Starrag machines make an impact at Porsche facility

The Droop & Rein machines at Porsche

In recent years, Germany-based Porsche Werkzeugbau GmbH has taken its mechanical machining to the highest technical level. Two large Droop+Rein FOGS high-speed machining centres supplied by Starrag, play a major part in this. The machines are used to finish press tools for moulding skin parts of vehicle bodies with the utmost precision and produce surfaces with grinding quality.

A premium car manufacturer should have a premium engineering plant. To ensure that the Porsche Werkzeugbau Schwarzenberg plant fulfils all of the pre-conditions for this, 40 million euros was invested over the last five years to develop the tool-engineering plant to the highest possible level following the transition from Kuka.

Among other things, the Schwarzenberg-based company used this money to set up two new production halls, purchase a new 25,000kN servo press for production-related tool try-outs and invest in retrofitting and new machine tools — including two Starrag HSC centres of the Droop+Rein FOGS 35 68 N40C type. In addition to this, mechanical production was completely overhauled and the production processes were restructured.

All of these measures were based on a solid foundation. Beforehand, the whole tool-engineering team analysed the entire process chain, from development and design to procurement, production and try-out of the tools, right through to the transfer to the operator pressing plant — including a fitness test for the pool of machines. An important detail that came to light was the realisation that when it came to mechanical machining, there was potential for improvement in the finishing of the tools, particularly in terms of accuracy, speed and surface quality.

Pictured right: angular head on the Droop+Rein machine at Porsche for accessing difficult to reach surfaces

Marco Franz, head of mechanical production, points out some important reasons for this: “For some years, we have been experiencing a shift towards lightweight construction and, as the Porsche competence centre for forming technology in the moulding skin area, we were commissioned to develop the tools required for the conversion to an aluminium body.

During forming, aluminium sheets respond much more sensitively than steel, which created big challenges for us when it came to tool engineering. Our existing machines were not able to meet some of the requirements.” Two new high-speed machining centres for finishing the pressing moulds should fulfil these new requirements regarding surface quality, accuracy and speed.

According to Mr Franz, they should also help to make the process more efficient and economical, as well as greatly improving quality. The list of requirements contains fundamental preconditions such as process stability, availability and ease of maintenance, as well as characteristics that cannot be compromised, such as temperature stability. In addition, future-oriented specifications for the performance data of the spindles were defined.

Lutz Kramer, head of tool and mould making, explained: “When you buy this kind of machine, you expect to use it for the next 10 to 20 years. During this time, several generations of new milling tools were developed that are more efficient and allow higher speeds. We must take all of this into account at the early tendering process.”

With their list of requirements, the Porsche Werkzeugbau decision-makers set a high bar — and came to realise that not many manufacturers would be able to meet the requested standards. Mr Kramer said: “During the technical discussions that we held with many different providers, we were soon able to tell who was able and willing to overcome the challenges. Some machine providers withdrew at an early stage, others were later unable to offer any acceptable solutions.”

Pictured left: the angular head is perfect for contouring large parts

Ultimately, the decision-makers opted for two Starrag Droop+Rein FOGS 35 68 N40C high-speed machining centres. These five-axis machine tools are designed for finishing, have an overhead gantry and offer traverse paths of 6.8 x 3.5 x 1.5m along the X, Y and Z axes. The equipment includes a tool magazine with a total of 100 places that can be filled with HSK32, HSK63 and SK50 retainers, as well as an eccentric fork head with a 40kW milling spindle and a high torque of 700Nm, and three different motor milling spindles with speeds of 18,000, 40,000 and 65,000rev/min. In addition, for one of the two machines, Marco Franz ordered a special 6,000rev/min angular head that can be used for machining areas that are difficult to access.

Mr Kramer added: “We did not just want to buy a machine, we wanted a solution for our tool manufacturing process. That is one of the main reasons why Starrag seemed to be the right partner. With the Droop+Rein specialists, we had the impression right from the start that they were listening to us, wanted to understand us and the thinking behind our processes, and wanted to use their expertise to help us.”

Once the machines were installed, the two companies began fine-tuning the workflow together, taking into account the characteristics of CAM, the control system and the machine. This was necessary because achieving the best surface quality requires a high level of accuracy on the cutting path, even at the highest feed speeds. For this, it was important to illuminate the interfaces between CAM and NC and to work with experts from the control system manufacturer to optimally adjust their functionalities in line with the process requirements.

Pictured right: the Droop+Rein has 100 tool positions available

This approach and the combination of sophisticated drive optimisation in conjunction with the proven machine technology of the FOGS series was extremely successful. The result was not only a first-class surface finish but also a very high level of accuracy — at the highest path speeds of up to 16m/min on the free-form surface. The processes have been running stable ever since.

Mr Kramer said: “It took a long time to achieve stable processes, especially when it came to the forming tools for aluminium parts because there are many factors influencing this. But Starrag assisted us until we were sure we had reached our goal.”

Mr Franz added: “I would not have believed it at the start but Starrag was able to implement more than 90% of our wish list. So it was a good decision to invest in the new Droop+Rein FOGS machines. We are now faster and more precise and achieve — depending on the component and the geometry — outstanding surface accuracy.”

Mr Kramer concluded: “We are proud of what we have achieved with Starrag. With the Droop+Rein FOGS machines, we have attained the desired improvement in quality and were also able to reduce the manual bench work of the toolmakers by roughly 20% in the follow-up process.”