BASIC MACHINING Foreword MIC has produced this book for us in its Industrial Maintenance Journeyman Programme and it is specifically designed to introduce the basics of maintenance.

This book is intended for use as a reference text to be supplemented by notes and explanations and does not stand alone.

Compilation of this book was completed with standard published material, Tel-A-Train and resource personnel at MIC. No claim is made to the ownership of any material contained herein.

THIS BOOK IS NOT FOR SALE

REFERENCE TEXT USED TABLE OF CONTENTS

1. Measurement 3

2. Hand Tools 7

3. Drills and Drilling Machines 22

4. Mills and Milling Machines 24

5. Grinding and Grinding Machines 43

6. Lathes 55

2 MEASUREMENT

VERNIERS

The Vernier Caliper is one of the most versatile precision measuring instruments available to the skilled metal working craftsman. Most toolmakers and prefer at least one good vernier and a few foe their own personal use on the job.

A is the name given to any scale making use of the difference between two scales which are nearly, but not quite alike, for obtaining small differences.

The difference between the smallest division on the fixed scale and the smallest division on the Vernier or slid- ing scale is the key basis on which the vernier caliper works.this difference is the accuracy of the Vernier.

METRIC VERNIERS

Vernier (1) On the fixed scale, real standard dimensions are accurately engraved.The smallest division is 0.5mm. On the Vernier scale 12mm is divided into 25 equal parts. The smallest division is therefore 12/25 = 0.48mm. Difference in divisions – 0.50 – 0.48 = 0.02mm. Vernier (2) On the fixed scale the smallest division is 1mm. On the Vernier scale 49mm is divided into 50 parts. The smallest division on the vernier scale is therefore 49/50 = 0.98mm. Difference in division = 1.00 – 0.98 = 0.02mm.

ENGLISH VERNIERS:

Vernier (1) On the fixed 1” is divided into 10 parts, then each tenth is further sub-divided into 4 parts 1/40 = 0.025. On the Vernier scale 6/10 is divided into 25 parts 6/10 – 25 = 6/10 × 1/25 = 0.024” Difference – 0.025 – 0.024 = 0.001”. Vernier (2) Fixed Scale: 1” is divided into 20 parts = 1/20 = 0.05” VERNIER SCALE: 49 divisions on the fixed scale, (which is 49 × 0.05 = 2.450”) is divided into 50 parts. The smallest division on the Vernier scale is 2.150 ÷ 50 = 0.049” Difference – 0.050 – 0.049 = 0.001”

GENERALLY –

The accuracy of metric verniers is therefore = 0.02mm. The accuracy of English verniers is therefore = 0.001”

3 HOW TO READ ANY VERNIER SCALE

1. Note the real dimension on the fixed scale which the zero on the vernier scale has already passed. i.e. the number of whole divisions on the fixed scale to the left of the zero on the vernier scale. (These divisions maybe whole millimetres or half millimetres 0.10”, 0.05”, 0.025” depending on how the fixed scale is divided).

2. Locate the line on the vernier scale which coincide exactly with a line on the fixed scale.

3. Check the number of divisions on the vernier scale between the zero and the line which coincides.

4. Multiply this number by the accuracy.

5. Add to the previous noted dimension.

This is also applicable to the height .

Measuring Exercises.

Review Questions.

RANGE

Internal measurements (error).

Provision for setting dividers.

Clamp nuts, fine adjustments.

4 VERNIER CALIPER

Vernier are precision tools used to make accurate measurements to within 0.02 mm or 0.001 ins. They consist of an L-shaped bar and movable jaws which are graduated on both sides, one side for taking outside measurements and the other for inside measurements. The bar contains the main graduations and the vernier graduations are on the movable jaw.

Limitations arise through application of the vernier caliper due to the fact that they depend a great deal upon the skill and the capabilities of the user. Feel is essential in determining when the jaws of the instrumen5t are truly along the line of measurement. This is especially true with inside measurements, whereby touch and manipula- tions determine when the maximum distance on diameter is obtained. Feel is a necessity in attaining precise measurements, and in addition, undue forces will cause excessive wear or damage to the instrument. Wear and manipulative factors have a direct influence on reliable measurements and one must be certain to read the proper scale.

Figure 1. VERNIER CALIPER

THE

When parts are to be measured to the second placed of decimal in the metric system, or the third place in the English system, the micrometer is commonly used.

Sleeve (with main scale) Spindle

Ratchet Knob Lock Thimble (with rotating Vernier scale) Frame

Figure 6. MICROMETER

5 WORKING PRINCIPLES AND CONSTRUCTION

A spindle with external thread for measuring and internal thread at the end for fixing the thimble to the spindle.

The barrel has the internal measuring threads and is a one piece unit with the frame. Over the frame is a thin walled sleeve with the graduations in inches or mm etched longitudinally. This sleeve can be turned with a spe- cial spanner for small adjustment. There is also a nut for adjusting the longitudinal position of the thimble.

The ratchet is mounted on the screw which holds the thimble and spindle together and held in position by a small screw. The ratchet slips when a certain fixed amount is applied. This gives consistent readings.

The right end of the thimble sometimes called the rim is divided into 50 equal divisions around its periphery. The measuring threads are very accurate and are precision ground with a pitch of 0.5 mm, hence for each revo- lution the spindle moves 0.5 mm. Therefore one division on the thimble is

= 0.5 = 1 = 0.01 mm this is the accuracy of the micrometer. 50 100 The barrel is graduated in mmm and 1//2 mm. Reading the micrometer (0-25 mm) (1) Note the number of whole millimetre visible between 0 (zero) on the sleeve and the end of the barrel. (2) Check to see if a 1/2mm division is visible before the end of the barrel on the lower half of the thimble graduations, if so add .5 to the whole mms in (1) (3) Read off from the thimble the amount of hundreds below the horizontal centre mark on the sleeve and add to the previous amount

ENGLISH MICROMETERS

The measuring threads are 40 T.P.I hence a pitch of 1/40” = 0.025 25 threads The thimble is divided into 25 parts therefore one division on the thimble is = 0.025 = 1 = 0.001 one thread 25 1000 This is the accuracy of the inch micrometer. The barrel is divided (graduated) in 1/10” which is further divided into four parts hence each division is 1/40 = 0.025

Reading the micrometer 0 – 1”.

(1) Record the tenths – the numbered lines before the thimble edge (2) Record the amount (number) of division between the last numbered line and the edge of the thimble and multiply them by 0.025. (3) Record the thousandth on the thimble below the horizontal centre mark on the sleeve (4) Add 1, 2 and 3

6 HAND TOOLS

FILES

A is a hand cutting tool, made of high carbon tool steel, used to remove surplus metal and to produce a fin- ished surface.

Files may be procured in a variety of shapes and types, therefore the student is required to secure the necessary knowledge, not only to distinguish one file from another, but also to determine their particular use.

EDGE TANG

BLADE TOE HEEL LENGTH HANDLE

SHOULDER

Length – the distance from heel to toe Body – is that part of the file that does the cutting. It comprises of the faces and edges of the file which are made up of a large number of cutting edges. Heel – is the uncut portion of the file Toe – the extreme end of the file opposite the heel Tang – pointed portion at the end of the heel used for securing handle to the file Note: The length of the file is measured by length of body (exclusive of the tang).

