is a possibility that cutting waste will offset the advantage of lower price per square foot. Sometimes sizes of furniture parts can be selected for little or no waste in cutting from stock-size panels. For example, a 4' x 8' sheet will only produce one part 48 1/4" x 24 1/4", but will produce four parts 47 3/4" x 23 3b It. One frequent application of this idea is in kitchen cabinets. The exact distance wall cabinets stick out from the wall is not too important as long as they are all the same. Consequently, it might be wise to construct these cabinets to use end panels just under 12 inches or just under 16 inches, to permit fitting the four-foot width of stock panels without ripping waste. In a similar way, base cabinets could be laid out for end panels just under 24 inches.

Similar care in planning can improve the uti ization of all panel type materials such as particleboard and hardboard.

Machining and Assembly

It was stated at the beginning of this chapter that comparison of alternates for labor cost should include all departments where the labor cost would be different for the different alternatives. Theoretically, this requires labor estimates so that the figures can be compared. In practice, a product engineer can often select the alternative with lowest labor costs by judging from his own experience. The possibilities for labor savings are so many and so diverse that it is hopeless to try to cover them all in a short discussion, but a few specific illustrations will be given as a guide and a stimulus to imaginative thinking in product engineering from the aspect of minimum 1 abor cost.

In general it pays to do considerable extra machining if it will reduce the cost of assembly or fitting. Most machine operations are fast--five to fifty pieces a minute; most assembly operations are slower--one to several minutes per piece. It often pays to machine for accurate location of parts in assembly. This can eliminate assembly jigs or gages which often slow down assembly. If two parts are jointed with a double dowel, there is only one way the parts can go together, and that is the right way (provided machining is accurate).

A mortise and tenon construction often accomplishes the same purpose as a dowel but involves less labor. For the joint between the front rails and post of a chest, the machine operations are compared here. There is no appreciable difference in assembly labor.

134 Mortise & Tenon Dowe 1

1. Doubl e-end tenoner. 1. Double-trim saw. Cut post Trim post to length to finished length (30 pieces per minute). (30 pieces per minute).

2. Double-end tenoner. 2. Double-trim saw. Cut rail Trim to length and cut to finished length (50 pieces per minute). (50 pieces per minute).

3. Multiple mortiser. 3. Boring machine. Bore post (15 pieces per minute). for dowels (20 pieces per minute).

4. Boring machine. Bore rail for dowels, both ends (8 pieces per minute) .

5. Dowel driving machine. Pre-pin two dowels in one part (8 pieces per minute).

If a double-end trim and boring machine were available, the dowel construction would involve fewer machining operations.

1. Double-trim saw. Cut post to finished length (30 pieces per minute).

2. Double-end trim and boring machine. Cut rail to finished length and bore for dowels (20 pieces per minute).

3. Boring machine. Bore post for dowel s (20 pieces per minute).

4. Dowel driving machine. Pre-pin one part (20 strokes per minute).

One radio cabinet manufacturer avoided machined joints as much as possible and substituted flat joints to be glued side-grain-to-side-grain. He then worked out assembly jigs to locate the parts and apply pressure to the glued joints. He also incorporated electrodes into his jigs for fast cure of the heat-setting glue by connecting the electrodes to high-frequency electric

135 generators. With this construction, many of his cabinet parts were rectangular so that the machining cost was very low. He had a big investment in assembly jigs but was satisfied that the labor savings justified the investment. Compared to a line of ordinary household furniture, he had very few different cabinets in his line and, as a result, ran very large manufacturing lots of each. His procedure might well be good economy for his operation but poor economy for a furniture factory with many items in the line, frequent changes of product, and small er manufacturing 1 ot sizes.

Sometimes the product engineer can save set-up time in machining by specifying two or more parts in such a way that one set up of a machine will take care of several parts. On a chest of drawers, it might be possible to specify front rails, back rails, and dust frame ends of identical width, thickness, and cross section. In such a case, one moulder set up would run all three. Frequently the top front rail needs no groove whereas the other front rails need a groove in the back edge for a dust bottom. While it is not needed, a groove in the back edge of the top front rail would permit it to be run on the same moulder set up as the other front rails. Often drawer sides and backs can have identical cross sections requiring only one moulder set up. In plants where manufacturing lots are small, set-up time is a large percentage of total machining time. However, set-up time is of less relative importance with large manufacturing lots. Some machines, such as the ripsaw or planer, are set up quickly while other machines require longer set-up times and are more expensive to operate.

In general, the lowest assembly labor cost is achieved by working out construction to permit as many simple sub-assemblies as possible. The sub- assemblies are then combined with a few individual parts into a final assembly operation. Many sub-assemblies can be made in low-cost, flat clamps such as sash clamps or end clamps. These clamps are convenient and fast to operate. In addition it helps if the construction can be designed in such a way that sub-assemblies and final assembly can be clamped with pressure in only one direction. Figure I shows two constructions for a frame. A will assemble with clamp pressure in one direction, C-C; B requires pressure in two directions, D-D and E-E.

D D

t ItD D A E3 Figure I:Two frame constructions requiring different clamping in assembly.

136 Where appearance will permit, it is well to avoid flush joints. Even with accurate machining, flush joints never assemble as ''true flush," but must be sanded or machined flush after assembly. Figure I1 shows two constructions for the joint between a case end and a front post. In (A) the joint has an intentional offset of about 1/16" at C. In (B) the parts are machined theoretically flush at C. But to get true flush, the end must be sanded flush after assembly.

A B

Figure 11: Constructions for joint between a case end and front post.

Figure I11 illustrates another flush situation. It is a mitered and doweled frame. If the two parts are machined and doweled as in (A), the edge will not be true flush at point C. If the parts are machined and assembled like solid lines in (B), the edge of the assembled frame can be run on a shaper form as shown by the dotted line, and the edge will be true flush at point C.

137 m

C C A

Figure 111: Mitered and doweled frame with machining done before assembly (A) and after assembly (B).

Construction requirements will often determine whether or not machining is required after sub-assembling. It is generally wise to avoid machining after sub-assembly if possible, but one should plan on doing some. Plant layout and supervision are involved in this decision. For machining after sub-assembly, one procedure is to assemble in the cabinet room and then haul the sub-assemblies back to the machine room. Another way is to sub-assemble in the machine room. Still another way is to insert the needed machines in the assembly line in the cabinet room. Plants can operate very well with any one or a combination of these procedures. The feasibility of machining after sub-assembly can be determined only after a careful study of all of the factors. One example of how construction can affect machining after assembly is a frame door with a raised panel. Figure IV illustrates this. The door frame must be drum sanded to a flush face after assembly. In (A) the panel can be assembled in the frame and the frame drum sanded flush with its face because the drum sander would cut on the line C-C. But the face of the panel would have to be sanded before assembly. In (6) if the panel is assembled in the frame, the face of the frame carinot be sanded flush because the drum sander cuts on the line C-C and would sand the panel without making contact with the face of the frame at any point.

138 C- -C c-

A 6

Figure IV: Alternative constructions for a paneled door frame.

Fitti na

The cabinet room is usually the place where assembling is done, but it is also the place where considerable fitting has to be done. Drawers and doors generally need fitting, and the product engineer should do what he can to minimize the labor cost of fitting.

Assume that the drawer opening in a chest is 30" x 8" and that the drawer front shuts into this opening. It is evident that a drawer front 30" x 8" will not slide easily into an opening that size. Assume that appearance permits a uniform clearance of 1/16" between the case and the drawer front at the top edge and the two ends of the drawer front. If machining and assembling are perfectly accurate and perfectly square, a drawer front 29 7/8" x 7 15/16" will give the uniform clearance without touching, and the drawer front would slide easily into the opening. If machining and assembly are poorly done, there will be many instances where the clearance will not be uniform but will vary from too small to too big. To get a uniform clearance, each drawer front will need handfitting with a plane to the particular drawer opening into which it is to slide. If dimensional deviations are extreme, it might be necessary to machine the drawer from 30" x 8" or even

139 or even larger in order to provide enough wood so that the drawer front can be handfitted to an opening that is too big or off square. Here is an instance where the product engineer needs to know the process capabilities in his plant and the degree of accuracy that can be expected. Otherwise, he cannot determine the proper size to which the drawer front should be machined. The objective is to select a drawer-front size which will permit a satisfactory appearance of the clearance around the drawer front with a minimum amount of handfitting in the cabinet room. Some plants make a low- price product that permits a fairly wide clearance and one that is not too uniform. If such a plant is reasonably careful about the dimensional accu- rac of parts and about squareness, a drawer front can be machined a little sma 7 ler than the opening, and handfitting is virtually eliminated. A little different situation applies to a china cabinet with a door consisting of a frame with glass set in the frame. If the opening for the door is 30" x 22" and a 1/16'' clearance is desired on all four edges between the door and the opening, the door should theoretically be 29 7/8 I' x 21 7/8". If the frame has mitered corners, the two door stiles should be 29 7/8" long and the two rails should be 21 7/8" long. However, the door will not be precision square if the parts are machined to these lengths and then assembled. The product engineer should plan the door about l/2" oversize in both directions. After the door is assembled, it should be put through double- rip and double-trim operations to get overall door size precision of 29 7/8" x 21 7/8" and to assure precision squareness. This will reduce handfitting more than the extra machining labor and will minimize overall 1 abor cost.

There are many other examples, but the two just discussed illustrate what is meant by minimizing labor in handfitting.

A1 lowances for Manufacturing Variation If all machining were of perfect precision, it would be idea . It is well known that, in practice, there will be small variations from specified dimensions. If these small variations slow down assembly, requ re extra handfitting or result in an unsatisfactory quality, the product engineer should use his ingenuity to develop constructions that will assemble easily in spite of small manufacturing variations.

To guide thinking along these lines, all dimensions of furniture parts can be considered to be in one of three categories:

1. Noncritical.

2. Critical.

3. Intermediate.

140 Noncritical Dimensions

These are dimensions where considerable variation (up to 1 /8 ") from specified dimensions will not affect assembly. A few examples follow.

If a chest of drawers has an overhanging top which is screwed to the case after the case comes out of the case clamp, variations in length or width of the top do not affect assembly.

On a chest of drawers where the apron is nailed to the bottom front rail and between the front posts, the width or thickness of the apron is noncritical, and variations will not affect assembly. However, the length of the apron is critical, as the distance between the front posts is determined by the shoulder length of the bottom front rail. If the apron is longer than this shoulder length, it will not go between the posts, and if it is shorter, there will be a clearance between the end of the apron and the posts. If the bottom edge of this apron is bandsawn, the shape of the bandsawing is noncritical because nothing fits against it.

In a chair factory, if the assembled chairs go over a leveling saw, the length of the posts is noncritical because the leveling saw will cut off the long post and level the chair.

Critical Dimensions

Most critical dimensions involve two parts in a male and female type of joint. Examples are dowel, mortise and tenon, tenon and groove. If the diameter of a dowel is much less than that of the hole into which it goes, wood-to-wood contact is not established, and the glue joint will have poor strength. It will assemble easily but will be a poor quality joint. If the diameter of the dowel is larger than the hole, the joint will be difficult to assemble. In addition, there is a risk of splitting the piece which has the hole in it. A mortise and tenon joint is similar to the dowel. If the mortise is 3/4" x 3/4", both the width and thickness of the tenon must be neither too small nor too large in order to get a good joint and avoid splitting. With tenon and groove, the width of the groove and thickness of the tenon are critical, but the width of the tenon is not critical because the edges of the tenon do not come in contact with the ends of the groove.

Shoulder lengths of front rails on a chest of drawers are fairly critical. If the various front rails on a chest have different should lengths, some will show a clearance on the front of the case between the front rail and post, or else the case front will be out of square. These shoulder lengths need not be held to quite as close tolerances as the members of a male and female joint, but variations should certainly not be more than -+ 1/32t1.

Miter angles on a mitered frame are critical if the miter shows on the face of a frame. Inaccurate angles assemble easily enough, but visible clearance in a mitered joint destroy quality. In chair joints with two dowels, the distance between centers of the two holes in a piece is critical. Varia- tions slow down assembly.

141 For male and female joints, the production facilities in metal-working industries generally use gauges. If the male member fits its gauge and the female member fits its gauge, the joint will assemble all right. This is the only correct way to manufacture interchangeable parts. Woodworkers do not use gauges as much as they should. Some furniture plants approximate this procedure with one which insures proper assembly on one manufacturing lot, although parts from this lot might not assemble properly with parts from the next lot. The procedure can be illustrated by a mortise and tenon joint. It is difficult to adjust the size of hole cut by a hollow chisel mortiser, so mortises are cut to whatever size the chisel cuts. A sample piece with mortises cut is often used as a gauge for setting rail thickness at the moulder (tenon thickness is the same as rail thickness) and width of the tenon at the double-end tenoner. Both moulder and tenoner have rapid and easy adjustments that permit very small change in dimension.

Intermediate Dimensions

In general, this category involves dimensions which can vary up to + 1 /32" without hurting quality of product or slowing down assembly if the product engineer foresees the situation and makes proper provisions inhis construction and in the manufacturing information from which the plant works. He often makes intentional a1 1 owance for manufacturing variations.

An example is a dowel joint. Whereas the diameter of the dowel is critical and the diameter of holes and distance between them are critical, the length of the dowel and the depth of the hole are not critical. For instance, assume dowels 2" long which are supposed to go 1" into each piece to be joined. Theoretically each hole should be 1" deep. But if the holes are specified 1" deep, suppose the depth of the holes is 1/32" scant, or the dowel is l/32" too long. The end of the dowels will hit the bottom of the holes before the shoulders pull up tight, and the result is an ugly-looking crack. If the holes are specified 1 1/16" deep, the shoulders will pull tight in spite of t l/32" variation in depth of the holes or length of the dowel. This is an example of an intentional allowance for manufacturing variation. A similar situation exists with tenon and groove construction. If the groove is intentionally cut 1/16" deeper than the length of the tenon, the shoulders will pull up tight in spite of t l/32" variation in either the depth of the groove or the length of the Tenon. This applies whether the groove is cut with a grooving saw or with a router. This works well as long as the edge of the frame does not show. But if it shows, there is an unsightly hole between the end of the tenon and the bottom of the groove, so something else must be done.

With a hollow chisel mortiser, a different situation develops. Due to the nature of the cutting tool, the mortiser does not cut a square mortise clear to the bottom of the hole. The last /8" or more at the bottom of the hole is a round cut made by the bit inside the hollow chisel but not squared out by the chisel. Also, the mortiser frequently leaves chips in the bottom of the hole which will not shake out but will require hand work and time to remove. In order to get the shoulders to pull up tight on a hollow chisel mortise and tenon construction, it is wise to intentionally cut the mortise at least 1/8" deeper than the length of the tenon; 1/411 is better.

142 Another intentional allowance for manufacturing variation is a dust frame such as in Figure V. Assume the long rails are 30" x 2 1/2" x 3/4" and the short rails are 15 3/4" x 2 1/2" x 3/4", 15" B.S. All grooves are 3/16" wide and 7/16" deep. The short rails, as billed, will have tenons 3/8" long. The question arises as to what size should the dust panel be. First, consider the width of the dust panel. Since it must slip easily into the opening A-A, determine the size of the hole as follows:

End rails, B.S. 15" Depth of front groove 7 /16 'I Depth of back groove 7 /16 I'

Size of opening A-A 15 7/8 'I

1A Figure V: Construction of a dust frame.

To maintain an additional margin of safety, specify the width of the dust panel as 1/8" less, which would make it 15 3/4" wide. Next consider the length of the dust panel. Since it must fit into the opening B-B, determine this dimension by calculating C-C and then adding the depth of the two grooves.

Overall length of front and back rails 30 I' Minus width of 2 end rails 5 Dimension C-C 25" Add depth of 2 grooves /8 'I

Size of opening B-B 25 7/8 I'

To provide the same margin of safety in length as was done for the width, deduct 1/8'' from the length, and we get 25 3/4". So the dust pane would be specified as 25 3/4" x 15 3/4".

Now consider the thickness of the panel. The grooves into which the panel goes are specified 3/16'' wide. If we specify a panel 3/16" thick assembly will be difficult or impossible if manufacturing variation results in panels a little too thick or in grooves a little too narrow. The strength of the

143 frame does not depend on a tight joint between panel and groove; strength of the frame depends on a glued tenon and groove joint between dust frame end rails and front and back rails. In order to assure fast, easy assembly, the dust panel should be specified a little thinner than the width of the groove. There are two commonly used ways to accomplish this. One is to specify a thinner panel , such as 3/20". The other way is to specify a panel 3/16" thick but specify sanding the panel to fit the grooves with a fast, cheap drum sander operation. Sanding is done on the back of the panel to avoid risk of sanding through the face veneer. Occasional sanding through the back veneer does no harm because it does not show after assembly.

The above examples are by no means the only situations where an intentional allowance should be made for manufacturing variations. But they illustrate the basic principles. Where the strength or appearance or other performance characteristics of an assembly require machining accuracy closer than -+ 1/32", the product engineer should plan for what accuracy is needed. In many cases, the performance of the product is not any less for making intentional allowances, but the final labor cost has been reduced significantly.

Fool proof ing

An English professor asked a class what was the most important thing about a business letter. One student replied, "Make it easy to understand." The professor said, "NO, make it difficult to misunderstand, and that is not the same thing, gentlemen." So in furniture construction, foolproofing does not mean making it easy for the man in the plant to do the right job; it means making it difficult to do the job wrong. The following examples will illustrate what is meant.

If a drawer side has identical dove tail cuts at both ends, it can be used as either a right hand or a left hand by turning it end for end. If they are not identical, half the drawer sides must be machined right handed and half left handed; a part machined for a right-hand drawer side will not fit into the assembly as a left-hand side.

Figure VI (see next page) shows the side seat rail of a chair. (A) shows a rail which can be used for either right or left by turning it end for end and upside down. (6) shows a rail where the right hand rail must be machined differently from the left hand rail.

The simple addition of one rounded bottom edge converts B to A. In construction 6, if it is desired to make 1,000 chairs, 2,000 side seat rails would be specified. But if the machine man forgets about rights and lefts and machines all 2,000 rails alike, you cannot assemble a single chair because you will have all rights and no lefts or vice versa. He cannot forget with construction A because the part will work for either side.

144 A B

Figure VI: Two types of construction in side seat rail for chair.

Often a part such as a dust panel works out near y a square, such as 12 l/4" x-12". It is hard for the assembler to tell at a glance which is length and which is width. If possible, it is better to work out the construction so that the panel is exactly a square like 12 1/4" x 12 1/41', or else quite a bit different, like 12 l/'t'' x 14". With dust panels, this can often be done by changing the width of the dust frame end rails or the width of a center munt if there is one. With drawer bottoms, it can often be done by a small change in the length of the drawer sides. A similar situation often arises with tenons. If the tenon is almost on center, but not quite, the assembler can easily assembly it upside down and get the result in Figure VII(A), whereas Figure VII(B) is what was intended. If the tenon and groove were centered instead of off-center, upside down-assembly would make no difference as shown in Figure VII(C).

wmA B

Figure VII: Off-center tenons may result in assembly errors not encountered with centered tenons.

145 Another advantage of a centered tenon is that it permits the assembler to select the best face and place it outside where it will show. Exposed end rails on a chest are such a situation. In this case, it would be well to drum sand both faces so that either could be used as the exposed face.

One other thing often helps in foolproofing. A man is more likely to make mistakes in reading a rule for 25/32" than for 3/4" or even l3/16". It is not always possible, but where it can be done, it is wise to specify fractions as simple as possible and to avoid fractions in thirty-seconds. As he acquires experience, the product engineer will find many other ways in which his construction can be foolproofed. The guiding principle is "Make it hard for the operators to make mistakes."

146 CHAPTER 10

PERMANENT SPEC F I CAT ONS

In the manufacture of any product, it is customary to have someone plan how the product is to be made. In all except the smallest factories, this planning function is distinct from the manufacturing process. Planning involves transmitting the necessary manufacturing information from the planner to the people who will erform the actual manufacturing operation. Ideally, this information shou rd be understandable, specific, and complete; production workers should be able to use it to produce the planned product without ask- ing further questions. However, furniture manufacturers typically fa1 1 short of this ideal in the matter of written or drawn manufacturing information. If production mistakes are to be avoided, it is necessary that correct, explicit information be available to those responsible for production.

A single item, such as a dresser, may be made from dozens of parts, each requiring many manufacturing operations. No one can be expected to keep all this information in his head, so it is very important that records be made. -The form of such records is not near1 as im ortant as the fact that records ---a re TG?li$ %~t~dT€eTl’hdpT&b ~e~s~p~i~ developing these records. The procedure can be modified to meet the requirements of individual factories.

There are four documents needed for an item of casegoods.

1. The Production Drawing (see Figure I). This is an accurately dimensioned line drawing which shows what the furniture looks like.

2. The Bill of Materials (Figure 11). This record is a list of all of the parts and the materials used.

3. The Veneer Bill (Figure VI). This is a specialized bill of materials that lists all the plywood parts on an item and furnishes details for each ply of every part.

4. The Route Sheet (Figure IX). Each part listed on the Bill of Materials has its own Route Sheet which records -in sequence all of the manufacturing operations necessary to prepare the part for the assembly department.

Production Drawing

No two factories use the same procedure with respect to production drawings. In order to be specific, the following discussion pertains to a production drawing similar to Figure I. Many plants deviate substantially from this type of drawing, and some use no production drawings at 911; however, drawings similar to Figure I are in actual use and have proven satisfactory in a number of plants.

147 I TopViw with Dtawrr

Si& vi* Trd viw I

Figure I: Production drawing of a night stand.

148 I I I 'rf. cn d d LL 0

t--I W W I v)

w 0y/I z3 I- U a

Figure 11: Bill of Materials for night stand,

149 In a general way, production drawings follow the standard orthographic project ion practices of engi neering graphics (or mechanical drawing) in that they show the customary front, side, and top views and cross sections. But furniture drawings deviate from established practices for engineering drawings. Some of the frequent deviations are noted below.

Furniture production drawings are usually made full size not to a reduced scale. This simplifies the making of patterns for bandsawing, shaping, and turning. Most curves in furniture are free hand curves, not arcs of circles. It is a difficult job to change the scale on a freehand curve and still keep the shape of the curve accurate. With full-scale drawings, no scale change is invol ved.

An article of furniture is generally drawn assembled, and frequently there are no separate parts drawings. Views of furniture can be half views to a center line if the whole view is symmetrical with respect to that center line. In Figure I, half views of the front, top, and top section give as much information as would full views. The side view is full because it is not symmetrical to a center line.

The guiding principle to follow in furniture drawing is clear communication: --Make it -----easy to read and understand. Use dimension lines with arrow heads and figures to provide information not given on the Bill of Materials. This includes such things as overall dimensions, clearances, and allowances. The idea of reference planes plays an important part in locating dimensions that are useful on a drawing. Always specify dimensions ---from one side (reference plane) --of a part or assembly. Intionto dimensions, notes frequently appear on drawings. In some cases, a brief note can explain something better than an additional drawing.

When used in conjunction with the Bill of Materials, a furniture drawing should give all the information needed to perform correctly any machining operation on any part. If the necessary information is drawn in one place, there is no point in showing the same information somewhere else on the drawing. In fact, it may possibly increase errors to have the same information drawn in two places. Often it becomes necessary to change a part. In such a case, it is easy to change a drawing in one place and overlook changing it in another. This causes the drawing to give conflicting information depending on where you look on the drawing. Do not feel obligated to draw lines in any view of a drawing just because there are edges there. One should show all required information --once and -~not again unless -its dupli- cation makesthe-- drawing easier --to read. Assembly drawings are very hard to read if dotted lines are drawn wherever there is a hidden edge; therefore, hidden lines should be used s aringl substituting cross sections where important features are to bKt- used, hidden ---lines are not always dashed. In the top view of Figure I, the end panel is shown in solid lines even though it cannot be seen because it is hidden by the night stand top. On the other hand, the grooves in the drawer parts for the drawer bottom are dotted. The criterion is to make the drawing easier to read rather than blindly follow a set of rules dictated by arbitrary drawi ng convent ions.

150 In making a drawing on paper, we know that the paper will shrink and swell with changes in weather. Consequently, one should not depend on measuring the drawing for dimensions greater than three inches. Many plants allow machine operators and pattern makers to measure drawings for dimensions less than three inches. This procedure leaves room for errors, whereas written dimensions eliminate the need to take dimensions directly from a drawing. If drawings are to be measured for patterns and set ups, the dimensions should be accurate to within 1/64", in which case the paper will not change enough with weather to affect the measurement of small dimensions.

All parts of the drawing and the size figure on the Bill of Materials should be consistent in the information given. Changes should be noted everywhere they occur as -a change --in one part -- often requires changes ---in other parts. In summary, a production drawing should accomplish the following:

1. Be easy to read and to understand.

2. Give all the necessary machining information for every operation on each part (when used in connection with the Bill of Materials).

3. Be accurate to within l/64" for dimensions under three inches.

Independent and Dependent Dimensions

It is important to select the right dimensions to be given. This will help in the fitting of the case. An example will show this.

Figure I11

In a drawing of a rail (by itself), should we give all of the above dimensions? Suppose we do. Then a machine operator might cut the rail to finished length, 14 1/4tt, and another machine operator might cut four notches, each 5/8" long. These machining steps are entirely consistent with the drawing as given in Figure I11 , yet the nbetween-shoulderstI or B.S.

151 length of 13" has the greatest potential error in it because we could have had a minus error in the length (14 1/4") and a plus error in the notches. If all these errors are merely 1/64", the B.S. length is 3/64" short, and the drawers will not fit.

Notice how much more logical the dimensioning is in Figure IV. Now the drawing tells the operators that it is not important that the tenons are 5/8" exactly, but that it is important that the B.S. length is 13".

The 5/8" is a dependent dimension. Just recognizing that this is so is very helpful in making the right decisions. We should not rely on the machine room operators' having a sixth sense for knowing which dimensions are more important than others.

Figure IV

Bill of Materials

The Bill of Materials (sometimes called the stock bill) is generally printed on a sheet of paper larger than 8 /2" by 11". Figure I1 shows a typical Bill of Materials reduced in size.

The overall headings at the top typically identify the type of form (Bill of Materials), name of the article (or piece) of furniture to which the information is related (Bill Sheet of One --Night Stand), the article identification number (No. 402-636 where -402 identifies a particular group or suite and 636 identifies a night stand with a door), and a sheet reference that includKthe total number of Bill of Materials sheets used to describe the parts in -one article (Sheet --1 of 2 ). The vertical columns are used to categorize information pertinent to the description of each part, and each part is described on a separate one of these horizontal lines ruled on the Bill of Materials sheet. Although the headings of these vertical columns may vary due to specific requirements of a particular plant or company, typical headings include the following (see Figure 11):

Part number Finish B.S. Parts per rough blank Parts per article Rough length Net rough footage Part name Rough width (per rough blank) Finish length Rough thickness Net rough footage Finish width Species (per arti cl e) Finish thickness Moulder (Sketch)

152 In addition to, or in the place of, some of these columns, particular applications may find use for column headings such as "Grade," "Sanding," or "Remarks .'I The column headings used in this example are discussed in the following paragraphs.

Part Number

In addition to the Part Name (see below), most plants now have some system of part numbers th37faG"Ttates part identification for data processing, particularly ifelectronic data processing is uti1ized in production and/or business processing. Depending on the use, a part number may be a small number (l-Top) or a large number (402-636-101-Top) to identify a part of a particular article of furniture. It is apparent that the latter case is a number that identifies a particular part of a particular article of a particular group or suite (see "articlexntification numberIn this ex- the last three digits need to be in the "Part Number'' column ~~~~~'t%rt~exn~ationnu* appears at the Wf the sheet. Some companies will carry one and the same part number for both the double dresser end panel rail and the triple dresser end panel rail because these parts are identical. Especially with common items, such as drawer sides, one part may have very many uses. To give it a separate number for each use requires more production paperwork. To reduce machine set-up time, the production people search the floor for the separate orders of the same part. It is easier on them if these parts carry the same number and the production control office has given them an order for the total of all applications in the same cutting.

The advent of computerized production control systems makes the one-part-one number (regardless of how many end uses) system the most logical one for the future.

Parts Per Article

These figures represent the quantities of each part required for one article (or piece) of furniture, such as a night stand. When a shop order is issued to make 100 night stands, these quantities are multiplied by 100 in issuing Route Sheets (or tickets) for each part to accompany. the shop order.

If two parts are the same size and are machined alike except for one machine operation, they should not be listed as one item, but two. An exception is illustrated in Figure I1 "2 Front Posts." The mortises and groove for the left-hand post require different machine set ups than for right-hand posts, but the two posts are combined in one item, and the "Parts per Article" co!umr! shows (L & R). The assembled case has a left-hand end pane! and a right-hand one, but the "Parts per Article" column does not show "L & R". This is because a machined end panel will assemble properly on either side of the case; the left panel and right panel are identical. The notation "L & R" is sometimes carried in the "Part Name" column.

153 Part Name

Unfortunately there is no standard nomenclature in the furniture industry. Each factory has its own language. However, it is generally possible to select names which are reasonably short, but still describe the part so that it can be identified. Function and position are characteristics that often provide clues for part names. Another helpful convention, which is sometimes used, is to number similar parts from the top of a case. In a 4- drawer chest, the drawer fronts can be named, starting from the top, as: #1 Drawer Front #2 Drawer Front #3 Drawer Front #4 Drawer Front

If it helps to make the BOM easier to read, the product engineer drawing up the BOM should not hesitate to use composite names. "Top back bottom" may not be elegant, but the top back of a chair is easily found, and the bottom part of it causes. no problems to the reader if identified this way.

Finish Dimensions

The Bill of Materials (Figure 11) assumes the practice of giving length first , width second, and thickness third (L,W,T). Dimensional units ('I for inches) are -not shown on the Bill of Materials as it is understood that all dimensions given are in inches. Sizes listed for solid parts are the dimensions after the part is com letel machined but before it is sanded. The finish~me?ETo=fFa3X!FE7&rived from xculationsbase-d-e production drawing of an article of furniture.

Fini sh Length

In most plants, length refers to the dimension in the direction with the grain of the face of the part. "Finish Length" is the finished lxhof the part after it has been machined and is ready for assembly. For example, Figure I1 gives the end panel as 18 x 11 l/2 x 1/2 (L,W,T).

This means that the grain of the face of the panel runs the long way and would be vertical in the assembled night stand. If it were desired to run the grain horizontally, the size would be billed as 11 1/2 x 18 x 1/2 (L,W,T)

154 Finish Width

"Finish Width" generally refers to the overall width in the direction across the grain of the face of a part when fully machined and ready for assembly. Practices vary for bandsawn parts that are not straight, such as the back posts on box seat dining chairs (Chapter 15). Some factories specify the finished width of the widest distance across the part.

For solid parts, the choice of what is width and what is thickness is often not at all obvious. It is a good practice (not universally in use, however) to name the width that dimension that came out of the width of the board.

In this example, the selection of rough width was based on cost; 7/8 x 5/4 will be a little bit cheaper than 1 1/4 x 4/4.

Fini sh Thickness

This is generally the overall thickness of the part when fully machined and ready for assembly. Most factories machine to these finish lengths, widths, and thicknesses before sanding. Whatever material is removed by sanding reduces the finished sizes slightly below those listed on the Bill of Mate- rials. In other words, the Bill of Materials includes finished sizes before sandi ng.

155 B.S.

This is an abbreviation for "between shoulders." It applies to cuts made on the tenon machine where there is a definite shoulder. Figures I11 and IV showed an example. Most parts have similar tenons on both ends as in Figure IV. In this case, the length of the tenon is easily calculated as follows:

Finished length 14 1/4 B.S. - 13

Length of two tenons 11/4 Length of one tenon: 1 lpI + 2 = /8 For some parts, such as a mirror frame rail, the desired tenon is like Figure V. In such a situation, the part could be bill 15 x 2 1/2 x 3pI; 12 B.S. face; 13 B.S. back.

Figure V: Illustration of dimensions on a mirror frame rail.

Drawer guides often have a tenon on one end with the other end cut off square. In this case, the part could be billed 14 x 2 x 7/16; 13 3pI B.S./1 end. Sometimes there are tenons on both ends, but they are not alike. Such a situation can be covered by showing figures for finished size and for shoulder length (B.S.) by adding in the "Part Name" or "Remarks" column a notation such as "Tenon per Drawing."

