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Exploring Drafting
Dimensioning Circles,
Holes, and Arcs
Many products are manufactured from parts
that contain circles, cylindrical solids, round
holes, or arcs in their design, Figure 10-17. These
shapes are usually created using one or more
manufacturing processes. Processes such as
cutting, reaming, boring, turning, drilling,
spotfacing, counterboring, and countersinking
are used to produce round shapes, Figure 10-18.
When dimensioning circles, holes, and
arcs, it is important that the information given
conveys the proper manufacturing procedure.
The information may describe either a drilling
or cutting procedure. If certain guidelines
are followed, the correct procedure will be
conveyed, and the correct tool will be chosen
for the job.
When dimensioning an arc greater than
180°, dimension it as a diameter. This conveys
a drilling procedure. When the drawing is
used to make the part, the correct size and
type of tool will be selected for manufacture.
When dimensioning an arc 180° or smaller,
dimension it as a radius. This conveys a
cutting procedure. When the drawing is used
to make the part, the correct layout will be
completed and the appropriate cutting tool
will be selected for the job.
General Rules for Inch
Dimensioning
As previously discussed, mechanical
drawings based on the US Customary system
are most commonly dimensioned in decimal
inches. Decimal inch dimensioning should
conform to the following rules:
1. A zero should not be used before the
decimal point for values less than 1″.
For example:
.6, not 0.6
2. In drawings with geometric dimensioning
and tolerancing, a dimension is expressed
to the same number of decimal places as
its tolerance. Geometric dimensioning
and tolerancing is discussed later in this
chapter. In inch tolerance dimensioning,
zeros are added to the right of the
decimal point for decimal inch values
where necessary. Hence, if the tolerance
is in three-place decimals, all dimensions
will be in three places:
.375 and .500, not .375 and .5
Fractional inch dimensioning is less common
than decimal inch dimensioning in mechanical
drafting. However, this type of dimensioning
should conform to the following rule:
1. All fractions should be reduced to their
lowest common denominator:
1/2, not 4/8 or 8/16
3/4, not 6/8 or 12/16
24
1
21/4 2
1
4
2
1
4
21
4
Preferred
Avoid
Goodheart-Willcox Publisher
Figure 10-16. Numerals and fractions must be drawn
to the proper size and orientation.
American Foundrymen’s Society, Inc.
Figure 10-17. These automobile castings are typical of
manufactured parts that contain circles, round holes,
and arcs in their design.
Copyright Goodheart-Willcox Co., Inc.