by Mike Finn
A viable option for shops that machine more than just gears
Today’s multi-tasking machines continue to command a presence in the world of gear cutting as viable processing alternatives for low to medium volume production. What started with five axis cutting of larger diameter ring-gear sets has now ratcheted up to power skiving gears using the highly synchronized rotation of a multi-tasking machine’s milling spindle and C-axis.
Unlike high volume dedicated skiving systems, multi-tasking machines provide the flexibility to perform other part operations. Shops can, for instance, turn a part’s I.D. and O.D. as well as 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 one day can machine completely different, non-geared components the next.
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 gear cutting in-house, shops also gain complete control over part processing, as well as delivery times because outside vendor turnaround times are no longer an issue.
Of all the challenges involved with machining gear sets, imparting the correct involute form on the pinion gear is the most daunting, and in machining, Mazak uses several proprietary techniques to minimize close out error. In doing so, the company is able to determine the minimal amount of stock to leave on the pinion gear to correct for warping from the heat treating process, yet also leave enough stock to fulfill the case hardened depth requirement. Removing too much material during the finish machining operation can compromise the gear’s case hardened depth, thus affecting the gear’s operation and its working life.
While multi-tasking machines can generate consistently precise matched large spiral bevel gear sets, the process is also dependent on a well-maintained machine tool. Worn and out of tolerance machines will be unable to drive consistently precise five axis toolpaths, which is key to accurate gear set machining, as well as any other five axis machining application for that matter. Machines must be reliable, square and deliver repeatable motion.
For the necessary speed and accuracy, multi-tasking machines intended for power skiving gears must have integral motor C axes. Those that are gear driven are unable to achieve high enough rpms and, simply by the nature of their design, allow for too much play. 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.
Regardless of the tooling and processes used, interest continues to grow in cutting gears on multi-tasking machines. And many shops involved in key industries such as heavy equipment, motorsports and others currently benefit from doing so for their low-to-medium volume gear production. Others that benefit are those shops that want to bring gear cutting back in-house and those that are simply unable to justify the cost of replacing worn out existing pieces of dedicated gear cutting equipment. But, it should be noted that dedicated gear cutting equipment is not going away. Multi-tasking machines simply provide another option for those shops doing smaller batches of gear cutting in addition to other part processing operations. SMT
Mike Finn is a process development engineer at Mazak Corp.