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THERMODYNAMICS

Thermodynamics is the science that deals with the relationship between heat

and work. Thermodynamics is the study of 3 E's Energy, Equilibrium and Entropy.

Basic Definitions :

System:

It is a definite quantity of matter of fixed mass and identity bounded by a

closed surface. All things other than the system is surroundings ( Both space and

matter ). There are three types of systems.

1. Closed system - There is no mass transfer between the system and surroundings. But their is energy transfer. E.g.. compression of a gas in a

piston cylinder.

2. Open system - Both mass and energy transfer takes place. It is classified into steady and unsteady flows. Eg. Turbine

3. Isolated system - No mass and no energy transfer takes place. E.g. All

subsystem of a power producing system

Sorroundings:

All things other than the system that are outside the wall that interact with the

system in question is called as surroundings. There are different types of walls that

are used to separate the system from the surroundings. They are rigid wall, diathermal

wall and adiabatic walls. A rigid wall does not permit the volume of the system to

change. A diathermal wall is one that will make it possible for the system to

communicate thermally with its surroundings. Two systems separated with a

diathermal wall is said to be in thermal contact. An adiabatic wall is the one that is

impermeable to thermal energy. Such a wall cuts of the thermal interaction between

between a system and surroundings.

Properties:

Properties are used to identify the state of the system and solely dependent

upon the state of system and not upon how the state was reached. A quantity is a

property if it has a exact differential. A quantity can be called a property of the

system if the changes in the value between two equilibrium states of system is same.

Properties may be directly observable or indirectly observable characteristic of a

system. Two properties, namely the temperature and entropy are unique to

thermodynamics. There are two types of properties. They are

Extensive state properties: Here the value of entire system is equal to sum of

the values of the parts of the system. They are dependent upon the mass. E.g..

Total Volume, total energy

Intensive state properties: The value of the entire system is not equal to the

sum of the parts of the system. These properties are not dependent upon the

mass. E.g.. Temperature, pressure, Density etc.

Path and point functions :

This is with reference to a system being taken

from state 1 to 2. There may be any three quasi static

process A, B, and C. Area below the curve gives the

amount of work involved in each case.

Thus the value of work depends upon the path

and not on the end state of the process. Hence work (

and also heat ) are path functions. On the contrary

thermodynamic property are point functions. These are definite values for a given

state. The change in property is independent of the path and depends on only the

initial and final states ( Exact differential )

Process:

Whenever a system undergoes a change, process is said to have taken place.

There are different types of process. They are

1. Reversible process: Is the one in which both the system and surroundings return to their original state. All real time process are irreversible. Process are

irreversible due to turbulence, temperature gradient and Friction. In a

reversible process there should be no viscous force or coulomb friction in the

system

2. Cyclic Process: The end states are identical. The system undergoes a series of change and returns to original condition.

3. Quasi-static Process: The system departs from the equilibrium condition only infinitesimally.

4. Adiabatic Process : There is no heat flow between the system and

surroundings. ( = 0 )

Work and Heat:

Work is the energy in transition in which the energy flows from the system to

the surroundings.

Heat is the energy in transition which flows from one body to another body on

account of the temperature difference between the two bodies. Unit of heat is Joule

Both the Heat and work are Transient Phenomena, Boundary Phenomena and

Path functions.

Derivation for displacement work:

This derivation is valid only for quasi static

process. Consider a cylinder of area 'a' and length of

the piston is 'l'. The piston moves due to gas

pressure. Between section 1 and 2, the value of

pressure and volume is P and V. When the piston

moves the force acting on the piston is

F = pressure x area = P.a

Work done = Force x distance moved = F.dl = P.a.dl

We know that area x length = volume. Hence a.dl = v. Thus the above

equation for work done becomes P.v. Thus when a piston moves from 1 to 2 the

amount of work done is given by dw = Pdv

Internal energy:

A system undergoes a change of state in which both heat transfer and work

transfer are involve. The net energy accumulated is stored in the system. It is denoted

by the symbol U, it includes all form of energy other than kinetic and potential

energy.

Q - Heat to the system.

W - Work from the system.

( Q - W ) is the net energy stored in the system.

This ( Q - W ) is neither heat or work and is given the name, internal energy of

system. The internal energy is just a form of energy like the potential energy of an

object at some height above the earth, or the kinetic energy of an object in motion. In

the same way that potential energy can be converted to kinetic energy while

conserving the total energy of the system, the internal energy of a thermodynamic

system can be converted to either kinetic or potential energy. Like potential energy,

the internal energy can be stored in the system.

Entropy:

Entropy means transformation. It increases with the addition of heat and vice

versa. Change in entropy can be defined. Over a small range the increase or decrease

in entropy when multiplied with absolute temperature, gives the heat absorbed or heat

rejected. For any reversible process, the change in Entropy of system and

surroundings is Zero.

Entropy is the index of unavailability of energy. Energy that goes down the

sink is less available for any useful work. Entropy changes are accompanied by heat

transfers. But may also take place with out the transfer of heat. In a reversible

process, if the entropy of the system increases, then the entropy of surroundings

decreases by a equal amount. Entropy is a property like T and V.

Change in entropy of a system along two equilibrium states can be obtained by

taking the system along any reversible path connecting the states, dividing the heat

added at each point with the temperature and summing the quotients.

Energy:

It is the capacity to produce effect. There are two types of energy. They are

stored energy ( E.g.. Potential energy, Kinetic energy and Internal energy ) and

Transient energy ( Heat, work and electric energy ).

Power:

The rate of energy transfer is called as power. The unit is watts. 1 W = 1 J/s =

1 Nm/s

Throttling :

The fluid expands from high pressure to low pressure without doing any work.

There is no change in KE and PE. Hence there is no heat transfer.

Nozzles and Diffusers:

Nozzles increases the kinetic energy of flowing fluid by creating a pressure

drop. But in diffusers, the pressure is increased and Kinetic energy is decreased.

Carnot's Cycle:

It is a reversible cycle in which the ideal gas receives heat at one temperature

and rejects heat at another temperature. There are 2 isothermal and 2 reversible

adiabatic process. Efficiency of carnots cycle is given by

= W / Qa = ( Qa - Qr ) / Qa

Enthalpy:

Of a substance is defined as the sum of internal energy and flow work. h = u +

pv.

Graham's Law of Diffusion of Gas:

It states that the rate of diffusion of a gas is inversely proportional to square

root of density.

Laws In Thermodynamics:

Zeroth Law of Thermodynamics:

If two bodies are in equilibrium with a third body, then the two bodies are in

equilibrium with each other. Through this concept, the temperature of the system may

be measured by bringing it into thermal equilibrium with a thermometer. Following

the conversion factors between various temperatures.

R = F + 459.67

K = C + 273.15

K = 1.8 R

First Law of Thermodynamics:

This law deals with conservation of energy, which states that energy can

neither be created not destroyed, but can be changed from one form to another.

Whenever a system under goes a cyclic change the algebraic sum of work

transfer is proportional to the algebraic sum of heat transfer. Work and heat are inter

convertible.

First law could be said as law of internal energy. However the drawback in

this law is that it does not tell anything about direction of heat flow.

Second Law of Thermodynamics:

For an isolated system, only those processes can take place for which the

entropy of the system increases or remains constant. Second law could be called as

law of entropy. In this there are two statements.

Lord Kelvin and Max Planck's statement of the Second Law: It is

impossible to construct a device operating in a cycle for the sole purpose of extracting

heat from a reservoir and changing it into an equal amount of work without rejecting a

part of the heat. i.e. it is impossible to devise a machine that converts 100% of heat

into work. i.e. The universe is cooling down.

Clausius' statement of the Second Law: It is impossible to construct a

device that operating in a cycle will produce no effect other than the transfer of heat

from a cooler to a hotter body. The spontaneous flow of heat from a colder body to a

hotter body is impossible.

