200 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
be inserted through the datum feature to establish
the tertiary datum axis. It should be noted that the
slotted bushing cannot verify the perpendicularity
or position tolerances applied to datum feature C.
The slotted bushing only serves to correctly estab-
lish datum C as it is referenced in the position tol-
erance for the three holes.
Customized Datum Feature References
There are six degrees of freedom including x,
y, z translations along the coordinate axes and u, v,
w rotations around the coordinate axes. These six
degrees of freedom are constrained to the extent
required by the datum feature references in a fea-
ture control frame. Each datum feature reference
constrains all possible degrees of freedom that
are not already constrained by higher precedence
datums and that can be constrained by its geometry.
Wherever it is necessary to take exception to
the default constraints of the datum references,
the desired degrees of freedom are noted follow-
ing each of the datum feature references. The nota-
tions are made using lowercase letters. For these
notations to have meaning, the coordinate axes
are identifi ed by labeling them on the drawing or
in the CAD model. Uppercase letters are used for
axis labels.
Noting Translational Degrees of Freedom
The coordinate axes X, Y, and Z are labeled,
when needed, using uppercase letters. The trans-
lational and rotational degrees of freedom are not
typically labeled because the translational degrees
of freedom are understood to be parallel to the axes
with the rotational degrees of freedom around
those axes. See Figure 6-62. When there are two
or more coordinate axis systems created by mul-
tiple datum reference frames, then the applicable
datum reference frame is noted adjacent to each
coordinate axis direction label. The applicable
datum reference frame is placed within brackets.
Specifying Applicable Degrees of Freedom
in Datum References
It is not always desirable for a datum feature
to constrain all the possible degrees of freedom.
Should it be desired to limit the degrees of free-
dom constrained by a datum feature reference,
then the applicable degrees of freedom need to be
Goodheart-Willcox Publisher
Figure 6-61. The translation symbol applied to a datum
feature reference permits the simulator to move in
a manner that is appropriate in relation to higher
precedence datums.
Goodheart-Willcox Publisher
Figure 6-62. The applicable datum references are
noted when labeling the axes of more than one datum
reference frame.