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GM's Metal Fabricating Division Stamps its Approval of CAD/CAM

Over the years, General Motors has worked on replacing its manual, paper-based design process with a math-based, electronic system. At the company's Metal Fabricating Division, the benefits are fantastic.

General Motors Corporation (GM) has been designing die stamping tools for, oh, about 80 years. Up until a few years ago, the process started with full-size car body blueprints and product drawings tacked onto a wall. The wholewall. A roof outer flange die, for example, could easily involve 35 drawings, each 20-ft. long. These paper drawings begat more paper drawings—and hand calculations as tipped parts and dies were designed and traced in. Tip parts are the cams that transfer the vertical motion of the press into angular or horizontal motion in the die.

Along the way, clay models of vehicle parts begat "master models" out of wood. Wood models begat plaster casts. On these, "addendum" surfaces—such as unfolded flanges—would be added. Tracer milling, a process similar to cutting a spare key at the hardware store, used the casted tooling aids to create a lead-alloy based soft die (e.g., kirksite die).

"We made those for every die," says Mark Stevens, General Director of Dies at the Metal Fabricating Div. of General Motors (Troy, MI). "Now we no longer make those soft dies. Instead, we prove out all of our processes in the math system."

development from part design to finished panel piece
This series of pictures shows the development, from part design to finished panel piece, of the hood outer for the new Buick LeSabre. This screen display shows the Unigraphics part design for the hood outer for the new Buick LeSabre. Included are the developed flanges, trim line, and the addendum and binder surfaces. This file is used to generate the DNC cutter paths for the contoured or sculptured shape surfaces, as well as to profile around the trim line. (Source: Metal Fabricating Div., General Motors Corp.)
Lower half of solid model
This view is the lower half of the solid model for the die to stamp out the hood outer (without the binder). (Source: Metal Fabricating Div., General Motors Corp.)
upper half of die design
The upper half of the die design for the hood outer. (Source: Metal Fabricating Div., General Motors Corp.)

Math Stamped Into Metal

The math system Stevens is talking about is the Unigraphics computer-aided design (CAD) system from Unigraphics Solutions Inc. (Maryland Heights, MO). Unigraphics is used throughout GM to design parts. But, explains Stevens, Metal Fab is not designing parts per se. Instead, the stamping engineers are using the math data produced by the stylist, then applying special programs to analyze these designs for formability. Unigraphics is then used to design the stamping tools, design the castings to make those tools, and generate the associated machining programs.

To make the new dies, the stamping engineers begin with "old" die designs. They first look at the last five die designs for that product in the division's technical library. This library contains feedback from the die shops and stamping plants about constructing dies or producing parts from the stamping tools for previous car designs.

The engineers then apply what they learned to the standard design geometries associated with the new part designs. What results is a B-spline surface model of the new stamping tool.

What used to be contained in 20-ft. drawings is now displayed as a complete solid model from any of the 275 17-in. graphics workstations in the Metal Fab engineering group. As required, a die engineer can cut sections all over the model for different views. "When you're designing solid models, you're actually viewing the object, not a representation of the object. It can be articulated, moved around, sectioned, and opened up," says Stevens.

The solid model includes detailed information, such as the thickness of the part. This thickness eventually becomes the offset in the cutter path for making the upper and lower dies, namely the cavity and punch sides of the dies. The model also provides the basis for calculating die weight. GM used to calculate that weight for the foundry, which needs to know how much iron to prepare for the die mold. No longer. Unigraphics does all the calculations.

As required, the stamping engineers use special software to pick parts, unfold flanges, and build the upper and lower binder surfaces that clamp the sheet metal while it's stamped—and stretched—into shape. The software also helps the division design cutting dies, cam flange dies, and piercing dies to do all the secondary operations.

Die and product engineers use finite element analysis software to test the formability of the die and the die stamping process itself. For example, the software can tell the engineers how much springback the metal part will exhibit. This helps determine whether a flat metal sheet will tear or wrinkle as it's stamped into a finished part.

The solid models are then transmitted to the five tooling centers, where tooling is made using a lost foam process. At the tooling centers, Unigraphics computer-aided manufacturing (CAM) modules generate the cutter paths for cutting the finished castings. The resulting cutter location (CL) files are downloaded to direct numerical control (DNC) machine tools. No paper drawings are involved in this process. Moreover, the dies themselves are virtually untouched by human hands as they go from DNC tool to assembly into a stamping press for tryout. (The only hand work done on those dies is for minor polishing, such as to smooth out some radii, according to Stevens.)

PC-based view-and-markup workstations on the plant floor in the die shops let people look at the solid models. "This lets us work without having paper drawings," says Stevens. As needed, die and product engineers, and machinists can "cut" sections of the model, rotate it, and get dimensions of whatever they need from the model. Granted, the design files are huge, measuring several dozens of megabytes, making viewing and rotating models painfully slow. So, in the view-and-markup system, the user is actually seeing a shaded image of a surface model, which is really an extract from the master solid model database. In the CAD/CAM room, where there are graphics workstations, the engineers are interacting with the real solid model data.

