Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (45 page)

Part Modeling 213

Referring to Figure 8.18, there is a hole in the part that is located paramet-
rically with respect to the height and width of the part. This is indicated by the
equations shown where the dimension value would typically be for the location
of the hole. For instance, the horizontal location of the hole is shown as the
W*2/3 (or two-thirds of the rectangular width). This means that whatever the
width of the rectangular section of the part, the hole’s horizontal dimension will
be ²⁄₃
that amount and thus the hole will keeps its proper location based on the
width parameter. There is a similar relationship shown for the height of the part
and the vertical location of the hole. This same capability can be used to “match”
dimensions (where 2 dimensions forced to be the same); in this case, the equation
would be something like Dimension1 = Dimension2.

Using these relationships or equations is obviously going to be helpful as
the design progresses or as new parts similar to this need to be created. Most
likely, the most important design issue for this part is the overall width and
height; the hole is simply placed in an appropriate location based on those overall
dimensions. This is a very common occurrence in the design process. There are
often important global characteristics that need to be considered as variable, but
then the smaller details are driven by these global characteristics.

As usual, it can take a fair amount of effort and planning to implement these
relationships for a complicated part, but it can save a great deal of time and effort
later in the design process. This technique should be used whenever possible.

FIGURE
8.18

A simple example of parametric dimensions.

214 Chapter 8

8.5 SECTION PROPERTIES AND BEHAVIOR

Recall from the previous chapter that part modeling may involve creating sec-
tions. When using a 3-D CAD system, understanding how sections behave can be
valuable in creating robust part models as well as understanding why some parts
are not created as expected. Also, sections provide some useful geometric proper-
ties. As usual, some CAD systems will make using sections an explicit activity,
while others may hide this mathematical foundation from the user.

The first issue to consider in the behavior of sections is whether they are
open or closed. A closed section means there are no breaks in the geometry that
forms the section. Since the section is like a cross section for a 3-D feature, one
can imagine that feature can’t be solid if there is a break in the section (see Figure
7.13). Of course, it really isn’t a section if it isn’t closed, but the CAD system
may refer to an ’open section’ just the same.

Another important behavior to consider with creating sections is that not all
the geometry created in the sketch plane must be used in the section and/or 3-D
feature. It may be desirable to create some basic geometry (particularly to get
good constraint behavior) that is not going to be included in the section, and thus
not in the 3-D feature. Figure 8.19 shows a sketch plane with some geometry cre-
ated for a new feature, and although the new feature will have rounded corners
(since the bold geometry indicating the section excludes the sharp corners), the
sketch still retains its “sharp” corners. This is important since the dimensional
constraints shown are to the ends of these corners. So the width and height of the

FIGURE
8.19
Construction geometry (light lines) used for constraints, but not section
building (heavy lines).

Part Modeling 215

rectangle can be the driving dimensions,
and the fillets can adjust accordingly.
But again, it is the section that is going to be used to create the new 3-D feature.
The other parts of the lines that are not part of the section simply remain as con-
struction geometry that is controlled by the part (assuming the CAD system han-
dles this approach).

If one considers a closed section as containing or bounding an area, and
this area can be used to create a solid feature (since there are no breaks in the
section), there are some situations that should not be allowed for sections. One
such situation is “self-intersecting.” Referring to Figure 8.20, it is seen that the
section is crossing over itself. This is a problem since if one imagines this section
being extruded to form a 3-D feature, there is going to be a singularity problem
where an edge is going to be part of two separate, but just touching volumes. Or,
the system may consider a part to have negative volume, or it may not be able to
resolve what side of the faces are inside the part and which are outside. In any
case, the system needs to correct this situation by rearranging the entities, or it
should force the user to correct the self-intersecting situation.

There are a number of properties associated with sections (at least proper
sections that are supposed to be closed). Not surprisingly, these are known as sec-
tion properties. Table 8.5 lists the various values that can be calculated by the
CAD system based on a section. Keep in mind that these are all 2-D properties
that are derived from the section that has been created on the sketch plane. Later

FIGURE
8.20

A self-intersecting section.

216 Chapter 8

TABLE
8.5

Geometric Properties Derived from a Section

Property Description

Area A closed section will define an area. This may include composite

section behavior where holes or other geometry subtract from

the overall area.

