MANUFACTURING OF GEARS
Gear
manufacturing refers
to the making of gears. Gears can be manufactured by a
variety of processes, including casting, forging, extrusion, powder metallurgy, and blanking. As a general rule, however, machining
is applied to achieve the final dimensions, shape and surface finish in the
gear. The initial operations that produce a semi finishing part ready for gear
machining as referred to as blanking operations; the starting product in
gear machining is called a gear blank.
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of
the desired shape, and then allowed to solidify. The solidified part is also
known as a casting, which is ejected or broken out of the mold to
complete the process.
Forging is a manufacturing
process involving
the shaping of metal using localized compressive forces.
Extrusion is a process used to create objects of a fixed
cross-sectional profile. A material is pushed through a die of the desired
cross-section.
Powder
metallurgy (PM)
is a term covering a wide range of ways in which materials or components are
made from metal powders. PM processes can avoid, or greatly reduce, the need to
use metal removal processes, thereby drastically reducing yield losses in
manufacture and often resulting in lower costs.
Blanking and piercing are shearing processes in which a punch and die are used to modify webs. The tooling and processes are the
same between the two, only the terminology is different: in blanking the
punched out piece is used and called a blank; in piercing the punched
out piece is scrap.
Selection of materials
The gear material should have the following
properties
- High
tensile strength to
prevent failure against static loads
- High
endurance strength to withstand dynamic loads
- Low
coefficient of friction
- Good
manufacturability
Gear manufacturing processes
There are multiple ways in which gear blanks can be
shaped through the cutting and finishing processes.
Gear forming
In gear form cutting, the cutting edge of the
cutting tool has a shape identical with the shape of the space between the gear
teeth. Two machining operations, milling and broaching can be employed to form cut gear teeth.
Form milling
In form milling, the cutter called a form cutter travels axially
along the length of the gear tooth at the appropriate depth to produce the gear
tooth. After each tooth is cut, the cutter is withdrawn, the gear blank is
rotated, and the cutter proceeds to cut another tooth. The process continues
until all teeth are cut.
Broaching
Broaching can also be used to produce gear teeth and is
particularly applicable to internal teeth. The process is rapid and produces
fine surface finish with high dimensional accuracy. However, because broaches
are expensive and a separate broach is required for each size of gear, this
method is suitable mainly for high-quality production.
Gear generation
In gear generating, the tooth flanks are obtained
as an outline of the subsequent positions of the cutter, which resembles in
shape the mating gear in the gear pair. There are two machining processes
employed shaping and milling. There are several modifications of these
processes for different cutting tool used.
Gear hobbing
Gear hobbing is a
machining process in which gear teeth are
progressively generated by a series of cuts with a helical cutting tool. All
motions in hobbing are rotary, and the hob and gear blank rotate continuously
as in two gears meshing until all teeth are cut.
Finishing operations
As produced by any of the process
described, the surface finish and dimensional accuracy may not be accurate
enough for certain applications. Several finishing operations are available,
including the conventional process of shaving, and a number of abrasive
operations, including grinding, honing, and lapping.
Honing is an abrasive machining process that produces a precision surface on a metal work piece by scrubbing an abrasive stone against it along a controlled path.
Lapping is a machining process, in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine.
American Gear Manufacturers Association
The American Gear Manufacturers Association or AGMA is
the trade group of
companies in manufacturing gears and gearing. AGMA was founded in 1916; there
are currently about 430 AGMA member companies.
