160 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
and datums. It is important to understand the dif-
ference between them.
Datum feature references are the letters
shown in a feature control frame or possibly in a
drawing note. They tell us what features on the
part to use in establishing the datum reference
frame (coordinate system).
Datum features are physical features on a
part. Datum features are identifi ed on a drawing
by the use of datum feature symbols or datum tar-
gets. A hole is an example of something that can
be used as a datum feature because it is something
that can be seen or touched. Centerlines, center
planes, and theoretical axes are not identifi ed as
datum features because they do not physically
exist.
A datum is different than a datum feature.
A datum is a theoretically perfect point, line, or
plane. These theoretical geometric entities are
located by the physical datum features that are
identifi ed on the drawing. As an example, a datum
axis may be established from a hole that is a datum
feature. The tolerance requirement in the feature
control frame is actually located or oriented to the
theoretical datums.
The question often arises as to why datum
features are needed. Why not simply identify the
datum, such as a datum axis? The reason is that
an axis, or datum plane, may pass through mul-
tiple features. It would not be clear how a datum
axis is to be established when it passes through a
shaft that has multiple diameters. To avoid con-
fusion, the specifi c diameter (or diameters) that
establish the datum axis are identifi ed as a datum
feature. See Figure 6-2. The upper segment of the
fi gure shows a datum feature symbol incorrectly
M
M
M
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Figure 6-1. Datums and datum feature references make it possible to defi ne relationships between features.
Datum features identified
Datum feature unknown
Incorrect placement
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Figure 6-2. Datum feature identifi cation provides
a clear defi nition of what features to use when
establishing a datum.