Gears may have internal or external teeth and are available in forms that typically relate to axis positions, which include:
• Parallel axes, which pertain to two or more tangent pitch shafts using either spur gears-the most common type of gear, or helical gears.
• Intersection axes using straight bevel gears, spiral bevel gears, or zerol bevel gears.
• Non-intersecting, non-parallel axes, which refer to worms and worm gears, crossed helical gears, and hypoid gears.
Racks and pinions which are used to convert rotary motion to linear motion are other familiar gear types.
Materials used in gear manufacture may include steel-which is the most common material, and various nonferrous materials including plastics and composites. Manufacturing methods include: machining, forging, casting, stamping, powder-metallurgy techniques, and plastic injection molding.
Of these, machining is themost common manufacturing method used. Gear machining is classified into two categories:
Gear generating involves gear cutting through the relative motion of a rotating cutting tool and the generating, or rotational, motion of the workpiece. The two primary generating processes are hobbing and shaping.
Hobbing uses a helically fluted cutting tool called a hob. Both the hob and the workpiece rotate as the hob is fed axially across the gear blank. Hobbing is limited to producing external gear teeth on spur and helical gears. Hobbing can be performed on a single gear blank, but also allows for stacking of multiple workpieces, increasing production rates.
Shaping produces gears by rotating the workpiece in contact with a reciprocating cutting tool. The cutter may be pinion shaped, a multi-tooth rack-shaped cutter, or a single-point cutting tool.
Gear form-cutting uses formed cutting tools that have the actual shape, or profile, desired in the finished gear. The two primary form-cutting methods are broaching and milling.
Broaching is the fastest method of machining gears and is performed using a multi-tooth cutting tool called a broach. Each tooth on the broach is generally higher than the preceding tooth. As a result, the depth of cut increases with each tooth as the broaching operation progresses. Broaching is typically used to produce internal gear teeth. External teeth can be broached using “pot broaching”. In this process a hollow broaching tool, called the pot, is used to cut the gear teeth.
Milling is a basic machining process which uses the relative motion between a rotating, multi-edge cutter and a workpiece to cut individual gear teeth. A variation of the process, called “gashing”, is used to produce large, coarse-pitch gears. Gashing is used on heavy-duty milling machines and involves plunging the rotating cutter into a blank for rapid metal removal.
Gears & Gear Manufacture
After gear manufacture, those require a number of finishing operations. Finishing operations include heat
treatment and final dimensional and surface finishing. This finishing can be accomplished using:
Shaving is performed with a cutter having the exact shape of the finished gear tooth. Only small amounts of
material are removed by a rolling and reciprocating action. The process is fast but generally expensive due
to the cost of machinery and tooling. Shaving is typically performed prior to heat treating.
Grinding sometimes serves as an initial gear production process, but is most often employed for gear
finishing. Grinding is classified as either form grinding or involute-generation grinding.
Form grinding uses wheels having the exact shape of the tooth spacing. The grinding wheels are either
vitrified-bond wheels, which require periodic re-dressing, or Cubic Boron Nitride (CBN) wheels, which can
last hundreds of times longer than vitrified wheels without dressing.
Involute-generation grinding refers to a grinding wheel or wheels used to finish the gear tooth by axially
rotating the workpiece while it is reciprocated in an angular direction, which in turn is determined by the type
of gear being finished. This type of grinding is performed either intermittently or continuously. Intermittent
grinding uses tooth profiles dressed on cup wheels, or on one or two single-rib wheels. Each tooth is ground
individually, then the next is indexed to the wheel. Continuous grinding uses grinding wheels with the rack
profile dressed helically on the outside diameter. Both the grinding wheel and the work turn in timed
relationship for continuous finishing.
Honing involves the meshing of the gear teeth in a cross axis relationship with a plastic, abrasive
impregnated gear shaped tool. The tool traverses the tooth surface in a back and forth movement parallel to
the workpiece axis. Honing polishes the gear tooth surface and can be used to correct minor errors in gear
Tung Yen’s Gear Manufacture
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