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Home , Bolt (fastener), Screw thread, Tap and die

A ’s Primer On Cutting Threads By Diane Falkenhagen (1998 text only)

Definition:

Screw threads are the uniform helical ridges which spiral around the exterior of a cylinder or cone, as in a or screw (called male threads), and around the interior of a cylindrical , as in a (called female threads). Screw threads are primarily used as and connectors -- the concept is based on the principle of the wedge as a holding device. Threads can be cut by hand or machine.1 The process of cutting internal threads is called tapping; the process of cutting external threads is called threading.

Fig. 1. Internal (Female) Threads Fig. 2. External (Male) Screw Threads

Most threads are right-handed -- meaning the threaded part advances in a clockwise direction and retracts in a counter-clockwise direction -- but left-handed threads can also be made.2 The nomenclature for screw threads is extensive and includes terms such as pitch, lead, helix angle, axis of thread, etc. The metalsmith interested in learning how to make simple threaded parts, however, need only know the few elements described in this paper.

Fig. 3. Basic Screw Thread Elements (image from: ftp://ftp.prenhall.com/pub/esm/general_engineering.s-046/giesecke/pdf/Chapt11%20.pdf)

Some Uses For Screw Threads In Jewelry- Making And Metalsmithing:

 Nuts and bolts to fasten parts or layers of materials together, especially in mixed media constructions where heat would be detrimental.  Nuts and bolts where soldering or gluing would be undesirable.  Nuts and bolts as alternatives to other mechanical (cold) connections, such as rivets or tabs.  To connect parts or materials which must be disassembled for cleaning.  Threaded clasps and catches such as a tubing catch.  Threaded hinge pins, as in eyeglasses, in which the pin allows movement and can be removed, but will not easily fall out of place.

1 Screw threads can also be formed through displacement in a chipless process known as cold , or they can be fabricated with a wire-coiling technique. 2 Instructional information in this article is restricted to cutting right-hand threads.

Page 1 of 8/Cutting Screw Threads/Falkenhagen  To make small mechanical devices for transmitting motion.

Some Advantages To Cutting Your Own Screw Threads:

 You can make continuous threaded rod, or individual nuts and bolts, from non-ferrous such as silver, copper, and gold (copper is problematic both because of its softness and toughness).  You can make nuts and bolts in sizes or styles not commercially available.  You can tap a cast or fabricated object for fastening other parts to it.  You can thread some and composite materials.

Some Trivia On The Evolution Of Screw Threads:

It is thought that the screw thread dates to the 3rd Century B.C when the Greek mathematician and inventor, , developed a wooden screw-like device for raising water. Early nuts and bolts, like those used during the Renaissance, were forged to shape and the threads were filed by hand. Leonardo da Vinci (1452-1519) had sketched his ideas for a machine that would make screw threads, but thread cutting machinery was not developed until the mid 16th century. and interchangeability came about with the invention of the steam engine during the 18th century, a factor that contributed greatly to the . Since British engineer patented the first standardized thread system in 18363, so many different types of systems, series and classes of screw threads have evolved it is astounding. The two most common standards used in the United States today are the Unified (National) Thread System, adopted by NATO in 19484, and the American National Screw Thread Standard which, in its original form, became a standard in 1924.5 Both standards are based on measurement in inches. The American National Screw Thread Standard (Sometimes referred to simply as the American National Standard or the National Thread System) has two primary series: National Coarse (NC) which has fewer threads per inch and is for general purpose threading; and National Fine (NF) which has more threads per inch and is used where precision is important. The Unified Thread System is very similar to the American National Standard and also has a coarse (UNC) and fine series (UNF). Despite some differences in variation of tolerances, threads cut to specifications of the American National Standard have the same thread form and are interchangeable with those cut according to the Unified Thread System.

Fig. 4. Archimedean Screw

3 The British Standard Whitworth Thread System. 4 The Unified System was developed in response to problems experienced by the incompatibility of threads used by allies during World War II. 5 The American National Screw Thread Standard, formerly known as the United States Standard, has undergone many modifications since 1924.

