Tap and Die - 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 or plug 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 ("tap chamfer") 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 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. The number of tapered threads typically ranges from 8 to 10. 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 somewhat smaller than the tap's 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 number of threads per inch (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 result produces a tap drill size that 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.

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.