me6004 unconventional machining processes l t p …...• wire cut electrical discharge machining. 8...

REGULATION :2013 ACADEMIC YEAR : 2018-2019

JIT-JEPPIAAR/MECH/Mr.M.K.KARTHIK & Mr. S.KANNAN/IIIrdYr/SEM 06/ME6004 /UNCONVENTIONAL

MACHINING PROCESSES UNIT 1-5/QB+Keys/Ver1.0

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ME6004 UNCONVENTIONAL MACHINING PROCESSES L T P C

3 0 0 3

OBJECTIVES:

To learn about various unconventional machining processes, the various process parameters and

their influence on performance and their applications

UNIT I INTRODUCTION 6

Unconventional machining Process – Need – classification – Brief overview .

UNIT II MECHANICAL ENERGY BASED PROCESSES 9

Abrasive Jet Machining – Water Jet Machining – Abrasive Water Jet Machining – Ultrasonic

Machining.(AJM, WJM, AWJM and USM). Working Principles – equipment used – Process

parameters – MRR- Applications.

UNIT III ELECTRICAL ENERGY BASED PROCESSES 9

Electric Discharge Machining (EDM)- working Principle-equipments-Process Parameters-

Surface Finish and MRR- electrode / Tool – Power and control Circuits-Tool Wear – Dielectric –

Flushing – Wire cut EDM – Applications.

UNIT IV CHEMICAL AND ELECTRO-CHEMICAL ENERGY BASED PROCESSES 11

Chemical machining and Electro-Chemical machining (CHM and ECM)-Etchants – Maskant -

techniques of applying maskants - Process Parameters – Surface finish and MRR-Applications.

Principles of ECM- equipments-Surface Roughness and MRR Electrical circuit-Process

Parameters- ECG and ECH - Applications.

UNIT V THERMAL ENERGY BASED PROCESSES 10

Laser Beam machining and drilling (LBM), plasma Arc machining (PAM) and Electron Beam

Machining (EBM). Principles – Equipment –Types - Beam control techniques – Applications.

TOTAL: 45 PERIODS

OUTCOMES:

Upon completion of this course, the students can able to demonstrate different unconventional

machining processes and know the influence of difference process parameters on the

performance and their applications.

TEXT BOOKS:

1. Vijay.K. Jain “Advanced Machining Processes” Allied Publishers Pvt. Ltd., New Delhi, 2007

2. Pandey P.C. and Shan H.S. “Modern Machining Processes” Tata McGraw-Hill, New Delhi,

2007.

REFERENCES:

1. Benedict. G.F. “Nontraditional Manufacturing Processes”, Marcel Dekker Inc., New York,

1987.

thods of Machining”, Chapman and Hall, London, 1998.




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3. Paul De Garmo, J.T.Black, and Ronald.A.Kohser, “Material and Processes in Manufacturing”

Prentice Hall of India Pvt. Ltd., 8thEdition, New Delhi , 2001.




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Subject Code:ME6004 Year/Semester: III /06

Subject Name:Unconventional Machining Processes

Subject Handler:S.Kannan&M.K.Karthik

UNIT I - INTRODUCTION

Unconventional machining Process – Need – classification – Brief overview.

PART * A

Q.No. Questions

1.

What are the industrial needs for UCM process? (May/June 2009/16) (Nov/Dec 2016 ) BTL2

Many of the materials such as carbides, stainless steel, nitralloy, hastalloy which have high

strength to weight ratio, hardness and heat resisting qualities used in aerospace and nuclear

industries. To obtain degree of accuracy and surface finish UCM process are used.

2

How non-traditional machining processes are classified? (Nov/Dec 2013)(May/Jun

2016)BTL1

Mechanical Energy based processes, Electrical Energy based processes, Chemical and Electro

Chemical Energy based processes, and Thermal Energy based processes.

3

What are the various cost factors that should be analyzed in unconventional machining

processes? (Nov/Dec 2010)BTL2

Factors are: Capital cost, Tooling cost, Power requirement, Metal removal rate efficiency, Tool

consumption.

4

What are the advantages and limitation of UCM process? (Nov/Dec 2012)(April/May2017)

BTL1

Advantages:

• To increases productivity

• Close tolerance is possible

Limitations:

• It is more expensive

• Metal removal rate is slow

5

Why is unconventional mechanical machining process not popular? (Nov/Dec 2009) BTL1

• Unconventional machining processes are more expensive

• Metal removal rate is slow

• AJM, CHM, PAM, and EBM are not commercially economical processes.

6

What are the advantages of UCM processes? (Nov/Dec 2015)(May/June 2016) BTL1

• The machined surfaces do not have any residual stresses.

• Harder and difficult to machine materials can be machined by this process

• It reduces number of rejected components

• It increases productivity.

7

List the unconventional machining based on Electrical energy. (May/June2012) BTL1

• Electro Discharge Machining

• Wire Cut Electrical Discharge Machining.

8

Why UCM process is not so effective on soft metals like aluminum? (May/June

2007)(April/May 2008) BTL2

UCM process is not so effective on soft metal like aluminum because it is very difficult to

maintain the close tolerance and accuracy desired.




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9

List the UCM process based on Chemical energy. (Nov/Dec 2015) BTL1

• Chemical machining and

• Electro Chemical machining.

10

Name the important factors that should be considered during the selection of an

unconventional machining process for a given job. (April/May 2011) BTL1

• Physical Parameters,

• Shapes to be machined,

• Process capability,

• Economic consideration.

11

Suggest a suitable UCM process to cut a thin glass plate into 2 pieces. (April/May

2017)BTL3

• Process like ECM, EDM, PAM, EBM are ruled out because they are suitable to machine

only electrically materials

• WJM, AWJM, USM can be used for machining.

• Based on accuracy required any one process can be selected.

12

What are the characteristics of unconventional machining process? (Nov/Dec 2007) BTL1

• The unconventional machining processes do not employ a conventional tool for metal

removal; instead, they directly utilize some form of energy for metal machinery.

• The tool material need not be harder than the work piece material.

13

Compare conventional and unconventional machining process. (April/May 2015)(Nov/Dec

2016) BTL2

• Tool and workpiece have physical contact

• High hardness materials like WC, Ti, etc (having more than 45 HRC) cant machined.

• Poor surface finish

• Low accuracy and poor tolerance.

• Contour profile can’t be produced.

14

What are the importance of unconventional machining? (May/June2006) BTL1

• Machining hard and brittle material,

• To machine high shape complexity

• Fulfill high surface integrity and precision requirements.

15

List the different unconventional machining process based on thermal energy. (April/May

2017) BTL1

• Plasma arc machining

• Laser beam machining

• Electron beam machining.

16

Name any four variables in Abrasive Jet Machining that influence the metal removal rate?

(Nov/Dec 2006) BTL1

• Standoff distance,

• Abrasive flow rate,

• Gas pressure

• Mixing ratio.

17

List the advantages of electric discharge machining. (Nov/Dec 2006) BTL1

• Electrically conductive material can be machined,

• Complicated geometry can be produced

• MRR high when compared to other unconventional machining process.




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18

What are the applications of WJM? (May/Jun 2007) BTL2

• To produce prototype parts efficiently,

• Doesn’t leave burr

• To produce complex geometry parts.

19

Write the importance of surface machining in machining operations. (May/June 2014)

BTL2

Surface finish is more important in machining components because it reduces the friction when

the components make contact with the other.

