Definitions

Taps and Dies

Tap and die

Taps and dies are cutting tools used to create screw threads in solid substances including, but not limited to, metal, wood, and plastic. A tap is used to cut the female portion of the mating pair (e.g. a nut). A die is used to cut the male portion of the mating pair (e.g. a bolt). The process of cutting the threads in a hole is called "tapping" the hole. The process of cutting with a die is called "threading" (or sometimes "chasing", although "chasing" as a technical term can have restricted meanings).

History

While modern taps and dies 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 evolutionary 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. The splinters having been sanded off, the remaining parts were 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 toolmaking), using such tools as lathes and files. 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 businesses specializing in them gradually supplanted most such in-house work. 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 first and 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. Greenfield's descendant companies comprise the majority of the U.S.'s modern domestic tap and die industry.

Tap

A tap cuts 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:

  • Bottoming Tap: The tap illustrated in the top of the image has a continuous cutting edge with no taper. This feature enables a bottoming tap to cut threads to the bottom of a blind hole. A bottoming tap is never used to cut threads in an unthreaded hole, as the cutting edges lack the taper required to successfully start into such a hole.
  • Plug Tap: Also known as an intermediate tap (US only - In Australia and Britain, the Bottoming tap is the Plug), 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. Plug taps are the most commonly used type of tap.
  • Taper Tap: The small tap illustrated at the bottom of the image is similar to a plug 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. A taper tap is most often used when the material to be tapped is difficult to work (e.g., alloy steel) or the tap is of a very small diameter and thus prone to breakage.

The above illustrated taps are generally referred to as hand taps, since they are, by design, intended to be manually operated. During operation, it is necessary with a hand tap to periodically reverse rotation to break the chip formed during the cutting process, thus preventing an effect called "crowding" that may cause breakage. Periodic reversing is usually not practical when power tapping is involved, and thus has led to the development of taps suitable for continuous rotation in the cutting direction.

The most common type of power driven tap is the "spiral point" plug tap (also referred to as a "gun tap"), whose cutting edges are angularly displaced relative to the tap centerline. This feature causes the tap to continuously break the chip and eject it into the flutes, preventing crowding. Another version of the spiral point plug tap is the spiral flute tap, whose flutes resemble those of a twist drill. Spiral flute taps are widely used in high speed, automatic tapping operations due to their ability to work well in blind holes.

Whether manual or automatic, the processing of tapping begins with forming and slightly countersinking a hole (usually by drilling) with a diameter slightly 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:

TD = MD - (1/N)

where TD is the tap drill size, MD is the major diameter of the tap (e.g., 3/8 inch for a 3/8"-16 tap), and N is the number of threads per inch (16 in the case of a 3/8"-16 tap). For a 3/8"-16 tap, the above formula would produce 5/16 as a result, which is the correct tap drill diameter for a 3/8"-16 tap. The result produces a tap drill size that results in an approximate 75 percent thread (recommended for most applications).

With soft or average hardness materials, such as plastic, aluminum or carbon steel, the common practice is to use a plug tap to cut the threads. If the threads are to extend to the bottom of a blind hole, the 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.

In metal working, the use of a tap lubricant is essential to achieve cleanly formed threads and to minimize friction. Failure to use the correct lubricant may result in ragged threads, as well as a substantial increase in the amount of torque required to turn the tap, possibly resulting in breakage.

Practical use and safety with taps is discussed in the tap wrench article.

Machine tapping

Tapping is essentially the internal threading of a hole. This may either be achieved by 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 M/c, pillar type drill M/c, vertical milling machines, HMCs, VMCs. Machine tapping is faster, generally more accurate as 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 & inconsistent quality of tapping.

Research has shown that the important reasons causing tap breakage are as follows:

  • Tap related problems:
    • Wearing of tap cannot be easily quantified (use of worn out taps)
    • Use of tap with improper tap geometry for a particular application.
    • Use of non standard/inferior quality taps.
  • Chip clogging
  • Tapping does not follow the pre-tap hole (misalignment)
  • Mismatch of machine feed & tap feed may cause the tap to break in tension/compression.
  • Use of improper cutting fluid.
  • No safety mechanism to limit torque below torque breakage value of tap.

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 & CNC tool holders. Addressed in detail in section below.

Precautions to be taken while tapping

  1. Taps have self feed (Taps once engaged get pulled into the job) due to helical angle & hence length compensation arrangement is required in tool holder to avoid breakage of tap.
  2. High carbon taps should not be used as they cannot sustain high speed. These were used for hand tapping earlier.
  3. Only HS taps should be used.
  4. In order to retract tap from the hole, the machine should be reversible.
  5. Proper cutting compound should be used during tapping.

Tool holders for tapping operations

Various tool holders may be used for tapping depending on the actual requirement of the user.

  • Tapping attachments: these may be normal (available is a range of tap sizes) or quick change
  • Quick change drilling & tapping chucks (variations available for both CNC & conventional tools)
  • Rigid tapping attachments (for CNC)

Generally the following features are required of tapping holders:

  • Twin chucking: tap is held both, on diameter as well as on the square thus giving it positive drive.
  • Safety clutch: The built in safety mechanism, operates as soon as the set torque limit is crossed & save the tap from breakage.
  • Float radial parallel: small misalignments are taken care of by this float.
  • Length compensation: built in length compensation takes care of small push or pull to the spindle or feed difference.

