Used with permission from Computer-Aided Engineering, March, 1999

Illuminating solids and surfaces. This leash light, invented and designed by Altitude Inc. for Black and Decker, was designed using PTC's CDRS for the outer surface and Pro/ENGINEER for the interior components. It combines a flashlight with a retractable leash to make it easier and safer to walk a dog at night.

Solids and Surfaces Become One

by Robert Mills

Mechanical CAD software that use strictly solid modeling techniques appear to be quickly giving way to systems that also include surface modeling capabilities. Surface modeling is concerned only about the definition of the skin of a model and is used – among other reasons – for designing and manufacturing aesthetic shapes. The inclusion of surface modeling functions into solid modeling packages is nothing new, but the trend is accelerating. Evidence abounds:

Indeed, it's difficult to find a vendor of 3D modeling software for mechanical design and manufacturing that doesn't include at least some solid and surface modeling capabilities. "In order to be competitive, all systems will include surface and solid modeling; those that do not will go by the wayside," predicts Mike Hansen, director of product marketing at Spatial.

Why Are Solids So Popular?

About everyone agrees that solid modeling has proved its usefulness and appropriateness for just about any application or process in mechanical design, analysis, and manufacturing. "Real engineering parts are solid, so there is always a benefit to using a solid modeler to design them," says Gary Stoll, vice president of engineering at Visionary Design Systems.

Solid modeling has become the primary CAD technology for mechanical design. One of its main advantages is that the technology accounts for the connectivity of the various geometries (called topology) – something that a surface or wireframe system cannot do. Solid modeling can be used to create the bulk of components in a mechanical system, even thin-shelled parts such as those made of sheet metal or plastic. The advantages are many, including the ability to determine mass properties, perform structural analysis, or create STL files for rapid prototyping. What's more, solid modeling can perform some complex operations such as shelling that cannot be done with surfaces.

In fact, the swiftly increasing capabilities of solid modeling systems is blurring the distinction between solid and surface modeling. "People tend to think of surface modeling as a tool for designing freeform shapes and solid modeling for designing simple, prismatic shapes. I think that's completely wrong," says George Allen, chief technologist at Unigraphics Solutions Inc., who reports that the firm has customers using its solid modeling systems to model medical devices, turbine blades, and even sunglasses – objects traditionally the exclusive realm of surface modeling.

Indeed, some functions that used to require surface modeling are now being added to packages that are primarily solid modelers, John McEleney, vice president of marketing at SolidWorks Corp. cites the example of splitting a surface for mold design. "A user can create the splitting surface with SolidsWorks 98Plus surfacing tools and use the resulting surface to split a solid model for the parting lines of the mold."

Why Add Surfaces?

The addition of integrated surface modeling functions into solids modeling, however, vastly broadens the capabilities of the software. "The benefit of using both solid and surface modeling is that you have the best of both worlds," says Peter Brooks, vice president of mechanical at Bentley Systems Inc.

Adds Michael Crown, product development manager at Varimetrix Corp. "A true hybrid modeling system provides the functionality to design a beautiful, ergonomic body with surface modeling tools and in the same environment, a powerful and intelligent set of solid modeling tools to convert it into a thin-shelled case, split it in half, and add all the required bosses, ribs, and flanges."

There are, indeed, many situations that are probably handled better by surface modeling than with solids. For starters, providers of hybrid modeling packages point to the ability to work with a wider range of data as a key benefit. Precisely because surface modelers don't have to worry about topology, it's easier for them to work with IGES and other imported data. Once in the system, the data can then be made into a solid. Geoff Hedges, product marketing manager at CoCreate Software Inc., explains, "To allow collaboration between OEMs and their subcontractors, and to leverage legacy data, it is essential to incorporate existing data into the solid modeling system."

What's more, there are some geometries that are easier to create using surface modeling, even some apparently simple shapes. Rael Morris, director of product marketing at Think3 Inc. (formerly CadLab), says, "There are many not-so-complex shapes that are difficult to construct using a solid modeler."

Also, surface modeling provides another bag of tools for creating desired geometry. "Surface operations provide a backup to solids operations," explains Jesse Luis, product marketing manager at Ashlar Inc. "In a solids-only package, you would be stuck."

