Tapered hand taps are cutting tools that are employed when the machinist wishes to produce a threaded hole in metal and plastic materials, the thread that results from the operation will be identical to the thread on the screw or bolt that is intended to be used in the assembly. The tap is slightly tapered; it begins with a narrow point and increases in size until it reaches the dimensions necessary to cut the specified thread form. Hand taps can quite often have eight to 10 threads, only the last five or six actually cut the thread to size, the first few threads are designed to lead the tap into the pre-drilled hole so the thread will be in perfect alignment.
The circular cutting motion that is required to cut the thread is usually done by hand; the machinist uses a holding device which ensures that the thread is cut accurately. The holding device is called a “T wrench,” the design of the wrench allows different size hand taps to be used in the same wrench. The top of the tap is cut square during its manufacture, this square is held firmly in the T-wrench which is adjustable to suit squares are various dimensions.
If the threaded hole is to go completely through the metal or plastic, a tapered hand tap can be used to complete the entire operation, however, if the threaded hole does not go completely through, but is blind, then a bottoming tap will be used to cut the threads right to the bottom of the blond hole.
Although there are electro-mechanical devices that can be used to cut the thread using a conventional hand tap, they are rarely used on a work site, most on site machinists prefer the conventional T-wrench as it allows them to start the thread perfectly and they can control precisely the cutting speed they want. Hand taps were first developed in the early 1800s and they have never changed in shape or form.
Tapping the hole properly does take a significant amount of skill, not only to get the tap started perfectly straight, but the remove the cuttings from the hole as the process proceeds. It is important that the tap be backed-off during the process; this breaks the chips and pulls them out of the hole. Every time the tap is removed from the hole during the cutting process provides an opportunity for errors to take place.
