Describes various types of gear manufacturing methods like gear forming, gear shaping, gear hobbing and various gear finishing methods.

1. Gear Manufacturing Methods
PREPARED BY KAUSHAL PATEL

2. Methods Commonly Used for Gear Manufacturing
Gears can be manufactured by most of manufacturing processes discussed so far (casting,
forging, extrusion, powder metallurgy). But as a rule, 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.
Two principal methods of gear manufacturing included.
1. Gear forming, and
2. Gear generating
Each method includes a number of machining processes, the major of them included in this
section.
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3. 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.
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4. 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withdrawn, the gear blank is rotated (indexed), and the cutter proceeds to cut another tooth.
The process continues until all teeth are cut.
Each cutter is designed to cut a range of tooth numbers. The precision of the form-cut tooth
profile depends on the accuracy of the cutter and the machine and its stiffness.
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6. Cont.…
In form milling, indexing of the gear blank is required to cut all the teeth. Indexing is the
process of evenly dividing the circumference of a gear blank into equally spaced divisions. The
index head of the indexing fixture is used for this purpose.
The index fixture consists of an index head (also dividing head, gear cutting attachment) and
footstock, which is similar to the tailstock of a lathe. The index head and footstock attach to the
worktable of the milling machine. An index plate containing graduations is used to control the
rotation of the index head spindle. Gear blanks are held between centres by the index head
spindle and footstock. Work pieces may also be held in a chuck mounted to the index head
spindle or may be fitted directly into the taper spindle recess of some indexing fixtures.
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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.
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8. Gear Generating
In 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. Milling with a hob (gear hobbing)
2. Gear shaping with a pinion-shaped cutter, or
3. Gear shaping with a rack-shaped cutter.
Cutters and blanks rotate in a timed relationship: a proportional feed rate between them is
maintained. Gear generating is used for high production runs and for finishing cuts.
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10. 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.
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12. Cont.…
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 action of the hobbing machine (also gear hobber) is shown in the figures. 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.
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13. Shaping with Pinion Shaped Cutter
This modification of the gear shaping process is defined as a process for generating gear teeth by a
rotating and reciprocating pinion-shaped cutter.
The cutter axis is parallel to the gear axis. The cutter rotates slowly in timed relationship with the
gear blank at the same pitch-cycle velocity, with an axial primary reciprocating motion, to produce
the gear teeth.
A train of gears provides the required relative motion between the cutter shaft and the gear-blank
shaft. Cutting may take place either at the down stroke or upstroke of the machine. Because the
clearance required for cutter travel is small, gear shaping is suitable for gears that are located close to
obstructing surfaces such as flanges.
The tool is called gear cutter and resembles in shape the mating gear from the conjugate gear pair,
the other gear being the blank.
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.
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15. Shaping with Rack Shaped Cutter
In the gear shaping with rack-shaped cutter, gear teeth are generated by a cutting tool called a
rack shaper. The rack shaper reciprocates parallel to the axis of the gear axis. It moves slowly
linearly with the gear blank rotation at the same pitch-cycle velocity.
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.
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17. Gear Finishing
In order for gears to operate efficiently and without noise at a high speed for a reasonable life
span it is important that the profile of the gear teeth be accurate, smooth, without any
irregularities, projections or nicks.
Gear produced by most processes are found not up to the mark in many cases.
Use of gears with inaccurate or rough profile leads to noisy operation, unequal loading of gears
teeth, faster wear and early failure of the gears.
The finishing operation is intended to perform the following functions :
• Correct any error of profile and pitch.
• Eliminate any after effect of heat treatment.
• Ensure proper concentricity of the pitch circle and central hole.
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18. Gear Shaving
In this method, the gear teeth are finished by making the gear run at high speed in mesh and
pressed against a hardened gear shaving cutter.
Sharp edge of the shaving cutter scrape small amount of metal from the surface of the teeth
removing any surface irregularities and correcting any errors.
Two types of gear shaving cutters are in common use : rotary and rack type.
The rotary type of cutter is a gear with serrations or grooves on its flanks.
Cutter is mounted on mandrel and rotated at surface speed of 2 m/sec.
Work piece is loaded between live centres, raised to the level of the cutter reciprocated at a
low speed. Reciprocating action is to ensure that the shaving operation extends over the entire
length of the gear tooth.
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20. Cont.…
The rack type shaving operation is carried out on rack type shaving machines using a rack type
cutter.
The cutter is reciprocated at high speed in mesh with the work piece.
At the same time the work piece is reciprocated sideways and fed into the cutter.
This type of cutter gives more accurate gear teeth than the rotary cutter because an accurate
rack is more easy to produce.
It also have full contact with the work piece with compared to a rotary cutter.
But rack type of shaving cutters cannot be used for very large gears.
Gear shaving is a fast and rapid production process producing accurate teeth profile on
unhardened gear.
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21. Gear Grinding
Gear grinding is most accurate methods of finishing gear teeth.
It is most suitable for finishing of hardened gears which cannot be finished by shaving.
Heat treatment often causes considerable distortion and oxide film formation which needs
deeper cuts for complete removal.
Gear grinding can accomplish this job easily because of the abrasive action with better
accuracy.
Gear grinding is done in three different ways,
1. Form wheel grinding
2. Threaded wheel grinding
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22. Form Wheel Grinding
•Form wheel grinding is done with the help of a grinding wheel shaped to the exact profile of a
gear tooth space like a disc type form milling cutter.
•The work piece is reciprocated under the grinding wheel which is plunge fed into the work gear
to the work gear to the desired depth.
•The teeth are finished one by one and after one tooth is finished to the desired size the blank is
indexed to the next tooth space as in the form milling operation.
•Spur, helical, bevel and worm gears are finished by this method.
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24. Threaded Wheel Grinding
•Threaded wheel finishing operation uses a wheel on which a helical thread has been developed.
•The wheel is rotated about its own axis to give a cutting speed of 20-30 m/sec and also given a
feeding motion of 0.5 to 0.6 mm/rev. of the work piece along the axis of the work piece.
•The work piece is also given a rotational movement in mesh with the wheel and a periodic in
feed towards the wheel.
•This method is very fast but a lot of time is required to prepare the grinding wheel.
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26. Thank You
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