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ME 2252
MANUFACTURING
TECHNOLOGY – II
ABRASIVE PROCESSES AND GEAR CUTTING
By
Dr. V.S.SENTHIL KUMAR,
Associate Professor
Department of Mechanical Engineering
CEG Campus, Anna University,
Chennai - 25
UNIT IV ABRASIVE PROCESSES AND GEAR CUTTING
• Abrasive processes: grinding wheel – specifications and selection, types of grinding process – cylindrical grinding, surface grinding, centreless grinding – honing, lapping, super finishing, polishing and buffing, abrasive jet machining - Gear cutting, forming,generation, shaping, hobbing.
GEAR MANUFACTURING -INTRODUCTION
•Gear manufacturing can be divided into two
categories namely forming and machining as
shown in flow chart in Fig 1.
•Forming consists of direct casting, molding,
drawing, or extrusion of tooth forms in
molten, powdered, or heat softened
materials and machining involves roughing
and finishing operations.
Fig 1. Categories of gear manufacturing process
FORMING GEAR TEETH •In all tooth-forming operations, the teeth on
the gear are formed all at once from a mold
or die into which the tooth shapes have been
machined.
•Accuracy of the teeth is entirely dependent
on the quality of the die or mold.
•Most of these methods have high tooling
costs making them suitable only for high
production quantities.
1. Casting Sand casting, die casting and investment
casting are the casting processes that are
best suited for gears and are shown in fig 2.
Fig 2. Casting processes
1a. Sand Casting
Characteristics:
Cheaper, low quality gear in small numbers
The tooling costs are reasonable. Poor
Surface finish and dimensional accuracy
Due to low precision and high backlash, they
are noisy.
They are suited for non- critical applications
Applications: (without finishing
operation)
Toys, small appliances, cement-mixer
barrels, hoist gearbox of dam gate
lifting mechanism, hand operated crane
etc.,
Materials:
C I, cast steel, bronzes, brass and
ceramics.
1b. Die casting
Characteristics:
Better surface finish and accuracy (tooth
spacing and concentricity) High tooling costs
Suited for large scale production Applications.
Applications:
Used in instruments, cameras, business
machines, washing machines, gear pumps,
small speed reducers, and lawn movers.
Materials: Zinc, aluminium and brass..
1c. Investment casting
Characteristics:
Reasonably accurate gears
Applicable for a variety of materials
Refractory mould material
Allows high melt-temperature materials
Accuracy depends on the original master
pattern used for the mold.
Materials: Tool steel, nitriding steel, monel,
beryllium copper.
2. Sintering or P/M process The powder metallurgy technique used for
gear manufacture is shown in fig 3.
Characteristics:
Accuracy similar to die-cast gears
Material properties can be Tailor made
Typically suited for small sized gears
Economical for large lot size only
Fig 3. Process chart for P/M gear manufacture
The components manufactured by P/M
technique are shown in Fig 4.
Fig. 4. Components manufactured by sintering
3. Injection Molding •Injection molding is used to make
nonmetallic gears in various thermoplastics
such as nylon and acetal.
•These are low precision gears in small sizes
but have the advantages of low cost and the
ability to be run without lubricant at light
loads.
Injection Molding Applications:
Used in cameras, projectors, wind shield
wipers, speedometer, lawn sprinklers,
washing machine.
Materials:
Nylon, cellulose acetate, polystyrene,
polyimide, phenolics.
4. Extruding •Extruding is used to form teeth on long rods,
which are then cut into usable lengths and
machined for bores and keyways etc.
•Nonferrous materials such as aluminum and
copper alloys are commonly extruded rather
than steels.
•This result in good surface finishes with
clean edges and pore free dense structure
with higher strength.
Extruding Materials:
Aluminum, copper, naval brass, architectural
bronze and phosphor bronze.
Applications:
Splined hollow & solid shafts, sector gears
are extruded
5. Cold Drawing
•Cold drawing forms teeth on steel rods by
drawing them through hardened dies.
•The cold working increases strength and
reduces ductility.
•The rods are then cut into usable lengths and
machined for bores and keyways, etc.
6. Stamping
•Sheet metal can be stamped with tooth
shapes to form low precision gears at low cost
in high quantities.
•The surface finish and accuracy of these gears
are poor.
Applications:
Toy gears, hand operated machine gears for
slow speed mechanism.
6 a. Precision stamping In precision stamping, the dies are made of
higher precision with close tolerances wherein
the stamped gears will not have burrs.
Applications:
Clock gears, watch gears etc.