Also it gives good appearance. Generally in aerospace and medical applications good surface

finish is required.

20

Enlist the requirement that demands the use of advanced machining process? (April/May

2008)BTL1

Machining newly developed material with high strength, hardness & toughness, to avoid surface

damage,To machine complex geometry components.

21

List the different unconventional machining process based on thermal energy. (April/May

2017) BTL1

• Plasma arc machining,

• Laser beam machining

• Electron beam machining.

22

What are the major characteristics of conventional machining? BTL2

Macroscopic chip formed due to shear deformation, Material removal takes place due to

application of cutting force, cutting tool harder than the work piece & mechanism of material

removal is shearing.

23

List some of the Hybrid processes. BTL1

• Electro chemical grinding,

Electro chemical spark machining,

• Electro chemical arc machining,

• Electro discharge abrasive grinding,

• Electro chemical honing.

PART * B

1

a) Classify the unconventional machining process on the basis of the type of energy

employed. Also, state the mechanism of material removal, transfer media and energy

sources used. (8M) BTL1Answer: Page 1.3 – Dr.S.Senthil.

Unconventional machining processes are classified as follows,

i). Based on the type of enérgyrequired to shape the material (2M)

• Thermal energy methods

• Electrical energy methods

• Electro chemical energy methods

• Chemical energy methods

• Mechanical energy methods

ii). Based on the mechanism involved in the process (2M)

• Erosion ‘

• Ionic dissolution’




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• Vaporisation

iii). Source of energy required for material removal (2M)

• Hydrostatic. pressure

• High current density

• High voltage

• Ionised material

iv). Medium of transfer of energies (2M)

• High voltage particles

• Electrolyte

• Electron

• Hot gases

b) Name the important factors that should be considered during the selection of

unconventional machining process for a given job.(Nov/Dec2012, May/June 2016,

April/May 2017) (5 M) BTL2Answer: Page 1.5 – Dr.S.Senthil.

All methods are not suitable for all materials. Depending onthe material to be machined,

following methods can be used as shown in the table,(5M)

S.No. Material Mothod of Machining

1 Non metals like ceramics,

plastics and glass

USM, AJM, EBM, LBM

2 Refractories USM, AJM, EDM, EBM

3 Titanium EDM

4 SuperAlloys AJM, ECM, EDM, PAM

5 Steel ECM, CHM, EDM, PAM

2

Discuss about the energy source and mode of metal removal for modern machining

methods. (May/June 2016) (13 M) BTL4Answer: Page 5.2 – Dr.S.Senthil.

Electro-Chemical Machining: (3M)




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Chemical Machining: (3M)

Mechanical Machining: (3M)




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Thermo-Electric Machining: (4M)




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3

Compare and contrast the various unconventional machining process on the basis of type of

energy employed, material removal rate, transfer media and economical aspects.

(Nov/Dec2013) (13 M) BTL3Answer: Page1.4 – Dr.S.Senthil.

i) Thermal energy m¢thods: (3M)

In these methods, heat energy is concentrated on a small area of the work piece to melt and

vaporize the tiny bits of work material.The required shape. is obtained by the continued repetition

of this process.

Examples:

• Laser Beam Machining (LBM)

• Plasma Arc Machining (PAM)

• Electronic Beam Machining (EBM)

• Ion Beam Machining (IBM)

(ii) Electrical energy methods (2M)

In these methods, electrical energy is directly used to cut the material to get the final shape and

size.

Example :

• Electro Discharge Maching (EDM)

• Wire Cut Electrical Discharge Machining (WCEDM)

iii) Electro chemical energy methods (3M)

In these methods, material is removed by ion displacement of the work piece material in contact

with a chemical solution. ‘

Example :




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• Electro Chemical Maching (ECM)

• Electro Chemical Grinding (ECG)

• Electro Chemical Honing (ECH)

• Electro Chemical Deburring (ECD)

iv) Chemical energy methods(2M)

These methods involve controlled etching of the work piecematerial in contact with a chemical

solution.

Example: ‘ ‘

• Chemical Machining

v) Mechanical energy methods (3M)

in mechanical energy methods, the material is removed bymechanical erosion of the work piece

material.

Example:

• Ultrasonic Machining (USM)

• Abrasive Jet Machining (AJ M)

• Water jet Machining (WJM)

4

Explain about the physical parameters and shapes to be machined criteria for the process

selection in unconventional machining process. (Nov/Dec 2013) (13 M) BTL1Answer: Page

1.6 – Dr.S.Senthil.

Physical parameters(6M)

The physical parameters of different unconventional machining processes are given in the

following table.




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Shapes to be machined: (7M)

‘ The application of the unconventional machining processes is also influenced by the shape and

size of the work piece to be produced.

For producing micro holes

- LBM is best suited.

For producing small holes ‘

- EBM is well suited.

For deep holes (L/D > 20 ) and contour machining

- ECM is best suited.

For shallow holes

- USM and EDM are well suited.

For Precision through cavities in.work pieces

' - USM and EDM are best suited

For honing

- ECM is well suited.

For etching small portions ’

‘ - ECM and EDM.are well suited.

For Grinding.

- AJM and EDM are best suited.

For deburring

- USM and AJM are well suited.

For threading




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- EDM is best suited.

For clean, rapid cuts and profiles‘

- PAM is well suited.

FQP Shallow pocketing

- AJM is well suited.

5

Explain about the process capability and process economics criteria for the process selection

in unconventional machining process. 13M (Nov/Dec 2010)BTL4Answer: Page 1.8 –

Dr.S.Senthil.

Process capability (or) Machining Characteristics: (6M)

The machining Characteristics can be analyzed with respect to

• Metal removal rate obtained

• Tolerance maintained

• Surface finish obtained

• Depth of surface damage

• Power required for machining

The following table gives the typical values of the variousunconventional machining

charafiefistics

Process Economy: (7M)

The economics of the various processes are analyzed by considering the following points

• Capital cost.

• Tooling’ cost.

• Power requirement

• Metal removal rate efficiency.

• Tool consumption.

The following table gives the process economy of unconventionalmachining processes




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.

6

List out the demerits of conventional machining process and the advantages of

Unconventional machining process. (May/June2012) (13 M) BTL3Answer: Page 1.9 –

Dr.S.Senthil.

DEMERITS OF CONVENTIONAL MACHINING PROCESSES: (8M)

• In conventional machining, metal is removed by chip formation which is an expensive

and difficult process.

• Chips produced during this process are unwanted by products.

• Removal of these chips and their disposal are recycling in a very tedious procedures,

involving energy and money.

• Very large cutting forces are involved in this process. So, proper holding of the

workpiece is most important.

• Due to the large cutting forces and large amount of heat generated between the tool and

the workpiece interface, undesirable deformation and residual stresses are developed in

te workpiece.

• It is not possible to produce chips by conventional machining process for delicate

components like semi conductor.

ADVANTAGES OF UNCONVENTIONAL MACHINING PROCESSES: (5M)

• It increases productivity.

• It reduces number of rejected components.

• Close tolerance is possible.

• The tool material need not be harder than work piece material as in conventional

machining.

• Harder and difficult to machine materials can be machined by this process.

• The machined surface do not have any residual Stresses




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PART * C

1

List out the features of unconventional machining process. (Nov/Dec 2015,May/Jun 2016)

(15 M)BTL3Answer:.

FEATURES OF UNCONVENTIONAL MACHINING PROCESS:

• Material is removed from the workpiece without physical contact.

