A tap is a tool, usually of very hard metal, which is used to cut threads into the sides of a predrilled hole. These threads accepts a threaded fastener such as a machine screw or bolt (fastener). A die (tool) makes the corresponding threads on a rod to fit into the hole.
Tapping can be done by hand or motor power. In general, a machine, such as a drill press, is preferable in order to keep the tap in perfect vertical alignment with the hole, and to maintain a steady level of pressure. Besides the problem of making irregular threads, an out-of-alignment tap can break the tap, which is made from hard but brittle material.
The most common means of hand-powering a tap is by use of a tap wrench, which has an adjustable set of jaws for holding the tap, and a T-shaped handle for turning the wrench-tap assembly. Taps usually have a square head that is gripped by the jaws.
Three variants of a tap may be needed for a particular threading project. Taper taps are optimized for starting the threaded hole, plug taps for the body of the hole, and bottoming taps for the end.
During a tapping operation, the taps must be withdrawn periodically so the waste metal that they have removed can be cleared from the hole. The hole also needs to be kept lubricated, and the taps themselves usually lightly coated with oil when stored.