Chapter 7 Computer-Aided Drafting and Design
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revolving operations are common in many
CAD modeling programs and are discussed
in Chapter 13.
Once the base feature is created, additional
features, such as holes and bosses, are added
to complete the part model. See Figure 7-31.
The base feature is referred to as the parent
feature. Each feature that is built onto a parent
feature is called a child. A parent can exist
without the child, but the child cannot exist
without the parent feature. Therefore, when
modeling the part, it is important to start with
the most fundamental shape and add features
in a sequence that refl ects the function of
the part. Common types of features include
“cutouts” such as holes or slots. There are also
more advanced features, such as rib features
and shell features. Another way to model
additional features is to create linear, circular,
or mirror patterns of existing features.
Often, the existing surfaces of a solid model
are used as working planes for creating feature
sketches. For example, the bottom surface
of the drawer knob in Figure 7-31 is used to
create the sketch for the hole cutout. Sketches
can also be created on one of the default sketch
planes or on a user-created construction plane,
typically referred to as a reference plane.
Features can be edited by modifying the
feature dimensions, or parameters, associated
with the corresponding feature. This is often
suffi cient for minor changes to a part. More
complex edits can be made by modifying the
sketch associated with the feature. When using
this method, the entire part updates to refl ect
changes made to the sketch geometry.
Design Intent
Because of the different processes involved
and the nature of feature-based modeling,
design plays a key role in parametric modeling.
It is important to consider the design intent
throughout the modeling process and plan each
stage accordingly. For example, the sketch
for a base feature should be made on the
appropriate sketch plane. The selected sketch
plane establishes the orientation of the part
When a sketch contains constraints that
are in confl ict, the sketch status is overdefi ned.
The sketch geometry changes to a different
color, such as red. This indicates that certain
constraints cannot be solved by the program
and a modeling error exists.
Creating Features
When creating features, modeling mode is
active in the program and feature modeling
tools are available. As previously discussed,
creating the base feature is usually the fi rst step
in constructing a part model. The base feature
represents the most basic shape of the part and
is usually created from a sketch. A common
way to create a base feature from a sketch is
to extrude the sketch using the Extrude tool.
Extruding a 2D sketch converts it to a 3D solid
by “sweeping” the sketch along the Z axis to
a given depth. See Figure 7-30. The resulting
feature is parametric and can be edited to
change the extrusion depth as needed. Another
way to create a base feature from a sketch is to
revolve the sketch geometry about an axis. In
this case, the sketch is normally a 2D profi le of
a cylindrical object. The Revolve tool is used
to create revolved features. Extruding and
Goodheart-Willcox Publisher
Figure 7-30. The sketch from Figure 7-29 is extruded
to create a base feature. The base feature forms the
basic shape of a drawer knob for a desk.
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