Chapter 2 Dimensioning and Tolerancing Symbology 29
Copyright Goodheart-Willcox Co., Inc.
Several methods exist for dimensioning
tapered parts. When the amount of taper per unit
of length is specifi ed, a taper symbol is used. See
Figure 2-15. The symbol appears as a cone on an
axis. The taper symbol precedes the taper value
and its tolerance. The specifi ed taper value in Fig-
ure 2-15 indicates a change in diameter of .18″ plus
or minus .02″ per inch along the axis (.18±.02:1).
Geometric Tolerancing Symbols
Tolerances are applied on drawings or in CAD
models to defi ne the allowable variations for the
part features. Tolerances are required because it is
not possible, effi cient, or cost effective to produce
every part to an exact size.
The utilization of symbology for the specifi ca-
tion of tolerances has been evolving from a time
before MIL-STD-8. Initially, control of form, orien-
tation, and position was achieved through notes
placed on the drawing. As the inconsistencies and
vagueness in notations became an obvious com-
munication obstacle, the utilization of symbols
began to replace notations. Symbology is now uti-
lized for a majority of specifi ed tolerances. Only
occasionally is a tolerance needed that requires
notations to supplement the existing symbology.
Symbol Shape and Size
Tolerancing symbols, like general dimension-
ing symbols, have shapes that logically connect
to the associated control. Correct utilization of
standardized symbols provides a clear tolerance
specifi cation. Utilization of nonstandard symbols
or misapplication of standard symbols can cause
tolerances to be ambiguous.
Standardized symbols exist for specifying
form, orientation, profi le, runout, and location tol-
erances. There are additional symbols for clarifying
and modifying tolerances. Each symbol has a rec-
ommended size that is related to the general char-
acter height used on the drawing or in the CAD
model. See Figure 2-16. Illustrated symbol pro-
portions include dimension values related to the
variable X. The value of X represents the character
height on the drawing or in the CAD model where
the symbol is to be used.
Form tolerance symbols are used for specify-
ing requirements that apply to individual features.
In some special cases, multiple features may be
noted as acting as one continuous feature. Form
tolerances may be applied to those continuous
features and treated as though only one feature
exists, but this may create confusion and is not
recommended. The application and interpreta-
tion of form tolerance specifi cations is explained
in Chapter 5.
The form tolerance symbols are straightness,
fl atness, circularity, and cylindricity. The straight-
ness symbol is a short, straight line. Because the
symbol has the same shape as the desired control,
it is easily identifi ed. The fl atness symbol is a par-
allelogram. This symbol can be thought of as an
oblique view of a fl at surface. A circle is used for a
circularity symbol. As with the straightness sym-
bol, this symbol resembles the indicated control.
The cylindricity symbol is a circle with two tan-
gent lines.
The symbols for specifi cation of orientation
tolerances are perpendicularity, parallelism, and
angularity. See
Figure 2-16. The perpendicular-
ity symbol is indicated by two perpendicular
lines. The parallelism symbol is indicated by two
inclined parallel lines. The angularity symbol is
two lines drawn to form a 30° angle.
There are two types of profi le tolerances. See
Figure 2-16. The profi le of a line symbol (also
called line profi le symbol) is a semicircle. The pro-
fi le of a surface symbol (also called surface profi le
symbol) is also indicated by a semicircle, but it has
a horizontal line drawn across the bottom to dis-
tinguish it from the profi le of a line symbol.
There are two types of runout tolerance sym-
bols. See Figure 2-16. The circular runout symbol
is a single arrow. The arrowhead may be fi lled
solid, or it may be left unfi lled. The total runout
symbol is composed of two arrows connected by a
horizontal line. The total runout symbol shown in
the fi gure was fi rst made a requirement in the 1982
issue of the dimensioning standard.
There are three types of location tolerances
and each has its own symbol. They are position,
symmetry, and concentricity.
ΔD
Taper
=
.18)
1.00
1.00
D
d (
D
−
d = ΔD
Goodheart-Willcox Publisher
Figure 2-15. Taper on a diameter may be dimensioned
using the taper symbol.