Rough Dimensions

Just as "Finish Dimensions" usually reflect the dimensions of a part after machining but before sanding, "Rough Dimensions" usually indicate the dimensions of a rough blank after the rough mill but before the machine room from which finished parts are machirred. For rough thickness, we use the nominal thickness of the lumber, such as 4/4, 6/4, etc. These dimensions are derived from the finish dimensions by using a series of allowances. These allowances are usually standardized for a given factory or company, but may vary from one company to another. The dimensions of a rough blank often reflect multiple parts (a rough blank from which more than one finished part is machined) .

156 Rough Length

"Rough" 1 ength is the measurement in the direction of the grain of a rough blank from which a finished part or parts are produced. Although specific practices vary, a "rule-of-thumb" allowance for obtaining rough length is to add to the finish length --one inch for ---solid wood parts and between-one-half inch and --one inch for plywood parts. This provides adequately for the need to trim square ends or for the mismatching veneer layers when laying up plywood. On the Bill of Materials, this length allowance shows as:

-+ UL - 636

PAAT PARTS PER PART NAME NO ARTICLE IO/ 1 TOP 102 //2 ro? CORE /03 2 FRONT PO57

When considering multiple parts from a single rough blank, an additional allowance is provided for saw kerf(s) necessary to separate the parts (usually 3/16t' per kerf). In Figure 11, the drawer tilt (Part Number 107) is a solid wood part and serves as an example to illustrate that, for a given rough blank,

Rough length = (number of parts in length direction x finish length of each part) + (number of saw kerfs x 3/16") + one inch allowance.

In this particular case,

Rough length = (2 x 10") t (1 x 3/16") t 1" = 21 1/4".

Note that rough length is rounded to the nearest 1/4't. The multiple parts are indicated in the "Parts per Rough Blank" column.

Rough Width Rough width is the measurement across --the grain of a rough blank frcm which a finished part or parts are produced. The old rule-of-thumb allowance for obtaining the rough width is to add to the finish width one-fourth inch for --solid wood parts and between one-half inch and one inch for plywood parts. One reason for the smaller allowance in rough wmh- solid parts is the increased precision of the ripping versus cut-off operations in the rough mill.

157 In the industry today, more and more people are becoming aware of the fact that l/kil is not always needed and, for many cases, is downright wasteful. One factory runs back rails through the moulder taking no cut off the back and only 3/32" from the face that is grooved. This simple procedure has saved about 24,000 Bft. of lumber per year.

The fact that the veneer plies are just as likely to be mismatched in width as in length is the basis for the equal allowances of rough length and rough width for plywood parts. These rough width allowances are reflected as follows on the Bill of Materials.

+U,L - 636

' PARTNAME

Mu1 tiD1 e Width

Solid parts of widths 2" and less are normally ripped from one board. They are called "solid" which is confusing. It would be better to call them "whole." Wood parts 6" and wider are normally glued up. In many plants, parts from 3 to 6" wide are billed out from glued-up stock. Gluing up solid panels and re-ripping allows the use of random width components and improves the yield. The widths of 2 to 3" are often manufactured whole and glued up, as the raw material quality allows. First the ripsawyers rip all the full width they can get, then convert what remains to random width to be glued UP.

When stock is edge glued, it is a good practice to glue up the maximum width of the gluing equipment. For a clamp carrier (glue reel), this is usually 32".

Rough Thickness

Rough thickness is also determined differently for solid versus plywood parts. For solid parts, variables, such as exposed versus hidden must be considered in order to derive the rough thickness from the finish thickness. A rule of thumb is to add one-fourth- --inch to -the finish thickness. This provides adequate material to consistently clean up a part even if it is to be exposed. For less demanding surfaces, such as interior parts, a 3/16i' allowance may be used. The sum of the finish thickness plus allowance is then rounded off to the next higher standard thickness, namely, 5/8, 4/4,

158 5/4, 6/4, or 8/4, and then stated using one of these fractions in the "Rough Thickness" column. Thicknesses of 10/4, 12/4, 14/4, and 16/4 have become increasingly rare. They are encountered as squares. Most companies will glue up 4/4 or 6/4 stock to make the thick parts. A 5 1/2" finished thickness bed post very commonly has five or six glue lines in it. A typical example from the Bill of Materials in Figure I1 is the part numbered 110.

The drawer back has a finish thickness of 7/16" which can be machined from 5/8 rough lumber. For the same reason, 3/8" is another rather common thickness for drawer sides and backs. In contrast to this, a specification of 1/2" for the finish thickness would necessitate the use of 4/4 lumber and result in considerable waste of material and machining cost because of the volume of wood that would have to be removed. Note that the drawer runner (Part No. 117) requires 4/4 rough lumber to machine to the 5/8" finish thickness.

Another special consideration relative to rough thickness is that the net --rough board footage calculations -are identical ----for both 5/8 and -4/4 lumbw the rough thickness is considered to be a multiplier of 1. The price per thousand board feet does, however, reflect a difference in thickness. For other thicknesses, such as 5/4, 6/4, etc., the fractional rough thickness is the multiplier for board foot calculations.

In upholstery frames, the majority of the structured members will not be exposed. The main interest is sufficient strength, and a completely smooth surface is not required. Since these parts do not need to be planed clear, it is common practice to specify 7/8" finish thickness out of 4/4 or 1 1/8" finish thickness out of 5/4 lumber.

For plywood, the rough and finish thicknesses are the same, both being determined by the number and thickness of plies (see "Finish Sizes" under the Veneer Bill" discussion).

Species

For each part, this column shows which species of wood is to be used, such as ''poplar," "oak," ''gum," "5-ply mahogany face," etc. For interior parts where species is not important, some plants will write "Any1' in the "Species" column. This permits the plant to use whatever scrap or salvage is available.

Moulder

This column is used for a freehand sketch of the cross section that is to be

159 cut on the moulder. It is not to scale, nor is it accurate, but is simply intended to show the moulder operator what to look for on the production drawing. In case of plain rectangular cross section, it is common practice to show the initials "S4S". This is an abbreviation for "surfaced four sides." If a part is not to be run at the moulder, the space in the "Moul- der'' column on the Bill of Materials is left blank.

Parts oer Rouah Blank

The title of this column is very descriptive of its content; it simply tells how many finished parts are to be machined from one rough blank; the number in this column must agree with the dimensional differences between the "Finish" and "Rough" dimensions. If each rough blank produces only one finished part, the "Parts per Rough Blank" column is usually left blank rather than placing a "I" in it. This procedure facilitates identifying multiple-part rough blanks when scanning the Bill of Materials for them.

The word "Make1' is often used as a heading for this column since it tells how many finished parts one rough blank will "make."

Net Rough Footage per Rough Blank

Another title for this column could be "Net Rough Board Feet per Rough Blank." The value here reflects the volume of wood in the rough blank as described by the rough dimensions and is derived by the following:

Rough Length x Rough Width x Rough Thickness 144 = Board Feet

This is obviously a straightforward board foot calculation with two important points to remember:

1. Since the three dimensions are expressed in inches and there are 144 cubic inches in a board foot, the result is a volume measure expressed in --board Feet as opposed to an area measure in square feet.

2. The base numbers for this calculation are the rough dimensions on the Bill of Materials.

These conditions apply to parts made of solid lumber. Since the footage column is also used for plywood parts, the entries are then in square feet. The thickness of plywood and fiberboard panels is never considered in the footage calculations.

Net Rough Footage Per Article

Like the "Net Rough Footage Per Rough Blank," this too is a volume measure expressed in board feet and is based on the rough dimensions. The "Net Rough Footage Per Article" can be calculated as follows:

Net Rough Footage Net Rough Footage Number Parts - Per Per X Per Article Part Article

160 A brief look at the Bill of Materials column headings will reveal that all three of the values necessary for this calculation are available.

In this example, the net rough footage is calculated as follows:

Net Rough Footage/Rough Blank = 36 32 = 8.000 144

- 8*ooo x 2 = 1.455 -- 11

(The number 11 may appear to be a strange choice, but it has a very sound reason. We bandsaw a number of back posts from a panel. There is a large piece of waste material left over. To minimize the waste per chair, we maximize the number of back posts per panel. This means that the panel width is always close to the capacity of the edge gluing equipment, in this case, 32".)

Footage Figures and Cost of Manufacturing

The Bill of Materials is, in the first place, a document to support manufacturing, but it is also the basis of the product cost structure. For this reason, we must bear in mind someaccounting-likepr'inciples, to make sure we account -for, everything without counting anything twice.

The plywood department should be viewed as a separate cost center. Looking then at a rough end/machine room as one cost center anKp-department as another, we need to know which cost center will be charged with each of the various materials needed. One basic rules applies: The raw material appears on the Bill for the department where the raw material is issued.

161 A number of examples are given below to explain the application of the rules.

This example shows a simple fiberboard top made from a bought-to-size rough dimension. The material is issued in the machine room. Hence the footage appears on the Bi'll of Materials. The footage is calculated as follows:

Net Rough Footage/Rough Blank = = 4.087

Net Rough FootagejArticl e --- 4*087 x 1 = 4.087 1

Now compare this with the following Bill of Materials entry.

Here everything appears to be the same except for the footage columns which are left blank. The reason is that, in the second example, we are dealing with a 3-ply veneered top; and the face, back, and core are listed on the Veneer Bi 11 . 162 Vclvttn IJIL

Even on the Veneer Bill, the top has no rough footage entry. The reason is that the materials issued are the oak veneer, the MDF (medium density fiberboard), and the poplar back veneer. We do not want to count the constituent plies -and the resulting panel. That wom be counting the same thing twice.

Now look at another example:

/ 3544 3.5# 1 8.300 4.300

In this example, the core is made up of three components. Because it is "built" in the machine room, it is "billed" on the BOM, which is essentially a machine room document. Notice that the top has no rough footage for the same reason as before. The top core has no footage either because the material is assigned in the following two lines, #26 and #27.

163 The corresponding Veneer Bi11 : c,vcL,, u,

NET SQUARE

In contrast to the previous example, this Veneer Bill shows no footage for the core. The reason is that the core came from the machine room "free of charge." But both veneers were issued in the Veneer and Plywood Department and show up on the Veneer Bill.

The next examble is identical in principle to the last except that a) the top is made in double size; b) the top core center is cut from a stock-size panel (a standard for this company); c) the panel is 5-ply; and d) the panel is 7/8'' thick. . d. -- VI- IVIH I I

164 For all of these examples, one simple rule applies. Wherever the material is issued from raw material stock, put the footage on that BILL. For lumber parts, this is always the Bill of Materials. For veneer parts, this is always the Veneer Bill.

For a material like particleboard or MDF, it depends. For part 8 in the second example, the MDF core was issued from stock to the Veneer and Plywood Department, so it showed up on the Veneer Bill only, and naturally the footage is charged there . In the other examples, the MDF core center rough blank is issued in the machine room, and the footage shows up on the Bill of Materia1 s.

Grade

This column indicates for each part what defects (if any) will be acceptable in that part. Basically there are only two grades, clear and sound.

In specifying grade for parts, disregard lumber grades such as FAS or #1 common. These lumber grades do not apply to parts. Clear means free from all defects. In many species, there is a difference in color between sap wood and heart wood. In ordinary usage, clear would permit either sap wood or heart wood or a mixture of both. Unselected color is not a defect under the term clear. If selected color is required, some descriptive term should be added such as ''clear white" or "clear red," "clear sap" or "clear heart." If a part will be exposed on all sides, it is listed as clear. If only one face shows, it is listed as llclear 1 face." Other possibilities are "clear 1 edge," ''clear face and edges," etc. It is wise to specify the lowest acce table grade for each part. This oft?%permTtG use of stx which hrxhave-been-ted. Some latitude is customarv in inter- preting clear. Drawer sides may be specified clear but need nocbe quite as clear as tops or bed panels. This is usually a matter of interpretation best handled by comparison with approved examples present on the manufacturing floor.

The term sound signifies that small defects are permissible, but they must not impair the strength or function of the part. The back rail on the chest or the hidden rails of an upholstered frame would be examples where ''sound" grade would probably be acceptable. If the lowest acceptable grade is specified for each part, it generally permits some saving in lumber waste and consequently in cost.

Remarks

Many Bills of Materials have a "Remarks" column, but there is no standard practice. It could be used for Yenon 1 end." Another frequent use is the notation "R & L", an abbreviation for "right and left." Two front posts might be billed as one item even though the mortising on the right post is different from the left post. The notation warns machine operators to make separate set ups for rights and lefts, even though finish size is the same.

A separate item should be listed on the Bill of Materials if there is any difference in size or machining (except for rights and lefts mentioned above). If two drawer fronts are the same size but are machined differently,

165 they should be listed as two separate items, not as one. However, if a four-drawer chest had five front rails and five back rails that were identical in size and if all machining was identical, they would be listed as one item, "10 front & back rails."

In this case, the "Remarks" column might read "front, gum clear 1 edge; back, any species sound." Most plants would list these as two separate items.

Veneer Bi11

In the discussion of rough footage, we have already described some aspects of the Veneer Bill. This is a listing of all the plies that are used in pressing plywood or laminated veneer parts. Some of these plies may be processed within the plant (such as the face veneers), or they may be purchased ready for use in making plywood (crossbands).

This extra detail may be presented on the Bill of Materials, but it is much preferred to have a separate document, the Veneer Bill. A Veneer Bill will list all of the plywood parts on an article and furnish details for each ply of every part.

Column headings on the Veneer Bill will vary, but the following discussion presents those that are shown in Figure VI.

Part Number

The part number on the Veneer Bill is taken from the Bill of Materials. This means the panel listing on the Veneer Bill carries a number but the individual veneer pl ies do not.

Parts Der Article

This identifies the number of parts required to make one article (piece) of furniture.

Parts per Panel

Each major line item will describe a plywood panel. This panel may make one or more parts. The end panel (part 106) makes two ends. Multiple parts substantially reduce handling in the Veneer and Plywood Department and may also result in a more uniform appearance.

Panels Der Article

Since in many instances one panel will make more than one part, it is advisable to include the number of rough-size panels necessary to make one article. For example, it takes one rough-size panel to make two end panels for the night stand, so one rough-size panel is required per article. Since the cupboard door panel makes two doors, but only one door is required per article, one-half of a rough-size panel is required per article.

166 I BILL SHEET OF ONE N. C. STATE UNIVERSITY m4n 7.5TAND SHEET / OF 3 NO. 402 - 636 VENEER BILL I I I PART PARTS PARTS PANELS PLIES FINISH NET SQUARE NO. I PER I PER I PER I PER I DESCRIPTION SPECIES FEET PER REMARKS 1(BOM) ARTICLE PANEL ARTICLE PANEL LlWlT PANEL I ARTICLE z I cn t 7 ID < ..H < ID 3 ID ID 7 II II I / I /t BACK z 4 A

-h 0 7 II II I / I // /IFACE 3 4. cn 3- II cc II In 6 9, 3 .Q

11 I I1 I Plies per Panel

This identifies the nature of the construction of the part. For a major line item (top, end panel, etc.), it tells the total number of component parts in the panel. For a component part (face, core, crossband, back), it tells how many of that component part are required for production of one panel.

Description

This identifies the part or component.

Finish Sizes

These are generally the overall sizes of the part when ful mach,ned and ready for assembly.

Finish thickness results from the sum of all plies used, assuming that sanding is not considered. A typical 5-ply construction for a 13/16" finish thickness woul d be as fol 1ows :

/32 I' Face

1 /16 'I Crossband

5/8" Core

-\l/32 " Back

Figure VII: Five-ply panel.

Using this construction, the thickness is increased or decreased by changing the core thickness. For example, a 1" thick panel would have a 13/16" core. Face, back, and crossbands sum to /16 'I. The concept is the same when using constructions other than 5-ply.

One point to remember is that face, crossband, and back thicknesses vary. Some firms use 1/28'' face veneers rather than 1/32". In some instances like this, the 1/28" is assumed to be 1/32'' when considering thickness; that is, a 5-ply panel having a 5 /8 I' core, 1 /16 I' crossbands, and 1 /28 I' face and back may be considered a 13/16" thick panel. Firms occasionally work with measurements to thirty-seconds of an inch, but rarely to smaller increments.

Rough Sizes

Procedures used in making plywood have an effect on machining allowances. Firms that have good control of veneer slippage and squareness can generally

168 get by with l/2" to 1" allowance between rough and finish dimensions (length and width). Thus, a top that is to have a finished size of 17 3/411 x 14 5/8" would call for a rough size of 183/4" x 15 5/8", using a 1" allowance for machining. Thickness does not change; rough thickness equals finished thickness for plywood parts.

When a plywood panel is designed to make two or more finished parts, the allowance may or may not be affected. For example, consider the cupboard door (Part 113). Finish door size is 11 1/2" x 12 7/8", and rough size 24 l/4" x 13 7/8". The door is made double length. The total allowance provided is:

This practice will vary depending on machining requirements and other factors.

Examination of the example Veneer Bill (Figure VI) reveals that finish sizes are not given for the individual components of each part (panel). The general rule is to list both rough and finish dimensions for a part, but only rough dimensions for components (face, back, etc.) since the rough- sized components are assembled into a panel before being trimmed to the finish dimensions of the part.

Species

For each ply, the species is listed.

Net Square Footage per Panel

This is the square footage determined by using the stated rough dimensions; that is:

Net Square Footage per Article

This is also determined using rough dimensions, but takes into account the number of parts per panel and parts per article to get the panels per article. For example, consider the following: Item Sq .Ft ./Panel Panels/Article* Sq.Ft./Article

Top Core 3.91 1/2 1.95 Dust Panel 3.67 i /4 0.42

169 Remarks

This column is used for special information when needed. For example, a fancy face will often be documented with a sketch. The sketch number may be entered in the "Remarks" column.

Cal cul ate the Dimensions

When establishing dimensions for use on the Bill of Materials, they should be calculated whenever possible instead of a measured dimension from the drawing. There are several reasons for this.

1. It is easy to read the wrong dimension off a rule.

2. The drawing may be inaccurate.

3. The drawing may have been accurate when drawn, but the paper has shrunk.

4. If the dimensions are first calculated, a fast check is then to measure the drawing. If the measurement corresponds to the calculated figure, it is probably correct.

A few examples will be given to illustrate what is meant by calculating the dimensions.

To start out, it is helpful to establish round figures or easy fractions for the following:

B.S. front and back of the case 18"

B.S. ends of the case 14 l/2I1

Vertical dimensions of openings in the case front 1st drawer opening 6 'I 2nd drawer opening 7 3rd drawer opening 7

Overall height of case 29 3 r+ I'

Overall width of case 22 1 /z I'

Overall depth of case 18 1 'I

170 - -let-

Figure VIII: Dimensions related to length of drawer opening

Dimensions Related to the Length of the Drawer Opening

Once the drawer opening is defined, a large number of other dimensions follow from it.

a. Drawer front length. This equals the opening less 2 x clearance or:

b. Drawer front length, measured on the inside. The dovetails of the drawers are sanded after assembly to clean up glue squeeze out. To allow for this operation, the back ends of drawer fronts are shaped with a step in it. This offset is 1/8" in the example shown in Figure VIII. The resulting dimension is:

17 7/8 I' - (2 x 1 ,% ") = 17 5 /S 'I.

c. The width of the assembled drawer, measured on the inside, is now found by subtracting 2 x the drawer side thickness:

17 5/8 - (2 x 7/16) = 16 3/4".

171 d. It then follows that the drawer back length can be found by adding 2 x the length of the dovetail.

16 3/4" + (2 x 5/16) = 17 3/8".

e. If the grooves for the drawer bottom are 1/4" deep, the opening for the bottom is:

f. The drawer bottom must have a clearance, say 1/16" each side. The drawer bottom width is:

The Bill of Materials will show the dimensions underlined in Figure VIII.

18" as the B.S. length for the rails 17 7/8" as the length of the drawer front 17 3/8" as the length of the drawer back 17 1/8" as the width of the drawer bottom

Note that only the 18" dimension can be freely chosen. The other three dimensions have to be calculated.

Transmitting Information to the Plant

For specifying details of machining and dimensions not given on the Bill of Materials, there are several commonly used practices. Three of these are:

1. Copies of the production drawing of the assembled item are kept at convenient places in the machine room.

2. Actual samples of correctly machined parts are conveniently located and used as patterns. These are often painted red to reduce the danger of losing them in the plant or assemblying them into some article of furniture.

3. A production information sheet or tag is provided for every part listed on the Bill of Materials. It gives, at least, the finished size and other dimensions stated on the Bill of Materials as well as the quantity of the part. In addition to these basics, it often relates quite a bit of supplemental information. Generally, this sheet or tag is placed on the truckload of stock when it leaves the rough mill and stays with that truckload until machining is completed.

172 Some combination of the above three may also be used. For example, patterns may be used as guides for set up in addition to a tag that follows the parts from the rough mill through the machine room.

In order to better understand a typical system, a detailed explanation of the Route Sheet follows below.

Route Sheet

When a cutting of furniture is to be manufactured, most factories issue a set of production information sheets. Both the form and the descriptive term used for this information sheet vary. The form may be anything from a small card or tag up to a full-sized 11" x 17" sheet. Typical names include "ticket," ''tag," "route ticket," "shop order," and "Route Sheet." The example used here will use the term "Route Sheet" to refer to such an information sheet. An example is shown in Figure IX.

A Route Sheet is provided for each part listed on the Bill of Materials. It gives complete information for making that part including dimensions, materials, board footage, sequence of operations, and drawings. It may also show standard times and revisions, as well as veneer and plywood information.

The example in Figure IX is a master sheet. When a cutting for a specific number of articles is ordered, a cflpy of the master Route Sheet for each part is made and the "WANTED" and JOB NUMBER" information is then entered in the proper blank spaces before the copies are issued to the plant. Since the Route Sheet lists a sequence of operations, it usually accompanies the truckload of parts for identification as well as routing purposes in the machi ne room.

The following information is taken directly from the Bill of Materials:

Article Number Article Name Part Number Part Name Material (Species) Parts per Rough Blank Parts per Article Rough Dimensions (Length, Width, Thick) Finish Dimensions (Length, Width, Thick and B.S.) Rough Board Footage per Rough Blank Rough Board Footage per Article

The following paragraphs explain the meaning and source of information for the remaining topics on the Route Sheet.

173 N. C. STATE UNIVERSITY ROUTE SHEET 1 ARTICLE NUMBER PART NUMEER JOB \ET ROUGHSF WANTED 402 - 636 402-6536-/06 NUMBER PER ARTICLI ARTICLES ARTICLE NAME PART NAME N/GH757+WD *2 PYjleTl/t/cf RqlA. .2/ ROUGH BUNKS MATERIAL ROUGH LEN TH ROUGH WIDTH ROUGH THICK. RBfT./R. BLANK A. BLANKS/ARTICU -4 POPLp 152 2 .a / FHlSH PARTS PARTS PER FINISH LENGTH FINISH WIDTH FINISH THICK. B. S. R BFT. / PART RCUGH BLANK ARTICLE / /4aL e I3 -21 oPERL\TIoN VENEER MA%M DESCRIPTION Of- OPERATION NUMBER STATION PLY FACE CUTOFF CROSSBANDS BACK

I I 12 I '3 I I 14 I I

I 15 I I 1

I '8 I I 19 I I I 20 I I 1

Figure IX: Route Sheet.

174 Rough Blanks per Article

The value for this is obtained from the equation:

Parts/Article R. Blanks/Article = Parts Rough B1 ank

Note that, since this reflects the number of rough blanks for one complete article, the numerical value may be a whole number, a fraction, or a mixed number.

Rouah Board Footaae Der Part

This is also a simple mathematical relationship obtained from:

NOTE: All of the preceding discussion relative to the Route Sheet reflects quantities necessary to produce -one complete article of furniture. In contrast, the following discussion of "Job Number" and "Wanted" information relates to a specific cutting order of -a quantity -of articles.

Job Number

The job number is a number assigned to a particular cutting order to facilitate the identification of all parts of that cutting order as they move through the machine room. Although arbitrary, it is usually assigned in sequence as cutting orders are issued to the plant. Some manufacturers also use a repeating cycle of different colored tags or Route Sheets in addition to the job number to make it even easier to spot truckloads of a particular cutting order and improve the flow of materials in the machine room.

Wanted

The "Wanted" section of the Route Sheet refl ects the cutting-order size. "Articles" is the total number of completed articles of furniture to be assembled, finished and shipped. Using this as a base, the number of "Finish Parts'' is simply:

Finish Parts = (Wanted: Article) x Parts per Article.

For example, if a copy of the master Route Sheet in Figure IX were issued for a cutting order of 350 night stands, the "Wanted: Articles" would indicate "350", and since there is only one "#2 Parting Rail" per case, the "Wanted: Finish Parts" would reflect 350 x 1 = 350.

175 The determination of the number of "Rough Blanks" is not quite so straightforward. Due to internal defects, set-up trials, machine or operator errors, etc., the original number of parts may show a significant attrition. For this reason, an ''overrun" policy is needed. Overrun policies vary from plant to plant. Some plants will cut 10% extra, no matter what it is or how many. Others have a graduated scale. A fair policy is 5 pieces + 3% of the number of parts wanted for the bulk of the parts. For a few critical parts, such as severe bendings or fragile turnings, the 3% can be increased to 5 or 10%. It is highly desirable that these numbers be given to the plant and that the overrun not be left to someone's judgment. Practice has shown that the fear of subsequent shortage orders has led to an intuitive policy of "let's make sure we have enough," resulting in very high overruns and waste.

The above policy would result in 366 rough blanks for the parting rail.

Description of Operation

A sequential listing of the various machining operations through which a part must be processed is listed here. The sequence is identified by the "Operation Number." The "Machine or Station" column provides space for a number or an abbreviation to identify the location of processing.

Veneer

If the part has veneered surfaces or is plywood, information rel.ative to the number of plies, species, patterns, and quantity can be included on the rough sheet as necessary.

Drawings

The entire lower right-hand corner of the Route Sheet is for sketches or drawings as necessary to provide a precise image of the part. In some operations, the drawing appears on the right half of an 11" x 17" sheet, is folded under, and placed in a transparent plastic pocket for protection. Others choose to print or photocopy a drawing onto the back of the route sheet copy that is issued to the plant. In the majority of cases, the paper size does not permit a full-scale drawing to show critical cross sections, etc., full scale. For a drawer front, a small sketch may show the location of hardware holes, and full-scale drawing of one end can be used to show the location of dovetails and groove and detail of the molder pattern.

Handling of Changes

Life would be much simpler ifthere were no changes, but they are often un- avoidable. The possible reasons for chany-ing drawings ana bili sheets include:

Design change Correction of errors Change of material Cost reduct ion Change in equi pment.

176 For whatever reasons, we have to be very careful when a change takes place. Up to what date (or cutting) are we producing the "old" version? What happens to the old overstock? The worst thing that can happen is that different people in the organization believe different versions to be correct.

Therefore, it is recommended that all drawings and bill sheets be dated. Then a notice can be distributed to the effect that cutting number 17 will be made according to drawing and bill sheets dated 5-20-82, and some person can be assigned the responsibility of finding the old overstock and having it re-machined (or scrapped). It helps enormously if the distribution of documents is known and limited to the minimum number of people. If someone insists on having a complete set of drawings and bill sheets, that person must process and file all changes.

177 178 CHAPTER 11

CASEGOODS

The term casegoods is part of the special language of the furniture industry. Although everyone in the industry knows what it means, it is hard to define precisely. In general, it means pieces of furniture designed primarily for storage with drawers, shelves, or cupboards . Examples are bedroom pieces, such as chests, dressers, night stands, and dining room pieces, such as buffets and china. Kitchen cabinets and T.V. consoles are not usual ly considered casegoods.

The basic principles of furniture construction also apply to casegoods, and it may be helpful in this chapter to point out some specific applications of these principles to casegoods construction. These applications will be discussed under the fol 1 owing headings :

Case body

Fa1 se frames and bases

Case Body

Loose Cases

One of the main problems encountered with casegoods is called "loose cases." This is lack of rigidity in the case body so that openings for drawers or doors do not stay square but are deformed into a parallelogram. This means that the drawers or doors which are rectangles do not properly fit into the openings which are no longer rectangles but parallelograms. This can be caused by the case's sitting on an uneven floor or by pushing the case to move it. This subject of "loose cases" will be discussed in more detail in Chapter 12.

Laying Out a Case-

One of the first steps in making a production drawing and Bill of Materials on a case is to lay out the case vertically in a front view. This can be done from a sample, a designer's small scale sketch or a designer's full- size detail drawing. Figure I shows a designer's sketch of a chest of drawers. Measuring the sketch with a rule results in the figures given in Table I under the column "As Measured." Measurements are taken vertically, starting with the upper face of the top and going down to the floor.

179 L 7

I/ c3 II

Size: 18" x 32" x 50" Scale: 1/8" = 1'-0" Figure I: Designer's sketch of chest of drawers.

180 I I AS FIRST MEASUR ED ADJUSTMENT

Upper top (thickness) 3 /4 It

Top front rail (thickness) 1 /2 3 /4 It

#1 Drawer opening (width) Front rail (thickness) False rail Front rail

#2 Drawer opening (width) 4 3 3/4" Front rail (thickness) 1 /2 It 3 /4 11

#3 Drawer opening (width) 6 5 3/4" Front rail (thickness) 1 /2 I1 3 /4 11 #4 Drawer opening (width) 7 Front rail (thickness) 1 /2

#5 Drawer opening (width) 8 Front rail (thickness) 1 /2 II

#6 Drawer opening (width) Front rail (thickness) Apron (width)

Total

Table I

It will be noticed that the measured figures add up to the total height of the chest, which is specified as 50 inches. We do not want to make front rails 1/2" thick but 3/4". A closer look at the sketch shows that the drawer fronts are probably supposed to be lip construction so that the top lip of the drawer front would lap over part of the front rail above it. If this overlap were 1/4", the drawer opening would be 1/4" less than the width of the drawer front. If all front rails were 1/4" thicker and all drawer openings were 1/4" narrower, the figures would be as shown in the column "First Adjustment." These figures add up to 50 12". The height of 50" specified on the designer's sketch is the way the chest would be advertised and sold. Many factories are satisfied if the actual overall size of the cases is within lP1'of the nomina: sizes. If this wew the sitwtion, the sizes listed in "First Adjustment'' column would be acceptable for billing out the chest. But if factory policy were to make the actual size exactly as advertised, the apron could be made 1/211 narrower without seriously changing the overall appearance of the chest. Or better yet, it could lap the bottom front rail 1/2f' instead of not lapping at all. This would result in the actual height of the chest being 50" instead of 50 /2 'I, and the figures in "First Adjustment" column would be as listed. The apron and

181 bottom front rail would be assembled as in Figure II(A) instead of that shown in Figure II(B). Figure II(A) with the overlapped construction permits finish nails (set and puttied) to hold the apron tight to the front rail, resulting in better construction than that in II(B) (which would show an opening C if either the front rail or the apron was not straight).

Figure 11: Alternate assemblies of apron and bottom front rail.

Case Ends

All cases must have ends. In general , there are two types of case ends: 1. Sub-assembly of 2 posts, end panel , and auxiliary parts.

2. One-piece end and possible auxiliary parts.

With the sub-assembled end, there are generally no serious problems in working out satisfactory joints between the parts of the sub-assembly. If possible, these joints should be designed so that the end can be clamped in a flat clamp with pressure in only one direction in order to simplify equipment and save time at the operation where the sub-assembly is clamped. A typical clamp operation is illustrated in Figure 111.

The fixed back bar of the clamp is A. The movable front bar of the clamp B is actuated by an air cylinder to exert pressure in the direction of the arrow. Both bars are mounted on a flat table or base structure so that the case end D can be laid flat on the table between the clamp bars, A and 9. At C are the adjustable stop dogs attached to the clamp bars, They should be set so that, when the case end is laid in place and driven against the

182 'A ._

Figure 111: End panel in a flat clamp.

------. c--I I ------I p C

Figure IV: Case end with thin panel and end rails (A) and with a thick panel (B).

183 In either situation illustrated in FSgure IV, there should be a clearance between the bottom of the groove and the edge of the panel in order to insure that, in clamping, the only contact will be at D-D between the two side rails and the shoulders of the panel or end rail. This eliminates unwanted open joints on the outside face of the finished case end. It also insures that the sub-assembled width of the case end will be uniform, controlled only by the shoulder measurement plus the thickness of two posts. With a clearance at C, dimensions of the tongue and groove (length and depth) do not require close control to insure proper assembly o the panel and posts.

