horizontal would be parallel or perpendicular (or
nearly so) to a prominent visible outline, the angle
should be changed to 30°, to 60°, or some other
angle. If two adjacent sectioned surfaces are
shown, the hatching should be in opposite
directions, as shown in figure 5-34, view B. If still
a third surface is included, it should be hatched
at another suitable angle to make the surface
clearly stand out separately from the other
surfaces (figure 5-34, view C). Note that the
hatching lines on one surface are not permitted
to meet those on an adjacent surface.
In drawing section lining, use a sharp,
medium-grade pencil (H or 2H). Space the lines
as uniformly as possible by eye. As a rule,
spacing of the lines should be as generous as
possible, yet close enough to distinguish the
sectioned surface clearly. For average drawings,
space the lines about 3/32 in. or more apart.
Diagonal hatching on an auxiliary section
should be drawn at 45 degrees to the horizontal,
with respect to the section. Figure 5-35 shows this
rule.
In a revolution or other view of an object in
other than the normal position, the diagonal
hatching on a section should be drawn at
45 degrees to the horizontal or vertical axis of the
object as it appears in the revolution. Figure 5-36
shows this rule.
Axonometric Projection
Axonometric single-plane projection is
another way of showing an object in all three
Figure 5-35.-Diagonal hatching on an auxiliary section.
Figure 5-36.-Diagonal hatching on a revolution.
dimensions in a single view. Theoretically,
axonometric projection is orthographic projection
in that only one plane is used and the projection
lines are perpendicular to the plane of projections.
It is the object itself, rather than the projection
lines, that is inclined to the plane of projection.
ISOMETRIC PROJECTION AND ISOMET-
RIC DRAWING.— Figure 5-37 shows a cube
projected by ISOMETRIC PROJECTION, the
most frequently used type of axonometric
projection. The cube is inclined so that all of its
surfaces make the same angle (35°16´) with the
plane of projection.
As a result of this
inclination, the length of each of the edges shown
in the projection is somewhat shorter than the
actual length of the edge on the object itself. This
reduction is called FORESHORTENING. The
degree of reduction amounts to the ratio of 1 to
the cosine of 35°16´, or 1/0.8165. This means that
if an edge on the cube is 1 in, long, the projected
edge will be 0.8165 in. long. As all of the surfaces
make the same angle with the plane of projection,
the edges all foreshorten in the same ratio.
Therefore, one scale can be used for the entire
layout; hence the term isometric, which literally
means “one-scale.”
Figure 5-38 shows an isometric projection as
it would look to an observer whose line of sight
was perpendicular to the plane of projection. Note
5-20
