Positional Tolerance on Threads
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Posted by: twhohen ® 07/16/2008, 15:51:00 Author Profile eMail author Edit

I'm using true position on a single part to control the ID of a cylindrical feature and a coaxial internal thread. That is, I want to make a threaded feature and a cylindrical feature coaxial. I'm making the non-threaded feature a datum, and applying true position to the threaded feature. I'm debating applying the tolerance to the pitch diameter or the minor diameter of the internal thread. Is anybody aware of the benefits/drawbacks of each strategy? To me, it seems like it would be easier to inspect to the minor diameter, but on the other hand, it seems like the coaxial requirement would be better satisfied by applying the tolerance to the pitch diameter, since it is probably a better indicator of where the mating part will align itself. Any comments or ideas are appreciated. Thanks, Travis

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Re: Positional Tolerance on Threads
Re: Positional Tolerance on Threads -- twhohen	Post Reply	Top of thread	Forum

Posted by: Kelly Bramble ® 07/16/2008, 19:26:04 Author Profile eMail author Edit

Depending on the type of thread and the thread tolerance (minor, major, pitch diameter)controlling the axis derived from the pitch cylinder is usually best. For measurement, if quality has a thread gauge then applying the tolerance to the pitch dia. is no big deal. If a thread gage is not available and the thread location and quality tolerances are loose then using the minor for an internal thread and the major diameter for an external diameter should work fine. BTW, True position is where a feature is actually built and position tolerance is the specification.

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