Chapter 6 Datums and Datum Feature References 191
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The given feature control frame includes ref-
erences to secondary datum feature B and tertiary
datum feature C. Secondary datum feature B is
identifi ed by two datum target points. Tertiary
datum feature C is identifi ed by one datum target
line. The target line could be replaced by a target
point if desired.
Regardless of whether target points, lines, or areas
are used, the datum precedence must be considered prior
to deciding the number of targets to show on a feature.
In some applications, not all surfaces are
machined when using cast, forged, or molded
parts. There are also applications where all sur-
faces are machined. See Figure 6-47. When possi-
ble, it is a good practice to locate machined datums
from cast datums. If no material is machined from
the cast datum features, it is possible to confi rm
that machined surfaces are properly located rela-
tive to the cast datums. If material is removed
from the cast datums, it is not possible to measure
from them on the fi nal part to determine how well
the machined surfaces are located relative to the
cast datums.
It is sometimes necessary to use a feature as a
primary datum in one tolerance specifi cation, and
a secondary datum in another. If datum targets
are specifi ed on the surface, the dual use of one
datum feature requires some special attention. See
Figure 6-48. One way of using a single feature for
two levels of datum precedence is to identify the
surface with two datum letters. One of the datum
letters is to identify the primary reference and the
second datum letter is for the secondary reference.
More than one datum letter should only be applied
to a single feature when using datum targets.
Figure 6-48 shows two feature control
frames. In the fi rst, datum feature A is referenced
as a primary datum and is identifi ed with datum
target points. Because datum feature A is primary,
at least three datum target points must be defi ned.
The second feature control frame requires the
same surface be used as a secondary datum. To
accomplish this, the surface is given a second iden-
tifi cation, datum feature D. Datum feature D is a
secondary datum and is therefore defi ned using
two target points, one of which shares a common
location with target point A1. It is not required that
any of the points share a common location, but one
or both of them may.
Although the same datum feature may be ref-
erenced at two levels of datum precedence in dif-
ferent feature control frames, the same datum letter
is not normally used to indicate both the primary
reference in one feature control frame and sec-
ondary reference in another feature control frame
when using datum targets. The reason is simple. If
three points are used to establish primary datum
A, a reference to the same datum feature A as a
secondary reference would not indicate which two
of the three points create the secondary datum.
However, if any two of the three points are allowable
for a secondary datum reference, then a reference to
datum feature A would be allowed, although it could
potentially result in some confusion. A note could be
used to explain the requirements.
There are situations where only the primary
datum is needed, such as for a perpendicularity
tolerance, yet the target locations for the primary
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Figure 6-47. Machined datum features are often located
relative to cast features using datum targets.
Datums A and D are established
by the same feature, but are
targeted according to datum
precedence
Optional
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Figure 6-48. The same surface may be identifi ed with
more than one datum letter when using datum targets.