Chapter 10 Dimensioning
261
Different conventions may be used for
dimensioning radii depending on the situation.
For instance, if several concentric arcs are being
dimensioned with radii greater than 1″ but
increasing in size in relatively small increments,
the drafter may choose not to use the common
method for large radii (radii 1″ and larger) but
rather one of the others to improve clarity.
As with diameter dimensioning, the key is to
select the style that most clearly does the job in
any given situation. There are many acceptable
conventions for radius dimensioning. The
conventions in Figure 10-26 are not the only
ones that can be used. Consult your instructor
for advice on whether one you have created is
acceptable.
When placing location and size dimensions
on parts, the sizes of round holes and
cylindrical parts are dimensioned with respect
to primary center points, never with respect to
When dimensioning arcs, the radius of the
arc is given, Figure 10-26. The capital letter R
indicates that the dimension is a radius. It is
placed before the dimension. The methods shown
in Figure  10-26 are common. Each method
corresponds to a specifi c range of radii for the
feature. While dimensioning standards vary,
these practices are typical (the guidelines
shown are not ASME standards).
The dimensioning method shown in
Figure 10-26A is commonly used for radii of
1″ or greater. The method shown in Figure 10-26B
is commonly used for radii ranging from 3/8″
to 1″. The methods shown in Figure 10-26C are
commonly used for radii smaller than 3/8″.
15.0
∅50.0
∅50.0
∅25.0
∅35.0
∅45.0
∅40.0
∅40.0
A B
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Figure 10-23. Recommended ways to dimension cylindrical objects and concentric circles. A—Dimensions for a
cylinder are placed in the side view. B—When placing dimensions in this manner, only half a view is needed for
the side view.
Preferred Avoid
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Figure 10-24. Placing leaders to dimension circles.
The leader should radiate from the center of the object.
Avoid using leaders oriented at lower angular values.
Preferred Avoid
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Figure 10-25. The tip of an arrowhead for any leader
never touches a primary center point.
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