Feature-Based Modeling, for Surfaces

Feb. 21, 2008
A new scheme helps speed the design of sculptured surfaces.

Simon Alford
P roject Manager,
NX Product Definition
Siemens PLM Software
Plano, Tex.

Edited by Leslie Gordon

In highly competitive industries such as automotive, it is increasingly design that differentiates a vehicle from its competitors. And because automotive manufacturers are pressured to get cars to market faster, design development must go quickly as well.

However, it can be difficult to meet this need for process speed. Design can be a slow, iterative process. For example as a vehicle design matures, there is a gradual shift from physical clay replicas to 3D CAD models, with a continuous refinement of vehicle form. So what can be done to accelerate the process?

For many years now, mechanical design has used parametric software that stores commands such as Extrudes and Blends as features in the Part History. This lets users quickly generate and modify data via rules or parameters. Until now, it has been widely perceived this approach is not practical to generate the free-form organic shapes found in much of surface design. Historically, vehicle surface-data has been designed on special, stand-alone software. Any changes to the vehicle form resulted in lengthy CAD rework. Deleting and recreating data was often the only feasible route forward.

But now programs such as NX Industrial Design use parametric features to create free-form designs for such curvy products as cell-phone bodies and toothbrush handles as well as automotive bodies. Users type in parameters to control values (such as length and radius) and assign associative constraints (such as tangency between adjacent curves). This ensures modifications propagate throughout the entire part file. Furthermore, users can choose when to store these features to control file complexity.

Surfacing Commands
Give the same Class-A model (Class-A is an industry term used to describe high-quality surfaces, typically found in vehicle design, that incorporate aesthetic and engineering criteria) to several designers, and they will probably produce different, but correct results Even so, because Class-A surfacing is procedural, designers repeatedly use lengthy sequences of common commands. Storing these commands as features lets users “roll back” in time to change a feature, and the model subsequently updates. Editing part history avoids repetitively recreating lengthy manual modeling procedures. The software also contains single features, such as Styled Blends and Sweeps & Corners, that provide embedded design “sculpting” directly inside of a feature to further speed designs.

Features can also provide alternative themes for a model — a common requirement in design. Traditionally, designers modify a copy of the part. However in NX, features can simply be added to the part and grouped together, while preserving the underlying geometry. In design reviews, these feature “groups” can be toggled on and off to illustrate alternative designs. Features also assist in recognizing the “cause and effect” of change to a model, for example, changing surface boundaries during initial creation and refining surface highlights near completion.

A hands-on experience
Surface modeling is not an exact science, it’s a hands-on experience. Designers only begin to ascertain the subtlety of the reference form once they start overlaying surfaces. As a model matures, surface data continually changes as designers move boundaries for required transitions between surfaces. Feature-based modeling lends itself well to this iterative practice. The combination of features, parameters, and associativity provide continuous feedback throughout the design process.

To capture design intent, users typically import styled sketches in the form of raster images or STL facet data as the reference geometry. NX uses a hybrid modeling approach, creating Bezier and Nurbs data for both Class-A (typically Bezier) and conceptual workflows (typically Nurbs). Surfaces are generated using a combination of curve-based (sweeping or lofting), free-form (surface and curve “control-point” editing) and surface solid-modeling techniques (a Boolean “solid” model from a 2D Sketch).

Also critical for design — the capability to diagnose and validate surface quality. This includes such engineering and aesthetic properties as legal constraints, manufacturing criteria, and surface curvature, highlights, and reflections. The software provides diagnostic and visualization tools that produce realistic virtual showrooms as well as photorealistic renderings for reviews and marketing material.

An Offset Curve feature (above) is used to create the initial wheel-arch “eyebrow” surfaces.


The Offset Curve is modified (left) and the “eyebrow surfaces” update accordingly.


Curvature continuous surfaces (G3 continuity) ensure seamless, smooth transitions between surfaces. Tools in the software let users diagnose and validate surface quality.


Feature-based modeling allows the rapid change of the Styled Blend and Silhouette Flange without lengthy model rework.


A Silhouette Flange feature generates the first Flange and Fillet along the wheelarch return edge.


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