180 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
Number of Datum Feature References
If the datum features on a part are fl at fea-
tures and a three-plane datum reference frame
is desired, then it is necessary to reference three
datums. See Figure 6-31. Proper specifi cation of a
position tolerance for the holes in the given part
requires that three datum feature references be
shown. Three surfaces on the part are selected to
serve as datum features. The selection of surfaces
is made through the process already described.
References to the datums are inserted in the posi-
tion tolerance specifi cation. Datum simulation for
the referenced datums is shown in Figure 6-32.
If one of the datum features is a feature of size,
then it is not always necessary to reference three
datums to establish the three planes in the datum
reference frame. See Figure 6-33. The position
Two point contact
Tertiary datum
simulator
One point
contact
Secondary
datum
simulator
Primary
datum
simulator
Three point
contact
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Figure 6-30. The specifi ed order of precedence sets the
requirements for how the workpiece must be located
in the datum reference frame.
datum simulator
Secondary
simulator
Primary datum
Datum reference frame
Simulated datums
C
B
A
Tertiary datum simulator
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Figure 6-31. Three datum surfaces may be used to
establish a datum reference frame.
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Figure 6-32. Three fl at tooling surfaces may be used to
simulate the datum reference frame that is created by
three datum feature surfaces.
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Figure 6-33. One datum feature of size and one datum
surface may be used to establish a datum reference
frame.