• Material removal rate is independent of the hardness of the workpiece.

• Cutting forces are independent of the hardness of the workpiece.

• Tool material need not be harder than the work material.

• Almost any work material, irrespective of its hardness and strength, can be machined.

• Tool wear is negligible.

• No burr is left on the workpiece.

• No residual stresses are left on the surface machined.

• Entire contour or desired shape can be obtained in one stage or in one setting. This is

possible since material removal takes place uniformly over the entire area

simultaneously.

• Surface integrity of the surfaces produced by modern machining methods is superior.

• Intricately shaped contours and fine machining of precision holes are possible.

• Modern finishing methods can be integrated easily with microprocessors and numerical

controls for better control of the process.

2

Discuss about the different type of metal removal for different types of unconventional

machining processes. (May/Jun 2014) (15 M) BTL4Answer:

S.No Machining Processes

Predominant mechanism of metal

removal (or

) Physical phenomenon involved during

machining

1 Conventional Machining Method (Eg:

Turning, Milling etc) Shear failure of work material

2 Abrasive Jet Machining / Water Jet

Machining Erosion of work material

3 Ultrasonic Machining

Cavitations phenomenon

Erosion of work material

Brittle fracture of work material

4 Chemical Machining (or) Chemilling

Etching

Corrosion

Vaporization

5 Electro Chemical Machining Anodic dissolution of workpiece

Ion displacement

6 Electro Chemical Grinding

Anodic dissolution of work

material

Ion displacement

Erosion of work material (due to

grinding action of abrasive wheel)

7 Spark Erosion Machining Vaporization of work material

8 Electron Beam Machining Vaporization of work material




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9 Laser Beam Machining Vaporization of work material

10 Ion Beam Machining Vaporization of work material

Fusion of work material

11 Plasma Arc Machining Fusion of work material


JIT-JEPPIAAR/MECH/Mr.M.K.KARTHIK & Mr. S.KANNAN/IIIrdYr/SEM 06/ME6004/UNCONVENTIONAL MACHINING

PROCESSES /UNIT 1-5/QB+Keys/Ver1.0

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UNIT II - MECHANICAL ENERGY BASED PROCESSES

Abrasive Jet Machining – Water Jet Machining – Abrasive Water Jet Machining –

UltrasonicMachining.(AJM, WJM, AWJM and USM). Working Principles – equipment used –

Processparameters – MRR- Applications.

PART * A

Q.No. Questions

1.

What are the abrasives used in AJM process? (Nov/Dec 2015,2016)(May/Jun 2017)BTL 2

The various abrasive particles used in AJM process are Aluminium Oxide, Silicon Carbide, glass

powder, Dolomite and specially prepared sodium bicarbonate.

2 What are the desirable properties of carrier gas in AJM? (Nov/Dec 2012)BTL 2

It should be cheap, non-toxic, easily available and should dry quickly

3

List the applications of WJM process (April/May 2015)(Nov/Dec 2016)

This process is very convenient for cutting relatively softer and non-metallic materials like paper

boards, plastics, wood rubber, leather, fibre glass etc.,

4

What is meant by transducer? (Nov/Dec 2012) BTL 2

It is a device which converts one form of energy into another form of energy. Ex: Piezoelectric

transducer

5

What is the effect of abrasive grain size on machining rate in USM? (May/June2009) BTL 2

Material removal rate and surface finish are greatly influenced by grit of the abrasive. For

roughing work operation, grit size of 200-400 are used for finishing operation, grit size of 800-

1000 are used.

6

Write the typical applications of ultrasonic machining. (Nov/Dec 2015)(April/May 2017)

BTL 2

Holes as small as 0.1mm can be drilled, Precise and intricate shaped articles can be machined, it

is used for making tungsten carbide and diamond wire drawing dies and dies for forging and

extrusion processes.

7

What is the principle of ultrasonic machining process? (May/June 2016, April/May2015)

BTL 2

In this process, slurry of small abrasive particles are forced against the workpiece by means of a

vibration tool and a causes the removal of metal from the workpiece in the form of extremely

small chips.

8

What are the parameters that influence the material removal rate in USM? (May/June

2016) BTL 2

Tool material, Frequency Range, Stand-off distance, Amplitude of tool is the parameters that

influence the material removal rate in ultrasonic machining process.

9

Explain water jet machining process. (May/June2013) BTL 3

In this process, high pressure and high velocity stream of water is used to cut the relatively softer

and non-metallic materials like paper board, wood, plastics, rubber, fibre glass, leather etc.,

10

What are the factors that affect the material removal rate in AJM process?

(Nov/Dec2013)(May/Jun 2016) BTL 1

Mass flow rate, Abrasive grain size, Gas pressure, Velocity of abrasive particles, mixing ratio and

Nozzle tip clearance.

11 State the advantages of USM. (May/Jun 2014) BTL 2

Adv: Noiseless operation, Metal removal cost is low and Good surface finish




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12

Explain the abrasives used in USM process. (Nov/Dec 2016) BTL 2

The most commonly used abrasives are boron carbide, silicon carbide, aluminium oxide and

diamond. Boron is the most expensive material and is best suited to cutting of tungsten carbide,

tool steels etc., Aluminium oxide is the softest abrasive and it is used for machining glass

ceramics.

13

Reuse of abrasives is not recommended in Abrasive jet machining process. Why? (May/Jun

2016) BTL 2

Once the abrasive is used, it loses its sharpness, which result in poor material removal and

Surface finish.

14

Why Abrasive jet machining is not recommended to machine ductile material? (April/May

2017) BTL 2

The abrasive materials tend to embed on the work piece because of the ductile nature of the

material.

15 List any four variables in AJM that influence the MRR. (Nov/Dec 2006) BTL 1

Standoff distance, abrasive flow rate, gas pressure & mixing ratio.

16 List the process parameter of WJM. (May/Jun 2007) BTL 1

Pressure of water, mass low rate of water, size of abrasive particles & abrasive flow rate.

17 Write the formula for MRR for ductile and brittle material in AJM. (May/Jun 2014) BTL 1

MRR =x.Z.d3.V3/2(p/12Hw)3/4

18 Mention the application of catcher in WJM. (April/May 2008) BTL 2

To absorb the residual energy of the water jet and dissipate the same.

19

What type of operation can be performed in abrasive jet machining? (Nov/Dec 2008) BTL 2

Cut narrow slots of range 0.12 to 0.25 mm, good surface finish of range 0.25 to 1.25 microns,

surface damage is low.

20

What are the major elements of USM equipment? (May/June 2009)BTL 2

Power supply generator, acoustic head transducer, tool feed mechanism, abrasive feed

mechanism.

21

How do the operating parameter affect the machining process in AJM? (Nov/Dec 2010)

BTL 2

Plastic,Nylon,Teflon components can be deburred and polished, Metallic mold cavities can be

cleaned by this process & Glue and paints can be removed from painting and leather objects.

22

How is ultrasonic machining different from conventional grinding? (Nov/Dec 2010) BTL 2

USM: Abrasive slurry is used to remove material, tool vibrated at high frequency

Grinding: Abrasive particles are used to remove the material, abrasive wheel is rotated at high

speed.

23 Mention the function of horn in ultrasonic machining. (April/May 2017) BTL 1

To connect the tool to the transducer, amplifies the amplitude of vibration.

24 What are the major elements of USM? (Nov/Dec 2008) BTL 1

Power supply, transducer, tool feed mechanism & abrasive slurry.

