Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (39 page)

These systems have developed over a long period of intense research and devel-
opment, and often academic terminology and techniques creep into the software.
Users that have not been engrossed in the systems over this long time are often
blind-sided. Hopefully, the remainder of this chapter will expose some of these
pitfalls. Third, 3-D CAD can not be beaten. The intrinsic advantages of a com-
plete geometric model of a product are simply too great to bother using 2-D any
more for any serious mechanical design activity. Once a 3-D model exists, it can
be interrogated, studied, analyzed, and sliced and diced in a way that not even a
physical prototype can equal. So there is no reason to fight the 3-D approach.

7.2 TERMINOLOGY

Although the glossary section of this work provides a list of terms with respect to
3-D CAD, it is important to define some of the most basic terminology at
this point.

172 Chapter 7

7.2.1 Model Terminology

Model
The most basic item within a 3-D CAD system is the 3-D model (or
referred to as just a model). A model is the 3-D computer graphics based object
that users create and interact with. It most often refers to a part model, but it can
also mean an assembly model, a surface model, or an analytical or finite element
model. Keep in mind that it is referred to as a model because it is based upon
underlying mathematical equations. Although these equations and their parame-
ters are usually invisible to the user, they dictate what construction methods will
or will not work, and whether the model will be robust or fragile. So, it is vital to
keep this basic mathematical nature in mind.

Part model
As mentioned, this is a model of a single component or part (often
referred to as a detail in older drafting systems). A part model is self-contained in
that it can be stored in the CAD system by itself, and the underlying mathemati-
cal equations are totally sufficient to regenerate and display the part. Figure 7.1
shows a part model.

Feature
A feature is a self-contained segment of a part. For example, in Figure
7.1, the circular region that sticks out of the front of the part is a feature. It is a
type of feature called a protrusion. The slotted region that cuts through the part
from top to bottom is also a feature. It is a type of feature called a cut or cut-out.
Sometimes the features are not obvious. The part shown in Figure 7.1 has some
rounded or smooth edges; the process of adding this rounding is also a feature of
the part. It is an example of a feature called a fillet (pronounced “fill-it”).

FIGURE
7.1

Example of a part model.

3-D CAD 173

Assembly model
This is a model made up of a collection of part models. The
assembly model is often just a list of names of part models and where they are
located with respect to some global or master origin (where X, Y, and Z are zero).
Although the assembly model will appear to contain many parts, each of those
parts would still be a model in their own right, and the assembly model is merely
pointing to or referencing those part models. Figure 7.2 shows an assembly
model. It has 2 part models of a pedal
and a crank part model connects them.
Solid model
This is a type of part model that is assumed to have volume. There
are no open faces or free edges on a solid model; all the faces of the model are
connected to other faces. Figure 7.1 is an example of a solid model. For a long
time, what is now called 3-D CAD technology was actually referred to as solid
modeling; this was to differentiate it from 3-D CAD systems that used a tech-
nique known as wireframe. However, now it is assumed that a 3-D CAD system
will almost always be creating solid models.

Surface model
A type of part model that does not have volume. This is a
model that is made up of surfaces that do not totally connect at all the edges. It is
open, and it appears to be made up of paper thin pieces. Figure 7.3 shows an ex-
ample of a surface model.

FIGURE
7.2

Example of an assembly model.

174 Chapter 7

FIGURE
7.3

Example of a surface model.

7.2.2 Geometric Terminology

Section
A section can be thought of as a cross section of a part model or a cross
section of a feature of a part model. A section that is closed encloses a 2-D area in
much the same way that a solid model encloses a volume. Often, CAD systems
refer to section properties. These properties include area, the centroid (basically
the geometric center or CG of the section), and inertias (a calculation that
weights the distribution of the area with respect to an axis). Figure 7.4 shows an
example of a section. The section is the bold line that looks like the basic cross-
sectional shape of the part shown in Figure 7.1.

Vector
A vector is a mathematical device for indicating that something has a
direction as well as a value. It can be thought of as an arrow. The longer the ar-
row, the larger the value that the vector signifies. The direction, then, is indicated
by the direction the arrow points. A commonly known vector quantity is a force.
A force being exerted on something has a definite direction, and it has a value
(i.e. how much force in Newtons or pounds-force). Figure 7.5 shows an example
of a vector. In this example, the length or magnitude of the vector is 163. A com-
mon use of the vector in 3-D CAD is to indicate a direction for a construction
(such as, protrude in “this direction”). It can also be used to show a distance with
a direction. In Figure 7.5, this vector could indicate the distance to an arbitrary
location on a part as 163 mm. This 163 mm could also be shown as having com-
ponents in the X, Y, and Z direction. These components would indicate what the
change in distance is from one point in space to another (these changes may also
be referred to as delta X, delta Y, and delta Z).

