Kedu and All,
Back to the discussion of Fig. 4-21 from Y14.5-2009. It was mentioned a while back that ISO would allow the use of A(M)-B. Would that be better? Is it even definable? I'm actually not sure, because I've never thought through the details of how a multiple datum feature (now called a common datum feature in 2018) would work with mixed modifiers or mixed feature types. Here is what I would think:
-Feature A, being referenced at MMB, would need to be enveloped by its TGC
-Feature B, being referenced at RMB, would need maximum contact with its TGC
-Features A and B should have equal precedence, with neither taking precedence over the other
For the as-produced part with LMC-sized feature A in Fig. 4-21(c), A(M)-B appears to be straightforward. The part could sit on the gage exactly as shown, with clearance on A and full contact on B.
For the configuration with MMC-sized feature A in 4-21(d), to me A(M)-B is not as straightforward. Should the part sit on the gage exactly as ahown, with full 4-point contact on A and 1-point contact on B? It seems like A is being given higher precedence.
What if the reference was A-B? How should the part in 4-21(d) sit on the gage in that instance? We would try to get maximum contact on both A and B, without one taking precedence over the other. This should mean that there would be less than full 4-point contact on A, and more than 1-point contact on B. But if that were the case, then feature A is given less priority in A-B than it was in A(M)-B. In other words, applying the MMB modifier to the A reference gives it higher priority. At the very least, this goes against the usual "hierarchy" of constraint, where RMB is more restrictive and MMB is less restrictive.
It is also possible that if the multiple datum feature is comprised of both a planar feature and a feature of size, then equal precedence cannot be achieved. It may also depend on how the datum features are toleranced relative to each other. Fig. 4-21 has a perpendicularity on B w.r.t. A. Does that need to be taken into account for the A(M)-B reference? If the perpendicularity had been on A w.r.t B, then we would have taken that into account in the TGC size for A in the A(M)-B reference.
At the end of the day, I don't think that Y14.5's example-based definitions provide enough detail (i.e. rules) on precedence or datum feature / TGC contact for us to really know how an A(M)-B reference would work. I wonder how ISO defines it.
Evan Janeshewski
Axymetrix Quality Engineering Inc.