PART * B

1

Discuss the working and effects of various process parameters of abrasive jet machining on

accuracy and MRR (May/June 2014, 2016)(13 M) BTL 4Answer: Page 2.5 – Dr.S.Senthil.

Metal Removal Rate Process Parameters:(1 M)

The metal removal rate depends upon the following parameters.

• Mass flow rate.




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• Abrasive grain size.

• Gas pressure.‘

• Velocity of abrasive particles.

• Mixing ratio.

• Nozzle tip clearance.

1. Mass Flow Rate:(2 M)

At particular pressure, the material removal rate increases with theabrasive flow rate. But. after

reaching an optimum value, the materialremoval rate decreases with further increase in abrasive

flow rate. This is due to the fact that the mass flow rate of gas or air decreases with the increase of

abrasive flow rate.

2. Abrasive Grain Size:(2 M)

The various abrasive particles used in AJM process are aluminiumoxide (A1203), silicon carbide

(SiC), glass powder, dolomite andspecially prepared sodium bicarbonate.

Aluminium oxide is a general purpose abrasive and is used in sizesof l0, 25 and 50 microns.

Silicon carbide is used for faster cutting onextremely hard materials. It is used in sizes of 25 and

50 microns. Dolomite -of 200 grit size is found suitable for light cleaning andetching. Glass

powder of 0.30 to 0.60 mm are used for light polishingand deburring.

In general larger Sizes are used for rapid removal rate whilesmaller sizes are used for

good surface finish and precision.

3. Gas Pressure(2 M)

The metal removal rate increases with increase in gas or airpressure as shown in Fig.2.6.




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4. Velocity of Abrasive Particles:(2 M)

The metal removal rate increases with the increase of velocity ofabrasive particles as shown in

Fig.2.7.

5. Mixing Ratio:(2 M)

Mixing ratio is defined as the ratio of mass flow rate of abrasive tothe mass flow rate of gas.

Mixing ratio =Mass flow rate of abrasive

𝑀𝑎𝑠𝑠 𝑓𝑙𝑜𝑤 𝑟𝑎𝑡𝑒 𝑜𝑓 𝑔𝑎𝑠

Metal removal rate first increases with the increaseof mixing ratio, upto certain limit after that it

decreases gradually as shown in Fig.2.8.




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6. Nozzle Tip Clearance or Stand-off Distance: (2 M)

The distance between the nozzle tip and the workpiece has greatinfluence on the diameter of cut,

its shape, size and also on the rate ofmaterial removal.

The material removalvrate first increases with the increase of tipclearance from workpiece upto a

certain limit after that it’remainsunchanged for a, certain tip clearance and then decreases

gradually asshown in Fig.2.9.

2

a) Explain with a neat sketch, the working principle of ultrasonic machining process.

(May/June2013/16)(April/May 2017 (9 M)BTL 4Answer: Page 2.19 – Dr.S.Senthil.

Ultrasonic Waves: (2M)

Ultrasonic machining is onekind of grinding method. Itis alsoknown as ultrasonic grinding or

impact grinding. The term ultrasonicrefers to waves of high frequency. Human ear can hear the

soundwaves between 20 Hz to 20 kHz. This rangeis known as audiblerange. The sound waves

which have frequencies less than the audiblerange are called infrasonic waves. The sound waves

havingfrequencies above the audible range are known as ultrasonic waves.

Principle Of USM:(3M)

In this machining method, a slurry of small abrasive particles are forced against the workpiece by

means of a vibrating tool and it causes the removal of metal from the workpiece in the form of

extremely small chips.




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(4 M)

b) Discuss about various applications of USM process. (May/June2013/16)(April/May 2017)

(4 M) BTL 4Answer: Page 2.41 – Dr.S.Senthil.

Applicationsof USM: (4 M)

• Holes as small as 0.1 mm can be drilled.

• Precise and intricateshaped articles can be. machined.

• It has been efficiently applied to machine glass, ceramics,tungsten, precision mineral

stones, etc.

• It is used for making tungsten carbide and diamond wiredrawing dies and dies for forging

and extrusion processes.

• Several machining operations like drilling, grinding, turning,threading, profiling, etc., on all

materials both conducting andnon-conducting.

3

Write down the construction and working principle of WJM (May/June2012) (13 M)BTL

Answer: Page 2.13 – Dr.S.Senthil.

Construction: (4M)

• It consists of pump, accumulator, control valve, regulating chamber, nozzle, etc.

• A pump or intensifier is used to raise the pressure of water. Pressure normally used in the

system are in the range of 1500 to4000 N / mm2.

• Since the cutting action may not be continuous, the accumulator is used to store the water

and also it helps in eliminating pulsation.

• Nozzle is used to increase the velocity of the Water jet. The nozzle is made up of sintered

diamond, tungsten Carbide or synthetic sapphire.‘ The exit diameter of the nozzle is in the

range of 0.05 to 0.35 mm and the exit velocity of the water jet from the nozzle varies upto

920 m/s.

• A regulating chamber is incorporated in the line to control the flow of water jet to the

nozzle.




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(5M)

Working:(4M)

• The working principle of water jet machining is similar to that of abrasive jet machining.

• A pump or intensifier is ‘used to increase the pressure of the water and the water passes on

to accumulator from from the pump.

• Water under pressure from a hydraulic accumulator is passed through the orifice of a nozzle

to increase its velocity.

• When the high velocity of water jet co mes out of the nozzle and strikes the work material,

its kinetic energy is converted into pressure energy including high stresses in the work

material.

• When this induced stress exceeds the ultimate shear stress of the material, small chips of

the material get loosened and fresh surface is exposed.

4

Write short notes on the following related to USM a) types of transducer used in USM b)

Magnetostriction transducer. (May/June 2014) (13 M) BTL 4 Answer: Page 2.23 –

Dr.S.Senthil.

TYPES OF TRANSDUCER:(2 M)

The following types of transducers are used in ultrasonicmachining processes for the production

of ultrasonic waves.

• Magnetostriction transducer.

• Piezoelectric transducer

MAGNETOSTRICTION TRANSDUCER

Principle: (4 M)

When a rod of ferromagnetic material such as iron ornickel is kept in a magnetic field parallel to

its length, the rod suffers achange in its length. The change in length is independent of

thedirection of the magnetic field and depends only on the magnitude of

the field and nature of the material. This phenomenon is known asMagnetostriction effect.




6-23

Construction: (7 M)

The arrangement of magnetostriction transducer isshown in Fig.2.l4.

• A rod of (AB) ferromagnetic materials such as iron or nickel is clamped at the middle.

• The two ends (A and B) of the rod is wound by the coil L1 and L2, The coil L1 wound on

the right hand portion of the rod along with a variable capacitor C1. The coil L2 wound on

the left hand portion of the rod is connected to the grid circuit.

• The frequency of the oscillatory circuit is adjusted by the variable capacitor (C1) and the

current is noted by the milli-ammeter.

• The low tension battery (LT) and high tension battery (HT) are provided to produce current

in the grid circuit.

• Working :

• The filament in the grid circuit is heated by the low tension battery (LT). This causes the

production of electrons and these electrons are accelerated to a very high velocity by using

high tension battery (HT). So, the altemating current is produced in the circuit (L, C).

• This alternating current passing through the coil L1 produces an altemating magnetic field

along the length of the rod.

• The result is, the rod starts to vibrate due to the magnetostrictive effect. The vibrations of




6-24

the rod create ultrasonic waves which are sent out as shown in Fig.2.l4.

