190 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
on position tolerances. The same principle applies
to profi le tolerances, and to a combination of posi-
tion and profi le tolerances.
Datum Targets Based on Order of
Precedence
The datum precedence expressed in a feature
control frame must be considered when defi ning
datum targets on a drawing. See Figure 6-46. A
two hole pattern on the given part has a position
tolerance that references primary datum feature
A, secondary datum feature B, and tertiary datum
feature C.
Datum targets are used to identify datum
features on the given part because the cast sur-
faces are considered too uneven for use as datum
surfaces. Datum feature symbols may be used in
addition to the datum target symbols, but they are
not required.
The feature control frame on the given draw-
ing includes a reference to primary datum feature
A. Because target points are being used, a mini-
mum of three targets is required to establish pri-
mary datum A. For a small rigid part, no more
than three targets should be used unless notes are
used to explain that only three of the targets must
make contact.
Same datums, same
order of precedence,
same material boundary
modifiers modifiers
same material boundary
order of precedence,
Same datums, same
Datum C is identified and used
to clock the hole locations
Datum C is omitted and the single pattern
requirement establishes clocking locations
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Figure 6-45. Groups of features toleranced relative to the same datum reference frame act as a single pattern.
Datum feature
symbols permitted
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Figure 6-46. The number of targets used for each
datum feature depends on the order of precedence for
the datum.