c. Figure 1-29 is a typical dimensioned
drawing of a bearing journal and can be used to
illustrate the proper way to make measurements from
the points indicated by the dimension lines on the
print. Let's consider the measurements required to
locate the two mounting holes. If you were using this
print and were drilling the mounting holes, you
should notice in the front view that the center of the
right hole is located 1 inch from the center of the
large hole. The top view shows that the center of the
left hole is 2 inches from the center of the right hole.
This method of making measurements introduces two
possible sources of error. There is some tolerance in
the location of the right hand mounting hole, and
some tolerance in the measurement you make to
locate the left-hand mounting hole. Any other system
of measurements will introduce additional sources of
error. For example, it can be determined from the
dimensions that the mounting holes are located 1/4
inch from the edges and the centers could be located
in this manner. However, this introduces three
sources for error. There is some tolerance in the
width of the journal (the 2 1/2-inch dimension) and
some tolerance in the location of the centers of the
two mounting holes. These three sources for errors
could accumulate and result in a significant error in
the distance between the two mounting holes. This
Figure 1-28. Exploded view.
distance is critical to the mounting of the journal. It
is also possible to locate the center of one hole 1/4
inches, and fractions of an inch. Engineering
inch from the edge and then measure the 2 inches to
drawings often give dimensions in feet and decimal
the center of the other hole. This procedure also
fractions of a foot. Metric dimensions are used on
introduces three possible sources of error. There is
drawings of European origin, most drawings related
some tolerance in the width of the journal, in the
to optical equipment, and a growing percentage of
location of one hole, and in the measurement made to
American machine drawings. During construction,
locate the second hole. This method reduces the
you should use measuring instruments calibrated in
sources of error in the distance between the mounting
the same system as used on the construction prints to
holes. However, it does introduce possible error in
eliminate the chance of error in conversion.
the location of the main hole. It may not be centered
between the mounting holes.
These examples
b. The placement of dimensions on a drawing
emphasize the importance of the location of the
is not arbitrary. They are placed to indicate which
dimensions when making the measurements in
dimensions are specified in the design of the object.
construction.
Measurements for construction should always be
made from the points indicated by the dimension
lines on the print.
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