In forming the teeth of worm gears, the greater part of the space is cut out by a stocking cutter or roughing cutter, which is adjusted at a proper angle, according to the pitch of the worm which the worm gear is to fit, and gradually sunk into the face of the worm-gear blank so as to form the spaces between the teeth. This revolving cutter will not produce the correct form for the teeth, as they must fit the sides of the worm thread. Recourse is therefore had to a cutter called a hob, which is shown in Fig. 278. This cutter is in effect a worm, across the threads of which are formed deep grooves, thereby producing cutting faces as shown in the engraving. Each of the teeth shown is relieved, or backed off, so that when the faces of the teeth become dulled by use and are ground, the accurate form of the teeth is not changed. This hob is mounted in the exact position that the worm is to take with reference to the worm gear, except that the centers of the spindle carrying the hob, and the arbor carrying the worm gear, are slightly farther apart, and so arranged as to be brought to the exact distance apart, as the hob shapes the teeth of the worm gear. This operation, called hobbing, is performed by the rotation of the hob, which, acting as a screw and producing the rotation of the worm gear, forms the teeth by its cutting action. In some cases the worm gear is positively rotated by suitable gearing. Previous gashing is then unnecessary.

Fig. 278. Hob for Forming Teeth of Worm Geara.

In Fig. 279 is shown the usual form of a rotating cutter for producing the involute form of gear teeth. The teeth of these cutters are relieved, or backed off, so that their form is not changed when ground upon the face after they have become dulled by use.