Flexible Measuring Arm Expands BMW's Measurement Process

A portable, lightweight measuring system helps in the production of BMW 3-Series cars—out on the production line itself.

"Bimmers" are truly enjoyable cars, and for good reason. The Bavarian carmaker, BMW, takes pride in producing consistently smart, well-engineered automobiles. As a result, BMW has a following worldwide among both the moderately and the genuinely wealthy.

Part of what makes a Bimmer a high-quality car is that from stamping to final assembly, high-precision measuring technology ensures the accuracy in manufacturing these vehicles. Yet a year ago, BMW focused on updating the metrology processes for producing its 3-Series cars at its Regensburg plant in Germany. Why? Too many measuring applications existed right on the plant floor and could not be carried back to the quality control (QC) department. Plus, many of these applications, particularly on the production line, were beyond easy access by conventional coordinate measuring machine (CMM) systems.

Given these realities, BMW aimed for flexible and mobile metrology. The automaker got both by adding a portable articulated measuring arm to its arsenal of metrology equipment.

Now to verify part design and quality, BMW's QC department brings its measuring equipment to the parts and assemblies on the production floor. This yields obvious time savings in responding to part and assembly production problems. It also ensures the production of high-quality parts, proper fit and finish, and ultimately to high-quality cars.

QC integrated with production
BMW's 3-Series automobiles debuted in 1998. The series includes passenger cars, convertibles, touring cars, 4-wheel drive vehicles, and the M3 motor sport cars. Produced in Munich and Regensburg, the car consists of 2,700 part numbers. About a fourth of these parts are manufactured in-house; the rest come from almost 500 suppliers. About a third of the total value of purchased parts come from outside Germany.

The Regensburg plant is an automated, just-in-time operation located about 70 miles north of Munich. It includes a new, $320-million body shop and a stamping plant that was updated for about $90 million. About 475 robots weld the car together in approximately 5,000 spots.

The 134 people in the plant's QC department monitor this entire production process—from individual parts and subassemblies to finished vehicles. This is not an after-the-fact monitoring process; BMW's QC process begins at the very early stages of a car's design. Officially called, in English, the Functional Measuring Concept (FMK), the process ensures the protection of tolerances and FMK's universal applicability to product development.

To do this, QC personnel work with all applicable design and manufacturing departments in specifying permissible tolerances as well as FMK values for functionally critical points in a car's production. These points provide the basis for an applicable measuring protocol, including the specification of the necessary measuring equipment to meet that protocol. BMW's suppliers are also required to adhere to this protocol, thereby expediting the quality verification of components.

To guard against changes in the dimensional accuracy of parts while assembling the new 3-Series car, QC began reviewing its approach to metrology, especially the measuring equipment. In the past, QC measured and verified automotive parts in a facility separate from production. While the facility's QC focus was a plus, transporting parts to and from QC and production was time-consuming and costly, it increased the potential for part damage, and the separation created a wall between QC and production.

A better approach was necessary—one that brought QC to where the action was: the production floor. In fact, mobility became a key consideration in choosing new measuring equipment because of the numerous locations in production with metrology requirements, such as for assembly fixtures, assembled modules, and finishing operations. Among the various options available in metrology equipment, BMW saw mobile measuring arms as a practical alternative to the fixed, stationary measuring systems in the QC facility.

Going mobile
BMW-Regensburg selected the FaroArm Gold 12 CMM system from FARO Technologies, Inc. (Orlando, FL). This FaroArm is a lightweight, portable, multi-axis measurement arm that can reach up to 12.14 feet (3.7 meters). Weighing just 31 pounds (14 kilograms), the measuring system can be moved to and set up for immediate use anywhere in the manufacturing plant within an hour.

The FaroArm uses precision encoders to accurately measure the exact position of the arm's probe tip, regardless of the user's approach to the measured part or assembly. BMW acknowledges that the arm is not as accurate as a stationary, computer-controlled CMM; however, at an accuracy of ± 0.0078 inch (± 0.2 mm), the arm is more than sufficient for most of BMW's applications. Patented internal temperature compensation negates the necessity for climate control.

