Gear Manufacturing Processes

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Gear manufacturing can be divided into two categories namely forming and machining as shown in flow chart in Fig 5.1. Forming consists of direct casting, molding, drawing, or extrusion of tooth forms in molten, powdered, or heat softened materials and machining involves roughing and finishing operations. They are discussed in the different sections of
this chapter.

Characteristics: In all tooth-forming operations, t he teeth on the gear are formed all at once from a mold or die into which the tooth shapes have been machined. The accuracy of the teeth is entirely dependent on the quality of the die or mold and in general is much less than that can be obtained from roughing or finishing methods. Most of these methods have high tooling costs making them suitable only for high production quantities. The various forming techniques are discussed below in detail:


Sand casting, die casting and investment casting are the casting processes that are best suited for gears and are shown in fig 5.2. T hey are explained in the following sections:

Sand Casting
The characteristics of sand cast gears are,
? Cheaper low quality gear in small numbers
? The tooling costs are reasonable
? Poor Surface finish and dimensional accuracy
? Due to low precision and high backlash, they are noisy.
? They are suited for non- critical applications

Applications: (without finishing operation)
Sand casting is used for gear manufacture which are used in variety of applications such as for toys, small appliances, cement-mixer barrels, hoist gearbox of dam gate lifting mechanism, hand operated crane etc.

The materials that can be sand cast are C I, cast steel, bronzes, br ass and ceramics. The process is confined to large gears that are machined later to required accuracy.

Die casting
The characteristics of die cast gears are,
? Better surface finish and accuracy (tooth spacing and concentricity)
? High tooling costs
? Suited for large scale production Applications:

Gears that are die cast are used in instruments, cameras, business machines, washing machines, gear pumps, small speed reducers, and lawn movers. Fig. 5.3 shows gears that are manufactured by die casting.
Materials used to manufacture these gears are zinc, aluminium and brass. The gears made from this process are not used for high speeds and heavy toot h loading. They are normally applied for small size gears.

Investment casting or lost wax process
The characteristics of gears that are manufactured by investment casting are,
? Reasonably accurate gears
? Applicable for a variety of materials
? Refractory mould material
? Allows high melt-temperature materials
? Accuracy depends on the original master pattern used for the mold.
Tool steel, nitriding steel, monel, beryllium copper are the mate rials that can be investment casted for the manufacture of gears. The process is used onl y if no other process is suitable since production cost is high. Fig 5.4 shows a wire twister stellite gear which mates with a rack made by IC. Complicated shape makes it economical to produce by investment casting process.

Sintering or P/M process :
The powder metallurgy technique used for gear manufacture is shown in fig 5.5.
? Accuracy similar to die-cast gears
? Material properties can be Tailor made
? Typically suited for small sized gears
? Economical for large lot size only


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