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MMC (M Symbol) for datum in FCF

MMC (M Symbol) for datum in FCF

MMC (M Symbol) for datum in FCF

If any clearance hole is a datum the corresponding feature control frames have MMC (M symbol) next to datum?

RE: MMC (M Symbol) for datum in FCF

Your question is not clear Please expand on your issue.

RE: MMC (M Symbol) for datum in FCF

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.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: MMC (M Symbol) for datum in FCF


I am new to the forum and would not have read "into" the statement as you did (to create a response). Thanks for the "instruction".

RE: MMC (M Symbol) for datum in FCF

mkcski, welcome. Many questions on this forum tend to be a bit lacking in details and asking for more info while a good idea may not always get the extra info you need - especially if a language barrier exists.

Folks like Belanger that can learn to read between the lines or extrapolate from the limited info, and then give a reasoned response clearly stating any assumption (e.g. referencing if they are ASME or ISO...) are often among the most tipped members.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: MMC (M Symbol) for datum in FCF


Thanks for the input. I have a Senior GDTP certification (#0199) and have taught/consulted GDT in the South Central PA region for over 30 years. I have a day job as a Mfg Engr making very large hydro turbine machinery. Given this, my GDT teaching/consulting time is limited to evenings (after office hours) within and hours' drive. I look forward to "peeking" at the forum on a more frequent basis and maybe offer a post or two if I can add anything to the discussion.

RE: MMC (M Symbol) for datum in FCF

One may also come to a conclusion that OP wants to get a confirmation that if a clearance hole is datum feature, it should always be referenced at MMB in a feature control frame.

If that is the case, it is of course not true. J-P (Belanger) gave one reason, but the other may simply be a function of the part. There are cases where from a functional standpoint it is better to have datum clearance hole referenced at RMB (or even LMB) in a feature control frame, even though the feature control frame could technically have the datum clearance hole referenced at MMB.

As it was already mentioned, OP's clarification is most likely needed.

RE: MMC (M Symbol) for datum in FCF

Yes, that's perfectly legal. It means that the datum is simulated at its "maximum material boundary." Notice that datum A in your case can't have an M modifier because datum A is just a plane with no such thing as maximum material.

To verify the position tolerance of the lower-left hole, you'd have to set the part down onto datum feature A (set it on a table or surface plate) and then put a fixed-size pin into the upper-left hole. (I can't tell you what size pin because I don't know the tolerance on the .500 hole size.)

Here's what it boils down to: When tolerancing to a datum that has an M after it, they are saying that there may be allowable looseness. Depending on the size of those four holes, the entire part may be able to jiggle around the mating pins (what is called "datum shift"). However, unlike the M after a geometric tolerance number, this datum shift can't be directly added to the position tolerance.

Two other side notes -- Yours is an academic example, but in the real world it's often the case where all four holes are lumped together under a single position tolerance; this allows for them to all create a single datum B.
The other side note -- you've stumbled upon one of the more difficult things in GD&T to really understand glasses

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