Another attraction of the FaroArm is its costs, primarily its one-time acquisition cost. While CMMs typically start at $250,000, the measuring arm costs about $50,000—an increasingly worthwhile investment in light of the measuring arm's capabilities.

The measuring arm is simple to use. The arm is moved on a special transport cart directly to the measuring site. There, the arm is readied, which is mostly locating where it is in space. Because the coordinate system for vehicle production is located in the subfloor, and therefore not readily accessible, the FaroArm uses a local alignment system to determine its location. The arm's coordinate system is based on predetermined points on a vehicle that were defined during vehicle development and duly noted on the applicable drawings contained in BMW's Catia computer-aided design (CAD) system.

By using the point-and-click touch probe on the measuring arm, critical known points and surfaces are touched, their location in known space captured. BMW's standard surface measuring software compares these actual coordinates against the standard values stored in Catia. The software computes deviations from the standard value and presents them immediately when the touch probe contacts the measured surface.

These computations can be done on-line through the plant's local area network (LAN) or, if no local access exists to the LAN, through the arm's German-based VDAFS neutral interface to digital versatile disc (DVD) storage that contains the CAD solid model. In either case, the requisite solid model data is always available at any measuring location.

The results—the deviation data—helps in accurately determining the source of potential manufacturing problems.

In operation
Because QC is using BMW's standard measuring software, little additional training was needed for using the FaroArm. As a result, the FaroArm was soon introduced throughout the plant.

One quick money-saving hit with the FaroArm involved eliminating a stubborn fit on the 3-Series convertible. In production, convertible hardtops have to fit any convertible body coming down the line. Equally important, fit and wind noise levels have to meet BMW's strict standards. Unfortunately, while hardtops were easily installed, they were not tight in all cases. To solve this, remembers Manfred Huber of BMW-Regensburg QC, inspectors set up the FaroArm on the master body-in-white to examine and check the hardtop's mounting hardware, the matching components on the car, and the sealing channel for the windows. They then used the FaroArm to examine the window and hardtop fit on twenty finished vehicles. Last, they used the FaroArm to check the location and security of the attached mounting points for the hardtop.

The results were unmistakable: The prepositioning of the car's side windows was causing production problems. QC was able to optimize the prepositioning of these windows in such a way that not only solved the fit problem, it also became a better way to mount the hardtop.

Another measuring application involved checking the location of headlights in the car's front-end subassembly. Obviously, the subassembly could not be brought to the QC department. Instead, QC and its metrology equipment had to go to the assembly line. Using FaroArm, headlight location is now verified at the final assembly site. BMW already has plans to use FaroArm to measure other aspects of a vehicle while it's in production and relatively inaccessible to measure using a CMM.

The FaroArm also helps BMW's suppliers. Many suppliers bring their parts to BMW-Regensburg for verification by in-house QC personnel. The flexibility of the FaroArm lets QC measure these parts inside the master body-in-white. In so doing, QC and the suppliers can easily determine if errors or out-of-tolerance situations exist in the supplier's part or the BMW design.

Huber sees additional uses for FaroArm, especially for measuring gaps and seams. At present, many of these are checked visually or with handheld feeler gages because fixed touch probes have great difficulty in properly locating the highest point of an edge. In the case of a seam, the tip of the touch probe may be too big to make accurate contact; precise seam dimensions can now only be measured with CMMs. The current FARO approach to this problem uses a touch probe adapter produced by Renishaw for high-end CMMs. Huber hopes eventually to outfit a FaroArm with a laser scanner and camera to automate and more accurately measure gaps and seams right there on the production line.

The consistent theme in all of these measuring applications is having the flexibility to control dimensional quality throughout a manufacturing plant—and not just in a special QC room. BMW got that, once it chose a portable, measure-anywhere CMM tool.

FARO Technologies Inc., 125 Technology Park, Lake Mary, FL 32746-6204. Tel: 800-736-0234; Fax: 407-562-5221.