With posts, there is generally plenty of wood for making jo nts between posts and front rails or between posts and back rails. It is desirable to make tenons on these rails as long as can be made with the amount of wood available for a mortise. Long tenons give a more rigid joint than short tenons and minimize "loose case" trouble. A note of caution is in order, however. The depth E of the mortise in Figure V(B) is greater than D in Figure V(A), and this should result in a very rigid case. It will, but it has one disadvantage. In sub-assembling the case end, glue is put in the groove G to hold tenons or panel. Some of this glue can run into the mortise in Figure V(B). This glue will harden before the case ends are used in final assembly at the case clamp. If there is enough of this hardened glue in the bottom of the mortise, it may prevent shoulders on front rails from pulling up tight at the case clamp.

A B

Figure V: Mortise and groove cut in post.

184 One-piece case ends are usually of solid edge-glued lumber or of plywood, 3/4" or 13/16" thick. Since they are not sub-assembled some of the problems involved in sub-assembled ends are avoided, but new ones may be encountered. Solid edge-glued lumber will shrink and swell considerably in width. (Re- view Chapter 6 for the principles of shrink and swell .) This shrink and swell mean that, wherever anything is attached to the case end, consideration should be given as to whether some form of "floating construction" is needed to avoid trouble. Shrink and swell are not a problem in plywood case ends.

Whether plywood or solid, there is not much room in the 3b4' or l3/16" thickness of a case end for a deep mortise to permit a long tenon on front and back rails. Consequently, there is more tendency toward loose cases than with posts which permit longer tenons.

Dust Frames

Dust frames generally consist of a 4-piece frame with tenon and groove joints. The same grooves enclose the edges of dust panels and serve as female joint members for tenons. Dust frames are generally put together as sub-assemblies, sometimes with tenon joints glued, but often without glue. Once the dust frame is in place in the case body, there is very little, if any, stress on the tenon joints with one exception. If case ends are solid, they will shrink and swell across grain; the dust frames will shrink and swell much less along the grain. Unless some sort of floating construction is used to attach the dust frames to the case ends, the swelling of the case ends can break apart the tenon glue joints. Even with non-swelling plywood case ends, there is a problem due to manufacturing variation. Figure VI illustrates this problem.

1 1 TOP VIEW !J5eIEND VZW Figure VI: Dust frame with tenons for fitting into mortises.

185 Assume that tenons on the front and back rails of the dust frame are supposed to be a tight fit in mortises to avoid loose-case trouble. Theoreti- cally the distance between tenons A-A would be the same as the distance between mortises 8-6, and the sub-assembled dust frame would fit into mortises in the case end. But due to manufacturing variations, these two distances frequently will come l/32" or so different from each other instead of being the same. This makes assembly difficult and slows it down. Some shops avoid gluing the dust frame so that, if distance B-B is a little greater than A-A (the non-glued joint in the dust frame at C), C permits both tenons to fit into the corresponding mortises without undue driving or force. But distance B-B could just as easily be a little less than A-A instead of a little greater. To handle such a situation, some shops intentionally make the shoulder distance on the dust-frame end rail (C-C) about 1/16" shorter than the theoretical dimension and do not glue joints C-C.

The above procedure means that generally there is a small open space at C between shoulders of the dust-frame ends and the edges of the front or back rails. Since these joints are not exposed on the outside of the case, some shops do not consider this objectionable.

An alternate solution for manufacturing variations is to intentionally make the tenon on the back rails about l/8" narrower than the mortise as shown in Figure VII. This permits easy assembly even ifdust-frame joints at C-C are glued. The thickness of the tenon (vertically) is machined for a tight fit in the mortise. Such tenons do not give as much rigidity to the case as tight tenons on the back rails, but if the back panel is securely glued and nailed to the case, some shops feel that a tight tenon on the back rails is not necessary and that the back panel will prevent case distortion.

- ?

GLUE-

Figure VII: Undersized tenon on back rail to facilitate assembly operation..

186 It would seem that these methods for hand1 ing manufacturing variations would also take care of shrink and swell of solid case ends. But while -t l/32" to -+ l/16" will generally take care of manufacturing variations, solid case ends generally shrink or swell considerably more than this amount so additional precautions are generally necessary for shrink and swell.

Although one of the most common methods of attaching dust frames to case ends is to use tenons on front and back rails going into mortises in the case end, many other constructions are possible, and the product engineer can use his imagination and ingenuity in working out methods of attachment.

It is generally convenient to let the dust-frame end also serve two other purposes :

1. Support the bottom edge of the drawer side.

2. Act as a drawer tilt for the next drawer below.

Figure VI11 illustrates this. Above dust frame (A) are two narrow drawers mounted side by side; below it, is one wide drawer. Dust-frame ends act as drawer tilts at B for the drawer below the dust frame.

Figure VIII: End rail.

187 Dust-frame ends A and center munt C support the bottom edge of the drawer sides above them at D, E. It is good design to make dust-frame ends and the center munt wide enough so that the drawer sides above them ride on solid wood the full thickness of the dust frame as at B instead of bearing down over the groove as at D. With the design as at D, there is some likelihood that the l/4" (approximately) of wood will split or crack and furnish poor support for the drawer. The center munt at D should be wider than drawn in Figure VIII.

Backs

Case backs are usually of thin plywood or hardboard. Except on office desks or room dividers, they are placed against a wall in a home and are not considered exposed surfaces, so we are not fussy about their appearance.

The case back should, of course, cover the body of a case so that the whole interior of the case forms one or more dust-proof compartments. In practice, a compartment is considered commercially "dust proof" even if it has small cracks up to about /16 'I wide. This permits small allowances for ease of assembly.

In china cabinets and some other pieces of furniture, the case back is visible from the front when the door is opened. The inside face of the case back is considered exposed and is finished.

One of the most important functions of a case back is to help hold the case tight and square. The back panel itself will always be square even if it is very thin. But if it is loosely attached to the case, the case can deform "off-square." For this reason, nailing or stapling is not as dependable as gluing the back to the case and then nailing or stapling to hold until the glue sets. Nails or staples by themselves frequently crush the wood or bend slightly and permit a small amount of sideways motion.

Generally the parts of a case to which the back is attached have their grain running parallel to the joint between these parts and the case back. In such situations, there is no problem of shrink or swell, so a tight joint is best. But if grain should run at right angles, a floating attachment may become necessary.

A1 1owances

When case bodies have wide parts of solid edge-glued lumber, it is very important to watch carefully for places where the shrink and swell of these parts might cause trouble. F1 oating construction or other precautions against trouble must be used when these parts do actually shrink or swell. It is also important to brace such wide parts against warpage. Frequently such parts are finished an the outside but ant OR the inside. This makes them susceptible to cupping as the atmospheric humidity changes because the unfinished face will pick up or give off moisture much faster than the finished face.

188 Allowances for manufacturing variations are thought about on individual parts but are often overlooked on sub-assemblies. There will be assembly trouble whenever the opening is smaller than what goes into it. This is very evident with the diameter of a dowel and the diameter of the hole it goes into. But it is easy to overlook in sub-assemblies. Figures VI and VI1 illustrated one such situation. Figure IX illustrates another situation.

I* I4 "-4

Figure IX: Cupboard bottom fitted into open mortises in posts.

Assume a plywood cupboard bottom is 3/4" thick, the corners of which are fit into open mortises in the four posts. Assume that the end panel is machined 14" B.S. and the mortises are machined 1/2t' from A to B and 314" in a vertical direction. Assume that the case end has been sub-assembled. At final assembly, the hole into which the cupboard bottom must go in A-A is 14" + l/2It + 1/2" = 15". If dimension C on the cupboard bottom is machined 15", it will theoretically go into the hole. But suppose the 14" B.S. dimension is undersized , or the dimension AB of the mortises is a trifle scant. The cupboard bottom is bigger than the opening, and it will not go in easily at final assembly,. So it is preferable to machine dimension, C 14 15/16", or even 14 7/8", instead of 15". Or better yet, machine C to 15"; mortise the front post with AB = /2 'I, and mortise the back post with AB = 5/8 'I. This would permit driving the cupboard bottom tight to the mortise in the front post so that the face of the front post and edge of the cupboard bottom would De flush (or offset 1/15" if preferred). All variation would be in the back where it would not be objectionable.

189 False Frames

Figure I shows a false frame with a shaped edge separating the upper and lower sections of a deck chest. While the sketch does not show construction at the bottom, there could be a similar false frame between the chest itself and a sub-assembly of aprons.

If appearance is especially important, this false frame can have front corners mitered and held together with dowels, splines, or clamp nails. Back corners can be tenon and groove. Figure X illustrates this.

I I I I I I I 1 I

Figure X: False frame with mitered and doweled front corners.

When appearance is not a prime consideration, the miters could be eliminated as in Figure XI (shown on the following page). The front joints could still be doweled and the back joints tenon and groove. The groove for dust bottom could be cut in the front rail with a shaper and could stop an inch or so from each end so that the groove would not show on the edges at A-A.

190 I I I- I I I I I I I I I

A- -A

~ ~~~

Figure XI: False frame without mitered corners. If tenon and groove construction is used as in Figure XII, the groove should be deeper than the length of the tenon to insure that shoulders will fit tightly at B and 6 because these joints are exposed. But then a hole would show in the edge at C. This is unacceptable because the edge is also exposed. Some shops will patch these holes by gluing in little tapered wooden wedges. In both Figure XI and Figure XII, construction is such that end grain of the front rail shows at both ends of the sub-assembled false frame. It takes stain a different color from side grain. Figure X shows no end grain on the three shaped edges. All three constructions avoid showing end grain of the back rail on the shaped edges.

Figure XII: False frame with tenon and groove construction.

191 With Figure X, it is possible to moulder run the shaped edge on front rail and end rails. If machining is accurate, then, after assembly, mould sanding on a belt of the three edges of the frame will flush the shapes at the mitered corners. With Figures XI and XII, it is almost hopeless to pre- shape the edges of the parts and have them assemble with the moulds near enough to flush for an economical job of true flushing at the mould sanding operation or the belt sander. A preferred procedure is to bill parts in such a way that the frame assembles about l/2 too wide and too long. After assembly, put the frame over the double-end tenoner so that trim saws cut to precise finished length and finished width, get corners precision square, and sha e the edges with cope spindles. This procedure can also be used on Figure !and should be used if it is important to have the frame precision square.

False frames are usually drum sanded after sub-assembly to get precision flush at the joints on exposed faces of the frames. No assembly can be counted upon to come precision flush.

Separate Bases

In Figure I the case ends could run to the floor and the aprons be mitered around the case as in Figure XIII. This is the construction assumed in the section of this chapter titled "Laying Out a Case." An alternate construction would be to make a four piece sub-assembly of aprons and attach it to the under face of a false frame by boring and counterboring the aprons

- BACK RAIL

END CASE ENDS * €ND - c APRON APRON

FRONT RAIL \ FRONT APRON /

Figure XIII: Apron attached to outside of case; corners are mitered.

192 for screws as shown in Figure XIV. If the design called for an appearance as in Figure XV, wrap-around aprons would be impossible and false frame with base under it would probably be best.

FALSE FRAME

I I {I a I AI n n

APRON 1 SUB-ASSEMBLY

Figure XIV: Apron made up as sub-assembly and attached to the under face of the false frame.

Figure XV: Design of stand with apron attached beneath false frame.

193 Many designs of modern or contemporary styling are designed with no aprons and with legs at a slant as in Figure XVI. There is such a variety of designs that each must be analyzed as a special case. The main problem is to get adequate strength in the attachment of the legs to the case. Leverages are generally adverse at the point of attachment, and adequate strength of attachment is difficult to attain. Another construction problem lies in getting adequate strength and rigidity in the part of the case which acts as a platform for the legs and to which the legs are attached. Rigidity is important here because even, if the case does not fall down, it must not be

Figure XVI: Contemporary design which omits the apron.

Figure XVII shows one way to attach round modern legs to a case. A round tenon is turned on the leg with sufficient diameter and length to have adequate strength against pulling out.

Figure XVII: Modern design legs used in case construction.

194 Standard hardware is available for attaching modern legs by running a hanger bolt into the top end of the leg and screwing the other end of the hanger bolt into a steel plate. The plate is screwed to the case. Leverage action tends to pull the hanger bolt out of the wood. For this reason, the hanger bolt should be long enough that it won't strip the thread in the wood and pull out. Remember that the holding power of screw threads in wood is much poorer if going into end grain than if it is going into side grain. Figure XVI I I shows this construction.

Figure XVIII: Standard hardware for attaching modern design legs.

V-Groove Construction

One of the most exciting developments in furniture construction is the groove-folding technique now being used in some casegoods. Sometimes called U-groove construction, the technique involves the application of vinyl veneer to particleboard and subsequent machining of U-grooves in the panel to permit folding into a desired configuration. Figure XIX shows one of the many constructions made possible by the groove-fold technique. The top drawing shows a panel after grooving, while the bottom drawing shows the same panel folded to make the desired configuration, in this case, the front edge of a speaker cabinet.

The basic element in the construction is the precision V-groove cut just deep enough in the particleboard to enable the vinyl to act as a hinge and fold to allow gluing of the adjacent edges that form a corner. Perhaps the best appl ication of U-groove construction found to date is in miter jointed speaker cabinets where a single panel can be grooved and folded into a complete encl osure.

195 Figure XIX: V-Groove construction.

The V-groove system can achieve tremendous cost savings in labor and materials; however, furniture must be designed for the system by a designer who understands the method. In addition, the technique offers several other advantages.

1. Panels require no further sanding or finishing; corners a re a1 igned perfectly.

2. A cabinet with a solid visual appearance is constructed through the use of wrap-around veneer.

3. There is a substantial reduction in the number of parts. 4. The simp1 ified fl ow-through manufacturing process elimi - nates many rough end and machine room operations.

5. The possibility of KD shipment of sub-assemblies occurs.

Wood veneer can be used with this system. However, vinyl tape must be applied where the V-grooves are to be cut. The tape serves as a hinge and permits the panel to be folded and glued. Miter joints formed in this manner are far superior in appearance to conventional joints. With wood veneer, the groove-fold technique requires much longer veneers, thereby creating some problems with utilization. Thus, wood veneer performs well in V-groove construction, but vinyl veneer is excellent in such applications.

196 The V-groove system is not limited to the production of box-shaped cabinets, but is capable of being integrated into full -scale casegoods. The only limitation is the imagination of the furniture designer and the product engineer.

Usabl e Storage Volume

An interesting study was conducted by students in the Furniture Manufac- turing and Management curriculum at North Carolina State University to determine the actual usable space contained in a piece of casegoods furniture. Data were collected on 180 cases from 21 different manufacturers. Measurements were made to compare the total volume of a case with the total volume of its drawers. The results showed an industry average of 50.4 percent total usable space in casegoods. While this may not seem important now, there is reason to believe that it may have a significant effect on furniture construction in the future with increased emphasis on compactness and more efficient utilization of space.

Knock-Down Construction

The bulky nature of furniture and the rapid increase in shipping costs have caused considerable effort to be placed on developing constructions for casegoods that permit the furniture to be shipped partly knocked down and be assembled at destination. Substantial savings in shipping costs have re- sul ted from knock-down (KD) construction for dining and occasional tables, bookcases and hutches, mirrors and various types of institutional furniture.

European furniture manufacturers have done a great deal more than Americans with KD casegoods. They have developed some ingenious fastening hardware. KD construction has become more accepted in this country and will continue to grow in popularity as new and different applications are found. Knock down furniture offers the product engineer an opportunity to real ize savings in assembly and finishing operations in addition to savings derived from reduced shi pping cost s.

Acknowl edgment

Acknowledgment is made to the following organization through whose courtesy and by whose permission illustrations in this chapter have been reproduced.

Victor Wood Products and Supply Co., Grand Rapids, Michigan.

197 i --

198 CHAPTER 12

CASEGOODS - RIGIDITY

The problem of "loose cases'' was mentioned in Chapter 11. The causes and cures for loose cases are somewhat obscure and not too much is known for sure about them. Some interesting theoretical work, which bears on this problem, has been done in England. Experimental work at N.C. State Univer- sity checked these theories to determine their applicability to practice in American furniture manufacture. Through the cooperation and with the permission of The Furniture Development Council of England, Chapter 1 of its book, Design Manual for Cabinet Furniture, is reproduced in full. This chapter presents the theories.

Simple Facts About Cabinet Rigidity1

1. What Happens When a Cabinet Distorts?

An article of cabinet furniture can, in most cases, be considered as an open box. Such a box has the property that when it is pushed or pulled enough to alter its shape, it always deforms into the same shape, no matter what the directions of the loads applied to it (see Figure I).

Figure I: The same shape resu ts from different distorting forces.

When the distortion so Produced i studied, it is found that all the faces of the box (including the open face) have twisted. Faces which were originally flat are now no longer flat. In addition, the open face, which

The Furniture Development Council, Design Manual -for Cabinet Furniture. (London: Pergamon Press, 1958) , pp. 3-18.

199 may have been rectangular at the start, tends to become diamond shaped, the corner angles become greater and less than true right angles and straight members around the opening tend to become slightly curved.

Unless the distorting forces which are applied are large, the deformations which are produced will be comparatively small and may not be easy to detect without careful measurement. Their effects usually become evident in furniture from the binding of doors or from relative movement of door edges.

The First Principle

If an open box can deform in only one way and that way includes change of shape of the opening and also distortion of the joints and members which form the edge of the opening, then any structural alteration which stiffens these members will therefore make the box stiffer.

This method of stiffening is somewhat in line with traditional practice in which cabinets made with plywood are stiffened by using center or side pilasters. Use of pilasters goes some way towards restricting the change of shape of the opening, but to be fully effective, the top and bottom members should be stiff enough to resist bending and the joints between these members and the pilasters should be rigid.

These effects can easily be demonstrated by making a simple cardboard model with five flat sides and one open face. The edge joints can be made with gummed paper strip (see Figure 11). Much can be learned from a model of this nature. It will be found that, when handled, the box will distort easily. It will be seen that each flat face becomes twisted and that the opening distorts to a diamond shape similar to that shown in Figure I.

Gummed strip

Stiff mrd

Figure I I: Cardboard model . If now, two narrow strips of cardboard are added to represent side pilaster (and attached by gummed paper), it will be found that the box has been stiffened considerably. Distortion of the opening can still occur, however,

2 00 by serpentine bending of the edges at the top and the bottom of the opening and by change of angle at the joints where these edges meet the pilasters.

If a strip is now added to the model to prevent bending of the top edges and also to prevent change of the corner angles (see Figure 111), it will be found that the whole box becomes remarkably stiff.

Glw overlap at A1

P,gure 11: Model w th open face stiffened by p lasters and linte I.

These experiments suggest that one principle for making a piece of cabinet furniture rigid is to stiffen up the face which contains the door or other opening so that the corner joints cannot change their angles and so that the side, top and bottom members cannot bend.

The Second Principle

Since the only way in which an open box can distort includes the twisting of each face, any structural alteration which resists the twisting of any face will resist the distortion of the whole box and will have the effect of making the box stiff.

This may easily be demonstrated with the cardboard model. While one face is completely open and not reinforced as in the first principle, the box has very little stiffness. If now one face (any face) is prevented from twisting by fastening it to a board by means of four drawing pins placed one at each corner and pushed well home (see Figure IV), the box will be found to be stiff.

Figure IV: Model stiffened by forcing one face to remain flat.

201 A more realistic way of demonstrating this method of stiffening is to build a "closed box" on the bottom, top or one side of the model. This is shown in Figure V. This ''closed box" is extremely difficult to twist and so serves a similar purpose to the board in Figure IV.

c JV

Figure V: Model stiffened by a closed box.

This suggests that a second general principle for stiffening cabinet furniture is to make at least one face very stiff n resisting tw sting.

Economy in Materials

Further examination of the cardboard model, while it is being distorted, will show that each corner edge where two panels are joined together remains straight even though the panels are twisting. This shows that, since corner members do not bend, the bending stiffness of such members does not contribute to the stiffness of the whole. These corner members do, however, twist and, in so far as they resist twisting, they contribute to the stiffness of the cabinet, but research measurements have shown that this contribution is so small as to be negligible.

The practical conclusion is that inside corner rails and stiles, linings, etc., are of little use for stiffening a cabinet and, for economy of materials, their size should be as small as possible consistent with the making of an efficient joint between adjacent panels or faces.

It is important to realize that, since some of the distortions which nave been described are small , any slackness in an important joint due to a local defect in gluing or due to use of screwed instead of a glued joint might up- set the above rules and might introduce unwanted flexibility in the cabinet.

202 2. Stiffening the Open Face

Since an open faced box, of the type we are considering, cannot distort without the rectangular open face becoming lozenge or diamond shape, any stiffening of the open face which prevents the diagonals from changing in length will stiffen the whole box.

If forces are applied to opposite corners of an open face, tending to make one of the diagonals either longer or shorter, a diamond type distortion will be produced which is similar to that resulting from twisting the box.

Thus, in examining the structural suitability of various frames for the stiffening of the open face, we can conveniently select them on the basis of their effectiveness in resisting diagonal forces.

To stiffen a rectangular opening, the framework surrounding the opening should be such that two opposite corners are so rigidly connected that the distance between them cannot be changed except by large forces. For the stiffening to be effective, the minimum requirements are that two adjacent side members shall be stiff in bending and that the joint between them shall resist any change in angle.

In figure VI(A), the stiffening fails because two adjacent sides are not stiff in bending. In Figure VI(B), the stiffening fails because the joint at A between the adjacent stiff members can change its angle. Figure VI(C) meets the minimum requirements (of two stiff adjacent sides with a stiff joint between them) and the frame is effectively stiffened.

A B C

Figure VI

Naturally, added stiffness will be obtained by making all front edge members stiff in bending and all front corner joints stiff, assuming the open face is at the front. This does not apply, however, to the internal members or the joints between them since, as has been shown, the internal framing does not contribute to the stiffness of the box, but only serves to hold the sides together.

203 It will also be seen that corner brackets, which give stiff joints, will serve little useful purpose in stiffening the frame unless they are placed at weak joints between stiff members such as that at A.

Practi cal Exampl es

One common and very effective means for stiffening an open face is the use of pilasters. In Figure VII, the pilasters A and B may be regarded as side members very stiff in bending in the plane of the open face. In order to meet the minimum requirements for effective stiffening, either the top member C or the bottom member D (preferably both) should be stiff in bending and the joints between all these members should be as stiff as possible.

Sometimes the pilaster is central between two doors as in Figure VIII. In this case, the open face of the box may be regarded as having two openings side by side. Provided that each opening is stiffened, the whole face will be stiff. Again, the minimum requirement will be met if either the top or bottom cross member is stiff in bending and if the joint between this and the pilaster is really stiff.

A similar form of stiffening might be used for a sideboard as in Figure IX, where the bottom member is made stiff in bending by the addition of an apron B. Again, it is essential that there shall be a really stiff joint between the pilaster A and the apron B.

Figure VII: Side pilasters. Figure VTII: Central pilaster.

Figure IX: Pilaster attached to apron.

204 Various typical forms for stiffening frames for open faces may be used. These are modifications of those already described and each can satisfy requirements for effective stiffening.

One form is the simple IIL" frame shown in Figure X. If another stiff side member is added, the common "U" frame is obtained as in Figure XI. Since a frame is equally stiff right side up or upside down, the 'IU" frame may be inverted as in Figure XII.

Figure X: "L" frame. Figurem XI : 'IU" frame.

Figure XII : Inverted "U".

An inverted "U" frame may have the vertical member central in which case it becomes a "T" frame as in Figure XIII. If the bottom member is stiff as well as the top member, it becomes an "I" frame as in Figure XIV and if this frame is on its side, it becomes an IIH" frame as in Figure XV.

Figure XIII: "T" frame. Figure XIV: "I" frame.

Figure XV: "H" frame.

205 Sometimes part of the frame may be concealed by doors, etc., as in Figure XVI, or it may be external to the carcass as in Figure XVII.

Figure XVI: Part of frame concealed Figure XVII: External frame. by door.

It is again emphasized that the frames will only be effective if the joints between their members are rigid.

For the most efficient use of the avaiable material, the bending stiffness of the various members surrounding the opening should be related to the type and proportions of the frame. These relations, for frames made from wood of solid rectangular cross section, are given in Chapter 4, together with other calculations related to the various front frame reinforcements.

It should be noted that since the stiffness of internal frame members con- tributes so little to the stiffness of the whole box, it is of little importance whether or not these members are directly jointed to each other or to the frame round the open face.

3. Stiffening of One or More Sides

It has already been explained that distortion of an open faced box may be prevented by stiffening the framework around the open face or by preventing one (at least) of the other sides from twisting. The amount of stiffening which can be provided for the open face is frequently limited where the design demands the largest possible opening free from obstruction and where external stiffening cannot be used. In such cases the desirable stiffening of the box may be obtained by making one or more of the remaining faces stiff in resisting twisting.

Four methods for making a panel resist twisting forces are now described. These are: Solid panel; Closed box; Crossed rib reinforcement; and Pyramid reinforcement.

206 a. Solid Panel

The simplest form of panel which will resist twisting is one made from a thick, solid piece of material (Figure XVIII). To save weight, the panel may be of sandwich construction with a light core but with faces of stronger and more rigid material (Figure XIX). Where sandwich material is used, special attention should be given to making adequate joints between each face of the sandwich and the edges of adjoining panels.

b. Closed Box

The closed box has greater stiffening effect than a solid panel for a given weight of material, and the greater the depth of the box, the more efficient it will be (Figure XX). The closed box has not often been used for stiffening purposes but in many conventional designs it could be incorporated without any great alteration.

Figure XVIII: Solid panel. Figure XIX: Sandwich panel.

Figure XX: Closed box.

Wherever there is an unused space (as, for example, in a plinth), it may be possible to close the space with a panel of plywood and thus obtain the additional stiffening effect of a closed box (see Figure XXI). If plywood is used, its effect will be greatest if it is laid with the grain at an angle of 45" to the edges of the panel.

Stiffening effect greatest if plywood laid wllh groin direction at 45" to thc edge t--

Figure XXI: Closed box with ply at 45'.

207 c. Crossed Rib Reinforcement

If a rectangular sheet of cardboard or thin plywood is twisted in the hands, three things will be noticed.

Firstly, the edges (AB, AD, BC, CD in Figure XXII) remain straight; sec- ondly, the diagonals (AC, BD) become curved in opposite directions; thirdly, the lines on the panel which become most curved are those at 45" to the edge (such as AF, BE).

Figure XXII : Twisted panel.

If two beams could be laid along the diagonals and attached to the panel at the center and the ends, the curvature of the diagonals would be resisted and the panel would be greatly stiffened against twisting (see Figure XXIII). In practice, the beams could take the form of two intersecting ribs. The central joint should be so designed that neither rib is unduly weakened in bending. If the ribs are ''halved" together, the rib that is cut away on the side furthest from the panel may need to be reinforced by a capping strip as shown in Figure XXIV, but an alternative method is to cut the notches so that only one-sixth of the depth is cut away on the rib that has the notch furthest from the panel (Figure XXV).

Figure XXIII: Crossed rib Figure XXIV: Halved joint with reinforcement. capping stri p.

Figure XXV: Joint to give balanced stiffness.

2 08 Since the direction of the greatest curvature of the unstiffened panel is at 45" to the edges, the ribs are most effective on a square panel where they lie at 45" to the edge of the panel. Therefore, if the length of the panel is more than one and a half times the width, it is better to treat the panel as if it were two panels with separate crossed-rib bracing for each as shown in Figure XXVI.

i F

Figure XXVI : Double crossed-ri b reinforcement.

The stiffness of the structure increases rapidly with increase of the depth of the reinforcing ribs, but only proportionally to increase in their thickness. Thus, for example, if the depth h of the ribs is doubled, the stiffness increases nearly eight times, but if the thickness w is doubled, the stiffness is increased to less than twice its original value. This is explained in Chapter IV.

It may also be noted that as the edges of the unstiffened panel remain straight when the panel is twisted, ribs placed around the edges of a panel, for instance, in the form of rectangular framing or an open plinth, do practically nothing to prevent twisting of the panel (see Figure XXVII).

Figure XXVII: Rectangular frame - no stiffening effect.

2 09 d. Pyramid Reinforcement

It has already been pointed out that when a panel is twisted, its four corners cease to be in the same plane. If these corners could be forced to remain in the same plane, it would not be possible to twist the panel. A further method of doing this is the pyramid reinforcement, shown in Figure XXVI 11.

This reinforcement consists of four inclined members meeting at a point and resembling the edges of a pyramid. The free ends of the members are firmly anchored to the four corners of the panel. Provided the angle between each member and the panel is at least 20" (a slope of 1 in 2 3/4), the stiffening effect of this type of structure for a given weight can compare favorably with other forms of reinforcement (see Figure XXIX).

Figure XXVIII: Pyramid bracing. Figure XXIX: Diagram of pyramid bracing.

Since the forces (pull and push) in the four inclined members must be considerable if they are to restrain any twisting of the panel, all the joints must be robust and well designed.

This reinforcement is obviously something of a challenge to the ingenuity of the production engineer, but it is included here as a means of achieving stiffness with the greatest economy of materials.

210 e. Twist Is Caused by Relative Movement of Opposing Faces

If the closed box or other construction is to be used efficiently to stiffen a cabinet article, it is necessary to understand clearly and exactly what the stiffening is intended to do.

In Figure XXX, three faces, A, By and Cy which might form part of a box or cabinet, are shown. The face C can be twisted by the attached panels A and B rotating in their own planes and in opposite directions. This is what happens when a cabinet deforms although the motions in an actual article of furniture may be so small as to not be easily seen by the eye.

Figure XXX: Panel C twisted by rotation of panels A and B.

If the face C is stiffened to resist twisting, the faces A and B will not be able to rotate, as shown in Figure XXXI. It will be seen that the stiffenincr will be effective only if there is a riqid joint between the stiffened"pane1 C and each of the" panels A and B whose relative motion is being restrained.

Figure XXXI: Relative movement of A and B restricted.

211 It will also be seen that the stiffening need not necessarily extend over the whole face so long as there is still good attachment to the two opposing faces which it connects (Figure XXXII).

Figure XXXII: Relative movement of A and B restricted.

The stiffening structure may occupy any position between the opposing faces (Figures XXXI I I and XXXIV) .

Figure XXXIII: Relative movement of A and B restricted.

Figure XXXIV: Relative movement of A and B still restricted.

212 If, however, the stiffening structure fails to make efficient contact with both the opposing faces (Figure XXXV), its stiffening effect may be largely or completely lost.

Figure XXXV: Relative movement of A and B not fully restricted. Similar rules apply to the other methods of making a panel difficult to twist. For instance, the crossed-rib reinforcement shown in Figure XXXVI will not be fully effective, as the reinforcing ribs do not extend to the ends. The arrangement shown in Figure XXXVII , however, would be quite e ffecti ve .

Figure XXXVI: Wrong. Figure XXXVII: Right.

213 214 CHAPTER 13

DRAWERS

Dovetailing

Although dovetail joints are not necessarily the strongest joints to use for the corners of drawers, they are good joints and, with modern dovetail machines, cost very little more than other joints. Furthermore, the viewpoint is firmly established among retailers that dovetails are the best joints. One exception is with kitchen cabinets, but most drawer corners are dovetail joints. Chapter 21, "Dovetailer," in Production Woodworking Equi pment by Rudol ph Wi11 ard is recommended for review.

The product engineer should lay out the exact size and exact location of dovetailing for each drawer. Below are some of the points to observe.

The male dovetail on the drawer side should cover the groove for the drawer bottom which is cut in the drawer front (and in the drawer back if there are male dovetails at the back end of the drawer side). Plan location of the dovetails as in Figure I(A), not as in Figure I(B).

A B

Figure I: Dovetail in drawer side located to cover the groove in drawer front.

Avoid locating dovetails so that they leave a delicate piece at the edge of the drawer side which is likely to split off during assembly as seen at C in Figure 11.

215 Figure 11: Small section of dovetail at end of joint is easily damaged in assembly.

If poss ble, avoid eaving part of a dovetail hole in the drawer front which is not over d bv a male dovetail as seen at D in Figure 111. With certain sizes of drawer openings, this cannot be avoided except by removing one cutter from the dovetail machine before making the female cut in the drawer front. This can be done, but it is better to avoid it if possible.

