Chapter 6 Datums and Datum Feature References 179
Copyright Goodheart-Willcox Co., Inc.
symbols should not be placed on a drawing unless
references to the datum features are made. Datum
feature references may appear in feature control
frames or in notes.
The number of datum feature references
shown in a feature control frame depends on the
type of tolerance specifi cation and the level of con-
trol being specifi ed. Some tolerance types, such
as parallelism, may need only one datum feature
reference. Position tolerances generally require
more than one datum feature reference. Regard-
less of the tolerance type, the number and selec-
tion of datum feature references is determined on
the basis of the degrees of freedom that need to be
constrained. Datum feature reference guidelines
provide fl exibility for identifying the needed num-
ber and type of datums.
Selection of the appropriate number of
datums is one step in completing the datum fea-
ture reference portion of a feature control frame.
The order in which to reference them must also
be determined because the order of precedence
impacts how the part is located in the datum refer-
ence frame.
Order of Precedence
If a feature control frame includes references
to three planar datum features (A primary, B sec-
ondary, and C tertiary), then a single unique datum
reference frame is created. Any tolerance specifi -
cation that references datum features A, B, and C
in the same order of precedence will be measured
relative to the same datum reference frame.
Any tolerance specifi cation that changes the
order of precedence for the datums results in a
different datum reference frame. See Figure 6-29.
By varying the order of three datum feature ref-
erences, it is possible to create six datum refer-
ence frames. Assuming the correct three datum
features are selected, only one of the six possible
datum reference frames is optimum for a specifi c
tolerance.
Care must be taken to select the correct datum
features and to reference them in the most appro-
priate order of precedence. Datum selection and the
order of precedence are determined on the basis of
design function and manufacturing considerations.
The given fi gure shows datum features A, B,
and C on a relatively simple part. The design func-
tion requires that datum feature A sit fl at on a mat-
ing part when two bolts are passed through the
holes. Making datum A primary ensures that at
least three points will be in contact with the mat-
ing part. Using this surface as primary also makes
sense for fabrication and inspection of the two holes.
Edge faces on the part are identifi ed as sec-
ondary and tertiary datum features. Datum fea-
ture B is selected to be secondary because it is the
longest surface and will ensure alignment of parts
in the assembly. This surface is also better than
datum feature C for stabilizing the part in fabrica-
tion. Selection of datum feature A as primary, and
B as secondary, results in datum feature C being
the tertiary reference.
The best order of datum precedence for this
part is A primary, B secondary, and C tertiary.
There are fi ve alternate orders of precedence that
could be used, but none of them meet the design
or manufacturing needs.
Based on the selection of datum A as primary,
datum B as secondary, and datum C as tertiary, the
workpiece can be located in a tool or machine setup
as shown in Figure 6-30. Three mutually perpen-
dicular datum simulators are set up for locating the
workpiece. Primary datum A must make contact
on at least three points. Secondary datum B must
make contact on at least two points. Tertiary datum
C must make contact on at least one point.
A physical tool is not required to establish the
datum reference frame for a part. The datum refer-
ence frame may be mathematically established uti-
lizing point data measured on the datum features.
Selected order
Possible orders
of specification
Primary
Secondary
Tertiary
Goodheart-Willcox Publisher
Figure 6-29. The order of precedence for datum
feature references affects the requirements shown in
the feature control frame.