Third Law of Thermodynamics:

It introduces the concept of absolute entropy. It states that the total entropy of

pure substances approaches 0o as the absolute temperature approaches 0o. ( It is

impossible to reach the absolute zero of temperature in any physical process. )

Thermodynamic Equilibrium:

When a collection of matter experiences no more changes in all its properties,

then it is in a state of thermodynamic equilibrium. But a real system is never in

equilibrium. To attain thermodynamic equilibrium, Mechanical, Chemical and

Thermal equilibrium should first be obtained. When a system has no unbalanced

force within it and when the force its exerts on its boundary is balanced by external

force, the system is said to be in Mechanical equilibrium. When the temperature of

the system is uniform throughout and is equal to the temperature of the surroundings,

the system is said to be in thermal equilibrium. When the chemical composition of a

system will remain unchanged, the system is said to be in chemical equilibrium.

Thermodynamic reservoirs:

There are three different types of thermodynamic reservoirs. They are work

reservoir, heat reservoir and Matter reservoir.

Work reservoir:

It is a device that we may employ to keep track of the amount of work done by

or done to a given thermodynamic system. It is a body in which every unit of energy

crossing the boundary is work energy. A work reservoir might be visualized as a

perfectly elastic spring that is compressed by the work done on it by a system, or as a

weight that is raised as the system does work upon the reservoir and lowered as the

reservoir does not work on the system.

Heat reservoir:

It serves as a heat source or heat sink, in the analysis of thermodynamic

problems. It can be considered as a body with large energy capacity so that its

temperature remains constant when heat flows into or out of it. The atmosphere

around the earth and the ocean may be considered as heat reservoirs.

Matter reservoir:

Matter, as well as heat and work can cross the boundary of an open system, the

surroundings of an open system may be imagined to contain only heat and work

reservoirs but also one or more matter reservoirs to supply and receive matter. A

matter reservoir is considered to be sufficiently larger than the system so that the

reservoir itself remains in a given equilibrium state. The atmosphere around the earth

may be considered as a matter reservoir supplying air to the engines of our

automobiles and to air separation plants.

Important Thermodynamic Process:

The below mentioned process uses the concept of U = Q - W

Process Significance /

Example Implications

Pictorial

Representation

Isobaric

Process

Pressure is Constant

( P = 0)

Gas heated in a

cylinder fitted with a

movable frictionless

piston. The pressure

the atmosphere and

the pressure due to the

weight of the piston

remains constant as

the gas heats up and

expands.

U is zero in a constant

pressure process. For an

ideal gas, constant pressure

work is W = PdV =

P V

Heat that flows into the

system causes the

temperature to rise. Q = m

Cp T = mR( T2 - T1 )

Isothermal

Process

Temperature is

constant ( T = 0)

The gas in a cylinder

is compressed slowly

enough that heat

flows out of the gas at

the same rate at which

is being done on the

gas.

For an Ideal gas U is a

function of the

temperature, Hence U is

zero since T = 0.

Since U = 0 then W = Q.

P1V1 = P2V2 = nRT, for an

isothermal process.

Work done W = PV ln(

V2/V1 ) which is also the

equation for Q.

Isochoric

process

Volume is constant

( V = 0)

Heating of a gas in a

rigid, closed

container.

No work is done on the gas

because W = PdV = P

( 0 ) = 0. This implies

that U= Q = m Cv T.

V1 = V2 = nRT1/P1 =

nRT2/P2, the ideal gas law

for constant volume

process.

Adiabatic

process

No heat flows into or

out of the system ( Q

= 0 )

Compression of a Gas

in an Insulated

Cylinder.

U = W ( Since Q = 0 ).

Hence any temperature rise

or fall is due to the work

done or by the gas alone.

W = (P1V1 - P2V2) / ( - 1)

Isentropic

process ( Rev.

Adiabatic

process )

Entropy is constant

( S = 0)

A heat engine in

which the working

fluid undergoes an

adiabatic reversible

cyclic process.

Any isentropic process is

also adiabatic since U

= dQ/Tand Q =

0. However, not all

adiabatic process are

isentropic.

For a reversible heat

engine, not only the change

in entropy of the working

fluid must be zero but

also U of the

environment (heat

reservoirs) must also be

zero.

Polytropic

process PVn is constant

Compression or

Expansion of a gas in

a real system such as

a Turbine.

n = 0 for Isobaric process

since PV0= P = constant.

n = 1 for Isothermal

process since PV1 = PV =

NKT = constant.

n = for Isovolumetric

process and

n = for Adiabatic

process.

Specific Heat:

It is the heat required to raise the temperature of unit mass of substance by one

degree. There are two types, they are specific heat at constant volume ( Cv )and

Specific heat at constant pressure ( Cp ). Its unit is J/Kg/K

For air Cp = 0.24 J/Kg/K and Cv = 0.171 J/Kg/K

The ratio of Cp / Cv = Gamma. and Cp - Cv = R / j

Gas Laws:

There are 5 gas laws. All perfect gases obey all gas laws under all conditions

of pressure and temperature.

1. Boyle's law : At constant temperature PV = C. The magnitude of C depends upon the volume of the gas.

2. Charles lay : At constant pressure V T.

3. Gay - Lussac law : At constant volume P T. 4. Joules law : Change of internal energy is directly proportional to the change in

temperature.

5. Avagadro law : Equal volumes of all gases under the same pressure and

temperature contain equal number of molecules.

Ideal Gas Real Gas

Obeys the equation of state at all conditions of

pressure and temperature.

Obeys the equation of state at all conditions of

Pressure and temperature, except at the point

where Pressure approaches absolute Zero.

The gases cannot be liquefied or solidified Can be solidified and liquefied.

Specific heat values are constant Not so, Varies with temperature and pressure.

Ideal gas equation : PV = mRT where

P is in N / m2 V is in m3 T is in K R is gas constant in Nm

/ Kg oK

Following are the assumptions for a ideal gas

Molecules occupy a negligible volume fraction.

Long range forces of attraction between the particles are negligible.

Assumptions of Kinetic Theory

Large number of molecules ~ their motion can be treated statistically.

Molecules are in continuous and rapid motion which is random, colliding

with each other and the walls of the vessel very frequently, the collision

beingelastic.

Pressure originates from the summation of large number of reacting forces as

the molecules bounce off the walls.

Combustion chamber:

Combustion Chambers convert the chemical energy stored in a liquid or

gaseous fuel to an enthalpy increase in the gas passing through them. Usually, the gas

is air, but it could be any gas with the proper components to react with the fuel. A

combustion chamber requires one initial spark to begin the combustion of the fuel in

the chamber. After that, the chamber will function as long as it has fresh fuel and gas.

The fuel combusts, or burns, in the chamber. This combustion releases large amounts

of energy to be absorbed by the gas. This increases the temperature and enthalpy of

the gas.

REFERENCES:

1. Engineering Thermodynamics, Francis F. Huang.

2. Engineering Thermodynamics, P. K. Nag.

THEORY OF METAL CUTTING

The metal cutting is done by a relative motion between the work piece and the

hard edge of a cutting tool. Metal cutting could be done either by a single point

cutting tool or a multi point cutting tool. There are two basic types of metal cutting

by a single point cutting tool. They are orthogonal and oblique metal cutting. If the

cutting face of the tool is at 90o to the direction of the tool travel the cutting action is

called as orthogonal cutting. If the cutting face of the tool is inclined at less than

90o to the path of the tool then the cutting action is called as oblique cutting. The

differences between orthogonal and oblique cutting is given below

Orthogonal metal cutting Oblique metal cutting

Cutting edge of the tool is

perpendicular to the direction of

tool travel.

The cutting edge is inclined at an

angle less than 90o to the

direction of tool travel.

The direction of chip flow is

perpendicular to the cutting

The chip flows on the tool face

making an angle.

edge.

The chip coils in a tight flat spiral The chip flows side ways in a

long curl.

For same feed and depth of cut

the force which shears the metal

acts on a smaller areas. So the

life of the tool is less.

The cutting force acts on larger

area and so tool life is more.

Produces sharp corners. Produces a chamfer at the end

of the cut

Smaller length of cutting edge is

in contact with the work.

For the same depth of cut

greater length of cutting edge is

in contact with the work.

Generally parting off in lathe,

broaching and slotting

operations are done in this

method.

This method of cutting is used in

almost all machining operations.

Elements of Metal Cutting :

Cutting speed : It is the distance traveled by work surface related to the cutting edge

of Tool

v = dN / 1000 m / min

Feed (s) : The motion of cutting edge of tool with reference to one revolution of work

piece.

