QUESTION
If any clearance hole is a datum the corresponding feature control frames have MMC (M symbol) next to datum?
REPLIES
Belanger
Yes, a hole that is a datum (well, a datum feature) can then be referenced in other feature control frames with an M (technically called MMB, if one is using the ASME standard). However, there are exceptions: a callout for concentricity, symmetry, circular runout, or total runout cannot use the M or L anywhere in the feature control frame, even on a datum letter that is referring to a hole. That’s just because of the nature of those symbols, not a fault in the datum.
SOURCE
https://www.eng-tips.com/viewthread.cfm?qid=405031
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