Gear Production

SEP 2016

Gear Production

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6—GEAR Production Supplement Hobbing—When it comes to aggressive material removal while machining spur and helical gears as well as pinions, hobbing is the gold standard, Mr. Fussell says. He describes the hob he uses as a fully coated, multi-start cutting tool that is cylindrical in shape with helical cutting teeth. These teeth have gashes along the length of the hob that aid in both cutting and chip removal. A specialized form of milling, hobbing can rough out gears at a speed that enables Schafer to produce a million gears a year quickly enough to avoid creating a backlog in the machining process (a significant number of Schafer's gears are manufactured using the standard "hob, heat treat and finish" grind technique). At the same time, not everyone requires a super-fine surface finish like that produced for industries such as aerospace. In such cases where a fine finish is not required, perfectly functional pinions can be hobbed and then heat treated without finish grinding. Schafer's approach to harnessing the power of hobbing is to first equip each machine with automation so that a single operator can run as many as four machines. When hobbing is conducted properly, the accuracy is such that just 0.004 inch per flank of material is left to be removed after heat treat, which the grinders can handle in less than a 1-minute cycle on many parts. To meet its production goals, Schafer has 15 to 20 hobbers running at a time, primarily machining 8620, 4140 and 9310 alloy steels, as well as bronze and stainless steel. Some machines are equipped with a spline roller, which basically "crushes" the spline into the pinion shaft, and others have either automatic deburring machines in the cell or make use of an option to mount deburring tools directly onto the hobber. Despite its many attributes, however, hobbing can't be used to cut gear teeth with an adjacent shoulder due to the shape of the hob and clearance issues, Mr. Fussell points out. This is where shaping comes in. Shaping—This machining process creates gear teeth by using a cutter that is applied to the workpiece in a continuous rotating movement along the same plane. The four basic types of cutters—disks, hubs, shanks and helical cutters—actually mimic the forms of the gears they're used to produce. The cutter is located on one spindle and the workpiece revolves around the tool on another spindle in a reciprocating motion that duplicates the gear tooth form of the cutter. The process is used to produce internal gears, external gears and gear-pinion arrangements. It is also the only way to address the aforementioned situation when gear teeth are located near an adjacent shoulder, which no other process can produce competitively, resulting in a higher cost per part than other processes due to its somewhat complicated tooling. Both machine tools and cutting tools have been greatly improved upon in recent years, Mr. Fussell says. Newer CNC shapers and machine Hardened 8620 steel was used for this helical gear. Its teeth were cut by hobbing and finished by gear grinding after heat treating. This helical gear is made of a ductile iron casting, with gear teeth cut by hobbing and then finished by shaving. F E A T U R E

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