Figure 111: Part of dovetail joint left uncovered.

One method for laying out dovetailing is to accurately draw the dovetails on a piece of tracing paper, then superimpose the tracing paper on a drawing of the side view of the drawer to find the best location for dovetails. The dovetails must be drawn in such a way that the production machine can cut

216 them. Avoid a representation as illustrated in Figure IV. The machine cannot cut that way. Shaded portion A is cut in the drawer front by the dovetailer bit; shaded portion B is cut out of the drawer side by the same bit. Consequently, A and B must be both exactly the same size and shape.

Figure IV: Incorrect way of representing a dovetail joint.

Some factories prefer to dovetail so that the bottom edges of drawer sides project down about l/32" below the drawer front. This keeps the drawer front from bumping the corner of the front rail below it. See Figure V at E. Other factories drive a thumbtack or nylon glide into the front rail, positioned so that the bottom edge of the drawer side rides on the glide and raises the drawer front slightly above the front rail. See Figure V at F.

In either case, the drawer front will ' not strike the corner of the front rail at G as the drawer is closed.

Figure V: AI ternate constructions to prevent drawer fronts from striking the front rails.

217 "Boxed-in" Construction

Formerly, many factories nailed the drawer bottom to the drawer back as shown in Figure VI(A). More recently, the drawer bottom is fitted into a groove in the drawer back as shown in Figure VI(B). This type of drawer is called, in the trade, a "boxed-in" drawer and is generally considered a higher quality construction. Actually it probably costs no more and is general ly recommended.

DRAWER DRAW E u BOTTOM 60 'T T 0M A B Figure VI: Two types of drawer construction (B) represents the "boxed-i nl' drawer

Tilts and Guides

Drawers are supposed to slide out of and into cases. They should move freely and easily at all times. This requires some clearance between drawer and case. But if there is too much clearance, the action of the drawer is loose and "sloppy." People want drawers to run free but not "sloppy." To get proper sliding action of a drawer, its motion must be controlled in a horizontal direction and also in a vertical direction.

Motion control in a horizontal direction is generally obtained by using drawer guides. There are two general types of drawer guides and many variations. The two types are:

1. Side guides.

2. Center guides.

218 Figure VI1 shows one type of side guide. This guide furnishes a flat horizontal surface upon which the bottom edge of the drawer side runs. It also has a vertical flange which limits movement of the drawer sideways as soon as the face of the drawer side comes into contact with the flange. Prac- tices vary, but the clearance at A between guide and drawer side is usually about l/16" to l/8". In a case end construction using posts and a thin panel, this guide can also help to hold the case end together. The bottom edge of this also acts as a drawer tilt for the drawer immediately below it. Figure VIII, on the following page, illustrates this drawer tilt action; A shows the drawer closed, and B shows the drawer open. The drawer is shown

SIDE GUIDE

Figure VII: Position of a drawer side guide.

219 - I I. ------k t2 m - -1 I I C-/ I I I I I I I 7- d A -

Figure VIII: Illustration of drawer tilt action. by dotted lines. When the drawer is closed, there is a clearance at C (Figures VI1 and VIII) between the top back corner of the drawer and the side guide above it. When the drawer is opened more than halfway, the front end of the drawer will drop down pivoting around the front corner of the front rail. This pivoting action causes the back end of the drawer to rise up until the top back corner of the drawer hits the drawer tilt at D.

220 This stops the falling motion of the front of the drawer. In Figure VIII, the side guide acts as a drawer tilt. The type of side guide just described lets one part (the side guide) perform three functions:

1. Furnish a flat horizontal surface on which the drawer sides can slide.

2. Control sideways motion of the drawer by side-guide flanges guiding against the face of the drawer side.

3. Control the amount which front of the drawer can drop.

Stated briefly, these three functions are to support the drawer, guide it and control the tilt.

In Figures VI1 and VIII, the clearance at C is called the "tilt allowance." It should be large enough so that, when the drawer sides swell the maximum amount in damp weather, there will still be l/16" or so of clearance to insure that the drawer will open. On the other hand, it is well not to have the clearance more than is needed. The greater the clearance, the more the drawer front will drop when the drawer is opened. People do not like to have drawers drop more than is necessary. But if a choice must be made, it is better to have the drawer drop a little too much than to have it swell shut in the case.

The side guide just described works out quite well for cases that have no dust proofing or that have only one dust panel at the bottom of the case. They do not work out as well for fully dust proof cases with dust panels between all drawers.

With side guides, it is important to have the case square and to have front and back rail shoulder measurements accurate because fundamentally it is the inside of the case which guides the drawers and controls horizontal motion.

Figure IX shows a common type of center guide. The center guide is of rectangular cross section and is attached to a frame which may or may not have a dust panel. The frame is fastened to the case ends. The end rails of the frame furnish the flat horizontal surface on which the drawer sides run. The bottom face of the end rails of the frame act as drawer tilts for the next drawer below the frame. The guide consists of two pieces, the center guide, which is attached to the frame, and the drawer runner, which is attached to the drawer. The interaction of these two parts controls the horizontal motion of the drawer but performs no other function. Clearance should be left at A. The drawer should run on the bottom edge of the drawer sides. Without clearance A, any slight variation can cause the drawer to ride on the single contact between the guide and runner, and the drawer will pivot on this point instead of riding firm on the two drawer sides. There must be clearance at B-B to permit free sliding of the drawer runner (attached to the drawer) on the center guide (attached to the case). Prac- tice varies as to amount of this clearance. One common practice is to make the groove in the runner 1/16" wider than the width of %he guide. This allows a theoretical l/32" on each side. Too great a clearance allows excessive sideways motion of the drawer which is undesirable. Too small a

221 CENTER GUIDE d

A *

FRONT RAiL

DRAWER RUNNER

I 1-

CEN~ERGUIDE FRONT VIEW WITH DRAWER

Figure IX: Illustration of center guide and drawer runner.

222 clearance runs the risk of binding between guide and runner if either is crooked or if machining is inaccurate. It is practical with this sort of center guide to hold sideways motions of the drawer to a considerably smaller amount than with the side guides previously discussed. Then too, a tradition has grown up in the trade that center guides are a higher-quality feature than side guides. This is not necessarily true, but the tradition is quite firmly implanted. With center guides, it is possible to leave any amount of clearance between drawer sides and posts or case ends. A large clearance reduces the danger of drawers binding if either case or drawer is out of square, or if drawer sides warp, or if dimensions are inaccurate. Except for very special reasons to the contrary, 1/8" is satisfactory clearance between drawer sides and posts or case ends. With the side guides discussed, this would give l/4" sideways motion of the drawer as compared with 1 /16" sideways motion with center guide.

An alternate construction for the drawer runner is a nylon clip. This clip is stapled or screwed to the drawer back. It guides the back of the drawer but not the front. It requires a T-shaped drawer guide. For guiding the front, there are available L-shaped pieces of nylon which attach in the corner between front rail and post. Figure X illustrates the use of nylon clips and Figure XI gives a pictorial view of the clip and a pad. In using nylon clips for low priced cases without dust proofing, the center guide can be attached to the back of the case and the dust frame is eliminated. The drawer rests on 3-point support. At the front, the two nylon IIL" pieces support the drawer sides; at the back, the nylon clip rests on the center guide. In this situation, there is no clearance at the back of the drawer.

Figure X: Nylon clip used as drawer runner.

223 The same kind of guide is often used with a formed sheet-metal drawer runner. This gives guiding front and back and very effectively keeps the drawer from tilting. Thus a 5" drawer back can be used in a 5 3p 'I opening. When used in conjunction with metal drawer runners, it is common practice to dip the drawer guides in wax.

In office desks and sometimes in other furniture, a different type of side guide is used. Figure XI1 illustrates the principles of this guide. The guide is mortised into or otherwise attached to the front and back posts. The drawer side has a moulder-cut groove machined into it. The drawer is supported at A. Clearance at B provides guide action for horizontal control. Clearance at C provides drawer tilt control. This sort of guide requires a fairly thick drawer side so that the groove into the drawer side does not weaken it too much so it will split. A minimum drawer side thickness is about 7/16", with 1/2" being better. Many office desks use 1/2" or 9/16" drawer sides made out of 3 p I' rough lumber.

Office furniture file drawers and kitchen cabinet drawers are oftentimes heavily loaded and see very active use. The trend is to use side guides with two telescoping pieces of metal in combination with nylon rollers. These guides provide support, horizontal guiding, and tilt control. The guide hardware costs much more than other guides discussed, but the drawers do operate very freely. Recently a few hardware manufacturers have developed center guides to accomplish the same result.

Figure XI: Pad and drawer runner made of nylon.

2 24 Figure XII: Drawer side-gu de construction used in off ce furniture.

Drawer Stops

When an open drawer is closed, it will keep going until it strikes a %top" of some kind. For appearance, it is generally important that the drawer stop at some precise location. Hence there is the need for something to act as a drawer stop.

If drawers have lip drawer fronts, the lips on the drawer front will stop the drawer when they hit the case front. With center guides as illustrated in Figure IX, the front end of the center guide would stop the drawer when the inside face of the drawer front struck the front end of the guide. By accurately controlling the distance A in Figure IX, the drawer can be stopped in whatever position is desired. If the drawer front is long, the ends of the drawer front may not be flush with the front of the case. This is due to the necessary side play in the center guide which permits the drawer front ends to move in or out in spite of the accurate control of position at the center when the drawer front hits the center guide. In such a situation, some shops install a glue block behind each drawer side when the drawer is located in the exact position desired. These blocks are pushed up tight to the back corner of the drawer. This must be done before the back panel is applied to the case, or else access to their location is impossi ble.

Other solutions are possible, but the main thing to remember Is that every drawer needs some arrangement to stop the drawer in the desired position when it is fully closed.

225 Shrink and Swell

Since drawer sides and backs are generally of solid wood, they swell. But proper allowance between the top edge of the drawer sides and the drawer tilt can avoid interference between the drawer and the case. The space allowed must be more than the increase in the dimension of the drawer due to swelling.

But drawer fronts involve a different situation. If the drawer front is plywood, it will not swell, so there is no problem. But if sales specifications require "solid," the drawer fronts cannot be plywood. Being solid, they will shrink and swell. The opening into which the drawer front fits does not change in size. If appearance design permits, the best solution is a lip construction for the drawer front. Figure XIII(A) illustrates this construction in a solid drawer front. Figure XIII(B) shows a plywood drawer front which fits into the opening in the case. Tilt allowance at D prevents drawer side swelling against the front rail above it and is the same for each type of drawer. This tilt allowance can be as large as is needed because it is not visible from the outside of the case. Allowance at C between the drawer front and the front rail above it can be small for good outside appearance because the plywood drawer front does not swell.

RAIL

CASE END CASE END --

IJ -A 0

Figure XIII: Two types of construction for drawer fronts.

226 As the solid front swells, the lip on its top edge simply laps a little farther up on the front rail, and the appearance is still all right. If the drawer front shrinks, the lip laps the front rail less. It is important that the lip should lap the front rail at least 1/16" when the maximum amount of shrinkage of the drawer front occurs. Otherwise an opening would show between the lip and the front rail at maximum shrinkage.

Lips are not necessary on the ends of the drawer fronts to take care of shrink and swell because the drawer fronts swell in width but not in length. However, most factories that use lips on the top edge of the drawer front also use similar lips on both ends of the drawer front mainly for appearance.

Some design specifications call for a solid wood drawer front which must fit between the rails as in Figure XIII(6). In this situation, the product engineer must increase the allowance at C to allow for the increase in the dimension of the drawer front due to swelling. The visible crack will be more obvious, but there is no alternative if the drawer front is not to bind in a humid envi ronment.

French Dovetai1 s

An alternate construction for attaching drawer sides to a drawer front involves the use of a french dovetail. Figure XIV shows the front and bottom views of a joint made with french dovetail. The male portion of the joint is machined on the drawer side with a double-end tenoner while the corresponding female groove can be cut into the drawer front with a router.

I I I I

LI11, TOP VIEW

Figure XIV: French dovetail construction.

227 French dovetails can be machined much faster than regular dovetails, and they are considerably cheaper. However, they do not position as accurately as do regular dovetails. French dovetails are particularly well suited for use with lipped drawer fronts because the joint formed is not exposed on the end and lip construction is used mainly on less expensive lines of furniture. A french dovetail permits the entire thickness of the drawer front to be in front of the case. Thus a 3/4" drawer front looks "heavy," whereas, with regular dovetails, a thickness of 1 1/4" would be required to get the same 1 ook. When pro'perly machined with clos@ attention paid to to1erances, french dovetails are every bit as good as regular dovetails. They are by far the best means of attaching drawer sides to plastic drawer fronts.

P1 astic Drawer Interiors

Several plastic manufacturers have come out with a one-piece molded drawer interior. It takes the place of drawer sides, backs, and bottoms and is designed so that any kind of wood drawer front can be attached to it. While it seems like a good idea, it has not become very popular. Apparently consumers do not prefer plastic drawer interiors to wood. The cost of a plastic drawer is equal to or greater than the cost of an all-wood drawer. Tooling cost of making molds for a plastic drawer is high, and as with most mold constructions, any change in the size of the drawer requires a new mold. This tooling cost appears to be a fundamental drawback to using plastic drawer interiors. Another problem experienced with plastic drawers and drawer interiors is electrostatic attraction of dust. Everyday dusting with a dry dust cloth builds up a static electricity charge on plastic furniture parts which attracts dust, whereas dusting with a damp cloth eliminates the problem completely. It is quite possible that plastic drawer interiors will come into wide use in the future. So far they have only been used in significant quantities in kitchen cabinets.

Miscel 1 aneous Drawer Construct ions

The Europeans have developed some interesting drawer constructions for use in their casegoods. They use side guides on all of the drawers. Figure XV and Figure XVI illustrate two of the new drawer side constructions being used in Europe.

Figure XV shows a drawer side made out of extruded plastic. The extruded shape is designed to accept a drawer bottom on one side and side guides on the other. It can be cut into desired lengths for drawer sides and backs. Drawer sides are fastened to a back with a specially designed plastic dowel and an adhesive. The sides are attached to a front by means of plastic dowels which can be adjusted to fit almost any spacing of the dowel hole. This type of drawer construction is strong and virtually trouble free; however, it may experience difficulty gaining acceptance from domestic furniture manufacturers except in the area of institutional furniture.

228 Figure XV: Extruded plastic drawer side.

A B Figure XVI: Vinyl-clad, particleboard drawer sides and back.

229 Figure XVI presents a concept more than a construction. High-quality flakeboard (the height and thickness of a drawer side and the length of two sides and a back) is run through a moulder to receive grooves for a bottom panel and side guides. After the moulder, the piece is sanded and covered with a layer of vinyl veneer printed in a desired grain pattern. The piece is then run through a double-end tenoner where it receives two V-cuts that form miter joints when folded and french dovetails on each end. Figure XVI(A) shows the piece after tenoning. Glue is applied in the V-grooves and then folded to receive a drawer bottom. Finally, a drawer front is slipped on the french dovetails, and you have a three piece drawer as shown in Figure XVI(B). The construction is very simple with the vinyl veneer farming a hinge between the back and sides until the glue sets. It eliminates a great deal of work now being done on drawers and produces a clean-looking drawer at the same time. For low-priced furniture, this is becoming the most popul ar construct ion.

Acknowledgment

Acknowledgment is made to the following organization through whose courtesy and by whose permission illustrations in this chapter have been reproduced.

Ronthor Reiss Corporation, New York, New York,

230 CHAPTER 14

CUPBOARDS AND DOORS

Cupboards

Many case pieces, such as buffets and china closets, have cupboards. Some cupboards are open front, such as dining room hutch cabinets and some have doors. Cupboards as such do not generally introduce any new construction problems. Interiors are usually defect free and are finished because they are visible when the doors are open.

The bottom of a cupboard should ordinarily be flush with the bottom of the front opening or a little higher. Housewives like to dust cupboard bottoms and it is harder to dust if the bottom is lower than the front opening. In some constructions a thin 3-ply panel is used on top of a frame to form the cupboard bottom. This involves machining the panel to fit the opening into which it fits. Some shops will make an intentional allowance by machining the panel about 1/16" smaller than the inside of the case. This avoids de- lay in assembly if dimensions are not precisely as specified, but it also leaves a small crack between the bottom panel and the case ends. Other shops will specify machining without an allowance and will hand fit any oversize panels.

She1 ves

Many cupboards have shelves. If the shelves are fixed in definite positions, they are attached to the case ends. If the shelves are to be adjustable, there is stock hardware available to provide supports. One form is uprights with holes for various shelf positions and clips to fit into these holes and support the shelf. The uprights are screwed to the cupboard ends. In another construction, holes are bored in the ends of the cupboard, and clips are available with round shank to fit the bored holes. The projecting end of the clips supports the shelves. Figure I shows such a clip made of plastic. Metal clips are also available.

Figure I: Plastic clip for shelf support.

231 Whether fixed or adjustable, all shelves should be thick enough so that they do not sag noticeably under load in normal household use. The longer the unsupported span of a shelf, the more it will have a tendency to sag in the middle. No fixed figures can be given for the proper thickness of shelves. It depends on load and span.

If fixed shelves are somewhat warped, it is not serious because they will usually straighten out when assembled into the case. But with adjustable shelves there is nothing to hold them flat. They are supported at four corners, and if the shelf has spiral warp, it will pivot on two of its supports. Adjustable shelves should be made of some material that will stay flat. Edge-glued solid gum would be a poor choice because it is subject to spiral warp. Plywood with a lumber core or particleboard core usually stays flat. Plywood with veneer core is not as likely to do so. Edge-glued poplar, basswood, or maple is fairly dependable about staying flat.

With veneer or particleboard as the core, the front edge does not look very good. Since this edge is visible, something should be done to improve the appearance of this edge. This problem was discussed in Chapter 4. The veneer plies that run perpendicular to the length of the shelf do not contribute much to the stiffness of the shelf. The amount of sag that may be expected in a plywood bookshelf is almost double that of a solid shelf of the same weight. Particleboard and fiberboard without a face and back veneer can sometimes show triple the amount of sag of solid lumber construction . This problem is severe in 36" and 42" bookshelves that are unsupported. The consumer has the option of turning the shelf upside down once the sagging is noticeable.

Door Openings

If the cupboard has a door or doors which fit into a door opening, a serious problem of ''loose cases'' may occur. Such a construction needs an extra rigid case because, if the door opening deforms off square or off size, the door will bind in the opening. In extreme instances, an opened door may not close into the opening at all. Chapter 12 discussed ways to construct a rigid case. The bigger the door, the greater the likelihood of trouble if the door opening deforms due to loose-case construction. Wardrobes are notorious troublemakers in this respect and require special precautions.

Doors Most furniture doors are one of two types -- flush doors or frame doors.

Flush Doors

Flush doors are generally made of veneered particleboard. It is no problem to machine a door to accurate length and width with precise 90-degree corners if the door is run to finished length and width on a double-end tenoner. The most difficult problem is warpage of the door. Here again, the bigger the door, the greater the chance of trouble from warpage. Methods of making plywood which will stay flat were discussed in Chapter 4.

232 If sales specifications call for "solid" wood such as maple or cherry, doors must be of edge-glued lumber, not plywood. This aggravates the problem of flat doors. It also introduces a new problem, namely, shrink and swell of the door while the door opening in the case does not change in size. Lip construction for the door is one answer to shrink and swell of doors. The construction is similar to lip drawer fronts as discussed in Chapter 13. This construction is frequently used on kitchen cabinets even though the doors may be of plywood. By making the lips deep, the door opening can deform considerably off square or off size without causing the door to bind in the opening. Kitchen cabinets are often deformed by carpenters attaching them to walls which are not flat.

Warpage shows up rather badly on a design which requires two doors that are supposed to close flush with each other. The slightest warpage of either door is very objectionable. This construction should be avoided if possible. If it must be used, trouble can be minimized by using a bead strip between the doors to break up the flush surface. The bead strip can be attached to one door. Figure II(A) shows doors without bead strip; Figure II (B ) , with bead stri p. .-

Figure II: Bead strip used to relieve flush surface on cabinet front.

233 Frame Doors

Frame doors usually have glass, or a panel, or a combination of glass and grill. Glass or glass and grill are common on china closet and secretary doors. This center is surrounded by a frame, and the whole sub-assembly constitutes the door.

Usually the individual parts of a door frame are made of solid wood instead of plywood. Warpage is just as serious on a frame door as on a flush door. It can be caused by spiral warp of any one frame member, so the frame members should be a species of wood with very little tendency to spiral warp. Gum is likely to give trouble. Frame doors are often quite flexible so that a strong catch can often pull the warp out of such a door when it is closed. But this is still not very satisfactory. The door should be flat and stay flat.

Much trouble can be avoided if extra precautions are followed during the entire manufacturing process. This kind of care would start with the selection of straight-grained lumber that is well kiln dried and equalized. Ripsawyers, moulder and tenoner operators should be taught to lay aside questionable blanks. The little extra cost will be more than offset by fewer problems in the cabinet room and fewer customer complaints.

If the corner joints of a frame door are tenon and groove joints, spiral warp of the door can be caused by tenons not being parallel to the face of the stock being tenoned. The same is true if grooves are not parallel to the face of the stock. These things can happen even if each member of the frame is flat ; mi s-machi ning can cause spi ral warp.

Precision length, width, and 90' corners are just as important in a frame door as in a flush door. Even if parts are accurately machined, it is very difficult to assemble them so that the frame is precise in size and squareness. Consequently, frame doors should be machined and assembled about 112 inch too long and too wide. This permits putting the assembled door over the double-end tenoner for a double rip and a double-trim operation to get accurate dimensions and squareness of the door.

Frame doors involve another problem of precision machining that does not apply to flush doors. Regardless of precision machining of parts of a door frame, when the frame is assembled, the parts will not assemble with flush faces at the door corners. In order to get a frame door flush enough to apply finish, it is necessary to sand the face of the door frame flush. Many shops also sand the back flush. This is generally done on a triple drum or wide belt sander. Such an operation is cheap and really flushes the surface but it leaves cross scratches on some of the rails. The final sanding belt should be 150 or 180 grit so as to leave only very fine sanding scratches.

Joints between members of a frame door may be maae in many ways. Miters are often used in connection with dowels, or splines, or clamp nails to hold the miters together. This makes a nice look ng door, but t is higher in cost than tenon and groove. Figure 111 on the following page illustrates a joint for a mitered and doweled door frame.

234 Figure 111: Door frame with mitered and doweled corner.

Figure IV shows a simple form of tenon and groove joint for a door frame. With this construction, the vertical door stiles and horizontal door rails are all run on the moulder to the cross section shown in Figure IV. It has

I I I I I ----r------I* I I I It

FigureII IV: Tenon and groove for a door frame.

235 a rabbett on the back to receive a piece of glass after the door is finished. The glass can be held in the door frame by a moulding nailed into the rabbett or by special clips. The horizontal rails are routed from the moulder to the double-end tenoner, which cuts finished length and tenons on both ends as illustrated in Figure V.

Figure V: Tenon machined in horizontal rail of door frame.

The vertical stiles are taken from the moulder to the double-end tenoner, which cuts finished length on trim saws and groove with cope spindles resulting in a cut like that in Figure VI. The construction is mechanically strong, and labor cost is very low, but it results in a rather crude looking frame .

Figure VI: Groove machined in vertical stile of door frame.

Figure VI1 shows a minor modification of this construction which adds some decorative effect. The machine operations are the same except that on the horizontal rails the cope heads on the double-end tenoner carry shaped knives which fit around the decorative moulder cut on the vertical stiles.

2 36 I I

Figure VI1 Door frame section showing moulder cut added to improve appearance.

Figure VI11 shows a construction where a panel is to be assembled inside the door frame before f nishing instead of a glass after finishing.

I I I I I------I I

Figure VIII: Door frame fitted with a panel.

237 Many other door frame constructions are possible; the examples cited here simply indicate some possibilities.

A thin, flat panel introduces no serious problem in a frame door. Figure VI11 shows one way to handle construction. There is one caution, however, about processing such a door. Once the panel is assembled into the door, it is impossible to machine sand the faces of the panel. Therefore, it is important to sand the panel ready for finish before assembling into the frame.

Some designs call for a raised panel; a raised panel is one with a shaper cut around the edges. This is popular on Early American and French Provin- cial styles. Two different situations arise with raised panels as shown in Figure IX(A) and (9). In Figure IX(A), the face of the raised panel does not come out as far as the face of the frame, so it can be handled like a flat panel. But in IX(B), the face of the raised panel comes out farther than the face of the frame. This makes it possible to sand the face of the panel after assembly into the door, but it makes it impossible to drum sand the face of the frame after assembly. This is a serious drawback. Frame joints will not assemble flush enough to finish even with precision machining. Consequently, all frames should be sanded on the face after assembly to flush the joints. A drum sander or wide belt is the low cost way to do this. In Figure IX(B), the frame joints have to be sanded flush on a hand block belt sander or some other slow process. Construction like Figure IX(B) is not recommended because of the excess cost of flush sanding the corners of the frame.

I I ------A -----I --n lL 7-\ ,7-" / 1 \ 1 0

Figure IX: Frame door with two types of raised panels.

2 38 Raised door panels are usually made of solid edge-glued lumber instead of plywood because it is difficult to get good appearance of the edges if they are shaped into plywood. If the door panels are solid, it introduces the problem of a panel which tends to shrink and swell surrounded by a frame whose dimensions hold relatively constant. Figure X illustrates such a situation. Enough clearance should be provided between the edge of the panel and the bottom of the groove at A to absorb the maximum amount of swell of the panel in damp weather. No allowance for swelling is necessary at B because the panel does not swell in length, but in width. The panel should be glued into the groove at C but nowhere else. This will hold the panel in proper position in the frame but will still permit movement in swelling to each side. In dry weather, if the panel shrinks, there will be no tendency to tear up the frame or split the panel, but there is the risk of a white line showing at D where the stain did not get on t6 panel. This can be avoided by staining edges of the panel before assembly into the frame. It can also be avoided by machining the frame with a rabbett instead of a groove to receive the panel. The panel is finished separately and put into the frame with moulding like a piece of glass after the whole piece is f ini shed. A A

\ / L-----_-___ J i

D- --D

i-- t - t. Figure X: Construction of raised panel door with clearance for panel to swell across the grain.

2 39 It will be noticed that Figure X shows the edges of the door beveled. Some shops do this whether it is a flush door or a frame door. Figure XI illustrates the reason. If the door is hinged at A, the edges R and C of the door will swing on circles whose center is A when the door is opened. If the clearance between C and the door opening is small, point B will interfere with the door's opening, and the door will not open freely. A beveled edge moves point B back to where it will not bind.

Figure XI: Method of hinging door that requires clearance bevel on 'opposite edge.

Door StoDs and Catches

As is the case with drawer fronts, some provision should be made to stop a door in the proper position to give it a proper appearance with the door closed. As with drawer fronts, a lip door automatically stops itself in the proper position. Where doors shut into the opening, something else is needed for a door stop. Some catches that hold the door shut also act as door stops. Most magnetic catches will do this. Sometimes a wood strip attached to the case serves as a door stop.

Doors generally need some sort of lock or catch to hold them closed. There are two general types of catches. The first type is a positive latch or lock which must be released by turning a key, pushing a button, or some other unlatching action. Figure XI1 shows such a catch. This releases from the front of the door, so it can be used on a single door.

Figure XI11 shows an "elbuw catch.'' An application would be to a secretary desk. The book compartment has two doors. One door is held shut by an elbow catch. The other door has a lock and key which locks it to the first door. To use an elbow catch, it is necessary to be able to reach around behind the door in order to release the elbow catch. This requires two doors in an opening so one door can be opened to give access to the elbow catch on the other door.

240 Figure XII: Door latch.

Figure XIII: Elbow catch.

There is a trend toward wider use of door catches of the second type where the catch does not need to be released but the door can be opened simply by pulling it open. Figure XIV shows three such catches.

Figure XIV: Spring catch (A); Bullet catch (B); and Magnetic catch (C).

241 Hinai nq

Although sliding doors are occasionally used in furniture, most doors swing on hinges. There are a large variety of hinges available from hardware manufacturers. Only a few will be discussed here. The most commonly used hinge is the butt hinge (see Figure XV). They come in a large variety of sizes, different swaging (shown in Figure XVI on the following page), equal and unequal wings, tight pin and removable pin, and even decorative designs for application on the outside of the case as is frequently done on kitchen cabinets.

Figure XV: Butt hinge.

One basic principle applies to nearly all hinge applications; namely, the motion of the part hinged is in a circle where the center is the pin of the hinge. This principle is self-evident and simple, but many product engineers have run into problems because they disregarded it. When a case piece with a door or lid is drawn, the product engineer should check both the closed and the opened position of the door or lid to make sure it will work properly (refer to Figure XVII). 1 1 AIDC LOSE0 C A P, Figure XVII: Illustration of correct (A) and incorrect (B) door mounting.

242 SWAGING OF HINGES

Where it is desirable to have the wings of a hinge close together with a minimum of space between them, a forming operation called "Swaging" is necessary on one or bath wings. This operation slightly increases the hinge width: Exact increases depend on the amount of swaging required.

NOT .SWAGED

Regular manner in which hinge wings are furnished unless otherwise ordered.

BOTH WINGS HALF-SWAGED

The common practice when a minimum of space is desired between wings, is to order hinge with each wing half swaged.

ONE WING HALF-SWAGED-ONE WING FLAT

This method reduces the opening normally present in unswaged hinge about fifty per cent.

ONE WING FULL SWAGED-ONE WING FLAT

This operation reduces distance between wings to a minimum.

DIRECTION OF OPERATlON

Direction of hinge operation is determined from the outside of the door to which the hinge is applied. If, as you foce the outside of a closed door, it is 0 to open to the right, u right hand hinge is required. If it is to open to the left, a left hand hinge is required.

LEFT HAND DOOR

Figure XVI: Terminology relating to swaging of hinges.

243 In (A) the door clears the opening, while in (B) the door jams against the do01 opening at C when the door is opened. The only difference between (A) and (B) is that the center of the pin of the hinge is brought forward in (A) Another example would be the hinging of a leaf on a drop-leaf table. Figure XVIII shows a simple construction which looks good when the leaf is up, but looks rather crude when the leaf is down. Figure XIX(A) shows a construction which looks just as good with the leaf up as it does with the leaf down.

Figure XVIII: Simple drop-leaf construction.

TOP LE IF TOP LEAF

I -7A -"- B

Figure XIX: Right method (A) and wrong method (B) of using knuckle joint in drop-leaf construction.

The knuckle joint shown in Figure XIX(A) is machined so that the curves in both top and leaf are quarter circles with the center being the pin of the hinge. Figure XIX(B) has the same machined joint as (A) but is incorrectly hinged. The wrong location of the hinge pin ruins the appearance when the leaf is down.

A good device for checking hinging is to draw the piece with the door in the closed position, then locate the hinge on the drawing. With a small piece of tracing paper, trace the door, hinge, and opening. Then push one point of a pair of dividers through the tracing at the center of the hinge pin. Next rotate the tracing until the traced door is in the open position. This

244 shows quickly whether the proposed hinging will operate properly. It is surprising how often changes need to be made after this tracing paper test.

Figure XX (on the following page) shows butt hinges which are available with a built-in stop feature which prevents the door from opening more than a pre-determined angle. They have no stop effect on the door in the closed position, only in the opened position of the door. Sometimes the stop feature is helpful to prevent the door's striking the body of the case, which may result in scarring the finish on the door or case. One precaution should be mentioned here. With the door open against the hinge stop, there is a big leverage action such that any pressure on the door is multiplied many times acting to bend the hinge or to pull out the screws which attach the hinge. In some applications this will make the use of this type of stop undesirable.

Shown in Figure XXI are two styles of knife hinges. These are available in a wide variety of sizes and shapes. They are quite widely used on cabinets for televisions and record players. In the finished article, less of the hinge shows than when using the butt hinge.

Figure XXI Knife hinges.

For knife hinges, the basic princip e remains the same. All rotation of the door is in a circle with the center being the pin of the hinge. This principle is basic in hinging, and if it is kept in mind, it permits the product engineer to choose from a tremendous variety of hinges which are available as stock items frcn the hardware manufacturers;

Keeping in mind the above principle, it will be realized that, although selection of the proper hinge is important, it is equally important that the hinge be quite accurately located on both wood parts to which it is attached. For this reason, it is often desirable to machine a cavity to receive and to locate one or both wings of the hinge.