• The longitudinal expansion and contraction of the rod (AB) produces an e.m.f in the coil

L2. This induced e.m.f is fed to the grid circuit continuously as a feedback. In this way, the

current is built up and the vibrations of the rod is maintained.

• When the frequency of the oscillatory circuit is equal to the frequency of the vibrating rod,

resonance occurs. At resonance, the rod vibrates vigorously and ultrasonic waves are

produced with very high frequencies.

Frequency of the oscillatory circuit = Frequency of the vibrating rod

1

2𝜋√𝐿1𝐶1

=1

2𝑙√

𝐸

𝜌

where,

E — Young’s modulus of the rod material, N/m2,

l - Length of the rod, m,

𝜌 — Density of the rod material, kg/m3.

5

Mention the Advantages and disadvantages of AJM. (Nov/Dec 2013) BTL 4 Answer: Page


Advantages Of AJM:(6 M)

• This process is suitable for cutting all materials. Even diamond can be machined by using

diamond as abrasive.

• There is no heat generation during this process. So, thermal damage to the workpiece is

avoided.

• Very thin and brittle materials can be cut without any risk of breaking.

• There is no direct contact between the tool and workpiece.

• Low initial investment.

• Good surface finish.

• It can be used to cut intricate hole shapes in hard and brittle materials.

Disadvantages Of AJM:(7 M)

• Material removal rate is slow.

• Soft material cannot be machined.

• Machining accuracy is poor.

• Nozzle wear rate is high.

• The abrasive powder used in this process cannot be reused because of decrease of cutting

capacity and clogging if the nozzle due to contamination.

• There is always a danger of abrasive particles getting embedded in the workpiece. So,

cleaning is essential afier the operation.

• It requires some kind of dust collection system.

6

Write a brief note on Feed Mechanism of material removal in USM. (April/May 2017) (13

M)BTL 4Answer: Page 2.31 – Dr.S.Senthil.

Feed Mechanism: (1 M)

Feed system is used to apply the static load between the tool and workpiece during ultrasonic

machining operation. The basic requirements of tool mechanism are as follows :

• The tool should be moved slowly to prevent breaking.

• The tool has to come back to its initial position afterfinishing its machining operation.




6-25

There are three types of feed mechanism which are used in ultrasonic machining processes. They

are:

• Gravity feed mechanism.

• Spring loaded feed mechanism.

• Pneumatic (or) Hydraulic feed mechanism.

Gravity Feed Mechanism:(4 M)

Fig.2.l9 shows the operation of gravity feed mechanism. In thismechanism, counter weights are

used to apply the 1oad to the headthrough a pulley as shown in Fig. In order to reduce the

friction, ballbearings are used. Gravity feed mechanism is generally preferredbecause of its

simple construction.

Spring Loaded Feed Mechanism:(4 M)

The arrangement of spring loaded feed mechanism is shown below. In this mechanism, Spring

Pressure is used to feed the tool during machining operation. This type of mechanism is also

preferred because of its sensitivity and compactness.

Pneumatic or Hydraulic Feed Mechanism:(4 M)

In order to get high feed rate, pneumatic feed mechanism is used.




6-26

7

Describe the apparatus, construction and working principle of AJM. (Nov/Dec 2016)(13

M)BTL 4Answer: Page 2.2 – Dr.S.Senthil.

Construction:(6 M)

The schematic arrangement of abrasive Jet machineis shown in Fig.2.2.

It consists of mixing chamber,nozzle, Pressure gauge, hopper,filter, compressor, vibrating

device, regulator, etc.

• The gases generally used in this process are nitrogen, carbon dioxide or compressed air.

• The various abrasive particles used in this process are aluminium oxide, silicon carbide,

glass powder, dolomite and specially prepared sodium bicarbonate. '

• Aluminium oxide (A1203) is a general purpose abrasive and it is used in sizes of 10, 25 and

50 micron. Silicon carbide (SiC) is used for faster cutting on extremely hard materials. It is

used in sizes of 25 and 50 microns. Dolomite of 200 grit size is found suitable for light

cleaning and etching. Glass powder of diameter 0.30 to,0.60 mm are used for light polishing

and deburring.

• As the nozzle is subjected to a great degree of abrasion wear, it is made up of hard materials




6-27

such as tungsten carbide, synthetic sapphire (ceramic), etc., to reduce the wear rate.

• Nozzles made of tungsten carbide have an average life of 12 to 20 hrs whereas Synthetic

Sapphire nozzle have an average life of 300 hours. Nozzle tip clearance from work is kept at

a distance of 0.25 to 0.75 mm.

• The abrasive Powder feed rate is controlled by the amplitude of the vibration mixing

chamber. A pressure regulator controls the gas or air flow and pressure. To control the size

and shape of the cut either the workpiece or the nozzle is moved by a well designed

mechanism such as cam mechanism, pantograph mechanicsm etc.,

Working: (7 M)

• Dry air or gas (N2 or CO2) is entered into the compressor through a filter where the

pressure of air or gas is increased.

• The pressure of the air varies from 2 kg /cm2 to 8 kg/cm2.

• Compressed air or high pressure gas is supplied to the mixing chamber though a pipe line.

This pipe line carries a pressure gauge and a regulator to control the air or gas flow and its

pressure.

• The fine abrasive particles are collected in the hopper and fed into the mixing chamber. A

regulator is incorporated in the lineto control the flow of abrasive particles.

• The mixture of pressurised air and abrasive particles from the mixing chamber flows into

the nozzle at a considerable speed.

• Nozzle is used to increase the speed of the abrasive particles and it is increased upto 300 m /

s.

• This high speed stream of abrasive particles from the nozzle, impact the workpiece to be

machined. Due to repeated impacts, small chips of material get loosened and a fresh surface

is exposed.

• A vibrator is fixed at the bottom of the mixing chamber. When it vibrates, the amplitude of

the vibrations controls the flow of abrasive particles.

• This process is. widely used for machining hard and brittle materials, non-metallic materials

(germanium, glass, ceramics and mica) of thin sections. This process is capable of

performing drilling, cutting, deburring, etching and cleaning the

surfaces.

• Abrasive Jet Machining (AJM) process differs from sand blflsllng process. AJM is basically

meant for metal removal with the use of small abrasive particles, whereas the sand blasting

process is a surface cleaning process which does not involve any metal cutting.

PART * C

1

Explain in detail about the piezoelectric transducer used in the ultrasonic machining

process. (May/June2013) (15 M)BTL 4Answer: Page 2.26 – Dr.S.Senthil.

Piezoelectric Transducer:(4 M)

Piezoelectric transducer is more efficient than magnetostrictiontransducer. Therefore, modern

ultrasonic transducers are of this type.It is based on the piezoelectric effect.

Piezoelectric Effect

When mechanical force is applied to one pair of opposite faces ofcertain crystals like quartz,

tourmaline, etc., equal and oppositeelectrical charges appear across its other faces.

But ultrasonic wave generation is based on inverse piezoelectriceffect.

Inverse Piezoelectric Effect

When an a.c. voltage is applied across the piezoelectric crystal, it starts vibrating at the frequency




6-28

of the applied voltage.

Construction :(5 M)

The arrangement of Piezoelectric transducer is shown inFig.2.l5.

• It consists of primary and secondary circuits. The primary circuit is arranged with coils

L1and L2.

