A tap is a tool for cutting inside or internal threads in holes so that the holes will hold tightly the bolts, screws, or studs which may be screwed into them. Taps are generally made from hammered round bar steel. After being drawn nearly to size, they are heated to a low, red heat, and covered with lime or ashes, that they may cool slowly. This process softens the metal and takes out the strains, which occur in iron or steel after it is hammered. The outside surface or skin, where the hammer blows affect the iron most, is subjected to the greatest strain, or as it is called "initial tension." There are many styles of taps, the most common being standard hand-taps, boiler taps, stay bolt taps, pipe taps, and machine screw taps.
Tap drills are drills used to make the proper sized hole for a standard tap, leaving the hole small enough in diameter to permit of threads being made by the teeth of the tap. For example/the size of tap drill for a 5 /16 in. screw is .24 in. in diameter; for a 1/2 in. screw tap it is .4 in., leaving .1 in. for the diameter of threads on both sides of the hole. The size of a drill's or reamer's outside diameter and the size of a tap is the diameter outside of the threads, and not the size at the bottom of the threads.
Standard hand-taps are found in sets of three. Figure 119a is called a taper tap and is used to start the thread in the drilled hole; Fig. 119b is called a plug tap, and Fig. 119c, a bottoming tap. The plug tap will finish the thread if the hole goes through the piece, but if the hole "bottoms" or only goes part of the way through, the bottoming tap must be used to cut a full thread the entire depth of the hole.
The word "standard" means that the number of threads to the inch is United States standard, and all taps made to this standard are exactly alike.