Hello, Carl!
Regarding the true center of a threaded shaft, if the thread was single point turned
and threaded in the same setup on the lathe it can be then be assumed to be on true center of the thread. If not all bets are out the window.
Just me Two cents worth!
Dick
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http://www.homemadetools.net/forum/?OFF-SET-tailstock-center-65965#post105972
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OldToolmaker