28 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
The countersink symbol is V-shaped; it has
the appearance of a countersink. See Figure 2-9.
When specifying a countersink, the countersink
symbol appears before the diameter symbol.
A dimension value enclosed in parentheses
is a reference value. See Figure 2-10. The proper
usage of reference dimensions is explained in
Chapter 3. If the reference dimension is a diam-
eter or radius, it is optional whether the associated
symbol may be inside or outside the parentheses.
A past drawing practice used the abbreviation
REF as a suffi x to the reference dimension value.
The arc length symbol indicates that a given
dimension defi nes an arc length. The symbol is
an arc drawn above the dimension value. See Fig-
ure 2-11. Past practice used a notation to indicate
an arc length.
A square symbol is used for dimensions that
apply across the fl ats on a square shape. See Fig-
ure 2-12. The square symbol precedes the dimen-
sion value. A space is not used between the square
and the dimension value. Previous practice prior
to 1982 used the abbreviation SQ as a suffi x to the
dimension.
An origin symbol is available for those appli-
cations where it is necessary to indicate the ori-
gin of a dimension. A small circle is placed at the
terminating point of the dimension line that is to
serve as the origin. See Figure 2-13. The origin
symbol replaces the arrowhead on the end of the
dimension line that is at the origin.
The slope on a fl at surface may be specifi ed
using the slope symbol and a slope value. See Fig-
ure 2-14. The slope value is the amount of change
in surface height per unit of distance along the base
line from which height measurements are made. If
the change in height is specifi ed in inches, then the
unit length is one inch. The slope symbol is placed
before the slope value. A complete slope specifi ca-
tion includes both the amount of slope and tolerance
on the slope per inch along the base line. The slope
specifi cation is written as a ratio. The specifi ed slope
value in Figure 2-14 indicates a rise of .20″ plus or
minus .01″ per inch of distance (.20±.01:1).
CAD model
Drawing
Goodheart-Willcox Publisher
Figure 2-10. Parentheses around a number identify it
as a reference dimension.
Goodheart-Willcox Publisher
Figure 2-11. The arc length symbol is applied to
indicate the length of an arc. Without the arc length
symbol, the dimension could be mistakenly read as the
width of the part.
1.00
1.00
=
.20)
Slope
ΔH
ΔH
(
Goodheart-Willcox Publisher
Figure 2-12. The distance across the fl ats of a square
shape is indicated using a square symbol.
Goodheart-Willcox Publisher
Figure 2-13. The origin symbol is used to indicate the
surface from which measurements are to be made.
Goodheart-Willcox Publisher
Figure 2-14. Slope symbol application.
Previous Page Next Page