Page 2 of 8/Cutting Screw Threads/Falkenhagen (Image from http://www.nt.ntnu.no/users/haugwarb/DropBox/The%20Dutch%20Windmill%20Stokhuyzen%201962.htm)

The :

Taps and Dies, the precision tools used to cut screw threads, can be purchased individually or in sets. Standard taps and dies are made of either high-speed steel or carbon steel; carbon steel is fine for hand-cutting operations. Taps have sharp cutting-threads which are fluted. They are held in an adjustable tap and are used for cutting interior threads. Dies are the counterparts to taps, and are used to cut exterior threads. They are held in a circular diestock or other holding device, and are numbered to correspond to their tap counterpart. Screw Plates are alternatives to individual dies; they are heavy-, tempered steel plates pierced with a series of sharp thread-cutting holes which progressively increase in size. Small screw plates are well suited to jewelry work. Tap are bits used to drill holes intended to be tapped and Clearance drills are drill bits used to drill holes large enough for a bolt or threaded rod to pass through freely. A tap extractor is a fingered device with a sliding collar which is held in a and used to remove broken taps from holes.6

Fig. 5. Tapered, Fluted Tap Fig. 6. Straight Tap Wrench Fig. 7. T-Handle Wrench

Fig. 8. Round Split Fig. 9. Diestock

Taps: There are three basic types of hand taps, the tapered tap (used as a starter tap or for threading through an open hole), the plug tap (for closed or blind holes that do not go all the way through the material), and the bottoming tap (used after the taper and plug taps, in that order, for tapping all the way to the bottom of a blind hole). The difference between the three types of taps is the number of threads which are ground to a taper at the end of the tap, where most of the cutting action is performed. The hollowed flutes that run along the length of a tap form the cutting edges and serve as a channel for the loose chips of metal to escape through. Standard taps have three to four flutes, depending on tap size. Taps are hard and brittle, and will break easily if forced; they should always be used with a cutting lubricant. The end of the tap is held firmly in either a T-handle or a straight-handle tap wrench. The T-handle wrench is normally used for smaller taps. Miniature taps can be held in a pin .

6 Attempts at extracting taps are not always successful, it is best to avoid breakage.

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Fig. 10. Tap In Straight-Handle Tap Wrench

Dies: There are many types of cutting dies, but the adjustable round split die is the most common. It is a circular steel die with sharp internal cutting-threads, a split down its side, and a small screw for adjusting cutting depth. the adjusting screw in forces the die to open and thus cuts an oversize thread; turning the screw out allows the die to close, thus cutting an undersized thread. As in a tap, flutes or grooves cut away from the die’s threads form a cutting edge and provide space for chips of metal to escape during the thread-cutting process. The diestock for a round split die has a set of guides which adjust to the diameter of the rod to be threaded. The diestock may also have a small screw for further adjusting the depth of cut. Before inserting a die into a diestock, make depth adjustments to the die (if in doubt, start with a more open die -- you can always re-cut the threads if the fit is too snug), and adjust the guides on the top of the diestock so that the rod feeds through not quite touching the guides. Secure the die in place by tightening the on the outside of the diestock. Notice that the cutting threads of a die have a smaller opening on one side and taper to a larger opening on the other side. The die should be positioned in the diestock with the smaller-diameter opening (usually the numbered side) toward the inside of the diestock, and the larger opening (sometimes inscribed with “START FROM THIS SIDE”) down, toward the material to be threaded.

Fig. 11. Round Split Die Secured In Diestock

Tap Drills: Tap drills are ordinary drill bits in sizes appropriate for standard sizes. The size of a necessary to drill a hole to be tapped is determined by the size of the tap itself and the desired amount of full-depth thread. Standard tap drill charts give drill sizes for average applications, and usually indicate the percentage of full-depth thread the chart is based on.7 Using the nearest fractional sized drill bit will change the percentage of full-depth thread and may or may not be satisfactory, depending on the holding power needed. A thread with 55% to 75% thread depth is adequate for normal circumstances. An internal thread with a 75% depth has about 95% of the strength of a full-depth thread. The power required to turn a tap for a 100% thread depth is three times greater than that required for one with 75% depth, often resulting in excessive tap breakage; 100 % thread depth therefore is not desirable.

Determining The Proper Tap, Die, Drill, And Rod Size:

It is very important to coordinate your thread-cutting tools. A good tap drill chart will supply you with all of the information you need: tap size, major thread , tap drill size and clearance drill size. The size of a tap is stamped on its shank, while the size of a die is usually stamped on its top side. Standard (non-metric) tap and die sizes are indicated by a set of numbers and letters which refer to diameter, number of threads per inch, and thread series. Smaller taps and dies are distinguished by machine screw sizes which are designated by gauge numbers such as 0000, 000, 00, 0, 1, 2, 3 and so forth up to 12.8 A tap or die labeled 2-56 NC,

7 100% thread depth, or Full-depth thread, is the absolute mating of threads with no clearance (play) between the threaded parts. Most tap drill charts are calculated on a 75% full-depth thread. 8 Gauge numbers for miniature taps and dies do not correspond to wire gauge measurements.

