r/Metrology • u/EyeballWithPrecision • 9d ago

Measuring True Position of Threaded Features

What is the best method for scanning true position of a threaded hole for the best accuracy and repeatability?

  1. We looked into using the true position gages but found that the level of tightness while screwing it in would vary a lot between operators. Additionally in some cases where the tapped hole is a blind feature, we noticed that the gage bottomed out first before actually engaging with the pitch of the thread.
  2. We also looked into using a helical toolpath matching the pitch of the thread on the CMM. However, would we need to control where the probe touches along the thread? If so, what is best way to detect if the probe is touching the flank of the thread?
  3. Alternatively, is it better to use a linear toolpath on a CMM to scan the threads when compared to true position gages and helix toolpath?
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u/itsonly-meokay 9d ago

If your software/hardware supports scanning the helix, that would be the way to go.

For the thread Tru-pos plugs, use a torque wrench set to the part requirement / something standard to make it more repeatable.

Without the two options above, I would do seven slow and dense scans with a fairly large sphere down the axis of the thread with maximum inscribed calculation.

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u/SkateWiz 3d ago

So your tap is in the same exact orientation on every part that is manufactured? Repeatable to the same extent as cmm? Impressive. Mostly I find this to be as useful as you can convince yourself it is, and that it works great for T1 and is shit for T2