Taps and dies are tools used to create screw threads, which is called threading. Many are cutting tools; others are forming tools. A tap is used to cut or form the female portion of the mating pair (e.g. a nut). A die is used to cut or form the male portion of the mating pair (e.g. a bolt). The process of cutting or forming threads using a tap is called tapping, whereas the process using a die is called threading.
Both tools can be used to clean up a thread, which is called chasing. However, using an ordinary tap or die to clean threads generally removes some material, which results in looser, weaker threads. Because of this, machinists generally clean threads with special taps and dies—called chasers—made for that purpose. Chasers are made of softer materials and don't cut new threads. However they still fit tighter than actual fasteners, and are fluted like regular taps and dies so debris can escape. Car mechanics, for example, use chasers on spark plug threads, to remove corrosion and carbon build-up.
While modern nuts and bolts are routinely made of metal, this was not the case in earlier ages, when woodworking tools were employed to fashion very large wooden bolts and nuts for use in winches, windmills, watermills, and flour mills of the Middle Ages; the ease of cutting and replacing wooden parts was balanced by the need to resist large amounts of torque, and bear up against ever heavier loads of weight. As the loads grew ever heavier, bigger and stronger bolts were needed to resist breakage. Some nuts and bolts were measured by the foot or yard. This development eventually led to a complete replacement of wood parts with metal parts of an identical measure. When a wooden part broke, it usually snapped, ripped, or tore. With the splinters having been sanded off, the remaining parts were reassembled, encased in a makeshift mold of clay, and molten metal poured into the mold, so that an identical replacement could be made on the spot.
Die and tap from Löffelholz-Codex (Nuremberg 1505)
Opened die from Löffelholz-Codex (Nuremberg 1505)
Metalworking taps and dies were often made by their users during the 18th and 19th centuries (especially if the user was skilled in tool making), using such tools as lathes and files for the shaping, and the smithy for hardening and tempering. Thus builders of, for example, locomotives, firearms, or textile machinery were likely to make their own taps and dies. During the 19th century the machining industries evolved greatly, and the practice of buying taps and dies from suppliers specializing in them gradually supplanted most such in-house work. Joseph Clement was one such early vendor of taps and dies, starting in 1828. With the introduction of more advanced milling practice in the 1860s and 1870s, tasks such as cutting a tap's flutes with a hand file became a thing of the past. In the early 20th century, thread-grinding practice went through significant evolution, further advancing the state of the art (and applied science) of cutting screw threads, including those of taps and dies.
During the 19th and 20th centuries, thread standardization was evolving simultaneously with the techniques of thread generation, including taps and dies.
The largest tap and die company to exist in the United States was Greenfield Tap & Die (GTD) of Greenfield, Massachusetts. GTD was so vital to the Allied war effort from 1940–1945 that anti-aircraft guns were placed around its campus in anticipation of possible Axis air attack. The GTD brand is now a part of Widia Products Group.
A tap cuts or forms a thread on the inside surface of a hole, creating a female surface that functions like a nut. The three taps in the image illustrate the basic types commonly used by most machinists:
Whether manual or automatic, the processing of tapping begins with forming (usually by drilling) and slightly countersinking a hole to a diameter somewhat smaller than the tap's major diameter. The correct hole diameter is listed on a drill and tap size chart, a standard reference in many machine shops. The proper diameter for the drill is called the tap drill size. Without a tap drill chart, you can compute the correct tap drill diameter with:
where is the tap drill size, is the major diameter of the tap (e.g., ⅜ inch for a ⅜"-16 tap), and is the thread pitch (16 in the case of a ⅜"-16 tap). For a ⅜"-16 tap, the above formula would produce 5⁄16 as a result, which is the correct tap drill diameter for a ⅜"-16 tap. The above formula ultimately results in an approximate 75 percent thread.
The correct tap drill diameter for metric-sized taps is computed with:
where is the tap drill size, is the major diameter of the tap (e.g., 10 mm for a M10×1.5 tap), and pitch is the pitch of the thread (1.5 mm in the case of a standard M10 tap) and so the correct drill size is 8.5 mm. This works for both fine and coarse pitches, and also produces an approximate 75 percent thread.
