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Grinding is the process of removing metal by the application of abrasives which are bonded to

form a rotating wheel. When the moving abrasive particles contact the work piece, they act as

tiny cutting tools, each particle cutting a tiny chip from the work piece.

It is a common error to believe that grinding abrasive wheels remove material by a rubbing

action; actually, the process is as much a cutting action as drilling, milling, and lathe turning.

The grinding machine supports and rotates the grinding abrasive wheel and often supports

and positions the work piece in proper relation to the wheel.

The grinding machine is used for roughing and finishing flat, cylindrical, and conical surfaces;

finishing internal cylinders or bores; forming and sharpening cutting tools; snagging or

removing rough projections from castings and tampings; and cleaning, polishing, and buffing

surfaces. Once strictly a finishing machine, modem production grinding machines is used for

complete roughing and finishing of certain classes of work.

Grinding

Grinding Operations

Grinding machines

Centerless-Grinding Operations Rounded external grinding machining

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GRINDING OPERATIONS FOR VARIOUS

WORKPIECE GEOMETRIES

(a) cylindrical surfaces, (b) conical surfaces, (c) fillets on a shaft, (d) helical

profiles, (e) concave shape, (f) cutting off or slotting with thin wheels, and

(g) internal grinding

Typical structure of a grinding wheel.

Four Types of Surface Grinding

(a) horizontal spindle with reciprocating worktable, (b) horizontal spindle

with rotating worktable, (c) vertical spindle with reciprocating worktable,

(d) vertical spindle with rotating worktable.

Surface Grinder

Surface grinder with horizontal spindle and reciprocating worktable (most common grinder type).

Cylindrical Grinding

Two types of cylindrical grinding: (a) external, and (b) internal.

Centerless Grinding

External centerless grinding.

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WHEEL WEAR

The three main mechanisms which are recognized as the principle causes of wheel wear are:

1) Grain fracture2) Attritions wear3) Bond Fracture

GRAIN FRACTURE: Occurs when a portion of grain breaks off and the rest remains bonded to the wheel. The fractured edges become the new cutting edges. Tendency of grain to fracture is called friability'. Higher friability occurs when large cutting forces are acting on the grain and ultimtely cause wheel wear.

ATTRITION WEAR: mainly caused due to diffusion, friction and chemical reactions between the abrasive material and work at high temperature. Involves dulling of grains , resulting in flat spots and round edges.

BOND FRACTURE: Occurs when individual grains are pulled out of bonding material. This generally occurs when grains have become dull due to attrition wear and the resulting cutting force is excessive.

TYPICAL WEAR CURVE FOR GRINDING WHEEL

1 Initial breakdown2 - Uniform wear3 Increased wear

WHEEL DRESSING

When the wheel is in the third region of wear curve it must be resharpened by a procedure called wheel dressing. It consists of:1)Breaking off the dulled grits on the outside of the grinding wheel in order to expose fresh grains2) Removing the chips which have clogged in the wheel.

This is done by a rotating disc, an abrasive stick or another grinding wheel operating at high speed., held against the wheel being dressed as it rotates.

TRUING

An alternative process which not only sharpens the wheel but also restores its cylindrical shape and ensures that it is straight across its outside perimeter.

It ascertains that the wheel is axially and radially aligned and there are no run outs.

For most wheels truing and dressing is done simultaneously.

LAPPING

Lapping is a finishing operation

Lapping is performed either manually or by

machine

Hand lapping is done with abrasive powder as

lapping medium

machine lapping is done either with abrasive

powder or with bonded abrasive wheel

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Lapping Process

(a) Schematic illustration of the lapping process.

(b) Production lapping on flat surfaces.

(c) Production lapping on cylindrical surfacesLapping Machine

Polishing Pads

Hows it work?

Lapping involves bringing the surfaces of the work pieces in contact with a tool called a lap.

Loose, fine abrasive powder mixes with oil and is used between the lap and the workpiece.

The work is moved into contact with the lap over an ever changing path in a continually criss-cross pattern

In general, lapping reduces the height of high spots on the surfaces of parts by means of a rubbing action

WORKING PRINCIPLE OF VERTICAL LAPPING

Its has 2 wheel, lower one rotates and the upper one

does not

The w/ps are loaded in rings or cages on the rotary

lower plate, which rotates about vertical axis

At the same time the rings also rotates along with

the w/ps in their own positions.

The combination of these provides a gyratory motion

to the w/ps, due to which entire surfaces of the 2

plates are covered

PARAMETERS

Pressure

Magnitudes of pressure are recommended for lapping

For soft material 0.07-0.2 kg/cm2

For hard material 0.7 kg/cm2

Speed

It varies from 1.5 m/sec to 4.0 m/sec

Vehicle materials for lapping

Machine oil

grease

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Abrasives of lapping:

Al2O3 and SiC, grain size 5~100m

Cr2O3, grain size 1~2 m

B4C3, grain size 5-60 m

Diamond, grain size 0.5~5 m

Technical parameters affecting lapping processes are

unit pressure

the grain size of abrasive

concentration of abrasive in the vehicle

lapping speed

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fig 1. Effect of abrasive content on MRR

fig 2. Effect of lapping pressure on surface roughness & MRR

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Fig 1 shows that to maximize the MRR, an optimum

lapping pressure and abrasive concentration in the

vehicle have to be chosen

The effect of unit pressure on MRR and surface

roughness is shown in Fig 2

It is shown in the figure 2 that unit pressure in the

range of p1-p2 gives the best values for MRR and

roughness of the lapped surface

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The variation in MRR and surface roughness with grain size of abrasive are shown in Fig.3

grain size corresponding to permissible surface roughness and maximum MRR may be different

Primary consideration is made on the permissible surface roughness in selecting abrasive grain size

Effect of abrasive grain size on

surface roughness and MRR

HONING

Honing is a finishing operation used to improve the form

tolerance of an internal cylindrical surface

The tool used, called a hone, is a bonded abrasive stone

made in the form of a sticks

They are held against the work surface with controlled

light pressure, usually exercised by small springs

The tool brushes along the cylindrical part surface by

rotating, and moving up-and-down along its axis.

