I hope this is as simple as I think it is. I have a part the calls out the true position of a 5/16-18 ID threaded hole with MMC. I think that the MMC should come off of the minor diameter. Onr of the owners and the QA manager think it should come off of the pitch diameter. Other than the fact we don't have a good way to measure the pitch diameter in house I think they are wrong. What are the prevailing winds on minor or pitch for MMC on a ID threaded hole?
If you ABSOLUTELY MUST - Pitch diameter of threaded holes CAN be obtained using flexbar reprorubber and casting it. Can be dificult to remove if not released properly ar if you don't add a key handle to help. But, as a rule I do not put MMC on threaded holes. Way to expensive to measure and the bonus values are so low. No return on investment.
I believe if it is not specified, pitch diameter is used.
In practice, if it is minor diameter (for internal) you will need to gage the diameter with something other than the CMM (gage pin?).
I actually never apply MMC to threaded holes. If it is in tolerance without adding the MMC bonus, great. If not, I note that it is not possible to measure the diameter (it's always pitch diameter here) so it is not possible to report if it is in tolerance.
By 'not possible' I mean that it is not possible for me to measure it with the tools I have at my disposal. Obviously, it is possible for someone, somewhere, with better tools.
I once found the tolerances for threads, to apply MMC is almost a moot point. Look at it this way, if they were that concerned about location, they probably wouldn't be using a screw or bolt to locate it. It just goes back to some Engineers not having a frickin clue. I ignore MMC on a thread period. If you take a screw and thread it in, can it wiggle to the left or right ? NO, its self centering. I can't believe we even need to have these discussions because of the lack of understanding by the people making the drawings, it disgusts me.
Pitch Diameter... buy those true posi gages, great investment. Or a cheap fix, get a socket cap screw, tap shank, anything you can thread into it, still, could lose some from the slop, but far better than the minor Ø.
I once found the tolerances for threads, to apply MMC is almost a moot point. Look at it this way, if they were that concerned about location, they probably wouldn't be using a screw or bolt to locate it. It just goes back to some Engineers not having a frickin clue. I ignore MMC on a thread period. If you take a screw and thread it in, can it wiggle to the left or right ? NO, its self centering. I can't believe we even need to have these discussions because of the lack of understanding by the people making the drawings, it disgusts me.
Reading this thread from 08 to gain some guidance as to why MMC is on our print & how the H E double hockey sticks I am going to make that work & wouldn't ya know.....! The issue you spoke on many years ago still exists today.
In most cases, the MMC should come off the pitch diameter, since that's the designed point of engagement.
This can be handled several ways:
1) Comment to measure manually, input PD, and use legacy dimensions with a formula in the tolerance field, based on input value (for some reason this field could not accept a formula in 2022, but I haven't had time to investigate).
2) If access is possible, all CMMs can measure pitch diameter directly, with a little creative coding (I normally perform the task this way)
3) just don't use MMC, but comment that this must be calculated manually, in cases of borderline failures
4) I have also seen programmers simply assume PD is nominal and add half the PD tolerance as a bonus (this is wrong)
I have always assumed that designers put this in to predict if a fastener can get started, rather than predict the final assembled position. Same concept as starting every bolt on a car part, just a couple threads before tightening, so you can jiggle the part around, rather than throwing wrenches.
