Chapter 6 Datums and Datum Feature References 171
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Using a surface to establish a datum depends
on two things. First, the surface must be identifi ed
as a datum feature. Second, it must be referenced
in a feature control frame or note. When a datum
feature surface is identifi ed, that identifi cation
does not by itself indicate how the datum feature
is used. It is the reference to the datum feature, in a
feature control frame or note, that determines how
the datum is used.
Order of Precedence
The order in which datum feature references
are made establishes an order of precedence for the
datums. Refer back to Figure 6-3. The datum fea-
ture references are always read from left to right.
The fi rst datum is primary, the second is second-
ary, and the third is tertiary. The primary datum
feature does all that its geometry can do in regard
to constraining degrees of freedom. The second-
ary datum feature can only do what the primary
datum feature has not already done. It cannot
override or change what has already been done
by the primary datum feature. Finally, the tertiary
datum feature can only do what the primary and
secondary datum features have not already done.
Datum features referenced in a feature con-
trol frame establish a datum reference frame. The
datum reference frame is located and oriented
according to the order of precedence specifi ed for
the datums. It is important to remember that the
datum reference frame is always perfect, regard-
less of the condition of the datum features.
If two feature control frames both reference
the same datums in the same order of precedence
with the same material boundary modifi ers, then
both tolerances are related to the same datum
reference frame. If two separate feature control
frames refer to different datum features, then
those tolerances each establish separate datum ref-
erence frames. Even if one or two common datum
feature references are shown, one changed datum
feature reference creates a different datum refer-
ence frame. As an example, two different datum
reference frames are created if one feature control
frame references datum features A, B, and C, and
another references datum features A, B, and D.
In addition, two datum reference frames are
created when the same datum features are refer-
enced in a different order of precedence. As an
example, datum features A, B, and C establish a
different datum reference frame than datum fea-
tures B, A, and C.
Primary Datum
The fi rst datum feature reference in a fea-
ture control frame is to the primary datum. See
Figure 6-19. The primary datum feature estab-
lishes the location of the fi rst plane of the datum
reference frame if the primary datum feature is
a fl at surface. Establishing the location of the pri-
mary plane is important because the other two
planes of the datum reference frame are perpen-
dicular to it.
A position tolerance referencing datum fea-
ture A as the primary datum feature is shown in
Primary datum plane A
Three point contact
Workpiece
Surface plate
Primary datum
feature only
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Figure 6-19. A primary datum plane is located by the
primary datum feature on a part.