7. Preforming
For close die forging the feed stock has to be
very near to the net shape and this is obtained
by performing.
8. Forging The steps in forging process are represented
in fig 5.
Fig 5. Procedure for forging of gears
Machining This is most widely used gear manufacturing
method.
Gear blank of accurate size and shape is first
prepared by cutting it from metal stock.
The gear blank can also be the metal
casting.
Gear is prepared by cutting one by one tooth
in the gear blank of desired shape and size
along it periphery.
Different gear cutting methods are used
in this category.
Two principal methods of gear manufacturing include
- gear forming, and
- gear generation.
Each method includes a number of machining 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:
Forming is sub-divided into milling by disc
cutters and milling by end mill cutter which
are having the shape of tooth space.
Form milling by disc cutter:
The disc cutter shape conforms to the gear
tooth space.
Each gear needs a separate cutter. However,
with 8 to 10 standard cutters, gears from 12
to 120 teeth can be cut with fair accuracy.
Tooth is cut one by one by plunging the
rotating cutter into the blank as shown in fig
6.
Fig 6. Form milling by disc cutter & milling of a helical gear.
Form milling by end mill
cutter: The end mill cutter shape conforms to tooth
spacing.
Each tooth is cut at a time and then indexed
for next tooth space for cutting.
A set of 10 cutters will do for 12 to 120 teeth
gears. It is suited for a small volume
production of low precision gears. The form
milling by end mill cutter is shown in fig 7.
Fig 7. Form milling by end mill cutter
To reduce costs, the same cutter is often
used for the multiple-sized gears resulting in
profile errors for all but one number of teeth.
Form milling method is the least accurate of
all the roughing methods.
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.
Broaching the teeth of a gear segment by
horizontal external broaching in one pass.
Gear generation 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,
milling with a hob (gear hobbing),
gear shaping with a pinion-shaped cutter,
or
gear shaping with a rack-shaped cutter.
Gear generating is used for high production
runs and for finishing cuts.
Gear Cutting by Gear Shaper This process uses a pinion shaped cutter
carrying clearance on the tooth face and
sides and a hole at its centre for mounting it
on a stub arbor or spindle of the machine.
The cutter is mounted by keeping its axis in
vertical position.
It is also made reciprocating along the
vertical axis up and down with adjustable
and pre-decide amplitude.
Gear Cutting by Gear Shaper The cutter and the gear blank both are set to
rotate at very low rpm about their respective
axis.
The relative rpm of both (cutter and blank)
can be fixed to any of the available value
with the help of a gear train.
This way all the cutting teeth of cutter come
is action one-by-one giving sufficient time for
their cooling and incorporating a longer tool
life.
Generating action of a gear-shaper cutter
The principle of gear cutting by this process
as explained above is depicted in the Figure
8. The main parameters to be controlled in
the process are described below.
Figure 8: Process of Gear Cutting by Shaper Cutter
Cutting Speed •The downward movement of the cutter
is the cutting stroke and its speed
(linear) with which it comes down is the
cutting speed.
•After the completion of cutting stroke,
cutter comes back to its top position
which is called return stroke.
•There is no cutting in the return stroke.
Indexing Motion
• Slow rotations of the gear cutter and
workpiece provide the circular feed to the
operation.
• These two rpms are adjusted with the help
of a change gear mechanism. The rpm are
relatively adjusted such that each rotation of
the cutter the gear blank revolves through
n/N revolution.
where n = Number of teeth of cutter, and
N = Number of teeth to be cut on the blank.
Depth of Cut
• The required depth is maintained
gradually by cutting the teeth into two
or three pass.
• In each successive pass, the depth of
cut is increased as compared to its
previous path.
• This gradual increase in depth of cut
takes place by increasing the value of
linear feed in return stroke.
The whole of this process is carried out in a
gear shaping machine which is shown in
Figure 9.
Figure 9. Setup for Gear Shaping Machine
Advantages of Gear Shaping Process
(a) Shorter product cycle time and suitable
for making medium and large sized gears in
mass production.
(b) Different types of gears can be made
except worm and worm wheels.
(c) Close tolerance in gear cutting can be
maintained.
(d) Accuracy and repeatability of gear tooth profile can be maintained comfortably.
Limitations
(a) It cannot be used to make worm and work
wheel which is a particular type of gear.
(b) There is no cutting in the return stroke of
the gear cutter, so there is a need to make
return stroke faster than the cutting stroke.
(c) In case of cutting of helical gears, a
specially designed guide containing a
particular helix and helix angle,
corresponding to the teeth to be made, is
always needed on urgent basis.
