_MARKING DIMENSIONS._
The dimensions of mechanical drawings are best marked in red ink so that
they will show plainly, and that the lines denoting the points at which
the dimension is given shall not be confounded with the lines of the
drawing.
The dimension figures should be as large as the drawing will
conveniently admit; and should be marked at every point at which a
shoulder or change of form or dimension occurs, except in the case of
straight tapers which have their dimensions marked at each end of the
taper.
In the case of a single piece standing by itself the dimension figures
may be marked all standing one way, so as to be read without changing
the position of the operator or requiring to turn the drawing around.
This is done in Figure 115, which represents the drawing of a key. The
figures are here placed outside the drawing in all cases where it can be
done, which, in the case of a small drawing, leaves the same clearer.
[Illustration: Fig. 115.]
In Figure 116 the dimensions are marked, running parallel to the
dimension for which they are given, so that all measures of length stand
lengthwise, and those of breadth across the drawing.
[Illustration: Fig. 116.]
Figure 117 represents a key with a sharp-cornered step in it. Here the
two dimensions forming the steps cannot both be coincident with it;
hence they are marked as near to it as convenient, it being understood
that they apply to the step, and not to one side of it. When the step
has a round instead of a sharp corner, the radius of the arc of the
corner may be marked, as shown in Figure 118.
[Illustration: Fig. 117.]
Figure 119 represents a key drawn in perspective, so that all the
dimensions may be marked on one view. Perspective sketches may be used
for single pieces, as they denote the shape of the piece more clearly to
the eye. On account of the skill required in their production, they are
not, however, used in mechanical drawing, except as in the case of
Patent-Office or similar drawings, where the form and construction
rather than the dimension is the information sought to be conveyed.
[Illustration: Fig. 118.]
[Illustration: Fig. 119.]