General Question

JoeyDesignsStuff's avatar

What does "UNC" stand for in a mechanical drawing / drafting context?

Asked by JoeyDesignsStuff (756points) February 23rd, 2009

As in, ”.250—.20 UNC x .30 deep,” referring to a pair of holes in a surface.

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1 Answer

Harp's avatar

It’s a screw thread designation: “Unified Coarse”, which refers to the standard for thread sizes and shape used for machine screws in the US. The ”.250” is the nominal diameter of the screw that will fit the hole and the “20” (not ”.20”) is the pitch, or number of threads per inch. In other words, the hole is first drilled with a number 7 drill bit, then tapped using a 1/4–20 tap.

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