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The Volkswagen (Wolfsburg, Germany) foundry in Hanover, Germany, produces more than 1.5 million cylinder heads and 1.3 million intake manifolds each year for Volkswagen, Audi and Bentley engine manufacturing. In Hanover, these aluminum engine parts are manufactured using the ‘hot box’ sand core production method.
Nonstop use of the same casting molds in the manufacture of thousands of sand core units inevitably leads to slight wear of the molds. To monitor the quality of sand core production equipment, Volkswagen quality assurance engineers regularly verify the geometry of sand core parts. They subject a predefined number of sand core samples to full 3-D geometric inspection. A coordinate measuring machine (CMM)-mounted Metris (Brighton, MI) LC50 laser scanner scans the entire surface of the parts, capturing tens of thousands of point measurements per second.
High-Speed, Large-Part ScanningThe Metris LC50 laser stripe scanner was developed specifically for scanning large parts at high speed, and the complete inspection-to-reporting procedure takes about 30 minutes for most free-form parts. The interface between the Metris laser scanner and the CMM is a five-axis Renishaw (Hoffman Estates, IL) PH10. This unit allows the laser scanner head to be positioned at different angles, providing access to difficult-to-reach surface sections.
Volkswagen metrology engineers use Metris Focus software to define the CMM-operated path the scanner follows during the scanning procedure. Engineers either program the scan path online at the CMM or offline on the basis of the computer-aided design (CAD) representation of the part. Compared with tactile probe programming, scan path definition for a CMM-mounted laser scanner is fairly straightforward. This is because the CMM only needs to move the scanner from one point to another to perform a partial scan, following a linear or polygonal motion path. The result of a sand core scan is a cloud of millions of measured points, which represents a digital 3-D copy of the scanned surface.
After performing point cloud processing actions-such as point reduction, meshing, shading and feature detection-Volkswagen engineers align the measured sand core surface to the original CAD model. When the measured and nominal sand core shapes are aligned for best fit, an automatic comparison is run between both.
As color-coded areas indicate sections that are out of geometric tolerance, surface irregularities immediately attract attention. This enables engineers to analyze the wear of the casting molds that produce the sand cores. The information is saved as part of graphic inspection reports, which can be shared among internal and external parties.
Easing Quality Assurance“Metris laser scanning is very effective in evaluating the geometric quality of the entire sand core parts,” says Frank Jeltsch, metrology technician at Volkswagen in Hanover, Germany. “The sand grain surface makes it difficult to perform reliable touch probe measurements. Therefore, noncontact Metris technology is ideally suited to support efficient and accurate quality control on sand core structures.”
The Metris laser scanner offers measurement accuracy of approximately 20 microns, far beyond the 200-micron diameter of a sand grain.
“This digital graphic approach enables us to really capture the texture of the sand core surface and detect the slightest degree of wear to the tool that forms the sand cores,” says Jeltsch. “Now we literally see the entire shape of the part, and with part-to-CAD comparison we are able to drill down to consistently trace local geometric surface imperfections. This kind of quality assurance is an important asset in maintaining high-quality manufacturing of sand core and aluminum parts.”
Besides offering a more comprehensive level of geometric quality assurance, the Metris laser scanner has made quality assurance more efficient. The tactile inspection process previously used delivered satisfactory results only after much work. Tactile measurements only provided geometric information for selected inspection locations, as the touch probe was unable to access all areas. Based on this partial information, serious effort was required to numerically detect surface imperfections.
Improving Production MethodologyLaser scanning also helped to establish improved sand core production methodology. After deploying Metris laser scanning, Volkswagen quality assurance engineers identified slight bending in sand core structures that produce engine water jackets. Further investigation revealed that water jacket sand cores had the tendency to bend slightly during sand core production.
“Laser scanning results were indispensible in tracing the bending,” says Jeltsch. “And during the evaluation of potential solutions to overcome the bending, laser scanning helped us pick the solution that yielded the best results.”
The process change resulted in accurate and straight engine water jacket sand core units. “In this way, sand core prototype geometry can be optimized on the sand core level rather than on aluminum parts produced on the basis of sand cores,” says Jeltsch. “During a longer period, this avoids iteration cycles during the aluminum casting stage, translating into reduced machining scrap as well as considerable savings in terms of cost, time and effort.”