Chapter 10 Dimensioning
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dimensioned in decimal inches. Decimal inch
dimensioning is convenient because decimals
are easier to add, subtract, multiply, and divide.
However, fractional inch dimensioning is still
widely employed. This type of dimensioning is
primarily used in architectural drafting, but it
is also sometimes used in engineering drawings.
Decimal inch and fractional inch dimensioning
are both used in examples in this text.
Fractional inch units are often used when
making drawings of objects made from materials
that typically cannot be machined to very fi ne
tolerances (such as wood). Decimal inches are
generally used for drawings of objects made
from materials such as metal or plastic. These
materials can be machined to very fi ne tolerances.
When using the SI Metric system, drawings
are commonly dimensioned in millimeters (mm).
There are special rules for dimensioning metric
drawings. Metric dimensioning is discussed
later in this chapter.
Unidirectional and Aligned
Dimensioning
Dimensions are placed on the drawing
in one of two orientations. They are drawn
in either a unidirectional or aligned manner,
Figure  10-2. Unidirectional dimensioning is
preferred.
In unidirectional dimensioning, dimensions
are placed horizontally so that they are read
from the bottom of the drawing. In aligned
dimensioning, dimensions are placed parallel
to the dimension line. The numerals are read
from the bottom and from the right side of the
drawing.
Regardless of the method used, dimensions
shown with leaders are lettered parallel to the
bottom of the drawing. The same is true for all
notes.
Dimensioning a Drawing
From your study of the alphabet of lines,
you will remember that special lines are used
for dimensioning, Figure 10-3. These include
dimension lines, extension lines, and leaders.
The dimension line is a thin black line
used to indicate linear distance as it relates to
a given object. It is drawn to the same weight
as a centerline. It should be thin enough to
contrast with the visible lines. It is usually
broken near the center for the insertion of the
actual lettered dimension. Although this is
the most common practice, placement of the
lettered dimension may vary. In some cases,
the lettered dimension may be placed outside
the extension lines. Refer to Figure 10-1.
The dimension line is capped at each
end with arrowheads or some other type
of terminator. The type of terminator used
generally depends on the drafting discipline.
For example, tick marks are used as dimension
line terminators on architectural drawings.
5
2
2
1 _
2
2
1
Unidirectional Dimensioning
(Preferred)
5
2
2
1 _
2
2
1
Aligned Dimensioning
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Figure 10-2. The two accepted methods for dimensioning drawings. The unidirectional method is preferred.
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