Tapping case studies with typical examples of tapping operations in various environments are shown on source machinetoolaid.com

Die

The die cuts a thread on a preformed cylindrical rod, which creates a male threaded piece which functions like a bolt. The dies shown are

  • top left: an older split die, with top adjusting screw
  • bottom left: a one piece die with top adjusting screw
  • center: a one piece die with side adjusting screw (barely visible on the full image)
  • right: two dies without adjusting screws

A cylindrical blank, which is usually slightly less than the required diameter, is machined with a taper (chamfer) at the threaded end. This chamfer allows the die to ease onto the blank before it cuts a sufficient thread to pull itself along.

The adjusting screws allow the die to be compressed or expanded to accommodate slight variations in size, due to material, manufacture, or die sharpness. The two rightmost dies shown in the image have no adjusting screws. However the die holder can exert pressure and decrease the size if required.

Each tool is used independently, but are usually sold in paired sets of both types, one die and three taps. Some sets may provide a lesser number of taps. The common sets shown are designed for hand operation, but different types such as helical or spiral may be used in production tools such as CNC machining tools.

Tap drill bit size table

Imperial Tap & drill bit size table Metric Tap & drill bit size table
Tap Fractional Drill Bit Number Drill Bit Letter Drill Bit
0-80 3/64 - -
1-64''' - 53 -
2-56 - 50 -
3-48 - 47 -
4-40 3/32 43 -
5-40 - 38 -
6-32 7/64 36 -
8-32 - 29 -
10-24 5/32 25 -
10-32 5/32 21 -
12-24 11/64 16 -
1/4-20 13/64 7 -
1/4-28 7/32 3 -
5/16-18 17/64 - F
5/16-24 - - I
3/8-16 5/16 - -
3/8-24 21/64 - Q
7/16-14 23/64 - U
7/16-20 25/64 - -
1/2-13 27/64 - -
1/2-20 29/64 - -
9/16-12 31/64 - -
9/16-18 33/64 - -
5/8-11 17/32 - -
5/8-18 37/64 - -
3/4-10 21/32 - -
3/4-16 11/16 - -
Drill sizes are for 75% depth of thread.
Tap Metric Drill Imperial Drill
3 mm x 0.5 2.5 mm -
4 mm x 0.7 3.4 mm -
5 mm x 0.8 4.3 mm -
6 mm x 1.0 5.0 mm -
7 mm x 1.0 6.1 mm 15/64
8 mm x 1.25 6.9 mm 17/64
8 mm x 1.0 7.1 mm -
10 mm x 1.5 8.7 mm -
10 mm x 1.25 8.8 mm 11/32
10 mm x 1.0 9.1 mm -
12 mm x 1.75 10.5 mm -
12 mm x 1.5 10.7 mm 27/64
14 mm x 2.0 12.2 mm -
14 mm x 1.5 12.7 mm -
16 mm x 2.0 14.2 mm 35/64
16 mm x 1.5 14.7 mm -
Drill sizes are for 75% depth of thread.

Pipe

Threaded pipe is often used in plumbing and pneumatic applications. Because pipe joints must form a seal, the threaded portion is slightly conical rather than cylindrical. As a result, threaded pipe requires specialized taps and dies. Conventional pipe threads must be assembled with pipe dope in order to achieve a leak proof seal.

A modified form of the basic pipe thread shape is the Dry-Seal thread. The Dry-Seal thread is formed so that during assembly, the tips of the male threads are slightly crushed into the roots of the female threads, effecting, in theory, a liquid-tight fit. In practice, a small amount of pipe dope is usually necessary to assure a pressure-tight seal, and to prevent galling of the mating parts.

BSP (British Standard Pipe) parallel threads are available in sizes 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, 1 inch. They are also available in Tapered thread-form and called BSPT (British Standard Pipe Tapered) for British pipe sizes. North American equivalents to BSPT are called NPT (National Pipe Tapered), and range from 1/16 inch through large integral sizes. Although BSPT and NPT are functionally identical, they are not mechanically interchangeable.

Rolled threads

Not all threads are made by cutting. In large production runs or where great strength is required, the threads may be rolled, both for the male and female portions.

In the case of the tap there are no cutting edges but instead the tap is lobed. The tap is forced into the hole and the material is deformed by the lobes into the required thread form. The male portion (bolt) is fed between rollers that have the full thread form ground into their outer diameter. The action of feeding the rollers into the work piece deforms the material into the required shape.

Rolled threads have the advantage of increased strength (the material flows into shape, similar to forging) along with reduced material cost as the bar or rod used is actually smaller than the finished size due to the material squeezing into shape.

A Roll tap is especially effective when driven by machinery, whether in a hand-fed vertical mill or in fully automatic machinery. Since the tap does not produce chips, there is no need to periodically back out the tap to clear away chips, which in a cutting tap can jam and cause tap breakage. Also, since no chips are produced the tap need not be reverse driven a short distance to break the chips, as is done when hand tapping with a cutting tap. Thus the roll tap can be driven in one run to the full length of the tap, but with precaution to not overdrive in depth when the hole is not completely through the workpiece. Note that the tap drill size differers from that used for a cutting tap and that proper lubrication is essential.

Other uses

An ordinary tap and die can also be used for repairing threads in stripped holes or bolts. Die nuts are dies made for cleaning up old threads, they have no split for resizing and are made from a hexagonal bar so that a wrench or shifter spanner can be used to apply them (rather than the purpose built die wrench). Dienuts are only for cleaning threads (not for cutting threads).

References

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