Chapter 7 Computer-Aided Drafting and Design
173
into an assembly (or into several subassemblies
and a top-level assembly). The second approach
is referred to as top-down modeling. In this
approach, the assembly fi le is created fi rst
and each part is modeled within the assembly
fi le until the assembly is complete. Each part
is still saved as a separate fi le. This technique
is useful for larger assembly models where
it helps to use the assembly geometry for
locating parts accurately. The third approach
to assembly modeling involves a combination
of bottom-up and top-down techniques. This
approach can be used when most of the parts
have been constructed independently and
additional parts need to be created as the
assembly is being built.
Adding Assembly Constraints
As parts are added into an assembly, they
are positioned accurately at the appropriate
locations. Then, constraints are added to the
parts. Constraining the parts is required to
control the degrees of freedom (DOF) of each
part. Each part has six degrees of freedom.
There are three degrees of linear movement
along the X, Y, and Z axes and three degrees
of rotary motion about the X, Y, and Z axes.
When a part is fully constrained, all degrees of
freedom are removed. By adding the appropriate
constraints, it is possible to accurately control
the functional motion of parts in the assembly.
equation is used to control a dimension using
the value of another dimension. For example,
an equation can be used to constrain the length
of a part so that it is always equal to twice the
width. This association is maintained when
the part is edited. In similar fashion, a hole
diameter can be constrained with an equation
so that it remains proportional to the overall
length of the part when the length is changed.
When designing a part, it is often helpful to
plan ahead and identify the most appropriate
modeling tools to use to create features. For
example, if a part is symmetrical, features can
be patterned or mirrored to save time. If a
feature is to be created by revolving a sketch,
it is important to identify the correct profi le
and determine where the revolution axis will
be positioned.
Careful planning in the early stages of
modeling helps maintain design intent in later
stages, such as when assembling parts into
an assembly model. Assembly modeling is
discussed next. Design intent also applies to
a variety of other design factors, such as
material selection, how the part or assembly
will be manufactured, and maintainability of
the design.
Assembly Modeling
An assembly model consists of two or
more individual parts or subassemblies. A
subassembly is a smaller unit of parts that must
be assembled together before being added to a
larger assembly. See Figure 7-32. For example,
an automobile engine must be assembled
before it is attached to the frame. In assembly
modeling, individual parts are assembled by
precisely locating and constraining the parts.
The assembly is saved as a separate fi le from
the part fi les. The part fi les are linked to the
assembly fi le so that any edits made to a part
fi le are refl ected in the assembly fi le.
There are three different approaches to
creating an assembly model. The fi rst approach
is referred to as bottom-up modeling. In this
approach, each part is created individually.
Then, the parts are combined by inserting them
Goodheart-Willcox Publisher
Figure 7-32. A subassembly consists of two or more
parts. This subassembly is made up of two parts, a
screw body and a pin, and is used in an assembly of a
C-clamp.
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