CHAPTER XI SHAPING OF GEAR TEETH Another general process used for the production of gear teeth consists of shaping or planing the tooth forms. The earlier machines of this type used a form tool of the shape of the tooth space or a standard shaping tool with a template to control its position. These methods are used today, chiefly on very large gears. Where a template type of shaping machine is used, a series of templates corresponding to a series of formed milling cutters can be used, and corrections in the settings of these templates to compensate for any theoretical error in the form can be made exactly as previously described for formed milling cutters. The same is also true when a formed shaping tool is employed. Fundamentally, these two shaping methods are identical to form milling of gears. The only difference is that a shaping tool is substituted for the formed milling cutter. Another method of shaping gear teeth consists of a molding or generating process on the principle of two gears in mesh, where one of the gears is a reciprocating cutter while the other is the gear blank that is being molded to form. This process was developed by the Fellows Gear Shaper Company. In Fig. 169, one of these machines is illustrated. The gear blank is held on an arbor while the cutter is mounted on a spindle carried in the reciprocating ram of the machine. The cutter and work spindles are connected by means of gearing, and each is provided with an index wheel and worm. Change gears are used to obtain the proper relation between the number of teeth in the cutter and the number of teeth in the gear being cut. In operation, the cutter and work rotate slowly together, thus producing on the gear blank the conjugate form to the teeth on the cutter. Here, as with hobbing, the accuracy of the product depends upon three main factors: the accuracy of the machine, the accu- racy of the cutter, and the carefulness of the operator in mounting the gear blanks and in resharpening the cutters.