When General Motors Corp. (GM) engineers want to make sure a vehicle is fitting together properly, or find out why if it’s not, they often turn to a group of five men in Warren, Michigan, who are equipped with the latest in portable coordinate measurement machine (PCMM) and laser scanning technology.

Decades of buyer surveys have shown GM, and the rest of the auto industry, that the way the sheet-metal skins of cars fit together—the “fits and finishes” and overall appearance—is a huge factor in the decision to buy any given car. “As we push to achieve better fits and finishes, we need to constantly validate our tolerance and variation stack-ups,” said Jesse Ortega, director of vehicle architecture and design check.

The team in Warren—GM’s Design Check and Vehicle Assessment team—plays a critical role in making sure the fits and finishes are exactly as designed. As part of the corporate engineering team in the sprawling Warren Vehicle Engineering Center (WVEC), their main tasks are “vehicle integration” and “design integration.” The team constantly checks fits and finishes, helping strengthen and refine virtual builds based on the GM Math Model.

When there is a problem, the design-check team quickly provides precise dimensional data to engineers and designers assigned to virtual design validation. The Design Check team has many other dimensional measurement and analysis tasks, including:
• What-if studies.
• Fielding a steady stream of design-check measurements from the body, chassis and powertrain engineers who are the core of WVEC.
• Examining vehicles from competitors that may have gained an edge in some area.

“Those color maps also show how whether any section of the surface is out of tolerance and, if so, how far out. That kind of data is very handy for stamping-die rework”

John C. Sturgis, Design Check team liaison engineer and team leader, explains that “for us, this is all about the physical integration of as-designed parts, how they fit together, rather than focusing on the dimensional measurements of individual parts.” Those kinds of inspections are always done at the production line—and are always referenced back to the math model. Sturgis is a former product design engineer himself.

In the WVEC, the primary tools for Sturgis’s team are a pair of portable CMM dimensional measurement systems—inspection “arms” with 8- and 12-foot-diameter working envelopes. Built by ROMER, Farmington Hills, Michigan, these arms provide nearly limitless flexibility in measurements. They replace manual methods such as rulers, tape measures and height gages. They also do away with the need to trek back and forth to a stationary CMM, a machine that is usually in an isolated location.

Both of the WVEC’s ROMER 3000i systems are equipped with the aptly named PowerINSPECT from DELCAM Inc., Windsor, Ontario, Canada. PowerINSPECT formats the data gathering, creates the coordinate reference systems, identifies key features, and provides the integration with the GM math model. ROMER provides pre-sales and post-sales support, application assistance, training and on-going telephone support for the complete CMM system.

The actual data is gathered as points with touch-probes or laser-scanners. For measuring large objects, such as the frame or underbody of an entire car rather than, for example, a door with its hinges mounted in its frame, the Design Check team also has a ROMER GridLOK™ system. GridLOK simplifies orienting the CMM to the car body while eliminating the need for “leapfrogging” with its unavoidable cumulative positioning error.

One of the Design Check team’s biggest measuring challenges is the underbody alignment of an entire car. This is a calibration check on the body framing weld-assembly system with which the car had just been built. “This was for a global alignment of the frame, corner to corner,” Sturgis said. “These inspections are what we call the ‘integration build activity.’” The team does not do inspections per se.

Team member Paul Bucilli noted that the practical accuracy of the portable CMMs as the WVEC team uses them is ±0.2 mm or ±0.008 inch. Adds Design Check team member Phil Karas, “this is where the math meets the molecules.”

Sturgis and his team leave nothing to chance. “We align to the vehicle with the GridLOK system to create a full body global alignment, the same as would be done with a fixed-axis CMM,” he said. “We use the body-build datums on the vehicle and if we need more we can glue on additional datum targets.

“Once we get all this set up with GridLOK and its data plotted into the car-body coordinate system with PowerINSPECT,” Sturgis said, “we take the car out into the shop where we can lift it up and get underneath to measure exhaust system locations, placements of bumpers and bumper fascia, and other parts of the underbody.”

“The latest version of PowerINSPECT combined with laser scanning [also known as laser probing] is very good for this kind of work,” added Jon Sajewski, also on the Design Check team. “The software preserves the body coordinates and the arm’s reference to the vehicle. That means we can use the same vehicle reference and coordinate systems even though these measuring jobs require two separate layouts.”

Another Design Check challenge is sheet metal fit-ups between doors, door frames and body pillars plus hinge alignments. Here the ROMER arm, working with the math model data downloaded with PowerINSPECT, is used to check a rectangular array of lines a few inches apart on all the exterior parts for accuracy. These kinds of measurements are often referred to as a “gap and flush” check but they are all part of GM’s “Class A” surface inspections for fits and finishes.

These scans are often color mapped, too, to show deviations from nominal as the surface is described in the math model or in a supplier’s CAD system. “Those color maps also show how whether any section of the surface is out of tolerance and, if so, how far out. That kind of data is very handy for stamping-die rework,” Sturgis added.

A frequent Design Check Team task is competitive analysis. In a recent project, Sajewski and another member of Sturgis’s team, Greg McDonald, touch-probed and partially digitized the front passenger compartment of a European-made luxury car.

McDonald explained: “We used the laser scanner to handle volumetric data generated from scanning hundreds of points all over the front of the passenger compartment” including the dashboard, steering wheel and the reach to the controls.

What GM gets from these efforts at the WVEC is assurance that vehicle bodies are fitting together as designed and, for those rare times when they don’t, reliable and easily understood dimensional data that points to where the problems lie. And in this context, the location of the Design Check team’s facility is significant: In the basement of GM’s largest corporate engineering facility.

“Almost everything the Design Check and Vehicle Assessment team does is tied to appearance,” said Jeffrey Holland, GM Design Center communications manager. “This is about what the appearance of our cars communicates, how the cars communicate it, and how they look when they communicate it. This is all very important to us. GM has a legacy of great designs and great designers. We are working to keep that legacy strong.”