Gear Shaping by Rack Shaped Cutter
• In this method, gear cutting is done by a rack
shaped cutter called rack type cutter. The
principle is illustrated in Figure 10.
• The working is similar to shaping process
done by gear type cutter.
• The process involves rotation (low rpm) of
the gear blank as the rack type cutter
reciprocates along a vertical line.
• Cutting is done only in the downward stroke,
the upward stroke is only a return movement.
• The main difference of this method
with the previous one is that once the
full length of the rack is utilized the
gear cutting of operation is stopped to
bring the gear blank to its starting
position so that another pass of gear
cutting can be started.
• So this operation is intermittent for
cutting larger gears having large
number of teeth over their periphery.
Figure 10. Gear Cutting by Rack Shaped Cutter
Rack Planning Process
• This process is used for shaping of spur
and helical gear teeth with the help of a
rack type cutter.
• In this process the gear blank is mounted
on a horizontal aims and rotated
impertinently.
• At the same time the gear blank is kept in
mesh with a reciprocating rack type cutter.
• The process is shown in Figure 11.
• The teeth cutter gradually removes
material to cut the teeth and to make the
required profile.
• The whole operation includes some
important operations. These are feeding
cutter into the blank, rolling the blank
intermittently and keeping cutter in mesh
with the rolling gear blank.
• After each mesh the gear blank is rolled by
an amount equal to one pitch of gear
tooth.
After each cutting, the rack is withdrawn and
re-meshed after the rotation of gear blank.
Figure 11. Gear Shaping by Rack Type Cutter
Gear Hobbing Process
• Gear hobbing is done by using a multipoint
cutting tool called gear hob.
• It looks like a worm gear having a number
of straight flutes all around its periphery
parallel to its axis.
• In gear hobbing operation, the hob is
rotated at a suitable rpm and
simultaneously fed to the gear blank.
• The gear blank is also kept as revolving.
• Rpm of both, gear blank and gear hob are
so synchronized that for each revolution of
gear hob the gear blank rotates by a
distance equal to one pitch distance of the
gear to be cut.
• Motion of both gear blank and hob are
maintained continuously and steady.
• A gear hob is shown in Figure 12 and the
process of gear hobbing is illustrated in
Figure 13.
Figure 12. Gear Hob. Fig 13. Process of Gear Hobbing
• The hob teeth behave like screw
threads, having a definite helix angle.
• During operation the hob is tilted to
helix angle so that its cutting edges
remain square with the gear blank.
• Gear hobbing is used for making a
wide variety of gears like spur gear,
helical, hearing-bone, splines and
gear sprockets, etc.
Figure 14. (a) Schematic illustration of gear cutting with a hob.
(b) Production of worm gear through hobbing.
• Three important parameters to be
controlled in the process of gear hobbing
are indexing movement, feed rate and
angle between the axis of gear blank and
gear hobbing tool (gear hob).
• If a helical gear is to be cut, the hob axis is
set at an inclination equal to the sum of the
helix angle of the hob and the helix angle
of the helical gear.
• Proper gear arrangement is used to
maintain rpm ratio of gear blank and hob.
• The arms of hob are set at an inclination
equal to the helix angle of the hob with the
vertical axis of the blank.
Figure 15. Setup for Gear Hobbing Machine
• The process of gear hobbing is
classified into different types
according to the directions of feeding
the hob for gear cutting.
Hobbing with Axial Feed
• In this process the gear hob is fed
against the gear blank along the face
of the blank and parallel to its axis.
This is used to make spur and helical
gears.
Hobbing with Radial Feed
• In this method the hob and gear blanks
are set with their axis normal to each
other.
• The rotating hob is fed against the gear
blank in radial direction or perpendicular
to the axis of gear blank.
• This method is used to make the worm
wheels.
Hobbing with Tangential Feed
• This is also used for cutting teeth on
worm wheel.
• In this case, the hob is held with its axis
horizontal but at right angle to the axis
of the blank.
• The hob is set at full depth of the tooth
and then fed forward axially. The hob is
fed tangential to the face of gear blank.
Advantages of gear hobbing process
• (a) Gear hobbing is a fast and continuous
process so it is realized as economical
process as compared to other gear
generation processes.
• (b) Lower production cycle time, i.e. faster
production rate.
• (c) The process has a larger variability’s in
the following of sense as compared to
other gear machining processes.
(i) Capable to make wide variety of gears like
spur gear, helical gears, worms, splines,
sprockets, etc.