CLASSIFICATION OF FILES

Files may be divided into two classes: (1) Single cut files Those files have a series of parallel teeth running diagonally across the width of the surface as shown in Fig. 1. This group included Mill Lathe and files. Single cut files are used when a smooth surface is desired or where hard materials are to be finished. (2) Double cut files These files have 2 courses of teeth crossing each other, one course being finer than the other. These rows produce hun- dreds of cutting teeth which makes for fast removal of stock and easy clearing of chips. Both single and double cut files are manufactured in various degrees of coarseness. This is indicated by the terms bas- tard, second cut and smooth. Files are manufactured in many shapes and cross sections. These are indicated by the terms, hand, mill, , round, half round, triangular etc. 7 FILING PRACTICE

Filing is a highly skilled and accurate phase of the machine trade. Skill in filing can only be acquired through constant practice.

If the following rules are observed in the early stages of practice filing, the operation will require much less ef- fort and the quality of the finished work will be of higher order.

The Filing Positions

1. Hold the file handle in the right hand, the index finger pointing toward the toe of the file.

2. Support the toe of the file with the thumb and forefinger of the left hand. Do not press heavily with the left hand as this will rock the file and produce a rounded surface.

3. Assume a comfortable stance while filing, the left door forward of the right so that the body will be well balanced.

To File a Flat Surface (Note: A FILE MUST NEVER BE USED WITHOUT A HANDLE)

1. Place the work in the vice, the surface to be filed should be aout 3/8” above the jaws. (Use soft jaws to protect finished work). 2. Select the file suited to your particular. 3. Grasp the files as illustrated above. 4. Apply pressure on the forward stroke, release pressure on return. Note: These proper filing stroke is produced by the hands and the arms – do not sway the whole body backward and forward as you file. 5. Strive for a straight horizontal motion to prevent motion to prevent rounding of the surface. 8 6. Do not rub the surface of the work with your hands as this will leave a thin film of grease and the file cannot cut properly. 7. File at about 50 to 60 strokes per minute – too fast a stroke only dulls the file and tires the operator. 8. Clean the face of the file frequently with a file card to prevent small chips from scratching the surface. 9. Remove the work from the periodically to check for flatness and squareness of the files surfaces.

Draw filing serves three purposes, it removes tool marks, gives a flat smooth surface and makes a grain line -fin ish to the length of the work-piece. When draw filing only a single cut file is used because it cutting action will give a much smoother finish than a double cut file.

To Draw File: Use only as a finishing operation. 1. Select a single cut file. Be sure it is clean. 2. Clamp work high enough in vise so that fingers will be clear of vise jaws when filing. 3. Place file on work-piece and hols as shown in diagram so that index fingers are firmly pressing the file flat against the surface to be filed. This will prevent the file from rocking and giving a rounded surface. 4. Draw file back and forth over work surface until desiredfinish is obtained. Check periodically for flatness using a straight edge. Note: If the file seems to be dull use the area near the heel since this area generally does not get the wear and tear the rest of the file gets and will therefore remain somewhat sharper.

9 COMMON SHAPES OF FILES

MILL FLAT HAND PILLAR

SQUARE ROUND 3-SQUARE HALF ROUND

CROCKET CROSSING WARDING BARRETTE KNIFE

CARE OF FILE

Files should be cleaned with a file card during and after use. This will help to keep the teeth clean and promote smoother cutting.

NB. Chalk is often used to prevent “pining” during filing; ensure that file is cleaned thoroughly immediately after to prevent corrosion occurring.

10 11 12 TOOLS

THE is made of hundred round steel of 3mm to 6mm diameter (used for marking layout lines and scribing fits). It is about 20 cm long with a pencil like point on one end and a hooked end on the other; the centre has a knurled portion for griping efficiency.

PRICK Is usually made of round section cast steel, sometimes of octagonal section carbon steel ranging from 8mm to 12mm in diameter about 90 to 100mm long. Knurled in about 12mm from the head and 20mm from the point. Used for punching layout lines so that they may be more visible and permanent. The included angle at the point is 30º-60º.

CENTRE PUNCH Of the same features as the prick punch but the included angle at the critical point is about 90º. Used for marking the centre point of holes to be drilled for the purpose of giving a start to the drill; marking the centre of circles preparatory to scribing circles or arcs as in layout and construction. The point is hardened with the head being left soft.

DIVIDERS Are designated by the maximum span to which they may be extended ranging in steps of 50mm usually from 150 to 300mm used for scribing circles and arcs and for marking off lengths and applied construction. The span is adjusted by a nut and screw arrangement fixed to the legs.

TRAMMELS Are applied to lengths which extend beyond the range of dividers. Two adjustable legs are arranged in a bow about 350mm long which may be adjusted to any required length within its range, used in the same way as the divider.

HERMAPHRODITE CALIPERS – (Jennies or odd-leg) A useful tool being half clincher and half caliper; used for scribing lines parallel to a straight edge or circular disc or bar, also used for locating the centre of round sectional stock.

TRY SQUARE Has a steel blade riverted in the solid stock and ground to form a true right angle. Low classes of squares may be used for scribing lines at right angles to surfaces but those of the high grade are specifically for reference only. A good square is valuable and should be treated with as much care as a watch. When using a square, care should be taken to ensure that its blade is held perpendicular to the surface being tested or error may occur. FEELER GAUGES Consists of a series of steel blades or leaves, having thickness ranging 1 – 0.02 – 1mm. Used for gauging small distances and clearances or measuring small slots and keyways.

COMBINATION SQUARE Consists of a blade which may be used in conjunction with any one of three heads. The various heads enables the tool to be used as a square, a or a gauge for marking lines passing through the centre of round bars. When a high degree of accuracy is required for squareness, a solid stock is recommended in preference to the square of the combination set.

13 THE VARIOUS HEADS ARE: (1) The square head – Used in very much the same way as the try square but is applicable to less precise work (2) Centre head: Used for locating the centre of round stock. (3) The protractor head: Used for locating lines at any angle to an edge.

VEE BLOCKS Are used for holding round stock when marking out and drilling, they are usually made in pairs. The large sizes are made of cast iron and the smaller sizes of mild steel; case hardened and ground and used in conjunction with “4” clamps which is accommodated by slots cut on the sides of the “V” block.

SURFACE PLATE A flat surface is one of the fundamentals of workshop engineering and although most of the surfaces we produces we produces are flat enough for their purposes, most of them would not be from the precision precision engineer’s standard of flatness. The which has a surface of proved flatness is the criterion of the workshop. Used for testing flatness, squareness and is also the datum for marking out and checking operations in conjunction with the surface gauge, and dial indicators.

ANGLE PLATE Cast from cast iron and machined to a true right angle, used for supporting a surface at right angles to the surface of the table, and is provided with a series of holes and slots for accommodating the bolts necessary to secure articles to it.

PARALLEL STRIPS These are useful for supporting work on the marking-off table, and may be of cast iron smoothly machined, scraped or ground, or of steel which when hardened and ground makes very durable strips. The corresponding widths of a pair of strips must be exactly similar dimensions. The strips must be perfectly parallel and all their faces square; their lengths are relatively unimportant.

SURFACE GAUGES OR SCRIBING BLOCK A surface gauge is used in conjunction with a marking-off table to mark out lines parallel to a true surface. This gauge consists of an accurately machined block base with an adjustable pillar and attachment for holding a scriber. There is an additional fine adjustment thumb screw for the pillar. In the base may be fitted to frictionally held pins which can be pushed down to slide along the edge of the marking off table, it also has a scriber which may be adjusted to any height within the range of the pillar.

