Chapter 9 Multiview Drawings 211
the original solid mass. The resulting subtrac-
tions created by these operations represent
negative mass of the object. Hence, negative
mass can be defined as the areas of unoccu-
pied mass that exist within the overall limits
of the original mass. See Figure 9-8.
For objects with more complex features,
identifying the positive and negative mass
becomes more difficult. Objects made from
thin material, such as sheet metal, require
bending. This process does not remove mate-
rial. However, the process of bending does
create a feature in an object. There are other
operations used to create features that do
not require removing material. Regardless
of what operations are required to create an
object, a thorough understanding of positive
and negative mass helps in identifying object
features.
Primary and
Secondary Views
When creating multiview drawings,
every feature should be represented in every
view. If the feature is visible in the given view,
it is drawn with object lines. If the feature is
invisible in the given view, it is drawn with
hidden lines. Sometimes, in special situa-
tions, a feature may be partially visible and
partially invisible from a particular point of
Third-angle
projection
First-angle
projection
Figure 9-7 The appropriate symbol is placed
on engineering drawings to show the method of
projection used.
Top view
Front view
Object feature
Negative mass
Positive mass
Side view
Figure 9-8 The entire solid object in this multiview drawing
represents positive mass. The hole, which is the only feature of the
object, represents the negative mass of the object.