GMN Paul Müller Industrie GmbH manufactures spindles for machine tools as custom manufacturing at the highest level. To do justice to this claim in its own manufacturing in Nuremberg as well, the manufacturing specialists rely on the long-lasting and precision clamping devices from Röhm.

So often, it is the small things in life that have the greatest effects. “What is the use of a high-precision 5- axis machining center if the component does not sit on the machine correctly,” says Erich Stecher. The experienced master in parts manufacturing at GMN Paul Müller Industrie GmbH in Nuremberg knows what he is talking about. “Everything has to work properly during setup so that downtime for the expensive machines can be kept to a minimum, and we also have to be able to transfer the precision to our components.” The Nuremberg-based spindle manufacturer's requirements are by no means modest: “After all, the components are installed in either our very powerful or very fast-rotating spindles,” says Stecher.

Founded in 1908, today GMN is in family hands and is being managed in the fourth generation. Around 450 employees develop and manufacture high-precision ball bearings, sprag clutches, non-contact seals and also machine spindles exclusively at the company’s headquarters in Nuremberg. This includes, for example, the new generation of higher-performance milling spindles for high-volume machining of aluminum of the HCS 230 series. For the latest model of the series, the HCS 230 – 30000/150, the continuous output was raised from 120 to 150 kW. This provides speeds from 17,300 up to a maximum of 30,000 rpm achieving torque of up to 83 Nm. The spindle is equipped with an HSK-A63 tool holder with a contact face of 80 mm.

The high-performance spindles are delivered completely enclosed so that the machine manufacturer can install the spindles immediately and only has to flange-mount the connections. For the milling specialists, this results in a mainly round range of components with batch sizes of 1 to 10. “Our batches are mostly not much bigger, because the spindles are not used in mass products,” says Stecher knowingly. “Therefore, we have to be able to react flexibly with our clamping devices to be able to reliably put the entire product range onto the machine.”

In use with flying colors for 15 years

The Nuremberg company has now relied on vices from Röhm for more than 15 years. “We have already had six vices in use for years, and I can’t remember that any have ever failed,” emphasizes Stecher. They are the NCCompact centric vices of the RKZ series. The vices are distinguished by a very stable steel base body and a large clamping range. Furthermore, the vices that are optimized for 5-sided machining have high repeatability and constant clamping force. The center clamping accuracy is ± 0.01 mm with a clamping repeat accuracy of 0.01 mm. The clamping thread of the spindle is protected against dirt and shavings in every position of the master jaw. As a series feature, the vice is equipped with stepped top jaws for components up to 180 mm. “Our components can reach diameters of up to 280 millimeters. Therefore, it was clear that standard jaws would not suffice,” says Stecher. They initially had a two-part round gripper made for the larger diameters. However, additional collars were required to be able to clamp all components with it. This clamping situation was too laborious for the milling specialists, on the one hand, and ultimately, the quality was not satisfactory. “We rely on high quality on the machine side as well for our precision manufacturing.”, says Stecher. "Hence, a 5-axis DMC 100 U Duoblock milling center from DMG with a GMN spindle is used in this case. The clamping devices must be able to keep up here.”

Good experience with the manufacturer

Due to the many years of good experience, it was quickly clear that we would ask the manufacturer Röhm for a clamping solution for the big diameters as well. This was a real technological challenge to the responsible Röhm account manager, Gerhard Häutle. “Having a customer who wanted to use our vices to clamp parts that were in those dimensions might come up every ten years. But our designers very quickly came up with a solution that, despite the very large overhang of around 284 millimeters, still guaranteed repeat accuracy of one hundredth and a clamping force of four tons." With two prism-shaped clamping sets, all diameters from 180 to 234 mm are now securely clamped, and with the second set, diameters from 234 to 281 mm are securely clamped. In the process, the ground clamping bars provide the necessary precision, and the clamping jaws are still easy to change thanks to only having six screws per jaw.

“With the clamping jaws, we can react much more flexibly,” says Stecher, “and in addition, we also increased the precision of the components. That is because with the 3-sided machining that is now possible, we can completely machine our parts with just one clamping process. This has tremendous benefits, especially for the spindle sleeves. For example, hole patterns can be milled on the two end faces in one operation. Ultimately, we get considerably higher accuracy of the faces to each other.”