2004

Add Geometric Tolerances | 559
Material Conditions
The second compartment contains the tolerance value. Depending on the
control type, the tolerance value is preceded by a diameter symbol and fol-
lowed by a material condition symbol.
Material conditions apply to features that can vary in size:
■ At maximum material condition (symbol M, also known as MMC), a feature
contains the maximum amount of material stated in the limits.
■ At MMC, a hole has minimum diameter, whereas a shaft has maximum
diameter. At least material condition (symbol L, also known as LMC), a fea-
ture contains the minimum amount of material stated in the limits. At
LMC, a hole has maximum diameter, whereas a shaft has minimum
diameter.
■ Regardless of feature size (symbol S, also known as RFS) means a feature can
be any size within the stated limits.
Datum Reference Frames
The tolerance values in the feature control frame are followed by up to three
optional datum reference letters and their modifying symbols. A datum is a
theoretically exact point, axis, or plane from which you make measurements
and verify dimensions. Usually, two or three mutually perpendicular planes
perform this task best. These are jointly called the datum reference frame.
The illustration shows a datum reference frame verifying the dimensions of
the part.
primary datum plane
secondary datum
plane
tertiary datum plane