tap

tap, a screwlike tool that has threads like a bolt and two, three, or four longitudinal flutes or grooves and that is used to cut screw threads in a nut or a hole. The interruption of the continuity of the threads by the flutes creates cutting edges; the threads behind the cutting edges may be circular arcs or they may be relieved or backed off to produce sharper cutting edges. Hand taps are made in sets of three: the taper tap, the plug tap, and the bottoming tap. The taper tap has 7 to 10 chamfered threads; the plug tap has 3 to 5 threads chamfered; while the bottoming tap has one thread chamfered. Hand taps have short square-ended shanks and are used for both machine and hand tapping. Taps are usually made either of carbon tool steel or high-speed steel. Cut-thread taps are threaded to size before hardening; ground-thread taps are finish-threaded after hardening. In hand tapping, the tap is screwed into the hole with a tap wrench; when a large number of holes are to be tapped, machine tapping is called for. Machines for tapping nuts use a long tap with a large-radius right-angle bend. The shank of the tap has a reduced diameter that allows the tapped nuts to slide up the shank after they are tapped; the nuts keep the tap centred and they slide off the end of the shank and are ejected from the machine. Machine tapping can also be done on a drill press with a special tapping attachment. When the tap is brought down against the work, a friction clutch is engaged with a bevel gear that rotates the tap in the cutting direction. As soon as the operator pulls back on the feed lever, the reverse clutch is engaged with another bevel gear that rotates in the opposite direction and the tap is backed out of the hole.