Depth of cut (t) : It is measured perpendicular to axis of work piece and in straight

turning in one pass. This can be estimated from the relation

t = ( D - d ) / 2 mm

Undeformed chip (Fc) : The cross sectional area of chip before it is removed from

work piece. it is equal to the product of feed and depth of cut.

Fc = s x t mm2

All tools have a major and minor cutting edge. The major cutting edge

removes bulk of material. Where as the minor cutting edge gives good surface finish.

Different types of chips produced during machining process :

When the tool advances into the work piece, the metal in front of the tool is

severely stressed. The cutting tool produces internal shearing action in the metal. The

metal below the cutting edge yields and flows plastically in the form of chip.

Compression of the metal under the tool takes place. When the ultimate stress of the

metal is exceeded, separation of metal takes place. The plastic flow takes place in a

localized area called as shear plane. The chip moves upward on the face of the tool.

There are three different types of chips. They are

1. Continuous chips, 2. Discontinuous chips and 3. Chips with built up edge.

Continuous chips :

The conditions that favor the production of continuous chips is small chip thickness, high cutting speed, sharp cutting edge, large rake angle in cutting tool and fine feed, smooth tool face and efficient lubricating system.

Such chips are produced while machining ductile materials like mild steel, copper and aluminum. Because of plastic deformation of ductile material long and continuous chips are produced.

This is desirable because it produces good surface finish, low power consumption and longer tool life.

These chips are difficult to handle and dispose off. Further the chips coil in a helix and curl around work and tool and may injure the operator when it is breaking. The tool face is in contact for a longer period resulting in more frictional heat. However this problem could be rectified by the use of chip breakers.

Chip breakers:

During machining, long and continuous chip will affect machining. It will

spoil tool, work and machine. It will also be difficult to remove metal and also

dangerous. The chip should be broken into small pieces for easy removal, safety and

to prevent damage to machine and work. The function of chip breakers is to reduce

the radius of curvature of chips and thus break it. The upper side of continuous chips

notches while the lower side which slides over the face tool is smooth and shiny. The

chips have the same thickness through.

Discontinuous chips :

These chips are produced when cutting more brittle materials like bronze, hard brass and gray cast iron.

Since there chips break up into small segments the friction between chip and tool reduces resulting in better surface finish.

These are convenient to handle and dispose off. Discontinuous chips are produced in ductile materials under the conditions

such as large chip thickness, low cutting speed, small rake angle of tool etc. Brittle materials lack the ductility necessary for appreciable plastic chip

deformation. The amount of deformation which the chip undergoes by deformation is limited by repeated fracturing.

If these chips are produced from brittle materials, then the surface finish is fair, power consumption is low and tool life is reasonable however with ductile materials the surface finish is poor and tool wear is excessive.

Chips with built up edge :

This is nothing but a small built up edge sticking to the nose of the cutting tool. These built up edge occurs with continuous chips.

When machining ductile materials due to conditions of high local temperature and extreme pressure the cutting zone and also high friction in the tool chip interface, there are possibilities of work material to weld to the cutting edge of tool and thus forming built up edges.

This weld metal is extremely hard and brittle. This welding may affect the cutting action of tool.

Successive layers are added to the build up edge. When this edge becomes large and unstable it is broken and part of it is carried up the face of the tool along with chip while remaining is left in the surface being machined. Thus contributing to the roughness of surface.

Thus the size of the built up edge, varies during the machining operation. It first increases, then decrease and again increases.

this built up edge protects the cutting edge of tool, thus changing the geometry of the cutting tool.

Low cutting speeds lead to the formation of built up edge, however with high cutting speeds associated with sintered carbide tools, the build up edge is negligible or does not exist.

Conditions favoring the formation of build up edge are low cutting speed, low rake angle, high feed and large depth of cut. This formation can be avoided by the use of coolants and taking light cuts at high speeds. This leads to the formation of crater on the surface of the tool.

Single point cutting tool:

Parts of a single point cutting tool:

Part Description

Shank It is the body of the tool which is ungrounded.

Face It is the surface over which the chip slides.

Base It is the bottom surface of the shank.

Flank It is the surface of the tool facing the work piece.

There are two flanks namely end flank and side

flank.

Cutting

edge

It is the junction of the face end the flanks. There

are two cutting edges namely side cutting edge and

end cutting edge.

Nose It is the junction of side and end cutting edges.

Important angles of a single point cutting tool:

Angle Details

Top rake

angle

It is also called as back rake angle. It is the slope

given to the face or the surface of the tool. This

slope is given from the nose along the length of the

tool.

Side rake

angle

It is the slope given to the face or top of the tool.

This slope is given from the nose along the width of

the tool. The rake angles help easy flow of chips

Relief angle These are the slopes ground downwards from the

cutting edges. These are two clearance angles

namely, side clearance angle and end clearance

angle. This is given in a tool to avoid rubbing of the

job on the tool.

Cutting edge

angle

There are two cutting edge angles namely side

cutting edge angle and end cutting edge angle. Side

cutting edge angle is the angle, the side cutting edge

makes with the axis of the tool. End cutting edge

angle is the angle, the end cutting edge makes with

the width of the tool.

Lip angle It is also called cutting angle. It is the angle between

the face and end surface of the tool.

Nose angle It is the angle between the side cutting edge and end

cutting edge.

Required properties of cutting tool material:

Hot hardness:

This is the ability of the material to with stand very high temperature without

loosing its cutting edge. The hardness of the tool material can be improved by adding

molybdenum, tungsten, vanadium, chromium etc which form hard carbides. High

hardness gives good wear resistance but poor mechanical shock resistance.

Wear resistance:

The ability of the tool to withstand wear is called as wear resistance. During

the process of machining, the tool is affected because of the abrasive action of the

work piece. If the tool does not have sufficient wear resistance then there are

possibilities of failure of cutting edge. Lack of chemical affinity between the tool and

work piece also improve wear resistance.

Toughness:

This property posses limitation on the hardness of the tool because of very

high hardness the material becomes brittle and weak.

Low friction:

In order to have a low tool wear and better surface finish the co-efficient of

friction between the tool and chip must be low. The thermal conductivity must be

high for quick removal of heat from chip tool interface.

In addition to the above, it must posses the following mentioned properties.

1. Mechanical and thermal shock resistance, 2. Ability to maintain the above properties at the high operating temperatures. 3. Should be easy to regrind and easy to weld the tool.

In addition to the above, high thermal shock resistance is also desirable. But

no single material fulfills all the above requirements.

Tool life:

It is an important factor in cutting tool performance. The tool can not cut

effectively for an unlimited period of time. It has a definite life. Tool life is the time

for which the tool will operate satisfactorily until it becomes blunt. It is the time

between two successive grinds. Following are the factors influencing tool life.

Cutting speed:

It has the greatest influence. When the cutting speed increases, the cutting

temperature increases. Due to this, hardness of the tool decreases. Hence the tool

flank wear and crater wear also occurs easily. The relation ship between tool life and

cutting speed is given by the Taylor's formula which states

VTn = C

V is the cutting speed in meters / minute

T is the tool life in minutes.

n depends on the tool and work.

C a constant.

Feed and depth of cut:

The tool life depends upon the amount of material removed by the tool per

minute. For a given cutting speed if the feed or depth of cut is increased, tool life will

be reduced.

Tool geometry:

Large rake angle reduces the tool cross section. Area of the tool which will

absorb heat is reduced. So the tool will become weak. Hence correct rake angle must

be used for longer tool life. If the cutting angle increases, more power will be

required for cutting. Clearance angle of 10o to 15o is optimal.

Other factors include the material of tool (Carbon steel, medium alloy steel,

high speed steel, molybdenum high speed steel, cobalt high speed steel, stellites,

carbides, ceramics and diamond are the commonly used tool materials.), use of cutting

fluids and work material.

Functions of cutting fluids:

1. To cool the tool and work piece and carry away the heat generated from cutting zone. It is essential to maintain a temperature of 200o C for carbon tools and

600o C for HSS.

2. At low speeds the surface finish obtained by using cutting fluids is better than what is obtained without using cutting fluids.