While surface modeling can be used for just about any design, it's most often used in ergonomics, industrial, aerodynamic, and conceptual design as well as styling. And its primary strength is its ability to create freeform shapes primarily for aesthetic design. "We believe that product design has really changed over the years. The needs of design engineers are being pushed by what we see is an aesthetic revolution," says Think3's Morris, explaining why he thinks surface modeling is so critical to the success of a 3D modeler.

Another area where surface modeling excels is in manufacturing, especially in complex milling applications. Because of this common need, several vendors of hybrid modelers also offer NC capabilities. "Quite often, supplemental geometry needs to be created for the purposes of NC toolpath creation. A CAM system with an integrated hybrid modeler is a tremendously powerful tool," explains Varimetrix's Crown.

Hardware capabilities are another notch in surface modeling's belt. "Solids take up a lot more room than surfaces," explains Mike LaLande, automotive domain leader at IBM Corp., which markets CATIA from Dassault Systemes. Even with today's low-cost, powerful systems, surface modeling can be a more interactive method of working with geometry.

Why Use Both?

In many cases, however, integrated solid and surface modeling is often required to completely model the entire product. "The integration of surfaces and solids inside Pro/ENGINEER is absolutely necessary to create the engineering aspects of surfaces of body panels and ribs, bosses, cuts, and offsets for the interior," explains Thomas Teger, manager of technical marketing, surfacing applications at Parametric Technology Corp.

Integrated surface and solid modeling has many benefits. The combination provides flexibility in making design changes through the use of parametrics, constraints, and associative operations, explains Roger Klemm, MCAD program manager at Autodesk. "If the surface geometry changes, the solid updates; if the solid changes; the surface operations update accordingly."

Indeed, Hedges of CoCreate cites the ability to use the right tool for the job as another advantage of using both technologies "Input from various people (users, analysts, other engineers) needs to be incorporated quickly in a solid model. Surfacing capabilities such as those implemented in SolidDesigner allow users to select a freeform surface and replace it with another one. The end result is still a solid model."

From an enterprise standpoint, the push is also on for integrated surface and solid modeling. By linking the two technologies – or having a system that can handle both – companies can create a smooth flow of information from concept design (which is often done primarily in surfaces) to detailed design, analysis, and simulation (which is typically done in a solids-based environment) to manufacturing (which often requires both surfaces and solids). Chris Cheek of SDRC says, "Our users generate revenue by designing products, not creating solid models. A combination of tools, including history support of surfacing commands, is necessary in order for our users to find benefit throughout their entire design process.

Not everyone needs both technologies, though. Developers usually say that some 20% to 30% of all 3D CAD users need surface modeling capabilities, depending on the industry. Says SolidWorks' McEleney, "Generally speaking, most people are not creating complex geometry which would require them to use surface modeling tools." Bentley's Brooks agrees. "The combination of solid and surface functionality probably won't show up for some time, basically because not all users need it," he says.

How does it Work?

Traditionally, surface modeling was done in a separate environment, with a separate user interface. The surface data could be imported into a solid modeler, perhaps, but that was about the extent of the integration.

Virtually every developer, however, says this type of separation between the two types of modeling is quickly becoming a thing of the past. Indeed, the race is on to provide a full set of solid and surface modeling capabilities in a single package with common commands and a common database. The goal is to minimize the differences between solid and surface modeling and further the use of the two technologies by single users.

One effort toward that goal is to provide the same kind of control over surfaces that users now have over solids, thanks to history-based modeling, features, and parametric capabilities. Cheek of SDRC explains, "We've chosen to eliminate as much chaos as possible by combining common solids and surface modeling capabilities such as constraints, history-tree support, and feature-creation tools."

Allen of Unigraphics says modern systems such as Unigraphics add connectivity to surface models. Users are then working with what Unigraphics calls "sheet" bodies, which are essentially a collection of surfaces connected by topological information. "All of the topological information that's there in a solid model is also in the sheet models," he explains.

Jesse Luis, marketing manger at Ashlar, adds, "When a new feature is needed, users should be able to access the right tool for creating that feature without sacrificing the creative process."