(ii) Process of required indexing is quite
simplified and capable to make any
number of teeth with consistent accuracy
of module.
(iii) Harringbone gear cam be generated by
gear hobbing exclusively.
(iv) Wide variety of batch size can be accommodated by this process.
• (d) Several gear blanks, mounted on
the same arbor, can be processed
simultaneously.
• (e) Lots of time is available to
dissipate the generated heat. There
is no over heating of cutting tool.
• Limitation of the process of gear
hobbing is manufacturing of internal
gears is not possible.
GEAR FINISHING OPERATIONS
• Surface of gear teeth produced by any of
the generating process is not accurate and
of good quality (smooth).
• Dimensional inaccuracies and rough
surface generated so become the source
of lot of noise, excessive wear, play and
backlash between the pair of gears in
mesh.
GEAR FINISHING OPERATIONS
• In order to overcome these problems
some finishing operations are
recommended for the produced gears.
• Sometimes poor quality of finish and
dimensional inaccuracies occur due to
hardening of a produced gear.
• So finishing operations are to be done at
last.
GEAR SHAVING
• Gear shaving is a process of finishing of
gear tooth by running it at very high rpm in
mesh with a gear shaving tool.
• Tool is of a type of rack or pinion having
hardened teeth provided with serrations.
• These serrations serve as cutting edges
which do a scrapping operation on the
mating faces of gear to be finished.
• Both are gears in mesh are pressed to
make proper mating contact.
Shaving operation is shown in fig 16.
Fig 16. External gear being shaved
GEAR BURNISHING
• The gear to be finished is mounted on a
vertical reciprocating shaft and it is kept in
mesh with three hardened burnishing
compatible gears.
• The burnishing gears are fed into the cut
gear and revalued few revaluations in both
the directions.
• Plastic deformation of irregularities in cold
state takes place to give smooth surface of
the gear.
• 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 Grinding
• In this operation abrasive grinding wheel of
a particular shape and geometry are used
for finishing of gear teeth.
• Gear to be finished is mounted and
reciprocated under the grinding wheel.
• Each of the gear teeth is subjected to
grinding operations this way.
• In grinding, a contoured grinding wheel is
run over machined surface of the gear teeth
using computer control.
Fig 17. shows grinding operations and
dressing of the wheel.
Fig 17. (a) Grinding the flanks only, (b) Grinding root and flanks,
(c) Grinding each flank separately with twin grinding wheels
and (d) Pantograph dressing of the wheel
Fig 18. Finishing gears by grinding: (a) form grinding with
shaped grinding wheels; (b) grinding by generating with two
wheels
• 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.
• Grinding operation for gears can be done
by profile grinding or form grinding as
shown in figs 19. and 20.
Fig 19. (a) Maag zero pressure angle profile grinding and (b)
Maag profile grinding
Fig 20.David Brown form grinding of worm threads
Lapping of a Gear • The process of lapping is used to improve
surface finish of already made teeth.
• Gear to be lapped is run under load in
mesh with cast iron toothed laps.
• Abrasive paste is introduced between the
teeth. It is mixed with oil and made to flow
through the teeth.
• One of the mating members (gear/ lapping
tool) is reciprocated axially along with the
revaluations.
Fig 21. Lapping operation for bevel gears
HONING • It is used for super finishing of the generated
gear teeth.
• Honing machines are generally used for this
operation.
• The hones are rubbed against the profile
generated on the gear tooth.
• Gear lapping and gear honing are the lost
finishing operations of a gear generation
process.
In the above gear finishing operations
some operations are based on metal
cutting by removing very small size of
chips like gear shaving, gear
grinding, lapping and honing and
some other operations like gear
burnishing, roll finishing and based
on finishing by plastic deformation of
metal.
CUTTING BEVEL GEARS Straight bevel gears are roughed out in one
cut with a form cutter on machines that
index automatically.
The gear is then finished to the proper shape
on a gear generator.
The reciprocate across the face of the bevel
gear as shown in fig 22 and 23.
The machines for spiral bevel gears operate
on essentially the same principle.
The spiral cutter is basically a face-milling
cutter that has a number of straight-sided cutting blades protruding from its periphery.
Figure 22. (a) Cutting a straight bevel-gear blank with two cutter. (b) Cutting a helical bevel gear
Fig 23: (a) Cutting a straight bevel-gear blank with two cutters. (b)
Cutting a spiral bevel gear with a single cutter.
Gear Manufacturing Cost as a
Function of Gear Quantity
Figure 24. Gear manufacturing cost as a function of gear quality.
The numbers along the vertical lines indicate tolerances