If you are a regular user of GD&T, you probably know that the ASME standard was recently revised (for details, see the blog entry below dated March 28, 2009). In today’s column, I’d like to introduce you to one of the changes.
The term “maximum material condition” or MMC has been around for a long time. This concept is invoked when the circled M symbol is placed in a feature control frame. Well, a new item for the 2009 standard is something very similar, called “maximum material boundary,” or MMB. Yet it is invoked using the same circled M symbol.
The reason this was introduced was to eliminate confusion when the M symbol is modifying a datum that has its own geometric tolerance. Consider the following example:
The position tolerance references datum feature B with the M symbol. But here’s the key: think about the size of a gage pin that would be inserted into the center hole (it should be attached to a flat plate that simulates datum A). It would not really be simulating the MMC of 22.0; instead, it would be 21.8 in order to accommodate the perpendicularity tolerance of 0.2. Thus the confusion — people would say “MMC” when discussing the datum, but had to understand that it wasn’t really the true MMC that would be simulated.
So the new standard uses the term “maximum material boundary” to differentiate it from the true “maximum material condition.” The picture above would not change, but in talking to someone you would say “the position tolerance of the OD is 0.4 at its MMC, relative to datum A and datum B at MMB.”
I suppose if there were no perpendicularity tolerance on the center hole, then you could interchangeably say “MMB” or “MMC,” since they would be the same (22.0). And by the way, the other choices of LMC or RFS on a datum reference also have their counterparts of “LMB” and “RMB” in the new standard.