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?
1) Pitch Ø controls the location, more specifically: "the
axis of the thread derived from the pitch cylinder"
2) It does make sense that threads can have MMC because as the pitch Ø on the male thread gets smaller and the pitch Ø on the female thread gets bigger there will be bonus tolerance.
3) Despite the bonus tolerance existing it is not a 1:1 calculation like it is with a clearance hole. The bonus tolerance of a thread is very small and difficult to measure/calculate.
4) Most importantly, the MMC allows for hard gaging. Without the MMC you would need to use some sort of expanding thread to find the pitch axis of the thread you were measuring.
5) When print calls for the Minor or Major to control location and has MMC that seems like a functional misuse and/or a misunderstanding of why MMC is there when the pitch Ø is controlling location.
It makes no sense to apply MMC on threaded parts they are self centreing and any theoretical bonus cannot be used due to the physical nature of the fixing
It makes no sense to apply MMC on threaded parts they are self centreing and any theoretical bonus cannot be used due to the physical nature of the fixing
