Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (46 page)

through the part, threads are cut into the cylindrical cutout’s walls.

Holes and slots remove volume from a part.

Fillets Fillets are rounded edges where two surfaces meet at an angle of some

sort. Fillets are generally a circular blend between the surfaces with a

radius specified, but other shapes are possible such as elliptical or other

conical sections.

Chamfers A chamfer is similar to the fillet, but instead of a rounded blending, there

is a new surface at a sharp angle to each of the two original surfaces.

This is often specified by giving an angle for the new surface with a

distance along an existing surface, or with two distances along the two

existing surfaces.

Ribs Ribs are protrusions that connect two surfaces with a new segment of ma-

terial. A rib would specify a distance along each of the existing sur-

faces and then a triangular web connects the endpoints of these

distances at the vertex where the surfaces meet. Ribs add volume to a

part.

Bosses Bosses are protrusions that will have a circular or slotted cross section.

They may be totally solid, or they may have a hole within the cylindri-

cal feature. Bosses add volume to a part.

Patterns Patterns may be used on individual features or entire part models. As the

name implies, they are used to create a repeating sequence of the se-

lected geometry. The advantage of patterns is that one master set of ge-

ometry can drive the many copies that create the feature. This allows

many features to be changed by modifying the master. Patterns could

be applied to cutouts or protrusions, so they could remove or add vol-

ume to a part.

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Even though the creation of these features is automated, they should still be
included in the part history. This means, that the user needs to take care to apply
them at the right stage of the part construction to get the desired results.

8.8 3-D GEOMETRIC PROPERTIES FOR

PART MODELS

Earlier in this chapter, section properties were discussed as geometric informa-
tion that could be derived from sections or from the 2-D foundation for 3-D fea-
tures. However, even more valuable information can be derived from the
complete 3-D model. Table 8.8 lists some of these properties.

TABLE
8.8

Geometric Properties for “Complete” 3-D Models

Geometric property Description

Volume Clearly the most common geometric property for the part model

is volume. If a solid model has been properly created, then the

3-D CAD system should be able to immediately calculate the

volume of the part. This is immensely helpful in comparison

to 2-D drawings. The 3-D CAD system should be able to cal-

culate the volume even when the part is very complicated.
Mass/weight Assuming the 3-D CAD system permits the application of mate-

rial properties (such as density) to part models, then the mass

and/or weight of the part can also be immediately calculated.

This is another immensely valuable capability.

Surface area Even if a part is not a solid model, the total of all the surface’s

areas can be readily computed and shown to the user. The 3-D

CAD system may also indicate if any surface area is open (not

completely stitched). This means that there are free edges

(possibly shown graphically). This open surface area calcula-

tion can help verify whether a part is really solid or not.
“CG” and inertial
As with the section property’s inertia, there is a similar calcula-

properties
tion for 3-D models. It is a more involved calculation, though,

based on how volume is distributed in all 3 dimensions. There

should be at least 3 values shown (Ixx, Iyy, and Izz), and the

meaning of the x, y, z directions should be clearly defined. The

inertias will change based on their directions, and cross-prod-

uct inertias (such as Ixy, Iyz, Ixz) may appear. At the rotation of

x,y,z directions where the cross-product inertias are zero, iner-

tial principal axes are indicated. The location of the point

Part Modeling 221

TABLE
8.8

Continued

Geometric property Description

“CG” and inertial
properties
(cont’d)

where there is equal volume in every direction or regardless of
cross section is the center of the volume. If the part is made of
a consistently dense material (usually an appropriate assump-
tion for a part model), then this center point becomes the cen-
ter of mass, or depending on the terminology implied, it may
be referred to as the CG or Center of Gravity.

8.9 PART MODELING SUMMARY

Hopefully this chapter has given some valuable insight into how a 3-D CAD sys-
tem creates and manages part model geometry. Obviously, the particulars of how
to create these models is quite dependent on the particular CAD system being
used, but the issues presented here will appear likely regardless of the system.
The issues are pretty much inherent in the mathematics and logic that the systems
are based upon.

The remaining chapters will get into even more issues than build on the
basic ideas of this chapter. Reread sections of this chapter as needed to become
comfortable with the concepts within the context of one’s particular CAD system
(before proceeding any further with this work).

The following highlights the most important ideas:

2-D geometry techniques are still needed to help make 3-D part models.
3-D solid models are really a collection of paper-thin surfaces that are con-

nected at the edges (at least if the part is supposed to be solid).
The creation of a complete 3-D part model involves first making a base fea-

ture that starts the part, and then a sequence of operations or features are

performed on the base feature in a history or part history or feature list.
The most common means of creating a feature is a basic three-step process.

