Gear generating process

1. 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

2. Gear Shaping
Gear 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.

4. 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 (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

5. 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.

6. 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.

7. 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-quantity production.

8. 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

9. 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

10. Gear Grinding
Gears 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.

11. With a small amount of metal removal high surface finish is obtained.
Gear forming
Gear generating

12. Gear Burnishing
In 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