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?
Here, the MMC is primarily used to allow the use of hard gages (cylindrical pins, not bolts) in order to have some speed in the checking of the part (manual, no CMM).
The minor diameter is used for the MMC (internal thread). I never apply the MMC when checking these requirements in the CMM though. I do it without MMC.
You would need a johnson gauge to complete this task. I never apply MMC to a thread and have yet to get yelled at about it. I'm thinking everyone knows its dumb.
Technically speaking: threaded holes per any spec have strict functional tolerances per that spec.
No further bonus allowance (aside from datum shift) should be permissible if the threads reference any thread specification (UN UNC UNF UNEF UNS etc).
For example, ANSI B1.1 (B1.3 for external threads) controls all UN spec-d threads. and particular for your 5/16-18 internal thread, all dims should be as follows:
(source is this handy engineers edge calculator)
Screw Threads Calculator for Internal Unified Inch Screw (engineersedge.com)
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.