As unit 3 grinding

Grinding and Super finishing
25 March 2020 Dr.AbhishekSharma 1
By
Dr. Abhishek Sharma
UNIT -III
GL BAJAJ INSTITUTE OF TECHONOLOGY & MANAGEMENT
Manufacturing Process (KME-403)

• Grinding is a process of removing material by abrasive action of a
revolving wheel on the surface of a work-piece in order to bring it to
required shape and size.
• The wheel used for performing the grinding operation is known as
grinding wheel.
• It consists of sharp crystal called abrasive held together by a
binding material or bond.
• The wheel may be a single piece or solid type or may be
composed of several segments of abrasive blocks joined
together.25 March 2020 Dr.AbhishekSharma 2
Grinding

Wheel Materials
1. Abrasive: it is that material of grinding wheel which does the
cutting action
(a) NaturalAbrasive:
The common natural abrasives are sand stone, emery (50-
60% crystallineAL2O3+iron oxide) corundum (75-90%) crystalline
aluminum oxide + iron oxide), and diamond
The sand stone is used only for sharpening some wood-working tool
Diamond is used for dressing the grinding wheel and acts as an abrasive
material for hard material.

Artificial abrasive
These are manufactured under controlled conditions in closed electric
furnaces in order to avoid the introduction of impurities and to achieve
the necessary temperature for chemical reactions to take place.
Example: Silicon carbide(Sic),Aluminum oxide(Al2O3)
Artificial diamond:
The diamond produced through artificial means are quite
comparable to the natural diamond in their grinding
characteristics and give better result than the later.

Advantages and use of artificial Abrasive
The artificialabrasives have latelysuperseded the naturalabrasive for following reason
1. The controlled conditions in the electric furnace enable
uniformly in the product.
2. The quality of production and supply can easily be varied
according to the demands.
3. They have largely abolished the dependence on natural means to meet
the growing demand of more abrasive material in the modern
manufacturing process.
Bond is the substancethat acts as a binder to hold the abrasive grains together.

Material to be grind Recommended abrasive
Cast Iron Siliconcarbide
Soft brass and bronze
Glassandstone
Aluminium
Copper
Rubber
Leather
Plastics
Heat treated carbon and alloy
steel
Aluminium Oxide
MalleableIron
Hard brass and bronze
Mild steel and Wrought Iron

A bond is a substance that is employed to hold abrasive grains together in the form of
sharpening stonesor grinding wheels.
Several types of bonding materials used for making grinding wheels,
1-Vitrified bond
2-Silicate bond
3-Shellac bond
4-Resinoid bond
5-Rubber bond
6-Oxychloride bond
These bonds may be used with either silicon carbide or aluminum oxide.

1-Vitrified bonding process: vetrified wheels are made by bonding clay melted to a
glass like consistency with abrasive grains.
• The clay and abrasive grains are thoroughly mixed with sufficient
water to make uniform mixture.
• The fluid mixture is then poured in to the mould and allowed to dry.
• After cooling , the material trimmed to more perfect size and shape.
• It is then heated in a furnace to a temperature 1260 deg .
• When the burning proceed, the clay vitrifies --that is it fumes and forms a porcelain or
glass like substance that surroundsand connectsthe abrasive grains.
• It gives good strength and porosity to allow high stock removal.
• It is affected by heat, cool water or acids.
• Denoted by “V”25 March 2020 Dr.AbhishekSharma 8

2-Silicate bonding process:
Silicate bonding wheels are made by mixing abrasive grains with silica
of soda or water glass.
The mixture is packed in to moulds and allowed to dry.
The mould shapes are baked in a furnace at a temp of 260 degree for
several days.
The silicate bondded wheels are water proof.
Denoted by “S”

3- Shellac bonding process: shellac bond wheels are also
known as elastic bonded wheels.
In this process, the abrasive and shellac are mixed in heated containers
and then rolled or pressed in heated moulds.
Later the shapes are baked at a temp 150 deg.
The elasticity in this is grater than in other two types and it has
considerable strength
It is not for heavy duty.
Used for finishing chilled iron, cast iron and steel rolls
• Denoted by”E”

4-Resinoid bonding process: resinoid wheels are produced by mixing
abrasive grains with synthetic resins and other compounds.
• The mixture placed in moulds and heated at about 200 deg.
• At this temp the resin sets to hold the abrasive grains in wheel form.
• These wheels are used for purposes which require a strong, high speed
wheel.
• Denoted by “B”

5-Rubber bonding process: Rubber wheels are produced by mixing
abrasive grains with pure rubber and sulphur.
• The mixture rolled in to sheets and wheels are punched out of these
sheets on a punch press.
• These are less heat resistant and more dense then the resinoid wheels.
• Denoted by “R”

