In pitchgear machining, full cut hob is often used. Although the cutting amount of hob increases, the cutting force will not increase much and vibration will not be generated due to the small teeth. This method can make the top gear and the ring gear develop at the same time, keep the top circle and the base circle concentric, so that the gear has a more uniform radial clearance when meshing. The method of machining gear with full cutting hob can reduce the accuracy of the outer circle of the gear blank, and there is no burr on the outer diameter of the gear. At the same time, the tooth thickness can be ensured by measuring the diameter of the tooth top circle.
Most of the pitchgear hobs are single headed. Generally, when the number of hobs increases, the productivity will also increase, but the quality of the processed gear will be reduced. When the diameter of the hob is fixed, the increase of the number of heads will increase the helix angle, which will increase the hob modeling error. Because each thread can only process a certain number of slots, when the number of hob heads increases, the number of hob thread turns for processing each slot decreases, that is, the number of teeth to be cut decreases, so that the tooth surface finish of the gear to be processed decreases. As each tooth shape of the gear is cut by the same number of teeth on the hob, for single head hob, even if there is an error in the pitch, it does not affect the pitch of the processed gear. However, there are indexing errors in the multi head hob, which cause the pitch error of the hob itself. When machining the gear, these errors will be reflected on the workpiece, causing the pitch error of the processed gear. And the single head hob is easy to manufacture, while the multi head hob is troublesome.
However, it should be pointed out that the productivity of multi head hob is very high, so if the manufacturing accuracy of multi head hob can be improved, multi head hob will be widely used in gear machining.