Chapter 2 Dimensioning and Tolerancing Symbology 25
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The following example shows how the size of
a symbol can be calculated.
Drawing character size = .125
Illustrated symbol dimension = 1.5X
Symbol size 1.5
×
.125″ = .188″
Modern CAD systems include a library of
dimensioning symbols and automatically size
the symbols proportionally to the character size.
To maintain uniformity, only one character size is
used throughout a drawing or CAD model.
If you are using CAD software that does not
contain the needed library of dimensioning sym-
bols, a library of symbols should be created. A
library of symbols permits the appropriate symbol
to be pulled into a drawing without recreating the
symbol each time it is needed. In the rare situation
where drawings are created manually, templates
may be used to draw symbols that are correctly
proportioned.
Symbol Application
Symbol application requirements for draw-
ings and CAD models are standardized in ASME
Y14.5. Special application requirements for CAD
models are defi ned in ASME Y14.41. Correct
dimensioning practices defi ned by ASME Y14.5
and Y14.41 require placement of each symbol in a
specifi c location relative to the associated numeric
value or relative to the feature to which the toler-
ance applies. Correct placement is introduced in
this chapter and described in more detail through-
out the rest of this book.
The diameter symbol is a circle with a line
drawn through it. It always precedes the numeric
value for the diameter. See Figure 2-3. No space
is shown between the symbol and the number. If
Diameter Counterbore
Countersink
Deep
X
Origin
Square
Arc length
.3X
1.5X
Reference
Slope
X
Conical taper
2X
.5X
.5X
30°
X
15°
X
60°
X
90°
Spotface
X
2X
.3X
1.5X
X
X
.6X
60°
X
2X
.8X
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Figure 2-2. CAD software makes it easy to control symbol proportions. The proportions of symbols should be as
shown in this fi gure.
Or
Drawing
CAD model
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Figure 2-3. The diameter symbol immediately precedes
the number.