By Mike Finn, Development Engineer at Mazak Corporation
At IMTS 2014, Mazak showcased how larger sized 5-axis multi-tasking vertical machining centers could allow a shop to cost-effectively and quickly produce those occasional, low-volume spiral-bevel gear sets in-house. We machined a spiral bevel gear set that included a 29-tooth, 22"-diameter pinion gear and a 114-tooth, 6-ft-diameter ring gear on our INTEGREX e-1550V/10 Multi-Tasking 5-axis Machining Center. We completed the gear set in days as opposed to months and using only the one machine.
Most recently, however, we successfully power skived gears using the highly synchronized rotation of both the milling spindles and C-axes on our INTEGREX i-200, INTEGREX e-420 and INTEGREX i-630 Multi-Tasking Machine models. With these machines, we power skived gears in several sizes and types, including standard and I.D. splines in both straight and angled teeth patterns along with spur gears and helical gears.
Unlike high-volume dedicated gear skiving systems, multi-tasking machines provide the flexibility to perform other part operations as well. You could, for instance, turn a part’s I.D. and O.D. and cut its other mating features – all on the same machine that will also power skive the part’s gear tooth pattern. This significantly improves overall part accuracy because all the machined features run true to the gear teeth. Plus, the same multi-tasking machine that power skives a gear for you one day can machine completely different, non-geared components for you the next.
No one will dispute that dedicated skiving systems are fast, but multi-tasking machines are much more cost effective in comparison, especially when they eliminate the need to farm out occasional gear work. By keeping your occasional gear cutting in-house, you also gain complete control over part processing, as well as delivery times because outside vendor turnaround times are no longer an issue.
Synchronization and proper cutting ratio are key in power skiving. During the operation, the workpiece – mounted on a table or in a turning spindle depending on multi-tasking machine model – rotates in synchronization with that of the skiving tool. Rotation of the two also happens at a specific ratio dictated by the number of teeth being cut in the part and the number of teeth on the cutter itself.
Keep in mind that the power skiving operation is a precarious and parameter-specific process in that there is very little room for error. As both the workpiece and cutter masses spin, the slightest miscalculation in terms of synchronization will immediately cause chatter that will continue until you completely remove the cutter from the cut and make the proper adjustments.
Also there can be limitations in terms of rpm – how fast the C-axis can spin while maintaining proper synchronization with the milling spindle. Dedicated skiving machines typically offer higher synchronized-rpm capabilities. However, thanks to our new MAZATROL SmoothX CNC control technology, we continue to boost the speeds/rpms at which our multi-tasking machines maintain skiving synchronization. Synchronization at faster rpms helps shorten power skiving cycle times and allows a machine to cut a wider range of gear sizes.
Another aspect of power skiving is that workpieces rotate faster than in a hobbing operation. Consider, for instance, a 40-tooth gear would rotate at 8.75 rpm while using a single start hob rotating at 350 rpm. To power skive the same gear, the part would need to rotate at 218.75 rpm while using a 25-tooth cutter rotating at 350 rpm. For the necessary skiving speed and accuracy, our multi-tasking machines incorporate integral motor C axes.
Power skiving also requires a very rigid cutting tool-spindle interface with absolutely zero rotational play. If the tool moves at all, the result will be vibration that, again, will continue until the tool is completely removed from the cut.
In regards to workpieces, the intended gear pattern location should be such that it allows for interference-free machining. The multi-tasking machine’s spindle needs plenty of room to tilt – almost to a horizontal orientation between 7 and 15 degrees – and maneuver the skiving tool for cutting. Basically, the gear patterns must be close enough to the workpiece ends to provide ample room for either O.D. or I.D. power skiving.
As a final comment, dedicated gear cutting equipment is not going away. Multi-tasking machines simply provide you another option for doing smaller batches of gear cutting in addition to your other part processing operations.