Methods of Gear Cutting

Methods of Gear Cutting

Methods of Gear Cutting - The selection of the gear cutting method depends essentially on the established accuracy standards of various elements of the gears, as well as on the main requirements made to the gearing in operation, which in turn, depend upon the purpose of the gearing.

Gears may he cut by a number of methods as explained under; however, two general and most commonly used processes of cutting gears are the generating processes and the form cutting processes.

Gear cutting methods:

(a) Casting

Sand casting
Die casting
Investment casting 

(b) Stamping.
(c) Powder metallurgy.
(d) Extruding.
(e) Plastic molding.
(f) Rolling.
(g) Machining

(i) Form cutting processes

Form cutter in milling machine.
Form cutter in broaching machine.
Form cutter in shaper
(ii) Template process

(iii) Generating processes

Gear hobbing
Gear shaping
Rack planning
(iv)Shear cutting
(v) Broaching,

(a)Casting processes Sand cast gears have rough surfaces and are not accurate dimensionally. They are used only for services where the gear moves slowly and where noise and inaccuracy of motion can be tolerated.

Gears made by die casting are fairly accurate and have fair surface finish. They can be used to transmit light loads at moderate speeds.

Gears made by investment casting may be accurate and have good surface characteristics. They can be made of strong materials to permit their use in transmitting heavy loads.

The materials for gears made by die and investment casting processes are limited to low temperature melting metals and alloys; consequently, these gears do not have the. wearing qualities of heat treated steel gears.

(b) Stamping Stamping, although reasonably accurate, can be used only in making thin gears from sheet metal. Large quantities of gears are produced in a punch press. The thickness of such gears is limited to 1.5 mm. By shaving the gears after they are blanked, excellent accuracy can be achieved.

A wide variety of materials such as low and medium carbon steels, all brass alloys, aluminium alloys, high carbon and stainless steels can be stamped. Such gears are used in clocks, watches, meters and calculating machines.

(c) Powder metallurgy High quality gears, both as to dimensional accuracy and surface finish, can be made by the powder metallurgy process. Powder metallurgy process is employed only for small size gears, ordinarily less than 25 mm in diameter. However, larger and excellent gears now are being made by forging powder metallurgy performs. Gears made in this way have much greater density and strength than those made by ordinary powder metallurgy methods and require little or no finishing.

(d) Extruding process Quite accurate small sized gears can be made by the extrusion process. Long lengths of rod, having the cross-section of the desired gear are extruded. The individual gears are then sliced from this rod. The only materials suitable for making gears by extrusion process are

Brass - Aluminium alloys
Bronze - Magnesium alloys.

(e) Plastic molding Plastic gears, in large quantities, are made by this process. The quality of gears thus made is good and they are suitable only for light loads. The most common thermo plastic material used for making gears is nylon. Nylon gears are highly wear resistant. Moreover no lubrication is necessary.

For making gears, the thermoplastic materials are injection molded, Gears are produced by the compression molding process in the case of thermosetting materials. Accurate gears suitable for heavy loads frequently are machined out of laminated plastic materials. Plastic gears are used extensively mated with, or running between, two metal gears, because they have the quality of reducing noise.

(f) Rolling process Gears may be made both by hot rolling and cold rolling processes. Rolling process is applicable to any ductile material. Straight carbon steel, structural alloy steels and non lead brasses are commonly used materials for rolling, Rolling process has mainly been applicable to cutting worms and involute splines.

In rolling process, the workpiece lies between cylindrical rollers provided with teeth and mounted on a robust machine. The centre distance of the rollers is adjustable. Both the rollers are of the same diameter and have equal number of teeth and same module.

In hot rolling process, suitable for spur and helical gears, the workpiece is heated by induction heating and then it is rolled between the rollers while it is in the plastic state. Large deformations are possible by hot rolling process.

Currently a very large quantity of gears; worms, and splines are being made by cold roll forming. This process basically is the same as that by which screw threads are roll formed, except that in most cases the teeth cannot be formed in a single rotation of the forming rolls; the rolls are fed inward gradually during several revolutions.

Cold Roll Forming of Gears

Roll formed gears possess excellent accuracy. In addition severe cold working produces tooth faces that are, very smooth and harder than \hose on ordinary machined gears. They have excellent wear characteristics.

Cold roll forming is rapid, can be easily mechanized, nochips are produced, thus less material is needed and skilled labour is not required.