Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (41 page)

system recreate the 3-D part starting with the first step in

the history.

Rollback or midprocess Rolling back refers to viewing the start of a part model in

the middle of the history. This term may also refer to al-

lowing the creation of features or steps in the middle of the

history.

Reorder Reordering refers to changing the order that features or steps

appear in the history.

History-supported An operation or change to a part that is “tracked” in the his-

tory of the part. Some steps are actually shown on the part,

but cannot be found in the history, and thus cannot be ma-

nipulated as an independent feature of the part.

So, as one learns to master 3-D CAD, one needs to plan an approach or
strategy for part modeling. Sometimes, it can help to think about how the part
is going to be manufactured, but sometimes even this is not sufficient. Design-
ers need to gain enough experience with modeling their type of parts, and to be
able to look at or imagine the part and then mentally plan how to create the
3-D model. This can be a daunting task at times, but it can also be a fun
challenge.

7.4 CHAPTER EXERCISES

1. Try to determine when your company or university first started using
3-D CAD. Record the name of the software package that was adopted.

2. How much did the software cost per user (or “per seat”)?

3. Compare and contrast three of the currently available 3-D CAD sys-
tems. Look at issues such as their cost per user, special capabilities, technical
support, supported platforms, customer base, etc.

3-D CAD 187

7.5 CHAPTER REVIEW

In your own words, define the following terms:

1. Model 7. Vector
2. Part model 8. Normal vector
3. Feature 9. Unit vector
4. Assembly model 10. Surface
5. Surface model 11. Clipping
6. Section 12. Triad

8

Part Modeling

8.1 INTRODUCTION

Part modeling uses a 3-D CAD system to create a single model for a single, stand-
alone object. These models can be referred to as parts or part files. Also, design-
ers familiar with 2-D design will sometimes refer to these items as details
(meaning they were documented on a detail drawing). In this work, however, they
will be referred to as parts or part models.

Recall from the previous chapter the number of misconceptions often en-
countered by users of 3-D CAD systems. Try to keep these issues in mind during
the discussion of this chapter to see how they are relevant to part modeling in
general. These issues were: 1) one must often create 2-D geometric entities be-
fore 3-D features can be created; 2) the 3-D model is not really solid, but really a
set of paper thin surfaces that meet at common edges (the next chapter on surface
modeling discusses this in more detail); 3) a more complicated 3-D model is cre-
ated by following a series of steps called a history, and the system remembers and
tracks these steps.

188

Part Modeling 189

8.2 THE THREE BASIC STEPS

In many cases for many different 3-D CAD systems, there is a basic three-step
process that is employed in creating and then expanding a 3-D part model. Table
8.1 lists the 3 steps in a brief list.

Step 1 Selecting the Sketch Plane

As mentioned before, many 3-D features start with 2-D geometry. Obviously, this
2-D geometry cannot be created unless the CAD system knows what 2-D plane is
being employed. In a 2-D CAD system, this is no problem; one draws on the vir-
tual paper. In a 3-D CAD system, however, this is an important issue; one needs
to think about where to sketch in a 3-dimensional space.

So, Step 1 is concerned with picking or creating this sketch plane. This
sketch plane may be an existing face of a part (meaning that an additional feature
is being created on an existing part model), or the sketch plane may be an imagi-
nary flat space or plane shown on the screen, or the sketch plane may be a plane
from a blank or prototype part that just has axis planes, datums, or coordinate
system planes.

Step 2 Creating 2-D Geometry

Once the sketch plane is specified, the CAD system can now be used to create the
base or foundation 2-D geometry. For example, if one wants to make a cylinder,
then one needs to sketch a circle first. If one wants to make a block, then one
needs to sketch a rectangle. Figure 8.1 shows a sketch example. Keep in mind,
though, that a 3-D CAD system will go beyond the simple creation of these 2-D
geometric entities; it will also permit intelligent modeling or constraining for the
2-D geometry. Constraining is explained in more detail in a later section of this
chapter.

TABLE
8.1

The Three Basic Steps

Step Description

Step 1—Selecting the sketch plane Picking a plane to start from

Step 2—Creating 2-D geometry Sketching some base or foundation 2-D geometry

(see Figure 8.1)

Step 3—Making the feature Turning the base geometry into a 3-D feature (see

Figure 8.2)

190 Chapter 8

FIGURE
8.1

Creating or sketching 2-D geometry.

Step 3 Making the 3-D Feature

Once the 2-D geometry has been created (on the sketch plane, of course), then
this geometry can be used in Step 3 to create or expand the part model with new
3-D geometry. Basically this involves selecting what segments of the 2-D sketch
geometry (from Step 2) are to be used in making the new feature and then select-
ing the method to create the feature (protruding, revolving, sweeping, etc.). No-
tice that there are some horizontal construction lines in Figure 8.1 that are not
part of the bold lines (or section) that are used to create the feature shown in Fig-
ure 8.2. It is often best to leave these construction lines in the sketch permanently
(sketching is not an exercise of making a drawing).

