Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (49 page)

FIGURE
9.11

Example of a midsurface “abstraction.”

junction between the surfaces. This is a special type of violation of standard solid
models.

For proper stitching of a solid model, the edges of one surface must meet
the edges of other surfaces. And, all the edges must meet just one other edge.
This proper stitching creates what can be called a manifold part; the violation of
this principle such as at the midsurface can be called nonmanifold. Some CAD
systems will not allow the creation of nonmanifold parts (since they may not sup-
port the advanced analysis methods associated with something like a midsurface
abstraction).

Another example of an abstraction is using a surface as a sort of place
holder for a real feature. For example, a hole that is drilled into a plate may be
tapped (meaning that the screw threads are cut into the sides of the hole). These
threads are regularly not shown in the 3-D part model (since it can consume too
much time and computational resources). Instead, an extra (and nonmanifold)
surface may be created that is concentric with the hole. It merely indicates to de-
signers that a tapped hole is located there. It would also create an associative cir-
cle in the drawing to help indicate a drilled and tapped hole condition. Figure
9.12 shows an example of such an offset surface for a hole.

Surface Modeling 239

FIGURE
9.12

Example of an abstraction to simplify screw thread representation.

9.6 PART HISTORY ISSUES

The final issue presented with respect to surface modeling is how part history is
supported for the surface modeling process. Recall from normal part modeling
that the part history or part structure or feature list is simply the sequential list of
operations performed to create the part model and the relationships between the
various steps and features. Part history was fairly simple for the basic three-step
process where sketch plane selection is used to set up the connection between the
features (each feature basically being a single step in the history). But for surface
modeling, part history can present difficulties.

For instance, one needs to consider whether surface modeling operations
are tracked at all by the part history. In some CAD systems, each surface model-
ing operation merely creates or alters existing surfaces and there is no relation-
ship or tracking among the surfaces. In this case, the part model is really just a
list of surfaces. Each surface is identified in some way, but there is no attempt to
figure out how the various surfaces are related to each other.

This situation often arises when 3-D part models are translated between
CAD systems. The translated model may have no part history; no list of steps;
no constraints; no stitching. The part model may “look” correct since the sur-
faces are trimmed and in the right location; but there is no way to modify the

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size of a feature, for instance, by modifying a dimension somewhere in the part
model. Some CAD systems will refer to this history-less part model as an or-
phan or orphan part. All the parent/child relationships between the features
are lost.

In general, however, techniques such as lofting and sweeping are more
likely to be supported with part history than techniques such as surfaces by edges
or points. Note from Figure 9.4 that
sketch planes and dimensions can be used in
a technique such as lofting. To change the lofted part, the designer would find the
lofted feature in the part history and modify the needed dimensions. However,
looking at Figure 9.10, it would be more difficult to constrain this surface to be
connected to the points in space that created the surface. If this were done, one
could then move some of the points in space and the surface would change
accordingly.

In Figure 9.9, the surface was created from lines in 3-D space. Imagine that
these lines were really edges of other surfaces already created for a part model. In
this case, the new surface could be related to that existing part model. However,
the part history for this operation may or may not be supported. If it is supported,
then when the original edges change, the new surface constructed from the edges
would change automatically. However, if history is not supported for this opera-
tion, then when the original edges change, the surface would simply stay the
same and appear to tear away from the rest of the part model.

9.7 CHAPTER EXERCISES

1. Record whether your 3-D CAD system can show untrimmed surfaces.

2. Record whether the CAD system can calculate and display surface
properties such as curvature.

3. Create an open part model by removing a surface from a solid part
model. Record whether the surface removal appears in the part history.

4. Create an open section and attempt to extrude it to an open part.

5. Attempt to create a part model for a bicycle seat.

6. Attempt to create a hollow tube with three bends (the tube being
straight between the bends). Try to control the shape of the part with linear and
angular dimensions such that the tube can be changed by only modifying these
dimensions.

7. Record whether the CAD system can create a nonmanifold part (e.g. in
Figure 9.11 and 9.12).

8. Record the default or standard part tolerance for the CAD system.

Surface Modeling 241

9.8 CHAPTER REVIEW

1. Explain how a surface model can become a solid model.

2. What are some reasons for using a surface model instead of a solid

model?

3. What geometric entities are analyzed to determine if two surfaces can

be stitched?

4. List some uses for abstractions of parts. What risks could arise from

the use of the listed abstractions?

10

Assembly Modeling

10.1 INTRODUCTION

Assemblies are simply groups of parts that are assembled or brought together in
some fashion. They may be welded together (such as flat plates joined into a
beam), or they may be fastened together (such as bolts holding a wheel to an
axle), or they may just be parts that form a working unit (such as a barrel and rod
in a hydraulic cylinder). Assembly models, then, are simply a 3-D CAD–based
representation of these groups of parts. Figure 10.1 shows a example of an as-
sembly model.

