Types of Views 85
As mentioned earlier, the United States and Canada use the third-angle system of projection for
drawings, whereas other countries use a different system known as first-angle projection. The purpose
of introducing the ISO projection symbols is to indicate that there is a continuously increasing interna-
tional exchange of drawings for the production of interchangeable parts. Thus, the symbol indicates
whether the drawing follows the third- or first-angle projection system.
The ISO projection symbol, the notation on tolerances, and information on whether metric and/or
inch dimensions are used on the drawing should appear as notes either within the title block or adjacent
to it. The designation of projection type is not always included with the symbol on a drawing. It is rarely
used on architectural drawings but is usually included on engineering drawings.
When drawings are transferred from one convention to another, considerable confusion is encoun-
tered in drafting rooms and engineering departments. On engineering drawings, the projection angle is
denoted by an international symbol consisting of a truncated cone, respectively, for first-angle and third-
angle and whether the cone is to the right or left of the two concentric circles symbol. The 3D interpre-
tation of the symbol can be deduced by envisioning a solid truncated cone standing upright with its large
end on the floor and the small end upward. The top view is therefore two concentric circles (“doughnut”).
In particular, the fact that the inner circle is drawn with a solid line instead of dashed lines designates
this view as the top view, not the bottom view.
Both first-angle and third-angle projections result in the same six views; the difference between them
lies in their arrangement around the box.
5.4 OBLIQUE DRAWINGS.
Oblique projection is a simple form of parallel graphical projection used mainly for producing pictorial,
two-dimensional images of three-dimensional objects. Oblique drawings are similar to isometrics except
that the front view is shown in its true shape on the horizontal line—i.e., when drawing an object in
oblique, the front view is drawn flat (Figure 5.12). Thus, it projects an image by intersecting parallel rays
from the three-dimensional source object with the drawing surface. In oblique projection (as in ortho-
graphic projection), parallel lines from the source object produce parallel lines in the projected image.
The projectors intersect the projection plane at an oblique angle to produce the projected image, as op-
posed to the perpendicular angle used in orthographic projection.
A 45-degree angle is the most commonly used to draw the receding lines from the front view, but
other angles are acceptable. In an oblique sketch, circular lines that are parallel to the frontal plane of
projection are drawn at their true size and shape. Hence, circular features appear as circles and not as
ellipses. This is the main advantage of the oblique sketch. The three axes of the oblique sketch are
drawn at the horizontal, vertical, and a receding angle that can vary from 30 to 60 degrees.
Whereas an orthographic projection is a parallel projection in which the projectors are perpendicu-
lar to the plane of projection, an oblique projection is one in which the projectors are not perpendicular
to the plane of projection. With oblique projection all three dimensions of an object can be shown in a
single view.
Oblique drawing is a primitive form of 3D drawing and the easiest to master. It is not a true 3D sys-
tem but a two-dimensional view of an object with contrived depth. Instead of drawing the sides full size,
they are only drawn at half the depth, creating a suggested depth that adds an element of realism to the
object. Even with this contrived depth, oblique drawings look very unconvincing to the eye. In Figure 5.13
the side views are drawn at a 45-degree angle. In oblique projection the side views are typically fore-