6-Oxychloride bonding process: this process consists of mixing abrasive
grains with oxide and chloride of magnesium.
• The mixing of bond and abrasive is performed in the same way as for
vitrified bond wheel.
• These wheels are used in making wheels and wheel segments for disc
grinding operation.
• Denoted by “O”

GRIT SIZE
Small grit sizes produce better finishes
Harder work materials require smaller grain sizes to cut effectively
Softer materials require larger grit sizes
Larger grit sizes permit larger material removal rates
Grit size is measured using a screen mesh procedure
Smaller grit sizes indicated by larger numbers in the screen mesh
procedure and vice versa
Grain sizes in grinding wheels typically range between 10
(very coarse) and 250 (very fine)

GRADE
The grade of a bonded abrasive is a measure of its bond strength.
The grade is also referred to as the hardness of a bonded abrasive
A hard wheel has a stronger bond and/or a larger amount of bonding
material between the grains than a soft wheel
Structure of a bonded abrasive is a measure of its porosity

Specifying of grinding wheel
To maintain uniformity throughout the country in the system of making
grinding wheel, the Bureau of Indian Standard has devised a standard
system to be followed by all manufacturer.
According to the system, the various elements and characteristics of
grinding wheels are represented on all the wheels in a definite
sequence as follows: 1.Abrasive
2. Grain size
3. Grade
4. Structure
5. Bond

Example:
Agrinding wheel carrying the marking
125 x 25 x32 WA46 L4 V 17
125(mm)---wheel diameter
25(mm)--- Thickness of wheel
32 (mm)---Bore diameter
W---Manufacturer’s prefix to abrasive. It is optional.
A---Abrasive(Al2O3)
46 ----Grain size(medium)
L ---grade (medium)
4 --- Dense structure
V --- Vitrified bond
17----it is the suffix denoting the bond type of the manufacturer

Major and inevitable defects in grinding are glazing of grinding wheels. Its remedy
will be discussed later. After the continuoususe grinding wheel becomes dull or glazed.
Glazing of the wheel is a condition in which the face or cutting edge acquires a
glass like appearance. That is, the cutting points of the abrasives have become dull and
worn down to bond. Glazing makes the grinding face of the wheel smoother and that
stops the process of grinding.
Sometimes grinding wheel is left ‘loaded’.
In this situation its cuttingface is found being adhering with chips of metal.
The openings and pores of the wheel face are found filled with workpiece material
particals, preventing the grinding action.
Loading takes place while grinding workpiece of softer material.
Glazing

Dressing
The remedies of glazing and loading is dressing of grinding wheels.
Dressing removes the loading and breaks away the glazed surface so
that sharp abrasive particles can be formed again ready for grinding.
Different type of dressing operations are done on a grinding wheel.
One of them is the dressing with the help of star dresser. It consists of a number of
hardened steel wheels with sharp points on their periphery. The total is held against the
face of revolving wheel and moved across the face to dress the whole surface.
Another type of wheel dresser consists of a steel tube filled with a bonded
abrasive. The end of the tube is held against the wheel and moved across the
face.

Truing
Truing is the process of restoring the shape of
grinding wheel when it becomes worn and break
away at different points. Truing makes the wheel true
and concentric with the bore.

Selection of Grinding Wheel
1.Properties of the material to be machined i.e hardness, toughness,
strength.
2. Quality of surface finish required.
3. Grinding allowance provided on the work-piece i.e the amount of
stock to be removed.
4. Dimensional accuracy.
5. Method of grinding i.e wet or dry.
6. Rigidity, size and type of machine.
7. Relative sizes of wheel and job.
8. Type of grinding to be done.
9. Speed and feed of wheel.

Types of grinding Machine
Grinding machinesaccording to qualityof surfacefinish,may be classifiedas
1. Rough /Non-precisiongrinding
2. Precision grinding
Rough/Non-precision grinding
To remove more stock than other grinding
• Floor stand and bench grinders
• Portable and flexible shaft grinders
• Swing frame grinders
• Abrasive belt grinders

Floor stand and bench grinders
Used for snagging and off-hand grinding of various materials and cutting tools in tool
rooms, foundries and general repair shops. It carries a horizontal spindle having
grinding wheels mounted on both ends.
It can be suitably bolted on a bench at convenient height.
The floor or pedestal grinder is nothing but a bench grinder of the above type
mounted on a steel stand or pedestal of suitable height.
The horizontal spindle carrying the grinding wheel is normally an extension on both
side of the armature shaft of the motor.
These grinder are also be used for polishing by replacing the grinding wheel by
polishing wheel.

