Cutting Helical Gears on Hobbing Machine

In cutting spur gears with single start hob, the speed (rpm) of the table is equal to that of the hob divided by the number of teeth in the gear being cut. However when cutting helical gear this relationship cannot be maintained; but it is dependent on the lead of the gear, the hands of the hob and the gear. As the hob feeds axially, the gear has to be advanced or retarded depending on the hands of the hob and the gear so as to generate the helix form of the helical gear. This can be better explained by Fig.7. We consider two examples, one cutting a spur, and the second cutting a helical gear with the same hob and with lead L. The hob is fed axially in both the cases with feed rate f mm/rev. initially, for both the spur and helical gears, the radial line PB from the center of the hob tooth is linear with the radial line OA from the center of the tooth space generated in gear, and the points A and B are coincident. Suppose the hob has now traversed axially L/4 distance. For spur gear, the radial line PB and OA are in one axial plane. However for helical gear, OA has to be advanced ¼ revolution with respect to PB so as to generate the helix. Similarly, when the hob has traversed L/2 distance axially, the gear should be advanced by ½ revolution and so on. So when the hob has traversed full lead length L, for the spur gear, the gear (and the radial line PB) would have rotated by N revolutions; but for the helical gear, the gear would have rotated one extra revolution, i.e., it would have done N + 1 revolutions. This compensation to the table speed is given in many machines by differential gearing. Where differential arrangement is not provided, this has to be obtained by a certain relationship between the feed rate and index change gears.