Bar peeling on a grand scale

BGH machines forged bar steel with a diameter of up to 600 mm

For many years, bar peeling has been the preferred method at BGH Edelstahl Siegen for removing the oxide and roll skin as well as any surface damage from forged, cylindrical bars. With what is currently the largest peeling machine from the SMS group GmbH, even bar steel can now be efficiently peeled with a diameter of 600 mm. 

BGH Edelstahl in Siegen produces stainless steel and special alloys for maximum loads. In order to guarantee the desired surface quality, dimensional accuracy and roundness of their bar steels, BGH peels the bars prior to delivery. The result is semi-finished products that are further processed by turning shops. The bars therefore act as a raw material for mechanical engineering, the oil & gas industry and the chemical industry. Heavy-duty gearboxes for wind power plants are also made from the raw material from BGH. "Our customers demand a mechanically machined bar steel without a rough forged surface," explains Jost Kretzer, Operations Manager at BGH. In order to guarantee both the desired diameter tolerance and surface roughness, the oxide and roll skin is peeled off the forged bars.

When turning is used, it takes around 8 hours to fully machine a tool steel bar with a diameter of 600 mm and a length of 8 m. This can be cut to 30 minutes using the peeling machine from the SMS Group.

16 times faster than turning

"One of the main advantages of bar peeling is the simultaneous usage of several tools, which in turn enables higher feeds than with turning. When turning is used, it takes around 8 hours to fully machine a tool steel bar with a diameter of 600 mm and a length of 8 m, whereas this can be cut to 30 minutes with bar peeling. That is a huge difference," says Kretzer. A diameter tolerance of 0.5 mm and a surface roughness of Ra=0.5 µm are standard. But things are getting even finer and more accurate: "up to a tolerance grade of IT 11," clarifies Jost Kretzer. "We then send the bar steel through automatic ultrasonic testing machines, which enables us to check the internal properties and surface for defects. This means we can guarantee our customers 100% product quality."

This investment in one of the world's largest peeling plants is BGH's response to the changing market situation. High-quality goods are now being delivered in large diameters with machined surfaces and correspondingly high testing requirements. "We would not have been able to cover the higher sales volumes with our existing machines," explains Kretzer. "We had to make a decision: either 5 to 6 new turning machines or an additional peeling machine." Since half of the production volume at BGH was processed by the existing peeling machine and there was no alternative machine in the event of a failure, the decision fell in favour of an additional peeling machine. The new machine had to have certain properties, however. It had to be possible to machine bar steel with a total weight of 25 tonnes and a diameter range of up to 600 mm. "We approached the peeling machine manufacturer with these figures, as a machine of this size previously did not exist. Over a period of 2 years, we worked with the SMS Group to design this machine according to our requirements," recalls Jost Kretzer.

The cutter head provides space for 6 cartridge systems. The tandem system is equipped with a roughing and a finishing insert and is suitable for medium rough machining. The trio system is the first choice for larger depths of cut and maximum chip volume. Up to 18 indexable inserts are used simultaneously in this system.

Advancing into new dimensions and market segments

The SMS Group developed a completely new machining concept to suit BGH's requirements, which revolutionised the production of forged bar steel. The new type PMH 600 peeling machine does away with conventional turning technology and peels bar material up to a diameter of 630 mm and a length of 18 m. Bar peeling is therefore entering the diameter range of up to 600 mm for the first time, opening up brand new market segments. BGH can now provide the raw material for mechanical engineering, the oil & gas industry, the chemical industry as well as heavy-duty gearboxes for wind power plants efficiently, quickly and in high quality.

The main challenge when it comes to producing bright steels is the fact that materials that are difficult to machine, such as stainless steels and nickel-based alloys, are often processed here. The machines in the PMH series offer higher power reserves than their predecessors or conventional turning technology. Depending on customer requirements, the new PMH series can not only work according to the conventional principle with rigid guides, but also adapts to the curvature of the bars and removes only a minimal amount of steel.

Another key consideration was for the PMH 600 to implement an optimised operating cost concept throughout the service life of the machine in conjunction with the technical peeling innovations. Maintenance and tool change costs for the PMH 600 have been reduced considerably in comparison with large-scale peeling machines of the previous generation. The use of low-wear infeed rollers, for example, has eliminated the need for these to be regularly replaced. Changing worn rollers of a feeding mechanism not only wastes several thousand euros in material costs but also takes up to half a shift of working time, during which the machine cannot be used.

The tool change for the PHM from the SMS Group is also limited to the replacement of the tool cartridges, keeping the changeover time for the peeling tools to a minimum. In the case of conventional machines, the entire carriage unit has to be replaced. The entire adjustment of the machine can also be covered by just one tool. The new procedure is therefore not only up to sixteen times faster than turning technology while removing less material and adapting to the curvature of the forged bar, but it also opens up new market segments.

The tandem cartridge system, with the new hexagonal roughing insert and a finishing insert, is designed for medium machining. This enables depths of cut of up to ap 12 mm to be achieved.

Peeling inserts for the world's largest peeling machine

The investment in such a machine was not without risks for BGH. Previously there had been neither a peeling machine of this size nor a tool that could be used to achieve the high machining values. For this reason, BGH turned to the experts from Cutting Solutions by CERATIZIT, who offer a comprehensive range of peeling indexable inserts for the most diverse range of peeling applications and have extensive expertise in this area. The special requirements of BGH and the demands that one of the world's largest peeling machines places on the tools prompted CERATIZIT to develop completely new peeling indexable inserts. Product Manager Ronald Huber has fond memories of the development process: "The existing button insert had major disadvantages in terms of its positioning. We therefore remodelled the roughing insert with a multi-edged outer geometry in order to achieve improved machining properties in addition to precise indexing in the insert seat." According to Huber, good chip breakage is decisive when it comes to process-secure peeling. Five tonnes of chips are produced every hour during the peeling process, so it is important to prevent the build-up of long swarf. "The new hexagonal roughing insert not only enables us to guarantee optimum chip formation. It can also withstand extremely high feeds of up to 24 mm/U."

Pioneering work in the field of peeling

With the new hexagonal insert from CERATIZIT, BGH is achieving excellent results with easy to machine structural and tempering steels. In future, it will also be possible to machine higher-alloy materials with the new peeling insert types. Tests are continuously being carried out on the peeling machine in order to optimise the results. "We are hugely grateful to BGH for giving us the opportunity to test our peeling inserts on the machine. This is the only way for us to continuously further develop and improve our tools," explains Andreas Schätzl, Head of Engineering at CERATIZIT. "The development of our new hexagonal roughing insert was pioneering work in collaboration with BGH and the SMS Group that enabled us to enter new territory in the field of peeling, where no-one had gone before and where no-one but us has been since," emphasises Schätzl.

The hexagonal roughing inserts defy the extremely high machining values with optimum chip formation. High process security, the best surface quality and maximum chip production are among the outstanding features of these innovative inserts.

For Jost Kretzer, the trustful partnerships with the SMS Group as machine manufacturer and Cutting Solutions by CERATIZIT as the preferred tool supplier is the key to success. "If all those involved hadn't pulled together, we wouldn't have been able to achieve the same result," concludes the BGH Operations Manager. "We also have a duty as a user and must communicate any advantages and disadvantages openly and honestly. As long as the tool supplier and machine manufacturer are prepared to listen to these concerns, the results can be continuously improved. Everyone benefits from this," says Kretzer.