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?
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.
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.
