24 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
Introduction
Symbology is used to express dimensioning
and tolerancing requirements in engineering doc-
umentation, which includes engineering drawings
and computer-aided design (CAD) models. The
shapes of dimensioning and tolerancing symbols
provide a logical connection between the sym-
bol and the characteristic it represents. The easily
identifi able shapes of the symbols permit a stan-
dard symbol set to be used internationally without
the problems caused by differences in language.
It is important to create engineering drawings
that have clearly defi ned dimensional require-
ments. The symbology defi ned by ASME Y14.5
makes a clear and consistent expression of require-
ments possible. The same level of clarity and con-
sistency is diffi cult to achieve when notes are used
instead of symbols. The diffi culty with notations is
caused by differences in language mastery levels
and personal writing styles.
All tolerances are applied to part features, and
part features fall into two major categories. They
are either surfaces or features of size. As the mean-
ings of tolerancing symbols are explained, some of
them are limited to application on one of the two
feature types. As an example, position tolerances
may only be applied to features of size. Another
example is the symbol for profi le of a surface, and
it may only be applied to surfaces. The meaning of
a couple of tolerance symbols is altered depending
on whether the symbol is applied to a surface or a
feature of size.
There are two classes for features of size. Reg-
ular features of size are typically associated with
a single size dimension such as thickness, width,
length, or diameter. Regular features of size are
made of common geometric shapes with a single
size dimension such as a cylinder, circle, or slot.
Irregular features of size may be one enclosed
shape or a collection of features that in combina-
tion establish a size boundary. A hexagonal shape
may be toleranced in such a manner that it is
treated as an irregular feature of size. Throughout
this text, the term feature of size is general and is
applied to both regular and irregular features of
size.
General Symbols and
Abbreviations
Some dimension values shown on a drawing
require clarifi cation through the use of symbols.
As an example, a dimension applied to specify
the diameter of a circular feature, such as a hole,
includes a diameter symbol. The diameter symbol
makes it clear that the specifi ed value is a diam-
eter, and not a radius or spherical diameter.
Standard dimensioning symbols, for charac-
teristics such as a diameter, are used to provide
consistency regardless of who creates a drawing.
See Figure 2-1. The symbols currently defi ned by
ASME are shown. Where symbols are defi ned,
abbreviations for equivalent terms are no longer
included in the standard. The omission of abbre-
viations from the standard is a clear indication
that symbols are preferred on the drawing or CAD
model. The symbols may also be used in notes.
However, abbreviations are commonly used in
notes and this continues to be a fairly common
practice because the symbols are not always avail-
able as a text font.
Symbol size on a drawing or CAD model
is established relative to the character height of
dimension numerals. See Figure 2-2. The charac-
ter height is to be substituted for the X in each of
the symbol size formulas. The formulas shown in
the fi gure are recommended symbol sizes and are
not absolute requirements. Symbol size for use in
notes is not standardized.
Parameter
Diameter
Spherical diameter
Radius
Spherical radius
Counterbore
Spotface
Countersink
Deep
Dimension origin
Square
Reference
Places, times
Arc length
Slope
Conical taper
General Dimensioning Symbols
Controlled radius
Current
practice
Abbreviation
in notes
Goodheart-Willcox Publisher
Figure 2-1. Symbology is used in place of abbreviations
previously used for dimensioning.