120 Section 1 Introduction to Drafting
is a group of lines that represent the edges of a
3D model. A wireframe does not have a “skin”
and is less realistic than a surface model. Surface
models are used for rendering and animation.
However, surface modeling is not often used for
engineeringapplicationsbecauseasurfacemodel
does not have volume or mass properties.
Solid modeling creates 3D objects by gener- g
ating a volume. If you think of surface modeling
as blowing up a balloon to obtain a final shape, fi
solid modeling is obtaining the fi nal shape by fi
filling it with water. A solid model can be ana- fi
lyzed for mass, volume, material properties, and
many other types of data. Many CAD packages
that can produce solid models also allow you to
create cross sections, which is hard or impossible
to do with a surface model. In addition, a solid
model can be rendered and imported into many
animation software packages.
3D Views
CAD software that is 3D-capable typically
has a
Hide
command to remove lines that would
normally be hidden in the current view. These
are the lines that would be drawn as hidden
lines in a 2D drawing. Hiding lines can help
visualize the 3D model. The
Hide
command is
used in Figure 4-22.
In addition to hiding lines, you also need
to be able to see the objects from different
angles. It would be nearly impossible to create
a 3D model of any complexity only being able
to see a top view, for example. It may seem as if
each CAD program has its own unique way of
displaying different 3D views. Some CAD pro-
grams have preset isometric views. Others allow
Figure 4-21. A—An isometric drawing of a mechanical part. This is a 2D isometric drawing that appears to show
the object in three dimensions. B—When the isometric drawing is viewed from a different viewpoint, you can see
that it is two-dimensional.
A B
Figure 4-22. A—An isometric view of a 3D model.
B—When the 3D model is viewed from a different
viewpoint, you can see that it is truly three-dimensional.
(Eric K. Augspurger)
A
B