Page 4 of 8/Cutting Screw Threads/Falkenhagen for example, corresponds to a #2 machine screw, cuts 56 threads per inch and is a National Coarse thread series. Larger tools are numbered according to their measurement in inches. For example, a ¼ - 28 NF tap has a ¼ inch diameter, cuts 28 threads per inch, and is a National Fine thread series. The rod to be threaded should have a diameter equal to the major diameter of the desired finished thread, which in turn corresponds to the threaded hole it is to be screwed into. If you choose a 2-56 tap, you should begin with material that is .0860 inches in diameter (the major diameter of the desired screw thread). If you choose a ¼-28 tap, the material to be threaded should begin as ¼ inch (.250 inches) in diameter.

National Coarse and National Fine Threads and Tap Drills Tap Major Minor Tap Drill Tap Drill Decimal Clearance Clearance Drill Decimal Size Dia. Dia. Size Equiv. Drill Equiv. 2-56 .0860 .0628 50 .0700 42 .0935 NC 2-64 .0860 .0657 50 .0700 42 .0935 NF 3-48 .099 .0719 47 .0785 36 .1065 NC 3-56 .099 .0758 45 .0820 36 .1065 NF

Fig. 12. Example of a Tap Drill Chart showing machine screw sizes in two series: National Coarse and National Fine; The first number in the tap size represents the machine screw size; the second equals the number of threads per inch.

The Process:

Tapping (Cutting Internal Threads):

1. Fabricate or prepare the part to be drilled and tapped. Any drilled hole of the proper depth (deep enough to accommodate at least three thread crests) can be internally threaded. The inside of a length of heavy-wall tubing may also be tapped. 2. Consult a tap drill chart to determine the correct tap drill size for the tap you have chosen. (See figure 12.) Always start with a tapered tap. Make sure you also have the corresponding cutting die and correct size rod or tube for threading the male counterpart. 3. The part to be tapped should be held in a drill vise, or otherwise firmly secured with the drill bit carefully held at a right angle to the surface of the part being drilled. (See figure 13.) Take precautions to protect delicate surfaces from the jaws of the vise. 4. Drill the hole all the way through, or to the desired depth, always keeping the drill bit at a right angle to the work. 5. Next, mount the work in a bench vise or otherwise firmly secure it so that the hole is in an upright, vertical position. 6. Tightly secure the tap in the tap wrench. Place the tap in the opening of the drilled hole. With light pressure, slowly turn the tap clockwise, keeping it at a right angle to the surface of the part being tapped. (See figure 14.) Turn until resistance is felt, then turn the tap counter-clockwise by one quarter-turn. Repeat the clockwise rotation. Once the tap is started it will draw itself in and pressure is no longer needed. 7. After one or two rotations, remove the tap wrench, leaving the tap in the partially threaded hole. Place a ’s square against the tap shank to see if the tap is square to the work. Check from two positions. Having the proper angle is critical. (See figure15.) If the tap is not situated squarely, back it out and start again. 8. Once the proper angle is achieved, resume tapping in a clockwise direction. Every one or two rotations reverse the turn of the tap one-half turn to allow the metal chips to fall away through the open flutes of the tap. This makes the job easier, prolongs

Page 5 of 8/Cutting Screw Threads/Falkenhagen the life of the tap and prevents breakage. Always remember to use a good cutting lubricant and never force a tap. 9. If tapping to the bottom of a blind hole (one that doesn’t go all the way through), first use a tapered tap, then a plug tap, then a bottoming tap, in that order, to gradually thread the entire depth of the hole. It is very important to occasionally back the tap out completely and blow out the metal chips trapped in the bottom of the hole. (Wear eye protection. The metal chips are sharp -- don’t attempt to wipe them away with unprotected fingers.)

Fig. 13. Checking Drill Angle Fig. 14. Starting The Tap Fig. 15. Checking Tap Angle

Threading (Cutting External Threads):

1. Filing or grinding a 45° chamfer on the end of the rod or tube to be threaded will help in starting the thread. The depth of the chamfer should be about equivalent to the depth of the screw thread desired. (See figure 16.) 2. Select the proper threading die, adjust the cutting depth, adjust the diestock guides, and secure the die in the diestock. (See figure 17.) 3. Tightly secure the rod or material to be threaded in a bench vise. 4. Apply cutting lubricant to the die and the rod. 5. Place the die over the rod. Applying light, even pressure, turn the die clockwise to start the threading process. (See figure 18.) As soon as the die has drawn itself onto the rod (usually several rotations), use a small machinist’s square to see if the die is positioned at a 90º angle to the rod. (See figure 19.) If it is not, back the die off and start again. 6. Resume cutting in a clock-wise direction. Every two rotations reverse the die slightly to allow the metal chips to fall away. After several full rotations, back the die off completely and check the fit of the thread with the tapped female counterpart you have already made. 7. If the fit is too snug, try tightening the adjusting screw on the outside of the diestock. That action forces the die to close. If the proper adjustment cannot be made by the first method, remove the die from the diestock and loosen the adjusting screw on the die itself (that action allows the die to close also). If the fit is not snug enough, remove the die from the diestock and tighten the adjusting screw on the die to open the die more. 8. After the proper fit is achieved, continue cutting threads to the length desired. If the threads must be cut close to the shoulder of a bolt, the die may be backed off, inverted in the diestock and the last several threads completed with the die in the inverted position.