3-D CAD 175

FIGURE
7.4

Example of a section.

FIGURE
7.5

Example of a vector.

176 Chapter 7

Normal vector
A normal vector is a special case of a vector. In this case, the
vector’s direction is dictated by a surface. At some point on this surface, the nor-
mal vector points perfectly perpendicular from the surface. No matter what plane
is chosen that the vector lies in, the vector still points perpendicularly from the
surface at that plane’s view of the model. Figure 7.6 shows a normal vector; note
how it is perpendicular to the surface at the point where the vector touches the
surface.

Unit vector
A unit vector is another special case of a vector. In this case, the
magnitude or numerical value associated with the vector is 1. In this case, the
vector only indicates direction, not magnitude. This is a common operation in 3-
D CAD systems when the system needs the user to specify an arbitrary direction.
The CAD system may prompt the user for a unit vector.

Surface
A surface (in 3-D CAD anyway) is any arbitrary, infinitely thin bound-
ary. A surface can be totally flat (a planar surface). A surface can be very curved
or warped. It can have straight edges. It can have circular or curved edges. It is

FIGURE
7.6

Example of a normal vector.

3-D CAD 177

often based on something called a NURBS (which is explained in a little detail in
Chapter 9); so some systems or users will use the term NURBS interchangeably
with a surface. Suffice it to say that a surface is defined by some specific mathe-
matical equations. Figure 7.3 (an example of a surface model) shows 2 surfaces
that are connected together (the rounded front and the saddle-like body are each
surfaces).

7.2.3 Graphics Terminology

In addition to the general terminology of the 3-D CAD system, there is a certain
amount of terminology that has arisen from computer graphics technology. These
terms may have slightly different uses in different CAD systems, but the basic
meaning is usually the same as presented here.

Wireframe or wireframe view
The most obvious computer graphics term is
wireframe. This refers to a 3-D model that is not shown with its surfaces. Instead,
it just shows the edges of the surfaces. As mentioned earlier, this was the only
approach to 3-D CAD at one time, so for some users, wireframe will mean doing
3-D CAD without solid models. However, this work assumes that this is only a
concern for companies that have this older technology in their “legacy” systems
(meaning they are not used in the latest product development activities). Figure
7.7 shows the wireframe graphics on the right.

Solid or shaded view
This might be called the opposite of the wireframe dis-
play. Referring to displaying the 3-D graphics with the surfaces instead of the
edges, it shows the state of the design much more clearly than wireframe. How-
ever, when a designer needs to work on a part model’s interior, it is often very
helpful to switch back to the wireframe display. Figure 7.7 shows the solid view
or shaded graphics on the left. The important point is that this is just a graphics
issue; it does not change the nature of the part model, only how it appears to
the user.

Pan
The option of changing the view of the 3-D model on the screen by mov-
ing left and right, or up and down. This is usually done with the mouse and/or
keyboard.

Zoom
The option of changing the view of the 3-D model on the screen by ap-
pearing to get closer to or farther from the model.

Rotate
The option of changing the view of the 3-D model by rotating the
model with respect to various axes. Often the model can be rotated with respect
to the observer; or the observer’s point of view can be changed.

Clipping
Cutting away portions of the 3-D model to reveal internal surfaces.
This is different than actually using a cutout feature described earlier (which
changes the model). Clipping does not change the 3-D model; it only changes the
appearance of the model on the computer monitor. Figure 7.8 shows an example
of a clipping display.

178 Chapter 7

FIGURE
7.7

Comparison of shaded and wireframe display.

FIGURE
7.8

An example of a clipped display.

3-D CAD 179

FIGURE
7.9

Examples of faces, edges, and vertices.

Triad
A coordinate system that indicates the overall orientation of the 3-D
model being displayed. As the model is rotated or manipulated, a triad may be
shown on the screen to give the proper bearings of which directions are control-
ling the display. It appears as an X, Y, Z with some direction lines.

Face
A somewhat interchangeable term with surface. Usually, however, a face
implies that the surface is used in the construction of a solid model. Some users
may consider a face to be planar or flat, but often it can also be any curved or
warped surface as well. Figure 7.9 shows some highlighted faces.

Edge
Usually refers to the boundary of a surface. Although an edge is clearly
found at a sharp corner where faces meet, an edge would also be indicated by the
boundary of a surface model that does not meet any other surface. Figure 7.9
shows some highlighted edges.

Vertex
Refers to a point at the end of an edge. Again, a vertex certainly is
found at a corner where 3 faces meet (as in the corner of a box), but a vertex
could also be found where 2 separate open surfaces meet, and vertices would
even be found on a single open surface (at each of the edges of the surface).

7.3 DISPELLING COMMON MISCONCEPTIONS