Today, there is a wide variety of options
to consider when purchasing CAD software
including:
In principle, CAD could be applied throughout
the design process, but in practice its
impact on the early stages, where very imprecise
representations such as sketches are used
extensively, has been limited. There are
some new software programs currently available
which are trying to fill this niche. It
remains to be seen how effective they will
be and how widely they will be implemented.
2D
Drafting
In Mechanical Design, there are a few specific
options to look for when choosing a 2D drafting
package. They should provide: a complete
library of geometric entities; support for
Bezier curves, splines, and polylines; the
ability to define hatching patterns, perform
hatching within complex boundaries and perform
associative hatching and provide complete
dimensioning ability. Bill of materials
generation is a nice added feature for any
CAD package.
3D
Wireframe/Surface Modeling
3D wireframe and surface modeling are beginning
to fall out of favor with the introduction
of inexpensive solid modelers. This is partly
due to the translation difficulties entailed
with wireframe and surface models when trying
to incorporate models in analysis or manufacturing
software. Most advanced modelers utilize
surfacing for creating freeform surfaces
(surfaces that simultaneously curve in 3
directions), especially for industrial design.
3D
Constructive Solid Geometry (CSG) Solid
Modeling
Programs that are capable of solid modeling
can be much more powerful than simple wireframe
modelers. These programs are used to build
parts that are actually solid objects instead
of simply a wireframe outline of the part.
Since these parts are represented as solids,
they have volume, and if given a density
can have a weight and mass as well. The
computer can calculate many physical properties
of these parts, such as center of gravity
and moments of inertia. These calculations
can even be performed for irregularly shaped
parts, for which manual calculations would
be extremely difficult. Finite Element Analysis
techniques can also be used to perform stress
analyses of these parts.
Constructive Solid Geometry (CSG) uses
solid primitives (rectangular prisms,
spheres, cylinders, cones, etc.) and boolean
operations (unions, subtractions, intersections)
to create the solid model. The main drawback
to this type of modeling is the lack of
editing or redimensioning capabilities.
If there is a change in the design, the
model, in most cases, will have to be
reconstructed.
3D
Boundary Representation (Brep) Solid Modeling
Brep methods start with one or more wireframe
profiles, and create a solid model by extruding,
sweeping, revolving or skinning these profiles.
The boolean operations can also be used
on the profiles themselves and the solids
generated from these profiles. Solids can
also be created by combining surfaces, which
often have complex shapes, through a sewing
operation. This can be used, for example,
to create the body of an aerodynamic vehicle
such as an airplane, with its carefully
designed wing profiles. These two methods
can often be combined in order to create
the desired parts. Each of these methods
has its limitations, and parts which are
very difficult to create using just one
or the other method can be created much
more easily using a combination of both
methods. Thus, most commercial solid modeling
systems are hybrids using both CSG and Brep
methods.
3D
Hybrid Solid Modeling
3D
Featurebased Solid Modeling
Featuredbased modeling is quickly becoming
the preferred modeling method of mechanical
engineers everywhere. Featurebased modelers
allow operations such as creating holes,
fillets, chamfers, bosses, and pockets to
be associated with specific edges and faces.
When the edges or faces move because of
a regeneration, the feature operation moves
along with it, keeping the original relationships.
The choices made developing these models
are very important. If the features aren't
referenced correctly, they may not end up
in the correct place if the model is regenerated.
A feature that is located at an X and Y
offset from a corner of the face instead
of at the center of the face will not remain
at the center of the face when the model
is regenerated unless constraints are added
to the model that will change the X and
Y offsets to keep the feature at the center
of the face.
3D
Featurebased, Parametric Solid Modeling
In a parametric model, each entity, such
as a boolean primitive, a line or arc in
a wireframe, or a filleting operation, has
parameters associated with it. These parameters
control the various geometric properties
of the entity, such as the length, width
and height of a rectangular prism, or the
radius of a fillet. They also control the
locations of these entities within the model.
These parameters can be changed by the
operator as necessary to create the desired
part. Parametric modelers use a historybased
method to keep a record of how the model
was built. When the operator changes parameters
in the model and regenerates the part,
the program repeats the operations from
the history, using the new parameters,
to create the new solid. There are many
uses for this type of modeling including
testing various sizes of parts to determine
which is the "best" part by simply adjusting
the model parameters and regenerating
the part.
Some parametric modelers also allow
constraint equations to be added to the
models. These can be used to construct
relationships between parameters. If several
parameters always require the same value,
or a certain parameter depends on the
values of several others, this is the
best way to ensure that these relationships
are always correct.
These modelers allow other methods of
relating entities as well. Entities can
be located, for example, at the origin
of curves, at the end of lines or arcs,
at vertices, or at the midpoints of lines
and faces. They can also be located at
a distance or at the end of a vector from
these points. When the model is regenerated,
these relationships are maintained. Some
systems will also allow geometric constraints
between entities. These can require that
entities be, for example, parallel, tangent,
or perpendicular.
Parametric modeling is most efficient
working with designs which only undergo
dimensional changes rather than gross
geometric ones such as removal of a feature.
3D
Featurebased, Dynamic Solid Modeling
Dynamic Modeling uses flexible model creation
and refinement concepts to allow designers
to capture ideas and detail models quickly,
without focusing on the models' underlying
history. Basically, it allows creation of
features that are not fully dimensionally
constrained.
