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?
IMO it is one of those things that look "cool" on the print, but is very difficult, or impossable to measure. In order for you to measure this you need the minor diameter in order to calculate MMC, BUT if you do not measure exactly on the crest of the minor diameter then any bonus may be questioned.
I have used the locators in the past(before PC-DMIS), but now I hit on the threads. I have done several tests and have confidance that the 2 methods were within .0015" of one another.
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.
In my opinion since it is called out to the thread it is for full thread form which is not just the pitch but the complete AME that the thread specification allows.
For giggles I will toss in here that in the last few days there has been yet another discussion of this on the Yahoo Y14.5 user group. Some of those participating in the discussion where doing so while attending meetings reviewing comments on the proposed draft of the new standard. Some think MMC should not be applicable to threads because of the self-centering tendency of threaded features. Some think the only reason MMC is allowed and used on threads is to facilitate the manufacture and control of "Hard functional gages".
I myself measure the minor diameter of threaded holes using the pitch function of pc-demon. I only use a circle when the length of thread and or reach of the tip demands it. 90% of the time I use a cylinder and I make sure that the distance between the first and last hit is a multiple of the pitch and that when more than 2 rows are used the distance between the first and last is also a multiple of the number of rows. I almost never use pc-dmis to calculate MMC on a thread. I treat threads as RFS. If it comes down to the MMC determining if the thread is good or bad, I advocate a functional gage. On the few rare occasions that I have been ordered to put MMC on the thread on a report I have used the self-centering thread bushings. They are very time consuming and require program modification, but IMNSHO they are the best way to attempt at accurate reporting of MMC on a thread with a CMM. (How silly of me not to add), that only tells you the location. You still have to find a method to determine what the pitch diameter actually measures. For threaded holes. . . there is not a good one that I am aware of. For external threads there are pitch micrometers.
In my opinion since it is called out to the thread it is for full thread form which is not just the pitch but the complete AME that the thread specification allows.
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.
