Parametric and Feature-Based Modeling

Another feature of modern CAD systems is the ability to create parametric models. 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 that use a history-based method 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. Designers can test various sizes of parts to determine which is the ``best'' part for their use 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.

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. [Shah]

Last Modified: Wed Aug 28 14:41:29 EDT 1996