Centroid or “CG” The centroid is like the center of the area. It may also be referred to

as the Center of Gravity or the CG. Once a section is created, the

CAD system can do calculations to identify this point.
Intertia The inertia or Ixx, Iyy, Ixy
is a calculation that weights the distribu-

tion of the area with respect to some axis. This is often used to

assess a part’s ability to resist deformation about that axis. Keep

in mind that the axis needs to be clearly defined. Sometimes, it is

assumed to be through the centroid and then with respect to the

local horizontal and vertical axes. Hopefully this is clearly indi-

cated by the CAD system.

in this chapter, there is a list of 3-D geometric properties that are derived from the
complete solid part model.

8.6 THE BOOLEAN OPERATIONS

Up to this point, the creation of 3-D features has all been based on the idea that a
new feature is sketched on a plane that is attached in some way to an existing part
(whether that plane is a face of the 3-D part, a datum plane, a coordinate system,
etc.). This is the most important aspect of the features-based modeling approach.
However, there are some other techniques that may be included in 3-D CAD sys-
tems that do not require this approach at all. They do not necessarily require a
plane for sketching, or perhaps no plane at all. These techniques are generally
known as Boolean operations.

The term Boolean generally refers to the mathematics and logic of combi-
nations (unions, intersections, ANDs, and ORs, etc.). In this case, it is applied to
the combining of two separate 3-D part models. Boolean operations for 3-D mod-
els are listed in Table 8.6. Figure 8.21 shows three different parts that result from
the join, cut, and intersect
of two separate original parts shown.

Just like the normal features-based approach, the Boolean techniques should
be thought of as operating on a base feature of the part model. As such, these Bool-
ean operations create steps in the part history in some fashion. However, instead of
one long progression of a single line of operations (as with the pure features-based
approach), a Boolean operation can bring together two separate histories into a sin-
gle part with its own history. Each of the parts (with their histories) become subhis-

Part Modeling 217

TABLE
8.6

Boolean Operations

Boolean operation Description

Join This operation brings two separate 3-D models into a single new

model. This operation will usually determine where the two

models meet and create new edges and surfaces so that only one

single volume is bounded by the new model. If the two models

do not actually touch or interfere, then the new model will look

like two parts but is treated as a single part by the system.
Cut This operation cuts one 3-D model with another 3-D model and

results in another new 3-D model. This operation will be forced

to determine all the junctions to create new edges or surfaces. If

the two parts do not interfere, then a sort of nothing part is the

result (or the system does nothing).

Intersect This operation creates a new 3-D model using only the volume

that results from where two separate 3-D models are interfering

or intersecting.

FIGURE
8.21

Example of the join,
cut, and intersect
Boolean operations.

218 Chapter 8

tories to the overall new part history (or they could be thought of as subparts
connected into a master part). This can very helpful with complicated parts since it
can be broken down into simpler pieces that can be created first. It can also help
the part to regenerate more quickly. Figure 8.22 shows a part history that used a
Boolean technique to connect two parts together into one new part.

Keep in mind that the use of these Boolean techniques breaks the idea of
the basic three step process. There is no picking of a sketch plane to signify the
beginning of the new feature. Also, one needs to consider how the two separate
parts are positioned prior to the Boolean operation. Some CAD systems will al-
low the two parts to be positioned with respect to faces or planar surfaces that are
going to be shared by the new, larger part (such as a face where they can be
thought of as mated). In this case, sketch constraints (such as dimensions, paral-
lel, perpendicular, etc.) can be used since the separate parts are sharing a plane
(where 2-D sketch constraints can be applied). Other CAD systems may allow
the 2 parts to be just arbitrarily moved into position relative to each other and
have no constraints at all for their connection with each other (although this is
rather risky since there is no clear design intent stored with the part).

Since the use of the Boolean techniques imply that the user has a good
grasp of how the 3-D parts are made, it is a good idea to use the Boolean tech-
niques only after becoming pretty comfortable with the features-based approach
(and the basic three-step process).

FIGURE
8.22

A part history that used a Boolean operation.

Part Modeling 219

8.7 CREATING STANDARDIZED FEATURES

Although the Boolean operations can be important techniques for 3-D part mod-
eling, there are also other techniques that may not follow the basic three step pro-
cess. For instance, most CAD systems will provide packaged or simplified
methods to create very common features on 3-D part models. For example, holes,
fillets, and patterns may be created by just picking from some standard menus.
Table 8.7 lists a some of these standardized features.

TABLE
8.7

Some Often Automated Standardized Features

Feature Description

Holes/slots Beyond holes that cut through the entire volume of a part, the CAD sys-

tem may automate holes that are given countersink, counterbore, and

drilled and tapped. Countersink indicates a conical cut-out along an

edge of the hole. Counterbore indicates a cylindrical cutout along an

edge of the hole. Drilled and tapped means that after the hole is cut