Each method offers special characteristics relating to quality, production quantity, cost, material
and application.

17.1 Generation of Gear Teeth
Machining constitutes the most important method of generating gear teeth. It is suitable for high precision gears in both small and large quantities.

17.1.1 Rack generation — This is the basic method of producing involute teeth. The rack cutter forms conjugate tooth profiles on the blank as the rack and blank are given proper relative motion by the drive mechanism of the generating machine. As the rack traverses the gear blank, it is reciprocated across the blank face. Cutting edges on the rack teeth generate mating conjugate teeth on the blank. The chief disadvantage of this method is that the rack has a limited length which necessitates periodic indexing. This limits both operating speed and accuracy.

17.1.2 Hob generation — This is the most widely used method of cutting gear teeth. It is similar to rack generation except that the rack is in the form of a worm. Referring to Figure 1.39, the central section of the hob is identical to that of the worm and gear. The differences are that the thread of the hob is axially gashed or fluted in several places so as to form cutting edges, while the sides and top of these teeth are relieved behind the gash surface to permit proper cutting action. This arrangement, in eftect, gives an infinitely long rack so that cutting is both steady and continuous. To generate the full Width of the gear, the hob slowly traverses the face of the gear as it rotates. Thus, the hob has a basic rotary motion and a unidirectional traverse at right angles. Both movements are relatively simple to effect, resulting in a very accurate process.
A further advantage of hobbing is that the hob can be swiveled relative to the blank axis. This permits cutting helical gears of all angles with the same tooling.
With regard to accuracy, hobbing is superior to the other cutting processes. Gears can be directly hobbed to ultra-precision tolerances without resorting to any secondary refining processes.

17.1.3 Gear shaper generation — This process, unlike the other two, employs a gear-shaped cutter instead of a rack or the equivalent. Uke a rack cutter, a given gear-shaped cutter is conjugate to all tooth numbers of that pitch. Thus, a gear made as a cutting tool can generate the teeth of a blank when the two are rotated at proper speeds. The cutting tool can be imagined as a gear that axially traverses the blank with a reciprocating axial motion as it rotates. The teeth on the gear cutter are appropriately relieved to form cutting edges on one face.
Although the shaping process is not suitable for the direct cutting of ultra-precision gears and generally is not as highly rated as hobbing, it can produce precision quality gears. Usually it is a more rapid process than hobbing.
Two outstanding features of shaping involve shouldered and internal gears. Compound gears and shaft gears frequently are designed so compactly that a hob cutter interferes with adjacent material.
In such cases, shaping can be used since the stroke of the gear-shaped cutter requires very little round space on one side of the gear. For internal gears, the shaping process is the only basic method
of tooth generation.
The shaping process can be used for the generation of helical gears. However, each helix angle requires special tooling. Therefore, with regard to helical gears, shaping is not as convenient and is