198 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
Dimensions Related to a Datum
Reference Frame
As explained in Chapter 4 of this text, Rule #1
of ASME Y14.5 requires perfect form of a feature
when it is at maximum material condition. The
requirement for an individual feature to have per-
fect form does not require the feature to be in any
specifi c relationship to other features. Relation-
ships between features are normally specifi ed in
tolerance specifi cations such as orientation, posi-
tion, and runout.
It is also possible to control feature size and
orientation relative to a datum reference frame. See
Figure 6-60. This is done by including a note on
the drawing. The note may be general and apply
to all dimensions, or it may be a local note that is
applied to specifi c dimensions. If a general note is
used, make sure that dimensions extend from the
datum features. The note must specify the order
of precedence for datums to which dimensions are
referenced.
A local note is applied to the given drawing.
It states that NOTED DIMENSIONS ARE RELATED
TO DATUM A PRIMARY, DATUM B SECONDARY,
AND DATUM C TERTIARY. The note is placed in a
notes list, and the note number is placed adjacent
to the applicable dimensions.
The effect of dimensions related to a datum
reference frame is the creation of an envelope.
An envelope is a geometric boundary that con-
tains an external feature or is within an internal
feature. Normally, an envelope, when created by
tolerances, establishes one boundary that may not
be violated. Envelope measurements for the given
fi gure are made relative to the datum reference
Alternate
Circular target line
Goodheart-Willcox Publisher
Figure 6-58. Datum target areas may be used to defi ne the area of a cylinder that must be contacted to locate a
datum axis.
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Figure 6-59. Datum target types may be mixed to
locate a single datum plane.