3. To wash away the chips and keep the cutting region free. 4. It helps to keep the freshly machined surface bright by giving a protective

coating against atmospheric oxygen and thus protect the finished surface from

corrosion.

5. Cutting fluids improves machinability and reduces machining forces. 6. To prevent the expansion of work piece and 7. To cause the chips to break into small parts rather than remain as long ribbons

which are hot and sharp and difficult to remove from work piece.

Requirements of cutting fluid:

A cutting fluid should posses the following properties.

1. High heat absorption to remove the heat developed immediately, 2. Good lubricating properties to have a low coefficient of friction, 3. High flash point to avoid fire hazard, 4. Stability must be high to that it does not oxidize with air,

5. It must not react with chemical and must be neutral, 6. Odorless, so that at high temperatures, it does not give a bad smell, 7. Harmless to the skin of operators, 8. Harmless to the bearings, 9. Should not have a corrosive action on the machine or work piece, 10. Cutting tool must be transparent so that the cutting action could be observed, 11. Low viscosity to permit the free flow of the cutting tool and

12. It must be economic.

Choice of a cutting fluid depends upon type of operation, material of tool and

work piece, rate of metal removal and cost of cutting fluid.

Types of cutting fluids:

Water based cutting fluids:

In this water is mixed with soluble oil and soaps. Following are the important

characteristic features.

It is a excellent cooling medium having maximum amount of specific heat,

The disadvantage in using this is that it causes rust and corrosion,

But a mixture of water and oil provides the best lubricating properties

The ratio of oil to water is different for different machining process. The usual

ratio are

Operation Ratio

Turning 1:25

Milling 1:10

Drilling 1:25

Grinding 1:50

Oil based cutting fluids:

These are fixed oil and mineral oil. Fixed oil has greater oiliness to become

gummy and decompose when heated.

To combine stability of mineral oil with lubricating properties of fixed oils they

are often mixed.

There are different types of oil based cutting fluids. They are soluble oils,

straight fatty cutting oils, sulphurised and aqueous solution.

Following are the different types of cutting fluids based on different operating

conditions.

Straight mineral oils for light duty and high speed work.

Mineral oil for light and medium duty.

Mineral oil with extreme pressure additives, such that they are suitable

for heavy duty and

Mineral oil and extreme pressure additives for the heaviest duty.

Effect of cutting fluid on cutting speed, tool life and chip concentration:

Cutting speed:

These are not only used to carry away the heat generated by also because of

the lubricating effect of the fluid on the working surface of the tool. When a cutting

fluid is sued for machining touch material the productivity may be increased from

15% to 30% more when compared with dry operation. But using cutting fluids, high

speeds may be used.

Tool life:

By using cutting fluids effectively during machining operations the tool life

increases. Carbon steel rods have less heat resistant have maximum increase in tool

life for HSS it is around 25%.

Chip concentration:

Without the use of cutting fluid chips are accumulated near the work tool

interface and are difficult to remove because of its high temperature. By the use of

cutting fluid the temperature of the chip is reduced and also the chips are washed

away from the work tool interface.

Application of cutting fluids:

The cutting fluids may be applied to the cutting tool in the following ways.

1. By hand, using brush, 2. By means of drip tank and

3. By means of a pump.

For effective use of cutting fluid and for heavy and continuous cutting the

fluid should penetrate into the cutting zone. The following are the famous methods of

cutting fluid application.

1. Flood application (Hi-jet application):

Here there is a continuous stream of cutting fluid is directed to the cutting zone

with the help of nozzle. The used cutting fluid drops into a tank at the bottom.

Before it is re-circulated by the pump, it passes through many filters to remove chips

and dirt. In some applications the cutting fluid is supplied through the tool itself and

directed along the flank face of the tool. Though economic it is not adopted

universally because the high pressure jet may be dangerous to the operation.

2. Mist method of application:

In this the cutting fluid is atomized the order of 10 - 25 m. The mist is

sprayed on cutting zone at high velocities of about 300 mpm and more under high

pressure. This method is used in all cutting operation, but is generally more useful

with high hardness work materials. The benefits of this process are listed below.

Due to high velocity the heat is dispersed immediately and maintains desired

temperature gradient near tool surface.

The surface area of coolant is greater when compared to flood application and

hence increases the cooling capacity.

Due to expansion of the mist in the issuing nozzle, it temperature falls down

considerably.

The basic components of the system are

1. Air pump with air storage, 2. Cutting fluid container 3. Piping and 4. Spray nozzle.

Benefits of cutting fluids:

Cooling:

By flowing over a tool, chip and job a cutting fluid can remove heat and

reduce temperature at he cutting zone. This reduction in temperature leads in

increase in tool life and decrease in tool wear. The cooling effect is also important in

reducing thermal expansion and distortion of work piece. The cooling action also

bring about good surface finish, increase chip curl and reduces BUE formation.

Friction reduction:

A fluid passing through the cutting zone may be subjected to any one of the

following conditions.

High temperature approaching melting point,

Clean freshly produced surface and

High local pressure approaching the hardness of the metal cut.

Under these conditions the chip may be made to react wit the fluid fro form a

low shear strength solid lubricant. This thin layer prevents the formation of the weld

between the chip and the tool and hence reduces the co-efficient of friction between

chip and tool.

Reduce shear strength:

When the co-efficient of friction is reduced there is also a decrease in shear

work, sue to the resulting increase in shear angle. An increase in shear angle results in

a decrease in shear strain giving rise to smaller shear stress and hence the net result is

a decrease of shear energy per unit volume when cutting with an increased shear

angle.

Tool geometries:

There are two distinct tool geometries. The are positive and negative rake

angles. Positive is suitable for machining soft, ductile materials (like aluminum) and

negative is for cutting hard materials, where the cutting forces are high (Hard

material, high speed and feed).

Forces on a single point cutting tool :

Following are the three forces acting on a tool

1. Axial force 2. Tangential force and

3. Radial force.

In the above figure (a) is for orthogonal cutting and figure (b) is for oblique

cutting. Wattmeter is a indirect method for measuring cutting force. More exact

method is the use of dynamometer. Of the total heat generated during machining

process, given below is the rough heat distribution.

Chip carries 70 % of heat.

Work piece carries 15 % of heat and

Tool carries the remaining 15 % of heat generated.

Tool life :

It could be defined from any of the below mentioned criteria.

Volume of material removed between two successive tool grind.

Number of work piece machined between two successive tool grinds.

time of actual cutting between 2 successive tool grinds.

Tool failure occurs by chipping or breakage or wear ( Takes place by crater

formation or by flank wear ) or deformation.

Machinability : It could be evaluated by using

Tool life

mm3 of stock removed

Cutting force required.

Temperature of tool and chip.

Machinability Index ( % ) = ( Cutting speed of work piece for 20 mm Tool life ) / (

Cutting speed of SAE 1112 steel for 20 mm min tool life ) X 100.

TOOL FAILURE:

A tool is said to fail when it losses its usefulness though wear, breakage,

chipping and deformation. During the machining operation high temperatures are

reached and leads to the softening of tool point. At a high temperature localized phase

transformation occurs. This gives rise in residual stress due to which cracks appear on

tool point and it is more prone to failure. In some cases tool point may even melt and

is frequently accompanied by sparking and hence can be easily recognized.

Thermal cracking occurs when there is a steep temperature gradient due to

intermittent cutting. Failure can be reduced by the proper selection of cutting

parameters.

Wear of cutting tools:

Flank wear ( or edge wear ):

This type of wear takes place when machining materials like cast iron or when

the feed is less than 0.15 mm / rev. The worn region at the flank is called as

wear land. This wear land is measured with the help of brinell microscope.

The work and the tool are in contact at the cutting edge only. Usually wear

appears on the clearance face of the tool and is mainly the result of friction and

abrasion.

Flank wear is a flat portion worn behind the cutting edge, which eliminates

some clearance on relief.

Flank wear is a progressive form of detoriotion and will result in failure in spite

of best precautions.

There are three stages in flank wear. They are primary, secondary and tertiary

stage. In the primary stage wear is rapid due to high stress at tool point. In

secondary stage, wear is less and linear. In the third and final stage called as

the tertiary stage the wear increases leading to catastrophic failure.

