82 Print Reading for Industry
As presented earlier in the unit, in third-angle
projection, the object resides in the third angle of space.
So, the projection planes are considered to be between
the viewer and the object. The views are projected
toward the viewer onto the planes. See Figure 5-16.
When quadrants two and four are collapsed, the top
view appears above the front view.
In contrast, in first-angle projection, the object
resides in the first angle of space. So, the projection
planes are on the opposite sides of the object as the
viewer. In other words, the object is between the
viewer and the projection planes. See Figure 5-17.
The individual views are the same as those obtained
in third-angle projection, but their arrangement on
the drawing is different. In essence, the top view
ends up below the front view and a right side view
is on the left side of the front view.
In summary, the individual views are the same
for both angles of projection. The only difference
between the two types is the arrangement of views
on the drawing. The ASME and ISO standard symbols
to indicate first-angle and third-angle projection
are shown in Figure 5-18. One of the two versions
of the symbol should be included in the title block
for drawings that are read within the international
community.
Vi
ewe
r
Viewer
H
orizon
tal
Frontal
1
2
4
3
Figure 5-15.
The two types of projection are based on a theoretical
division of space into four quadrants. Quadrants two
and four are not used.
3
Third-Angle Projection Third-Angle Drawing
Front view
Top view
Vi
ewe
r
Viewer
H
orizo
ntal
Frontal
Figure 5-16.
In third-angle projection, the projection plane is considered to be between the viewer and the object and the views
are projected toward the viewer onto the plane.