A Metalsmith’s Primer On Cutting Screw 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 bolt or screw (called male threads), and around the interior of a cylindrical hole, as in a nut (called female threads). Screw threads are primarily used as fasteners 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 screw thread 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 machining process known as cold forming, 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 metals such as silver, copper, brass 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 plastics 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, Archimedes, developed a wooden screw-like device for raising water. Early metal 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. Mass production and interchangeability came about with the invention of the steam engine during the 18th century, a factor that contributed greatly to the Industrial Revolution. Since British engineer Joseph Whitworth 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 Tools: 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 wrench 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-gauge, 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 drills are drill 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 tap wrench 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 Die 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 square 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 diameter taps. Miniature taps can be held in a pin vise. 6 Attempts at extracting taps are not always successful, it is best to avoid breakage. Page 3 of 8/Cutting Screw Threads/Falkenhagen 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. Turning 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 screws 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 tap and die sizes. The size of a drill bit 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.