Abrasion by hard particles and inclusions in the work piece, shearing of micro

welds between tool and work material and abrasion by fragments of build up edge

plowing against the clearance face of the tool are some of the causes of this wear.

Crater wear ( or face wear ):

This is caused by the pressure of the chip as it slides up the face of the cutting

tool. Due to the pressure of the sliding chips the cool face wears out gradually.

On the faces of the tool there is a direct contact of tool with the chip. Wear

takes place in the form of cavity or crater, which as its origin above the cutting

edge.

The crater occurs on the rake face and does not actually reach the cutting edge

by ends near the nose.

This type of wear takes place when cutting ductile material. This wear

weakens the tool. Cutting temperature is increased. Friction and cutting force

will also increase. When the crater becomes large the tool will totally fail.

Severe abrasion between chip and tool interface and high temperature in the

tool-chip interface reaching the softening (or melting temperature) of tool resulting in

increased rate of wear. These are the two causes of crater wear.

To combat crater wear, tool manufacturers can increase the chemical stability

of the tool material, as when they added titanium carbide (TiC) to tungsten carbide

(WC) in the first successful steel-cutting carbide tool. Applying a hard coating to put a

hard, inert barrier between tool and work piece at high cutting speeds will also

minimize crater wear. Tool geometry can also make a difference. A positive-rake tool

will reduce tool pressure and decrease contact between the chip and the insert, and the

reduction in pressure and contact can reduce crater wear.

Nose wear:

This is similar to flank wear in certain operations like finish turning. It takes

place at the nose of the tool. When the nose of the tool is rough, abrasion and friction

between the tool and work piece will be high. Due to this, too much heat is

generated. Also more cutting force is required. As a result the nose of the tool wears

quickly. This is more pre-dominant than flank wear.

Breakage:

Because of high pressure acting on cutting edge of a tool there ay be

immediate failure. Breakage is usually attributed to mechanical shock, thermal shock,

thermal cracks and fatigue.

Chipping:

The cutting edge may crumble due to improper relief angle, excess clearance

and insufficient support of the tool. This could also happen if the work piece is very

hard. It is a microscopic form of breakage due to loss of many small particles caused

due to unhoned carbide edges, excessive vibration and chatter.

Deformation:

When a heavy load is applied close to the cutting edge of tool the surface

becomes indented while the adjacent face shows a bulge. Because of which crack

occurs on periphery of indentation and finally leads to failure.

NUMERICAL PROBLEMS

1. The useful tool life of a HSS tool at 18 m/min is 3 hours. Calculate the tool life

when the tool operates at 24 m/min.

Solution:

VTn = C

V = 18 m/min

T = 3 x 60 = 180 min

Constant C = 18 x ( 180 ) 0.125 = 34.45 ( Here n = 0.125 )

Now V = 24 m/min.

T = ( 34.45 / 24 ) 1/0.125

= 18 minutes.

GRINDING

Grinding is a finishing process used to improve surface finish, abrade hard

materials, and tighten the tolerance on flat and cylindrical surfaces by removing a

small amount of material. In grinding, an abrasive material rubs against the metal part

and removes tiny pieces of material. The abrasive material is typically on the surface

of a wheel or belt and abrades material in a way similar to sanding. On a microscopic

scale, the chip formation in grinding is the same as that found in other machining

processes. The abrasive action of grinding generates excessive heat so that flooding of

the cutting area with fluid is necessary. Following are the reasons for using grinding

operation.

The material is too hard to be machined economically. (The material may have been hardened in order to produce a low-wear finish, such as that in a bearing raceway.).

Tolerances required preclude machining. Grinding can produce flatness tolerances of less than 0.0025 mm (0.0001 in) on a 127 x 127 mm (5 x 5 in) steel surface if the surface is adequately supported.

Machining removes excessive material.

Principle of Operation:

To grind means to abrade, to war away by friction or to sharpen. In

manufacturing it refers to the removal of metal by an abrasive wheel rotating at high

speeds and working on the external or internal surface of a metallic or other part hard

enough to be abraded, rather than indented by the grinding wheel. The action of the

grinding wheel is similar to that of a milling cutter. The grinding wheel is composed

of many small abrasive particles bounded together, each one acting as a miniature

cutting point.

Grinding removes metal from the work piece in the form of small chips by the

mechanical action of abrasive particles bonded together in a grinding wheel.

Grinding operations :

Following are the different grinding operations that could be performed.

1. Grinding flat surface 2. Grinding vertical surface

3. Grinding slot 4. Grinding angular surfaces 5. Grinding a radius 6. Cutting off.

TYPES OF GRINDING MACHINES:

Grinding machines are designed principally for finishing parts having

cylindrical, flat or internal surfaces. The kind of surface machined largely determines

the type of grinding machine. Following is the classification of various types of

grinding machines.

1. Surface grinding machine:

It is a precision grinding machine to produce flat surfaces on a work piece. It

is more economical and practical method of accurately finished flat surfaces than

filling and scraping. The grinding is done on the circumference of the plain wheel.

Area of contact is less. Following are the different types of surface grinders. In

general, following are the parts of any grinding machine.

Base: It has a driving mechanism ( hydraulic device, tank and motor. ) It has column

at the back for supporting the wheel head.

Saddle: It is the frame. It carries the table in its cross wise movement. It is used to

give cross-feed to the work. It can be moved by hand feed or auto-feed.

Table: It is fitted on the saddle. It reciprocates along the guide ways to proved the

longitudinal feed to the work. It has 'T' slots for clamping purposes. It is moved by

hand or auto-feed.

Wheel head: It is mounted on the column. It can be moved vertically up and down to

accommodate work piece of different lengths. The wheel rotates at a constant speed

of 1500 m / min.

Horizontal spindle reciprocating table Horizontal spindle rotary table

Vertical spindle reciprocating table Vertical spindle rotary table

Specification of surface grinder:

Maximum diameter of the wheel that can be held one the spindle. Maximum size of the job that can be ground. The type of drive of the work table ( Hydraulic / electrical )

2. Centered Grinding:

Grinding for surfaces of rotation (axially symmetric surfaces) can be either

centered or centerless. Centered grinding involves fixturing the part on a spindle axis

as it is ground, as illustrated below. This configuration can be compared to fixturing a

part on a lathe with or without a tail stock. The abrasive material is on a grinding

wheel that rotates in a direction such that rolling or sliding contact occurs where the

wheel and work piece touch. Centered grinding is accurate and stable, but set-up takes

time and through-put suffers.

3. Centreless Grinding:

Center less grinding is similar to centered grinding except that there is no

spindle. This allows high through-put since parts can be quickly inserted and removed

from the process. Out of the two wheels the large wheel is the grinding wheel, and

the smaller one is the pressure wheel. In operation, the pressure exerted by the

grinding wheel on the work forces the work against the work rest and regulating

wheel. The regulating wheel is of rubber bonded abrasive having the frictional

characteristics to rotate the work at its own rotational speed.

The axial movement of the work piece past the grinding wheels is obtained, by

tilting the regulating wheel at a slight angel from horizontal. An angular adjustment

of 0o to 10o is provided in the machine for this purpose. There are three main types of

center less grinding.

Through-feed grinding:

In through-feed grinding, the part rotates between the grinding wheel and a

regulating wheel as shown below. For through-feed grinding, one or both wheels of

the centerless grinding machine are canted out of the horizontal plane, as shown

below. This imparts a horizontal velocity component to the work piece, so that outside

feed mechanisms are not necessary.

The grinding wheel is canted with respect to the other two axes so that a

component of its surface velocity pushes the part in the direction shown below. This

auto feeding characteristic is useful for rapidly processing many parts in quick

sequence. Because of the axial movement, through-feed parts can only have right

circular cylindrical ground surfaces. The wheel cannot be dressed to grind more

complex shapes.

In-Feed Grinding:

It is used for jobs that, because of a shoulder or some other obstruction on the

part, can only enter the machine so far and then, after the grinding is done, must be

with drawn. In-feed grinding differs from through-feed grinding in that the part is not

fed axially so that the ground surface does not need to be a right circular cylinder. The

grinding wheel can be dressed to accommodate the part. Once the work piece part is

in place, the grinding wheel is fed in radially.

