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.
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