Gear Production

SEP 2015

Gear Production

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16—GEAR Production Supplement F E A T U R E number of teeth being cut in the part and the number of teeth on the cutter itself. The power skiving operation is a precarious and parameter-specifc 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 the cutter is completely removed from the cut and adjustments are made. There can be limitations in terms of rpm—how fast the C axis can spin while maintaining proper synchronization with the milling spindle—with traditional gear cutting processes. Dedicated skiving machines typically offer higher synchronized-rpm capabilities. However, with its new Mazatrik SmoothX CNC control technology, Mazak continues to boost the speeds/rpms at which its multitasking machines maintain skiving synchronization. Synchronization at faster rpms helps shorten power skiving cycle times and allows the 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. To produce the necessary speed and accuracy, multitasking machines intended for power skiving gears must have integral motor C axes. Those that are gear driven are unable to achieve high enough rpm 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. Skiving tools, like gear hobbers, progressively generate all the gear teeth profles on a part simultaneously, as compared with machining each tooth individually with a standard milling cutter. However, there are clearance requirements that must be met for power skiving tools. The intended gear pattern location should be such that it allows for interference-free machining. The 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 OD or ID power skiving. Consider Power skiving is a gear shaping process in which the cutting tools are shaped like gear teeth, spinning and meshing with the work- piece to generate gear tooth geometry. Like hobbing, power skiving progressively generates gear tooth profles simultaneously.

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