Reasons for optimization:
Around 25,000 PTO shafts are manufactured in Grieskirchen every year. Pöttinger produces the power take-off shafts as splined shaft profiles with six teeth in the common sizes 1-3/8 inch and 1-3/4 inch. The main material used is steel grade 16 MnCr5, which is also well suited for subsequent inductive surface hardening. “We manufactured the gearing on a hobbing machine before switching to the tools from Horn. Since we are constantly optimising our production processes, we wanted to manufacture the PTO shaft in a single clamping operation,” explains Roland Grafe, Pöttinger’s work technician. In machining, Pöttinger is relying more and more on automated production cells. For unmanned and process-reliable machining, Grafe’s team was looking for a tool solution to mill the gear teeth of the power take-off shaft on a lathe with driven tools.
„We have received precise requirements from Pöttinger as to what the tool must be able to do and what other demands are placed on the machining process“, tells WEDCO technical field representative Alfons Kocher. One challenge was that the individual teeth including a chamfer were to be produced in one infeed. The problem, however, was the limited power of the driven tool. Another point was the possibility to regrind the tool up to four times.
Successfull cooperation: Roland Grafe talking to WEDCO representative Alfons Kocher.
Special cutting edge design
„To counteract the reduced machine performance, Horn’s technicians developed a special design of the individual teeth of the milling cutter with a cut division. The cutting edge has to be very efficient due to the machine power“, tells Kocher. In use, the six cutting edges of the tool always machine the left and right sides of the groove to be milled alternately. The cutting edges are ground with the required tooth profile including the profile of the root circle diameter. The expected hardening distortion of the PTO after heat treatment is already provided for in the design of the tool profile. Furthermore, the clearance angles of the individual teeth are designed so that they can be reground up to four times. Regrinding thus only changes the tool diameter and not the profile of the individual teeth.
HORN chose the DG tool changing head system as the tooling system. The screwable cutter head is centred via a taper face and with a centring pin. This interaction guarantees a change accuracy of the cutting head in the length of +/- 0.02 mm and in the run-out of 0.01 mm. This interface brings several advantages: high stability, stable support as well as the cutting head change without having to re-measure the tool. In addition, changing the cutting head is simple and user-friendly with a torque spanner. The interfaces of the cutting head and the basic holder are made of tool steel. The solid carbide tool cutting edge is brazed onto the tool head. This enables resource-saving handling of the carbide material and thus also enables a more favourable tool price.
In the field, the HORN technicians’ thoughts on tool selection and cutting design were confirmed. For the six teeth/grooves with a length of 87 mm and a depth of 3.3 mm, the cutter needs a machining time of 85 seconds. Machining takes place in full cut at a cutting speed of vc = 200 m/min. “Actually we are still milling in up-cut mode. With a more stable interface of the driven tool, we will switch the machining to climb milling. Then we can run the tool at an even higher output,” says Grafe. The tool complies with the required tolerances in the milling process. The required surface quality was Ra 3.2 µm. The HORN milling system produces a roughness depth of Ra 1.6 µm. The service life of the cutting edge also exceeds Pöttinger’s expectations. Until the cutting edges are reground, the milling system achieves a service life of 300 PTO shafts, i.e. 1,800 grooves. With the changeover to climb milling, the value can be increased even further.
Two different versions of the tool are already in use in Pöttinger’s production and further projects may follow. „This was Horn’s first tooling project in our production. The technical implementation and the advice provided by Wedco and Horn fully met our expectations. I am looking forward to further projects“, says Grafe.