Maximum precision – the exterior master jig

At Audi, the master jig is the main instrument used by Quality Assurance to attain a premium impression and perfect fits.

The task of the exterior master jig is to optimize and fine-tune dimensional fits on every vehicle project before a production launch. This involves coordinating the fits of all parts that are visible on the exterior with one another. These parts range from sheet metal panels such as doors, engine hoods and trunk lids to exterior trim parts such as bumpers, headlights, taillights, windows, mirrors, door handles, spoilers and decorative trim strips. The measurement and analysis fixtures of the exterior master jig are housed in a dedicated building together with the interior master jig.

Other tools – which are also fundamental to successful coordination – are used in this area before work is performed on the exterior master jig.

The joint master jig is used to check for dimensional conformity of the underbody and its individual components, such as the front and rear floors and to coordinate them to one another. It ensures that all components can be joined without stresses. This is necessary to be able to produce a high-quality body in the later production process, which is the foundation for the premium image of Audi vehicles.

Using what are known as exterior function cubes, experts analyze the vehicle’s exterior trim parts in a local zero-reference environment. Correction data is derived from this process, which is used to optimize components at an early phase. When they fit to the cubes, they are added to the exterior master jig for further fine tuning.

Work in the zero-reference environment of the exterior master jig begins about nine months before production launch. Previously inspected panel parts of the body’s exterior skin and the parts mounted to them, such as doors and hoods and lids, are built up on a frame constructed of solid aluminum profiles. Exterior trim parts are also added to the assembly to analyze the interplay of all body parts.

This gives specialists a stepwise method for perfecting the dimensional tolerances of parts, achieving flush fits and perfecting the lines of joints.

An additional fine-tuning step involves working with reference bodies that are equipped in a way similar to that of an exterior master jig. This is done so that the paint application can also be considered in a final fine-tuning step. Although the paint is only as much as 15 hundredths of a millimeter (0.04 inch) thick, by Audi’s way of thinking, even this plays a major role in dimensional optimization.

The seam pattern on the new Audi A3 does not always follow exact mathematics; in some zones, subjective impressions are also considered in the tuning process. Take the front bumper, for example, which is slightly offset to the rear at its transition to the fender so that the customer always perceives a harmonious transition when looking at it from above. Another example: the seams at the fuel filler door. The upper seam was intentionally made smaller than the lower one, to give the subjective impression of a better fit for the observer.

Along with dimensional conformity, the exterior master jig also yields information on color fidelity, gloss level and the tactile feel of sheet metal, aluminum and plastic parts. In addition, it provides information on the firm seating of all components and their easy assembly and whether optimization potential exists in the component concepts.

Audi specialists are always faced with exciting new challenges in their work at the exterior master jig – challenges they must overcome to fulfill stringent requirements for the overall impression of the vehicle’s exterior.

Different materials such as steel, aluminum, plastics, rubber and glass must be cleanly fitted to one another. The overall impression is only distinctive and high-end if all edges and their radii fit precisely to tenths of a millimeter; only then is the run of the tornado line from the fender over the door to the side wall frame pleasing to the eye.

The equipment, data, and prices specified refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.