Quality control for the series production of home appliances at Bosch and Siemens Hausgeräte GmbH (BSH) in Traunreut, Germany, used to be based on coordinate measuring machines (CMMs). Last year, the tactile measuring system was replaced by optical 3-D measuring technology from Steinbichler Optotechnik in Neubeuern, Germany. This increased the reliability of the production process and reduced the costs.
“Can you also precisely measure and analyze stamped sheet metal parts fully automatically with an optical system?” This is what instrument engineer Paul Spiel of Bosch and Siemens asked the staff of Steinbichler Optotechnik back in 2006. The seemingly simple question marked the beginning of intense cooperation between BSH and Steinbichler. Within one year, three robot-assisted, 3-D digitizing systems were installed in the measuring room of BSH’s’ central measurement and control department, in the press shop, and in production.
The Traunreut plant manufactures electric cookers, ovens, hubs, combination microwave ovens, and electric kettles. BSH had scheduled the production start of a new electric cooker series with increased design requirements for the second half of 2008. The cookers feature smaller clearances and offsets compared to the previous models. This requires reliable, high-accuracy measurements with individual tolerances of up to ±0.05 mm within the tolerance chain.
For the manufacturing ramp-up and the subsequent quality control monitoring of the series production, it was necessary to increase the measurement and control capacities. So, the quality assurance staff set out to tackle this issue in early 2007. The easy way out would have been to expand the existing tactile measuring system by adding a few more coordinate measuring machines. This would have meant known machinery, known procedures, and known technology. But it would also have meant no new insights, no improvements in process monitoring of series production, and a technological standstill at BSH.
After intense discussions between the two project teams, and after extensive accuracy and repeatability tests, including gauge repeatability and reliability (R&R) testing, Christoph Schmitt, BSH’s head of quality assurance, and the project team opted for the COMET 5 4M optical 3-D measuring system based on white-light fringe projection.
The decision was based on the following key criteria according to BSH project manager, Thomas Rychtarik:
• The high measuring speed saves a significant amount of time. The measurement results are available more quickly and allow an earlier correction of the process parameters in production (e.g., in the press shop) if deviations occur. The components are captured completely three-dimensionally. Two-dimensional deviations from the nominal dimensions can also be easily represented in this way (for example, contact surfaces for self-adhesive profiles or mounting frames).
• A 3-D data set is stored for each component. Any measuring point on a component can thus be inspected again at any time, without having to mount it in the measuring machine again.
• The system uses a noncontact optical measurement process. It is therefore ideal for measuring bonded components because it allows checking them for geometric deviations (e.g., position and tilting) before the adhesive has fully cured. Corrections to the bonding process can be made at an early stage.
• The high absolute measuring accuracy and repeatability achieved by the COMET 5 sensor system ensures reliable measurement data acquisition within the required individual tolerances. The tests for gauge R&R and test process capability have been successfully completed.
All three inspection systems have the same configuration and feature a COMET 5 white-light fringe projection system equipped with a 4-megapixel camera and a type 200 field of view (figure 1). The sensor is positioned by a Kuka robot with extension arm. The component to be measured is placed on a Kuka rotary table that is controlled as a seventh axis by the robot controller. The surface of the rotary table has a reference-point list based on photogrammetry measurements. This list is used for globally matching the individual 3-D images of the component surfaces.
As all three installations are configured with the same positions for rotary table, robot, and calibration plate, they can all run any measurement program created for the different components. This redundant design guarantees a high availability of the measurement, control, and test equipment. If one system cannot be used because of maintenance work, for example, the measurements required for quality control monitoring of the series production can be performed on the other two systems.
The first inspection system was installed in the measurement and control department, which has a specially prepared room for high-quality measurement tasks, including:
• Solving urgent problems by capturing the entire component surface
• Programming the measurement programs for the other systems (systems two and three)
The measuring room with system one is the central unit of the installation network. In other words, it is the master system on which the staff creates the measurement programs and test instructions for all components to be tested. This includes, for example, the component position on the rotary table, the robot paths, the rotary table position, the sensor position with reference to the component, creating the macros for the inspection software, creating the reports, and integrating the data in the BSH quality management database.
The second inspection system is used for the acceptance tests of sheet metal parts in prefabrication. It has been set up in a separate air-conditioned room in the press shop, about 15 m away from the eight presses (figures 2 and 3). Due to vibration in the press shop floor, the robot (with the sensor) and the rotary table had to be placed on a solid concrete slab.
The increased expenses for this construction are compensated for by allowing very short paths for the press shop staff. The second system is mainly used for the following tasks:
• Testing the first and last parts in the prefabrication plant
• Process capability studies from the current series production
• Serial measurements to solve production issues
The second inspection system has another special feature. As soon as the bar code reader (built into all three systems) has read the part number of the test sample, a separate monitor above the rotary table displays the component-specific adapter configuration. The test engineer takes the displayed adapters from the storage box underneath the monitor, sets them up at the indicated positions on the rotary table, positions the test component, and starts the automatic measuring procedure (figure 4).
The third system is used almost exclusively for the acceptance tests of bonding processes. In cooperation with developers from Steinbichler, a team headed by BSH quality assurance representative Florian Huber has worked out clamping concepts and measuring strategies that allow optical measurement of metal profiles bonded to glass surfaces while the adhesive has not yet cured, and without pretreatment by spraying. The key tasks of the third system include:
• Checking the dimensional accuracy of the glass bonding process
• Process capability studies from the current series production
• Serial measurements to solve production issues
The third inspection system also has a bar code reader for reading the component numbers. At this system, the users wear gloves to avoid fingerprints when handling the glass panes.
Figure 1: Instrument engineer Paul Spiel checks the test result on system one.
Figure 2: System two is located in the press shop.
Figure 3: Eight presses are in operation near system two.
Figure 4: The setup of the adapters is automatically displayed for the test engineers.
The measurement system network of automated optical 3-D digitizing systems turned out to be money in the hand for BSH. Particularly the drastically reduced measurement time per sample, the availability of additional surface information in the test report, and consequently, the increased process reliability in the manufacturing process reduced not only the cost per test, but also waste.
“The Steinbichler solution pays for itself every day,” says Christoph Schmitt. The included COMETinspect inspection software also provides a viewer application with which the design staff can perform nominal-actual value comparisons (measurement data for CAD data) on their own computers. “Just gathering measurement data and reports won’t do. We need a statistically substantiated basis for our decisions,” says Schmitt. This requirement is met by providing an interface between the Steinbichler software and the Q-DAS quality management software.
Steinbichler Optotechnik develops, manufactures, and markets highly precise optical metrology systems with corresponding software solutions for a wide range of applications including 3-D digitizing, surface inspection, tire testing, shearography-nondestructive testing, and deformation & vibration analysis. Steinbichler’s Optotechnik’s technology partners include API Inc., EOS, Geomagic, Innovmetric, INUS/Rapidform, Leica, Metrologic Group, and Tebis. Steinbichler Optotechnik’s hallmark is cutting edge technologies that are also user-friendly, efficient, and resource saving.
