How to select the best CAD modeler

Most engineers are familiar with geometric modelers with their solids and surfaces and Nurbs-based curves. Yet there are many cases where traditional geometric modelers are too slow or not a good fit for the design task at hand. Examples include modeling textures; shelling complex models; modifying and using scan data; designing complex organic shapes and artistic, aesthetic forms for manufacturing; adding highly sculptural detail to parametric CAD models; and combining hand-modeled and computer-generated forms.

Increasingly over time, the lines between different types of geometric modeling software have blurred, as each combines similar approaches and tools from the others. Newer, nontraditional approaches have accelerated this trend. For example, “hybrid” software from 3D-Coat combines what are called polygonal and voxel modeling. And PowerShape from Delcam, Salt Lake City, lets users model with solids, surfaces, and polygons. In addition, Freeform, 3D organic design software from Sensable, Wilmington, Mass., combines voxels, solids, surfaces, and polygons.

The combination of multiple geometry types in one package shortens design time and eliminates the need to learn multiple pieces of software with different user interfaces and ways of working. It also reduces the number of challenges inherent in getting separate pieces of software to efficiently talk to each other. These modelers complement traditional CAD packages.

As this blending of modeling approaches continues, engineers and designers need a working understanding of the advantages of each geometry format, when and how to combine them, and where to best apply them. Of course, the capability to model efficiently is crucial to streamlining the design and manufacturing of new products.

Different geometries solve different problems
Let’s take a look at four different geometry types, their pros and cons, the workflows they facilitate, and the types of products they suit.

Nurbs is short for “nonuniform rational B-splines,” with “B-splines” being the important bit. Years ago, manufacturing companies needed a way to mathematically capture the nonrectilinear shapes found in car exteriors, ships hulls, and airplanes in a way that was accurate and could be reliably repeated. It was easy to define straight lines and arcs, but a freely defined curve was another thing. In 1959, French engineer Pierre Bezier and French physicist and mathematician Paul de Casteljau, working independently, came up with a way of using control points to define and control a free curve.

Methods were also invented to control the curve with control points directly on the curve as opposed to on a control net. This made the definition and control of these curves more intuitive. The method works well to define straight lines, arcs, and circles as well as curvy shapes. Defining a 3D surface just extends this principle.

Extrapolating these simple examples into a fully finished design requires that the model follow the abstract rules inherent in Nurbs modeling. Mostly, the CAD software controls these rules, working behind the scene. The upside is that as long as the software follows the rules carefully, it can generate models that won’t fail. The downside: Sometimes the rules become so limiting or so time consuming to follow, the designer is forced to compromise his or her design, or give control to others later in the manufacturing workflow, for example, in applying textures.