With soft or average hardness materials, such as plastic, aluminum or mild steel, common practice is to use an intermediate (plug) tap to cut the threads. If the threads must extend to the bottom of a blind hole, the machinist uses an intermediate (plug) tap to cut threads until the point of the tap reaches bottom, and then switches to a bottoming tap to finish. The machinist must frequently eject chips to avoid jamming or breaking the tap. With hard materials, the machinist may start with a taper tap, whose less severe diameter transition reduces the torque required to cut threads. To threads to the bottom of a blind hole, the machinist follows the taper tap with an intermediate (plug) tap, and then a bottoming tap to finish.
Tapping may either be achieved by a hand tapping by using a set of taps (first tap, second tap & final (finish) tap) or using a machine to do the tapping, such as a lathe, radial drilling machine, bench type drill machine, pillar type drill machine, vertical milling machines, HMCs, VMCs. Machine tapping is faster, and generally more accurate because human error is eliminated. Final tapping is achieved with single tap.
Although in general machine tapping is more accurate, tapping operations have traditionally been very tricky to execute due to frequent tap breakage and inconsistent quality of tapping.
Common reasons for tap breakage are:
To overcome these problems, special tool holders are required to minimize the chances of tap breakage during tapping. These are usually classified as conventional tool holders and CNC tool holders.
Various tool holders may be used for tapping depending on the requirements of the user:
The biggest problem with simple hand-tapping is accurately aligning the tap with the hole so that they are coaxial—in other words, going in straight instead of on an angle. The operator must get this alignment close to ideal to produce good threads and not break the tap. The deeper the thread depth, the more pronounced the effect of the angular error. With a depth of 1 or 2 diameters, it matters little. With depths beyond 2 diameters, the error becomes too pronounced to ignore. Another fact about alignment is that the first thread cut or two establishes the direction that the rest of the threads will follow. You can't correct the angle after the first thread or two.
To help with this alignment task, several kinds of jigs and fixtures can be used to provide the correct geometry (i.e., accurate coaxiality with the hole) without having to use freehand skill to approximate it:
Generally the following features are required of tapping holders:
Tapping case studies with typical examples of tapping operations in various environments are shown on source machinetoolaid.com 
Double-lead taps and insert taps need different speeds and feeds, and different starting hole diameters than other taps.
|Imperial tap and drill bit size table||Metric tap and drill bit size table|||
A die cuts an external thread on cylindrical material, such as a rod, which creates a male threaded piece that functions like a bolt. Dies are generally made in two styles: solid and adjustable. An adjustable die may be adjusted either by an integrated screw or by a set of screws set in to the die holder (termed a "die stock"). Integral adjusting screws may be arranged to work axially, where the movement of the adjusting screw into a threaded hole in the die forces the slit section of the die open, or tangentially where a screw threaded in to one side of the slit bears against the opposite side of the slit. Dies without integrated screws are adjusted inside the die stock by radially-arranged screws. Two screws in the stock bear in to indentations on either side of the slit, tending to squeeze the slit closed, whilst a third screw with a tapered tip screws in to the slit forcing it open. Working these three screws against each other adjusts the die.
Integrated screws appear to be common in the US but are almost unknown in the UK and Europe.
The dies shown in the image to the right are adjustable:
Solid dies cut a nominal thread form and depth, whose accuracy is subject to the precision the die was made with, and the effects of wear. Adjustable dies can be slightly compressed or expanded to provide some compensation for wear, or to achieve different classes of thread fit (class A, B and more rarely, C). Adjustable taps also exist but are not common. These have a tip that is split through the flutes and an axial screw which forces the cutting edges slightly apart.
The work piece (blank) to be threaded, which is usually slightly smaller in diameter than the die's major diameter, is given a slight taper (chamfer) at the end that is to be threaded. This chamfer helps center the die on the blank and reduces the force required to start the thread cutting. Once the die has started, it self-feeds. Periodic reversal of the die is often required to break the chip and prevent crowding.
Die nuts, also known as rethreading dies, are dies made for cleaning up damaged threads, have no split for resizing and are made from a hexagonal bar so that a wrench may be used to turn them. The process of repairing damaged threads is referred to as "chasing." Rethreading dies cannot be used to cut new threads as they lack chip forming teeth. However the external profile of a die does not strictly map to its function. Manufacturers of dies have produced models in a hex form which are intended for the creation of new threads. These appear identical to solid dies in all aspects besides the external shape. Hexagonal thread cutting dies are used with a die stock with hexagonal holding features.
The use of a suitable lubricant is essential with most tapping and threading operations. Recommended lubricants for some common materials are as follows: