Machine Design

In a solid world, surfaces still matter

Surface modelers are the best way to handle complex shapes

Stuart Watson
CAD Development Director
Delcam plc
Birmingham, U.K.

This face mask was modeled with Delcam's PowerShape, a hybrid modeler that combines solid and surface-modeling capabilities. Typical applications include tooling and prototypes, and designing packaging, footwear, ceramics, and other products with complex shapes.

Solid-modeling systems have grown to wider use ever since Parametric Technology launched Pro/Engineer more than a decade ago. The more recent launch and success of various midrange modelers, especially SolidWorks and Solid Edge, has further strengthened the market for solid-based CAD. For a time it appeared that solid modeling would be the only form of modern CAD. However, an increasing number of users now find that surface-modeling capabilities are more important in many applications.

To appreciate why solid modeling appears so attractive and why surface modeling is still relevant, it helps to start with a brief history. In the early days, most CAD modelers were surface modelers. These build models by shaping a "skin" over a component. They could generate most any shape a designer might imagine, but were historically difficult to use.

For example, users had to know the surface modeler's hierarchy of geometrical objects, such as surfaces with internal control-curves, trim curves, and so on. Each has control points plus tangent vectors or knots and weights. Users also had to learn a set of edit commands for each of these objects, and there are extra commands that link or stitch individual surfaces together to form the complete, continuous skin over the model. In short, surface modelers involve a major learning curve.

Solid modelers, on the other hand, are easier to understand since they often work with only a single class of object -- the solid. Manipulating the solid, in turn, requires only three basic editing commands (at least conceptually), the Boolean operations of subtraction, addition, and intersection. In addition, solid modelers, in most cases, do not break individual surfaces apart, so commands to link skin surfaces are unnecessary.

Modern solid modelers actually combine the Boolean operations in various ways to provide many more than just three editing options. Even so, they are generally much easier to learn and use than surface modelers because they deal with fewer types of objects, and have far fewer editing functions. Also, with only three basic edit operations it is feasible to maintain a history of operations and build associativity into the modeling process. This ability to change a parameter and have the rest of the model update automatically makes solid modelers ideal for conceptual design where designers must try out various "what-if" scenarios.

Simple, easy-to-use attributes combined with a parameterized, associative architecture is why solids have become the predominant technology for modeling engineering parts. They are suited for most components that merely perform a function. In such cases, shape is often fairly simple and well within solid-modeling capabilities. Also, the exact shape is often not a key consideration, so restrictions imposed by the software are not critical.

However, despite their wide use, solid modelers are by no means perfect for every application. For example, solid modelers are not particularly adept at handling complex geometries with free-form, sculptured surfaces. Such shapes are essential for any product where aesthetics or ergonomics are important. Thus, despite the prominence of solid modelers, there remains a need for surface modeling.

For instance, toolmakers need surface models to handle the complex shapes in plastics molds, metal die-casting equipment, and so on. While the surface-CAD software market is smaller than the mass-market for solid modelers, it is still significant, and one in which solid modelers have been able to make comparatively little headway.

Interest in surface modeling also comes from product designers across a wide range of industries, including packaging, footwear, ceramics, and toys, as well as those involved with aerospace and automotive components, and housings for electrical goods. The trend is toward more-complex shapes because of a growing need for product differentiation and the increased importance of ergonomics.

Surface modeling remains the only method that can interactively model part geometry. This is a key factor in any product where appearance outweighs functional requirements. Companies also realize that problems result when models are just approximations with complex details left out. Virtual prototyping and digital mock-ups need accurate geometry to work effectively. Surface modeling provides that accuracy.

Manufacturing developments have also played a part in a wider demand for surface models. At one end of the scale, larger, more-complex parts are replacing a number of smaller, simpler components to speed assembly and cut costs. At the other extreme, miniaturization, for example in mobile phones, has required more-complex shapes to allow closer packing of components.

In response, CAD developers are trying to give solid modelers the ability to handle more-complex shapes.

Other companies have added solids functionality to their surface modelers. These hybrid modelers aim to offer the speed and simplicity of solids with the power and flexibility of surfaces. It remains to be seen whether adding solids to a surface modeler, or vice versa, will give the most effective solution.

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