After completion of 1 or two teeth, the blank and gear rack for Machine Tool Industry cutter stop feeding and the cutter is withdrawn and indexed back to its starting position, thus enabling a short rack cutter of a practical size to be used. Cutter is once again fed back again to depth and cycle is repeated. Number of teeth is controlled by the device gearing, and pitch and pressure position by the rack cutter. This technique is utilized for generation of external spur gears, being ideally suited for cutting large, dual helical gears. For making helical tooth, the cutter slides are inclined at the gear tooth helix angle.
The hob is fed into the gear blank to the proper depth and the two are rotated together as though in mesh. The teeth of the hob cut into the work piece in successive purchase and each in a somewhat different position. Each hob tooth cuts its own profile depending on the form of cutter , but the accumulation of these straight cuts generates a curved type of the gear teeth, thus the name generating procedure. One rotation of the work completes the cutting upto certain depth upto which hob is usually fed unless the gear has a wide face.
This methodis specifically adopted to cutting large teeth which are challenging to cut by formed cutter, and also to cut bevel-gear teeth. It isn’t widely used at present.
In gear planing process, the cutter consists of true involute rack which reciprocates over the face of the blank and the blank rotates in the right relationship to the longitudinal movement of the cutter as if both roll together as a rack and pinion. At first the cutter is usually fed into complete tooth depth with cutter reciprocating and blank stationary. Involute shape is produced as the blank rotates and involute rack cutter feeds longitudinally.
In the other method, both roughening and completing cuts are taken with single pointed tools. The utilization of the formed tool for finishing is usually impracticable for the bigger pitches which are completed by an individual pointed tool. The number of cuts required depends upon how big is the tooth, quantity of share to be removed, and the type of material.