A drunken thread has a helix that does not advance at a uniform rate. Inasmuch as this error occurs within one turn of a thread it is not normally detected by conventional linear measurements.
To allow for normal wear an H-3 tap is ground from +.001 to +.0015 over the basic pitch diameter for a 1/4-28 UNF thread.
Since the "Effective Pitch Diamter" increases by a mathematical constant of .00017 for each .0001 of helical path error and the Go Thread Plug Gage has a drunkenness problem of .0005, (as shown above) this gage will begin rejecting good parts before the tap is fully worn. A simple calculation shows that .00085 of the H-3 tap life is consumed by the gage.
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