Telford, England--Managing tolerances in a design--making sure there are no holes or gaps as the design moves into production--is one of the most vexing challenges engineers face.

The truth is it's one that software alone can't solve, says David Corbishley, CAD/CAM systems manager for NATO contractor Alvis Vehicles Limited. Only an engineer can find the right set of compromises for the solution, he says.

But, he adds, software can certainly help by enabling engineers to visualize problems.

"3D modeling does provide a facility to quickly assemble numerous virtual prototypes and or mock-ups to enable design and manufacturing engineers to evaluate a variety of potential solutions early in the design process," Corbishley says.

That's what he and his colleagues at Alvis have learned in several recent complex military projects.

Among those projects is engineering work on the Piranha and the Warrior 2000 armored personnel vehicles (APV). The company licensed the design of the Piranha from Mowag of Switzerland and modified its hull, or basic shell structure. Armed forces in several countries, including Oman and Qatar, use the Piranha. Alvis developed the Warrior 2000 from scratch for use by British and Kuwait armed forces.

In the first versions of some of its military vehicles, there were several tolerance problems, says CAD Coordinator Steve Booth. For example, he says, early designs from Mowag were on paper not computer, and when manufacturing tried to assemble the plates, or panels, they found they didn't fit. Using Bravo 3D solid modeling from Applicon (Ann Arbor, MI) for later versions solved the problem. "We could virtually build the product on the computer and highlight problems before production," he says.

"Vehicle models can have as many as 5000 3D solid components, and four or five times that number of solid shapes," Corbishley says. He reports that the company did art-to-part design on the Warrior 2000 with the Bravo suite of products, including Bravo NC and NCG. Additionally, engineers used BravoFRAME to manage the design process and control engineers' access to the CAD models, drawings, and all non-CAD data.

By enabling engineers to foresee tolerance problems before production, 3D solid modeling has helped Alvis engineers save time in product development, Corbishley asserts. For example, he says they cut the build time for the Piranha hull in half.

Alvis made the move to 3D solid modeling a few years back when the company upgraded its computer hardware. Engineers conducted benchmark tests with six software vendors. Among the most important criteria: the ability to use the data to cut metal, or manufacture product, and to create and manage assemblies.

"We selected Bravo because it can handle large complex assemblies with relatively modest hardware requirements," says Corbishley. "The art-to-part solution works, and the product provides built-in tools that allow us to manage the design cycle and control costs early in the design stages."

He says that manufacturing gets a boost because it can generate toolpaths on solids without the need of data translations. Manufacturing directly from the model reportedly reduces the number of detail drawings.

The company still uses some 2D CAD for electrical schematics and block diagrams, primarily Medusa, from Parametric Technology Corp., and Bravo 2D. Other software vendors Alvis works with include IBM/Dassault and SDRC.

Corbishley's goal is to eliminate drawings for all parts Alvis manufactures. But, for all the benefits Alvis has derived from use of 3D solid modeling to date, he still finds opposition to 3D. "Many members of senior management around the world still think that CAD is no more than an electronic drawing board," Corbishley says.

What they want from CAD

Engineers aren't shy about stating their needs. To find out what they want from CAD products, Design News surveyed five readers, all of whom are happy with the software they use now. Here is what they said:

Constantinos Minos, mechanical engineer at General Electric (Schnectady, NY) uses 3D solid modeling from Unigraphics Solutions on the Unix platform to design fan blades, turbines, and "all kinds of rotating machinery." The advantages of 3D that he sees: full descriptions of geometry. "2D models can't describe the geometry," he says. "They force you to make too many assumptions." His wish list: full compatibility between CAD and finite element analysis. "We perform very sophisticated analyses with ANSYS, LS Dyna and PATRAN," he says. "I want to be able to put the analysis mesh directly on the CAD model. Sometimes we can do that, but sometimes we can't."

Greg Butcher, CAD manager, Wabash Magnetics, (Huntington, IN) uses SDRC's I-DEAS Master Series 6.0 for design of electronic sensors. "Solids has improved communication between designers, production engineers and toolmakers," he says. It hasn't sped up the design process, he says, but he believes solids speeds downstream applications like CAM. On his wish list: general interoperability. "We get CAD models in from customers who use other software and read them into I-DEAS through an IGES translator. But, sometimes we have to re-stitch the model, and we can't change dimensions, so we start over in I-DEAS."

Tom Wogoman, project engineer at Wabash Magnetics, doesn't use 3D. He uses AutoCAD 14 2D CAD to update prints with design changes. He would like to have 3D capabilities, though. "3D would do dimensions much faster," he says. His need: "An AutoCAD LT-type version of a solids program that's inexpensive so all I would have to do is simple editing." He would also like a simple, basic solids program for doing presentations. "Presenting cross-sectional 2D views takes a lot of explaining," he says.

John Asseff, design engineer, Zoeller Pump Co. (Louisville, KY), has used Pro/ENGINEER from Parametric Technology Corp. (Waltham, MA). "3D modeling is great, but it changes the way you engineer," he says. "Before, we would take a 2D print to the shop and they would make a part out of metal. Now, we send the file to a rapid prototyping service and we get a test part. It's a prototype and helps us explain the design." Among his biggest needs: a way to get a 2D print from a 3D model.

John Horwath, engineer at Wright Laboratory (Dayton, OH), does not use 3D. He uses only 2D for electrical schematics. What he wants: a low-price 3D package (under $2500), and better interoperability between CAD packages.