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