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Fig. 16. Rod With 45 º Chamfered End Fig. 17. Diestock Guides Properly Adjusted For Rod

Fig. 18 Starting The Threads Fig. 19. Checking The Angle Of The Rod

Cutting Miniature Threads:

Screw plates and miniature tap and die sets are more suitable to jewelry making than larger machinist’s versions. Miniature taps and dies can be purchased individually, or in sets. Common miniature sizes range from 0 to 12 in fine or coarse threads, but taps and dies for cutting threads as small as 0000 exist. A practical miniature tap and die ensemble for the jeweler or metalsmith who only occasionally needs to thread a small part, consists of the following: carbon steel taps sizes 00- 90, 0-80, 1-72, 2-56, three-prong adjustable dies in the same sizes, and the J.I. Morris E-Z Tap and Die Holder. (See figure 14.) These miniature tools are available through the sources listed below. The threading process with these miniature taps and dies is basically the same as with standard taps and dies, but the tools are different. The E-Z holder described above is actually a specially-bored pin vise with four tap bushings to hold the four different tap sizes. The tap is inserted into its bushing, and the bushing into the holder. A small tightens against the tap and bushing, holding both in place. The E-Z holder also serves as a die holder. A steel ring, which slides up and down on the tubular cutting die allows for adjustment of thread depth by constricting or expanding the die jaws

Fig. 20. E-Z Tap And Die Holder Fig. 21 Miniature Tap and Three-Pronged Die

Page 7 of 8/Cutting Screw Threads/Falkenhagen National Coarse and National Fine Threads and Tap Drills Tap Major Minor Tap Drill Tap Drill Decimal Clearance Clearance Drill Decimal Size Dia. Dia. Size Equiv. Drill Equiv. 00-90 .047 .0628 50 .0700 42 .0935 0-80 .060 .0657 50 .0700 42 .0935 1-72 .073 .0719 47 .0785 36 .1065 2-56 .085 .0758 45 .0820 36 .1065

Fig. 22. Tap Drill Chart for E-Z Tap And Die Set

Some Helpful Hints For First Time Tap and Die Users:  Always verify that you have chosen the correct size tap, tap drill, die and rod by cutting some practice threads first.  Have a decimal-equivalent chart for drill bits and a good or digital caliper handy.  Keep a large variety of wire, rod and heavy-wall tubing in stock.  Know the recommended cutting lubricant for the material you are cutting and use it liberally. Soluble oil is good for most non-ferrous metals.  Never use excessive force. Miniature taps break very easily.  If hand-held pin vise is used for holding a rod or tap, tightened the pin vise well with vise grips.  When tapping with a tap in the E-Z Tap and Die Holder, secure the tap firmly in the holder so that it will not slip and turn as you’re trying to cut.  An alternative way of starting a tap and making sure it is positioned squarely with the work, is to secure the tap in a drill press and turn the spindle by hand. (Do not try tapping with drill motor on!!) After the tap is started, remove it from the drill press and finish tapping by hand.  When cutting threaded rod from soft materials, like silver, use a black permanent marker to draw a straight line down the length of rod. This will allow you see if the rod is trying to twist or turn during the cutting procedure.  When threading rod for one project, make extra and store it marked with the thread size for future projects.

Sources:

J.I. Morris Company http://www.jimorrisco.com/ Ph: 508-764-4394 Miniature taps, dies, E-Z Tap and Die Holder, threaded rod, miniature nuts, bolts, washers, screws, etc.

Reactive Metals Studios, Inc. www.reactivemetals.com Ph: 1-800-876-3434 or 520-634-3434 J.I. Morris E-Z Tap and Die Set, miniature threaded rod, miniature nuts, bolts, washers, etc.

Small Parts, Inc. www.smallparts.com Ph: 1-800-220-4242 Miniature taps and dies, metric tap and die sets, threaded rod, miniature nuts, bolts, washers, quality measuring instruments, metal rod and tubing and many other items of interest to the metalsmith.

Bibliography:

Machine Technology; McCarthy, Willard and Smith, Robert E. McKnight and McKnight Publishing Co., Bloomington, IL; 1968.

Machinery’s Handbook 25, Twenty-Fifth Edition; Green, Robert E., Editor; Industrial Press, New York, 1996.

Machining Fundamentals; Walker, John R.; The Goodheart-Willcox Company, Inc.; Illinois; 1993.

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