14 It is a manual tool consisting of a steel arch on which a saw is mounted (blades may be of high speed or carbon steels indented and tempered). The blade has perforations at its ends for fixing on to the arch by means of pins placed at the supports. The frame has a fixed support, and an adjustable one which is cylindrical and threaded, carrying a butterfly nut to tighten the blade (fig. 1).

The is used to cut materials and to make or begin grooves.

Characteristics and Structure: The arch of the saw is characteristic because it can be regulated and adjusted according to the length of the blade. It has a screw with a wing nut which tightens the saw blade. For its manipulation it is equipped with a handle made of wood, plastic or fibre, or metal. The blade is characterized by: Its length, which commonly measures 8”, 10” or 12” from centre to centre of the holes; by the width, which is generally of ½”; by the num- ber of teeth per inch, which is generally of 18, 24 or 32t/1” (fig. 2).

15 The saw has set teeth, which are lateral displacements of the teeth in an alternated as shown in fig: 3 to 7.

SUMMARY Saw arch – carbon steel tempered, tooth blade – high speed or carbon steel handle – wood, plastic or fibre

Characteristics: Length – width – number of teeth per inch Selection: According to the thickness of the material (more than 2 pitches of the teeth); according to the type of the material (large number of teeth for hard materials).

16 Blades must be strained slightly in the frame and slow firm and steady strokes (50 strokes/min) should be used, lifting the blade slightly on the return stroke.

Breakage of blades may be caused by the following:- (1) rapid and eratic strokes (2) too much pressure (3) blades held too loosely in the frame (4) work not held firmly in the vice

Solid metals should be cut with a good pressure and thin sheet and tubes with light pressure. Insufficient pressure at the start of the cut may cause the teeth to glaze the work, and so rub away their egde.

The is a tool of impact, consisting of a carbon steel block with a wooden handle. The part with which the hammering is done is tempered. The hammer is used in the majority of industrial activities, such as: mechanics in general, civil engineering, construction and the like. The hammer is characterized by its shape and weight. By its shape: Ball-peen hammer (fig. 1). Peen (figs. 2, 3 and 4) These are the most common types used in mechanic shops.

17 By its weight: The weight varies between 200 to 1000 grammes. Conditions of use: A hammer to be used must have the handle in good condition and well fitted at the wedge. Care: Avoid hammering with the handle and do not use it as a lever.

The MALLET is a tool of impact, made up of a head of wood, aluminium, plastic, copper, lead or leather, and a wooden handle (figs. 5, 6 and 7) It is used to strike on work or materials whose surfaces must not be deformed by the effects of the blows. Plastic or copper heads can be replaced when torn (fig. 6).

18 Conditions of use: a) The mallet head must be well fitted into the handle free from burns. It must be only used on smooth sur- faces. b) It must be used only on smooth surfaces.

TECHNICAL VOCABULARY Mallet of Rolled Leather – Hide-faced Mallet; ride-hide mallet

A COLD

1. The chisel is forged to shape and normalized by heating by heating to a dull red in the smith’s hearth and then removing it and allowing free cooling in the open air. Normalising removes any internal stresses set up in the metal by the hammering. 2. The chisel is filed to shape, draw filed, and finished with emery cloth. 3. Hardening the chisel is performed by heating 8 or 10 cm of the cutting edge half of the chisel to a cherry red colour in daylight and then quickly quenching it in a bath of water. The chisel may be heated in the smith’s hearth or with the blowpipe. 4. Tempering the chisel is carried out by polishing one surface with emery cloth. It is then held in the and heated at a spot about 8 cm from the cutting edge with a blowpipe. Colours will begin to form and travel towards the cutting edge. When the light purple reaches the edge, the chisel is quickly quenched in a bath of water. Tempering removes the brittleness and sets the tool to its correct degree of hardness. 5. The chisel is polished with emery cloth. (It is too hard to file, and would ruin the file teeth.)

Another method of hardening and tempering is to combine the two processes into one. The 8 to 10 cm of the cutting edge half is heated to a cherry and then the first half only of the heated portion is quenched.The tip is quickly polished with emery cloth so that the colours can be seen as they travel from the reservoir of heat left in the shank. When the light purple reaches the cutting edge the chisel is quenched. Small articles can be hardened and tempered by placing them on a hot plate.

19

Chisels are made of octagonal steel. They are forged to shape, then hardened and tempered. The four main types are illustrated and are known collectively as cold chisels because they are used to cut cold metal.

The flat chisel is used for general purposes, chipping, cutting sheet and plate metal, and removing surplus metal from surfaces.

The cross-cut chisel is used for cutting square grooves such as slots, channels and keyways. The cutting edge is slightly wider than the supporting metal, to provide clearance.

The cutting edge of the chisel is ground half-round. It is used for cutting grooves and for drawing over the cen- tres of holes the have run off during drilling

The chisel is drawn down to a square section.

20 The flat chisel. A diagram of a chisel point in the action of cutting at Fig. 111(a), where the angles of rake and clearance are indicated is usual, the point is ground symmetrical, the rake and clearance depend upon the angle if inclination (A) between the chisel and surface. A clearance of about 10º would be suitable so that for a 60º point would be necessary to hold the chisel with angle A = 10º + 30º = 40º the rate would then be 90º - 40º + 30º) = 20º. This is shown a (b) cutting a hard and brittle metals calls for less rake than soft metals, the angle must be less for the softer metals, and the following table give able values for the angles:

As an exercise, the reader should calculate the angle (A) at (WORD MISSING) of the above chisels should be sloped to give a clearance of 10º, and find the cutting rake.

21 DRILLS AND DRILLING MACHINES

DRILLING

Probably one of the first mechanical devices developed was a drill to bore holes in various materials.The princi- ple of a rotating tool making a hole in various materials is the one in which all drill presses operate.

The drilling machine is essential in any metal working shop. Fundamentally, a drilling machine consists of a spindle which turns the drill and which can be advanced into the work, either automatically or by hand, and a work table which holds the workpiece rigidly in position as the hole is drilled.

A drilling machine is used primarily to produce holes in metal, however, operations such as tapping, reaming, counterboring, countersinking, boring and spot facing can also be performed.

Basically a drilling machine is made up of a base, column, table and drilling head.

Base: The base, usually made of cast iron, provides stability for the machine and also rigid mounting for the column.

Column: The column is an accurate cylindrical post which fits into the base, the table, which is fitted to the column, maybe adjusted to any point between the base and head. The head of the drill press is mounted near the top of the column.

Table: The table is used to support the work piece to be machined. The table, whose surface is at 90º to the column maybe raised, lowered, and swivelled around the column. On some models it is possible to tilt the table in any direction for drilling holes at an angle. Slots are provided in most tables to allow the clamping of jigs, fixtures or large jobs.

Drilling Head: The head, mounted close to the top of the column, contains the mechanism which is used to revolve the cutting tool and advance it into the work-piece. The spindle which is the round shaft that holds and drives the cutting tool, is housed in the spindle sleeve or quill. The spindle sleeve does not revolve but slides up and down inside the head to provide a down feed for the cutting tool. The end of the spindle may have a tapered hole to hold taper shank tools.

FEED RATES FOR DRILLING

The feed rate for drilling is governed primarily by the size of the drill and by the material being drilled. Other factors that also affect the feed rate that can be used are the work-piece configuration, the rigidly of the machine tool and the work-piece set up, and the length of the chisel edge.

