Insert threads Automated thread forming

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FIBRO GmbH

FAULHABER Drive Systems

Inserting threads into thousands of thick sheet metal parts is not trivial. It becomes even trickier when tight cycle times have to be met and there is a lack of space in the pressing tool.

More automation! This demand comes from many company managements. After all, it's about increasing process reliability, reducing costs, and tackling the shortage of skilled workers. This was also the case at Interstuhl in Meßstetten-Tieringen (Germany). The company was founded in 1961 when Wilhelm and Werner Link developed the first work chair with the Bi-Regulette.

From their village smithy, they have since become a manufacturer of chairs for office and home office, in production and in the lab, as well as for gaming. The independent, family-run company employs around 1,000 people worldwide and yet remains committed to its headquarters in Meßstetten-Tieringen. Added to this is a high level of production depth, which is unique in the industry. For example, Interstuhl manufactures all metal parts themselves.

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There is potential for optimization in the bottleneck process

The location in Germany certainly has its own requirements. Among other things, more and more automation is necessary to remain competitive. An automation project focused on how to incorporate threads more efficiently into armrest carriers. Such an armrest carrier is found in about every second chair. The armrests are screwed onto it—if desired. Until now, punching and thread forming were two separate production steps. "Punching took 2.5 seconds and thread forming, which was done manually, took eleven seconds."

"This led to bottlenecks in production, creating a bottleneck," reports Michael Maier, tool designer at Interstuhl. When it became clear what potential lay behind the project, his department quickly took over the former trainee project for developing automation technology.

This is how to form threads safely and automatically

The implementation was not quite simple! "Thread forming was not possible with the existing press tools. Therefore, we had to build a completely new facility," said Torsten Dinser, head of toolmaking/prototype construction/fixture design at Interstuhl. The new facility is connected to the punching press via a good-part conveyor belt. A gripper independently detects when an armrest carrier arrives via the conveyor belt and positions it precisely on a transverse part conveyor. This guides the workpieces made of five-millimeter-thick sheet metal under a total of twelve electronic thread forming units of the Fetu type (for Fibro Electronic Tapping Unit)—six in a row from the left and six from the right side. Fibro specifically developed this electronic thread forming unit for punching and forming processes. It can be used independently in progressive and compound tools, presses, or punching and bending machines to introduce threads in a process-secure and controlled manner. This improves thread quality, increases process security, and enables faster and more cost-effective production. "The autonomous system allows for versatile and flexible use," explains Matthias Jörg, technical consultant in Fibro's field service. The achievable thread sizes range from M0.8 to M12. Additional sizes are available upon request. The electronic thread forming unit is easy to program and control. It also comes with its own process monitoring. The high forming speed ensures that the sheet metal processing operations continue to run economically and safely.

"We equipped the system with a total of 46 sensors for monitoring," explains Maier. The thread formers insert a total of twelve threads at 24 strokes per minute. "A part with twelve threads falls out of the system every 2.5 seconds," adds Sascha Seemann, who also works in fixture design at Interstuhl. Achieving this productivity was indeed associated with some work. "We gradually increased the number of strokes until the process still ran safely," Dinser recalls. Because Interstuhl reduced the armrest carrier variants to only two in the course of the project, the system can be retooled in less than 30 minutes if necessary. The employees almost daily retool the punching press itself because production is characterized by a high variety of parts.

Learned a lot for the next automation

From the initial ideas to the finished facility, the project took about a year and a half. Partly, this was due to delivery difficulties after the Corona pandemic—the necessary servomotors were simply not available. But that was not the only reason, as some things take time. Seemann: "It was a learning process for us in the field of automation technology." Therefore, it was clear from the start that the project would take time. Not least, it was complex to properly coordinate all the automation components. But after about three months of pure production time, all parties involved are very satisfied. So far, Interstuhl has manufactured approximately 136,000 armrest carriers with the new facility. The first 100,000 pieces were with M6 threads. "Meanwhile, production is also proceeding securely and reliably with M8 threads," Maier notes. Because some manual labor can be saved, the facility will pay for itself in just over two years. And Maier also praises the cooperation with Fibro: "Throughout the project, we always had good collaboration and could always openly discuss how to solve this or that problem."

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