How to Make OD Thread a Datum?

[[An...]]

How is everyone going about making an OD thread a datum? Scan up the side of the thread in multiple places to make your circle?

 

On our own parts we generally don't worry about concentricity when a bore and threaded OD are done during the same OP but we're running into some customer parts that want it checked.

[[Ze...]]

I use helical scans based on the pitch, using the largest stylus possible. There was an old lunch and learn that did a bunch of experiments with thread measurements on a CMM. Helical scans seem to work the best. You won't necessarily get accurate major/minor sizes, but the position is accurate. 

I know others use linear scans and report the MCC/MIC to get the diameters. Linear scans work pretty well also, but the probe has a greater chance to bounce across the threads, depending on the diameter of the stylus used and the speed of the scan. 

[[Ma...]]

This may be that LuncheoN Learn you are referring to.

Calypso - Threaded_Holes_metrology conf 2011.pdf

[[Pa...]]

I do this, but additionally self-center that helix.  On an OD thread use a max inscribed fit to best approximate PD location.  Initially many folks think this is backwards but consider what points you want to keep and what you want thrown out.  Max inscribed uses the points deepest in the root, which is what you want.

You don't need the largest available stylus if you're self-centering; in fact optimally you'd use something the same diameter as an appropriate thread wire. 

If you're not going to scan the root directly with self-centering and the correct pitch on the helix, almost none of my advice above is appropriate.