Measuring sheet metal parts in auto industry

By: Stefan R?ding, Witte Far East

A passenger car consists of about 600 different metal parts. To build a perfect car every one of those parts would have to be checked with regard to its geometrical dimensions, which with conventional methods and due to time constraints is inconceivable in series production. 

A frequent practice is therefore the execution of random sampling, for which parts are taken in regular intervals from the current manufacturing process and checked using a gauge or a tactile measuring machine. But while parts are being measured, manufacturing continues and incorrect parts with quality relevant issues are produced, which then have to be corrected or even scrapped.

In order to shorten the time required for manual part withdrawal and measurement, some car manufacturers are implementing so called conventional automation processes. These include palletting equipment, which helps to optimise and speed-up part handling. Parts are mechanically removed from running stamping or assembly lines, moved to loading stations equipped with appropriate measuring fixtures and hydraulically or pneumatically clamped. They are automatically transported to a measuring machine by a pallet exchange system. These often comprise of several loading stations, which are equipped with aluminium sandwich plates as a base mounted with a corresponding measuring fixture. After measurement, the part is automatically returned to the line. In comparison with manual unit withdrawal substantial time savings are achieved. However the problem of manufacturing further incorrect parts remains, although the quantity is reduced.

The increasing need to avoid process interruptions and to speed-up parallel quality controls is leading to an increase in importance of the use of contact-free measuring techniques.

While in the electronic industry for more than 20 years optical measuring techniques have been used, applications with contactfree measuring for checking sheet metal parts have only recently started to increase. Meanwhile, there are numerous suppliers of optical and/or contact-free measuring systems, which contribute to improved process efficiency and are used in production lines of different car manufacturers and their suppliers.

Increasingly optical measuring methods are used for the realtime determination of dimensions, such as diameter, width, height, gap and edge positions directly in the manufacturing process, i.e. for Inline measurement.

During photogrammetry objects are evaluated 2-dimensionally from different directions, digital pictures are processed and the 3D coordinates of the relevant geometry points are calculated.

The graphic data of the plates is compared with the original CAD part data via reference marks, which are mounted on the part or on the fixture. Cameras are integrated into the production line, so that units need not be removed and the process does not have to be interrupted. The time required is limited to passing the measuring station. Commonly used CCD cameras (Charge-Coupled Device) can capture surfaces up to 500 x 500mm within seconds. Cameras are usually placed at critical points, for instance at a point where two manufacturing lines join resulting in an assembled part. The cameras capture selected, quality relevant areas on the parts. Firmly installed, stationary cameras can lead to perspective distortion, which can prove critical with regard to evaluation of holes, radii or recess cuts. In practice, this problem is counteracted by using several cameras, which are positioned at different angles.

Consequential 100% contact-free measuring of all sheet metal parts is so far only rarely applied in automotive manufacturing. In order to limit the quantity of cameras required, sNIKE AIR FORCE