The hobbing process for cutting the teeth in spur and spiral gears is beginning to be very widely used. The principle of this method is shown diagrammatically in the accompanying illustration. In the lower part Of the illustration is shown an imaginary rack (in dotted lines); this rack is in mesh with the gear, the teeth of which are to be formed, and if the blank could be imagined as made Of a plastic material, the rack, if moved along as indicated by the arrow, while the gear rotated to correspond, would form theoretically correct teeth in the gear blank. The teeth of this rack coincide with the outlines of the worm shown in full lines, this latter having been set at such an angle as to make the teeth on its front side parallel with the axis Of the gear. In other words, it has been set at the angle of its helix, measured at the pitch line. This worm, when properly ﬂuted, forms the hob for cutting the gear teeth. It will be seen that the teeth of the hob, when set in this position, correspond with the teeth of the rack. If, now, the hob and blank be rotated at the ratio required by the number of threads in the hob and the number of teeth in the gear, this movement will cause the teeth of the hob to travel lengthwise in exactly the same way as the teeth of the imaginary rack would travel, if in mesh with the gear, the teeth of which are to be cut. It will thus be seen that the hob fulfills the requirements necessary for molding the teeth of the gear to the proper form. In practice the hob is rotated in the required ratio with the work, and fed gradually through it from one side Of the-face to the other. When it has passed through once, 'the work is completed. Of the great number of machines built during the past few years involving this principle, many are arranged for cutting Spiral gears as well as spur gears. Of course, all of the machines capable Of cutting spiral gears are capable of cutting spur gears also. The spiral gear-hobbing machine bears about the same relation to the plain spur gear-hobbing machine that the universal does to the plain mill ing machine. The added adjustments and mechanism required in each case tend to somewhat limit the capacity of the machine in tak ing heavy cuts although they add to its usefulness by extending the range of work It is capable of performing.