162 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
notations called a customized datum reference
frame. Customized datum reference frames are
explained later in this chapter. Uppercase letters
are used to identify the axes. See Figure 6-5.
Planes in a datum reference frame are always
mutually perpendicular and perfect. Although
a perfect datum reference frame is located by
imperfect datum features, the imperfections in the
features are not permitted to affect the datum ref-
erence frame. The planes of the datum reference
frame remain mutually perpendicular regardless
of the conditions of the datum features. This is
important, because it prevents the variation in the
datum features from affecting the measurement of
the toleranced features.
Datum feature references shown in a feature
control frame communicate which part features
are to be used to establish the datum reference
frame. See Figure 6-5. Selection of the features to
use is primarily driven by the functional require-
ments of the part, but the impact of datum feature
selection on fabrication and inspection methods
must also be considered.
Three surfaces on the given part are selected
to act as datum features A, B, and C. A feature
control frame shows the position tolerance that is
applied to the two holes. Datum features A, B, and
C are referenced in the feature control frame. The
tolerance specifi cation, as shown, requires that the
three datum features be used to locate a theoreti-
cally perfect datum reference frame. How they are
used will be explained in following paragraphs.
There are advantages to having a perfect
datum reference frame from which to make mea-
surements. One advantage is that surface variations
on the part will not impact the locations of holes.
Another advantage is that measurements may
always be made relative to a known coordinate
system. This is in contrast to making measure-
ments from surfaces that may not be perfectly
perpendicular.
A drawing of a fabricated part similar to the
one in Figure 6-5 is shown in Figure 6-6. The
upper segment of the fi gure shows that ques-
tions arise regarding the origin for measurements
when a drawing shows no datum features and
has no tolerances referencing the datum features.
It is unknown whether the measurements should
come from the high or low points on the edges of
the part. There are no good answers because there
is no standard to defi ne the meaning when datums
and tolerances are not correctly specifi ed. The
lower segment of the fi gure shows the same part
and the correct measurement origin when datum
features and tolerances are specifi ed as shown in
Figure 6-5. The measurements all originate from
the datum reference frame. Identifying datum fea-
tures and specifying tolerances that reference the
datum features is the only way to specify require-
ments that have only one correct interpretation.
Z
Y
X
w
v
u
Axis
Axis
Axis
Datum plane
Datum plane
Datum plane
Datum plane B
Datum plane A
Datum plane C
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Figure 6-4. A datum reference frame is theoretically
perfect and is made of three mutually perpendicular
planes.
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Figure 6-5. Datum feature references made in a feature
control frame determine how a part is located in the
datum reference frame.