Meeting the Challenges of Modern Body-In-White Inspection

Dirk Noffke, a metrology technician at DaimlerChrysler Bremen, inspects the dimensional accuracy of the left fender on a Mercedes SLK body-in-white. The comparison between the actual and CAD data is displayed in real time on an application PC running the PolyWorks Inspector software.

The almost complete absence of people is the first thing you notice while walking the endless corridors of DaimlerChrysler’s ultra-modern Bremen plant. Hundreds of robots go about their work with surgical precision. They move different sheet metal parts into place, weld them together, transport completed parts onto the next station, and transfer finished products overhead to maximize space utilization. Viewed from afar, the ubiquitous robots seem to be dancing. The few factory workers you do see use “old school” bicycles to jet between different parts of the factory: high-tech and low-tech harmoniously coexist here side-by-side. Many more daring body shapes can leap from the automotive designers’ drawing board to the assembly line due to significant technological progress in manufacturing processes. In the not-so-distant past, cars had relatively simple, square shapes, with individual parts fitting to one another along straightforward lines. On the other hand, modern cars boast far more complex shapes than the cars of yore.

The task of inspecting how automotive parts fit together has also become increasingly complex. DaimlerChrysler’s body-in-white (BIW) rollout and process optimization department is entrusted with the job of verifying how well the various body parts fit to one another. More importantly, based on their measurements, feedback is given to the production facilities so that targeted adjustments can be made on the tooling.

Looking for a perfect fit
Operational engineer Henning Siemers is involved in BIW measurement and inspection tasks. He explains, “Our job is to assure dimensional accuracy of the entire body-in-white. Of course, every BIW consists of many individual parts, and they must all fit perfectly. Our primary duty is to inspect the entire BIW, both individual parts and the whole vehicle, including the panel gaps, and then analyze the data we get. Based on the analysis we perform, we then go back to the production facilities and make adjustments to the tools and make sure that what comes out is an automobile whose parts fit together flawlessly.”

In the past, Siemers’ department has relied on hand-operated measurement instruments. However, the complex shapes of today’s cars and the ever-increasing tolerances have pushed such instruments to their limits. Siemers continues, “Viewed in retrospect, cars used to be made of almost square-like segments which used to fit nicely together without much hassle. Nowadays, cars are much more curvaceous, with round areas suddenly changing shapes and meeting other panels at all kinds of angles and lines. Plus, the much higher requirements for the nearly seamless fit between individual panels mandate that we work with very tight tolerances.”

Laser tracker–hand scanner combo delivers

Deviations from CAD data are visually displayed using the PolyWorks Inspector software, with deviations shown either using a color palette or vectors. The information is analyzed in order to yield correction instructions, which are then implemented directly on the tools.

“With the shapes of our vehicles getting more complex, we have been on the lookout for new technologies that deliver,” continues Siemers. “Our quality engineer, Karl-Heinz Boecker, has been attending the CONTROL trade show in Sinsheim, Germany, each year, and he had his first encounter with the Leica T-Scan system a couple years ago, when it was still just a prototype. Based on the announced specs, we knew this could be something for us, but we wanted to wait until we were positive that the product was truly ready for the market. The primary issue that we saw with many of the other scanning solutions out there was that the software could not keep up with the hardware. Collecting all those points is relatively simple, but what do you do afterwards? We were under the impression that many manufacturers did not award the same attention to the software as they did to the hardware.”

Boecker’s team arranged a product demonstration at their facilities. An LTD840 Laser Tracker from Leica Geosystems (Miamisburg, Ohio), coupled with a Leica T-Probe and Leica T-Scan, was chosen. The new system delivered. “It was immediately clear to us that the hand scanner and the PolyWorks software were a mighty combination. We already knew that the scanner was a great sensor, and we were also shown that the PolyWorks suite is a worthy match for it. The software is very capable, and we knew we would be on the right track.”

Change is good
Since the system delivery, the multitasking system has been used, among other things, to measure panel gaps, examine part curvatures, inspect reference holes, and similar tasks. Siemers has also been busy testing different applications for the hand scanner. “One of the new applications we have found is inline calibration. In the past, BIWs were inspected by being placed inside a grid station and measured using conventional CMM equipment. When you have 150 pallets and want to measure with high tolerances, you’re quickly reaching the limits with traditional methods. That’s why we wanted to be able to measure linearly, both the pallets and the tools, and this is where the Leica T-Scan will come into play. Another application of interest to us is the inspection of the entire robot path, and this is something we plan on doing in the near future.”

A Leica Laser Tracker follows the position of the Leica T-Probe, accounting not only for the 3-D coordinates of the “Walk-Around” probe but also keeping track of its pitch, yaw and roll. The position of the reference holes in the A-pillar is being checked. Outer sheathing will later attach to the holes located in the A-pillar.

Quality information = Good analysis
When asked about what has changed since the implementation of the metrology system, Siemers is very specific: “By relying on scanning, I am in the possession of much more useful information. The quality of part analysis has improved. We are able to get to the root of a problem much more quickly, therefore coming up with targeted solutions for what needs to be changed, for example, intervening at the tools at such and such point and changing the parameters based on the results seen in PolyWorks.”

The benefits of using a mobile CMM are manifold. “We really appreciate the portability,” says Siemers. “With the Leica equipment, I can gather all the information I need in just two or three hours and perform the analysis later. This way, I’m not blocking production for too long because the system lets us work in parallel. While one operator is taking the measurements, another one can already be working on sorting out the results.”

The modularity of the system is important to Siemers’ team. “We purchased a Leica T-Probe at the same time we got the rest of the system. Tactile wireless measurement is of great value to us when we measure easily twistable parts like the hood. We perform a quick measurement with the wireless probe to get a rough idea about the condition of the part. If we were to start scanning immediately, it would take longer to realize that the hood is twisted. With the probe, collecting just a few points will let us know fairly immediately how we are doing. And we are performing this within the same software, merely switching between the probe and the hand scanner.”

The Leica T-Probe/T-Scan combination measures within a range of 15 meters, thus giving the technicians a measurement volume of 30 meters. If measurement requirements should change in the future, Henning Siemers and his team are well-equipped for operating in a measurement volume large enough to accommodate several BIWs at once.

About DaimlerChrysler Bremen
DaimlerChrysler has a product portfolio including microcars, performance coupes, sedans and station wagons, versatile vans, and commercial vehicles. The second-largest DaimlerChrysler plant in Germany is the Bremen plant, which employs nearly 14,000 people and has a floor space of 850,000 square meters. The SL and the SLK roadsters as well as all versions of the C-class (sedan, station wagon, coupe, and convertible) are produced there.

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