This document discusses different types of CAD software programs available for engineers. It describes 2D drafting software, 3D wireframe and surface modeling programs, and various types of 3D solid modeling programs including constructive solid geometry (CSG), boundary representation (Brep), hybrid, feature-based, parametric, and dynamic modeling. Feature-based parametric modeling that allows for changing dimensions is highlighted as particularly useful for mechanical engineers.

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Engineering Software - CAD, CAM, FEA - CAD Software Programs
Introduction Types of CAD CAD Links CAD Vendors
Today, there is a wide variety of options to consider when purchasing CAD software including:
2D Drafting - Technical Illustration Software
3D Wireframe/Surface Modelers
3D Constructive Solid Geometry (CSG) Solid Modeling
3D Boundary Representation (Brep) Solid Modeling
3D Hybrid Solid Modeling
3D Feature-based Solid Modeling
3D Parametric, Feature-based Solid Modeling
3D Dynamic, Feature-based Solid Modeling
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
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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 free-form 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
CSG and Brep solid modeling 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.

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Most commercial solid modeling systems are hybrids using both CSG and Brep methods.
3D Feature-based Solid Modeling
Featured-based modeling is quickly becoming the preferred modeling method of mechanical
engineers everywhere. Feature-based 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 Feature-based, 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 history-based 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 Feature-based, 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

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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.
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