202 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
location and orientation relationships between the
simulators are unchanged. However, the simula-
tors must be fi xed in size to the appropriate MMB
of the datum feature. The lower half of the fi gure
shows the MMB modifi er on the datum feature
references. The simulators used to meet these
requirements are fi xed in size equal to the appli-
cable MMB envelopes.
The simulator for the primary datum feature
is equal to the maximum material size limit of the
shaft, because only a size tolerance is applied to
the primary datum feature. The secondary datum
simulator is constrained (fi xed in location and ori-
entation) relative to the higher precedence primary
datum feature simulator. The secondary datum
simulator is sized to the .148″ maximum material
boundary that is created by the position tolerance
on the keyseat. The part is free to move relative to
the simulators by an amount equal to the datum
feature variation from the MMB.
The concepts explained above are applicable
to datum feature references made at LMB except
that the simulators are established at the least
material boundary instead of the maximum mate-
rial boundary.
The simulators for the position tolerance on
the hole are made up of the following: To establish
the primary datum axis A, the smallest circum-
scribing cylinder is brought into contact with the
primary datum A feature. Because the shaft is a
primary datum feature, the simulator is a cylin-
der that is free to fi nd a best fi t to the datum fea-
ture. The simulator in this case is known as an
unrelated actual mating envelope. It is called
unrelated because it is not constrained (or related)
to any higher precedence datums.
For secondary datum B, the simulator is an
expanding pair of parallel planes that are cen-
tered on datum axis A. The planes expand equally
while centered on primary datum axis A. They
expand to make maximum possible contact with
datum feature B while still remaining centered on
and aligned with datum axis A. The planes form
a related actual mating envelope for datum fea-
ture B. The envelope is considered related because
it is constrained relative to a higher order of prece-
dence datum.
In the lower portion of Figure 6-64, the effects
of the MMB modifi er are shown. When the datum
feature references include the MMB modifi er, the
Datum feature references at MMB
Simulator fixed in location
and fixed in size at the
MMB of datum feature B
Simulator fixed in location expands
from MMB until maximum possible
contact is made (related actual
mating envelope)
Simulator contracts until the
unrelated actual mating
envelope is established
MMC specified
MMB specified
MMB specified
MMC specified
Simulator fixed at MMB
Datum feature references at RMB
RFS assumed
RMB assumed
RMB assumed
RFS assumed
Goodheart-Willcox Publisher
Figure 6-64. The applicable material boundary modifi er impacts the way in which the simulators establish the
datums using features of size.