We can identify a honed surface by looking for the helical

cross-hatched scratch marks on the part surface.

HONING MACHINES

Horizontal HM

Used for longer jobs such as gun barrels

It carry a horizontal spindle, on which is mounted the

honing tool

w/p is mounted on a table which reciprocates

hydraulically to work to and fro on the hone, which

rotates about its own axis also

Oscillating motion of the hone may be controlled

hydraulically or mechanically

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Vertical HM

Hold the work as well as tool in vertical positions

They are available in both single and multiple spindle

The spindle heads, and hence the tools, reciprocate

and not the work piece

Most of machines carry a hydraulic drive for their

spindle heads and the tools

Best suited for shorter jobs

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HONING TOOL

Schematic illustration of a honing tool to improve the surface finish of bored or ground holes

HONING MACHINING

Schematics of honing process showing the honing tool, how the abrasive

sticks are pressed against the work surface by springs, and the resulting

surface pattern

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In addition to the surface finish of about 0.1 micro meter

honing produces a characteristic crosshatched surface

that tends to retain lubrication during operation

Relatively slow speeds in the range of 10-30 m/s

A common application of honing is to finish the holes.

Typical examples include bores of IC engines, bearings,

hydraulic cylinders, and gun barrels

process parameters

The important parameters that affect material removal rate (MRR) and surface roughness (R) are:

(i) unit pressure, p

(ii) peripheral honing speed, Vc

(iii) honing time, T

The variation of MRR (Q) and R with unit pressure is shown in Fig.4

It is evident from the graph that the unit pressure should be selected so as to get minimum surface roughness with highest possible MRR

fig 4. Effect of honing pressure on MRR and surface finish

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increase of peripheral honing speed leads to

enhancement of material removal rate and decrease

in surface roughness

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Fig 6. shows that with honing time T, MRR decreases. On

the other hand, surface roughness decreases and after

attaining a minimum value again rises.

SUPER FINISHING

It is a finishing operation similar to honing

But it involves the use of a single abrasive stick and

grit size are generally small

The reciprocating motion of the stick is performed at

higher frequency and smaller amplitudes

A cutting fluid is used to cool the work surface and

wash away chips

The results of these condition is mirror like finishes

with surface roughness around 0.025 micro meter

Super finishing of a Cylindrical Part

Schematic illustration of the super finishing process for a cylindrical part: (a) cylindrical microhoning; (b) centerlessmicrohoning

PRINCIPLE OF OPERATION

One face of the abrasive block is given the shape of

the surface to be super finished

This block is held in suitable holder and place on the

work piece

Holder is spring loaded to provide a light Pressure on

the work surface

w/p rotates at slow speed ( 2 to 20 m/min)

As the work rotates, the abrasive block reciprocates

forward and backward at a rapid speed

Continue..

A suitable lubricant is used in the process

An oscillatory motion is obtain due to combination of

rotary of work and reciprocating motion of the block

With rubbing of the stone against the work surface,

results super finished surface

SF machines are available in horizontal and vertical

Most of SF surfaces are usually flat and cylindrical

Difference b/w honing and SF

It is possible to create dimensions i.e size through honing but SF employed only for obtaining high quality surface finish

Honing need only 2 motions whereas SF may involve many

Honing is mostly employed for finishing internal surfaces whereas SF is largely used for outside surface

Much low speed in SF

Work pressure is too low in SF

Length of stock is very short, usually 1.5 mm to 6 mm in SF

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POLISHING

Polishing is a finishing operation to improve the surface

finish

polishing wheel made of fabrics or leather and rotating at

high speed

The abrasive grains are glued to the outside periphery of

the polishing wheel

Operation is performed by means of abrasive coated

wheels or belts

Wheels used are disc shaped and termed as bobs and

mops

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Grit sizes of 20-80 are used for rough

polishing, 90-120 for finished polishing, and

above 120 for fine polishing

Polishing operation are often accomplished

manually

Wheels are operate at speed of 1200-1500

m/min

BUFFING

Buffing is a finishing operation similar to polishing, in

which abrasive grains are not glued to the wheel

wheels are contained in a buffing compound that is

pressed into the outside surface of the buffing wheel

while it rotates.

Wheel speed ranging 2400-5200 m/min

Polishing is used to remove scratches and burrs and

to smooth rough surfaces while buffing is used to

provide attractive surfaces with high lustre Schematics of the buffing operation

BURNISHING

This process for producing mirror like glazed and

smooth surfaces on metallic components

The burnishing process consists of pressing hardened

steel rolls or balls into the surface of the work piece

and imparting a feed motion to the same

These balls, rub over the metal surfaced in contact

under pressure or compress it remove the superficial

irregularities from there, producing a fine lustreScheme of ball burnishing

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SURFACE ROUGHNESS PRODUCED BY VARIOUS

PROCESSES REFERENCES

B.L. Juneja, Sekhon ,Nitin Seth, Metal cutting and

machine tool, new age international publishers, New

Delhi, 2009

A Bhattacharyya, Metal cutting Theory and Practice,

new central book agency ltd. Kolkata, 2004

B.S. Raghuwanshi, workshop technology Vol-II

(machine tool), dhanpat rai &co. , New Delhi