22 CUTTING SPEED FORMULAS

Most machining operations are conducted on machine tools having rotating spindles, and the cutting speed in feed per meters per minute must be converted to a spind;e speed or to revolutions per minute; This is accom- plished by use of the following formulas:

For metric use units only N = 1000V πD

For inch units only N = 12V π D

N = Spindle Speed: R.P.M. V = Cutting Speed: f.p.m, or m/min. D = Diameter: IN or mm (for turning, D is the outside Diameter or the work-piece. For milling and reaming, D is the Diameter of cutter.

SAFETY

Accidents happen every day in factories worldwide. The higher percentages of accidents are caused by the worker, the lesser by the employers. A lot is time and money is spent to increase the level of safety in the indus- try in so doing the loss of lives, limbs and time is greatly reduced.

Safety equipment is available in every industry and are to be used at all times in the work area. There are rules that must be followed”

1. Secure loose clothing, (Ties, sleeves.)

2. Remove all jewellery. (Watches, chains, rings.)

3. Always wear eye protections. (Specified for job)

4. Special care must be taken when handling power tools, sharp objects.

5. Report safety hazards as soon as they are noticed.

6. Report damage machinery and tools.

7. Report all accidents.

8. Do not operate machinery you know nothing off unless you are being supervised.

23 MILLING AND MILLING MACHINES

THE MILLING MACHINE 1. INRODUCTION

Milling may be defined as an operation whereby material is removed by rotation of one or more multi-toothed circular cutters in contact with the work which is fed into the cutter path either manually or automatically. Each individual cutter tooth, successfully removes an equal amount of metal, thus generating a smooth surface as the work progresses.

The more elaborate work usually encountered in modern production workshops is just an adaptation or combi- nation of the above fundamental principles.

It is seen that the milling operation is governed by the type of cutter being used, the purpose of the machine be- ing: - To control the cutter rotation by giving a range of speeds to suit general conditions - To provide means for holding work rigidly while the operation is in process - To control the rate at which the work is passed under the cutter

Milling operation can also be described as horizontal milling or vertical milling.

24 25 The horizontal machine is so named because the cutter sample spindle is the horizontal plane and the vertical machine has a vertical cutter spindle.

FEEDS

The rate of the table movement past the cutter is known as the rate of feed and the movement can be in three directions: longitudinal, cross and vertical. It is defined as the distance in inches (or millimetres) per minute that the work moves into the cutter. The rate used on milling machine depends on a variety of factors such as: - the depth and width of cut - the design or type of cutter - sharpness or the cutter - work-piece materials - strength and uniformity of the work-piece - type of finish and accuracy required - power and rigidity of the machine

As the work advances into the cutter, each successive tooth advances into the work an equal amount, producing chips of equal thickness. It is this thickness of the chips or the “feed per tooth” , along with the number of the teeth in the cutter, which form the basis for determining the rate of feed. The ideal rate of feed may be deter- mined as follows:

FEED = number of teeth in the cutter X recommended feed per tooth X r/min of the cutter

Under average operating conditions, it is suggested that the milling machine feed be set to approximately one- third or a half the amount calculated. The feed can then be gradually increased to the capacity of the machine and the finished desired. Table 1 and 2 give suggested feed/tooth for various types of milling cutters for rough- ing cuts under average conditions. For finish cuts, the feed/tooth would be reduced to one-half or even one third of the value shown.

26 CUTTING SPEEDS

One of the most important factors affecting the efficiency of a milling operation is cutter speed. If the cutter is run too slowly, valuable time will be wasted, while excessive speed results in loss of time in replacing and regrinding cutters. Somewhere between these two extreme is the efficient “Cutting Speed” for the material be- ing milled.

The cutting speed of a metal may be defined as the speed, in metres/min (or in surface feet/min), at which the metal may be machined efficiently. The cutter must be revolved at a specified number of revolutions per minute depending upon its diameter, to achieve the proper cutting speed. Since different metals vary in hardness, struc- ture and machinability, different cutting speeds must be used for such type of metal. The cutting speeds for the more common metals are shown in Table 3.

Spindle Speed (rpm) = CS × 1000 11 D CS in m

Speed may have to be altered because of the hardness of the hardness of the metal and/or the machine condition. Best results may be obtained if the following rules are observed:

- for longer cutter life, use the lower cutting speeds in the recommended range

- know the hardness of the material to be machined

- when starting a job, use the lower rough of the cutting speed gradually increase to the higher range if conditions permit

- if a fine finish is required, reduce the feed rather than increase the cutter speed

- the use of coolant properly applied, will generally produce a better finish and lengthen the life of the cutter, since it absorbs heat, acts as a lubricant and washes chip away.

27 MACHINING FUNDAMENTALS

Increase or decrease feed until feed until the desired surface finish is obtained. Feeds may be increased 100% or more, depending upon the rigidity of the machine and the power available, if carbide tipped cutters are used. Fig. 12-91. Recommended feed in inches per tooth (high-speed steel cutters).

Cutting Fluids

Cutting fluids serve several purposes. They carry away the heat generated during the machining operation; act as a lubricant and prevent the chips from sticking and fussing to the cutter teeth; and flush away chips.The lubricating qualities of the cutting fluids also influence the quality of the finish of the finish of the machined Fig. 12-93.

Work Holding Attachments One of the more important features of the milling machine is its adaptability to a large number of work holding attachments, each of which increases the usefulness of the machine.

Vises The Vise is probably the most widely used method of holding work for milling. The jaws are hardened to resist wear and ground for accuracy. The milling vise, like other work-holding attachments, is keyed to the table slot 28 with LUGS, Fig. 12-94. The FLANGED VISE, Fig. 12-95, has slotted flanges for fastening the vise of the table. The slots permit the vise to be mounted parallel to or at right angles to the spindle. The body of the SWIVEL VISE, Fig. 12-96, is similar to the flagged vise but is fitted with a circular base, graduated in degrees, Fig. 12-97, permitting it tp be locked at any angle to the spindle. The TOOLMAKERS’S UNIVERSAL VISE, Figs. 12-98 and 12-99, permits compound or double angles to be machined without complex or multiple setups.

Fig. 12-92. Rules for determining and feed

29 30 31 32 33 34 35 36 37 38 39 40 41 42 GRINDING AND GRINDING MACHINES

ANSI B7.1 Safety Requirements for “Use, Care and Protection of Abrasive Wheels”. For your safety, we sug- gest you benefit from the experience of others and carefully follow these rules. Post this near your grinding machine IMPROPER USE MAY CAUSE WARNING BREAKAGE AND SERIOUS INJURY DO DON’T 1 DO always HANDLE AND STORE wheels in a 1 DON’T use a cracked wheel or one that HAS BEEN CAREFUL manner. DROPPED or has become damaged. 2 DO VISUALLY INSPECT all wheels before mount- 2 DON’T FORCE a wheel onto the machine OR AL- ing for possible damage. TER the size of the mounting hole – if wheel won’t fit 3 DO CHECK MACHINE SPEED against the estab- the machine, get one that will. lished maximum safe operations speed marked on the 3 DON’T ever EXCEED MAXIMUM OPERATING wheel. SPEED established for the wheel. 4 DO CHECK MOUNTING FLANGES for equal and 4 DON’T use mounting flanges on which the bearing correct diameter. surfaces, ARE NOT CLEAN, FLAT AND FREE OF 5 DO USE MOUNTING BOTTLERS when supplied BURNS. with wheels. 5 DON’T TIGHTEN the mounting nut EXCESSIVE- 6 DO be sure WORK REST is properly adjusted. LY. (Center of wheel or above; no more than 1/8” away 6 DON’T grind on the SIDE OF THE WHEEL (see from wheel.) Safety Code B7.1 for exception). 7 DO always USE A SAFETY GUARD covering at 7 DON’T start the machine until the WHEEL least one-half of the grinding wheel. GUARD IS IN PLACE. 8 DO allow NEWLY MOUNTED WHEELS to run at 8 DON’T JAM work into the wheel. operating speed, with guard in place, for at least one 9 DON’T STAND DIRECTLY IN FRONT of a grind- minute before grinding. ing wheel whenever a grinder is started. 9 DO always WEAR SAFETY GLASSES or some 10 DON’T FORCE GRINDING so that motor slows type of eye protection when grinding. noticeably or work gets hot. 10 DO TURN OFF COOLANT before stopping wheel to avoid creating an out-of balance condition.