• Coil L1 is connected to the grid circuit and the coil L2 is connected to the plate circuit. The

frequency of the oscillatory circuit is varied by using the capacitor C1.

• The quartz crystal is placed between two metal plates A and B. The plates are connected to

the secondary (L3) of the transformer.

Working:(6 M)

• The filament in the grid circuit is heated by the low tension battery (LT). This causes the

production of electrons and these electrons are accelerated with a very high velocity by high

tension battery (HT). So, the altemating current is produced in the circuit.

• This alternating current is passed through the coil L1 and L2 of the primary circuit and it is

transferred to the secondary circuit (L3) due to transformer action from the secondary coi1

L3. This current is passed to the plates A and B and it leads the crystal to vibrate due to the

principle of inverse piezoelectric effect. The vibrations of the crystal create ultrasonic

waves.

• When the frequency of the oscillatory circuit is equal to the frequency of the vibrating

crystal, resonance occurs. At resonance, the crystal vibrates vigorously and ultrasonic

wavesare produced with very high frequencies.

Frequency of the oscillatory circuit = Frequency of the vibrating crystal

1

2𝜋√𝐿1𝐶1

=𝑃

2𝑙√

𝐸

𝜌

where,

l –length of the crystal, m

E - Young’s modulus of the crystal, N/m2,

𝜌 — Density of the crystal, kg/m3,

P = 1,2,3,4 etc-, for fundamental first over tone, second over tone respectively.




6-29

2

Explain the process parameters that affect USM. (Nov/Dec 2012)(15 M)BTL 4Answer: Page


Process Parameters:(3 M)

The various process parameters involved in USM methods are as follows :

• Metal removal rate.

• Tool material.

• Tool wear rate.

• Abrasive materials and abrasive slurry.

• Surface finish.

• Work material.

• Metal Removal Rate:(2 M)

The metal removal rate depends upon the following parameters.

• Mass flow rate.

• Abrasive grain size.

• Gas pressure.‘

• Velocity of abrasive particles.

• Mixing ratio.

• Nozzle tip clearance.

• Tool material: (2 M)

Generally, tough and ductile tool material is used in USM process. Low carbon steels and

stainless steels are commonly used as tool materials. Since very long tools cause

overstress, the tool should be short and rigid.

Hollow tool can be made with wall thickness greater than 0.5 to 0.8 mm. Side clearance to

the tool is of the order of 0.06 mm to 0.36 mm depending on grain size of the abrasive.

The USM process is a copying process in which the shape of the cutting tool is same as

that of the cavity produced.

• Tool wear:(2 M)

Tool wear ratio is defined as the ratio ofvolume of material removed from the work to the

volume of material eroded from the tool

𝑊𝑒𝑎𝑟 𝑟𝑎𝑡𝑖𝑜 =𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑓𝑟𝑜𝑚 𝑤𝑜𝑟𝑘

𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝑒𝑟𝑜𝑑𝑒𝑑 𝑓𝑟𝑜𝑚 𝑡𝑜𝑜𝑙

The wear ratio is approximated to 1.5:1 for tungsten Carbide(WC) workpiece, 100: 1 for

glaas,50:1 1 for quartz, 75:1 forceramics and l : 1 for hardened tool steel.

• Abrasive materials and Abrasive slurry: (2 M)

The most commonlyused /abrasives are boron carbide, silicon carbide, aluminium oxideand

diamond. Boron is the most expensive abrasive material and isbest suited for the cutting of‘

tungsten carbide, tool steels, etc.Aluminium oxide is the softest abrasive and it is used for

machiningglass and ceramics.

Material removal rate and surface finish are greatly influenced bygrit or grain size of the

abrasive. For roughing work operation, grit size of 200 — 400 are used and for finishing




6-30

operation, grit size of 800-1000 are used.

• Surface Finish: (2 M)

The maximum speed of penetration in soft and brittle materials such as soft ceramics are of

the order of200 mm/ min. Penetration rate is lower for hard and tough materials. Accuracy

of this process is 0.006 mm and surface finish upto0.02 to 0.8 micron value can be achieved.

• Work materials: (2 M)

Hard and brittle metals, non-metalslikeglass, ceramics, etc., andsemiconductors are used as

work material inUSM process. Wear ratio, average penetration rate and

maximummachining area of the different workpiece materials.

3

Explain the process parameters that affect WJM. (Nov/Dec 2012)(15 M)BTL 4Answer: Page


Process Parameters:(3 M)

The following process parameters are needed to utilise the WJMprocess successfully. .

• Material Removal Rate (MRR).

• Geometry and surface finish of work material.

• Wear rate of the nozzle.

Material Removal Rate (MRR):(4 M)

In water jet machining process, material removal rate is directlyproportional to the reactive force

(F) of the jet.

𝑀𝑅𝑅 𝛼 𝐹

𝑀𝑅𝑅 𝛼 𝑚 𝑥 𝑣

where, m — mass flow rate, andv — Jetvelocity.

Mass flow rate depends on nozzle diameter (d) and fluid pressure(p). Jet velocity depends on fluid

pressure.

𝑀𝑅𝑅 𝛼 𝑑 𝑥 𝑝

Stand-off distance (SOD) is the distance between the nozzle tipand the surface of the material

being machined.When MRR increases, the SOD also increases upto a certain limit

after that it remains unchanged for a certain tip distances and then falls gradually.

Geometry and surface finish of work material:(4 M)

Geometry and surface finish of work material mainly dependsupon the following parameters-

• Nozzle design,

• Jet velocity,

• Cutting speed,

• Depth of cut, and

• Properties of the material to be machined.

Wear Rate of the Nozzle:(4 M)

Nozzle wear rate depends upon the following factors.

• Hardness of the nozzle material,

• Pressure of the jet,

• Velocity of the jet, and

• Nozzle design.




6-31

UNIT III - ELECTRICAL ENERGY BASED PROCESSES

Electric Discharge Machining (EDM)- working Principle-equipments-Process Parameters-Surface Finish

and MRR- electrode / Tool – Power and control Circuits-Tool Wear – Dielectric – Flushing – Wire cut

EDM – Applications.

PART * A

Q.No. Questions

1.

Define electrical discharge machining. (April/May 2011) BTL 2

EDM is the controlled erosion of electrically conductive materials by the initiation of rapid and

repetitive spark discharge between the electrode tool to the cathode and work to anode separated

by a small gap kept in the path of dielectric medium. This process also called spark erosion.

2

What are functions of dielectric fluid used in EDM? (May/Jun 2014)(Nov/Dec

2016)(April/May 2017) BTL 2

It acts as an insulating medium, it cools the spark region and helps in keeping the tool and work

piece cool, it maintains a constant resistance across the gap, it carries away the eroded metal

particles.

3

List the basic requirement of dielectric fluid used in EDM. (May/June 2016) BTL 2

• Stable Dielectric strength.

• It should have optimum viscosity.

• It should have high flash point.

• It should be chemically stable at high temperature and neutral.

• 5. It should not emit toxic vapours.

4

What are the dielectric fluids commonly used in EDM? (Dec/Nov 2011)(April/May 2017)

BTL

Petroleum based hydrocarbon fluids, Paraffin, white sprite, transformer oil, Kerosene, mineral oil,

Ethylene glycol and water miscible compounds.

5

What are the prime requirements of tool material in EDM? (May/June 2013) BTL 1

It should be electrically conductive, good machinability, low erosion rate & low electrical

resistance.

6

Name some of the tool material used in EDM. (April/May 2011)(May/June 2014) BTL 1

• Copper, brass, alloys of Zinc &tin.