Precision grinders: according to the type surface generated or work
done they may be classified as:
(a)cylindrical grinding machine
(b) surface grinding machine
(c) Internal grinding machine
(d) Tool and cutter grinding machine

Surface grinding machine
Horizontal spindle and reciprocating table
Vertical spindle and reciprocating table
Horizontal spindle and rotary table
Vertical spindle and rotary table

Cylindrical Grinders
The workpiece is hold on centre by a chuck or in a suitable holding fixture, rotating it
about its axis and feeding a fast revolving grinding wheel
If the work surface to be ground is longer than the face width of the grinding wheel, the
work is traversed past the wheel or the wheel past the work.
Traversing of wheel or work is done either by hydraulic or mechanical power or by hand
Feed is given to the work or the wheel at the end of each traversing movement

In case of width of wheel face is more or equal to the length of work surface to be
ground, the wheel may be fed in with no traversing movement of it or that of work.
This is known as Plunge Grinding
The simplest and quite commonly used type of cylindrical grinder is tool post grinder
used on lathe
When wheels of large diameters are used, they can be mounted directlyon the motor shaft
For mounting small wheels, an auxiliary shaft is provided, which runs at a relatively
much higher speed than the motor.
Both external and internal cylindrical grinding can be done on lathe by this equipment
Types of cylindricalgrinding:
1. Plain cylindrical grinding
2. Universal cylindrical grinding
3. Centrelessgrinders

Centreless Grinders
Here the work is supportedby a combination of grinding wheel, regulating wheel and a
work rest blade
The relative movement of work-piece and two wheels shown in figure
• In operation, grinding operation is performed by grinding wheel only while the
function of regulating wheel is to provide the required support to the work piece
while it is pushed by cuttingpressure of grinding wheel
• It helps the work-piece to remain contact with the grinding wheel
• At the same time, required support from bottom is provided by work-rest as the
work-piece while rotating rests on the blade of worksheet
• The regulating the wheel essentially carries rubber bond and helps in the rotation of
work-piece due to friction.
The methods used for feeding the work are:
Through feed
Infeed
End feed

Internal grinders
Used for grinding internal surface of different holes, cylindrical, tapered etc
Types:
1. Plain internal grinders
2. Universal internal grinders
3. Chucking internal grinders
4. Planetary internal grinders
5. Centreless internal grinders

Universal internal grinders
Here the work head is mounted on a cross-slide which enables a cross feed
to the work head.
The work head can be swiveled though 90 degree
It is used to grind cylindrical and tapered holes, flat surfaces, outside
the cylindrical surfaces, convex and concave surface

Centreless internal grinders
The work-piece is supported between three rolls known as
pressure roll, supporting roll and a regulating roll
These rolls while supporting the work-piece also rotate the same
All rolls rotate in same direction while the work-piece and
grinding rotate in opposite direction
The grinding wheel remains contact with the internal surface of
the work-piece at the horizontal centerline of regulating wheel
It ensures that a uniform wall of thickness of the work-piece and
therefore concentricity of the ground internal hole with the
external surface
The pressure roll is so mounted that it can be swung to a slide to
allow loading and unloading of the work-piece

SUPER-FINISHING
The surface finishing process of producing highest class of surface finish using
bonded abrasives at low speeds, low unit pressures, variable multi-motion and proper
lubricationis known as super-finishing.
Factors Causing High Finish
• Small Unit pressure of the abrasive tool on the work surface, usually 2 to 2.5
kg/sq.cm
• Low CuttingSpeeds
• Oscillation of the abrasive stick which frees the abrasive sticks of the chips.
• Complex Path of theAbrasive GRAINS
• Low Working temperatureabove room temperaturedue to low heat generation.

Honing: it is an abrading process mostly used for polishing bored and
reamed holes and also cylindrical surfaces using bonded stones (called
hones)

MATERIALS HONED BY HONING MACHINE

Lapping: It is finishing process and is done
after grinding

POINTS LAPPING HONING
Form of Abrasive It uses loose fine abrasive
powder mixed with a
lubricantor bondedfine grit
abrasivewheels.
It uses fine grit bonded
abrasivesticks holds in a
holdingframe
Nature of work Flat ,cylindricalformed
surfaces
Cylindricalholes
Stock Removal Lesser Stock Removal More stock removal
Finish Better micro finish and
surface smoothness
Lesser surface
smoothness
Hole Geometry Cannot correct much of fine
hole geometry errors
Better for correcting fine
hole geometry errors
Difference Between lapping and honing

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