Audi A3 development: master jigs and color matching

From the official Press Release:

Exterior Master Jig

Front cubing measurements with CNC coordinate machine

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.

Optical measurements on joints 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.

Rear cubing measurements with CNC coordinate machine

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.

Interior Master Jig

The interior master jig is a body produced to a specific design level that does not exhibit any deviations at the points where interior components are mounted.

Measurements on instrument panel with coordinate machine

Ten months before production launch, the components are coordinated and fitted to one another using optical and tactile measurement methods.

Along with verifying functional criteria – free movement of parts, firm seating, ease of installation, harmonious actuation forces and sounds – the primary focus of measurement work with the interior master jig is on visual properties. All parts are inspected for grooves, voids and blisters.

Special attention is given to checking of seams, e.g. on the centre console that is made up of numerous individual components – from knee pads to the MMI terminal.

The four air nozzles in jet design at the front of the cockpit are highly complex parts. Each consists of over 30 individual components.

Visual surface inspection of a trim strip

Allowable tolerances are on the order of a few hundredths of a millimeter in the design feature for adjusting the air stream to spot mode or diffuse mode.

Therefore, the supplier assembles and checks every part in a 100 percent automated process.

A parts that helps to ensure optimal sitting comfort is the center armrest with its height and length adjustments. A high-quality aluminum die cast body with a two-component material joint ensures full adjustment acoustics.

Another highlight in the Audi A3: the three-dimensional decorative trim strips in the doors – they are embedded in a soft film without any gaps.

The complex mounting feature on the rear side was not released in the internal master jig process until it was perfectly tuned to assure an elegant impression and no noise.

The decorative ring on the gear shift grip is embedded with a precision of just a few hundredths of a millimeter to assure perfect tactile feel for the driver.

Audi has even implemented elaborate solutions in the luggage compartment of the new A3: when the cargo floor is folded upward, spring-loaded plastic latches hold it in place for the customer.

Discussing analysis results of functional cubing for door

Special cubes are used to check the fits of functionally relevant vehicle subassemblies. In the doors of the new A3, quality experts worked until pleasant-sounding window tracking and door closing acoustics were assured under all conditions.

This was technically implemented by features such as “post-tracking,” in which the window slides into the window guide starting at the front.

Only then did the window seal perfectly on the roof profile of the window guide with a constant insertion depth.

Another focal point was the double seal between the door and the body.

A continuous seal was attained with the help of color imprint methods and optical measurement technology. Only with this high level level of precision could the stringent quality requirements for closing noise, closing comfort and interior acoustics be met.

On the interior master jig, specialists also ensure that the door trim has precise, uniform gaps in its transition to the body door to ensure noise-free operation over the entire life of the vehicle at the customer. A new fastening concept with a flocked stop rail was coordinated and implemented.

Audi A3 – Color matching

Visual color evaluation of roofliner

The interior of the new Audi A3 integrates about 150 colored components; customers can choose from five color schemes.

All parts, from decorative trim strips to the carpeting, must be precisely matched and coordinated. Audi Quality Assurance coordinates them all and resolves any issues with suppliers.

The colored parts in the A3 interior are extremely diverse. They consist of 34 different types of semi-finished goods – such as fabrics, leathers and films – and ten types of plastic; they come from 45 suppliers across the globe.

This broad variety of materials results in many difficult neighboring interactions. For example, when smooth plastic meets textured surfaces, such as in the area of the center console, the different reflectance values of the materials can generate different, undesirable colors as perceived by an observer.

Color hue check and gloss measurements in cockpit

Special pigments are necessary to dye parts with different material chemistries in the same color hue. All components are dyed through, and many are also painted – some with high-gloss paint. In the new A3, for example, high-gloss components include the control panel for the climate control system and the frame of the MMI monitor. UV-stabilizers in the plastics prevent colors from bleaching out over a period of years.

Quality Assurance coordinates colors in its light studio whose lighting system can be adjusted to produce three different light environments: clear daylight, warm evening light or artificial light as in a showroom.

This is necessary, because individual materials give a different color impression under different lighting conditions. This effect – known as metamerism – must of course be avoided.

Visual surface inspection of a trim strip

All employees participating in color matching efforts must first pass a special vision test, because the optical measuring instruments at Quality Assurance – such as the spectral photometer – can only provide objective measured values of the surfaces.

Such instruments cannot measure the impression the color makes on a person, because only people can simultaneously detect and evaluate color, gloss level and surface texture. Therefore, if there is any doubt, in the end it is the subjective impression of trained employees that is the decisive factor in achieving perfect color harmony.

Color matching work is also performed in the finished, assembled interior, i.e. with components in their mounted positions and from the viewing perspective of the driver. This is done, because the appearance of certain components such as pillar trim changes due to the texture of the trim material.

(Source: Audi)