Gear generating

The tooth flanks are obtained (generated) as an outline of the subsequent

positions of the cutter, which resembles in shape the mating gear in the gear pair.

In gear generating, two machining processes are employed, shaping and milling.

There are several modifications of these processes for different cutting tool used

1. Gear shaping with a pinion-shaped cutter

2. Milling with a hob (gear hobbing)

3. Gear shaping with a rack-shaped cutter.

Gear Blank

Shaper

Gear ShapingGear shaping process is defined as a process for generating gear teeth by a

rotating and reciprocating pinion-shaped cutter.

It is a true shape-generation process in which the gear-shaped tool cuts itself

into mesh with the gear blank.

The cutter axis is parallel to the gear axis and cutting may take place either at

the down-stroke or upstroke of the machine.

A train of gears provides the required relative motion between the cutter shaft

and the gear-blank shaft.

Gear shaping is one of the most versatile of all gear cutting operations used to

produce internal gears, external gears, and integral gear-pinion arrangements.

Advantages of gear shaping with pinion-shaped cutter are the high dimensional

accuracy achieved and the not too expensive tool.

The process is applied for finishing operation in all types of production rates.

But any errors in one tooth of the shaper cutter will be directly transferred to the

gear.

Gear HobbingGear hobbing is a machining process in which gear teeth are progressively

generated by a series of cuts with a helical cutting tool (hob).

All motions in hobbing are rotary, and the hob and gear blank rotate continuously

as in two gears meshing until all teeth are cut.

When hobbing a spur gear, the angle between the hob and gear blank axes is

90° minus the lead angle at the hob threads.

For helical gears, the hob is set so that the helix angle of the hob is parallel with

the tooth direction of the gear being cut.

Additional movement along the tooth length is necessary in order to cut the

whole tooth length

The cutting of a gear by means of a hob is a continuous operation.

The hob and the gear blank are connected by a proper gearing so that they rotate

in mesh.

To start cutting a gear, the rotating hob is fed inward until the proper setting for

tooth depth is achieved, then cutting continues until the entire gear is finished.

Machines for cutting precise gears are generally CNC-type and often are housed in

temperature controlled rooms to avoid dimensional deformations.

The gear hob is a formed tooth milling cutter with helical teeth arranged like the

thread on a screw.

No repositioning of tool or blank is

required and each tooth is cut by multiple

hob-teeth, averaging out any tool errors.

Excellent surface finish is achieved by

this method and it is widely used for

production of gears.

Shaping with a Rack-Shaped Cutter

In the gear shaping with a rack-shaped cutter, gear teeth are generated by a

cutting tool called a rack shaper.

The hardened and sharpened rack is reciprocated along the axis of the gear

blank and fed into it while gear blank is being rotated so as to generate the involute

tooth on the gear blank.

RACK

Blank

The rack shaper is actually a segment of a rack.

Because it is not practical to have more than

6-12 teeth on a rack cutter, the cutter must be

disengaged at suitable intervals and returned

to the starting point, the gear blank meanwhile remaining fixed.

Advantages of this method involve a very high dimensional accuracy and cheap

cutting tool (the rack shaper’s teeth blanks are straight, which makes sharpening of

the tool easy).

The process can be used for low-quantity as well as high-quantity production of

spur and helix external gears.

BroachingBroaching 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-quantity production.

Finishing Processes As produced by any of the process described, the surface finish and

dimensional accuracy may not be accurate enough for certain applications.

Poor surface finish results in lot of noise, excessive wear, play or backlash even

leads to failure of tooth.

To overcome the defects finishing is necessary for smooth running of gears.

Finishing operations typically removes little or no material but improves

dimensional accuracy, surface finish.

The various finishing processes are :

Gear Shaving

Gear Grinding

Gear Burnishing

Gear Lapping

Gear honing

Gear Shaving

Shaving is similar to gear shaping in this gear is rotated at high speed in mesh

with a gear shaving tool which is in the form of a rack or pinion and also

reciprocated simultaneously.

The teeth of shaving tool are hardened accurately ground an their faces are

provided with serrations.

Serrations form the cutting edges and will perform scraping operation on the

mating faces of the gear teeth to be finished.

Shaving with rack type tool is used for small gears.

Serrations

Gear GrindingGears required to operate on high speeds an carry heavy loads are always

hardened, which naturally implies a small distortion on the teeth flanks.

In order to remove distortion and have accurate profiles on the teeth for smooth

running, grinding is essential.

Grinding is used to correct the heat-treatment distortion in gears hardened after

roughing.

Improvement in surface finish and error correction of earlier machining are added

advantages.

Two common methods used for grinding hardened gear teeth are forming and

generating.

In forming process the abrasive wheel is first trued to the required shape and size

of gear teeth.

The work is mounted between centers on an indexing head.

In generating process grinding is done by flat faces of grinding wheel.

With a small amount of metal removal high surface finish is obtained.

Gear forming

Gear generating

Gear BurnishingIn burnishing, a specially hardened gear is run over rough machined gear.

The high forces at the tooth interface cause plastic yielding of the gear tooth

surface which improves finish and work hardens the surface creating beneficial

compressive residual stresses.

Gear Lapping and Honing: Lapping and honing both employ an abrasive-impregnated gear or gear-shaped

tool that is run against the gear to abrade the surface.

In both cases, the abrasive tool drives the gear in what amounts to an accelerated

and controlled run-in to improve surface finish and the accuracy.

Lapping