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Exploring Drafting
dimen sioned using one of the methods shown
in Figure 10-19. The methods shown are recom-
mendations based on the size of the feature.
Note the different conventions used for placing
the leader with the dimension. Each method
corresponds to a specifi c diameter range for the
feature. While dimensioning standards vary,
The ASME drafting standard sets guidelines
on how to “call out,” or specify, a diameter
or radius dimension. The symbol ∅ indicates
that the dimension is a diameter. The symbol
is placed before the size.
Circles and round holes are dimensioned
by giving the diameter. They should be
∅2
1 _
2
A B C D E
∅.62
∅.62
∅
1
_
2
∅
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Figure 10-19. Circles and holes are dimensioned by giving the diameter. Different conventions are shown.
The need for standardization in
manufacturing, making drawings, and
dimensioning dates back many years. The
Industrial Revolution created a demand for
standards because of the need to manufacture
interchangeable parts and also because of
the existence of many different manufacturers.
Standards were adopted and used in Europe
before they came into use in the United States.
In the late 1700s, the SI Metric system was
developed by French scientists and instituted
as the world standard. The system was based
on the meter. It was made equal to one-ten
millionth of the distance from the North Pole
to the equator. Other units of measure in the
metric system, such as the liter and gram, were
based on the meter.
The SI Metric system was specifically
developed to address the need for
standardization. By contrast, the US Customary
system had evolved from the English system
of weights and measures. This system
standardized traditional units of measure, such
as the inch, foot, and pound.
Most of the world today uses the metric
system. Although it has yet to gain widespread
acceptance in the United States, there have
always been efforts to bring metric standards
into practice in the US. The SI Metric system
was legalized as the weights and measures
standard by the US in the late 1800s. Also, the
Metric Conversion Act of 1975 was intended
to facilitate the conversion of inch-based
standards to metric standards.
The metric system is used today by the
US in international commerce. Industry uses
drawings in metric units, and automobile
mechanics and other technicians must be
able to use metric tools. In all US automobiles,
speedometers have readouts in both miles per
hour and kilometers per hour. Can you think of
other examples where the metric system is used?
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