Because of the set up time involved for each part, in-feed grinding does not

have the high throughput of through-feed grinding. In-feed grinding is illustrated

below.

End-Feed Grinding:

In end-feed grinding, the part moves in axially between the grinding wheels,

stops for grinding, and then moves out again. The wheel can be dressed to form more

complex shapes, but the part can only get progressively smaller in diameter. End-feed

grinding is illustrated below.

Advantage:

Center less grinding is used when large quantities of the same part are

required. Production is high and cost are relatively low because there is not need to

drill center holes nor to mount the work in holding device. Almost an material can be

ground with this technique. Minimum time is lost in loading and unloading. Since no

axial force is acting on the work piece, long slender work pieces can be used without

being distorted.

Large grinding wheels are used and hence wear is less and minimum amount

of adjustment. A low order of skill is required to attend the centerless grinding much

of the time.

4. Cylindrical grinder:

It produces a cylindrical or conical shape on a work piece. The work piece is

mounted between centers or in a chuck and the face of the grinding wheel passes over

the external surface of the revolving work piece. There are two types of cylindrical

grinders. They are

Plain cylindrical grinders:

These are the machines that are designed for simple external grinding. The

wheel head is made to operate to and from the work table but cannot be swiveled.

The work table holds the work head and tail stock and can be swiveled for slight

tapers. The head stock is rigidly attached to the work table and cannot be swiveled. It

is located to the left of the operator. These grinders are used to produce

Plain or stepped surface, External cylinders. Tapers, Concave or convex radii, Under cuts and Form grinding by dressing the grinding wheel the desired shape.

Universal cylindrical grinders:

It is different from the above grinder in the sense that the wheel head can be

swiveled on its base and can be fed to and from the table. The upper work table can

be swiveled and is equipped with scales and adjusting screws for setting the table to

produce slight tapers. Steep tapers may be ground by swiveling the headstock on its

base. The universal grinding machine is a tool room machine.

5. Internal Grinder:

It is designed to facilitate the finishing of holes. There are three type of

internal grinders. They are

Work rotating type machine is commonly used in tool and die rooms. In this grinder, the wheel head may be stationary with a reciprocating work table or the wheel head may reciprocate and the work table remains stationery.

Planetary internal grinder is where the wheel spindle is arranged that besides rotating on its axis it can be made to run eccentrically, thus making it possible to grind large holes of varying diameter depending upon how much the wheel spindle is made to run eccentric. The work is mounted on a table which has vertical, horizontal and longitudinal adjustments similar to those of the plain milling machine.

Centreless internal grinder works on a roller chucking principle in which the rollers hold the work and impart the rotary motion to the work. The wheel head has reciprocating motion and may be fed in and out by hand. This machine issued for work of a repetitive nature.

6. Tool and cutter grinder:

In a machine shop, many of the operations are done by single point cutting

tools or multipoint cutting tools called as milling cutters. The cutting tools become

blunt and becomes important to carry out re-sharpening. This is done in tool rooms

where a tool and cutter grinder is sued for this purpose. A universal tool and cutter

grinder is used to re-sharpen reamers, taps, single point tools dies and punches. A tool

and cutter grinder is also used as a surface, grinding, cylindrical grinding and internal

grinding machine with the help of certain attachments.

GRINDING WHEELS:

A grinding wheel may be considered as a multipoint cutting tool with a cutting

action similar to that of a milling cutter except that the cutting points are irregularly

shaped and randomly distributed over the active face of the wheel. In order to make

the grinding wheel suitable for different work situations, the features such as abrasive,

grain size, grade, structure and bonding materials can be varied.

Those grains which actually perform the cutting operation are called active

grains. In peripheral grinding, each active grain removes a short chip of gradually

increasing thickness in a way that is similar to the action of a tooth on a slab milling

cutter. Because of irregular shape of the grains, there is considerable plowing action,

between each active grain and the new work surface. The plowing results in

progressive wear, causing the formation of worn areas on the active grains. As

grinding proceeds the number and size of these worn areas increase, thus increasing

the interference or friction, resulting in an increase in the force acting on the grain.

Eventually this force become large enough to tear the work grain from the bond of the

wheel and thus expose a new cutting edges. Thus grinding wheel has self sharpening

characteristics.

A grinding wheel consists of an abrasive that does the cutting and a bond that

holds the abrasive particles together. There are two types of abrasives. They

are Natural and Artificial abrasives. The natural abrasives are emery and corundum.

These are impure forms of aluminum oxide. Artificial abrasives are silicon carbide

and aluminum oxide. The abrasives are selected depending upon the materials to be

ground. Following are important criteria in grinding wheel manufacture.

Grain size: The number indicating the size of the grit represents the number of

openings in the sieve used to size the grain. Larger the grit size number, finer the grit.

Grade: Indicates the strength of the bond and, therefore the hardness of the wheel. In

a hard wheel the bond is strong and it securely anchors the grit in place, and therefore,

reduces the rate of wear. In a soft wheel, the bond is weak and he grit is easily

detached resulting in a high rate of wear.

Structure: This indicate the amount of bond present between the individual abrasive

grains, and the closeness of the individual grains to each other. An open structure will

cut more freely. That is, it will remove more material in a give time and produce less

heat.

Bond: Is a substance which, when mixed with abrasive grains holds them together,

enabling the mixture to be shaped in the form of the wheel, and after suitable

treatment to take on the form of the wheel and the necessary mechanical strength for

its work. The degree of hardness possessed by the bond is called as 'grade' of the

wheel, and this indicates the ability of the bond to hold the abrasive grains in the

wheel. There are several types of bonding materials used for making wheels.

Types of bonding:

Vitrified bonding ( V ):

Vitrify means to change into glass by heat and fusion. Thus when clay,

feldspar or flint are mixed with the abrasive grains and heated to 1200o C, the ceramic

material melts and forms a lass like coating and bonding agent for the grains. The

forming of wheels is mostly done by the puddled or pressed process.

In puddled process, the correct proportion of grain and bonding material are

mixed wet and poured into a molt to dry. The wheel is then shaped on a machine

operating on the principle of potters wheel. The wheel are then charged into a kiln for

the burning process which takes 2 - 3 weeks. In pressed process the grains and

bonding clay are mixed in a semi-dry state and the wheel moulded under pressure.

But this process the wheels can be made under better control as regards density,

giving a wider range of grades.

It has high porosity and strength which makes this type of wheel suitable for

high rate of stock removal. It is not adversely affected by water, acid, oils at ordinary

temperature conditions.

Silicate bonding ( S ):

Silicate wheels have a milder action and cut with less hardness than vitrified

wheels. For this reason they are suitable for grinding fine edge tools, cutlery etc.

Shellac bonding ( E ):

This is used for heavy duty, large diameter wheels where a fine finish is

required. These are expensive and comparatively very rare. They are used where

their exceptionally cool cutting abilities are essential to prevent burn damage or to

provide very fine finish. Applications include metallurgical sample cutting and Tool

& Cutter grinding for reclaiming broken slot and end mills. Shellac wheels may be

made to 3 mm or less in thickness. Shellac wheels posses considerable elasticity.

Rubber bonding ( R ):

This is used where a small degree of flexibility is required on the wheel as in

the cutting of the cutting off wheels. They produce good quality of cut with minimal

of burr formation. This could be uses in places where there is polishing of metals

such as ball bearing races and for cutoff wheels where burr and burn must be avoided.

Resinoid bonding ( B ):

This is used for high speed wheels. Such wheels are used in foundries for

dressing castings. Resinoid bond wheels are also used for cutting off parts. They are

strong enough to with stand considerable abuse. Resinoid bond is made from

powdered synthetic resin used as phenol formaldehyde. This is mixed pressed and

heated to 177o C. After cooling, this makes a wheel which is less brittle, tougher and

more flexible than the vitrified bond and which can be run up to 2900 m/min.

Wheel structure:

Wheel structure defines how "open" or "closed" the wheel surface is. An

"open" wheel is one with the grits spaced relatively far apart, a "closed" wheel is one

with the grits spaced close together. For conventional wheels, it is assigned a number,

normally between 1 [most closed] and 15 [most open]. It is a measure of the

percentage of grit by volume. The less volume of grit, the more open the wheel

structure is with more space for coolant and chip clearance.

