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List Gear manufacturing methods and explain any one in detail with neat sketch.

Toothed gears are indispensable elements in mechanical transmission of power and their accurate production made it necessary for engineers to develop ingenious tools and processes. The most common and accurate method of gear production is the machining processes.

Gear manufacturing methods can broadly classified as follows:

1. Casting
2. Plastic Molding
3. Rolling
4. Stamping
5. Extrusion
6. Powder metallurgy
7. Machining
   • Forming
   • Template method
   • Generating

Castings: Gears cast in sand molds include those which are made of cast iron and very large in size. They are hence inaccurate and used only for low RPM. The finishing of teeth is done by hand grinding process.

Plastic Molding: For light work, gears made from nylon are used which are only suitable for very less loads. Injection molding is a common technique to produce these gears. They are light in weight and give noiseless working.

Rolling: Here a hot gear blank is rolled against a master gear which produces the desired teeth on the hot blank similar to the master gear.

Stamping: It’s used for mass production of gears lesser than 1.5mm in thickness from sheet metal and used in household appliances.

Extrusion: Soft materials like brass, aluminium etc. can be extruded through a forming die such that the extruded shape will carry gear tooth shape all along its length. Smaller lengths are then cut from the bar to form gears.

Powder metallurgy: Used for producing very small but extremely accurate gears which are used in precision equipment’s.

Machining: It’s the most widely used method of gear manufacture. Blanks which are previously cast, forged or cut from a round bar are machined to produce teeth of desired shape and size on their periphery.

• Forming/Gear forming: It comprises of finish machining of the gear teeth to a predetermined tooth profile using form cutters, or a single point form cutting tool. Using a dividing head on a milling machine, spur gear, helical gear, bevels and worm gears can be cut. Rack can also be cut on the milling machine using a special attachment. On the milling machine the blank is mounted between the head and foot stock. The cutter is mounted on a shaft such that its axis is at right angle to axis of the work piece. The work piece is bought to the starting position and the knee is raised to give the required depth of cut. In a single pass only a single tooth is milled. The cutting operation is repeated until all the teeth’s are cut. For cutting helical gears the machine table is set on an angle according to the helix angle of the gear to be cut.

• Template forming: This method is used to form large gears. A suitable template attached to the work piece may serve as a guide for manually feeding a suitable cutter. Large straight tooth bevel gears are made by this method.

Generating method: This method enables to cut mathematically correct tooth profiles by means of relative motion between the cutters and the gear blanks. The basic fundamental behind this method is that two involute profiles with the same module will mesh together. This method is based on the involute system hence cycloidal gears cannot be made using this method.

Generating processes are of the following types:

i) Gear shaper process:

In this process a pinion shaped cutter is used. It carries a hole in the center for mounting on a spindle of the machine. The cutter is mounted with its axis vertical and is reciprocated up and down by sliding the spindle head along the vertical ways of the machine. In addition to the reciprocating motion, the cutter and the gear are rotated slowly about their own axes. The cutter in its rotation generates tooth profile of the blank. All gears cut by the same cutter will mesh correctly. This is an advantage over the forming process. Internal gears can also be cut using this method.

ii) Hobbing process:

In this process the gear blank s 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.