A tap is a metal-working tool used to cut threads for bolts and screws. A tap cuts threads into the sides of a smooth hole bored with a drill bit of a specific size relative to the size of the bolt or screw to be used.
Taps and Dies
A die cuts the male thread on a bolt or screw. A tap cuts the female thread into which the bolt or screw is turned to tighten pieces together. The female thread can be on the inner surface of a nut or a fixed metal object.
Machine screw sizes are described by two numbers. The first is a size designation, the second is pitch (threads per inch). A 14-20 tap cuts threads for a size 14 screw with a pitch of 20.
A 14-20 tap requires a #10 drill bit. An ANSI #10 drill bit’s diameter is 0.1935 inches.
A 1/4-20 tap is a different, slightly larger size than a 14-20 and should not be confused with it. This size requires an ANSI #7 drill bit, whose diameter is 0.2010 inches.
Kelvin O'Donahue has been writing since 1979, with work published in the "Arizona Geological Society Digest" and "Bulletin of the American Association of Petroleum Geologists," as well as online. O'Donahue holds a Master of Science in geology from the University of Arizona, and has worked in the oil industry since 1982.