245 -wF-

58-8823 8 58-1703 58-1 705

58-1704 CHART SHOWING VARIOUS DEGREES 58-1710 OF STOP AVAILABLE

Figure XX: Hinges with stops built into knuckle of joint.

246 By referring to Figure XVI, it will be noticed that, if neither wing of a butt hinge is swaged, the thickness of the hinge itself is about 3/16" or more. This introduces a problem which is illustrated in Figure XXII. Figure XXII(A) shows the situation where no hinge seat is cut into either the door or the door opening and the wings of the hinge are mounted on the flat face or edge. The hinge will function properly but there will be a gap of about 3/16" at D between the door and the case when the door is closed. An opening this wide may be unacceptable for appearance sake. If a closer fit is desired, it can be accomplished as in Figure XXII(B) by machining a hinge seat for each wing as at E-E. By controlling the depth of these hinge seats, the opening at D can be made any width desired. Both hinge seats are cut slightly shallower than half the thickness of the hinge. The same result can be accomplished by cutting only one hinge seat instead of two as in Figure XXII(C). In this instance the hinge seat is cut slightly shallower than the full thickness of the hinge.

D D

A B C

Figure XXII: Hinge seats are used to improve appearances.

247 There is one important difference between Figures XXII(8) and XXII(C). In (B), the vertical position of the door with respect to the opening is fixed by the machining of the hinge seats. There is no way to make vertical adjustments. This means that, if machining of parts or assembly of the case deviates from required dimensions, the crack at the top of the door will be a different width from that at the bottom. If the difference affects appearance too much to be acceptable, there is no easy way to make an adjustment. Figure XXII(C) permits vertical adjustment because there is no hinge seat that fixes the location of the hinge on the case. The operation of fitting and hanging doors requires considerable operator skill. Before deciding on how hinge seats should be cut, the product engineer should check with the cabinet room foreman to determine the degree of skills available and the preferred method of hanging doors.

What has been said about hinge seats for butt hinges also applies to an even greater extent to knife hinges because fitting and hanging a door with knife hinges is more difficult than with butt hinges.

In Europe the principal piece of casegoods is the wardrobe, which traditionally is tall and constructed KD (knock down). Hanging doors after the case has been set up in the customer’ s home is a tricky business. This has led to the development of special hinges. Figure XXIII shows such a hinge.

S.€

Figure XXIII: KD hinge.

‘he features generally found on these hinges are as follows:

1. The case end is bored with a shallow hole (A) of 32 mm diameter (1 1 /‘ti’). This hole receives the hinge in a force fit. There are machines on the market that bore the hole -and insert the hinge. Most often the hinge is further anchored with two screws.

2 48 2. The door is pre-bored, and a small pad (B) with ridges is installed with wood screws.

3. The hinge itself has an arm (C) which has corresponding ridges. With a machine screw, the arm (C) is attached to, (B). This attachment can be adjusted by small increments, typically, 0.5 mm (1/50"). Because of the ridges, the seating of (C) on (B) is firm and will slip. This arrangement allows for easy fitting of the doors. An acceptable fit is obtained even when the case is a little out of square.

4. The hinge mechanism (D) is complex and resembles a minia- ture excavation machine. The compound movement allows for doors to be installed entirely in front of the case. Even if the case has four or five doors, these can be installed very close together. They move away from the case before the door swivels very far. This allows several doors to be opened simultaneously . For the furniture and kitchen cabinet industry, hinges have been developed that open almost 90°, llOo, and 170'. These hinges general ly are spring-1 oaded and requi re no catches.

The main drawback of these hinges is their very substantial cost. Several European firms have establ ished supply or manufacturing companies in the USA and the use of these hinges will no doubt increase with time.

One final caution about hinged doors. Many retailers complain about large or heavy doors bending the hinges or pulling out the screws with which they are attached. Be sure to use hinges that are rugged enough to stand up in transport and in service and attach them with screws that are big enough and long enough to stay tight and will not pull out by stripping the thread in the wood.

Lids

Some pieces of furniture, such as secretaries and stereo cabinets, involve hinged lids. Lid problems are much like those already discussed for doors, but there are also a couple of new ones.

On a secretary, the lid holds itself in position when closed. But when open, it is used as a writing bed and needs to be "stopped" in a horizontal position. Furthermore, the user is likely to lean down on it so that the support needs to be fairly strong. Ordinary stop hinges are not very satisfactory, so many shops use "fall Supports." Figure XXIV illustrates an inexpensive type of fall support for use with a round-head screw in the slot; there are several other types.

249 Figure XXIV: Fall support.

On a stereo cabinet, the lid is often horizontal when closed. It is often desirable to have the lid open somewhat less than 90" so that it will not strike or fall against the wall behind it. This means that the lid will not remain in the open position because gravity will cause it to close. One way of solving this is to use supports similar to that shown in Figure XXV, which has a lock notch for the open position. When it is desired to close the lid, these lock notches have to be released by hand. A different approach is to have spring-loaded or friction hardware or both to balance the weight of the lid so that the lid will stay in any position from fully closed to fully open.

Figure XXV: Support with SOCK notch.

2 50 Acknowledgments

Acknowledgment is made to the following organizations through whose courtesy and by whose permission illustrations in this chapter have been reproduced.

Ronthor Reiss Corporation, New York, New York.

National Lock Company, Rockford, Illinois.

251 252 CHAPTER 15

CHAIRS

Few pieces of furniture have as much diversity of appearance as chairs. The diversity in chairs is not confined to surface decoration, but often extends to variations in the basic construction. It would be hopeless to cover all possible variations, however, an understanding of the construction of six types will cover the most important principles of construction. Figures I and I1 (shown on the following two pages) show eight representative designs. The names used are not strictly accurate but will do for identification of the construction types. A review of Chapter 28, "Chair Equipment," from Production Woodworking Equipment' is recommended as a prelude to this chapter.

Elements of Construction

Non-Rectangul ar Structure

Almost all chairs have a non-rectangular structure. In this respect, they differ from casegoods, most of which are basically rectangular in spite of a few curves. This causes a different procedure by the product engineer in working out construction details, making a permanent record of them, and transmitting manufacturing information to the shop. Chapter 10 discussed these procedures for casegoods, which can be summarized as follows:

1. Make an accurate Production Drawing,.

2. Make an accurate Bill of Materials.

3. Establish shoulder measurements for front and end, lay out front post or case end vertically, including drawer and door openings, and calculate other dimensions where possible instead of measuring the drawing.

Since chairs are non-rectangular, the conventional mechanical drawing of an assembled chair does not give true actual size or miter angle of many of the parts. True size of a line is shown in a mechanical drawing only if the line is parallel to the picture plane of a top view, end view, or front view. For this reason, many factories do not bother to make a production drawing of an assembled chair even though they do make such drawings of cases. Designers' drawings of chairs are usually made this way, but they are not intended to give accurate manufacturing information. They show primarily what the chair is supposed to look like when built.

' Rudolph Willard, Production Woodworking Equipment, 4th ed., (Raleigh, NC 27650: N.C. State University, 1980).

253 Box Seat-Dinner A Box Seat-Institutional B

Windsor C Bank of England D

Figure I: Representative designs of chairs.

2 54 Bentwood A Modified Bentwood B

Ladder Back C Contemporary D

Figure 11: Representative designs of chairs.

255 Probably the most commonly used procedure for developing a new design of chair is for the product engineer to study the designer's drawing, make notes, and then turn the designer's drawing over to a sample maker, along with points to watch or modify in making the sample. The sample maker then machines several sets of parts for the chair by cut and try, hand fitting, and general mechanical skill until he has sets of parts that will assemble into the desired chair without additional fitting. These sets of parts are then the basic information for permanent records and also for transmitting manufacturing information to the shop. One set can be kept by the product engineer as his permanent record.

For shop information, a set of the sample parts can accompany each production order. The sample parts, as mentioned in Chapter 10, are often painted red to avoid getting them mixed in with the production and to reduce the chance of losing them. But samples are lost very frequently, and this is one drawback of the procedure. In production, each machine operator sets up to duplicate the sample as nearly as possible. Since many machines that will cut miters have indicators graduated into degrees of angle, it helps set up if the sample has all miters marked with the number of degrees. This is not actually done as often as it should be.

Instead of using sample parts for shop information, some factories make an accurate drawing of each sample part. If this drawing is full size, the machine operator can set up according to the drawing, cut a piece, and lay it right on the drawing to see whether his set up is correct.

Dowel Construct ion

While not universally used, the most common form of joint for chair construction is the dowel joint and its modifications.

One of the big advantages of a dowel joint is that it permits the joining of two wood parts of almost any conceivable shape coming together at almost any conceivable angle. All that is required is to have two matching surfaces, one on each part where they are to join, with enough wood behind these surfaces to bore holes to receive the dowels. The joining surfaces are usually flat. With the dowel joint, two pieces can be joined end to end, end-to edge, end to face, edge to edge, edge to face, or face to face. Except for the matching surfaces (which are usually flat), the rest of either part may be straight or curved, in fact, may be of any shape. The two parts may be parallel to each other, at 90" to each other, or at any other conceivable angle. Since chairs are non-rectangular, many of the parts are not straight and many joints, at odd angles, including compound miter angles, so that dowel joints seem a logical choice.

256 Another advantage of a dowel joint is that it fixes accurately the position of each part with respect to the other. There is only one position in which the parts will go together; that position is determined by the boring for the holes for the dowels. This broad statement is not quite true of a single dowel joint. Figure 111 illustrates this on a chair stretcher doweled to a front post. The position of the stretcher with respect to the front post is fixed accurately in three directions as indicated by the straight arrows.

Figure 111: Chair stretcher joined to front post with single dowel.

The angle of the stretcher with respect to the front post is fixed accurately in two directions as indicated by the curved arrows. But the stretcher itself can rotate on its own axis and be assembled as in Figure III(B) instead of Figure III(A). If the joint were a square mortise and tenon, no such rotation would be possible. If the stretcher were wide enough, a double dowel joint would also prevent such rotation. Figure IV shows such a joint. If either the single dowel or double dowel is off center with respect to the stretcher, it is possible to assemble the stretcher

- Figure IV: Use of doub e dowe in joint.

257 upside down, in which case it will be in the wrong location. Figure V(A) shows the correct position as planned, while Figure V(B) shows the wrong position because the stretcher was assembled upside down. 'd - A Figure V: Double dowels not located symmetrically in the stretcher.

Unless there is good reason otherwise, it pays to center the dowels in order to avoid trouble in case of careless upside down assembly. This is not so important if the stretcher is at a miter angle with the post. Upside-down assembly is very evident, and the assembler will correct it. Figure VI shows this. In spite of these special instances, it is still true that a properly designed dowel joint can give accurate location of two parts with respect to each other.

Figure VI: Stretcher assembled with dowel joint at miter angle with post.

2 58 One limitation of dowel joints is that they require certain dimensions to be held within close tolerances. But this is also true of many other joints, such as mortise and tenon. In a multiple dowel joint, the following dimensions must be precise:

Diameter of holes Diameter of dowels Distance between centers of dowel holes.

If the diameter of the dowel is much smaller than that of the hole for it, a weak joint wi11 result because of poor wood-to-wood contact between dowel and hole. Glue bonds are strongest with positive wood-to-wood contact. If the diameter of the dowel is larger than the hole, there is a danger of splitting the part when the dowel is driven. There is also the danger of a starved glue joint because the big dowel wipes most of the glue off the contact surface when the dowel is driven. If the distance between centers of a double dowel joint is different in one part from that of the other part to which it is to be joined, the joint will not assemble.

Apart from center-to-center distance, the location of dowel holes in parts is not so critical. If the location of the hole in the post (Figure 111) were off r/16", the parts would still assemble, and no one would know the difference.

If the depth of the holes for a dowel is too small, there will be trouble. Assume a dowel two inches long with half its length to go into each piece. If the holes in the pieces are 15/16" deep, there would be a 1/8" crack between the parts when the dowel hit the bottom of both holes. But if the holes were 1 l/8" deep, the parts would assemble tight, and no one would know the difference. This means that the depth of the hole for a dowel can vary beyond the minimum depth. It is customary to specify holes 1/16'1 deeper than theoretically needed to provide for proper assemble in spite of small manufacturing variations in the depth of hole or in the length of dowel. This is another example of intentional allowances for manufacturing variations as was discussed in Chapter 9.

If one part is too narrow to use a double dowel and there is room for only a single dowel, it is frequently better to machine a round tenon on the end of the part itself than to bore for a separate dowel. The round tenon is of particular advantage where there is considerable miter on the end of the part. Figure VI1 shows a narrow stretcher designed for joining to a back

Figure VI1 : Use of dowel or machined-round tenon to join stretcher to back post.

2 59 post. In (A) the stretcher is mitered, then bored, then the dowel is inserted. In (5) a round tenon is machined right out of stock of the stretcher. This is done with a chucking machine, which is described in Chapter 28 of Production Woodworking Equipment. The chucking cutter not only cuts the tenon but also cuts the shoulder square to the tenon. Thus, one operation replaces three. In addition, the chucked tenon is stronger because boring for the dowel as in (A) permits only a short length of dowel in the hole, and even at that the hole comes close to the surface of the stretcher. Usually, chucked tenons are of larger diameter than separate dowels. This offsets the fact that the stretcher may be gum, which is not as strong as birch, beech, or maple, which are customary species used for making dowels.

As dowel joints are generally used, most of the external forces tending to break the joint are in such a direction that they try to pull the dowel out of the hole. Such a force is resisted by the glue bond between the dowel and the hole. In theory, this resistance can be calculated by multiplying the number of square inches of glue line by the strength of the glue bond in pounds per square inch. The glue line is the surface of a cylinder, so its area would be d x 1 where d is the diameter of the dowel and 1 is the distance it goes into the hole, both expressed in inches. The Wood Handbook2 published the strength of glue bonds. But these published figures are based on results of the standard glue shear test which simulates the gluing procedure in edge gluing. As discussed in Chapter 7, the gluing procedure in dowel joints results in a weaker glue bond due to grain orientation, wiping action, lack of control of gluing pressure and of thickness of glue line. No reliable figures are available for glue bond strength in order to accurately calculate the load-carrying capacity of a specific dowel joint. But certain general conclusions seem justifiable relative to the resistance of a dowel to pulling out of a hole.

1. Glue line area is proportional to dowel diameter, therefore, increased diameter should increase joint strength, probably in approximate proportion to diameter.

2. Glue line area is proportional to length of the dowel which goes into one hole, therefore, increase in this length should increase joint strength, probably in approximate proportion to length.

3. If the external force is not parallel to the dowel but at an angle to it, this force can be resolved into two components. One component can be parallel to the dowel and the other component, perpendicular to it. Both components are smaller than the total force. The component parallel to the dowel is the only one trying to pull the dowel out.

2 U.S. Department of Agriculture, Forest Service, Wood Handbook, by Forest Products Laboratory, Agriculture Handbook Noe 72, Rev, ed, (Washington, DC. 20302: GPO, 1974).

260 The other component tries to crush or shear the dowel. In an extreme situation where the external force is perpendicular to the dowel, there is no component parallel to the dowel. Since no force is trying to pull the dowel out, there is not stress on the glue bond. This is the situation where a lock dowel is used to lock a mortise and tenon joint.

4. Experiments carried out at N.C. State University indicate that it is likely that glue bond strength in a dowel joint is much lower than the handbook figures based on standard shear test, probably not over half and sometimes much less.

In dowel construction, it is customary to bore the holes in both parts at right angles to the face where these parts join. This insures that the axes of both holes will line up in a straight line. It also makes for easier clamping than if the holes were at some angle other than a right angle. A minor advantage is that it allows the boring bit to approach the stock at right angles. Several kinds of "trim and boring" machines are on the market which by their very nature insure that the angle of the dowel hole to the joint is always 90"

Some rough rules of thumb which are used for dowel sizes follow:

1. The dowel diameter should be not much over half the thickness of the thinnest piece into which it is driven. If the side seat rail of a chair is 3/4" thick, use 3/8" dowel or maybe 7/16". If the dowel is much bigger, there is a danger of splitting the rail when the dowel is driven. This rule is sometimes violated in case of chucked round tenons to get a stronger tenon, especially if the tenon is cross-grained because of being chucked on a miter.

2. The distance which a dowel goes into a part should not be less than twice the diameter of the dowel. Thus, a dowel 3/8" in diameter should go into each part not less than 3/4" which means that the length of the dowel should not be less than 1 1/2". If the dowel can be longer, the joint will be stronger. Sometimes there is not enough wood available to follow this rule. In this situation, make the dowel as long as possible.

3. The distance between centers for double dowel joint should be not less than the center spacing to which your particular two spindle boring machine can be set. It is much better to bore for double dowel with a two-spindle machine or a fixed-center cluster head attachment. Center distance is more accurate, and the time consumed in boring is 1 ess.

261 Most chair factories buy their dowels from dowel manufacturers instead of trying to make them. They can be bought to specified diameter and cut to specified length. Most cut-to-length dowels have a small chamfer on each end to facilitate starting the dowel into the hole driving it. This chamfer is usually about 1/16" so that a 1 1/2" dowel has a glue bond surface about 1 3/8" long instead of the full 1 1 /2 I' length of the dowel. Many dowels are made with a small spiral groove running around the surface. This is supposed to help distribute glue over the whole surface of the dowel. There are differences of opinion as to how effective spiral grooves are, but they do no harm and may do some good.

A superior alternative to the dowel joint for the critical side-rail-to-back post joint is mortise and tenon joint. Several makes of single-end tenoners exist that Droduce a tenon that resembles a chuck. It has a straight section in the middle and two half-cylinder shapes at the end. The matching mortise is cut on an oscillating router.

Figure VIII: Tenon for chair side rails.

The main advantage of this joint is that it incorporates several times more side-grain-to-side-grain blue line surface than a pair of dowels. The joint should be made with the A dimension of the tenon a few thousandths oversize and the B dimension of the tenon undersized a few thousandths for maximum strength.

Shrink, Swell and Warp

Except for chairs with a wood seat, most chair parts are narrow enough that shrink and swell are so small that no allowance need be made for them in laying out the construction details of a chair. But this is not true of warpage, especially spiral warp. If the front raiis of a case have spiral warp, the case construction is usually heavy enough that it will straighten the rails when the front rail tenons are driven into the mortises of the case ends. But in a chair, spiral warp of one part, such as a back seat rail is likely to distort the shape of the whole assembled chair. Spiral warp also often causes parts not to fit tightly at the joints even if the parts are correct-ly sized and mitered. If the parts are forced together by clamping, they are likely to spring apart when the clamp is released because the

262 the friction of the dowel joints is not enough to overcome the pull of the warped parts. Many chairs are made of gum or some species susceptible to warpage without any special precautions to remove or reduce the warpage.

Even with flat parts of correct size and shape, assembled chairs will not sit evenly on a level floor but will rest on three legs with the fourth leg off the floor anywhere from l/3.2" to 1/4" or even more. To cure this, most chair shops have a special machine called a "leveling saw." This machine cuts one long leg just the right amount so that all four legs make contact with flat floor instead of the chair's resting on three legs. It is a fast, low-cost operation. It requires no special provisions in the construction of a chair.

Comfort

While everyone admits that comfort should be an important requirement of every chair, there is wide disagreement about what constitutes comfort and how to get it. This problem is usually given to the furniture designer, not the product engineer. Yet some discussion of comfort is in order because of its importance. It is generally agreed that comfort is affected by several factors :

1. Desired posture. A chair which is comfortable when sitting and working at a desk would not be comfortable when used as a lounge chair or vice versa.

2. Height of the front of the seat from the floor makes a difference.

3. Pitch of the seat from front to back. A horizontal seat is seldom comfortable. It should pitch down toward the back.

4. Angle between the seat and back is important.

5. None of the above factors is independent. There seems to be an interrelationship among all. If one is changed, it will probably require changes in the others to achieve maxi mum comfort .

Considerable research work has been done here and abroad on comfort in chairs, but results do not agree too well with each other and apparently have not been widely accepted by chair manufacturers. Dr. Clara A. Ridder did extensive research, the results of which were published in 1959 under the title "Basic Design Measurements for Sitting."3 With the University of Arkansas' permission, quotatiofis in this chapter are from that bulletin.

3Agricultural Experiment Station. October, 1959. "Basic Design Measurements for Sitting," Bulletin 616. University of Arkansas, Fayetteville, Arkansas.

263 "Basic design measurements including the variations in size needed to support adults in common types of sitting postures were sought in this study.

The purpose was threefold: to uncover

1. The Basic Design Measurements for Sitting.

Such measurements are heights, depths, widths, angles, and curves of seats and backs of sitting devices; the placement of arm rests; the heights of tables where required as part of a seated activity; and the measurements for footstools when desired for a seated position.

All such basic design measurements were to be applicable to chairs, sofas, or benches regardless of type of materials or construction used and regardless of style or design.

2. The Basic Types of Sitting Positions.

Since a comfortable seated position differs with type of activity, basic design measurements were to be determined for common types of seated activities. The activities chosen were dining, writing, card playing, talking, and relaxing. Whether or not each of these activities requires different chairs for comfort should be indicated by the basic design measurements uncovered for each.

3. The Basic Sizes for Each of the Types of Sitting Positions.

Since a comfortable position for one person is not necessarily a comfortable position for another, it was a further purpose of the study to determine the variations in the basic design measurements selected for each type of sitting activity in order to establish a basis for the recommendation of sizes where needed.

In other words, the purpose was to uncover a single best size and shape of chair for the basic measurements and for the various types of sitting positions preferred by adult men and wcmen and the needed variations from this single best size I'i a good fit were to be obtainable by the majority.*

*This study was made in conjunction with a search conducted under Southern Regional Project S-8."

264 The basic design measurements for sitting were obtained by constructing an ingenious chair frame with a large number of plungers set close enough to each other that the faces of the plungers gave nearly a continuous surface for seat and for back. Each plunger was spring loaded with light pressure so that all would conform to the body of a person sitting in the chair. Heights and angles of seat and back were adjustable. One hundred and sixty- two people were selected by statistical procedures to be a valid sample of the adult population. Body measurements were made on these people, and as each person sat in the test chair, it was adjusted so as to give maximum comfort for that particular person.

Of the basic types of sitting positions, Dr. Ridder says:

"The Sitt ing Activities

Of the numerous activities for which seats are usually used, five were included in this study. These are (1) dining, (2) writing, (3) playing table games, (4) talking, and (5) re- 1 axing.

The same chair is often used for dining, writing, and games in American homes. However, the three were included in order to determine whether or not the subjects would select similar shapes for these three activities if given freedom of choice.

The talking position was explained to each subject as one in which he or she could easily see another seated person or perhaps a TV picture--a sort of 'sit up and talk' position.

The position that was most difficult to explain was the relaxing position. Some of the subjects seemed to think of this chair as a napping chair. The aim was not to determine the measurements for a lounging chair but rather for a comfortable 'easy' chair. The subject was told to think of the chair as one in which he might enjoy reading the paper or watching TV. It was to be the 'easiest' chair he would want in his living room but not a napping chair or lounging chair.''

Results as to sizes and shapes recommended for the three sitting positions are best summarized by diagrams from Builetin 616.

Figure IX is for dining, writing, and games. Figure X is for talking, viewing, and listening. Figure XI is for relaxing and reading. Figure XI1 compares the three shapes.

265 Figures IX through XI1 all show curved surfaces which would not be economically justifiable for low-priced dining chairs, but it is probable that straight lines which approximated the curves would give better comfort than other straight lines which deviated considerably from them.

I ’ Desired but I” not essentiol

Toble or desk

Shape of bench bock Arm rest ond of choir back T Iot center

chair bock

urve for moulded seot, 3.8 inches each side of center,; or curve for entire length of bench; or curve to be obtained with firm support when using spring or cushion majority ond construction for general use

Seot height for +individuals toller thon averoge

SCAIE V t w - Floor levels

Figure IX: Basic design measurements for chairs for dining, writing, or playing table games.

2 66 I I

hard bench bock. ond rhepe cushioned choir or rofo bock should maimtoin with firm ruppOr( Am rut T reo may be left free. If ncluded shdd DYIVC bock i Ot Ieort 01 far as dotted line Side of chair seer 1 t

twwe for moulded seat 5.8 inches eorh rids of center: or rurv;f~r entire length Sed height for of bench; or curve to be obtained with individuoli toller firm SUPPO~when using spring or cushion than overmg* Constwetiom

ri i

-.&iZ4''1- 1 flwr Ie*eI. 1

Figure X: Basic design measurements of chairs for talking, viewing, or listening.

267 I I

I //

ll -4'1-

Figure XI: Basic design measurements of chairs for relaxing or reading.

268 Figure XII: Relative shapes in space of three main types of support desired by adults for sitting.

People, of course, have very diverse sizes, one from the other; and it might seem hopeless for any one chair to be comfortable for all sizes of people. On this situation, Dr. Ridder comments as follows:

"Number of Sizes Recommended

One size is recommended for each of the three types of sitting positions as the best overall size for the majority of adults and for public or general use.

A second size for each of the three types of sitting positions is also recommended for adults of greater than average stature and for individual use.

A third size is NOT recommended for those of shorter than average statlire because the "vest over-all size (with its seat height and depth shorter than in present-day chairs) will fit the majority of short individuals very well indeed. Further- more, short individuals tend to choose other than the lowest seats when tall people are to be seated in the same room, probably in order to be able to communicate on a more equal 1 eve1 .

269 Small deviations from the recommended heights and depths and measurements which fall between the two recommended sizes would prove to be comfortable for a large number of people. The two definite heights for each type of chair, bench, or sofa are recommended as a general guide. The shapes of the chair seats and the chair backs and their relationships to each other in space are more important for real comfort in sitting than small deviations from preferred seat height.

It seems rather obvious that chairs, on the whole, should be designed and purchased for people rather than for men or for women according to body size because most chairs are used by a number of different people. However, some ethnic groups and some families tend to be either shorter or taller than average. Then, too, when a chair is to be used mainly by one individual , as an office desk chair and sometimes a reading or relaxing chair, it should fit that individual. Even though the majority who work in offices do not have the opportunity to select or to purchase a desk chair, along with a desk height, one individual often uses a given chair and a given desk for a number of years. Providing a choice between two sizes of chairs and two heights of desks would permit most people to work at a desk in a comfortable position."

Box Seat Chairs

Figure I(5) shows a typical box seat chair. Joints are general y dowel joints. We have already discussed in this chapter which d mensi ons need to be held to c ose tolerances in dowel construction. In box seat cha rs, the shape of the front edge of the back posts must conform closely to desired design. If this shape varies from what it is supposed to be, the side seat rails which are doweled to it will not assemble at the correct angle. In addition, the side stretchers will be either too long or too short to assemble properly because the whole chair is out of shape. Most shops get the proper contour of the front edge of the back posts by shaping them on a form which gives precise shape. The operation can be done on either a hand shaper or an automatic shaper.

Back Posts

The back posts on a box seat chair run the who1 e height of the chai r and are the basic structural unit. They serve the same function in the chair as the spine does in the human body.

Back posts shobld be as thick as can be dressed cleaii f~omthe thickness of rough lumber which is used. Many low-priced chairs use 4/4 lumber for back posts, but it is generally felt that using 5/4 results in a better chair though the cost is a little higher. One of the problems of chair construction is to have enough wood at the critical points to make a strong joint.

270 The most critical point is generally where the side seat rail and back seat rail join to the back post. The additional wood in the 5/4 post yields stronger construction throughout. Should the chair be tilted by the person seated on it, or dropped in a way that will tend to break the back legs, the extra strength will provide a greater margin against breakage or other damage.

Two methods of making back posts are widely used, and a third method is occasionally used.

In household chairs, the back posts are usually bandsawn from a wide blank which has been edge glued and planed on two sides. Many designs of back posts nest fairly well as shown in Figure XII. Post blanks are rough bandsawn from the wide blank. Final configuration of back post is shaped from each bandsawn blank using a shaper form. The shaded areas shown in Figure XI11 are waste. If the wide blank were 38" x 18" x 1 l/16" and made five post blanks, the product engineer would bill the back posts for one chair as follows:

Back Post 38 x 18 x 1 l/16. Makes 5. Shape to pattern.

For 100 chairs (200 back posts), 40 wide blanks would be required.

Figure XIII: Pattern for sawing back posts from a wide blank.

Some shops would incur extra labor but reduce waste by the procedure shown in Figure XIV. The wide blank has a glue joint edge machined at D and E and is bandsawn once as in Figure XIII(A). The two parts are then glued back together as in Figure XIII(B). The panel is then planed two sides and bandsawn into back post blanks as in Figure XIII(C). There is no waste material

271 -\ \

E D

-J A C

Figure XIV: Method of eliminating waste material in sawing post blanks from panel.

except that for the glue joint, provided the original width of the panel is an even multiple of the post blank width. Five blanks are shown in Figure XIV(C). With high-priced lumber such as walnut, the saving in material waste would probably more than offset the labor cost of the two extra operations (first bandsaw and then re-glue).

The use of tongue and groove glue joint for edge gluing back posts gives a poorer appearance than a flat joint. The bandsaw cut goes through the glue joint at such an angle that the tongue and groove look very much elongated and would show noticeably on the front and back edges of the back post.

A few shops object to the appearance of any glue joints in back posts. They avoid glue joints by marking full-length boards with a bandsaw pattern, nesting the posts around defects the best they can. In such cases, the board is not cut to rough length at the cut-off saw but is a full-length board (or maybe half length). Lumber waste is considerably higher than with edge-glued posts. Occasionally this procedure is used for fear of glue joint failure instead of for appearance reasons. But if glue joints are properly made, strength should be satisfactory with edge-glued posts.

For institutional and office chairs, the back posts are often steam bent from individual blanks the proper width for one back post. This is a higher cost method, but tradition in sales has been built up strongly for bent posts. It appears that with stock of suitable width and thickness, bandsawn back posts have adequate strength, ifthe curvature is not severe.

272 Joint Between Side Seat Rail and Back Post

This joint is probably the most important one in a box seat chair. If people never tilted back when sitting in a chair, the external forces trying to break this joint would be no worse than for other joints in the chair. But some persons will tilt back, and this fact may just as well be recognized.

I L15" I

Figure XV: Diagram of a chair joint where the side seat rail joins the back post.

If a downward load is applied on the seat while all the chair legs are resting on the floor, approximately a quarter of the load will try to shear the dowels of one joint. Against shear, the dowels have high strength. But if the chair is tilted back, the situation changes. Approximately half the load can be considered concentrated about 8 inches out from the back post. This will give a leverage action trying to rotate the side seat rail around a fulcrum point at B or E or in between. Rotation is resisted chiefly by the top dowel, but the load now tries to pull the dowel out of the hole instead of trying to shear it. Consequently, most of the strength of the joint depends on the glue bond between the top dowel and the back post. Several ways to increase the strength of this glue bond are apparent.

1. Increase square inches of glue line by increasing diameter of the dowel. But then there is a danger of splitting the seat rail when the dowel is driven.

2. Increase square inches of glue line by increasing length of the dowel. This is frequently practicable and probably should be done more often than it is.

273 3. Increase distance BD or ED. Increasing the length of this lever arm reduces the force exerted on the top dowel. This would require a wider seat rail. Seat rails should probably be made as wide as appearance will permit, and dowels should be as far apart as possible.

4. Do a better job of gluing. Most shops apply glue in the hole but not on the dowel. Operators frequently do not get glue spread onto all parts of the hole. Dowel driving machines simply squirt glue into the hole. The wiping action of the dowel going into the hole removes some of the glue even where it has been spread.

Although a double dowel, illustrated in Figure XV, is the sort of construction used most frequently for this critical joint, the mortise and tenon joint described on page 262 is. stronger. Alternately, constructions might be worked out so that a large part of the force would be resisted by wood structure instead of depending almost wholly on strength of the glue bond. It is desirable, of course, that an alternate construction should not cause much increase in cost, preferably no increase.

One other factor is probably important in the durability of this joint in service. In a home there will be a large variation of humidity between summer and winter. Several cycles between high and low moisture content of the wood will cause shrink and swell of the wood. The amount may be too small to be visible but still large enough to break or weaken a glue bond.