43 These are machines in which the operator grinds work, principally, in the sharpening of tools.

CONSTITUTION It is composed, generally of an electric motor, on the ends of which are attached two emery stones: one, made of coarse grain, serves to trim the work and the other, of fine grain, for finishing tool edges.

USUAL TYPES Pedestal Grinder (fig. 1). It is used in common rough-cutting and in the sharpening of manual and machine tools in general. The power of the standard electric motor is 1 H.P., with 1450 to 1750 RPM.

OBSERVATION There are pedestal grinders with 4 H.P motor power. They are used principally for coarse hewing and for trimming castings.

Parts of the pedestal grinder a) Pedestal – A grey cast iron structure which serves as a support and enables the fastening of the electric motor. b) Electric Motor – which rotates the emery stone. c) Stone Protector – it accumulates the particles loosened from the emery or, when it breaks, prevents the pieces from causing accidents. d) Work Support – It may be fastened at an appropriate angle, the important thing is to maintain, as the diameter of the stone diminishes, some clearance ( from 1-2 mm) so as to prevent small parts from getting in between the stone and the support. e) Visual Protector – Indicated in Fig. 1. It is the most practical for work in general. f) Coolant Container – Used to cool the tools made of tempered steel, preventing the heat produced by the friction between the tool and the emery stone from reducing the resistance of the cutting edge, in case they are annealed.

44 Bench Grinder (fig. 2). It is fastened to the beach and its electric motor has ¼ to ½ H.P power with 1450 to 2800 R.P.M. It is used for finishing and re-sharpening the cutting edge of the tools. In Fig. 3 we have a bench grinder for sharpening metallic car- bide tools.

CONDITIONS FOR USE Grinders and other related machines are the ones that cause most accidents. To avoid this, it is recommended that these points be ob- served: a – When the emery stone is mounted on the motor axle, the revo- lutions indicated on the stone should co-incide with or be a little greater than that of the motor; b – On fastening the emery stone, the hole should be exact and be perpendicular to the flat face; c – The curved surface of the stone should remain concentric with the motor axle. If this is not the case, on turning on the motor, vibra- tions and undulations would be experienced by the work.

In order to dress the grinding wheels, various types of special dressers are used: a – Dressers with tempered steel cutters, with angular shaped grooves (star-faced, fig. 4 or undulated fig. 5); Fig. 6 shows the correct position for the dresser to even off the surface of the grinding-wheel.

45 b – abrasive rod dresser

c – emery wheel dresser with a diamond tip (fig. 80. It is often used in the dressing the wheels on the grinding machine. It is also used in fine grain emery stones of the bench grinders. Figs. 9, and 10, demonstrate the correct position for machining the diameter of the emery stone. The cuts should be very fine and the size of the diamond should always be greater than the grain of the crushed emery stone so as to prevent it from being rooted out from the support.

GRINDING

Grinding is one of the fastest growing areas in the machine trade. Improved grinding machine construction has permitted the construction of parts to extremely fine tolerances with improved surface finishes and accuracy. Grinding has also, in many cases eliminated the need for conventional machining. Often the rough part is fin- ished in one grinding operation, thus eliminating the need for other machining processes. The role of grinding machines has changed over the years, initially they were used on hardened work and for truing hardened parts which had been distorted by heat treating. Today, grinding is applied extensively to the production of unhardened parts where high accuracy and surface finish are required.

46 THE GRINDING PROCESS

In the grinding process, the work piece is brought into contact with a revolving grinding wheel. Each small abrasive grain on the periphery of the wheel acts as an individual cutting wheel and removes a chip of metal (Fig. 1). As the abrasive grains become dull, the pressure and heat created between the wheel and the work piece cause the dull face to break away, leaving new sharp cutting edges.

47 Figure: 2. SURFACE GRINDING VERTICAL

48 Regardless of the grinding method used, cylindrical, centreless or surface grinding process is the same certain general rules will apply in all cases:

- Use a silicon carbide wheel for low tensile strength materials and aluminium oxide wheel for high tensile strength materials

- Use a hard wheel on soft material and a soft wheel on hard material

- If the wheel is too hard, increase the speed of the work or decease the speed of the wheel to make it act as a softer wheel

- If the wheel appears too soft or wears rapidly, decrease the speed of the work, or increase the speed of the wheel, but not above its recommended speed.

- A glazed wheel will affect the finish, accuracy and metal removal rate. The main cause of wheel glazing: 1. The wheel speed is too fast 2. The work speed is too slow 3. The wheel is too hard 4. The grain is too small 5. The structure is too dence which causes the wheel to load

- If the wheel wears too quickly the cause may be any of the following: 1. The wheel is too soft 2. The wheel speed is too slow 3. The work speed is too fast 4. The feed rate is too great 5. The face of the wheel is to narrow 6. The surface of the work is interrupted by holes or grooves

49 50 An industrial diamond, mounted in a suitable holder on the magnetic chuck is generally used to true and dress a grinding wheel.

WORKHOLDING DEVICES

1. Magnectic Chuck In some surface grinding operations the work may be held in a vice, on V-blocks or bolted directly to the table. However, most of the fer…. Work ground on a grinder is held on a magnetic chuck which is clamped to the table of the grinder.

2. Doubled-faced Tape Double faced tape is often used for holding thin, non-magnetic pieces on the chuck for grinding. The tape, hav- ing two adhesive sides, is placed between the chuck and work causing the work to be held securely enough for light grinding.

3. Special Fixtures These are often used to hold non-magnetic materials and odd-shaped work pieces, particularly when a large number of work pieces must be ground.

4. Surface Finish The finish produced by a surface grinder is important, and factors affecting it should be considered. Some parts that are ground do not require a fine surface finish and time should not be spent producing fine finishes if not required.

It should be noted that soft materials such as brass and aluminium will not permit as high as harder ferrous ma- terials. A much finer finish can be produced on hardened on hardened steel work pieces then can be produced on soft steel or cast iron.

51 5. Grinding Wheels Grinding wheels are composed of abrasive m,aterial held together with a suitable bond. The basic functions of grinding wheels are: - generation of cylindrical, flat and curved surfaces - removal of stock - production of highly finished surfaces - cutting off operations - production of sharp edges and points

For grinding wheels to function properly, they must be hard and tough, and the wheel surface must be capable of gradually breaking down to expose new sharp cutting edges to the material being ground.

The material components of a grinding wheel are the abrasive grain and the bond. However, there are other physical characteristics, such as grade and structure that must be considered in grinding wheel manufacture selection.

ABRASIVE GRAIN

The abrasive used in most grinding wheels is aluminium oxide or silicon carbide. The function of the abrasive is to remove material from the surface of the work being ground. Each abrasive grain on the working surface of a grinding wheel acts as a separate cutting tool and removes a small chip as it passes over the surface of the work. As the grain becomes dull, it fractures and presents a new sharp cutting edge to the material. The fracturing action reduces the heat of friction which would be caused if the grain become dull, producing a relatively cool cutting action. As a result of hundreds of thousands of individual grains all working on the surface of a grinding wheel, a smooth surface can be produced on the work piece.

