Gear Cutting Is All About Hobbing, Shaping and Grinding

There are several methods that are used in gear cutting. The first one is gear hobbing, where the hobber die and the gear blank are rotated at the same time and speed, when the profile of the hob is transferred on to the gear blank. A milling machine or a jig grinder is used to cut spur and other straight gears. This mechanism makes use of gear cutters that are numbered, along with indexing head or rotary table. The tooth count of the gear forms the basis in determining the number of gear cutters used, and any of the straight gears can be produced this way.

When machining a helical gear, or twist drill, on a machine that is operated manually, a true indexing fixture needs to be used. The indexing fixture would be required to be taken off from the drive worm, and fixed with a gear-train to the machine table, which can then be operated as a carriage on the lathe. With this kind of a system set up, while the machine table moves on the x-axis, the fixture would be moving at a fixed accurate incremental ratio with respect to the table. The machine table would move in a highly controlled manner via the index plate, producing a precision linear movement, such as, a vernier scale.

The gear-cutting tool comprises of a cutter body member, which has a circular shape, a front and a back surface, and an axis on which it rotates. An outer section of this cutter body has several individual projections, arranged equally spaced from each other around the cutter body. The spaces between the projections are so arranged that they can receive the slots for the cutting blades.

There are several machines which are used in gear cutting, and these are briefly mentioned below:

Hobbing Machine
This is a special kind of milling machine, which is used for cutting gears. It is the major machine with is essentially used for spur gears of involute form. This process uses the method of rotating the gear blank and the cutter at the same time, maintaining a fixed ratio between the cutter and the gear blank.

Gear Shaper
For cutting the internal and external gear teeth, a gear shaper machine tool is used. The name "shaper "comes from the action of the machine itself, where the concerned part is engaged on the forward stroke of the cutter, and pulls away from the part on the return stroke. The cutting tool of the shaper has the same pitch that of the gear to be cut.

Gear Grinder
This tool is used to automatically grind a helical gear. A drive source in this machine tool drives a rotating grinding wheel. A second rotating driving source is used to rotate a helical gear, meshed with the teeth on the grinding wheel. The third driving source in the machine tool displaces the helical gear and the second drive source simultaneously. Two pulse generators are used, which are connected to the first and the third drive sources. The pulse output from one is subtracted from the output of the other, and this differential is applied as the drive pulses for the second rotational drive.