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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
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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