Vitrified bond wheels naturally have a certain amount of porosity in their

structure. The porosity level can typically be up to 50%. The structure can be

artificially changed to increase the porosity level by introducing an additional material

when the grit and bond are mixed together before firing. This material is in particle

form of a specified size. During firing of the wheel, this material is removed to leave

pores of the same size as the original particles. This type of wheel is called an induced

porosity wheel. The wheel then contains natural porosity plus induced porosity as

shown in the figure. Induced porosity wheels provide additional space for chip

clearance and for coolant. They are particularly useful for grinding processes which

have a long arc of contact between wheel and component. For this reason, they are

used almost invariably for creep feed grinding. They are also used for the grinding of

rubbers, plastics and polyurethane.

Types of Lay:

Each method will produce a characteristic finished determined by the lay of

the surface of the work piece after the grinding operation. A straight wheel with

reciprocating motion produces fine straight lines on the work piece. Where as a cup

wheel with reciprocating motion will produce curving lines. A cup wheel with

rotating work piece will produce concentric circles.

Marking system for grinding wheels:

Standard wheel markings specify all the important wheel characteristics. The

marking system comprises of seven symbols which are arranged in the following

order.

E.g.. 51 - A46 H5V8

51 - Manufacturers symbol for abrasive

AA - Type of abrasive grit

46 - Grain size

H - Grade

5 - Structure

V - Type of bond

8 - Manufacturers own mark.

Specification of grinding wheels:

A grinding wheel is specified by the marking, shape, outside diameter, bore

diameter, thickness etc. A recessed wheel is specified with all the above given

particulars plus the diameter of the recess and the depth of the recess.

Selection of grinding wheel:

For grinding a job the right grinding wheel is to be selected. The selection of a

grinding wheel will depend on the following factors.

Material to be ground: For grinding high tensile material an aluminum oxide wheel,

and for low tensile material silicon, a carbide wheel should be selected. For grinding

hard materials a soft wheel and for grinding soft material, a hard wheel is chosen.

Amount of stock to be removed: When the stock of material to be removed is more

with heavy cuts select a coarse grain, open structured and hard grade wheels. For

removing less stock of material with light cut, select fine dense structured soft wheel.

Finish required: Rough finish requires coarse grains and open structure. High finish

requires fine grain and dense structure.

Area of contact: The are of contact depends on the size of the work piece, the grinding

wheel and the nature of operation. When the area of contact is more a soft grade and

coarse grain wheel is to be selected. For less area of contact select hard grade and fine

grain wheel.

Type of grinding operation: The selection of grinding wheel is affected by the

grinding operation to be done. The wheel shape and size are to be selected on the

basis of the grinding operation such as surface, cylindrical or tool grinding.

Wheel speed: Generally the speed at which a grinding wheel is to be used will be

marked on the wheel by the manufacturer. Select a soft wheel for high speed and a

hard wheel for low speed.

Work speed: Select a hard wheel for high work speed and a soft wheel for low work

speed.

Condition of the machine: For rigid and new machines, select a soft grade and open

structured wheel. For light and old machines, select a hard grade and dense structured

wheel.

Personal factor: A skilled person can do the operation effectively, even if there is a

slight deviation in the selection. But for a semi skilled labor, perfect selection is

essential.

Method of cooling: If better cooling is required select an open structured wheel.

Always the coolant should be directed at the cutting areas to minimize the heat and to

wash away the grain particles.

Balancing of grinding wheels:

When a new grinding wheel is used it should be checked for balancing. Most

manufacturers balance their wheels before selling them. For checking the balance of

the grinding wheels, it is mounted at the center of a perfect straight and round spindle,

the assembly then being rested on level knife-edge ways on a lathe bed or on a special

stand. For the test to be really satisfactory the wheel should be mounted on its won

spindle. The wheel is then rolled a little and left. Any out of balance will result in the

wheel coming to the rest with the heavy side underneath.

Balancing may be achieved by adding lead weight to the light side. This may

be accomplished by removing small amounts of the wheel beneath the flanges and

then filling the hole thus made with lead. The wheel is mounted on its own spindle

kept on knife edge ways, and again give a slight push, allowing it to roll back and

forth until it comes to rest, which it will do with the heavy portion of the wheel at the

bottom. Continue adding weight from the wheel, until it is balanced. This will be

evident when the wheel rolls to a gentle sop with no apparent tendency to roll

backward.

Types of grinding fluid:

There are 5 main types of grinding fluid. Of these four are water based and

the other is a neat oil. With the water based fluids, the main constituent is water with

a concentrate added to a specified percentage. The concentrate should always be

added to the water, rather than the other way round, so that a stable emulsion will be

formed.

1. Emulsion: The concentrate normally has an oil content of 30-80%. When mixed with water, oil droplets are formed and these are dispersed evenly throughout the fluid. Droplet size is typically 3-8 um, which gives the fluid a milky appearance.

2. Semi-synthetic: The concentrate contains both oil and a synthetic lubricant. The oil content is in the range 4-30%.

3. Micro-emulsion: The concentrate has an increased emulsifier system to reduce the oil droplet size to less than 2 um. This makes the fluid transparent. Oil content in the concentrate can be up to 60%.

4. Synthetic: The concentrate contains no oil and a clear solution is formed. It can contain non-mineral lubricity materials at levels between 0 and 60%. With no oil content, a rust inhibitor is an essential additive.

5. Neat oil: The main constituent is a mineral oil. The type of base oil determines the viscosity. The viscosity affects the power required from the coolant pump and friction losses in the pipe work. A higher viscosity requires more pumping power and loses more velocity through friction in the pipes. The type of base oil, and the viscosity, selected depends on the application. Values of viscosity can range from 2 to 100 cSt @ 40oC, with 80% of applications in the range 6 to 40 cSt @ 40oC. Additives are usually included, with the types of additive depending on the application.

GLAZING, LOADING, WHEEL DRESSING AND DRESSING TOOLS:

Glazing:

When the surface of a grinding wheel develops a smooth and shining

appearance, then it is said to be glazed. This indicates the abrasive particles on the

wheel face are not sharp. These are worked down to their bond level.

Loading:

When soft materials like aluminium, copper, lead etc are ground the metal

particles get clogged between the abrasive particles. This condition is called as

loading. The effects of glazing and loading are almost same. Following are the

effects.

Excessive cutting pressure between wheel and work. More heat generation, Burning of the ground surface, Poor surface finish, Inaccuracies in the size and shape of the work piece and Wheel breakage.

Causes of glazing:

Wrong selection of grade and size, High wheel speed, Feed too fine Dirty coolant

A glazed or loaded grinding wheel can be reused after removing the glazed or

loaded particles from the grinding wheel face.

Grinding wheel dressing:

Dressing is an operation to change the cutting action of a wheel or to

recondition its grinding surface. Mostly dressing and truing are done at the same

time. Grinding wheels should be dressed and trued regularly to improve

Work production, Wheel performance and Grinding economy.

Dressing Truing

Refers to the removing of clogs and

blunt abrasive grains from the

surface of the grinding wheel.

Dressing exposes the cutting edges

which restore the correct cutting

action of the wheel. Dressing is

done on a glazed or loaded wheel to

recondition it.

Refers to the shaping of the wheel

to make it run concentric with the

axis. When a new grinding wheel

is mounted, it must be trued

before use to remove the run out.

Truing is done on the wheel

which is out of shape due to long

use. Sometime a wheel is also

trued to change the shape of the

grinding wheel face for a specific

grinding operation like form

grinding.

There are three types of wheel dressers. They are

Diamond,

Steel and

Abrasive.

Dressing tools:

A diamond dressing tool has a hard diamond point mounted in a metal shank.

The shank is fitted in a tool holder for location on the grinding machine to perform

dressing. Diamond dressers are most effective for precision grinding wheels. The low

feed of a diamond dresser can glaze the wheel. They are specified by their weight in

carats. Usually 0.5 carat to 1 carat diamond is used for dressing up to 300 mm

diameter of wheels.

