tap

A tap cuts a thread INSIDE a hole. It is a hardened, fluted screw: drill a hole to the fastener's tap-drill size, start the tap square to the surface, and turn it a turn forward and half a turn back so the flutes clear the chips instead of packing them. The result is a threaded hole — a nut with no separate nut.

The paired tool is the die, a round or hexagonal nut of hardened steel with cutting teeth inside, turned down a rod to cut a thread on the OUTSIDE. A tap threads a hole, a die threads a rod. That single sentence is the boundary the Shop Information subtest asks about most often, and it usually appears as a stem with both words among the choices.

Taps come in sets — taper, plug and bottoming — because a taper tap starts easily while a bottoming tap cuts to the bottom of a blind hole. A tap is not a drill bit even though both turn in a drill: a bit removes the hole, a tap only cuts thread into a hole that already exists.