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.
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 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 (also known as swarf) formed during the cutting process, thus preventing an effect called "crowding" that may cause breakage.
The most common type of power driven tap is the "spiral point" plug 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 forward into the hole, preventing crowding. Spiral point taps are usually used in holes that go all the way through the material, so that the chips can escape. 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.