Table 8.2 lists the basic 3-D feature creation methods that usually require
initial 2-D geometry (later on there are some other methods that don’t actually
require 2-D sketching). Figure 8.3 shows what they create using a circle as the
initial 2-D geometry.

Recognizing and understanding the basic three-step process should be
helpful in optimizing the potential of 3-D part modeling. It is simple and logical,
and it lends itself to the understanding of how history-based modeling works.
Some CAD systems will make the three steps recognizable in the process of
modeling, while other CAD systems will not make it obvious. Regardless, the
user needs to understand that this process that may be occurring behind the
scenes.

Part Modeling 191

FIGURE
8.2

Making the feature.

TABLE
8.2

The Most Common Methods of Making a Feature

Method Description

Extruding Extruding means taking the 2-D geometry and creating surfaces that fol-

low along a single direction or vector. Depending on the user’s selec-

tion, this can be used to create a protrusion (a new positive volume) or

a cut (removing existing volume from a solid). This is probably the

most common 3-D feature creation method.

Revolving Revolving means taking the 2-D geometry and creating surfaces that re-

volve about an axis. If this is done to add a feature to an existing part,

then this operation can be used to add volume (as in a protrusion) or

remove volume (as in a cut).

Sweeping Sweeping means taking the 2-D geometry and creating surfaces by drag-

ging the geometry along some sort of arbitrary, but smooth path. A

typical example is a pipe or tube with bends.

192 Chapter 8

FIGURE
8.3
ing, sweeping.

Examples of the common methods of making a feature: extruding, revolv-

8.3 CREATING NEW FEATURES

The three basic steps just explained are going to be useful in not just starting or
creating a new part model, but also in expanding upon a part model. Although
one generally thinks of the word “feature” meaning something that is added onto
an existing part, “feature” frequently refers
to the first histo
ry step that starts a
part model. In this case, though, it can be referred to as the “base feature” or per-
haps the “base node” of a history tree. In any case, creating the part model with
the base feature is usually just the beginning of the part model. Although some
very simple parts (such as flat plates) might only require the base feature, most
3-D parts are going to require that new features be added onto the base feature.

To create these added features, one can often apply the three basic steps
once again. However, in Step 1, one must be careful to pick a sketch plane that is
related to or associated with the base feature. This may seem obvious, but many
new 3-D CAD users miss this point. If the 3-D CAD system allows the user to
create a base feature by just sketching out in space, then the user may be tempted
to just start sketching out in space a second time to make the next feature for the
part. However, this really skips Step 1 (selecting a sketch plane). If the user does
not select this plane carefully using geometry from an existing part model, the
system will simply assume that the user wants to start a second, separate part
model. Although it is possible to have these two part models connected later (us-
ing a technique known as a Boolean operation), it is usually preferable to have
new features automatically connected with the base feature. So, to do this, the

Part Modeling 193

user needs to be careful to pick a sketch plane (or to be careful to think of all
three steps in the three-step process).

8.3.1 Sketch Plane Based on a Face

The simplest way to be sure that the sketch plane is connected to the base feature
is to pick a face of the base feature. This face would be a flat surface (a planar
surface) on the existing part model. Figure 8.4 shows a sketch plane selected
from an existing face of a part.

8.3.2 Sketch Plane Based on a Datum Plane

Obviously, sometimes there are no visible faces of a part that supply a plane in
the orientation or location needed for the new feature. At other times, the use of a
face for a sketch plane is considered too risky. For instance, a face that could be
used as a sketch plane is not yet fully designed or is still subject to change. In
these cases, a different approach is needed for the sketch plane. Often, the solu-
tion is to create and/or select a sketch plane based on a datum, reference geome-
try, or construction geometry.

For drawings, a datum refers to a plane that is considered a foundation or
baseline for dimensions on a drawing. This plane is given a letter designation in a
notation system called Geometric Dimensioning and Tolerancing (GD&T). This
datum concept is easily transferred to the 3-D model for a selecting a face of the

FIGURE
8.4

An existing face as a new feature’s sketch plane.

194 Chapter 8

part. But this idea is often expanded upon in the 3-D CAD system to include
planes that can be created based on arbitrary X, Y, Z coordinates in space or
planes created based on various geometric techniques. For instance, a datum
plane can be created by selecting any three points on a part, or a plane can be
created by specifying 1 point and a normal vector to the plane. In each of these
cases, a plane is being created that can be used as a sketch plane, and it is created
based on geometric entities that already exist in the part. This is the essential con-
cept if one wants this new feature to be part of the history of the existing part.
Take care when creating these types of sketch planes that they are not just out in
space and unconnected to the existing part.