For appearance reasons, some chairs are designed in such a way that it is difficult, uncomfortable, or impossible to tilt back when sitting in them. Figure XVI illustrates this. If a vertical line were drawn from the back edge of the seat to the floor, assume that it hit the floor at point 6. If the distance between this point and the bottom of the back post (AB) is small, it will be easy to tilt back in the chair. If this distance is large, it is difficult to tilt back. No precise figures are available, but it seems probable that, if distance AB is 3 inches or less, the chair will tilt easily. If AB is 5 inches or more, it will be very difficult to tilt. If a chair will not encounter frequent tilting back in ordinary service, it does not need as strong a joint from side seat rail to back post as it would if tilted frequently.

2 74 I I I I I I A B

Figure XVI: Design of chair back post to discourage user from tilting chair.

Sometimes the curvature of the back post is severe enough that the top back is forward of the vertical plane through A. Such chairs are sometimes marketed as "wall -saver" models, for obvious reasons.

Corner B1 ocks

Most box seat chairs have four corner blocks. These blocks are fastened to the side seat rail and the back seat rail in the two back corners and from the side seat rail to the front seat rail in the two front corners. In the finished chair, they are not exposed parts because they are hidden by the seat and the seat rails. But they are important parts of a chair. Their functions are all or some of the following:

1. Brace the seat frame of the chair against distortion of shape.

2. Help in attaching slip seat by using screws through corner blocks into the plywood of the slip seat. This can also be done with solid wood seats although some shops prefer to attach the wood seat to the seat rails by screws.

3, Reinforce the critical joint between side seat rail and back post. This function seems a bit illogical. It would seem that the joint itself should be strong enough to stand up, but corner blocks would be a big factor in holding the chair together should the side rail-back post joint ever fai I.

275 Two forms of corner blocks are widely used. The most prevalent form is where the corner blocks are mitered and bored for screws. They are applied while the chair is in the clamp for final assembly and, being in the clamp, the seat frame is in the proper shape. Corner blocks are glued and screwed to the seat rails. The chair can then be removed from the clamp, and the shape of the seat frame will not distort because the corner blocks hold it.

The other form of corner block is also mitered, but instead of being bored for screws, it has multiple tenons cut on the mitered ends which fit into multiple grooves in the inside faces of the seat rails. It is applied while the chair is in the final assembly clamp. Both ends of the corner block are dipped in glue, and the corner block is driven into place a with hammer. Figure XVI illustrates this construction.

There are differences of opinion as to which is the better corner block, but the following comparisons are probably valid:

1. The multiple tenon block takes less time to apply and probably no greater time to machine since the multiple grooves in the seat rails can be run on the moulder. Some shops have special corner block machines which will machine the complete corner block from random length blanks if the blanks have been run to finished width and thickness. Such machines will make either type of corner block at a fraction of the labor cost of separate machining operations on ordinary machines.

2. Many shops machine the multiple tenon blocks to a 45- degree miter in spite of the fact that the chair seats do not have 90-degree angles. If the grooves are deep enough, there is considerable area of glue bond even though the miters are not an exact fit. This permits one standard glue block which fits front and back and fits different designs of chair seats. With glued and screwed corner blocks, the glue does almost no good unless the miters on the corner blocks fit the shape of the seat frame . 3. For the same reasons as in (2), the multiple tenon corner block is more foolproof in assembling. With glued and screwed blocks the glue does no good unless surfaces are actually in contact. This means that even though the blocks are accurately machined to the proper miters some care is needed when driving screws to attach the blocks. It is very easy to slip the block out of position when driving screws with the result that one end of the block hits the seat rail on the heel of the block and the other end hits the other seat rail on the toe of the block with no large area of wood contact for the glue to hold.

4. The multiple grooves in the seat rails leave less solid wood to bore for dowels. The result is either a smaller dowel or holes so close to the face that danger of splitting is increased.

276 5. When a multiple-tenon corner block fails under stress it fails completely and thereafter has no strength at all. It is either 100% tight or 100% loose. But screws fail gradually. If the glue bond fails with a screwed block the screws still retain considerable holding power as they gradually strip the thread in the wood and gradually pull out.

6. Of the two, the glued and screwed corner block is probably the strongest if the block itself is of adequate width and thickness, if the miters are accurately cut, and if the block is carefully applied. A third corner block has been developed by E. L. Clark at NCSU which may answer the problems inherent in the commonly used diagonal corner blocks. The NCSU corner block uses the principle of box joint. Figure XVII(A) shows the two short blocks prior to being positioned. Figure XVII(6) shows the blocks in the corner formed by the side rail and the back rail. Glue would be applied to the fingers of the joint and also to the faces that contact the rails. Staples or screws would hold the blocks in place while the glue line cures. The advantages of this new concept are as follows:

1. The 3/4 'I x /4 'I fingers result in a side-grain-to-side- grain gluing surface which is over four times that of the two dowels. There is also a mechanical advantage because the wood structure itself resists the forces trying to break this critical joint.

2. The side-grain gluing between the blocks and the rails results in a much stronger bond than the semi-end-grain gluing of diagonal blocks.

3. The NCSU block is described as "universal" because it ad- justs itself to any angle. There is no need for a precision-cut diagonal corner block.

4.. The adhered surfaces are brought close to the critical joint area so that torsional stresses are immediately transferred to the back rail rather than into the dowel joint.

5. There is no need for multiple grooves in the seat rails as required by the groove and tenon block. More wood remains in. the seat rails, so it may be possible to use 4/4 lumber rather than 514 for these components.

6. The corner block is very inexpensive because it is made of short pieces (7'' to 8") of 4/4 scrap. The box joint is machined on each end and then cut in half to make the two parts.

277 Figure XVI: Corner blocks with multiple tenons to match multiple grooves cut in side rails.

Figure XVII: NCSU corner block.

2 78 Stretchers

Some designs, such as Duncan Phyfe and certain French styles, do not permit stretchers. But if appearance permits, stretchers substantially improve the strength of a chair. Side stretchers distribute a large part of the load on the critical joint between side seat rail and back post so that the stretcher joints help resist the load when the chair is tilted. Stretchers between back posts strengthen the posts against sideways breakage when the chair is dropped or when someone tilts sideways in the chair.

Narrow stretchers generally use a chucked round tenon on the ends instead of boring for separate dowel.

Arms

Most dining chairs are sold in sets, often four side chairs and two arm chairs. The arm chair is frequently exactly like the side chair except that it has a slightly larger seat size and arm stumps and arms attached after the chair is assembled. In such a construction, the arm and arm stump are sub-assembled. The sub-assembly is then attached to the chair by fastening the back end of the arm to the back post and the bottom end of the arm stump to the side seat rail. The method of attachment is usually glue and screw. With such a chair, a person sitting in it sometimes pushes out against the arms in rising or sitting down. This exerts an unfavorable leverage on the joint between the arm stump and the side seat rail, so this joint should be made strong to resist such treatment.

Occasionally, arm chairs are designed so that the front post projects up above the level of the seat to the proper height to support the front end of the arm. In this situation, the front post replaces an arm stump. This construction makes a much stronger chair as the arm becomes another "stretcher." The side-rail -to-back-post joint is then reinforced one more way.

Assembly Procedure

As with case pieces, it is found that box seat chairs assemble cheapest if constructed for as many sub-assemblies as possible. Typical steps in assembly of the box seat institutional chair shown in Figure I would be as f 01 1ows:

1. Pre-pin dowels into the ends of all rails involved in dowel joints. This is frequently done in the machine room before sanding the parts.

2. Sub-assemble top and middle back rails with vertical back slats. Frequently these joints are not glued in order to be sure that dowels in the ends of back rails can be moved to match properly with holes in the back posts for these dowel s . 3. Using sub-assembly (2) , sub-assemble the chair back with back posts and back stretchers.

2 79 4. Sub-assemble front posts and front seat rail.

5. Sub-assemble stretchers. Sub-assembl ies (3) and (4) can be done in a flat horizontal clamp which is convenient and invol ves small investment.

6. Final assembly. Use sub-assemblies (3), (4), and (5) and add side seat rails. While assembled chair is in the clamp, attach corner blocks.

7. Attach seat.

8. Put assembled chair over leveling saw to level bottom ends of legs.

Seats

Most dining chairs have slip seats. Institutional chairs often have solid wood seats that are saddle cut on the top face for more comfort.

A typical slip seat has a base panel of 3/8'1 plywood cut to the shape of the seat frame and about 1/4" smaller all around the edge. The edges of the panel are often shaped with a quarter round on all edges of the top face of the panel. Above this panel is a layer of upholstery padding material with possibly a layer of cotton batting above it. The cover fabric is laid on top, lapped around the edges and stapled or tacked to the bottom face of the base panel. In practice, the slip seat is often built upside down in a form. This procedure is convenient and results in lower labor cost.

If a slip seat were attached to the chair before finishing, the finishing materials would ruin the cover fabric of the slip seat. So slip seats are attached after finishing. Inventories of dining room chairs are often seat- less, as the customer can specify many different fabrics. For this reason, one often finds a slip seat department inside the shipping department.

Since wood seats must be finished, they are attached to the chair before finishing and finished along with the rest of the chair.

Solid wood seats, being wide, are likely to shrink and swell, so the product engineer should consider this. Wood seats are generally attached by running screws up through the corner blocks into the seat or up through the seat rails as an alternate method of construction. It is good practice to bore holes for these screws bigger than the diameter of the screw in order to permit some horizontal movement of the seat before the screw binds against the hole.

Sometimes wood seats are designed to project over the back seat rail and between the back posts. Figure XVIII shows such a situation. Clearance should be planned at A-A between the seat and the back posts. Otherwise there is a danger that, when the seat swells, it will pry the back posts apart and loosen the joints between back seat rail and back posts.

280 A A

Figure XVIII: Rear section of wood seat designed to fit between the back posts with clearance for expansion.

Windsor Chai rs

While the term "Windsor" applies strictly to an appearance style, this style requires a construction entirely different from that of a box seat chair, and for the present discussion, all chairs of such construction will be referred to as "Windsor" even though not of that appearance style. (Some people use the term "hard seat chairs" to describe this category.)

Just as the back posts are the basic construction feature of a box seat chair, the wood seat is the basic construction feature of a Windsor chair.

The legs have round tenons turned or chucked on the top end, and these round tenons fit into holes bored in the bottom of the seat. This joint is the one that determines the rigidity of the chair when tilting in it. It will be seen in Figure XIX that very large forces are developed at the joint B due to leverage when the chair is tilted. Force at A has a leverage AB/BC against the joint. The leverage does not try to pull the tenon out of the hole as in a box seat chair, but it does try to crush the wood in the tenon and also to break the leg at B. The stretchers help distribute part of the effect of this force-to the front legs, and the direction of the force is not at right angles to AB. All this helps, but it is still true that the joint between the back leg and the seat is a critical joint. If the seat is thicker, distance BC can be increased. This lessens the leverage action and also permits more square inches of glue line, resulting in a stronger joint. Experience has shown that it is very difficult to get this joint strong enough to be satisfactory with a 4/4 seat; 5/4 is about minimum for acceptable strength, and 8/4 permits a sturdy and rugged construction.

281 Some of the old-time furniture makers had an excellent construction for this joint, but it took considerable extra labor time and is seldom used now. They bored the hole for the tenon through the seat. The round tenon was split with a bandsaw cut parallel to the axis of the tenon. They glued hole and tenon, drove the tenon through the seat, then drove a glued wedge into the bandsaw cut which spreads the tenon very tightly to the hole. After the glue had dried, they chiseled and sanded the wedge and tenon until it was flush with the top face of the seat. It was impossible to get this joint apart and very difficult to even loosen.

Figure XIX: Construction for attaching legs to seat of a Windsor-sty1 e chai r.

Some shops call the top back posts above the seat "pillars." The joint between the pillars and the seat is the same kind of joint as between the bottom back posts and seat. It is also subject to large stresses because of the leverage action from a force pushing against the chair back, such as occur when a person exerts pressure in leaning back. But with the pillars, there is an easy way to get extra strength. Figure XX illustrates this. n

Figure XX: Outline of Windsor chair seat and triangle formed by back structure.

282 The back edge of the seat can be curved, and holes can be bored parallel to this curve for the bottom ends of the vertical spindles. In this way, the whole back structure forms a triangle ABC in a side view of the chair. Without the triangle, any force backwards on the chair back would try to break off the pillar at A. But with the triangle, the force is distributed; and a very strong, rigid structure results. The greater the distance AC, the greater the strength and rigidity of the back structure.

Bank of England Chai rs

Here again, as with the Windsor chair, "Bank of England" is the name of an appearance style shown in Figure I(D), but it involves a unique construction which is different from the box seat or Windsor type of construction. This chair consists of a top unit and a bottom unit. Each unit is assembled complete. Then the units are joined by screws to make the complete chair.

The top unit consists of the seat, back, and seat aprons (if any), or a sub- frame under the seat (if one is used). No particularly new problems of construction are involved which have not been discussed except for the steam bending process required.

The bottom unit involves legs, bottom stretchers and top stretchers. This permits assembly of the bottom unit into a structure which is rigid and strong of itself before it is attached by screws to the bottom face of the seat. Once assembled to the seat, it is naturally even more rigid.

The conventional office swivel chair is a modification of this two-unit construction, but the swivel base is attached to the top unit by steel hardware instead of being joined directly to it with screws.

Frequently chairs of these types involve joints between parts with different grain orientation joined in such a way that one or more surfaces are flush at the joint. Differences in shrink or swell of the two parts may be too small to affect structural strength but still great enough to break the finish film. The break in the finish and the small change in surface height are noticeable to the touch and/or the eye. There is no pat answer to this problem, but it can be a serious one. Each situation must be studied as an indi vidual case.

Ladder Back Chairs

The term "ladder back" is the name of a style rather than a type of construction, but that name will be used here to indicate a construction. An example is shown in Figure II(C). As with the box seat chair, the frame of the ladder back chair is a strtietLiie by itself, and the seat is added to this structure after assembly of the chair frame. Posts are joined together by stretchers that are generally turnings or large dowels. The stretchers have a round tenon at each end. These tenons are either turned or chucked, but if the stretchers are dowels as in Figure II(C), the dowel itself can serve as a round tenon. Round tenons fit into holes bored in the posts. The four top stretchers constitute the seat frame. In Figure II(C), the

2 83 seat itself is a sub-assembly of wood slats and cleats. Oftentimes the seat will be hand woven around the top stretchers using rush or a paper substitute.

When a ladder back chair is tilted back, the forces on the joints are quite different from a box seat chair. With a box seat chair, the downward force on the seat (with front legs off the floor) tries to pull out the top dowel in the side seat rail due to leverage action. This force is resisted chiefly by the glue on the dowel. With a ladder back chair, the same force tries to break off the side stretcher across the grain. There is very little stress on the glue bond. Round tenons on stretchers are usually 3r+" diameter or larger and are relatively long. Consequently, these chairs have considerable strength against actual breakage at the critical point. But they are somewhat flexible and not as rigid as box seat chairs for moderate loads which are too small to break the glue bond at the critical joint in a box seat chair.

Some manufacturers of ladder back chairs intentionally use high-moisture content (15% or more) for the posts but have the stretchers very dry. Holes are bored for a tight drive fit for the stretchers. As the posts shrink by drying down to household humidities, the holes tend to shrink. Stretchers swell by picking up moisture from household humidity. The result is a tight shrink fit for the joints. Some factories do not even glue these joints but rather depend entirely on the shrink fit of the wood.

The use of posts with high-moisture content in order to get shrink fit introduces problems relative to finishing these parts. A high-class finish requires dry wood underneath. Of course, the assembled chairs could be dried before finishing, but this takes time and a lot of floor space, so it is seldom done.

One drawback of the ladder back type of construction is that it imposes fairly narrow limits upon the appearance design of the chair. Many appearance designs popular today cannot use the ladder back type of construction.

-Bentwood Chairs One of the really old constructions of chairs is the bentwood chair. It takes many forms, but Figure II(A) shows one that is fairly typical. These chairs use steam bentwood parts and emphasize reduction of the number of separate parts and of the number of joints involved in a chair. For example, the seat frame is bent out of one piece of wood which has its two ends joined together with a scarf joint. This one joint contrasts with eight joints required to make a seat frame in the conventional box seat chair. Most bentwood chairs are much more flexible than box seat construction, but they still stand up very well in service. The flexibility may even be an advantage in case a chair is dropped accidentally. The flexibility seems to distribute the shock stresses instead of concentrating them at joints. When dropped, a bentwood chair is likely to bounce and be no worse off while a box seat chair is more likely to break, generally at a joint.

284 The bentwood construction imposes some limits on appearance design. Certain styles and designs are simply not adaptable to a bentwood construction. However, the construction has a very definite useful place in the overall picture and should not be ignored.

From the viewpoint of manufacturing, there is a big difference, as far as equipment and processes are involved, between bentwood construction and box seat construction. As a consequence, most factories are equipped to make one but not the other. But there is nothing to prevent a chair factory from equipping for both constructions even though it is seldom done in actual practice.

The bentwood chair shown in Figure II(A) usually attaches the seat frame to the back posts by using a lag bolt or hanger bolt through the seat frame from the inside and into the back post. This joint is frequently reinforced by a stamped sheet steel piece of hardware which adds strength and rigidity to this joint. When tilted repeatedly, this joint has a tendency to loosen somewhat but resists actual failure for a long time. When it fails, it is usually by pulling the lag bolt out of the back post if the back post is of a species like soft maple. With hard species like beech, final failure is likely to be breakage of the bolt itself, possibly due to the fatigue effect of repeated stressing as the chair is tilted.

One feature that is a big aid in reinforcing this joint is the bent stretcher that is attached at one end to the lower part of one back post, curves up and is attached to the seat frame, curves down and is attached to a front post, continues to the other front post and then returns to the other side of the seat frame and the other back post. This stretcher forms a triangle structure from back post to seat frame. It is true that one side of the triangle is a curve instead of a straight line, but in spite of this, the triangle structure greatly reinforces the strength and rigidity of the critical joint between back post and seat frame. Some styles will not tolerate the appearance of this stretcher, but where it can be used, it adds much to the durability of a chair in service.

In an effort to achieve more flexibility in style and appearance, some factories have developed a compromise as illustrated in Figure II(B), the modified bentwood chair. The seat frame is horseshoe shaped. It is attached to the back posts as in the regular bentwood, but the two ends of the horseshoe are doweled to the front posts. A horseshoe-shaped stretcher is often used though other forms of stretchers are possible. This construction does not give the triangle structure to brace the chair against tilting. But if properly constructed and carefully made, it has proved quite satisfactory for durability in service.

Steam bending of solid wood is also widely used in making curved parts for many other styles previously discussed ii; this chapter (refer +-LW =;IlgiireS I and 11). The top back rails of Windsor, Bank of England, box seat, and ladder back styles lend themselves to this process as do the seat slats of the ladder back chair. Such parts are strong and attractive because they do not contain the glue lines that would be apparent if the parts were bandsawn from glue-up stock.

285 Some factories have developed constructions where the curved parts are not steam bent from solid wood, but are veneer laminates which are pressed to shape in press forms. The varieties of appearance and construction are almost end1 es~.

Modern-Sty1 e Chai rs

With the recent popularity of modern styling, many chairs have been designed that do not permit any of the construction types which have been discussed. The construction for each of these unusual designs must be worked out on an individual basis. The product engineer should keep in mind the stresses to which the chair will be subjected in service and should plan joints and other construction features to have adequate strength and rigidity so that the chair will give satisfactory durability in service.

Acknowledgments

Acknowledgment is made to the following organizations through whose courtesy and by whose permission illustrations in this chapter have been reproduced.

Liberty Chai r Company, Liberty, North Carolina.

Boling Chair Company, Siler City, North Carolina.

Nichols & Stone Company, Gardner, Massachusetts.

Thonet Industries, Inc., Statesville, North Carolina.

Troutman Chair Company, Troutman, North Carolina.

Stanley Furniture Company, Stanleytown, Virginia.

Agricultural Experiment Station, University of Arkansas Fayettevi 1 le, Arkansas.

286 CHAPTER 16

UPHOLSTERED FRAMES

Most frames for upholstered chairs and sofas are structurally very similar to box seat chairs although sizes, proportions and shapes are quite different.

Most of the parts of an upholstered frame are not visible in the finished article so that these parts can be sound grade instead of clear. Defects which do not impair the strength of the part are generally permissible. Such defects might be weather checks or other small checks, worm holes, small sound knots, and/or skip dressing.

Double-dowel construction is generally used for most joints. For this reason moisture content of the parts can be higher and can vary more than that for casegoods. High-moisture content gives a shrink effect on dry dowels. If the parts are not finished because they are not visible, moisture content can be higher than for box seat chairs where almost all parts are visible and are finished. If frames are a species of wood such as gum, there is likely to be warpage as the parts dry out, which can cause problems. But many good frames are made from air-dried lumber (without kiln drying) using a species such as ash, which stays flat in drying.

Many upholsterers consider certain properties important in the wood species used for frames. The wood should tack easily. It should not be so hard that it turns the needle point on the tack which would probably happen with hard maple. Neither should it be so soft that tacks do not have good holding power as in basswood. With the recent trend to substitute machine stapling for hand tacking in the upholstering operation, the importance of "tackability" of species used for frame parts is reduced. As long as a species is strong enough to stand the external and internal stresses, almost any species can be used for non-exposed wood parts. Therefore, the company's lumber-buying specifications will call for "mixed hardwoods." Ex- posed wood parts, of course, would be the same species depending on the wood 1 ook des ired.

Rigidity becomes more of a problem in upholstered frames than in box seat chairs. This is especially true for the seat rails. When seat springs are installed in an upholstered frame, they have to rest on a base. The most frequently used base for the seat springs is jute webbing which is stretched tight and tacked to the bottom edges of the seat rails. The stretching of the webbing exerts a considerable force sideways on the seat rails and tends to bend them inward. To get satisfactory rigidity, most shops use 5/4 lumber for seat rails instead of 4/4. Shops that use 4/4 are likely to dress it thicker than for box seat chairs, 7 18 !! or even 5 fi6 !! instead of 3 /4 ::. This can be done because moderate amounts of skip dressing are permissible in frame parts which are to be covered.

287 For long seat rails such as front and back of a sofa seat, it is customary to run stretchers between front and back seat rails to get rigidity. Some- times one stretcher is used half way between ends of the rail, sometimes two stretchers, each a third of the way from the end of the rail.

Since it is practically impossible to tilt back a sofa or lounge chair when sitting in it, the joint between side seat rail and back post is not subjected to the tilting stresses as in a box seat chair. This joint rarely gives trouble in an upholstered frame. Another reason for freedom from trouble with this joint is that side seat rails can be made wider than in box seat chairs because they are covered and do not affect appearance. Extra width gives extra strength with the double-dowel joint.

There is a category of upholstered chairs cal led "Occasional 'I or "Pull -Up" chairs. These are smaller than overstuffed lounge chairs and often show considerable exposed wood. They are more like box seat arm chairs with seat and back upholstered. Many of them tilt back easily, so the critical joint in the frame is more nearly like a box seat chair than the upholstered frames just discussed.

Corner blocks are usually used in upholstered frames at the four corners of the seat frame in much the same way as on box seat chairs. But on upholstered frames the multiple-tenon type of corner block is very seldom used. Most are mitered, glued, and reinforced with screws. In planning corner blocks for upholstered frames care must be exercised to assure that the corner blocks do not interfere with the seat springs which are to be installed inside the seat frame.

Tacking strips are generally needed as parts of an upholstered frame; they are not needed on box seat chairs. In upholstering, it is common practice to cut the fabric cover in one piece for the inside back, other pieces for inside arms, etc. Pieces of cover are applied and tacked separately. The function of tacking strips is to provide something to which these cover pieces can be conveniently tacked. These parts should be parts of the frame and be assembled with the frame. Location, size and shape of tacking strips should be decided on the basis of convenience to the upholsterer when he gets ready to tack a piece of cover. Tacking strips do not contribute to the strength or rigidity of the frame itself.

Front feet of upholstered frames are usually short in a vertical direction, frequently not measuring more than about 5 inches in height. These legs are usually very plain if the upholstery includes a skirt which hides the legs. If there is no skirt, the legs are often quite ornate, turned or carved. In making ornate front feet, especially in carving, it is convenient to handle small blocks. Then too, front feet are exposed and frequently of a high- priced species like walnut. It seems sensible to make then as separate parts and attach them to the seat frame. Some shops pre-finish them before attaching.

There are two prevalent methods of attaching the front feet. In the first method, the top of the foot is flat, maybe 3 inches square. Dvdel holes are bored in the bottom edges of the seat rails and corresponding holes, bored in the top face of the foot. The foot is glued and doweled to the frame.

2 88 The second method is to have a tail on the foot which projects up inside the seat rails, and this tail is glued and attached with screws to the seat rails thereby forming the joint between the frame and the front foot. This is probably stronger than the dowel joint, but many shops use dowel joint and seem to have no serious trouble with it. Short height of the foot reduces the stresses on this joint. Also, if the foot is wide and thick, or ifthe foot has wings at the top for appearance, it gives a larger platform on which the frame rests and reduces the stresses on the joint. In most instances the exposed bottom end of the back post of an upholstered piece is not a feature of appearance. In such situations the back post is usually a single piece as in a box seat chair, even if the front posts are separate pieces and are doweled to the seat frame. If necessary, the back posts can be separate and can be attached to the seat frame in the same ways as front posts. But the one-piece back post is cheaper and stronger.

Most upholstered pieces have arms. These must be part of the frame. Appearance dictates construction, but there are two constructions which are widely used. In both constructions the back end of the arm is attached to the back post by dowel or glue and screw or a combination. This joint is frequently hidden so that it can be reinforced by glue blocks: Folks are more likely to sit on the arm of an upholstered piece than on the arm of a dining chair so the joint should be strong enough to stand this treatment.

In one construction the front end of the arm is supported by the top end of an arm stump. The bottom end of the arm stump is attached to the side seat rail. This construction was discussed for box seat arm chairs. Where the joint is hidden in an upholstered piece the arm stump can generally be notched around the side seat rail. This gives much extra strength to carry the load of a person sitting on the arm.

In the other construction, there is a front post (not the front foot). The bottom end of this front post is flush with the bottom edge of the front seat rail. The end of the seat rail is doweled to the edge of the front post instead of being doweled to the side rail. Thus there is a front sub- assembly consisting of two front posts and a front seat rail. This sub- assembly is then doweled to the ends of the side seat rails. The front post extends up to the proper height to support the front end of the arm. Post and arm are doweled together in the position required by appearance design.

For appearance reasons, top back rails, arms, and front posts are often bandsawn to quite irregular shapes. In some upholstered chairs, such as the "barrel chair," there is scarcely one straight part in the frame; nearly all are bandsawn to irregular curves. One big advantage of dowel joints is that they require only a flat face on the end of one piece and a corresponding flat spot on the other piece where this end is supposed to joint to it. This permits great flexibility in designing curved parts without serioiisly complicating the problem of joining the two curved parts together.

New Concepts in Upholstered Frames

Designers like to use curves in frame designs, and there is much material waste and extensive machining is needed to make curved parts out of lumber.

289 Since the frames are covered by the upholstery, the material used is not important so long as it meets the structural requirements. Curved components are being made of plastics, curved plywood, paperboard, and even molded particleboard.

Curved plastic frame parts are commonly made from self-skinning polyurethane or polypropylene structural foam. Small parts, such as wing fillers with compound curves, are inexpensively made from cast polyester. Self-skinning polyurethane resembles a laminated core. The dense outer surfaces or skins are about l/16" thick with much lower density material in the center. This variable-density material is cast in a single operation. All of these plastics are formulated to hold upholstery staples well and have been proven to have sufficient structural strength to withstand normal use. The upholstery manufacturers who use these frames do not usually have in-house plastic operations, but instead they purchase them from specialty plastic molding suppliers. A big advantage of molded plastic frames is the greater degree of design freedom which is possible.

The papertube process is unique and lends itself particularly well to curved contemporary or modern upholstery designs. Paper is roll-fed onto a rotating mandrel shaped to the customer's specifications. The components can be produced to any thickness and any convex shape ranging from a small 10" by 10" square to a 60" diameter round. As the paper winds around the mandrel under carefully controlled speed, pressure and tension, a resin adhesive is sandwiched between each layer. The paper lamination is allowed to build up to whatever thickness is desired, typically from 3/8 'I to 1/2" thick. Specified shapes are then cut out of the tube by a bandsaw or water jet.

Curved plywood is light and strong, but by the very nature of the raw material and the presses, it cannot give us the compound curves and bulk obtainable in molded plastics. The multiple plies of veneer are spread with glue and assembled under pressure over and between curved forms of a specific design. Curved plywood can be used for non-exposed parts or be made with outer plies of face veneers to add esthetics to contemporary designs.

The Clark Uoholsterv Frame

Curved upholstery frames are costly in labor and material. Barrel backs, kidney shapes, curved top rails, fronts and arms have traditionally been made from heavy bandsawn blocks of wood. Such frames are heavy, weak, and expensive. A unique invention for making curved upholstery frames has been developed and patented by researchers at N.C. State University (see Figure I on the following page).

The new concept is very simple. The engineering principles could be compared to the principles of the light, but strong, airplane wing where two thin ''skins'' are held apart by a series of ribs. The two "skins" of the curved upholstery frame parts are made of several plies of inexpensive vefieer (I.€., 1/1081poplar) which are :aminated on forms to the required curvature. The unique construction feature is the use of bridging blocks which are glued and stapled between these rims. The bridging serves two functions:

290 1. The thin plywood arcs become a remarkably strong, but 1 ightweight, assembly.

2. The bridging blocks are located to serve mechanical purposes for attachment of springs and as locators for exact positioning of stumps, posts, legs, etc.

Figure I: Clark upholstery frame.

291 Designers and product engineers find that the frame offers a number of a dva ntage s . Superstructure parts (stumps, posts, legs, etc.) have tenons on their ends which are securely fastened into the gap between the rims. Weak dowel joints are eliminated.

Weight has been cut 30 to 40%. The board footage from the rough end will be from one quarter to one half of previous needs.

Standard frames, which have been re-engineered to use the concept, have shown cost reductions of 20 to 40%.

Wood is an excellent material in both performance and price for upholstery frames. Past practices have tended to overengineer the size of the parts and to underengineer the critical joints. Upholstery frames must withstand severe stresses in use, but creative product engineering can result in reduced costs while improving quality.

292 CHAPTER 17

BEDS AND TABLES

Most beds are simple structures and introduce few problems which have not already been discussed. Most beds consist of a headboard, a footboard, and a pair of side rails. These units are usually not fully assembled at the factory but are shipped from the factory as separate units and are not assembled into a bed until they are delivered into the customer's home. This means that the assembling of side rails to headboard and footboard should be a simple job which can be done by anyone with a minimum of tools. But when side rails are joined to the bed ends, they should form a rigid assembly that will not be wobbly or noisy due to loose joints. Special hardware is available to give ease of assembly and a tight joint.

Figure I shows a typical bed hook for use with a wood side rail. A slot is machined in the end of the bed rail to receive the bed hook, and two holes are bored for steel pins which lock the hook into place in the bed rail. The posts on the headboard and footboard are slotted to receive the part of the bed hook which protrudes beyond the end of the side rail. Holes are bored in the posts and steel pins driven into the holes. When ready to assemble the bed, the bed hook (which was attached to the side rail at the factory) is inserted into the slot in a post and pushed down. The taper on the hook pulls the end of the bed rail tightly against the face of the post. If the side rail is tapped down with a hammer, a solid tightly-fitted joint results, and the bed will not wobble or be noisy. To remove a side rail, simply tap it upward with a hammer, and the bed hook will release. It is important that the pins in the post do not go to the end of the slot in the hook at A in Figure I. If they do, the taper effect of the hook slot is lost, and there is nothing to insure a snug fit between side rail and post B.

Figure I: Bed hook used in the side rail.

293 It is also important that there be clearance between the end of the hook and the bottom of the slot in the post at C. If the hook hits the bottom of the slot, it spoils the tightening effect of the taper on the hook pulling against the pins in the post. The slot in the post at D should go high enough so that the hook can be inserted above the pins in the post and can catch on these pins when the side rail is pushed down. Figure I shows dimensions of 1" and l/2''. These are correct for most bed hooks. But if they do not result in a tight, satisfactory fit, change one or the other of these dimensions to get the desired tightening action of the taper hook against the pins in the post. A bedlock machine is available to cut the hook slot and bore for pins all in one handling, thereby reducing labor cost of machining.

