Toy tab technology at Mitsubishi Motors

The traditional method of assembling a vehicle is to have the body shell go through a series of stations along the assembly line. One of the established problems with this traditional system for automotive body shops has been how to construct all of the body shell in one framing fixture in order to improve body accuracy and time efficiencies, and ensure a quality product.

Generally the ability to construct all of the body shell in one framing fixture has not been accomplished without significant investment and cycle-time limitations through time taken to load all of the assemblies into the framing fixture.

Mitsubishi Motors Australia Limited (MMAL) has recently been successful in introducing a new vehicle assembly technology into its Tonsley Park plant in South Australia known as 'toy tabs', which has been applied to the build process of the all-new Mitsubishi 380 due for release in October 2005.

The toy tab concept is similar to the metal tabs used to hold metal toys together, which explains the name. In automotive manufacture the four metal tabs per side are formed as part of the body side outer panel, extending out from the weld flanges. When the body side assemblies are applied to the underbody in the preset fixture, the metal tabs of the body sides are formed over the corresponding flanges of the underbody, preventing the body sides from being dislodged during transfer.

Dedicated, two-stage crimping tools, designed by Kim Berry of MMAL tool design, are built into the preset fixture to fold the tabs through a pre-clinch and a final clinch process. At the next stage the rear shelf and front and rear roof rails are installed and similarly located on tabs to prevent dislodgement during transfer into the main framing fixture. Here the assemblies are all tack-welded together while held firmly by the locators, ensuring dimensional integrity of the body. Following this, the roof assembly is lowered onto the body and tack welded. The body is then released and transfers to the re-spot stations.

Typical body welding process

Traditionally, vehicle assembly would have meant that at each station workers weld the main body structure adding panels sequentially, in a series of framing fixtures. Each fixture would hold the panels to be welded in the 'correct' position, but there was always the risk of some variability or tolerance allowed. The cumulative effect of the tolerances in these fixtures resulted in decreased body accuracy in a vehicle.

Cycle-time constraints because of the manual welding content and operator safety resulted in a concept whereby the body was built up in stages. In the first station, the preset fixture, the body sides were loaded to the underbody and tack-welded into position. In the next station the front deck and rear shelf assemblies were loaded into position while additional welds were applied between the body sides and underbody. From here the body was moved to the main framing fixture where the front and rear roof rails were loaded in and, along with the previously loaded front deck and rear shelf assemblies, were tack-welded into the body. The main fixture is intended to establish the accuracy of the body by placing welds at strategic points of the assemblies, held by locaters, to control the triangulation of the body. Following this the body moved to a re-spot station where more welds were applied and then to the roof set fixture where the roof assembly was tack welded to the body structure.

The limitation of this system is that body accuracy requires a lot of work to be achieved during the launch period. Much fine-tuning of panels and locators was required to achieve a good fit condition for trim and exterior components. This required a high degree of input by production engineers, stamping engineers and the quality department to determine the critical panels and components needing to be tuned to achieve the required fit and finish levels to present to customers. Essentially, the pilot program had to be lengthy to allow for this fine-tuning.

Lengthy pilot programs impact on company profits because of the time line required before return on investment can be achieved. The automotive industry is continually striving to reduce lead times to improve returns and lessen the impact of interest rates.

Body accuracy must also be maintained throughout the model life of the vehicle. Having body accuracy influenced over several key welding fixtures and re-spot stations created a lot of work for body shop production engineers with continual fault finding and corrections. Minimisation of this ongoing activity is desirable to reduce the impact on production engineers' workload as well as improve the build consistency.

For the customer, the results were less tangible but could be related to engine vibration, fit and finish, body flexibility and on-road performance. All of these issues can arise due to vehicle assembly body accuracy issues. The flow-on effect to customers is always cost of the product and with vehicle rework, cost reduction opportunities and profit margins are reduced.

The benefits of toy tab technology

Use of this system has allowed Mitsubishi Motors to set aggressive body accuracy targets with the intention of giving downstream processes the best opportunity to achieve first-time capability. With the advantages of employing this new technology, MMAL has now set a target of 95 per cent body accuracy for the Mitsubishi 380.

Confidence in the 'toy tab' process and the advantages of welding all components together in the main framing fixture was such that it was also decided to adopt more stringent tolerance requirements on the body of the PS41. Traditionally, the standard dimensional check tolerance used has been ±1.5 mm. For the 380 it was decided to adopt a ±0.7 mm tolerance on critical areas, with ±1.0 mm tolerance on 79% of check points and ±1.5 mm on other check points for a total of 311 dimensional checks.

The body accuracy strategy for the Mitsubishi 380 has been extremely successful. Already quality standards of the pilot vehicles have surpassed anything produced at Tonsley Park previously. And as the vehicle progresses through the pilot build phases in preparation for volume production, the resultant body accuracy levels continue to achieve the targets set for each stage.

The benefits to customers will be a much better quality product with significantly improved standards of fit and finish, ride and quietness that is produced with very high levels of first time capability and significantly reduced levels of rework.

Mitsubishi Motors is confident that its new car, the Mitsubishi 380, will have a more consistent build, fewer rattle and squeak issues, fewer problems with fit and finish and more consistent seam welding, seam sealing and painting. Exterior parts fitment such as tail lamps, headlamps and bumper facias have been tuned with the body to achieve desired appearance results much more quickly than ever experienced in the past.

The Australian manufacturer is confident this will be the best quality car ever built in Australia, with the accuracy levels of the locally made large car, is already becoming a benchmark for the Mitsubishi Motors group worldwide.