186 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
In the given fi gure, datum feature B is ref-
erenced as a primary datum and the reference
includes an MMB modifi er, and no derived
median line or derived median plane form toler-
ance is shown. Datum axis B is therefore located at
the center of a simulator or tool that has a diameter
equal to the maximum material size of the shaft.
The maximum material size of the datum feature
is .878″ diameter. To locate the axis, the shaft is
inserted in a .878″ diameter hole. A shaft made
.878″ diameter will fi ll the hole, and the axis of the
hole and shaft will coincide. Any shaft made at a
smaller diameter is free to move inside the hole.
The amount of movement depends on the amount
of feature size departure from the MMB.
The MMB modifi er allows simulation of the
datum axis with a fi xed diameter gage. This can,
especially for high volume production, result in an
easier and less expensive inspection process than
the axis simulation methods required by a datum
feature reference made at RMB.
The three datum feature references in the
given position tolerance in Figure 6-31 defi ne
all three planes in the datum reference frame. In
Figure 6-41, only two datum feature references
are needed to establish an adequate datum refer-
ence frame. Datum axis B establishes the location
of two planes. Datum A locates the third plane.
Depending on the produced diameter of the
shaft, datum feature B may or may not establish
orientation of the datum reference frame before
datum feature A comes into contact with the
datum simulator. Consider a shaft made at .878″
diameter. If the .878″ diameter shaft is made with
zero perpendicularity variation, datum surface A
will be able to sit fl at on its simulator. If the .878″
diameter shaft is made with a perpendicular-
ity variation of .003″, datum surface A cannot sit
fl at on its simulator. Both conditions are accept-
able based on the tolerance specifi cation showing
datum B as the primary datum at MMB.
A reference to a primary datum feature of
size at MMB is appropriate for some of the pos-
sible applications of the given part. An MMB refer-
ence can be appropriate when a clearance fi t exists
between mating parts, and assembly of the parts is
the main concern.
RMB on a Secondary or Tertiary Datum
Feature Reference
A reference to a datum feature of size includes
a material boundary modifi er regardless of
whether the referenced datum feature is primary,
secondary, or tertiary. It is also applicable when the
datum feature is a surface that has a maximum and
minimum material boundary. RMB is assumed
for all datum feature references on all tolerances
unless the MMB or LMB symbol is shown.
The given part in Figure 6-42 has a position
tolerance with a reference to primary datum A.
It does not include a material boundary modifi er
and is therefore assumed to apply RMB. The refer-
ence to the secondary datum feature B also shows
no modifi er and is assumed to apply RMB.
The fi rst plane in the datum reference frame
for the shown position tolerance is established by
datum surface A. It is constrained in translation
along one axis, and constrained in rotation around
Simulator is expanded to
make contact regardless
of material boundary
Primary datum
surface A makes
three point contact
Datum axis B
Two perp planes
passing through
datum axis B
Datum plane A
RFS assumed
RMB assumed
Goodheart-Willcox Publisher
Figure 6-42. A secondary datum feature referenced at
RMB requires the datum be simulated by contacting
the surface of the datum feature.