204 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
The application of the MMC (or MMB) modi-
fi er on a surface was not permitted prior to the
ASME Y14.5-2009 standard. The application of
MMB on a fl at surface is a new concept based on
material boundaries created by profi le tolerances.
It should be expected that the application of MMB
on a surface will need to be explained to people
that have training or experience based on previous
dimensioning and tolerancing standards.
Material Boundary Modifi ers on Offset
Datum Features
A datum feature that is offset from a datum
axis is sometimes utilized to stop the rotation of a
part relative to higher precedence datums. The fol-
lowing explanation shows the signifi cance of the
material boundary modifi ers and other notations
when they are applied to an offset datum feature.
Figure 6-66 shows a part that has a hole that is
identifi ed as the primary datum feature, and that
establishes the primary datum axis. The second-
ary datum feature is a fl at surface that is offset
from the primary datum axis. This surface has a
profi le tolerance that is related to the higher pre-
cedence datum. A face surface perpendicular to
the primary datum axis is identifi ed as the tertiary
datum feature. There is a small hole perpendicular
to the primary datum axis that includes a position
tolerance and appropriate datum feature references.
When the datum feature references are made
RMB, the simulator for the primary datum is an
inscribing cylinder that expands to make maxi-
mum possible contact with the hole. The simula-
tor for the secondary datum feature is basically
RFS assumed
RMB assumed
Simulator expands until the
unrelated actual mating
envelope is established
Simulator progresses from
MMB toward LMB until
maximum possible contact
is made
Datum feature references at RMB
Datum feature references at MMB
MMB
LMB
MMB specified
LMB
MMB
Simulator fixed at MMB
Simulator fixed at MMB
MMB specified
Simulator fixed at basic
Simulator fixed at MMB
MMB
LMB
Datum feature references at basic
Basic noted
RFS assumed
RFS assumed
Goodheart-Willcox Publisher
Figure 6-66. The simulator for a surface that has material boundaries may be referenced at RMB, MMB, LMB, or at
the basic location of the datum feature.