Chapter 6 Datums and Datum Feature References 203
Copyright Goodheart-Willcox Co., Inc.
RMB, MMB, and LMB Application to
Datum Feature Surfaces
Application of material boundary modifi -
ers on a surface may be done only when material
boundaries are created for the surface. Material
boundaries may be created through the applica-
tion of a profi le tolerance. See Figure 6-65. A shaft
with a fl at is given. The diameter of the shaft is
identifi ed as datum feature A. The fl at has a pro-
fi le tolerance that is referenced to datum A. The
fl at is identifi ed as datum feature B. There is a hole
through the center of the shaft and parallel to the
fl at. The position tolerance on the hole references
primary datum feature A, secondary datum fea-
ture B, and tertiary datum feature C. The fi gure
shows the difference between referencing the pri-
mary and secondary datum features at RMB and
MMB.
When the datum feature references are at
RMB, the simulator for the primary datum is a
circumscribing cylinder contracted around the
shaft until it makes full contact. The simulator for
the fl at surface is a plane parallel to datum axis
A. This plane is fi xed in its orientation to datum
axis A and must also make contact with the sur-
face. To bring it into contact with the feature, the
simulator progresses from the MMB toward the
LMB until it makes maximum possible contact.
The simulator must remain within the allowable
profi le tolerance zone (between the two material
boundaries).
Application of the MMB modifi er on the pri-
mary datum feature reference requires the simula-
tor to be a cylinder with a diameter equal to the
MMB of the shaft. The MMB modifi er on the sec-
ondary datum feature reference requires the simu-
lator be a plane fi xed in orientation and location
relative to the primary simulator. The location of
the secondary datum simulator is at the maximum
material boundary created by the profi le tolerance.
RMB assumed
Datum feature references at MMB
tolerance
envelope is established
unrelated actual mating
Simulator contracts until the
possible contact is made
toward LMB until maximum
Simulator progresses from MMB
at MMB created by profile
Simulator fixed in location
Simulator fixed at MMB
MMB
LMB
LMB
MMB
MMB specified
MMC specified
Datum feature references at RMB
RFS assumed
RMB assumed
RFS assumed
Goodheart-Willcox Publisher
Figure 6-65. The applicable material boundary modifi er impacts the way in which the simulators establish the
datums using surfaces.