Tap and die
In the context of threading, taps and dies are two classes of tools used to create or repair screw threads. A tap is used to cut or form the female portion of the mating pair. A die is a cutting tool used to cut the male portion of the mating pair. 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 also be used to clean up or repair 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.
Taps and dies are available in many hardware stores, home improvement centers and automotive part retailers in common sizes and pipe sizes. Online industrial suppliers have an immense selection of various taps and dies. Manufacturers and DIY websites have instructions on how to use the tools.
History
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 even 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 16th and 17th centuries, 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 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 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.
Tap
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:;Bottoming tap :The tap illustrated in the top of the image has a continuous cutting edge with almost no taper — between 1 and 1.5 threads of taper is typical. This feature enables a bottoming tap to cut threads to the bottom of a blind hole. A bottoming tap is usually used to cut threads in a hole that has already been partially threaded using one of the more tapered types of tap; the tapered end of a bottoming tap is too short to successfully start into an unthreaded hole. In the US, they are commonly known as bottoming taps, but in Australia and Britain they are also known as plug taps.
;Intermediate tap, second tap, or plug tap:The tap illustrated in the middle of the image has tapered cutting edges, which assist in aligning and starting the tap into an untapped hole. The number of tapered threads typically ranges from 3 to 5. Plug taps are the most commonly used type of tap. In the US, they are commonly known as plug taps, whereas in Australia and Britain they are commonly known as second taps.
;Taper tap: The small tap illustrated at the bottom of the image is similar to an intermediate tap but has a more pronounced taper to the cutting edges. This feature gives the taper tap a very gradual cutting action that is less aggressive than that of the plug tap. The number of tapered threads typically ranges from 8 to 10. A taper tap is most often used when the material is difficult to work or the tap is of a very small diameter and thus prone to breakage.
;Power taps: The above taps are generally referred to as hand taps, since they are manually operated. During operation, the machinist must periodically reverse a hand tap to break the chip that forms from cutting. This prevents the cut material from crowding and breaking the tap.
;Forming tap: A forming tap, aka fluteless tap, cold forming or roll tap, functions quite differently than a cut tap, in that it forcefully displaces the metal into a thread shape upon being turned into the hole, instead of cutting metal from the sides of the hole as cutting taps do. A forming tap has lobes periodically spaced around the tap which actually do the thread forming as the tap is advanced into a properly sized hole, and the threads behind the lobes are slightly recessed to reduce contact friction. Since there is no need to provide space for evacuating chips, these recesses are much more subtle than a cutting tap's flutes and the tap appears very nearly just like a plain thread. Since the tap does not produce chips, there is no need to periodically back out the tap to clear away chips, which can jam and break a cutting tap if allowed to accumulate. The problem of accumulated chips is particularly acute in blind holes, so thread forming is particularly suited to such applications. Forming taps only work in malleable materials such as mild steel or aluminum. Formed threads are typically stronger than cut threads. Note that the tap drill size is larger than that used for a cutting tap as shown in most tap drill tables, and an accurate hole size is required; a slightly undersized hole can break the tap. Proper lubrication is essential because of the frictional forces involved, therefore a lubricating oil is used instead of cutting oil. Form taps are much stronger than cut taps, as can be seen by cross sections of the same size tap. This significantly lessens tap breakage. Form Taps last 3 to twenty times longer than cut taps.
Holes
Whether manual or automatic, the processing of tapping begins with forming and slightly countersinking a hole to a diameter somewhat smaller than the tap's major diameter. The correct cut tap 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, and is the thread pitch. For a -16 tap, the above formula would produce, which is the correct tap drill diameter. The above formula ultimately results in an approximate 75% thread.
Since metric threads specify the pitch directly, 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, and pitch is the pitch of the thread and so the correct drill size is 8.5 mm. This works for both fine and coarse pitches, and also produces an approximate 75% thread.
Tap sequence
With soft or average hardness materials, such as plastic, aluminum or mild steel, common practice is to use an intermediate tap to cut the threads. If the threads must extend to the bottom of a blind hole, the machinist uses an intermediate 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 tap, and then a bottoming tap to finish.Machine tapping
Tapping may either be achieved by a hand tapping by using a set of taps 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.
Tap Breakage
Common reasons for tap breakage are:- Tap-related problems:
- * Wearing of tap cannot be easily quantified
- * Use of tap with improper tap geometry for a particular application.
- * Use of non-standard or inferior quality taps.
- Clogging with chips.
- Misalignment between tap and hole.
- Over- or under-feeding the tap, causing breakage in tension or compression.
- Use of improper and/or insufficient cutting lubricant.
- Absence of a torque limiting feature.
- Improper or zero float for use with screw machines
- Improper spindle speed.
Tap breakage is common, removal is difficult. Various methods are used to remove broken taps, to include: Manually smashing the tap, Using a tap extractor, Milling, Grinding with an Omegadrill, TIG welding, and Electrical discharge machining.