One important factor to consider in grinding wheel manufacture and selection is the grain size. The size of the abrasive grain is important since undersize grains in the wheel will fail to do their share of the work, while over size grains will scratch the surface of the work.

The factors affecting the selection of grain sizes are:

- The type of finish desired: coarse grains are best suited for rapid removal of metal. Fine grains are used for producing smooth and accurate finishes

- The type of material being ground: generally coarse grains are used on soft material, while fine grains are used for hard materials

- The amount of material to be removed: where a large amount of material is to be removed and surface finish is not important, a coarse grain wheel should be used. For finish grinding, a fine gain wheel is recommended.

- The area of contact between the wheel and the work piece: if the area of contact is wide, a coarse-grain wheel is generally used. Fine grain wheels are used when the area of contact between the wheel and the work is small

52 BOND TYPES

The function of the bond is to hold the abrasive grains together in the form of a wheel. They are six common bond types used in grinding wheel manufacture: vetrified, resinoid, rubber, shellac, silicate and metal.

Vetrified: Made of clay feldspan, fuses at a high temperature and when cooled forms a glossy bond around each grain. These bonds are strong but break down readily on the wheel surface to expose new grains during the grinding operation. This bond is particularly suited to wheel used for the rapid removal of metal.

Resinoid: Synthetic resins are used as the bonding agents in resinoid wheels. They are cool cutting and re move stock rapidly. They are used for cutting off operations, snagging and rough grinding, as well as roll grinding.

Shellac Bond: These are used for producing high finishes on parts such as cutlery, cam shafts and paper mill rolls. They are not suitable for rough or heavy grinding.

Silicate bond: Rarely used in industry. It is used principally for large wheels and for small wheels where it is necessary to keep heat generation to a minimum.

Metal Bond: Metal bonds (generally non-ferrous) are used on diamond wheels and for electrolytic grinding operations where the current must pass through the wheel.

53 COMMON GRINDING WHEELS SHAPES AND APPLICATIONS

GRINDING OPERATIONS

- grinding flat surfaces - grinding shoulders - grinding angles and slots - form grinding

54 LATHES

SAFE PRACTICES IN LATHEWORK

Rotating chucks and work are the major dangers in lathework. Chips and the are also potential dangers if the operator does not pay attention to what is happening.

01. Permission should be obtained to operate the machine, at the discretion of the instructor.

02. Always roll up loose sleeves and remove ties, rings, watches, and so on before operating the machine.

03. Always wear specified eye protection.

04. Make all adjustments with the machine off.

05. When installing or removing chucks, place a safety board on the ways in case the chuck falls. This will prevent damage to both the chuck and ways.

06. When loosening a chuck on a threaded spindle, do not use a pry bar between the chuck jaws. It is too hard on the jaws. Use a on one of the jaws, with the jaw fully supported by the chuck.

07. Never leave a chuck wrench in a chuck. If the machine were turned on, the wrench could become a lethal projectile.

08. To check the clearance of the chuck and the work, rotate by hand before switching on the machine.

09. Always keep your hands away from moving parts.

10. Always keep your hands away from chips. They are hot, sharp and dangerous.

11. When filing, always use a file handle. It is also best to learn to file left-handed rather than by placing your left arm over the revolving chuck. Running the lathe in reverse and filing right-handed at the back of the lathe is poor practice because the controls are at the front of the lathe, out of reach. It is also best to remove the tool post assembly.

12. Do not measure work with the lathe running. It is poor practice.

13. Do not adjust the cutting tool with the machine running.

14. In general, it is poor practice to change gears while the lathe is running.

15. As a general rule of thumb, no more than 3 times a diameter of the work should be out of a chuck with out being supported by the tailstock or a steady rest.

16. Never walk away from a lathe when it is operating.

17. When the work is complete, shut off the power and clean the machine.

55 THE LATHE – TURNING

INTRODUCTION

Turning is the art of producing cylinders, cones, or rather surfaces of revolution by means of rotating the work about on axis and removing the surplus material by means of a cutting tool suitably guided. The machine on which such operations are performed is termed a lathe. Essentially it consists of a headstock carrying a spindle to which the work is connected in order that it may be rotated, means for driving the spindle, a bed carrying the headstock and the tool support as well as the tailstock.

The work may be concerned with the spindle in various ways. The simplest is by centres. The ends of the work are drilled by a specifically pointed drill leaving two conical depressions or “centres”. If the work is short as compared with its diameter it may be “chucked”. The chuck consists of a cylindrical body fixed to the spindle and carrying two, three or four radially adjustable members which may be arranged to grip the work and drive it against the resistance of the cutting tool.

In many lathes the spindle is hollow so that long bars may pass through it for operating on one end. In this case the chuck is used.

CUTTING TOOLS

Various material are used for making cutting tools:

1. Carbon Steel - Light finishing cuts - Machining soft materials - Fairly slow cutting speed - Cutting edge softens during cutting

2. High Speed Steel - Tough enough to withstand most cutting shocks - Retains its hardness at high speed - Cut most material satisfactory - Useful for general purpose work 56 3. Stellite - Withstands heat very well - Hard chilled castings and similar material

4. Tungsten Carbide (tip) - Hardest cutting material normally used - Higher speeds can be used

The two most common type of cutting tools are the high speed steel and tungsten carbide tipped tools.

High Speed Steel (HSS) - Work to great accuracy on small diameters - Turning small diameters, if the machine is not capable of a high RPM - Screw cutting - Intermittent turning

Tungsten Carbide Tool - Fast metal removal rate is required - Cutting hard and non-ferrous material such as cast iron and brass - General machining - Screw cutting pipe

WORKHOLDING AND SETTING

1. Three jaw chuck 2. Between centres 3. Four jaw chuck 4. Fixed and travelling steady

1. Three Jaw Chuck The advantages of a three jaw chuck is that the jaws are self-centering, enabling the work to be easily set cen- tral.

2. Between Centres The work is mounted between the live centre and the dead centre. Normally used for large length to diameter ratio work piece.

3. Four Jaw Chuck The four jaw chuck has jaws which are reversible and are adjusted independently. The advantages are: 1. Both symmetrical and irregular shaped work can be held 2. Work pieces may be set either concentrically or eccentrically 3. Greater holding potential than the three jaw chuck

4. Fixed and Travelling Steady Steadies are used for supporting long, slender work against the pressures of the work piece. There are two types of steadies used on a lathe: - Fixed steady - Travelling steady 57 Fixed Steady The fixed steady is clamped to the body of the lathe and supports the work piece being machined by means of three jaws at 120º to each other. As the steady is clamped to the machine bed the saddle cannot pass it. Thus a bar being turned must be machined at one end, reversed in the chuck, and then machined at the other end. The most common applications of the fixed steady are for: - Facing the end of long bars too large in diameters to pass through the machine spindle - Drilling, boring or tapping of tapping bars

Travelling Steady The travelling steady is fixed to the carriage of the lathe and travel along with the tool.The two steady points should be set to travel just behind the tool, so that the steady pads bear on the portion of the work which the tool has just machined. The most common application is for turning long shafts where it is inconvenient for the bar to be reversed in the chuck.

