Unit 5 Multiview Drawings 81
meanings a line in a multiview drawing can have,
whether it is visible or hidden. As you learn how to
read prints, keep these three meanings in mind.
First-Angle and Third-Angle
Projection
The system of projection explained earlier in
the unit is common throughout the United States.
The projection of a view is basically what the viewer
sees when looking at the object through the glass
projection plane. The glass box is unfolded in such
a way as to place the top view above the front view.
This is known as third-angle projection. In many
countries, especially in Europe, a slightly different
projection system is used that results in the views
being located differently.
The two projection systems used in industrial
drawings are identified as third-angle projection and
first-angle projection. These two types result from
a theoretical division of space into four quadrants
by vertical and horizontal planes, Figure 5-15. The
viewer of the four quadrants is considered to be in
front of the frontal plane and above the horizontal
plane. The views are arranged by folding the two
planes into one by collapsing the second and fourth
quadrants and the views are then seen from the
front. As a result of this, there are no second or
fourth angle projections. If there were, the views
would overlap.
Figure 5-13.
When rounded edges intersect curved surfaces,
a runout is used to show how the edge tails out.
Computer-generated views from a CAD model may
show this differently.
Figure 5-14.
This figure shows how to interpret several lines on a multiview drawing.
Runout
A = Edge view of a flat
or curved surface
B = Intersection of two
surfaces only (just an edge)
C = Maximum contour of a
curved surface
B
C
C
A
A
B
A A
C
A
C
A
C
A
B