For the maker of "the ultimate driving machine," quality assurance is more than a process - it defines the entire brand. As part of the company's never-ending quest for quality improvement, BMW has implemented new computer-aided inspection (CAI) technology at its engine-building plant in Steyr, Austria.
The plant builds more than 300 different engine variants on its assembly lines, producing nearly 600,000 a year. More than 900 specialists oversee quality assurance on the plant's production lines and operating units. Core engine-block components, such as the crankcase, crankshaft, connecting rods, and cylinder head are verified by spot checks in BMW Steyr's precision-measurement laboratory, where components must conform to tolerances of hundredths of a millimeter.
Faced with this kind of demand, BMW Steyr wanted to move beyond traditional inspection processes using coordinate measurement machines (CMMs). CMMs collect a sample of discrete points on a part one at a time. The process is slow and does not adequately address surface-to-surface inspection required to verify the accuracy of sheet metal or freeform surfaces. Results are recorded in a 2D geometric dimensioning and tolerance (GD&T) report that does not directly correlate to 3D CAD models of the part.
CAI can greatly increase speed and accuracy of the inspection process. It uses noncontact scanners to collect millions of data points in seconds. Software then processes the information automatically. BMW Steyr hired Westcam, an Austrian technology firm (www.westcam.at), to help the plant bypass the conventional inspection process and generate exact 3D digital models directly from scanned physical parts.
"BMW was searching for a technology to quickly digitize complex freeform surfaces in order to compare them with existing Catia CAD data," says Westcam's Norbert Blaha. The company helped BMW Steyr automate the inspection processes and establish an interactive loop among the design, manufacturing, and quality-control divisions, he adds.
The first step in the new CAI process is to capture accurate geometry and dimensions by placing target points, which are used to align multiple scans, on the surface of the existing engine component. The part is then scanned with a noncontact Atos white-light scanner from German-based GOM (www.gom.com), which generates a polygonal model.
Jagged edges, holes, and surfaces on the polygonal model are smoothed, and the model is cleaned to remove extraneous points or noise that might undermine the data. The completed files are then merged and saved in STL format.
The STL model is imported into Geomagic Qualify, CAI software from Raindrop Geomagic, Research Triangle Park, N.C. (www.geomagic.com). It automatically aligns and compares the STL model with original CAD data to pinpoint variations in the geometry and analyze how deviations might impact the part's functionality.
"We needed simple tools to show us where our parts differed from our designs, which is especially important in areas where engine components must fit together," says Walter Fasching, an engineer at BMW Steyr. The software's feature-based alignment automatically matches the models together quickly and accurately, he adds.
Even miniscule differences between the physical part and CAD data can result in performance flaws and inaccurate engineering analysis, so BMW Steyr imposes tolerances of 0.02 to 0.03 mm. If parts are found to differ from the original design by more than that, they are sent back for reworking.
The Geomagic software automatically generates reports on part discrepancies. Reports can be output in many standard formats, including HTML, PDF, Microsoft Word, and Excel, or as customizable graphics. The reports have proved to be useful tools for suppliers, quality engineers, and others in BMW Steyr's supply chain.
"This process has opened a whole new world for us," BMW's Fasching says. "The first report was finished earlier than expected, so the supplier could go ahead and send the parts for engine production, saving us a lot of time. The design engineers also have better information about the quality of parts, which will ultimately save the company money and produce a better end product for consumers."
Computer-aided inspection generates 3D digital models directly from scanned physical parts.