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A tap drill is a tool used to cut a cylindrical opening in a piece of stock, usually metal, prior to tapping it, which is the process of cutting threads in the inside surface to accept a screw. The tap itself is a hard metal cylinder with sharp threads, inserted into the hole created by the tap drill and turned to cut threads in the inside surface of the cylinder, forming the female half of a mating pair, such as a nut. The male half, a screw or a bolt, is formed by using specialized dies to cut threads onto pre-formed metal cylinders with a special die, in a process called threading.
Using screws, bolts, nuts, and similar devices to fasten different things together has been a staple of human industry since at least the Middle Ages, and perhaps earlier. While modern threaded fasteners are nearly always made of metal, the earliest threaded fasteners were made of wood. The size of these steadily increased as the loads they had to deal with, on such devices as ships and windmills, became larger and larger. The tools used to make the mating components — the taps and dies — were generally handmade of metal by the accomplished craftsmen who would use them.
In a modern machine shop, a tap drill is a hardened drill bit used to cut holes in metal. The most important consideration in selecting a tap drill is that it's the proper size for the tap that will be used to cut the threads. It's critical to remove as much metal from the hole as possible so there's no extra metal left for the tap to cut, which can significantly increase the stress on the tap. The tap drill is sized by its length, as well as by its diameter, because the hole drilled is countersunk; that is, it's slightly wider at the top than the rest of the hole. This countersink accommodates the tap, which is slightly tapered at its tip. This taper facilitates seating the tap prior to beginning the process of cutting the threads.
Many modern craftsmen pride themselves in their proficiency in tapping and threading their own fasteners for custom work. When fabricating from sheet metal, for example, it is frequently necessary to tap holes for nuts and screws to fasten the workpiece to other components. When this is done, the taps are matched to commercially-available bolts and screws. Hand tapping can be a time-consuming job, though, with the tap being frequently reverse-rotated to break off and remove the metal shards being created. If this isn't done, the shards can interfere with the tapping process. Thus, all the fasteners available commercially are machine-made, and most of those used in custom work are also machined.