• Hardened plain carbon steel

• Copper tungsten, silver tungsten, tungsten

• 4. Copper graphite and graphite.

7

Define over cut. (Dec/Nov 2012) BTL 2

It is the discharge by which the machined hole in the work piece exceeds the electrode size and is

determined by both the initiating voltage and the discharge energy.

8

Define Rehardening. (Dec/Nov 2013) BTL 2

While metal heated to a temperature above the critical and then rapidly cooled by the flowing

dielectric fluid the metal is rehardened.

9

What is recast metal? (May/June2009) BTL 2

Metal heated to a temperature above the melting point and which is not displaced by the action of

the spark discharge, re-solidifies as recast metal.

10 Explain electrode wear. (April/May2011) BTL 2




6-32

A crater is produced in the electrode, which is likewise dependent on the electrode material and

the energy of the discharge.

11

What are types of power supply circuits used in EDM? (Nov/Dec 2007) BTL 1

• R-C circuit.

• Rotary impulse generator.

• Controlled pulse (vacuum tube).

• Oscillator controlled pulse.

• Transistor pulsed circuit.

12

Why the servo controlled system is needed in EDM? (May/June 2016) BTL 3

EDM requires that a constant arc gap be maintained between the electrode and the work piece to

obtain maximum machining efficiency. Therefore EDM tool in corporate some form of servo

control.

13

Define wear ratio. (May/June2012) BTL 2

Wear ratio is defined as ratio between Work piece material removed and Loss of Electrode

material.

14

State the principle of EDM. (April/May2016) BTL 2

The working principle of EDM by the initiation of rapid and repetitive spark discharge between

the electrode tool to the cathode and work to anode separated by a small gap kept in the path of

dielectric medium. It is also known as spark erosion machining or electro- erosion machining.

15

List the application of wire cut EDM. (Nov/Dec 2015) BTL 1

• Fabrication of press tool and electrodes for EDM,

• To machine parts with large variety and small quantity,

• To machine special matterial that could not be machined in conventional machining.

16

Write short notes on stratified wire.BTL 2

A very thin wire of 0.02 to 0.3 mm made of brass or molybolenum having circular cross section is

used as a electrode. Such type of electrodes are named as stratified wire.

17

Write down the limitations of EDM. (Nov/Dec 2016) BTL 2

• Ellectrically non conductive material cannot be machined,

• Cannot produce sharp corners,

• MRR is lesser.

18

List the factors affecting MRR in EDM. (Nov/Dec2011)(April/May 2017) BTL 1

• Spark on time,

• Spark off time,

• Arc gap,

• Discharge current,

• Voltage and

• Duty cycle.

19

What are the functions of adaptive control used in EDM? (April/May 2008) BTL 3

• Possibility to run the EDM unattended overnight for several days,

• Monitors power supply parameters as well as does samling of electrical conditions and

protect the machine against arcing.

20

Distinguish between EDM and CNC wire cut EDM? (April/May 2011) BTL 2

EDM:Electrodes are shaped as mirror image of work piece, electrodes can be reused

WEDM:Uses thin wirebas its electrode for any shape of component, wire cannot be reused.

21 What are the feasible dielectric flushing technique applicable in EDM process? BTL 2

Open flushing, suction flushing, suction through work piece, suction through electrode, pressure




6-33

flushing, pressure through work piece, pressure through electrode, jet flushing, side flushing and

combined flushing.

22

Write down the advantages of EDM machining. (April/May 2011) BTL 1

• Electrical conductive material can be machined,

• Complicated geometries can be produced,

• No mechanical stress present.

PART * B

1

Describe with neat sketch relaxation generator used in EDM. (Nov/Dec 2011) (13M) BTL 4


Relaxation circuit:(5M)

The operation of Resistance – Capacitance (R-C) generator circuit. This type of generators are

quite common because of its simplicity and lower cost. In this system, Direct Current (DC) is

flowing through a resistor (R) and it charges the capacitor (C). The charged capacitor is

connected to the machine. When the voltage across the capacitor is sufficiently high (50 to

200V), dielectric medium breakdown occurs. So, the dielectric medium between the tool and

work is ionized and spark takes place. Millions of electrons are developed in each spark. During

sparking period, the voltage falls and it again rising (since the capacitor is charged again).

(4M)

Energy released per spark = E = ½ CVd2

Where,

C = Capacitor value

Vd = discharge voltage

= Vo [ 1- exp [-t/Rc] ]

Vo = DC source voltage

For maximum power delivery, the discharge voltage (Vd) should be 75% of the supply voltage

(Vo). (4M)

2

Describe the working principle of wire cut EDM process with a neat sketch. List down its

advantages, disadvantages and application? (May/June 2012,2016) (13M) BTL 4 Answer:

Page 3.20 – Dr.S.Senthil.

Construction: (3M)

• A very thin wire (0.02 to 0.3mm) made of brass or molybdenum having circular cross

section is used as a electrode (tool).




6-34

• The wire is stretched and moved between two rollers. The part of wire is eroded by the

spark.

• The prominent feature of a moving wire is that a complicated cutout can easily machined

without using a electrode.

• In consists of

i. Workpiece movement control unit.

ii. Workpiece mounting table.

iii. Wire drive section for accurately moving the wire at constant tension.

iv. Dielectric fluid supplying unit.

v. Power supplying unit.

(3M)

Working: (3M)

• Workpiece to be machined is mounted on the table which is operated by control unit.

• A very small hole is predrilled in the workpiece, through which a very thin wire made of

brass or molybdenum is passed as shown in fig. and this wire is operated by wire feed

mechanism.

• Dielectric fluid (distilled water) is passed over the workpiece and the wire (tool) by using

pump.

• When the DC supply is given to the circuit, spark is produced across the gap between the

wire and the workpiece.

• When the voltage across the gap becomes sufficiently large, the high power spark is

produced.

• This spark occurs in an interval of 10 to 30 microseconds and with a current density of 15

– 500A per mm2 approximately. So, thousands of spark discharge occur per second across

the very small gap between the wire and the workpiece, which results in increasing

temperature of about 10,000ºC.




6-35

• At this high pressure and temperature, workpiece metal is melted, eroded and some of it is

vaporized. The metal is thus removed in this way from the workpiece.

• The removed fine material particles are carried away by dielectric fluid circulated around

it.

ADVANTAGES OF WIRE CUT EDM PROCESS: (2M)

• Manufacturing Electrode:

In this process a very thin wire made of brass or molybdenum is used as the electrode

(tool) to machine the workpiece material. So, there is no need for manufacturing

electrodes (as in EDM) which are traditionally made by cutting and grinding by using an

expensive alloy of silver and tungsten. This feature is used to reduce the man – hour

requirements and ensures greater economy.

• Electrode wear:

During machining process, the wire electrode (tool) is constantly fed into the workpiece.

So the wear of tool is practically ignored.

• Surface finishing:

A very thin wire electrode is constantly fed into workpiece at speed of about 10 to 30

mm/s by wire feed mechanism. So machining is continued without any accumulation of

chips and gases. It gives high surface finish and reduces the manual finishing operating

time.

• Complicated shapes:

By using programme, complicated and very minute shapes can be efficiently machined.

So there is no need of skilled operators.

• Time utilization:

Since all the machine motions of wire cut EDM processes are controlled by NC, it can be

operated throughout the day without any fire hazards.