Other types of hardware are available to do the same thing as a bed hook. Most hardware is designed on the principle of a taper which forces a tight joint when the side rail is pushed down. However, some designs rely on a bolt and nut construction so that turning the bolt tightens the joint.

It is customary to attach a slat bearer to the inside of the bed rail along the bottom edge. It is usually a strip 3/4 'I thick and 1" to 1 1 /2 I' wide and is glued and attached with screws to the bed rail. The purpose of the slat bearer is to provide a ledge on which bed slats can rest. Most beds use three or more bed slats laid crossways of the bed inside of the side rails and resting on the slat bearers. The bedspring can rest on these slats. Most bed manufacturers do not furnish slats, the retailer buys them separate from the bed, often from a local planing mill.

Some bed manufacturers supply steel side rails which are bought from suppliers, instead of using a wood rail. Figure I1 shows a pair of steel side rails fastened together for shipment. The main body of the rail is an angle iron. A shoulder plate and a hook are fastened to each end of the rail. Such rails are fully as strong as wood rails and are cheaper, but some retailers object to their appearance.

Figure I11 shows a different type of steel rail. The body of the rail is formed from sheet steel so that it appears similar to a wood rail. Finish may be paint, with a color to match the wood headboard, or it may even have wood grain painted on it. This type of steel rail costs more than the angle iron type but looks better. Either has adequate strength.

294 Figure 111: Bed rails made from sheet steel.

Tables

While all exposed flat surfaces of furniture are supposed to be flat and smooth, there are degrees of flatness and of smoothness. A high-quality flat and smooth surface is more important on a table top than on almost any other furniture part. Tables are frequently placed under a window or in the room where a person looking at the table top catches the reflection of a window on the finished face of the table top. Under such conditions, the slightest deviation from a flat, smooth surface distorts the reflected image and calls attention to the imperfect surface of the table top. The higher the polish (sheen) of the finish, the more apparent the distortion. A top which would look all right with a dull rubbed finish may look wavy and un- i even if the finish is given a high-sheen polish. t- !- While it is more a problem of processing than of construction, it might be well to mention that sunken glue joints are more objectionable in table tops than in most other furniture parts due to the reflection effect. This is true whether the tops are plywood or solid edge-glued lumber. Good sanding of the face is also more important on table tops than on other furniture parts.

Table tops are frequently exposed to rougher treatment in service than most other furniture parts. Cigarette butts fall out of ash trays; liquids are

295 spilled; assorted items are placed on and removed from the top at frequent intervals increasing the danger of dents and abrasion. Plastic laminates, such as Formica, are being used widely for the surface of table tops. They are hard and tough, resist abrasion, heat, and liquids. They can be had in a wide variety of colors and patterns. These patterns are photographic reproductions in color and are available in patterns of fabric, marble, cabinet woods, and opaque solid colors. The reproduction is on the first layer of paper under the top surface of plastic resin so that color and pattern are protected by this resin coat. Plastic laminates are not cheap, but they are extremely durable. The wood grain reproductions look natural enough so that many people cannot tell whether the table top has actual walnut veneer or has a plastic laminate with walnut-grain pattern.

The laminate face is usually 1/16" or 1/32" thick and, being hard and strong, it would be natural to expect that, if it were glued to a panel with a wavy or rough surface, these defects would not show through on the top face of the laminate. But actually the reverse is true; rough core surfaces show more clearly with a plastic laminate face than with a wood veneer face. The reason is probably because veneer faces are sanded after being glued to a panel whereas the plastic laminate faces are not sanded. The roughness or waviness is probably present in the wood veneer face but is sanded out before finishing. The plastic laminates are pre-finished when bought.

Occasional Tab1 es

Occasional tables or novelty tables are terms used for small tables primarily used in furnishing living rooms, dens, and great rooms. Examples would be end tables, coffee tables, lamp tables, and corner tables. Such tables are customarily assembled at the factory and shipped as a complete unit. They are usually small and lightweight. Most of them do not involve any principles of construction other than those which have been previously discussed.

Dinette and Kitchen Tables

Changing 1 ife sty1 es and smal ler fami 1 ies have greatly increased the demand for dinette sets that are styled for informal dining in kitchens, "great rooms,'' etc. These tables are of a simple construction when compared with the larger dining room tables, which are discussed later in this chapter.

These are usually plain rectangular tables consisting of a top, four aprons, and four legs. The top is usually made in one piece with no provision for center table leaves. The tops are made from a wide variety of materials with emphasis on impervious surfaces that hold up well in everyday use. A very common top construction consists of a particleboard core surfaced with a melamine plastic laminate and edge banded with the matching laminate or a decorative metal or plastic extrusim. High-styled djilettes oftei; feature 6 thick-tempered glass top, and some kitchen tables are still made with a vitreous enameled steel top.

296 Dinette and kitchen tables are like dining tables or other large, heavy tables in that they can be shipped at substantially lower freight rate per 100 pounds if shipped knock down (KD) instead of set up. The railroads describe quite specifically what will be considered KD from the point of view of their rates. So it usually pays to check their descriptions and comply with their definitions of KD before planning the construction of large tables unless such compliance involves considerable extra cost in manufacture. These descriptions are in a publication called "Uniform Freight Classification." Any railroad can supply this publication.

There is a simple dinette table construction which gives a low-cost rigid table and also permits shipping KD. Figures IV and V are bottom views of this construction. Figure IV shows how a wood corner block is machined to fit the apron. Figure V shows the whole corner, but using a pressed-steel corner brace instead of wood. The four aprons and four corner braces are assembled using glued and screwed joints which are machined as shown in Figure IV. The sub-assembly is then attached to the top by screws leading

Figure IV: Bottom view of a corner block fitted to apron on kitchen table. through the aprons. The whole top assembly is shipped without attaching the legs. Legs are attached at destination. For either type of corner block, hanger bolts are inserted at the factory into lead holes bored in the legs. The hanger bolt when bought has one end threaded with wood screw thread similar to a lag bolt. The other end is a conventional thread which fits a wing nut. A low-cost machine is available to drive hanger bolts into legs. At destination, the leg is placed in position with the hanger bolt projecting through the hole in the corner brace. When the washer and wing nut are applied and the nut is tightened, the leg is drawn into position and held tight against the ends of the aprons. If the legs loosen through use, they can be tightened again by simply tightening the wing nut. Corner braces of stamped and formed steel are available if preferred to wood corner braces. Joints between the apron and leg could be doweled or mortise and tenon, but they would provide no special advantage and would not permit shipping KD.

297 TOP

Figure V: Bottom view of a pressed-steel corner brace.

Dini ng Tab1 es

These tables are frequently made using construction like the dinette table provided that such a construction will permit the appearance design which is wanted. But if appearance design specifies stretchers, legs in any position other than at the corners of the aprons, a different construction is needed. Some modern designs omit aprons. In such situations, the legs are frequently attached by using a hanger bolt in the top end of the leg and screwing it into a steel plate which, in turn, is attached with screws to the table top. Figure VI shows this construction. Table legs are usually much longer than case legs, so the leverage action on a table is much more severe than on a case. It is especially important to use a hanger bolt long enough so that it threads into sufficient wood in the end grain of the leg to prevent stripping the thread and pulling out.

With dining tables, there is usually no problem of strength. If the parts are big enough to look satisfactory, they are generally strong enough. But there frequently is a problem of rigidity in dining tables. Sometimes small legs are too flexible; sometimes the joints are not tight; sometimes the table slides have too much play. Any table with more than three legs rocks or pivots on an uneven floor. A common occurrence is to wedge a match book under one leg of a table in a restaurant to attempt to get the table to sit level and solid on the floor. Glides are available for the bottom end of the legs to alleviate this trouble. The glides are height adjustable.

Most dining tables consist of a top structure and one or more base structures. These are sub-assembliesi and each is a complete structure by itself. Frequently, these structures are shipped separate from each other in order to take advantage of KD freight rates. The base structures are assembled to the top structure at the retail store or in the consumer's home.

298 Figure VI: Metal plate used to join table leg to table top.

TOD Structure

If the top has aprons under it, they will be attached to the top at the factory. These aprons are usually stiff enough so that, when the screws used to attach the top are driven, the apron will straighten out warpage in the top. Consequently, warpage of the table tops is generally not a serious p rob1em.

A different situation applies to the leaves on a drop-leaf table. Here there are no aprons or cleats to hold the leaf flat. If drop leaves are solid rather than plywood, it is almost certain that there will be some warpage. The best you can do is to use exactly the same finish on face and . back of the drop leaves. This equalizes the two faces in the rate of pick- i -- ing up or giving off moisture and thereby reduces crookedness due to cupping. To reduce spiral warp, use a species which is not likely to warp spirally, if possible, or use narrow strips for edge gluing or both. If the drop leaves are plywood, use a balanced construction with corresponding plies either side of the core made of the same species and thickness of i veneer. If this is not sufficient, quarter-sl iced crossbanding may reduce - the trouble. Finish both faces alike as recommended for solid drop leaves.

If the top is of ywood, shrink and swell of the top is no problem. But if the top is of sol d edge-glued umber, it will shrink and swell so that a floating construct on is needed n attaching aprons.

299 One common form of floating construction is illustrated in Figure VII. A wood corner block is used, bored for two screws, one into the top and one into the apron. If the top shrinks or swells, it will tend to move screw A in the direction of the arrows. If screw B is not driven too tight, a moderate amount of this motion is possible, and the corner block will assume a position indicated by the dotted lines. But the corner blocks and screws will prevent the top from lifting away from the apron. Motion is permitted in the direction wanted and is prevented in the directions where no motion is wanted. For such a floating construction, be sure -not to glue the corner block; use screws only, no glue.

r4 TOP i

SIDE VIEW

Ill

Figure VII: Floating construction used in joining corner biock to apron and table top.

Chapter 6 has already discussed the shrink and swell problem involved in a solid top with end bands or with mitered bands on four edges.

300 Most dining tables are "extension tables." This means that the top structure has two halves. These halves can be pushed together for ordinary use, or they can be pulled apart and loose table leaves laid into the opening in order to make a longer table. The usual way to accomplish this is with table slides. Very few table manufacturers make their own slides. Figure VI11 shows a typical wood table slide, and Figure IX shows how a pair would be attached to a top structure. A table slide has two or more members with a telescoping action. The members are attached face-to-face with each other by double dovetail into the slides to prevent less than an acceptable lap between members when the slide is opened (distance A-B in Figure IX). This insures against breakage of the slide and gives rigidity in a vertical direction whether the table is closed or extended. One end of an outside member of the slide is attached to one half of the top; the other end of the other outside member is attached to the other half of the top. Telescoping action of the slides then permits the two half tops to be pushed together or pulled apart to provide space for leaves.

Figure VIII: Wooden table slide partly extended.

BOTTOM Vl€W

Figure IX: Table slides assembled to the two sections of table top.

301 The design of bases for extension tables sometimes involves stretchers or a single center base which must stay in the center of the table irrespective of whether the half-tops are closed or extended. This requires a different table slide which is called an "equalizer" slide (see Figure X). The middle section of the slide is attached to the table base, and one outside section is attached to each half-top. By the action of the rack and pinion, if one outside section moves seven inches in one direction, the other outside section will have to move seven inches in the opposite direction. This keeps the center section (and the base to which it is attached) always in the center of the two half-tops whether they are closed or extended. Equalizer slides must always have an uneven number of sections, such as three, five, etc. There must always be a center section with the same number of other sections on each side of it. But with plain table slides (non-equalizer), there can be any number of sections.

Figure X: "Equalizer" slide with rack and pinion attachment.

Table slides seem very simple, but there are tricks and pitfalls in specifying the proper slides for a specific table. Most slide manufacturers have encountered all these pitfalls and know how to avoid trouble, and most are prepared to engineer the correct slides for your tables. Figure XI shows a typical data form giving the information they need to properly select the right slides.

302 I bur Table Size 01 Fillers Type of Table Special Data Number TOP 1urnb.r Wldlh

711

B. WALTER & COMPANY, Inc. WABASH, INDIANA Mokers of table slides md extension table equipment since 1887

FIRM BY

SlREEl CllY SlAlf

Prin8.d I.Y S A.

Figure XI: Data form for use in selection of proper table slides.

Base Structure

Figure XI shows a variety of base structures. Most of them involve no unusual construction problems in the secticn mar the flmr, but many of them do involve problems at the top where the base structure is to be fastened to the top structure. These problems will be discussed in connection with "bridging."

One problem pertaining to base structures is illustrated in Figure XI(2, 5, and 6). Feet are attached to a center column and splay out at an angle

303 until they hit the floor. This arrangement gives severe leverage action trying to open up the joint between the foot and column at the heel of the joint. Hardly any two table factories handle this problem in the same way. Some factories buy a sheet steel "spider," which is available from hardware suppliers. This spider is attached by screws from below and is not visible. A screw goes into each foot through the spider. It seems to substantially reinforce the joints. Strength is a most important consideration in any design of the joint between foot and column. While there are a wide variety of designs possible, the product engineer should thoroughly test his design of this joint for strength before putting it into production.

Sometimes the foot itself, seen in Figure XI(6), gives breakage trouble if the bandsawn pattern results in a bad angle grain situation on a delicate foot. One way to strengthen such a foot is to use two outside plies of lumber with a center ply of lb'' veneer with grain crossing that of the outside plies. It costs more, but broken feet at destination are expensive and also cause customer ill will. The abpearance of the top edge of the foot is not as good because of the end grain of the center ply. Use of the stronger 3- ply leg is not an ideal situation, and it should be used only as a matter of necessity . Bridging

The major function of bridging in a table is to furnish a convenient way by which the base structure can be attached to the top structure when the table arrives at its destination. The bridging should give rigidity when the table is assembled. It should also provide for simple assembly requiring only common household tools, such as a wrench or screwdriver.

Bridging can also serve a second important function. It can brace the top of the base structure against transit damage in shipping. In Figure XI(3), the joints between stretchers and posts would be subject to breakage in transit unless something holds the top ends of the table legs in proper position. This could be provided in the design of packing, but it is often possible to construct the bridging so that it will brace the base structure against transit damage as well as serve for the attachment of base structure to top structure.

With table construction, such as in Figure XI(l), no bridging is needed. The base structure is like a kitchen table consisting of our separate legs which can be attached to the top structure with a wrench.

There are a wide variety of table designs and constructions, and each should be studied by itself for details of proper bridging construction. The product engineer can use much imagination, but he should be sure to accomplish rigidity and ease of assembly at destination. He should also decide whether to depend on bridging or package construction to protect against transit damage. These are the general principles, but a few examples may help in applying them.

304 Figure XI1 illustrates one of the most common forms of bridging for a dividing pedestal table, as sketched in Figure XI(2). A single rectangular wood part serves as the bridging for each pedestal. The bridging is long enough so that each end can be attached by screws to one member of each of the two table slides. These screws would be driven at destination, but holes in the bridging for screws are bored and countersunk at the factory. The top end of the pedestal is attached at the factory to the middle of the bridging piece. The bridging piece itself should be thick enough to give rigidity. The weight of the table is supported by the pedestal at one small section in the center of a long span of the bridging. Rigidity of the bridge against the vertical load would vary in proportion to the width of the bridging piece and with the cube of the thickness (see Chapter 3). Increasing thickness by one-third (from /4 I' to 1") would increase rigidity about '2 1 /3 times.

Figure XII: Illustration of bridging used on pedestal- type table.

Any force applied to the edge of the table top, as at A in Figure XII, exerts a severe leverage action trying to break the attachment between the top of the pedestal and the bridging. Screws through the bridging into the end grain of the pedestal are seldom strong enough to resist such a force, especially if someone puts his entire weight on the table. A joint similar to one type used in Windsor chairs (discussed in Chapter 15) makes a high- strength construction. A round tenon turned on the pedestal is inserted in a hole bored in the bridging after which the tenon is glued and wedged. Since the joint is not exposed, it is a cheap joint to make because no flushing or cleaning of the wedge is necessary. If the top of the pedestal is a square cross section, this joint could be reinforced at both sides with glued and screwed corner biocks similar to tnose used between table tops and aprons.

305 This double-pedestal base structure involves no transit damage problem requiring special features for bridging.

In Figure XI(3), the legs are assembled with stretchers. Thus they cannot move outward when the table is extended. The most straightforward construction for such a table is to build an open, shallow box on top of the legs. In this box are anchored the equalizer slides. The box has four small openings through which the slide can move when it is being extended.

Ha rdwa re

In addition to table slides, many stock items of hardware for tables are available. Catalogs are easily available upon request from the hardware manufacturers. Some of the functions that table hardware fulfills are 1 isted below:

Table pins. Inserted in edges of extension table tops and leaves to insure matching position of shaped outside edges.

Eveners. Metal clips for extension table tops and leaves to. insure flush top surfaces.

Table locks. On bottom faces of extension table half-tops to hold them firmly together in closed position.

Drop-leaf supports. To support and hold drop leaves flush with table top when drop leaves are raised up.

Folding-leaf hardware. On extension tables, to replace loose separate leaves with built-in leaves which can fold down out of the way when it is desired to close the table and use it without center leaves.

Card table hardware. To permit folding legs.

Acknowledgments

Acknowledgment is made to the following organizations through whose courtesy and by whose permission illustrations in this chapter have been reproduced.

J. G. Edelen Company, Inc., Baltimore, Maryland.

Metal Bed Rail Company, Inc., Lexington, North Carolina.

Victor Wood Products and Supply Company, Grand Rapids, Michigan.

B. Walter & Company, Inc., Wabash, Indiana.

306 CHAPTER 18

PACKAGING

In Chapter 2, it was stated that good product engineering involves constructing the product to meet a number of "in-use" conditions, one of which is transportation from manufacturer to retailer to consumer's home. This means that construction problems are not all solved when a piece of furniture is finished and ready to be shipped. It is important that furniture be packed in properly constructed packages to avoid the danger of damage in transit. In years past, most furniture was shipped in wooden crates, but today almost all of it is shipped in corrugated fiberboard cartons.

Regardless of the mode of transportation, there are several common types of transit damage; good package construction should protect against all of them.

1. There is the possibility that something else in a freight car or truck may press against a furniture package and puncture the carton. Resistance of cartons to this type of damage is evaluated by a standardized test for bursting strength of the corrugated board out of which the carton is constructed. Corrugated board is rated by number, such as "200-pound test." The numbers refer to the bursting test. A 275-pound test board is more highly resistant to puncture than a ZOO-pound test board.

2. If a finished surface of the furniture comes in contact with corrugated fiberboard, there is a good chance that vibration in transit will cause this material to act as sandpaper and cut through the finish. As a consequence, there should always be at least one layer of some non- abrasive material between the corrugated board and any finished surface that it might touch.

3. Vibration not only causes abrasive damage but can also cause structural, damage by loosening glue joints, etc. Interior packing in the form of corner pads and edge pro- tectors is used to minimize vibration and to provide space between the carton wall and the furniture. This interior packing introduces the danger of print marks or pressure marks on the finished surface. Prior to packing, the furniture finish should have cured long enough to be print resistant.

4. Furniture packayes often get rough treatment by freight handlers. If these shocks are transmitted through the package to the furniture itself, it can break off legs, break joints apart, etc. Sometimes the construction of furniture is rugged enough to withstand these shocks. Product engineering investigations to improve the strength of the furniture should be the first step in

307 reducing transit damage. Many package constructions provide for suspending the furniture within the package so that much of the shock is not transmitted to the furniture. Padding is often used between furniture and the corrugated box to absorb some of the shock.

5. Fragile parts require extra protection in package construction to avoid excessive damage. Examples are long, thin 1egs, del icate gal 1 eries , mi rrors , and gl ass tops.

6. Drawers and doors are likely to open in transit and be damaged. Means should be provided to keep them tightly closed.

7. Projecting hardware, such as drawer pulls, can be damaged and can also damage other furniture. It should, therefore, be protected. One solution would be to reverse the hardware to the inside of the drawer front. Another approach would be to ship it in a separate bag. However, customers (retailers) do not like to relocate and apply hardware, and there are often complaints about insuffi- cient or lost hardware. Wrapping the hardware with padding material is sometimes necessary.

Present-day package construction has evolved from two different concepts. The first concept used crating lumber and is called a "clearance pack" because no finished surface of the furniture came in contact with the package. The furniture was attached to the package by nailing through the packing into the unfinished back of the case. The legs were usually held up off the bottom of the package by a wood strip that passed under the body of the case and was attached to the ends of the package. The clearance pack required the outside of the package to be rigid enough to keep it from deforming under transit stresses, which is why wood crates made good clearance packs if properly constructed. They are still acceptable to the railroads but have been 1 argely rep1aced by 1 ower-cost cartons.

The second concept is called a "pressure pack." The "pressure pack" uses corrugated fiberboard cartons as the exterior of the package. The name de- rives from the fact that the furniture is held firmly in place by pressure on the surfaces'of the item. There are many variations in this most commonly used concept which challenge the ingenuity of the product engineer and the carton manufacturers. The following description illustrates the sequence of a packing process and how this package minimizes transit damage on a typical piece, such as a dresser.

1. Drawers and doors are held closed by padding or clips so they will not open in transit.

2. A non-abrasive paper or plastic pad is placed over the dresser top and wrapped a few inches over the edges.

3 08 Oftentimes, a second such pad is wrapped around the front and ends and is stapled to the back of the piece. These pads prevent abrasion of the finish and furnish some degree of cushioning.

3. The carton is slipped down over the wrapped dresser. Corner pads and edge pads (about 1" thick) are wedged between the edges of the dresser top and the sides and top flaps of the carton. The top flaps are folded into place and held shut by tape and/or glue and perhaps self- clinching staples. The closed top flaps hold the corner pads, etc. in place. The thick corner and edge pads hold the furniture away from the sides and top of the carton so that there is dead air space around the piece. This dead air space allows the carton to absorb blows that would otherwise dent the furniture. The thick pads also serve as shock absorbers in the event of rough handling.

4. The cartoned furniture, with the top closed, is turned upside down to expose the bottom flaps of the carton. For casegoods, such as a dresser, a diagonally braced rectangular skid is made of crating lumber and placed in the carton. The legs or feet of the furniture will ride on the skid during transit. The bottom flaps of the carton are folded onto and securely stapled to the skid. The skid gives rigidity to the bottom of the package and maintains a clearance between the walls of the carton and the furniture. For cartoned upholstered furniture, the bottom flaps are often full overlap, and no wood skid is used. The full flaps are taped and/or glued and sometimes stapled.

Freight C1 assification

Ra i1 road

Having spent large amounts of money over the years in claims for transit damage to furniture, the railroads made a study of furniture packaging and published detailed descriptions for many different types of furniture packages which they termed "F-Packs." These descriptions, together with a set of rules, ratings, and requirements, made up the Uniform Freight Classifi- cation 9, which was issued on May 31, 1968. However, the railroads have revised and upgraded the specifications. Up-toydate details of railroad packaging requirements are printed in the book "Uniform Freight C1 assification," which can be obtained from any railroad freight agent.

It should be emphasized that F-Package descriptions specify only the minimum amount of packaging required by the railroads. A1 1 packages shipped by rail must meet at least these minimum requirements. Because of the wide variety of types and styles of furniture, provisions could not be made for each individual piece. The F-Package specifications, therefore, are very general in nature and, in many cases, should be exceeded to assure safe shipment.

309 The F-Package descriptions should only be used as a general guide to packaging methods, as each package must be designed with regard to the physical characteristics of a specific piece of furniture.

Pages 324 to 328 of this chapter illustrate the kind of information found in the railroad specifications. Through the courtesy of and with the permission of "Furniture Packing" at 335 East Broadway, Gardner, Massachusetts, the foll owing pages were reproduced from its manual entitled "Furniture Pack a g in g .I' Motor Ca r r ier

The National Motor Freight Traffic Association issued National Motor Freight Classification A-10 on April 23, 1968. It contains nearly sixty general rules which apply to all freight moving in truck shipments. These rules cover such subjects as the use and making of shipping documents and bills of lading, general shipping container specifications, definition of terms, and provisions for special charges and rates. For all intents and purposes, motor carriers will accept any pack the rail roads will , as their package classification schemes are very similar; however, their requirements are not exactly identical.

Corrugated Board

Corrugated board is formed when one piece of corrugated or fluted fiberboard is joined to one or more pieces of flat fiberboard. The fluted material is called the corrugating medium (or more simply, "medium"), and the flat fiberboard is called the facing or linerboard.

When one facing is added to a single medium, the product is termed "singleface" corrugated board. A1 though not used extensively in furniture packaging, singleface corrugated board does find some application as a wrap for small, delicate parts and as hardware protection.

Figure I : Si ngleface.

The workhorse of the furniture industry, singlewall corrugated board, is the result of combin?'ng one corrugated medium witn two facings, one facing on each side of the medium. Singlewall corrugated board (sometimes referred to as doubleface) is used for all types of furniture packages as well as inner packaging components, such as corner pads, separators, dividers, partitions, and suspension forms. It would be a safe guess to say that at least 75%

310 to 80% of all corrugated board used for furniture packaging is of singlewall construction.

Figure 11: Singlewall.

By adding an additional corrugated medium and facing to singlewall board, a doublewall board is formed. When a stronger or more rigid box is needed, or when stacking strength is an especially critical requirement for the package, doublewall corrugated board fits the bill perfectly. Doublewall corrugated is also used extensively for three-ply or four-ply corrugated skids and platforms for tables and casegoods.

Figure I I I : Doubl ewal 1 .

Nearly all material used for the corrugated medium is made from paperboard produced from semi -chemical pul p. Semi -chemi cal pul p is derived through a mild chemical action on chips of hardwood, followed by a mechanical beating action. The use of semi-chemical pulp for medium stock allows the paper- maker to produce the medium as thin as possible, in addition to as heavy and stiff as possible. The industry standard for corrugated medium material is semi -chemi cal board cal iperi ng not 1 ess than .009 inches and weighing 26 pounds per 1,000 square feet (set by carrier regulations). The 33-pound and 36-pound mediums are also commonly used as heavy-duty mediums when more stacking strength and flat crush strength are desired.

When corrugated board was first developed for use as a packaging material, the distance between the top of one flute and the bottom of the next flute of the corrugated medium was approximately 3/16'i. To insure the greatest strength properties, the flutes were designed so that one linear foot of the board contained 36 flutes. This original 36 flutes per foot, 3/16ii high corrugated board was designed as "A-fl Uteii board and exhibited good cushioning propert Ies and excel I ent top-to-bottom compressi on strength.

Figure IV: A-Flute.

311 Some time later, packagers recognized the need for constructing a thinner corrugated board with greater compression strength in the side-to-side, rather than top-to-bottom, direction. After experimenting with various flute sizes, it was found that a flute approximately 3/32" high, spaced 50 flutes per foot would produce the desired qualities. This new flute configuration was dubbed "8-fl Ute."

Figure V: B-Flute.

In time, 'IC-flute" board was developed as somewhat of a compromise between A-flute and B-fl Ute board. C-flute board, containing approximately 42 flutes per foot, each 9/64" high, has properties which fall nearly midway between those exhibited by A-flute and 6-flute.

Figure VI: C-Flute.

Both A- and C-flute board are used extensively in the furniture industry, with C-flute board having a slight edge and B-flute board placing a poor third. For most purposes, both A- and C-flute are quite adequate for the type of protect ion desi red.

When stacking strength and top-to-bottom compression strength is warranted, e.g., for pedestal tables and suspension forms, A-flute board is the likely choice. When used as a cushioning material in corner and edge pads, there is no conclusive evidence to show which type Of flute configuration is better. Although it awaits technical verification, it can be hypothesized that A-flute is better for lightweight items (occasional tables, mirrors, etc.), while C-flute board will perform more satisfactorily and efficiently in combination with large, heavy items, such as dining room tables and most casegoods.

The final specification criterion for corrugated board is the grade, or bursting strength, of the finished piece of material. The bursting strength of corrugated board refers to the amount of hydraulic pressure, in pounds per square inch or "paints," required to push a distended rubber diaphragm through the finished board. There is no relationship, as many people be- 12-iieve, between the bursting strength of a particular type of board and the weight of the contents packed in a box made of the same board. Freight specifications often limit the amount of weight authorized to be shipped in a particular box, but the bursting strength refers to the properties of the board, not the properties of the finished box.

312 The bursting strength of the board is generally controlled by the basis weight (pounds per 1,000 square feet) of the facings. Almost all material used as corrugated board facing is a high-quality Kraft paperboard stock made from a very high percentage of virgin pulp. "Standard" basis weight of most corrugated board facings are 26, 33, 38, 42, 47, 69, and 90 pounds per 1,000 square feet. These standard facing weights are used in various combinations to obtain the desired bursting strength qualities of the board being produced. For example, to obtain a 200-pound test singlewall board, two 42-pound facings are generally used with the standard 26-pound medium. By using two 69-pound facings, the boxmaker is able to achieve a singlewall bursting strength of 275 pounds per square inch. A 350-pound test singlewall board generally is made from two 90-pound facings. For doublewall board, the common weights of facings are as follows: 200-pounds test - 42+26+26; 275-pound test - 42+26+42; 350-pound test - 42+42+42 (see Table I )*

FACING WEIGHT *OARD TEST I I I I I I Singl ewal 1 200 42 + 42 = 84 Singl ewall 275 69 + 69 = 138 Singl ewal 1 3 50 90 + 90 = 180 t I I Doubl ewal 1 200 42 + 26 + 26 = 94 Doubl ewal 1 275 42 + 26 t 42 = 110 Doubl ewal 1 3 50 42 + 42 + 42 = 126

I I I Table I: "Standard" weights of facings.

The various corrugated fiberboard styles and grades can be fabricated into a large variety of box styles. The furniture industry utilizes only three basic box styles for the majority of furniture items: the regular slotted container (RSC), the full overlap container (FOL), and the partial overlap or overlap slotted container (OSC). The basic difference in these styles is the relative length of the closure flaps as all flaps, except in special instances, are the same length (distance between score line and edge of flap). In the RSC (Figure VII) , the outside flaps just meet at the center of the box. The OSC flaps (Figure VIII) overlap each other at the center of the box, the overlapping portion usually being 2" to 4". The FOL box flaps (Figure IX) overlap each other completely, thus making the length of the fiap approximately the same as the width of the box.

313 / Figure VII : Regul ar sl otted container. (RSC 1

\ Figure VII I : Over1 ap sl otted contai ner. (OW

Figure IX: Ful 7 overlap container. (FOC)

314 By knowing what corrugated board is and what it can do, the product engineer can be assured of receiving exactly what he is paying for and can communicate his needs more fully to the box supplier. By knowing this and by ob- serving the following steps, he can be assured that the proper boxes for each particular product are produced:

1. Board Type/Grade

Determine the type and grade of board, e.g., 275-pound test singl ewall , 200-pound test doubl ewall , necessary to properly protect the item being packaged. Generally, this is specified in the various railroad and motor carrier freight classifications. Occasionally, however, these minimum specifications should be exceeded when packaging abnormally heavy or long items, or when severe distribution environments can be expected.

2. Flute Size

Flute size should be specified. As previously noted, generally A- and C- flute boards are comparable, but in some instances, one is more functional than the other.

3. Box Style and Dimensions

Style of box (RSC, FOL, OSC) is also an important consideration. Generally, RSC boxes are adequate for most applications. Some boxes are so narrow in width that an FOL is the only practical answer. When box flaps are glued, an OSC box will provide much greater protection than will an RSC.

The dimensions of the box should be specified according to its inside dimensions; length x width x depth. In boxmakers' terminology of the opening of the box, the length of the box is the largest dimension of the opening, the width is the shortest of the opening dimensions, and the depth is the remaining dimension of the box.

4. Time

Finally, be sure to give your box supplier a sufficient length of time to make and deliver your order. This is especially true when ordering small quantities, non- standard medium and facing stock, special die cuts, etc. A corrugator is an expensive and time-consuming machine to operate. To operate efficiently, the bcx ru~smust be scheduled to provide a minimum amount of down time and changeover periods. Thus, if a special order does not allow the corrugator sufficient lead time, additional expenses are incurred, which are more likely than not reflected in the finished cost of the completed box.

315 Pads and Padding

Pads and padding are easily the most versatile packaging materials used in the furniture industry today. Furniture pads and blankets provide an excellent means of protecting finished surfaces from rubs and abrasion. They help cushion furniture from damage-causing shocks. In some cases, wrappers make up the entire package - no crate and no corrugated box. Pads and padding can be used as fillers to take up void spaces in packages. They can be used to separate finished surfaces, secure KD parts in place, and are sometimes even used to wedge doors and drawers in place.