This die design process is repeated about 1,500 to 1,800 times a year. That's how many stamping tools Metal Fabricating Div. produces in a typical year for all of GM's North America assembly plants. In a big year, such as 1996, when many of GM's vehicles changed, Metal Fab made 2,200 new stamping tools.

The stampings produced with the dies the group creates include structural parts, rails, floor pans, part pans, as well as inner panels and skin panels (such as the doors and hood). Typically, the major panels, such as doors, roofs, or hoods, require three or four dies. A hood outer, for example, requires a draw die, a trim die, and a flange die. The largest die Metal Fab has designed weighed 40 tons.

Top-Down Approach to Development

The design approach being taken by the people in General Motors' Body Development and Portfolio Operations is going from assemblies to parts rather than from parts to assemblies. This is contrary to a traditional approach, which is, in effect, accumulative: the sum of the parts should equal the space of the whole. This GM approach is top-down, not bottom-up.

But what they are doing now, utilizing UG/WAVE software* from Unigraphics Solutions in vehicle development, is creating control structures and interface parameters for the modules that go into creating a total vehicle.

The control structures serve as a framework for the various elements that are needed to create the assembly. The interfaces for these modules help assure that there will be a fit between meeting modules in the entire assembly, or body-in-white, that there won't be interferences. Given that different groups are designing different modules—and it is likely that they are working in isolation (or they may conceivably be working for a supplier company)—one problem that can arise is that when their work is seemingly completed, one group's design may not work with another group's. Consequently, more design time is needed. Looking toward accelerating product development time, this lag is unacceptable.

Although the initial approach required to create the control structures and the interface can be on the order of weeks into months, according to a GM spokesman, the advantage is that these structures will become elements of a library. The libraries of math models can then be used for future programs, so that the designers won't have to "reinvent the wheel" (or the floor pan, toe pan, quarter panel...) from project to project. So the initial design time is, in a sense, an investment. Done once, it doesn't need to be repeated.

In cases where there are product design changes, then this can occur at a high level and then the different interfaces and criteria can be driven down to all of the project teams.

Bottom line: "What used to take days can now be reduced to minutes," according to GM's Roy DeBrabant, director of Design Process & CAD/CAM Integration.—GSV

Just Think of the Trees That Died for These Cars

"We used to make physical things just to make another physical thing," explains Stevens. "We made templates to make wood models to make plasters to make...

"We were never making the actual object until we were all done."

Now, the Metal Fab "stamps" parts electronically before any die is built. "We can adjust surfaces and springback, and change cam angles interactively within the solid model. All the prototype developmental work is done in math. We go from that math model right to the final die surface that's going to stamp the sheet metal part," continues Stevens.

One result of this is "incredibly accurate" dies compared to the multi-step manual process involving drawings, various models, and a couple of cutting processes. "Tolerances would build up in every step of that process. And when we were all done, we wanted this part to be like the original master mylar? If it was, it was by luck." Recently, on a 2000-year car model, Metal Fab witnessed a reduction in manufacturing-driven changes in the production tooling—a direct result of having confirmed the new designs before their release.

The math-based approach also lets GM add "more exotic sculpture to the vehicles," says Stevens. For instance, exotic cut lines can now appear between fenders and doors, front doors and rear doors, fenders and the hood. "We can put those cut lines in where in the past everything wanted to be square and straight because it was simpler to match up. It doesn't matter anymore because with DNC processes based on a math master, there is no stackup of tolerances."

GM also gains tremendous savings. Math-based CAD/CAM cuts 16 weeks out of the development process—along with a significant investment in physical proof tools, temporary tools, and other types of prototypes.

And GM gains physical space. Ten years ago, GM needed a 35,000-ft2warehouse to house all of the original design and engineering drawings for dies. The die drawings had to be archived for 10 years after a part came out of production. By the way, product engineering had its own warehouse for the product drawings.

Less than five years ago, the warehouse for die drawings was down to about 10,000 ft2. Today, it's less than 500 ft2. The only paper drawings archived today are of old tools that are producing service parts. Otherwise, all of the die design drawings are CAD based—archived and on-line so that the die design engineers can access them.

Reflecting back on the physical-based die design and production process versus the math-based approach, Stevens chuckles at the night-and-day contrast. "It took huge square footage. It took huge facilities. Hundreds and hundreds of people that were doing things that we don't even do anymore.

"Today, math-based CAD/CAM has evolved where it almost doesn't matter if you're a product engineer, a stylist, or a manufacturing engineer: you're all interacting graphically, using one system, the same type of hardware, and looking simultaneously at the same object."


*Unigraphics has extended the capabilities of UG/WAVE to die design. In this application, a model is created that includes knowledge of die designers. This model is referred to as a "die smart model." The key benefit is time. Once a model is created, it can be quickly modified to meet specific part requirements. Quickly means updates in minutes. So it is possible for die design to begin prior to the completion of die engineering—and perhaps even before the body panel is completed (i.e., a die smart model can be made for a class or size of vehicle, so it is possible to start working in the general space that the hood or quarter panel or whatever fits within). Dave Shook, Unigraphics Solutions Automotive Business Unit director, observes, "Die development represents one of the longest lead-time components of the entire automobile development process. A savings of even a few days would have a substantial effect, but the technology we're offering goes far beyond that."