Läpple Blechverarbeitung GmbH & Co. Bayern, a subsidiary founded in 1990, produces stampings and complete assemblies for the automotive industry. This company, based near Regensburg in the Upper Palatinate town of Teublitz, employs about 750 people, and has approximately 50,000 m² of manufacturing floorspace. The plant also has a large amount of undeveloped land available for expansion. In Teublitz, Läpple works with sheet metal up to 4.5 mm thick. The finished parts may have maximum dimensions of up to 3550 mm x 1650 mm. The plant processes about 40,000 tons of sheet metal per year. While Läpple’s customers come from all over the world, the German automotive industry as a whole plays a central role.

Output rising sharply

One of the most important customers is DaimlerChrysler AG, for whom Läpple produces the front end of the Mercedes CLK, for example. The finished parts for coupes are then shipped “just in time” by railroad to DaimlerChrysler in Bremen; parts for convertibles are transported to Karmann in Osnabrück.

Läpple was also tied into the simultaneous engineering process for the CLK front end. In 1996 the company had a production island with 18 KUKA robots constructed, originally designed for a production capacity of 170 complete front ends per day in two-shift operation. Market introduction and the sales success which followed caused demand to rise to 280 units a day. Läpple compensated for the increased workload by retrofitting the production island to include 21 robots and by going to three-shift operation. “Since our production volume has risen in the meantime to 430 – most of all because of the convertible that was added later – we have had to invest in a second production island, which went into operation in February 1999”, explains Dipl.-Ing. Werner Klenk, Managing Director of Läpple Blechverarbeitung Bayern, with reference to the sharp rise in production.

Both islands, each of which consists of two subassembly areas and a main line, are as far as possible identical; nevertheless, Läpple had the one that was built last optimized with regard to a few details. A power-and-free system for unloading the main line was installed, for example. Inline measurements and visual monitoring of the finished parts are integrated into the materials flow system. Parts are fed into all of the individual areas manually, while in the main line transport is carried out automatically by mobile fixtures and transfer units equipped with grippers.

Robots join 110 sheet metal parts

The new production island has 20 KUKA robots, of which five are employed in each of the subassembly areas for the right and left wheel housings of the front end. The individual components are then welded together in the main line to form the complete front end.

The 20 robots are made up of fourteen KR 125s and three KR 150s, all of which use pneumatic weld guns for resistance spot welding, plus one KR 15 for MAG arc welding and one KR 125 for stud welding. In addition there is one more KR 125 which works at the same time for both the new island and the one previously installed. This 125 kg payload robot, which is located between the two systems, welds various front end subassemblies such as the radiator bridge.

The robots employed in the new system join 110 sheet metal parts per front end; 26 of these are manufactured by Läpple especially for the CLK, while the others are the same as those used in the Mercedes C-class front end; these are made by DaimlerChrysler itself or by subcontractors. The robots make about 1,200 spot welds and 39 stud welds; in addition they weld seams with a combined length of 1100 mm.

“All in all, we use 80 KUKA robots at the Teublitz plant. Our highest priorities when selecting them were repeatability and availability. We want to avoid rejects – our reserve capacity is already completely taken up in any case, since we work in three shifts”, says Werner Klenk about these basic requirements. “The new production island has a manufacturing capacity of 280 parts per day. As far as the robots are concerned, we achieve an availability of nearly 100%. With reference to the entire island, including electrode changes, the figure is about 90%”.

Other decisive factors were the attractive price/performance ratio and the good experience that Läpple had already gained with robots from this manufacturer. The decision-makers also considered it important to concentrate on a single brand of robot, thus significantly reducing the number of spare parts that have to be stocked. Standardization also makes training and working with the machines easier.

New robot controller provides time savings

While the robot and controller were supplied by KUKA Roboter GmbH, Augsburg, the periphery was designed at Läpple’s headquarters. Robot programming was carried out by the user’s employees, and it was here that the Windows interface of the KUKA Control Panel proved its worth. “We thought for a long time about whether to use the new PC-based controller from KUKA, which meant having two different systems, or to stay with the old one for the sake of consistency. Although the conversion required more time and expense for programming, in the end it was the right decision”, says Werner Klenk in retrospect. “Today in this production island we benefit from significant time savings whenever we have to carry out troubleshooting or modify programs because of technical changes in the vehicle being produced.”

The robot and welding controllers communicate via Interbus with each other and with a PLC which functions as a master. The welding controllers use constant-current control and a stepper function to compensate for electrode wear, thus ensuring a reliable welding process. For this purpose, electrode tip dressers have been installed in the system. These redress the tips after a certain number of spot welds, restoring them to a usable shape.

Robots more cost-effective

In comparison with robots, manual welding would be less effective, since three human operators would have to be used in place of each robot. Moreover, the heavy weld guns are ergonomically inconvenient to handle. This disadvantage could necessitate additional rest breaks or even an exchange of operators during a shift. In addition, the robots maintain consistently higher quality, which further enhances the profitability of the production island. The payback period is thus very short; with regard to the robots, Läpple mentions a time of six months.

Author: Jürgen Warmbold, freelance technical journalist, 27327 Martfeld, Germany