straightedge is also excellent for trimming
blueprints and cutting heavy illustration board.
Steel straightedges are usually made of
stainless steel and are available in lengths of 15
in. to 72 in. The one included in the draftsman
kit is 42 in. long. Some have a beveled edge, like
the one shown in figure 2-9.
TRIANGLES
TRIANGLES are used in combination with
the T square or straightedge to draw vertical and
inclined lines. They are usually made of
transparent plastic, which allows you to see your
work underneath the triangles.
Triangles are referred to by the size of their
acute angles. Figure 2-10 shows two basic
drafting triangles: the 45° (each acute angle
measures 45°, and the 30°/60° (one acute angle
measures 30°; the other, 60°). The size of a 45°
triangle is designated by the length of the sides
that form the right angle (the sides are equal). The
size of a 30°/60° triangle is designated by the
length of the longest side that forms the right
angle. Sizes of both types of triangles range from
4 in. through 18 in. in 2-in. increments.
Like all other drafting equipment, triangles
must be kept in good condition. If plastic triangles
are dropped, their tips may be damaged. Also,
triangles may warp so that they do not lie flat
on the drawing surface, or the edge may deviate
from true straightness. To prevent warping or
29.277
Figure 2-10.-45° and 30°/60° drafting triangles.
chipping, you should always lay them flat or hang
them up when they are not in use. Since there is
seldom enough drawer space available to permit
laying triangles flat, it is best to develop the habit
of hanging them up. If the tips are bent, use a
sharp knife to cut off the damaged part. If the
triangle is warped, you may be able to bend it back
by hand. If this does not straighten it, leave the
triangle lying on a flat surface with weights on
it or hold the triangle to the opposite curvature
with weights. If the triangle becomes permanently
warped so that the drawing edges are curved or
the angles are no longer true, throw it away and
get another.
To test the straightness of a triangle, place it
against the T square and draw a vertical line, as
shown in figure 2-11. Then reverse the triangle
and draw another line along the same edge. If the
triangle is straight, the two lines will coincide; if
they don’t coincide, the error is half the resulting
space.
PROTRACTORS
PROTRACTORS are used for measuring and
laying off angles other than those that may be
drawn with the triangle or a combination of
triangles. Most of the work you will do involving
the use of the protractor will involve plotting
information obtained from field surveys.
Like the triangle, most protractors are made
of transparent plastic. They are available in
6-, 8-, and 10-in. sizes and are either circular or
semicircular in shape, as shown in figure 2-12.
45.126
Figure 2-11.-Testing a triangle for straightness.
2-10
