Shoulder to the wheel
Mercedes-Benz is using a shoulder milling cutter from Sandvik Coromant to produce a Mercedes van rear axle tube.
Mercedes-Benz Works in Kassel was having trouble with the manufacturing of the axle tube for a van in its current range. Trials revealed that turning the axle tube as planned was not a reliable process, and an alternative solution was necessary. Markus Groppe, Milling Products Manager at Sandvik Coromant Germany, says: 'Any downtime, even if it seems inconsequential in itself, results in high costs both in the long term and overall. Solutions to minimise downtime permanently have to be tailor-made to match the situation. Interruptions to the production process have to be analysed in detail, and need everyone involved to search for creative solutions.'
A wide variety of axle tubes are manufactured on automatic production lines. At the beginning of the process, the various parts to be mounted on the axle tube are joined to it, in this case by friction welding. This produces a double-bulge weld bead, which has to be made cylindrical in diameter for further manufacturing steps. Initially, the Kassel engineers tried to achieve this by conventional turning. However, when the weld bead was machined away, two circumferential burr rings were produced. These burr rings were a problem because an automated line cannot be stopped in mid-operation for manual removal of rings or chips. Process reliability and unmanned operation would be severely impaired, and it would no longer be possible to guarantee the output of the plant. To make matters more difficult, it was absolutely essential not to damage the external shape of the axle tube under any circumstances, as this would be a fracture and would lead inevitably to the part being rejected.
The purpose of optimising the machining process was to prevent the burr rings from forming, thereby reducing downtime. It was then left to the responsibility of the Tool Management team at the Mercedes-Benz Works in Kassel. The team is a group of some 30 employees who manage everything to do with tools: new planning, process development and production optimisation to job organisation, tool stock management and regrinding. Initial attempts with all kinds of turning strategies, and tools and inserts in all possible geometries, failed to bring the desired result. Carsten Klinge, the team leader at Sandvik Coromant responsible for Daimler and who belonged to the project team on the tool manufacturers' side says: 'The best ideas often come outside working hours. After a day of unsuccessful tests, the idea occurred to me on the way home of attempting turn milling.’
At that time Sandvik Coromant was carrying out market testing on the new CoroMill 490 shoulder milling cutter, which had already shown itself to be particularly easy-cutting with low power consumption. Groppe says: 'The CoroMill 490 is a particularly suitable milling cutter for use with drive units on lathes and smaller machining centres like the Montforts MultiTurn RNC-500 used here.'
However, to avoid burr rings completely, a special process was necessary; each friction weld bead was first machined down to the finished dimension at two diametrically opposite places. This means that the subsequent turn milling produced two semi-circular rings that simply fell off the work-piece. The turning/milling operation required a detailed preliminary study, since process parameters like the centre offset of the milling cutter diameter in the Y-direction had to be determined in order to obtain a cylindrical shape of the required length. Naturally, this also had to take into account the interfering contour of the working space of the machine and work-piece, in other words a kind of simultaneous engineering.
Tools with six GC1030 inserts and a Coromant Capto C4 tool holder are used, which guarantee the highest repeat positional accuracy when changing tools, and short tool changing times. A cutting speed of 275m/min turned out to be best. Jörg Eisenberg, tool planner at the Mercedes-Benz Kassel plant, says: 'The cutter is changed after 120 work-pieces. Although this is before the end of its service life, we wanted to be on the safe side.'
This apparently simple solution at first glance was only made possible by the knowledge gained of the best way to use the shoulder milling cutter, coupled with its capabilities for this particular special machining task. This knowledge has since been confirmed by an independent body; a comparison of milling cutter designs from well-known manufacturers by the Institut für Fertigungstechnik und Werkzeugmaschinen (Institute of Production Engineering and Machine Tools) at the University of Hannover certified that the Sandvik shoulder milling cutter had the lowest feed, normal feed and axial forces.
