36 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
symbol, and the size of the area is specifi ed inside
or adjacent to the datum target symbol.
A datum reference frame symbol is used to
indicate the three axes of a customized datum ref-
erence frame. See Figure 2-27. The symbol is made
up of arrows that indicate the axis direction, and
each of the three axes is labeled. The axes may be
labeled in more than one view. When multiple
datum reference frames exist, each of the axes may
be noted to indicate the applicable datum reference
frame. In the fi gure, the X axis is established by
datum feature references to B primary, C second-
ary, and E tertiary. The axis is noted as X [B,C,E].
Uppercase letters are used to identify the axes.
Basic Dimensions
Every surface, every feature of size, and every
feature location on a part is designed with an
intended form, orientation, and location. Dimension
values for these characteristics may be contained
within the CAD data without being displayed in
any view, or the dimension requirements may be
shown in the CAD model or on a drawing. Regard-
less of whether the dimensions are displayed or
determined by querying a CAD model, it is not
possible to build perfect parts to the “as designed”
dimensions. Perfect fabrication processes do not
exist; therefore, an amount of permissible variation
must be defi ned for every characteristic of the part.
The permissible variation is specifi ed through the
application of tolerances.
Permissible variation may be assigned by tol-
erances directly applied to a dimension, title block
tolerances, a general note, or through the use of
feature control frames. Geometric tolerances for
orientation and location require that the orienta-
tion and location be defi ned by basic dimensions.
A basic dimension is a theoretically exact
value for which there is normally a tolerance
shown in a feature control frame. There is no
directly applied tolerance on a basic dimension. A
basic dimension is not an indication that a feature
has zero tolerance. Wherever a basic dimension is
seen, it is known that a feature control frame, or
other control, identifi es the amount of tolerance for
the dimensioned feature.
Basic Dimension Symbol
A rectangle placed around a dimension iden-
tifi es that dimension as basic. See Figure 2-28.
Plus and minus tolerances do not apply to a basic
dimension, and a feature control frame, or other
control, is required to express tolerance for the
dimensioned feature.
Noted Basic Dimensions
Drawing notes may be used to indicate that all
untoleranced dimensions are basic. If all dimen-
sions on the drawing are basic, the following gen-
eral note may be used:
ALL DIMENSIONS ARE BASIC
Should it be necessary to use plus and minus
tolerances on some of the size dimensions, the
following general note may be used:
ALL UNTOLERANCED DIMENSIONS ARE BASIC
Title block tolerances do not apply to any dimension
if either of these notes is used.
Area
Line Point
2X
2X
Goodheart-Willcox Publisher
Figure 2-26. These are the three types of datum targets.
Datum reference frame identification
Labeling datum reference frame
Goodheart-Willcox Publisher
Figure 2-27. Datum reference frame symbol.
Goodheart-Willcox Publisher
Figure 2-28. A rectangle around a number identifi es it
as a basic dimension.