Steel dressers for dressing a grinding wheel have rotary cutting surfaces made

from hard steel. They are held in place against the grinding wheel by hand and moved

across the face of the grinding wheel to do the dressing. The tool rest or other rigid

support must be used during this operation.

When only light dressing is required abrasive sticks are used. There are

abrasive materials made in the form of square or round sticks or put in metal tubes for

convenient handling. This type of dresser is more common in tool and cutter grinders

where truing and dressing is necessary.

Measurement of Grinding process:

There are two types of measurement. Those that are necessary to check

component quality and those that can be used to check efficiency of the grinding

process.

Measuring quality: There are three main checks on component quality.

[1] Accuracy: This involves overall dimensions and profile shape.

[2] Surface finish: This is often specified as a value of a surface roughness parameter.

Ra is probably the most common, other parameters such as Rz and Rt are also used.

As well as conforming to a measured value, visual appearance is also important in

some applications. This may mean avoiding vibration or chatter marking and deep

scratches.

[3] Component material condition: In many grinding applications it is essential to

avoid grinding burn (also called grinding abuse). This usually means damage to the

material structure of the component. There are three degrees of abuse:

[a] Rehardening burn. This is the most severe type of grinding damage.

It produces a hard, brittle layer on the surface. This is often associated

with grinding cracks.

[b] Temper burn. This is a softening of the material through overheating

during grinding. It is less severe than re hardening burn. Requirements

vary from no temper burn allowed to no check required. In between,

specifications are sometimes laid down for the amount of surface

softening that can be allowed.

[c] Residual stress. Grinding can leave stresses in the component

material, even when there is no burn. This can be critical for certain

applications such as gears and bearings, since fatigue life can be

affected.

Rehardening burn and temper burn are commonly assessed using a Nital etch.

Temper burn shows up as a darker area. Re hardening burn shows up as a lighter area,

usually surrounded by an area of temper burn. Residual stress measurement is not

common, but may become more so, as component quality requirements become more

stringent. Specialized equipment is needed.

Measuring grinding efficiency:

The following are three ways in which grinding efficiency can be measured,

additional to the quality checks above. These have not traditionally been measured,

but the trend is to add these to quality checks as a way of improving the control of the

grinding process and as a means of ensuring defects do not occur, rather than leaving

inspection to discover them and then scrap the component.

[1] Grinding power: A measurement of grinding power will show how efficiently the

wheel is cutting. A blunt or worn wheel will tend to rub so creating friction and

increased grinding power. This can be used to indicate when dressing is required.

Grinding power can also be used to detect if burn is likely to occur, since in some

cases, the start of burn can be related to a specific level of grinding power.

[2] Grinding ratio: This is defined as the ratio of the volume of component material

removed to the volume of the wheel consumed in the process. It is therefore a measure

of the efficiency with which the wheel is being used. This measurement can be used to

check if the wheel specification is correct. A low grinding ratio may mean the wheel

is too soft and is therefore breaking down too easily under the grinding forces. Care is

needed here, as too hard a wheel can sometimes give a low grinding ratio as well. Too

hard a wheel encourages chips to stick to the wheel surface and this can cause grits to

fall out too soon.

[3] Vibration: Vibration can be caused by many factors including a low stiffness

machine, too high a work speed, too hard a wheel, faulty bearings, out-of-balance, etc.

It usually leads to more efficient cutting as the vibration gives a self-dressing effect.

However, it is detrimental to surface finish, wheel life and machine life. Also, it often

causes excessive noise.

Grinding speed, feed and depth of cut:

Grinding speed:

It is the rate of travel of the wheel surface past a point on the work piece.

Wheel speed is otherwise called surface speed. It is expressed in terms of meters per

second.

N = V x 1000 / x d

V - Surface speed in meters / second.

D - Diameter of the wheel in mm.

N - RPM of the machine spindle.

1000 - to convert mm to meters.

60 - to convert RPM to revolution per second.

Feed:

In grinding refers to the movement of the wheel per stroke across the work

surface. The feed in grinding depends on the work speed, wheel width and the finish

required. It is generally 3/4th to 2/3rd of the wheel face width for rough grinding and

1/4th to 1/8 of the wheel face width in case of the finish grinding. When feed is high

the wheel wear increases surface finish deteriorates and the dimensional accuracy of

the work piece is affected.

Depth of cut:

It is the thickness of the material removed in surface grinding for one cut.

Depth of cut depends on the cutting load, power of the machine and finish required.

Generally the depth of cut is 0.02 to 0.03 mm for rough cut and 0.005 to 0.01 mm for

finish cut.

UNITS AND CONSTANT

Definitions:

Nominal size:

The size designation used for general identification. The nominal size of a

shaft and a hole are the same. This value is often expressed as a fraction.

Basic size:

The exact theoretical size of a part. This is the value from which limit

dimensions are computed. Basic size is a four decimal place equivalent to the

nominal size. The number of significant digits imply the accuracy of the dimension.

example: nominal size = 1 1/4

basic size = 1.2500

Design size:

The ideal size for each component (shaft and hole) based upon a selected fit.

The difference between the design size of the shaft and the design size of the hole is

equal to the allowance of the fit. The design size of a part corresponds to

the Maximum Material Condition (MMC). That is, the largest shaft permitted by the

limits and the smallest hole. Emphasis is placed upon the design size in the writing of

the actual limit dimension, so the design size is placed in the top position of the pair.

Tolerance:

The total amount by which a dimension is allowed to vary. For fractional

linear dimensions we have assumed a bilateral tolerance of 1/64 inch. For the fit of a

shaft/hole combination, the tolerance is considered to be unilateral, that is, it is only

applied in one direction from design size of the part. Standards for limits and fits state

that tolerances are applied such that the hole size can only vary larger from design

size and the shaft size smaller.

Basic hole system:

Most common system for limit dimensions. In this system the design size of

the hole is taken to be equivalent to the basic size for the pair (see above). This

means that the lower (in size) limit of the hole dimension is equal to design size. The

basic hole system is more frequently used since most hole generating devices are of

fixed size (for example, drills, reams, etc.) When designing using purchased

components with fixed outer diameters (bearings, bushings, etc.) a basic shaft system

may be used.

Allowance:

The allowance is the intended difference in the sizes of mating parts. This

allowance may be: positive (indicated with a "+" symbol), which means there is

intended clearance between parts; negative("-"), for intentional interference: or

"zero allowance" if the two parts are intended to be the "same size".

Base and Supplementary Units

Quantity Unit Symbol

Length meter m

Mass kilogram kg

Time second s

Electric current ampere A

Thermodynamic temperature Kelvin K

Luminous intensity candela cd

Molecular substance mole mol

Plane angle radian rad

Solid angle steradian sr

Derived Units

Quantity Unit Symbol

Space and Time

Area square meter m

Volume cubic meter m

Velocity meter per second m/s

Acceleration meter per second per second m/s

Angular velocity radian per second rad/s

Angular acceleration radian per second per second rad/s

Frequency hertz Hz (cycle/s)

Rotational speed revolution per second

revolution per minute

r/s

r/m

Mechanics

Density kilogram per cubic meter kg/m

Momentum kilogram meter per second kgm/s

Moment of inertia kilogram meter squared kgm

Force newton N (kgm/s)

Torque, moment of force newton meter Nm

Energy, work, heat quantity joule J (Nm)

Power watt W (J/s)

Pressure, stress pascal Pa (N/m)

Heat

Customary temperature degree Celsius C

Thermal conductivity watt per meter Kelvin W/(mK)

Entropy joule per Kelvin J/K

Specific heat joule per kilogram Kelvin J/(kgK)

Light

Luminous flux lumen lm (cdsr)

Illumination lux lx (lm/m)

Luminance candela per square meter cd/m

Viscosity

Kinematic viscosity square meter per second m/s

Dynamic (absolute) viscosity pascal second Pas

Quantity Equivalent Dimensions S.I. units

Mass M Kilogram

(kg)

Length L Metre (m)

Time T Second (s)

Frequency cycles/unit time T-1 Hertz (Hz)

Area length x width L2 m2

Volume length x height x width L3 m3

Density Mass/unit volume ML-3 kg/m3

Velocity Distance/unit time LT-1 m/s

Acceleration Velocity/u