Agrees Think3's Morris, "We consider it important that a system be architected from the ground up to be able to handle any type of geometric entity in a non-modal way. If there aren't additional rules on when and how you can use the surface modeling functions, designers will use them."

Autodesk's Klemm concurs. "Surfacing and parametric solids evolved from two very different origins. With the on-going integration of these technologies, the differences in look and feel diminish."

Still, most experts agree that surfacing modeling remains, in at least some ways, a different breed than solid modeling and thus requires that users understand the techniques. "Solid modeling and surface modeling are fundamentally different ways of thinking about design," explains Michael Shook, chief operating officer of AeroHydro Inc., whose SurfaceWorks add-on to SolidWorks attempts to tie surface and solid modeling together.

The two types of modeling technologies, however, are not always needed by the same user, especially in large scale systems. Tom Toomey, CATIA design domain manager at IBM, explains that some of its customers use a master model approach. Here, one group creates surfaces, which can then be used a reference for a another group working in a master solid model.

Where's It Going?

Developers generally agree that integrated solid and surface modeling is, for the most part, the future of 3D mechanical design. "[The technology] is driven by the need to fulfill a wider range of customer requirements within a single system," says Autodesk's Klemm. Spatial's Hansen agrees. "The trend is to offer more modeling for a smaller price."

Ashlar's Luis, however, believes that simply having both modeling techniques in a single package is not enough. "The value comes from usability. This requires a seamless integration of surfaces and solids, where the user choose the right tool for the job without having to think about the technology."

Stoll of VDS has a slightly different view. "A solid modeler is really just a surface modeler that doesn't let you create gaps in the model, so the future is not surface and solid capabilities combined, but very freeform-shaped solids."

Still, some do not feel that all 3D modelers will have both surface and solid modeling. "The 3D design world is too large and has too many specialized niches for that to happen," says AeroHydro's Shook.

Solids and Surfaces at Ford

Automobiles are a special case when it comes to surfaces. The quality and appearance of Class A surfaces - that is, those visible to the customer - are an obvious and critical component in their perception of the vehicle. Because of the importance and complexity of these surfaces, most automobile manufacturers use specialized surface modeling software rather than solid modeling systems to design and analyze body panels and other parts with Class A surfaces.

Ford Motor Co., however, has found a way to integrate surface and solid modeling. Its system consists of a version of ICEM Surf from ICEM Technologies, called Advanced Freeform Surfacing (AFFS) that is integrated with SDRC I-DEAS Master Series, Ford's primary CAD/CAM system. The technology is part of Ford's famed C3P program, which is aimed at improving product quality and reducing product development time. ICEM Surf has been a part of the automotive development program since 1991 and today is used in the development of every new vehicle program.

In practice, explains Richard Riff, director of the C3P program, sketching and conceptual design are done in software from Alias|Wavefront. While Alias|Wavefront also uses surface modeling technologies, the concepts are usually still milled in clay so that they can be seen in the real world.

The next step is to create Class A surfaces using ICEM Surf AFFS. These surfaces are used for feasibility studies, which essentially determine the organization's ability to produce what has been designed. Riff explains, however, that the surface information is kept in the I-DEAS Master Series database, so that the feasibility studies and subsequent design can be done using that system and related applications. The integration works both ways, so that any changes that might require alteration of a Class A surface can be passed back to ICEM Surf AFFS.

To reduce costly design rework, a major goal of the C3P program, the company conducts Electronic Design Reviews. At this point, an engineer may discover the need for extra clearance in a certain area of a door. The challenge is to satisfy the design team's aesthetic needs while meeting engineering requirements. Ford's combined use of surfaces and solids help resolve these sometimes conflicting requirements.

An interesting side note to this story is that ICEM Technologies, once the CAD/CAM division of Control Data Systems, was bought by Parametric Technology Corp. in 1998. Subsequently, PTC has said it will combine its industrial design organization (which has roots back to a system known as CDRS) with ICEM Technologies to create a Center for Surfacing Excellence. The result will be a suite of surfacing tools. It doesn't concern Riff that ICEM is now owned by one of SDRC's competitors; in fact, he has hopes that PTC and its resources will make ICEM software even better.

Copyright 1999, Penton Media, Inc.
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