CHAPTER X HOBBING OF GEAR TEETH The method of producing gears by form milling is limited to gears that run at relatively low pitch-line velocities because of the errors in form that inevitably develop from distortion of the form cutter in hardening, inaccuracies in the form of the cutter itself, and errors in setting up the machine. These conditions and also the search for methods of more rapid production led to the development of the hobbing process. This process consists of revolving and advancing a worm-shaped cutter through a revolving blank. The ratio between the speed of the hob and that of the gear blank is determined by the number of threads or starts on the hob and the number of teeth to be cut in the gear blank. Thus, if a single-threaded hob is used to cut a 24-tooth gear, the hob would revolve 24 times while the gear blank revolves once. If a double-threaded hob were used, the hob would revolve 12 times while the gear blank revolved once. In general, the hob is set at such an angle in relation to the teeth of the gear being cut that the helix at the middle of the tooth of the hob is tangent to the sides of the gear teeth. In Fig. 148, the cutter head and work arbor of a typical hobbing machine is shown. Hobbing is a generating or molding process. A simple con- ception of this method is to consider any axial section of the hob as representing the basic rack. Successive axial sections, because of the lead or helix, in effect, will advance this basic rack in the direction of the axis of the hob as the hob is revolved. Thus, when the hob has completed a full revolution, the basic rack will have advanced one full-tooth interval. This process has the advantage of requiring but one hob to produce mating gears of any number of teeth. It also possesses the advantage over form mill- ing of enabling more effective tooth proportions to be used when desired, using the same cutting tool, while a special form cutter would be required in each individual case with form milling. For quantity production, in particular, hobbing has proven to be a rapid and effective method. All motions are continuous, and, except for the feeding, all motions are rotary. Such continuous rotary motions are usually conducive to rapid production. The many advantages of this method of production soon led to its wide adoption. But it soon became apparent that this newer process in itself did not eliminate all of the difficulties of producing accurate gears. There are still many problems to be solved before that end is reached, some in relation to the machines and others in relation to the hobs. We will first direct our attention to the problems in relation to the hobbing machines. Many types of hobbing machines are now on the market. All of them, however, follow certain general lines. The gear train between the hob and the work arbor, which carries the gear blanks, in its simplest form is shown in the diagram, Fig. 149. In addition to the movements shown, provision must be made to allow for the adjustment of the hob to suit both the diameter of the gear blank and the helix angle of the hob. Provision must also be made to obtain the necessary feeding of the hob through the gear blank.