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 will generally result in the removal of some material, which will result in looser and weaker threads. Because of this, threads are typically cleaned using special taps and dies made for this purpose, which are known as chasers. Chasers are made of softer materials and are not capable of cutting new threads, however they are still tighter fitting than actual fasteners and are fluted like regular taps and dies (to provide a means for debris like dirt and rust to escape). One particularly common use is for automotive spark plug threads, which often suffer from corrosion and a buildup of carbon.
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.
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.
The largest tap and die company to exist in the United States was Greenfield Tap & Die (GTD) of Greenfield, Massachusetts. GTD was so irreplaceably 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. 456+74635843546+6783
A tap cuts or forms a thread on the inside surface of a hole, creating a female surface which 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 may be determined by consulting a drill and tap size chart, a standard reference item found in many machine shops. If the hole is to be drilled, the proper diameter is called the tap drill size.
In lieu of a tap drill chart, it is possible with inch-sized taps to compute the correct tap drill diameter as follows:
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 as:
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, the common practice is to use an intermediate (plug) tap to cut the threads. If the threads are to extend to the bottom of a blind hole, the intermediate (plug) tap will be used to cut threads until the point of the tap reaches bottom, after which a bottoming tap will be used to finish the hole. Frequent ejection of the chips must be made in such an operation to avoid jamming and possibly breaking the tap. With hard materials, the machinist may start with a taper tap, whose less severe diameter transition reduces the amount of torque required to cut the threads. If threads are to be cut to the bottom of a blind hole, the taper tap will be followed by an intermediate (plug) tap and then a bottoming tap to finish the operation.
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:
In order 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 rather close to ideal in order to (a) produce good threads and (b) avoid tap breakage. The deeper the depth of thread, the more pronounced the effect of the angular error becomes. 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 this alignment task is that the first thread or two that is cut establishes the direction that the rest of the threads will follow. In other words, you can't make corrections to the angle once you have cut 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 
There are also double lead taps and insert taps which will need different speeds and feeds as well as starting hole diameters that are different from their regular parameters.
|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 which functions like a bolt. Dies are generally made in two styles: solid and adjustable. The dies shown to the right are adjustable:
Solid dies cut a nominal thread form and depth, whose accuracy is subject to the precision with which the die was made, as well as 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).
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 will self-feed. A 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.
The use of a suitable lubricant is essential with most tapping and threading operations. Recommended lubricants for some common materials are as follows: