34 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
Application to Threads
A geometric tolerance applied to a thread is
either placed adjacent to the thread specifi cation
or is attached to the threaded feature by a leader.
See Figure 2-23. Any tolerance in a feature control
frame that is applicable to a thread is by default
applicable to the pitch diameter and the resulting
pitch cylinder axis.
Tolerances applied to threads typically should
apply to the pitch diameter. Sometimes it is desir-
able to specify a geometric tolerance that controls
the major or minor diameter of a thread. This can
be done by noting the applicable diameter beneath
the feature control frame. See Figure 2-23. The fi rst
two examples are applicable to the pitch diameter.
This is the way that most threaded features are tol-
eranced. The two exceptions shown in the lower
portion of the fi gure show the tolerance noted as
applicable to the minor diameter on an internal
thread, and the major diameter on an external
thread.
Application to Gears
Geometric tolerances applied to gears are not
generally applied to the gear teeth. Past practices
of applying runout tolerances to gear teeth have
been replaced by other methods that are defi ned
by gear specifi c standards. Any desired toler-
ances on gear features other than the teeth may be
accomplished through application of appropriate
feature control frames.
Datum Identifi cation
Datum features are the surfaces and features
of size used to establish theoretically perfect datum
planes, datum axes, and datum points. Standard
symbols are used to identify datum features.
Datum Feature Symbol
The datum feature symbol consists of a square
with a single capital letter or a rectangle with mul-
tiple capital letters, a leader line extending to the
point of application, and a triangle at the end of
the leader line. See Figure 2-24. The datum feature
triangle is typically shown fi lled. The datum fea-
ture symbol and triangle may be used to identify a
surface or feature of size (either regular or irregu-
lar feature of size) as a datum feature. The given
example only shows the symbols as they apply to
surfaces. More information about application of
datum feature symbols on surfaces and features of
size is given in Chapter 6.
Datum letters are assigned alphabetically
using single letters A through Z, omitting letters I,
O, and Q. No two datum features on a single part
should have the same datum letter. If all the letters
of the alphabet are used and additional datum fea-
tures are identifi ed, a rectangle replaces the square
and double letters are used for the additional datum
features: AA–AZ then BA–BZ, etc.
Application to features of size
Or
Or
Goodheart-Willcox Publisher
Figure 2-22. Placement of a feature control frame
on the dimension line or near the dimension value
indicates a control on the feature of size.
Or
Or
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Figure 2-23. A notation must be placed under the
feature control frame when the intent is to control a
feature other than the pitch diameter of a threaded part.