A tap drill bit is a tool attachment specifically designed for the metalwork technique of tapping. The technique of tapping is utilized to create a threaded hole in a material into which a threaded bolt can be screwed. The attachment can also be used to repair, or rethread, a damaged fixing. A tapping tool can be used together with a die tool to create internal and external threads, respectively.
Specifically, a tap drill bit resembles a long bolt thread, which is inserted into a drill chuck to accelerate the process of forming an internal thread. These bits are usually constructed of hardened tungsten steel to allow the bit to be used several times to form threads in a variety of metals. A tap drill bit can form an internal thread, sometimes referred to as the female section of a coupling, such as that found in a standard nut.
A die bit is the partner attachment to a tap drill bit. It is used to form an external thread, or male section of a coupling, as one would find on a bolt. Tap bits and die bits are commonly paired to ensure that the male and female section of the coupling join correctly. Both types of bits also feature numbering systems based on the diameter of the resulting threads to allow matching pairs to be created.
There are three main types of tap drill bits used by metal machinists; these are known as the bottoming, intermediate, and taper tap. The first of these types of bit, the bottoming tap, features a flat head with very little tapering from the tip to the shaft of the bit. These types of tap drill bit can form a fully threaded hole in metal, where the hole does not exit the metal — an example of this is a piece of furniture where legs need to be screwed into a base or platform. To utilize this type of bit, a machinist needs to start the threading process with a tapered bit as the flat head of the bottoming tap cannot achieve sufficient purchase in an unthreaded hole.
Intermediate and taper taps both feature a more tapered tip than the bottoming tap. The most commonly used type of tap drill bit is the intermediate tap. The tapered cutting edge of these two types of bit are designed to assist in aligning the threads and commencing the tapping of unthreaded holes. The taper bit features a more gradual tapering than the intermediate bit, making it more suitable for working with harder materials, such as steel alloy.