CUTTING SPEEDS

Lathe work cutting speed may be defined as the rate at which a point on the work circumference travels past the cutting tool. It is always expressed in metres/mm (m/min) or feet/min (ft/min). If a cutting speed is too high, the cutting tool edge breaks down rapidly, resulting in time lost to recondition the tool. With too slow cutting speed, time will be lost for the machining operation, resulting in low production rates. Recommended cutting speeds for various materials using a High Speed Tool bit is shown in Table 1. These speeds may be varied slightly to suit factors such as condition of the machine, the type of work material and sand or hard spots in the metal.

The revolutions/min (r/min) at which the lathe should be set for cutting metals is as follows: Rev/min = Cutting speed (metres) Π × diameter of workpiece (metres)

= cutting speed × 12 inches Π × diameter of workpiece (inches)

58 LATHE FEED

The feed of a lathe is defined as the distance of the cutting tool advances along the length of the work every revolution of the spindle. Table 2 lists the recommended feeds for cutting various materials using high speed tool bit.

59 TURNING OPERATIONS

1. Parallel turning 2. Drilling holes and boring 3. Parting off 4. Knurling 5. Screw cutting Vee-form threads: - external - internal 6. Taper turning

When selecting cutting speed, feeds and depth of cut the following factors should be taken into consideration: 1. Type and hardness of the work material 2. The grade and shape of the cutting tool 3. The rigidity of the cutting tool 4. The rigidity of the work and the machine 5. The power rating of the machine Table 3 gives the recommended cutting speeds and feed for single point carbide tools.

60 TURNING OPERATIONS

Screwcutting Vee Form threads 1. Prepare workpiece (a) Turn diameter to be screwcut. (b) Undercut to provide a runout recess of thread depth and of approximately two pitches width

2. Set machine to cut required pitch of thread (a) Set levers in position as indicated on machine. (b) Determine at which positions of the screwcutting lever can be engaged

3. Set screwcutting tool (a) Check tool shape with the aid of a screwcutting gauge, e.g., 60º for unified threads, 55º for B.S.F. and B.S.W. threads. (b) Set topslide parallel to machine bed and clamp. (c) Set tool on correct centre height and clamp lightly. (d) Set tool point square to work centre line. Position the tool into an appropriate cut-out of the screwcutting gauge whilst holding the gauge in contact with the work diameter or face. Gently tap the tool to align tool form with setting. Clamp securely and re-check.

4. Position tool for first cut (a) Take up any backlash in the topslide by turning the hand- wheel clockwise. (b) Set topslide index to zero. (c) Set machine to run to run at slow r.p,m. (d) Feed the tool slowly in until contact is made with the diam- eter to be screwcut. Contact will be indicated by a fine ring being cut around the diameter. (e) Set cross-slide index to zero and clamp. (f) Retract tool from work surface.

5. Make a trial cut (a) Position the saddle to bring the tool approximately ¼” clear of the end of workpiece. (b) Re-index the cross-slide to zero. Apply cut of .003”. Memo- rize setting. (c) Apply coolant. (d) Apply a light engaging pressure to the screwcutting lever just before an engagement position is indicated on the screwcutting dial. Note: Ensure the lever engages fully or a false start will be made. Obtain the feel of the lever movement before commencing the work. (e) Disengage the screwcutting lever with a sharp single move- ment as the tool is seen to enter the run-out recess. (f) Retract tool from work. 61 6. Check pitch of thread (a) Select the correct pitch gauge and offer it to the lightly screwed work surface. Pitch of thread must correspond to that of gauge. (If incorrect check gear lever setting and/or the change gears of machine.)

SAFETY Isolate the machine before opening guard to inspect gearing.

7. Cut thread (a) Re-position tool beyond the end of the workpiece. (b) Re-index cross-sliide to apply a .003” cut. (c) Index topslide forward .001”. Note: This movement relieves the pressure on the trailing edge of the tool. A thin thread will result if topslide is moved much above a third of the cross-slide movement. (d) Engage screwcutting lever at appropriate screwcutting dial indication. (e) Disengage as tool enters run-out recess. (f) Retract tool from work. (g) Re-position for next cut. (h) Repeat these operations until within .005” of full cutting depth.

8. Check thread (a) Check for fit using a screw ring gauge or mating component.

9. Check thread Check for fit using screw plug gauge or mating component.

SAFETY Work to be stationary whilst checking.

10. Finish thread to size (a) Take fine cuts. Check for fit after each pass until full depth is reached. (b) At depth setting index topslide back, and with tool running along the thread take a very light cut off op- posite side of thread. (c) Run cut over length of thread. (d) Check thread. (e) Continue to take fine cuts until correct fit of gauge is obtained. Note: For left hand threads the lead-screw rotation is reversed causing the saddle to travel from left to right. The tool is set in the recess for the commencement of each cut.

62 SCREWCUTTING SQUARE AND ACME THREADS

General Note When producing a one off thread and nut it is best to aim to make one of the three fitting elements of the threads a close fit, whilst the other two are varied within limits, to give a required condition.

Three elements (1) Outside diameter of male thread: Make this the close fitting element, i.e., turn diameter to basic size. (2) Root diameter of male thread: Make this slightly smaller than basic size. (3) Width of thread form for both threads: Make tool widths slightly wider than the basic half pitch.

For the female thread (1) Bore to basic size. (2) Cut thread to depth to give required fit.

The screwcutting tool To avoid interference between tool flanks and the side walls of the threadform, the tool side clearance angles must be designed so that the tool can fit into the form it is cutting.The side clearance angle must be ground a few degrees larger than the helix angle of the thread being cut, and the trailing edge ground sufficiently to velar the thread while maintaining maximum tool strength.

63 Internal screwcutting 1. Prepare work (a) Machine bore to required depth and diameter. (b) Cut run-out recess at inner end of bore.

2. Set machine thread to cut required pitch of thread

3. Select internal screwcutting tool (a) Ensure shank will pass freely down bore. (b) Ensure tip is small enough to allow point to enter recess before fouling back face of work. (c) Check using screwcutting gauge that point angle is correct, e.g., 60º Unified threads, 55º B.S.F and B.S.W,

4. Set screwcutting tool (a) Set topslide parallel to machine and clamp. (b) Clamp tool lightly in toolpost slightly above true centre height. (This allows for downward spring when cutting). (c) Set tool point square to work centre line. (d) Position the tool into an appropriate cut-out of the screwcutting gauge whilst the gauge is held in contact with work diameter, or face. (e) Gently tap the tool to align tool form with setting gauge. (f) Clamp securely and re-check settings.

5. Position tool to take first cut. (a) Take up backlash in topslide. (b) Set topslide index to zero and clamp. (c) Set machine to run at a slow r.p.m (d) Enter tool point about ¼” into bore. Touch tool lightly on to bore wall, indicated by a fine ring being cut. (e) Set cross-slide index to zero and clamp. Note: When cutting internal threads it is more convenient to set the depth of thread on cross-slide index so that on reaching full depth the index read- ing is at zero. (f) Retract tool from work surface. Note: When working with little clearance between tool shank and bore wall, note the index reading as the tool moves clear of the work surface, i.e., after backlash has been taken up. Retract to this reading after each cut.

(g)Position tool with point adjacent to the run-out recess. Pencil mark the bed to indicate the position of the saddle. Note: Do not scribe the bed. (h)Slowly feed the tool into the recess to full cutting depth. Listen and look for any contact between shank and wood.

64 6. Make a trial cut (a) Re-position saddle to set tool ¼” clear of workface. (b) Index cross-slide outwards and apply a .003” cut. Memorize this setting. (c) Apply coolant. (d) Engage lever at appropriate dial reading. (e) As the saddle comes up to the pencil mark disengage the lever with a sharp single movement.

