MMC refers to maximum material condition, and when it follows a tolerance, then if the size of an actual feature is not equal to the maximum material condition (but still within the allowed limits of size), then the difference between the actual feature size and the maximum material condition is used as additional geometric tolerance called “bonus” tolerance.

LMC refers to least material condition, and when it follows a tolerance, then if the size of an actual feature is not equal to the least material condition (but still within the allowed limits of size), then the difference between the actual feature size and the least material condition is used as additional geometric tolerance called “bonus” tolerance.

Example of MMC

The position tolerance applies at MMC, meaning that a hole produced at size 14.3 would have to be located within a diameter of 0.3 mm of its theoretically exact location. If a hole were actually produced at 14.5 (LMC), then that larger hole would only need to be within a diameter of 0.5 mm (0.3 “stated” plus a “bonus” of 0.2 from the size variation) from its theoretically exact location.

Example of LMC

The feature control frame indicates that the 0.8 tolerance applies when the hole is at its least material condtion, or at size 13.5. At size 12.5, the available geometric tolerance would be 0.8 “stated tolerance” + (13.5-12.5 or 1.0 “bonus” tolerance) = 1.8 mm.

MMC vs. MMB

The size-related modifiers can also be used in feature control frames that reference datum features of size (and with the new 2009 standard, their definitions have been expanded to include some uses with surface datums). When placed after a datum reference, the M symbol referred to as MMB, for maximum material boundary indicates that the datum feature simulator should be produced at the datum feature’s maximum material boundary, starting with the appropriate datum feature size and adding or subtracting any applicable geometric tolerances.

Example of MMB

The MMB symbol (the circled M shown immediately to the right of datum A in the top feature control frame) indicates that the datum may be simulated with a fixed hole 28.3 mm in diameter (27.8 MMC size + 0.5 geo tolerance). A chuck or collet is not necessary to simulate the datum, and “datum shift” may occur on a part where datum feature A is sized toward the small end of the size limit and/or has axial straightness variation less than 0.5 mm.

In 2009, the Y14.5 standard kept the definition of the symbol following the stated tolerance as MMC, maximum material condition, but re-defined the symbol following the datum reference as MMB, maximum material boundary, even though it is the same, circled M symbol. The change helps differentiate between the M’s because they have different functions.