Chapter 6 Datums and Datum Feature References 187
Copyright Goodheart-Willcox Co., Inc.
two axes. As a result, the rotation of the other two
planes in the datum reference frame is constrained,
and the planes are required to be perpendicular to
datum plane A.
Datum feature B is a hole passing through the
part. Datum axis B is perpendicular to datum A
and it is located by the related actual mating envelope
of the hole. In this case, the related actual mating
envelope is a cylinder that must be perpendicular
to the primary datum and it is expanded until it
makes maximum possible contact without pull-
ing the part off the primary datum. Datum axis B
is at the axis of the related actual mating envelope.
Datum axis B locates the two remaining planes in
the datum reference frame (the axis constrains two
translational degrees of freedom). Datum axis B
must be perpendicular to datum plane A regardless
of any variation in the perpendicularity of the hole.
Care must be taken to expand the mandrel just
far enough to stop the part from moving. Expand-
ing it too far will align the center of the hole with
the mandrel axis and raise the primary datum fea-
ture off the datum simulator. If this happens, the
datum reference frame is not properly established.
As has been shown above, secondary datum
feature references at RMB are simulated differ-
ently than primary datum feature references at
RMB. As previously explained, a primary datum
feature reference at RMB requires that both trans-
lation and rotation of the datum relative to the part,
or the part relative to the datum, be constrained. A
secondary or tertiary datum feature reference at
RMB may only constrain the degrees of freedom
that are not already constrained by the higher
precedence datums. Any datum feature reference
made at RMB requires that the feature surface be
contacted to establish the datum reference frame.
MMB on a Secondary or Tertiary Datum
Feature Reference
A reference to a datum feature of size includes
either an implied or an explicitly specifi ed material
boundary modifi er regardless of whether the ref-
erenced datum is primary, secondary, or tertiary.
When the datum is to apply at MMB, the modifi er
must be shown. See Figure 6-43.
The given part has a position tolerance with a
reference to primary datum A. It does not show a
material boundary modifi er for the datum feature
A reference. The reference to secondary datum
feature B includes an MMB modifi er.
The fi rst plane in the datum reference frame
for the shown position tolerance is established by
datum surface A. The primary plane is constrained
in translation (located) along one axis, and con-
strained in rotation around two axes. As a result,
the rotation of the other two planes in the datum
reference frame is constrained, and the planes are
required to be perpendicular to datum plane A.
The locations of the second and third planes are
established by datum axis B. Datum axis B is per-
pendicular to datum plane A and located at the
center of the simulator for the datum B hole.
Datum plane A is established by a simulator
that is a fl at plate. Datum axis B is established by
a simulator that is a fi xed diameter cylinder. The
simulator in this example is a fi xed-diameter pin
that goes through the hole. The pin must be per-
pendicular to the plate, and it must have a diam-
eter equal to the maximum material boundary of
the hole. A reference to a secondary or tertiary datum
at MMB requires that the feature be picked up at its
Pro Tip
Mating Envelope Defi nitions
Actual mating envelope. A theoretically exact
geometric counterpart of the feature that is at
the maximum size that will fi t within the hole
or other internal feature or at the minimum
size that will encompass a shaft or other ex-
ternal feature.
Related actual mating envelope. An actual
mating envelope that is related to one or more
datums as required for the geometric toler-
ances that are applied to the feature.
Unrelated actual mating envelope. An actual
mating envelope that is not related to any da-
tums and that encompasses all size and form
variations on the feature.
The datum reference frame for the given part
may be established through the following datum
simulation methods. Primary datum feature A is
placed against a fl at surface to locate datum plane
A. An expanding mandrel extending perpendicu-
lar to the fl at plate is expanded until it makes con-
tact with the hole. It must expand only until the
part cannot slide on the fl at plate. This establishes
the location of datum axis B. Datum axis B is not
required to coincide with the derived median line
of the hole because the axis must be perpendicular
to the primary plane, and the hole may not be per-
fectly perpendicular to the primary plane because
it has a perpendicularity tolerance of .002″ diam-
eter applied to it.