10.2 CHARACTERISTICS OF AN

ASSEMBLY MODEL

There are a number of essential characteristics of assembly models contained in a
3-D CAD system. If these elements are not present, then one needs to question
whether the system is really doing assembly modeling at all. It may just be allow-
ing many 3-D part models to be shown simultaneously, but with no real intelli-
gence about the grouping of parts to the extent that it could be considered a real
assembly 3-D model. The remainder of this section presents three essential char-

242

Assembly Modeling 243

An example of a simplistic assembly model.

10.1

IGURE

F

244 Chapter 10

acteristics for the assembly model—assembly structure, positional information,
and instancing.

Naturally, assembly models are going to be dependent on 3-D modeling
methods. Although a 2-D assembly model would be theoretically possible in a 2-
D CAD system, assembly models are really only found in the context of 3-D
CAD systems. Realistic assembly design is simply a very three-dimensional
activity.

10.2.1 Assembly Structure

The first essential characteristic of the assembly model is that it must have a
structure. This is really just a list or perhaps a table (to use a database term). This
list or table simply indicates the collection of 3-D part models (or details) within
the assembly model. The assembly structure indicates what is actually in the as-
sembly model. Figure 10.2 shows a typical assembly structure. This list could
also be called the assembly hierarchy, skeleton, product structure or even a Bill of
Material (BOM) (although a BOM can have implications beyond assembly mod-
eling and is explained in greater depth later).

Although the assembly model has this structure, it does not does not really
contain any visible geometry of its own. It looks like it is a 3-D model in its own
right, but it is really getting this geometry via the part models. The assembly
model can be thought of as using its assembly structure as just a list of pointers to
the part models. The assembly model points to the part models, and the part mod-
els, in turn, are what is seen on the computer monitor. This is important since

FIGURE
10.2

Example of an assembly structure.

Assembly Modeling 245

changes or revisions to the parts need to be reflected in the assembly model (as
dictated by the overall design processes), and by using the pointer approach the
assembly update or regeneration can happen automatically. As the parts change,
the assembly shows these changes appropriately. The idea of pointers becomes
even more important with respect to the characteristic of instancing explained
below.

10.2.2 Assembly Positional Information

The second essential characteristic of the assembly model is that it uses some sort
of mechanism for intelligently tracking and storing positional information about
the 3-D part models. It is not enough just to know what is in the assembly. The
assembly model needs to know where each of the part models are located with
respect to the 3-D modeling space.

Of course, creation of just a part model entails little concern for its location
with respect to the modeling space; it doesn’t matter where it is located, the part
model still is the same (same volume, features, etc.). However, for the assembly
model where models are located is quite important. For example, having a part
incorrectly located in modeling space could mean that two parts collide or inter-
fere. If the design is not corrected, this could result in a costly production error.
The rest of this section discusses the basic positioning of the parts; a later section
expands on this concept with respect to assembly constraints.

First of all, the 3-D modeling space needs to be defined and quantified.
This includes the specification of the master or global origin. This is the location
of a point where the X, Y, and Z coordinates are all zero, and where the direction
of the X, Y, and Z directions can be defined (this point may then also be referred
to as the global or master coordinate system). Given this global origin, then, any
other point can be defined relative to that position with a set of X, Y, and Z coor-
dinates. These coordinates may have some unit system defined (such as mm,
inches, m, etc.), or the coordinates may just be normalized or unitless (numerical
values the user must interpret).

Now, for each part model in the assembly structure there should be an X, Y,
Z position stored and tracked. This gives the overall location of the part, or at
least one point on the part. However, location of one point is not a complete pic-
ture for the position of a particular part model in the model space. Beyond the
point location, there needs to be rotational information as well.

The rotational information would be X, Y, and Z rotation angles for the X,
Y, and Z axes or coordinate system that is found at the positions tracked for the
part. Some CAD systems will dictate and/or display the location of the part coor-
dinate system as the part model is created. Sometimes this coordinate system is
somewhat arbitrary for the part model; but at other times, users will use this coor-
dinate system as a source of datums or other aids during the part model construc-

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tion. Recall that the first feature for a part model is called the base feature, so this
part coordinate system may actually be the base feature and it may be referred to
as the base coordinate system or the base node in the part history. On the other
hand, some CAD systems will hide the point and/or part coordinate system loca-
tion from the user since the designer may only be interested in the position of the
part in the assembly relative to others parts in the assembly (not relative to the
global origin at all). In any case, whether the rotation angles or these part coordi-
nate systems are shown by the 3-D CAD system, these three X, Y, Z coordinates
and the three X, Y, Z rotation angles are what the 3-D CAD system is going to
use to keep track of the location of the part models.