7. Check pitch of thread (a) Select correct pitch gauge and offer to screwed surface. (b) Check gear lever settings and/or the change gears if pitch is in- correct.

8. Cut thread (a) Re-position saddle to set tool ¼” clear of work. (b) Index cross-slide outwards to apply a .003” to .005” cut. (c) Index topslide forward ⅓ the amount of cut applied on cross- slide. (d) Engage screwcutting lever at an appropriate dial reading. (e) As saddle reaches pencil mark disengage lever. (f) Retract tool from bore. (g) Repeat operation until within .005” of depth.

SAFETY Work to be stationary whilst checking.

9. Finish thread to size (a) Take fine cuts. Check for fit after each pass until depth is reached. (b) At depth setting, index topslide back with tool running along thread until a very light cut is taken off op- posite side of thread. (c) Run cut over length of thread. (d) Check thread. (e) Continue to take very fine cuts until correct fit of gauge is obtained. Note: For a left hand thread the leadscrew rotation is reversed causing the saddle to travel from left to right. Hence the tool is set in the run-out recess to commence each cut.

65 An alternative method of cutting vee-threads 1. Set tool (a) Set topslide at ½ thread angle and clamp, i.e., 27½º for B.S.F., 30º for Unified. (b) Set the tool square to work. (c) Set topslide dial to zero. (d) Touch tool on to work diameter. (e) Set cross-slide index to zero.

2. Cut thread (a) Apply a cut of .004” using topslipe. (b) Retract tool from run-out recess using cross-slide. (c) Re-position the tool for the next cut. Apply a cut of .004” with top- slide, set cross-slide to zero. Repeat to required depth of thread.

Screwcutting up to a shoulder Where no recess is permissible, as the tool approaches the shoulder it is nec- essary to disengage the screw cutting lever, and at the same mo,nt to retract the tool from the work with a quick movement of the cross-slide.

Taper Turning Using a Form Tool A method for producing short tapers of any angle either external or internal. External 1. Prepare workpiece (a) Mount workpiece in machine, keeping section to be tapered as close to support as possible. (b) Finish turn to length and diameter.

2. Select form tool (a) For 30º, 45º and 60º angles standard tools may be available. For other angles select tool approximating to required angle. (b) Check tool face is long enough to produce complete taper in one cut. 3. Set tool (a) Set topslide square an clamp. (b) Set tool on true centre height keeping tool overhang to a mini- mum. (c) Release toolpost. Set cutting edge to required angle using an an- gle gauge or protractor, registering from chuck or other suitable surface. Note: The tool will not cut the angle required unless it is set accurately on centre height. (d) Lock toolposts and re-check settings. Note: Where a self indexing toolpost is to be used for more than one of the fol- lowing procedure is more suitable: (a) Set topslide square and lock. (b) Lock toolpost. 66 (c) Set tool to approximate angle on true centre height and clamp lightly. Keep tool overhang to a minimum. (d) Set tool accurately to required angle using an angle gauge or protractor, registering from chuck or other suitable surface. Note: The tool will not cut the angle required unless it is positioned on true centre height. (e) Clamp tool securely.

4. Turn taper (a) Rotate work at cutting speed. (b) To produce a taper to a given diameter. (i)Touch outer end of tool cutting face lightly to work corner. (ii) Lock saddle. (iii) Set across-slide index to zero. (iv) Apply coolant. (v) Feed in to produce required diameter. (c) To produce a taper to a given length. (i) Touch leading end of tool cutting face lightly to work corner using topslide. (ii) Lock saddle. (iii) Set topslide index to zero. (iv) Apply coolant. (v) Feed along to required length. Note: Chatter marks tend to develop as the width of cut increases. These may be eliminated by a reduction in work speed.

67 Taper Turning Using the Compound Slide External taper 1. Ensure topslide movement will enable full length of taper to be turned. Note: As the angle of taper is increased the maximum axial length becomes progressively less. 2. Prepare workpiece Turn parallel diameter(s) to maximum diameter of taper. 3. Set topslide angle (a) Release topslide clamping screws. (b) Set topslide to half include angle of work taper. (c) Re-clamp topslide and re-check setting. 4. Set tool (a) Set tool on true centre height, normal to the surface to be turned. Note: A true taper will not result if the tool is set above, or below, centre height. (b) Bring the tool to working position and by using the topslide ensure the full taper length can be turned without obstruction. Note: When working with the tailstock it may be found necessary to position the handwheel towards the headstock.

SAFETY When set in this position ensure there is freedom to rotate before switching on. Keep hands clear of chuck or catchplate, when operating the topslide control. 5. Turn taper

(a) Set topslide to rearmost position. (b) Position saddle to clear tool of length to be tapered. (c) Lock saddle in position. (d) Rotate work at normal cutting speed. (e) Position tool to take a light cut. (f) Apply coolant. (g) Using topslide take the cut until tool runs out. Note: For fine angles, small changes in work diameters result in big changes in the taper length. On such work care must be taken to avoid the first cut being too large. (h) Return topslide to rear position, apply further cuts until taper reaches about ⅔ full length.

68 6. Check angle of taper (a) Make a along the work taper. (b) Holding a taper gauge firmly in contact with the work- piece rotate it about ¼ turn. (c) Observe where chalk line has been removed. Where com- plete line removed – taper is correct. Where part line covered – ta- per incorrect. (d) Where necessary reset topslide to correct error. Take light curls. Re-check.

7. Position tool for final cut to length (a) Position tool point required length from front face of workpiece. (b) Lock saddle. (c) Adjust cross-slide to allow tool to lightly trap a against workpiece.\ (d) Remove feeler gauge. (e) Index tool in thickness of feeler gauge. (f) Set cross-slide to zero. (g) Retract tool clear of work using cross-slide. (h) Return topslide to rearmost position. (i) Re-index cross slide to zero position. (j) Proceed with final cut.

Internal taper For an internal taper a similar procedure is followed using a suitably sized standard boring tool, setting the top- slide parallel to the taper being cut.

Taper Turning Using the Offset Tailstock A method limited to the production of slow tapers on work turned between centres. 1. Prepare work for turning between centres. 2. Mount work between centres. 3. Turn all parallel diameters and faces. 4. Calculate tailstock set-over Slow tapers are usually dimensioned as a given amount of taper on diameter per foot of length. Note: This set-over allows an approximate first setting to be made. Final adjustment is made by checking work to a taper gauge. 5. Set over tailstock (a) Remove workpiece from centres. (b) Release tailstock clamp and lightly tighten. (c) Release locking screw on rear face of tailstock. (d) Position a dial gauge to indicate amount of set-over. (e) Adjust the two opposing setting screws in the side faces of the tailstock base, to offset the tailstock the calculated amount as registered on the dial indicator.

69 Note: Adjust tailstock towards tool to turn taper smaller at tailstock end.

(f) Tighten the free setting screw. (g) Tighten rear lock screw. (h) Fully tighten tailstock clamp.

6. Remount work between centres

7. Turn taper Note: Proceed as for normal parallel turning. Cuts taken must be light otherwise offset tailstock centre will damage work centre. (a) Take roughing cuts to produce a taper long enough to check to gauge. (b) Adjust tailstock as necessary. Take light cut and re-check. (c) Finish turn tapered diameter. Note: The taper as set will only be reproduced on successive workpieces if the length between work centres remains constant.

8. Re-set tailstock in true plane.

70