To assure uniformity of the pads and padding, the minimum requirements and specifications have been carefully spelled out in Item 42352, Note 4 of the railroads' "Uniform Freight Classification" and as Item 79024 of the "National Motor Freight C1 assification." Since the pad specifications were outlined under Note 4 of UFC, Item 42352, they have become known in the furniture industry as "Note 4" pads.

Making an acceptable pad for the least cost has challenged the ingenuity of pad manufacturers. Different combinations of indented paper, cotton and cellul ose waddi ng, wood wool , molded pul pboard, etc. have been acceptable for many years. A relatively new development is the blanket or pad made of extruded, closed-cell foam sheeting made of polypropyl ene or polyethylene. These foamed plastic sheetings have received wide acceptance because they are lightweight, less bulky, and are easily applied in the packing department.

Too many furnituce manufacturers have not paid enough attention to the net cost of their packing operations. The total expense must include, not just the raw materials, but also total labor cost, dollars in material inventories, overhead costs for space, and the reduction of customer complaints. Since padding is used on virtually every piece of furniture, a thorough study of these materials and their application is justified.

C1 osure Materials (Tape, Staples, and Adhesives)

The choice of package closure is an important decision not to be passed over lightly. Many factors will influence the decision, and they must all be taken into consideration. It must be emphasized at the outset that there is no one "best" closure method. The proper choice of methods will, however, reflect the best method for a particular type of box, plant layout, production facility, handling and storage environment, and expense involved.

For use in packaging, there are only three basic types of closures with which the furniture manufacturer must generally concern himself: adhesives (or glue), staples, and tape. io make a wise, efficient, functional decision, the furniture packager must know exactly what each type of closure is, the various types available to him, and the advantages and disadvantages of each.

316 Tape

Probably the most common type of closure used today in the furniture industry is tape or tape in combination with glue. In general terms, tape may be considered any flexible material to which an adhesive is attached for the purpose of holding two surfaces together. Tapes are classified according to both the type of adhesive and the material upon which that adhesive is placed.

According to the type of adhesive used, most furniture packaging tapes may be classified as either gummed or pressure sensitive. Gummed tapes generally contain a dried ''gum," which becomes an active adhesive when activated by water and allows the bonding process between the tape and the substance to take place. Pressure-sensitive tapes, on the other hand, are pre- activated and require no water or other type of solvent before application. Pressure on the tape is all that is needed to bond pressure-sensitive tape to the taping substance.

Within the material classification, tapes used for furniture packaging are generally either paper or paper reinforced with strands of fiberglass, nylon, asphalt, etc. Both gummed and pressure-sensitive tapes are found reinforced and non-reinforced. Easily 90 percent of all box sealing tape used in the furniture industry is either reinforced or non-reinforced gummed paper tape.

Staples

Staples, unlike tape and glue, create a mechanical seal between the two sub- stances being joined. Being metal, the staple is able to penetrate both flaps of the box and bend over (or clinch) behind them to securely hold them together.

For purposes of this discussion, a distinction will be made between stitches and staples. The word stitches will be used to refer to that type of metal fastener produced from a continuous roll of metal wire as the box is being stapled. The staple, conversely, will refer to that type of metal fastener formed from pre-cut lengths of wire, usually partially formed and supplied either in "rolls" or %ticks."

Dimensions for stitches and staples are specified by width of the crown (top) and the length of the leg. Normally, all stitches have 1/211 crowns. Staples generally are supplied with the conventional l/2" crown or the more popular 1 l/4" "wide crown." The length of the leg is variable and depends, to a great extent, upon the thickness of the material being stapled. A common practice is to specify the length of the leg as being equal to the thickness of the material plus one-half the width of the crown. Thus, for A-flute corrugated board, the leg length of a standard i I/". wide-crown staple should be slightly over 1" long. For 6-flute board, the leg length should be approximately 7/8" long, and C-flute should be l5/16" long.

Adhesives

The most commonly used adhesives for furniture carton closures are dextrins, sodium silicate, and hot melts.

317 Dextrins are vegetable adhesives (starch based) which are generally water soluble and which can be purchased in either liquid or dry form. Borax is frequently added to improve adhesiveness and to reduce set-up or bonding time. Dextrins are relatively inexpensive and are the most widely used.

Sodium silicate (water glass) provides a moisture proof bond when dried. It is corrosive to some metals but is excellent for carton closures.

Relative newcomers in carton-closing adhesives are the holt melts. These resins are 100% solids which turn to liquids at high temperatures and then quickly turn to solids when their temperature falls below the melting point. The obvious advantage is that the glued carton flaps need to be held together for only a few seconds before bonding takes place. The quick bonding eliminates the need for self-clinching staples which hold the flaps together while other slower-setting adhesives achieve bond strength. Hot melts are dispensed through a heated glue gun, and then the flaps are immediately closed before the adhesive has time to cool and solidify.

There is no "best" type of closure for furniture packaging systems. Tape, staples, and adhesives all have their place. On many packages, all three closure systems may be used. One important point should be kept in mind that relates to the discussions of rigidity in Chapter 12. The carton will be much more rigid and resist distortion if one or more of the adjoining faces are stiff. If the overlapped flaps on the top and/or bottom of the carton are glued together, these faces will be very stiff. Closing these flaps with just tape or staples is nowhere near as effective. A little adhesive in addition to staples or tape is an inexpensive way to get a much better package.

New Ideas and Products

The money spent on packaging adds nothing to the actual value of the goods except to help assure that they arrive in the customer's home undamaged. Furniture manufacturers, packaging material suppliers, and freight carriers are constantly searching for more effective and cheaper ways to deliver undamaged goods.

Corner and edge pads were once made exclusively of multiple thicknesses of corrugated cardboard. Newly developed pads are being molded out of recon- stituted waste paper, expanded polystyrene, etc. The molding processes lend themselves to engineered shapes and variable densities, which may offer greater protection. Some delicate furniture items are actually being foamed in place within the carton. The furniture is tightly covered with a plastic film bag and placed in its carton. A foam dispenser fills the surrounding space with a solidifying foam that completely immobilizes and cushions the iten.

The strength and design of the interior packaging and bracing is often more important in reducing damage than the strength of the exterior walls of the carton. Items with long legs (8" or more) must be suspended within the carton so that there is no weight or side pressure on the legs. These supports are usually made of ingeniously die-cut and folded corrugated cardboard. A

3 18 pre-formed flexible fiberboard edge wrap known as "Sus-Rap" has been devel oped for protecting round tops, mirror frames, etc. Carton suppl iers will have numerous examples of interior packing to show to the product engineer.

Polyfilm bags are being used to cover upholstered furniture, particularly by big companies who ship these goods uncartoned in their own trucks. The transparent bags give good dust protection and enable the retailer to see and identify the merchandise.

Shrink films combine the advantages of the polyfilm bags with the ability to anchor the furniture to a rigid corrugated cardboard base tray. The polyethylene bag is placed over the piece and tucked around the bottom of the tray. The film is then shrunk by the application of heat which wraps the furniture to the tray. The tray is then fastened to the bottom flaps of a carton so that the film-wrapped furniture cannot contact the side walls or top of the container. This concept has been particularly effective on upholstered goods and in packing dining room type chairs. Distributors and dealers praise the technique saying: (1) it reveals concealed damage more readily than the conventional pack; (2) it eases warehouse identification; (3) it protects chairs against dirt and marring while providing full visi- bility for customer inspection; and (4) the tight wrap eliminates vibration.

Sh ipme n t P r epa rati on

A package may be well designed, tested, and executed, but all efforts will be of no use if the package is shipped to the wrong place, or if the wrong package is shipped, or if the handling or in-car hazards are excessive. Package marking and labeling and railcar loading and bracing, although frequently overlooked, are as much a part of packaging as the shipping con- t ainer it sel f . Marking and Labeling

To be entirely effective, a package must provide protection from the many different kinds of environmental hazards, such as over-the-road vibration, drops, impacts, dust, etc. When designing a package, the ''normal" transportation and handling hazards are taken into consideration. This, of course, assumes that the package will be handled the way it is supposed to be handled, $tored in the most advantageously protective position, and loaded and braced in a way that will minimize the potential damage hazards. The furniture manufacturer can control the packaging, but he can do little to control the handling and storage of his merchandise once it leaves his plant or warehouse. But although he cannot control the environment, he can influence the handling of his package through imaginative marking and labeling. There are basically three different types of marking and labeling on a furniture package with which the furniture manufacturer must be concerned: product identification; required markings; and special cautionary markings a nd in st. r uct ion s .

319 Far too often furniture packages fail to provide adequate identification of the item in the package. There are several common errors made in the area of product identification which can lead to confusion, delays, mis-routed shipments, and handling severity.

Probably one of the most common areas of confusion is the result of box standardization. In an attempt to standardize box sizes, a box is often designed for a number of different items with the style numbers of each item printed on the box. This is a sound economic practice, but caution must be exercised. Frequently, when packers fail to delete the numbers of the items which are not acked in the box, confusion results. Packers must be in- structed to ma I? e sure that one and only one style number is legible on a box.

A similar problem occurs when a box must be improvised, i.e., when a box for one item is used for a different item. Often, both the original printed style number and the new stenciled style number are marked on the box. In cases such as this, care must be taken to assure that all "wrong" style numbers are crossed off the box. The number of panels upon which the style number is printed or stenciled is also important. The more panels containing the product identification, the better handling the item will receive. If the product identification is printed on only one panel of the box, it frequently must be rolled, tipped, or turned around for a handler to find out what the article is.

Where the identification is located may also be determined by the way the item is stored. For example, mirrors, beds, and other long, narrow items are frequently stored on one of the long, narrow edges of the box. There- fore, to assure expedient identification, such packages should be identified on the top and edges.

Another problem frequently encountered with product identification is covering up or obliterating the pre-printed style number. This most often occurs when paper tape is being used as a package closure. The printing should be placed on the Dackaqe in a location that is not in close Droximitv to the area likely to be coiered with the sealing tape. In addition, packers must be cautioned to be aware of co ering up production identifications.

Coding and Code__ Readers.__-

Both rail and motor carriers require that certain markings be present on packages shipped by that part cular mode of transportation.

320 Item 42351, Note 2 of the "Uniform Freight Classification" and Item 79022 of the "National Motor Freight Classification" states that:

Packages IF, 2F, 3F, 6F, 7F, 8F Paragraph 3(b), IOF except for box springs or mattresses, 14F. 15F, 16F. 19F except when articles are KD flat the arrows and the word "UP" may be omitted, ZOF, 21F, 23F, 24F, 25F. 26F. 28F, 31F Para- graph (61, 33F, 34F, 36F, 37F, 38F, 39F. 40F, 41F, 42F, 43F, 45F, 46F, 47F. 49F. 50F. 51F. 52F. 53F, 54F, 55F. 58F, 59F, 60F, 61F, 62F. 64F, 66F, 68F. 69F, 70F. 71F, 72F, 73F, 75F, 77r, 8OF, 82F. 83F, 84F. 85F, 86F, 87F. 88F. 89F, 91F, 92F, 93F. 94F, 95F, 96F, 97F, 9BF, lOlF, 102F, 104F, and 106F, must be conspicuously marked on at least one panel as follows:

UP UP FURNITURE FURNITURE INCLUDING MIRRORS OR GLASS t FRAGILE , HANDLE WITH CARE FRAGILE HANDLE WITH CARE t

[Note: F-Pack numbers in BOLD TYPE are applicable only on truck shipments. Numbers in italics applicable only on rai I shipments.]

In addition, a package certificate, as specified under "UFC" Rule 41, Sec- tion 4, Note 5(c) and "NMFC" Item (Rule) 222, Section 4, Note 5(c) which is shown in Figure X, must be printed on the following packages.

IF, ZF, 3F, 6F. 7F, 8F Paragr jpti 3(5), IOF, 14F. 15F, I6F, 19F. 20F, 21F. 23F, 24F, 25F, L6F, 28F. 31F Paragraph (b), 33F, 34F, 36F, 37F, 38F, 39F, 40F, 41F, 42F, 4JT, 45F, 46F, 47F. 49F, 50F. 51F, 52F. 55F, 54F, 55F. 57F. 58F. 60F, 61F, 62F. 63F. 64F. 65F. 66F. 67F, 68F, 63F, 70F, 71F, 72F, 73F, 75F, 77F, 8OF. 81F, 82F. 83F, 84F. 85F, MF, 87F, 88F, 89F, 9.IF, 92F, 93F, 94F, 95F, WIT, 97F, 9PF, 101F, IOZF, 103F, lUdF, 106F, 205, 218, 826. 829, 958, 978, 1021, 1099, 1135, 2029.

[Note: Package numbers in BOLD TYPE are appl icable only on truck shipments. Numbers in itaZics applicable only on rail shipments.]

PACKAOE CERTIFICATE MIS B6X MCETS ALL C0ImI)CTIOW R€Ul[lIEkEIPS OF APPLICABLE FREIGHT CLASSIFICATION FOR PACKAGE NO. BURSTING TEST LBS PER Sa. IN. 1-F 200

Figure X: Package certificate.

321 Ca uti o n Ma r k in g. s

Some furniture is of such a nature or fragility that it is especially susceptible to damage. Even a very good package cannot prevent damage to this type of article if it is handled, stored, or loaded incorrectly. To assure that these items are handled properly, caution markings are uti1ized.

One type of caution marking that is frequently used concerns the opening instructions. Whenever it is critical that a package be opened in a particular manner, i.e., the finished surface of the article is near the "probable" cutting area, etc., opening instructions such as "CUT ON DOTTED LINE," "DO NOT CUT HERE," etc. should be used. Detailed opening instructions, if necessary and if different from what one would normally expect, should be printed in the vicinity of the "normal" opening area to assure that the instructions are seen.

The use of clamp trucks or "squeeze" trucks can oftentimes cause considerable damage to articles that are especially delicate or that are prone to easy case-racking. Some type of caution marking should be employed to warn operators not to use that type of handling equipment. A suggested marking that could be used is indicated below in Figure XI.

NO CLAMPS

Figure XI: Caution markings.

Figure XI is a suggested format for warning handlers that a glass panel, door, panel, etc. is located within the package. Generally, a label or marking that merely indicates that glass is contained in the package is aot sufficient warning. A caution mark should be placed on the package over each area containing glass.

322 Pedestal tables and bases, glass-topped servers, and tables with glass or marble inlays are especially prone to damage caused by handlers walking on the unsupported part of the package. There is virtually no way to prevent walking or standing on packages, but handlers should be made aware of those items where such behavior is not desirable. Figure XI1 is one example of how this can be accomplished. The marking should be placed directly over the critical area in question.

DO NOT STEP Figure XI1

Railcar Loading and Bracing

As a railcar moves from the manufacturer's plant or warehouse to its ultimate destination, it travels through a hazardous environment. It may travel over many miles of "rough" track, be subjected to severe back-and-forth swaying motions, and be "humped" in several different switching yards. A1 1 of these conditions expose the merchandise loaded inside the car to forces which can easily cause damage. To assure that damage is held to an absolute minimum, it is extremely important that the shipment be properly loaded and braced to prohibit as much movement within the car as possible. Rule 27 of the rail roads' "Uniform Freight Classification" explicitly states that "shippers must comply with carriers' rules regulating safe loading of freight and protection of equipment." Pamphlet No. 15, "Rules Regulating the Safe Loading of Carload Shipments of Furniture in Closed Cars and Pro- tection of Equipment,"l published by the Association of American Rail roads, outlines the proper methods of loading and bracing CL shipments of furniture.

1 Copies of this and other loading pamphlets may be obtained from: Freight Loading and Container Section, Association of American Railroads, 59 East Van Buren Street, Chicago, IL 60605.

323 Package Testing-

Nearly every day, the product engineer is confronted with a great number of decisions regarding the adequacy and effectiveness of his packaging. His decisions may very possibly mean the difference between the success and failure of a new suite; if it cannot be shipped, it cannot create sales. In any case, his decisions will affect his company's profit picture. If his decisions are right, the most economical package (1 owest possible amount of damage for the least amount of money spent) will be used. But if his decisions are not correct, he may cost his company thousands of extra dollars every year because of overpackaging or underpackaging with consequent damage to the product. The consequences of an incorrect decision dictate that the decision-making process must go beyond the realm of guesswork, hunches , or intuition. It is far too important a decision to be le t in the hands of the package materials supplier who does not really know your product and, realistically, whose sole function is to sell his goods Package testing provides the manufacturer with an effective decision-mak ng tool. Through package testing, many of the perplexing questions can be answered quickly, accurately, and consistently.

Package testing is an invaluable aid in the development of new packages and the evaluation of currently used packages and packaging materials. Scienti- f ical ly control 1 ed and designed package testing methods and equi pment have far too long been ignored by the furniture manufacturing industry. Several furniture manufacturers have established their own laboratories and are pre- shipment testing every piece they make.

Not only does package testing result in better, more functional packages; it often is a device whereby packaging costs can be reduced. Through package testing, overpackaging can be virtually eliminated. Also, as a side benefit of testing, quality control problems can often be solved. A great number of articles fail, not because the package is inadequate, but because of some inherent deficiency in construction design or manufacturing techniques. In nearly all of these cases, it is much easier (and always significantly less expensive) to make a change in construction than to change the packaging. Package testing can never be used to replace creative package design or the valuable services provided by the packaging materials suppliers. It can, however, be a valuable research device and decision-making tool for the furniture manufacturer in answering his heretofore "unanswerable" questions.

Mi scel 1 aneous

The whole subject of packaging is closely related to transportation, and the various means of getting goods from one place to another. We have seen some rapid developments in transportation over the past twenty years, and the next ten promise to be even more exciting. Air cargo, containerization, piggyback trailers, and electronic data processing are but a few of the new techniques being used to improve and speed up freight transportation. It behooves any product engineer to keep up-to-date on some of the new ideas in transportation which might possibly have an effect on the way furniture is packaged or constructed in the future.

3 24 In an attempt to reduce damage in transit and cut shipping expenses, many furniture manufacturers either own or lease a fleet of trucks rather than use a common carrier. There are many arguments pro and con for having one's own trucks, but the important thing to remember, as far as furniture construction is concerned, is that with your own trucks you can pack without regard to any standard classification which usually means reduced packaging costs. However, many manufacturers do not want to be in the trucking business, and they must pack according to common carrier specifications. For this reason, increased emphasis should be placed on package design and construct i on.

325 PACKAGE DESCRIPTION 2-F (1) FurDiture must be packed for shipment in conformity with Methbd No. 1, 2 or 3 below. Whether Method No. 1, 2 or 3 is used, furniture must be without legs or with legs not exceeding 8 inches in length except when cross sectional area of leg at smallest dimemion is not less than 2.25 square inches, legs may be 9 inches in length or when le s are of one piece solid extruded metal extending full height of piece, legs may extend not more than 10% inches bjow bottom of article. Finish of furniture must be entirely dry, and fragile projecting hardware, knobs or pulls must be removed or ade uately protected. Fragile galleries must be removed or protected by pads of sufficient thickness to provide lev2 top. (2) Where Method No. 1 or 2 is used, all finished surfaces.must be completely covered with blankets securely fastened to furniture. Blanketa must comply with specifications of Item 42352, Note 4, Paragraphs (11, (2), (31, (41, (51, (IO), (12), (14) or (23). Each top corner of article must be further protected by corm ated form not less than 4 inches long in all directions from ita inside corner, made of not less than two plies of doufle-faced corrugated fibreboard or one ply of double-wall corrugated fibreboard, or made of not less than two Lies of molded corrugated fibreboard, each ply backed with paper weighing not less than 26 pounds per 1,OOO sq. i.The molded corrugated fibreboard must weigh not less than 50 ounds per 1,OOO 8 It. and have between 22 and 24 flutes per foot, (OR a three-ply corner pad preformed to a rig!t angle, 9 inchesqong, and not less than 2% inches wide in all directions from ita inside corner, V-notched in center to permit folding.around corner, made of molded corrugated fibreboard weighing not less than 50 ounds per 1,OOO square feet having 22 to 24 flutes per foot, hacked with Kraft paper of not less than 50 pounds Basis weight. Note I.--Rlicre in furniturc packagcs reference is made to specific paragraphs of this Sote, blankets, see Sote 9, Item 42355. pads, see Sotc 9, Item 42355, bap or envelopes must comply with the following re uirementr. All specifications are minimum requirements and no allowance will be inadeqor manufacturing tolerances. (1) Macerated paper blankets constructed with kftpaper covers not less than 30 Ibs. basis weight. Kraft paper must be coated with adhesive on inner side of sheet. Filler of macerat.ed paper must be of uniform thickness overall. 13lankets must be sealed on four sides, and must weigh not less than 144 Ibs. per 1,OOO sq. ft. (2) Shredded paper packing blankets constructed with Kraft paper covers not less than 30 Ibs. basis weight. Filler of shredded paper must be evenly distributed in each blanket to correspond to a minimum weight of 288 Ibs. per. 1,OOO sq. ft. The shredded paper must be secured to outer paper wrapper to prevent shifting. (3) Wood-wool packing blankets constructed with Kraft pa er covers not less. than 30 pounds basis weight. The ed es of the paper must be rapped not less than % inch and be glued entire length of%lanket. The wood-wool must be not leas than 120 knife cuts per inch and must be manufactured of thoroughly seasoned timber, moisture content not exceeding 25 per rent,, oven dry basis, and must be evenly distributed. The wood-wool must be secured to outer paper wrapper to prevent shifting. Blankets must weigh not less than 144 Ibs. per 1,OOO sq. ft. (4) Wood fibre felt blankets not less than M inch thick, see Note 6, item 42353. Wood fibre in the blanket must weigh not less than 54 ounds per 1OOO square feet and must consist of new wood fibres, felted and bonged together into a homogenous mat of uniform thickness and surfaced on one side with Kraft paper basis weight not less than 30 pounds. (5) Creped cellulose wadding blankets, embossed. having a minimum thickness not less than % inch, see Sote 6. item 42353. surfacrll on outer side with Kraft paper basis weight not less than 30 Ibs. Cellulose wadcling in thc blanket must weigh not less than 56,lbs.per 1,00(1 s uare feet excrpt that when wadding contains no asphalt or other hinder and is nit& of not less than 607.sulphate fibre, the wcight per 1,OOO square feet, of the cellulose wadding in the blanket must be not less than 49 Ibs. (10) Cotton waddin cushioning blanket weighing not le- than 50 Ibs per 1,ooO rq. ft., glazed on bo& rides. (12) Shredded paper and wood-wool blankets constructed with Kraft paper covers not leas than 30 Ibs. basis weight. Filler must consist of wopaper shredded in widthr not more than pa ineh wide and loo/, wood-wool by volume, distributed evenly in each blankrt to correspond to a minimum weight of lil Ibs. per 1,OOO sq. ft. Filler must be secured to outer paper wrapper to prevent shifting. (14) Indented paper blanketa constNcted of four thicknews of indented aper, inden& tiom no arranged as to prevent nestin in each other, and two thic\newes of chip paper, not indented, armn ed one thicfness on to and one thickness on bottom of the four thicknesses of incflented paper. All six tEicknesses must be enclwed in a sleeve of machine glazed Kraft paper. Each thicknens of indented paper and chip aper must have a basis wei ht of not less than 60 Iba. and the machine lazed kraft muat have a basis wei itof not leu than 30 Ik. The blanket must ge not lean than W inch thick, see aote 6, item 42353. (23) Blankets or pads made of double-faced corrugated fibreboard not thinner than A-flu& the cormgating medium and each liner complying with the thickness and weight basin provided in Rule 41, Section 2, to which must be laminated creped cellulolle wadding. Wadding must be not less than .06 inch thick, weighing not less than 20 Ik.Der 1,ooO q.ft., aurf-4 00 QE~side with Mdftpaper, baais weight not than 26 Ibn.

NOTE-Where Method No. 1 or 2 is used each top corner of article must be protected by corrugated or molded corrugated Esrm. For specifications of forms, see paragraph (2) above.

326 PACKAGE DESCRIPTION 2-F -OD No. 8 (7) T of furniture murt k rotaated with non-abrmiw pad material and all 6n&hd"a iwludiag top mtmt %en be ooved with &aft papa? WeighiqJ not leu &% lb. pa? ream. CleJamwo of not leu than 1% inohm between furniture and inaide oontaiw w Ir murt be maintained b Wkrp layem of doubla-faaed corrugated boud or rob of oingldwd corrugated board, either not 1- than inahea diameter, glued at mu intamah to doubldacd corruga@ rtripr. Stripr murt be not lea than 8% inohm in width .nd murt be amangod on furniture 10 that container mll rwt on corrugated fornu on top md at the front and ddw. Stripr mutbemaredin plM0 With dhhpe, and: (8) Auniture must be in container made of doubldaaed corryted 5breboud tsrting not len than #w) lb. aompletdy cbveriq front back and ddea of funukw. When top ooatainer dow not haw u dotted or overlap om- rtrwtion doublefaaed corrugated Bbreboud cap tsrting not ken than %76 lk. m9c- top of furniture .ad 01ulap ddsr, front and baak not leu than 4 inahw. (0) Container mu& have fhngw or fl at bottom not 1- than 8 iwlfsr wide. Mturemurt rat on a four-pIeoe frame made of not lesa than %x!2Ech lumber, diagonally braced. +ArticIea with leg detached must bave bottom dgeu protected from direct contact with wood base frame by E&, adding or furniture ides. When frame xs mqde pt lumber not less than y inch thick and hss end lap io..& pt afl cornem each memgr beii receased with doinymemben80 that the joints form a flush dace,pints cllnch-nailed 4th not lass than three nail8 in each corner, iagonal brace may be omitted. Frame must fit into base of container and bpttom fl~or flr must fold over frame and be neourely nailed at each corner mth two nul8 and an additional nsll eve 16 lnches or gaction thereof of rimeter of bane skid. Nails must be coated and have hedo not lass than H inch%ameter or nail8 with wanhem 20, le^ than W inch diameter. In lieu of nailing, container fllrnges may be necured by two wood rumen nailed or stapled lengthwine of package. .Staph must be made from 16 gauge galvded flattened steel wire, coated, with crown not lass than mch and with le 1 inch diverging into the wood. Pack- must Etrapped with not lea than two me& #trap, not leas tf& h.015 inch.

\ \ flanges 3 inches wide

30 Ib. Kraft Paper

\ f / Corrugated clearance form

4/24/62

327 PACKA6E DESCRIPTION --2-F METHOD No. 1 In containera mrrde of double-faced corrugated fibreboard terthg not less thm 200 lbr. covering top and dl four rider of furniture. Container must have regular dotted top or other construction to provide a double thioknew of anme material at top. IQote.-Cap may twt not lean than #K) Ibr. when wood, fibre or metal b used under rtrapa to prevent cutting of cap. (4) Furniture md wrapper must fit into wood tray constructed of not kaa than %x2% inch lumber consirting of two piece# forming "em and four piecw on edge forming ridw and ends of tray except that furniture without lega or with rtron rturd le not exceeding 6 inchw in le0 th msy be eecurely fMhIed to &piece frame made of not lean thm %x2# inch YumEr, diae;onall braced, of rucg aize that no part of bmOF lege pro'ecta beyond skid. +Artic!w with le68 detached must have%ottom edges protected from direct contact with woodbase frame by pads padding or furmture glidea. When frame ie made of lumber not leea than inch thick and hss end lap joints at ad comem each member being receased with adjoining membera 80 that the jointe form a flush eurface, joints clinch-nailed with not lese than three nail8 in each comer, diagonal brace may. be omitted. Wrapper must be eecurely fastened or nailed to back of furniture and package must be strapped with not lea than two metal atrape not lees than s&.Ol6 inch.

k 200 LB TEST BOX

OPEN BOTTOM

furniture nurt bo padded

4/24/6 2 4PA

328 PACKAGE DESCRIPTION METHOD No. t 2-C (6) In aontaiwr made of doubldaaed aorrugated fibreboard htiq not less than a00 Ik. Wrrrpper mum have dotted or overlap top or other aonatruation to rovide a double thioknwa of name matend at top; muse bottom flangm or &pr not lew than 8 inahwi& and muat aover top and all four ddea of furniture. (6) Furniture must rest on a four-pieoe frame made of not lees than wsinoh lpber, diagonally braced. Artialea with legs detaahed must have bottom edr protested from direct contact-mth wood bane fr+e by pade , padding or furniture glider. When frame is m e of umber not lean than % inch tluak and hse end lap joints at ell aomers, eaah member behg reawed with adjoinin members so that the join? form a flush swfsce joints clinah-nailed with not lean than three nails in each comer, %agonal brace may be omitted. Frame must ht into bane of container and or flap of oonher must fold over frame and be aeaurely nailed or et8pIed at each comer with two ttaz and an additional ndevery 16 inches or fraction themdf, or an additional st8 le every 8 inohea or 5;:fraation thereof, of perimeter of bane frame. NaiL must be aoated and have he& not!= t&m %.inah.diameter or drwith wanhers of not lean than inch diameter. Staples must be made of % mah steel mre mth orown not less than inah, and with % inode diverging into the wood, or with crown not less than 1 inch and with inoh lege diverguy into the wood. Infeu of nailing or stapling, oontainer flyen or flap may be mured by two wood nu" mi ed or stapled lengthwise of pka~ Staplea must be made rom 16 paw galvarrired flat- tenad steel wire, gum aoated, mth orown not le& han moh and with legs 1% inah Lvergmg mto the wood, OR Furniture must reat on platform Nl imide dimenniona of aontainer made of doublewall corrugated fibreboud, the fibreboud aomplying with Beotiona 2 and 8 of Rule 4! for fibreboa@ testingnot less than 276 d,aonntruated of not 1- than two thioknwnw of mah board, oorrugatwua of one thiahat right anglen to %at of the other thiak- nem. Bbud must be soored and folded eo that not less than four thiaknessw of board parallel the long dimenoion of aontainer full lenfh, and feet or bottom of utide must Tt on. suah four thiakneaaw. When container hu bottom fiangee, ma flangw must be mlyglued over them entm area to aorrugated platform. When aom t.iner hm inner and outer flaw rwh fl.w must be ascured in aomlianae with Rule 41 but need not be gldto

200 LB. TEST BOX Corner Form I R.S. TOP

Bottom must be Diagonal Brace closed by applying wood cleats or May Be Elimi- large head nails. nated By Using End Lap Joints 3/4 Inch Thick Lumber NOTB- Corner form required over 7 blanket on each top corner.

4/24/6 2

329 PACKAGE DESCRIPTION 2-F METHOD No. 1 (8) In wrapper made of doublefaced corrugated fibreboard tenting not le^ than #)o lk. Wra er must cover front and rideo of furniture, overlapping back not less than 4 incheo. Double-faced corm ted #reboard cap teating not less than !2751k. (me Nota) must cover top of furniture and overlap sides, front anrback not less than 4 inches, OR In containem made of double-faced corrugated fibreboard teating not Ieaa than a00 lk. covering top and a11 four ridw of furniture. Contriner must have regular dottad top or other COUII~IC~~OOto provide a double thicknen of oame material at top. Note.-Cap may teot not lea than a00 Ibs. when wood, fibre or metal in uwd under strap8 to prevent cutting of cap. (4) huniture and wrapper must fit into wood tray c?natmoted of not leaa than %dg inch llrmber com.istiog of two piecer forming nmners and four pieces on edge formmg ridw and en& of tray except that furniture without leg, or with rtron sturd le not exceeding 5 inches in lyth may be securely fastened to Cpieoe frame made of not lea than Hx28 inch fumRr, diagonally braced, of suc size that no part of base 0' legs pro'ects beyond skid. Urticlea with lep detached must have bottom edges protected from direct contact with wood baae frame by pads padding or furniture glides. When frame is made of lumber not less than % inch thick and hae end lap joints at ali cornem each member being recewe$ with adjoining members so that the joints form a flush surface, joints clinch-nailed with not less than three mle in each comer, diagonal brace may be omitted. Wrapper must be securely fmtened or nailed to back of furniture and package must be strapped with not less than two metal straps not less than %.OK5 inch.

/ r

\ ?add I

\ NOTE- Corner form required over blanket on p corner.

S/a I 24 lurbor Tw .try8 $/a x ,015 4/2 4/6 2 .tool 4QA r

330