Threads, Taps, and Tapping - 2: Thread Tolerances
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
Just like for machining operations, internal and external threads must be machined within a specific tolerance zone.

There are three tolerance classes of fit:

1=lose, 2= medium, and 3 = tight.

The most commonly used in general manufacturing is the class 2 tolerance. However in the defense and aerospace industries the class 3 is more widely used.

An example would be 1/2 - 13 UNC-3B, a half inch diameter thread, thirteen threads per inch, coarse thread, and held to the class 3 tolerance. The B indicates that we have internal threads (A would be for external threads).

Tolerances should be looked up in the Machinery's Handbook but general formulas are available also.
end mills, lathe bits, c'sinks, c'bores, taps, reamers
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