The first step is selecting a plane for sketching; the second step is sketch-

ing the needed 2-D foundation for the 3-D feature; the third step is turn-

ing the 2-D foundation into the 3-D feature by selecting a process or

operation (e.g. extruding, revolving, protruding vs. cutting, etc.).
CAD systems usually offer a means of constraining the 2-D sketch geome-

try. This will at least include making dimensions. This may also include

the ability to have intelligence or parametric data applied to the geome-

222 Chapter 8

try so that they automatically update and change based on varying part

needs.

CAD systems also usually offer means of creating 3-D features that are not

based on the 2-D sketch plane operation at all. Some of these techniques

are Boolean operations that allow 2 separate part models to be connected

to each other to form a new larger part model. Another means for creat-

ing 3-D features without the sketch plane are to make standard features

such as holes, fillets, etc.

8.10 CHAPTER EXERCISES

1. Begin learning and using your 3-D CAD system. Refer to the docu-
mentation and tutorials supplied by the CAD system vendor, your company, or
your university.

2. Record whether the CAD system can follow the basic three-step pro-
cess discussed in this chapter.

3. Record whether coordinate systems can be connected with a part and
used for sketch planes.

4. Record whether new features (sketched on a sketch plane) can be cre-
ated that join
to the existing geometry of the part,
cut
the existing geometry of the
part, or intersect
with the existing geometry of the part.

5. Record whether features can be Added
(intersections of the 3-D geo-
metry are not determined).

6. Try the problem shown in Figure 8.10 where a cylinder (sketched as its
own features) grazes or is just tangent to the top of a block. Record how the CAD
system deals with the singularity.

7. Try the problem of sketching and constraining shown in Figure 8.16
where multiple distinct (but valid) solutions can be found.

8. Record whether the CAD system can take two separate part models
and use the Boolean operations of join, cut, or
intersect
to make them become
one part model.

8.11 CHAPTER REVIEW

1. What are the 3 basic steps that often are used in the features-based ap-

proach to part modeling?

2. What are some potential sources of planes that can be used for sketch-

ing? What are some of the advantages/disadvantages of these different

sources?

3. What are some situations where it may not be necessary to fully con-

strain the geometry in a part model?

Part Modeling 223

4. What is meant by stitching for 3-D part models?

5. Explain why an intersection
operation might be considered the result

of a Boolean “AND”? What Boolean would apply to join? What Bool-

ean would apply to cut?

6. Explain some advantages to using the Boolean operations that make

larger part models based on smaller part models?

9

Surface Modeling

9.1 INTRODUCTION

Surface modeling or surfacing
is often considered a separate activity within the
3-D CAD system. As mentioned before, normal solid modeling can really be
thought of as surface modeling as well. It is just that in solid modeling, the 3-D
CAD system is automatically doing all the surfacing functions and creating a
closed volume part where all the surfaces connect or stitch or sew up. This chap-
ter will look at this issue in more detail.

In surface modeling, the user takes on the burden of understanding how the
surfaces are created. The user has the choice of figuring out if, and when, the
surfaces are to be stitched together. The user has the choice of figuring out how
properties such as thickness or perhaps material side are going to be applied to
the part model. Clearly, this is going to make it more difficult to make basic 3-D
part models. On the other hand, surface modeling can succeed in making part
models where all other approaches fail.

Typical applications of surface modeling of parts include sculpted parts,
molded parts, and swept parts. Sculpted parts would be parts that are somewhat
free form or wavy. A car body panel would be a kind of sculpted part. These
kinds of parts have surfaces that are sometimes referred to as Class A surfaces
(surfaces the consumer first sees; they are not hidden within the product). It is

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Surface Modeling 225

certainly going to be difficult, if not impossible, to find a single flat surface on a
car body panel that could be used for sketching and extruding. So, clearly the
basic three-step process presented in the previous chapter is not going to be
useful.

Molded parts could also probably be considered sculpted, but they have the
added issue of being created in a mold (such as in injection molding). Their
surfaces need to be modeled in such a way that the mold can be split into two
halves, and the surfaces of the part must be drafted (having an angular slope so
that the part can be removed from the mold). Also, this is an interesting part
modeling application because the part can just be surfaces (it does not need to be
solid) since the mold cavity really only needs to have the outer skin of the part
model. Often, 3-D CAD is used to make the mold as well as the parts, and 3-D
CAD may even be used to interface with a package that creates the NC instruc-
tions to actually machine out the cavities for the mold. In this case, there might
be no real human intervention with the creation of the physical part at all, and
the mathematical definition of the surfaces carries right through to the final
product.