Types of CAD

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 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.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 history. Basically, it allows creation of features that are not fully dimensionally constrained.