Mumbai University > Mechanical Engineering > Sem 4 > Production Process II

Marks: 4M

Year: May 2015

In gear hobbing the gear blank is rolled with a rotating cutter called a hob. A majority of involute gears are manufactured by this method. A hob looks like a worm but carries a number of straight flutes, cut all around, parallel to its axis. This results in the production of separate cutting teeth and cutting edges. In operation the hob is rotated at a suitable speed and fed into the gear blank. The blank is also rotated simultaneously. The speeds of the two axes are so synchronized that the blank rotates through one pitch for each complete revolution of the hob. The hob teeth are just like screw head, i.e. having a definite helix angle. The hob is therefore tilted to its own helix angle while cutting the gear so that its teeth are square with the blank and produce a true involute shape.

Each cutting tooth of the hob is provided relief at its back to enable clearance. The cross section of very tooth across the helix resembles that of the rack teeth as shown in the figure. The operation of gear hobbing starts with the in-feed to the revolving hob till it reaches the required depth of the gear tooth, or else moving the gear blank towards the hob until the desired depth of the tooth is acquired. There is no pause in the rotation of the hob or the blank the operation is a continuous one.