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When a joint venture to make a revolutionary small car results in a body design sourced from five different suppliers, it takes a robust measuring system to ensure zero-defects assembly.
Achieving such consistently high accuracy in a zero-defect, mass-production environment has raised the bar for both manufacturing and quality control in the automotive sector. Each SMART car comprises 110 stampings sourced from five different suppliers. A total of 137 robots are involved in bonding, sealing, and making 2,455 spot welds per body; and these, as well as every device in the car’s 99 production stations, must be fine-tuned to ensure the reliable manufacture of 750 car bodies each day. According to Jean Saling, Magna’s quality manager, “Without a processd integrated and flexible quality control system, body production involving such complexity and precision could not be maintained reliably. Two laser-based cells assist us in warning if dimensions are drifting out of tolerance, but there is one thing these noncontact scanners cannot do: measure.” That is the task set for the two CMMs brought in by Magna from British manufacturer LK MSI. The HC-90 twin-column machines have measuring envelopes of 10' × 4.0' × 5.3' with a resolution of 0.00004" in each axis. They operate with an acceleration of 13,000 ft./min.2 to a top speed of 128 ft./min. One machine is located in a protected zone in the body production line, while the other is housed in a central quality laboratory.
During the life cycle of the SMART car, Magna anticipates cosmetic tweaks to the body shape as well as periodic major face-lifts; added to which there will probably be new models such as a convertible. According to Saling, each change entails considerable effort in production engineering and associated metrology. The ease of creating programs for the LK MSI machines using PC-based Windows software was one of the main reasons for choosing these particular CMMs. LK Metrology Systems Inc. - January 2000 |
The eye-catching, highly unusual SMART car, a joint project of Mercedes Benz and Swatch, may be one of the smallest cars ever to hit the road. However, that does not mean that its assembly will not demand great care and precision. Geometric accuracy of the SMART car’s bodywork must be held to ±0.5 mm (±0.020"); and a process capability of Cpk 1.33 dictates that the maximum allowable car-to-car deviation be just 0.125 mm (0.005"). To check that these and other requirements are met, first-tier subcontractor Magna has integrated two LK Metrology Systems Inc.
(LK MSI) coordinate measuring machines (CMMs) into its “body-in-white” production facility in
Hambach, France.
To utilize the capacity of the machines as flexibly as possible, the working areas may be loaded automatically; or they can accept test assemblies manually loaded from the opposite end to take advantage of free capacity. On every fourth body, 100 measuring points are checked. A complete “body-in-white” inspection comprising the measurement of 1,100 points is conducted on one assembly each shift to a tolerance 10% more accurate than is required at a cycle time of 40 minutes, which is about twice as fast as the production cycle. Additional measuring tasks are conducted as well, including regular audits and assembly checks.