• Straight holes:

The electrode wire is maintained at optimum tension by a unique wire tension control

mechanism. So, it prevents taper holes, barrel-shaped holes, wire breakage and wire

vibration.

• Rejection:

Rejection of material is minimized due to initial planning and checking the programme.

• Economical:

Since most of the programming can be easily done, it is economical for small batch

production, including prototypes.

• Cycle time:

Cycle time for die manufacture is shorter, as the whole work is done on one machine.

• Inspection time:

Inspection time for wire cut EDM process is reduced due to single piece construction of




6-36

dies with high positioning accuracy.

DISADVANTAGES: (2M)

• Capital cost is high.

• Cutting rate is slow.

• It is not suitable for large workpieces.

APLICATIONS: (1M)

The wire cut EDM process is best for the production of gears, tools, dies, rotors, turbine

blades and cams for all small to medium size batch production.

3

a) Mention the advantages and disadvantages of EDM process. (7M) BTL 4 Answer: Page

3.18 – Dr.S.Senthil.(May/June 2013)(April/May 2017)

ADVANTAGES OF EDM PROCESS: (4M)

• It can be used for machining various materials such as tungsten carbides, electrically

conductive materials, and other hard materials.

• It gives good surface finish.

• Machining of very thin section is possible.

• It does not leave any chips or burrs on the workpiece.

• It is well suited for complicated components.

• Since there is no cutting forces act on the job, error due to elastic deformation is

eliminated.

• High accuracy is obtained.

• Fine holes can be easily drilled.

• It is quicker process. So, harder materials can also be machined at much faster rate than

conventional machining.

• The process once setup does not need constant operators attention.

DISADVANTAGES (LIMITATIONS):(3M)

• It is only used for machining electrically conductive materials. So non-metallics such as

plastics, ceramics or glass cannot be machined by EDM.

• It is suitable only for machining small workpieces.

• Electrode wear and over cut are serious problems.

• Perfectly square corners cannot be made by EDM process.

• Metal removal rate is slow.

• Power requirement is very high.

• In many cases, the surface machined has been found to have micro cracks.

b) List down the characteristics of EDM process.(6M) BTL 4 Answer: Page 3.25–

Dr.S.Senthil.(May/June 2013)(April/May 2017)

Metal removal technique By using powerful electric spark

Work material Electrically conductive materials and alloys

Tool material Copper, yellow brass, alloy of zinc, copper, tungsten etc.,

Metal removal rate 15 to 80 mm3/s




6-37

Spark gap 0.005 to 0.05 mm

Spark frequency 200 – 500 KHz

Volts 30 -250V

Current 5 – 60 A

Temperature 10000˚C

Dielectric fluid Petroleum based hydrocarbon fluids, paraffin, white spirit etc.,

4

Explain the general arrangement of electrical discharge machining process and list out its

advantages, disadvantages and its application. (May/June 2016) (13M) BTL 4 Answer: Page


Working Principles Of EDM: (3M)

In electrical discharge machining (also known as spark erosion machining or electro-

erosion machining), metal removed by producing powerful electric spark discharge between the

tool (cathode) and the work material (anode). This principle is followed in this process.

Construction Of EDM:

(4M)

• The main components are the electric power supply, dielectric medium, work piece, tool

and a servo control mechanism.

• The work piece and the tool are electrically connected to a DC power supply.

• The work piece is connected to the positive terminal of the electric source, so that it

becomes the anode. The tool is connected to the negative terminal of the electric source,

so that it becomes the cathode.

• The tool and work piece are submerged in a dielectric fluid medium such as paraffin,

white spirit or transformer oil having poor electrical conductivity.




6-38

• The function of the servo mechanism is to maintain a very small gap, known as ‘spark

gap’ ranges of 0.005 to 0.05mm between the tool and the work piece. (3M)

Working:

• When the DC supply is given to the circuit, spark is produced across the gap between the

tool and the work piece.

• When the voltage across the gap becomes sufficiently large (more than 250V), the high

power spark is produced. So, the dielectric breaks down and electrons are emitted from

the cathode (tool) and the gap is ionized.

• This spark occurs in an interval of 10 to 30 microseconds and with a current density of 15

– 500A per mm2 approximately. So, thousands of spark discharge occur per second

across the gap between the tool and the work piece, which results in increasing

temperature of about 10,000ºC.

• At this high pressure and temperature, work piece metal is melted, eroded and some of it

is vaporized. In this way the metal is removed from the work piece.

• The removed fine material particles are carried away by dielectric fluid circulated around

it.

• The metal removal rate depends on the spark gap maintained. If anode and cathode are

made of same material, it has been found that the greatest erosion takes place at anode.

Therefore, in order to remove maximum metal and have minimum wear on the tool, the

tool is made as cathode the work piece as anode.

• When the voltage drops to about 12 volts, the spark discharge extinguishes and the

dielectric fluid once again becomes deionized. The condensers start to recharge and the

process repeats itself.(3M)

5

a)Explain the break down mechanism in EDM process. (6M) BTL 4 Answer: Page 3.9 –

Dr.S.Senthil.

Breakdown Mechanism:(6M)

• The cathode electrode is assumed to be source of producing electrons which are emitted

either by field effect or by schottky effect.

• The electrons liberated from the cathode are accelerated until they gain sufficient energy

to ionize the liquid molecules and initiate an electron avalanche.

• The applied field E, at which an electron avalanche can be initiated is given as

eEλ = chν

where,

e – charge

E – applied field

λ – mean free path of electron

c – velocity of light

ν – ionization quantum for the liquid molecule.

• This theory is used to magnitude the breakdown strength of hydrocarbons. But it does not

taking into account for the ignition delay observed between the applied voltage and

breakdown voltage.

• Breakdown in gas is introduced by collisional ionization of the molecules. But in liquid,

collisional ionization of the molecules by electrons is not possible due to insufficient




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kinetic energy of the electrons. In order to avoid this, a pre-breakdown electron current

flows from cathode to anode. This low current heats the liquid to form a vapour bubble of

sufficient pressure in between the electrodes. Then a spark is produced in the vapour

bubble according to the high pressure gas-discharge mechanism.

b) Discuss the various electrode material used in EDM process. (Nov/Dec 2015) (7M) BTL 4


Tool (Electrode) Materials:(2M)

• The tool materials generally used can be classified as metallic (copper, brass, copper-

tungsten etc), non-metallic materials (graphite) and combination of metallic and non-

metallic materials (copper-graphite).

• Copper, yellow brass, alloys of zinc, copper tungsten, silver tungsten, tungsten carbide

and graphite are used for tool materials.

• For commercial applications, copper is best suited for the machining, aluminium is used

for die-sinking and cast iron for rough machining.

The three most commonly used materials are given below.

i) Graphite: (2M)

Graphite is a nonmetallic which is generally used as a tool material in Electrical Discharge

Machining processes. A wide range of grades are available in graphite and these are used

for variety of applications.

A big advantage of graphite is though it is abrasive, it can be produced by several methods

like machining, moulding, milling, grinding etc. graphite can generally achieve better

metal removal rates and fine surface finishes than metallic tool materials. One

disadvantage of graphite is, it is costlier than copper.

ii) Copper: (1M)

Copper is a second choice for using as tool material in Electrical discharge Machining

processes. It can be produced by casting or machining. Copper tools with very complex

features are formed by chemical etching or electro forming.

iii) Copper-tungsten: (2M)

Copper-tungsten tool material is difficult to machine and it has low metal removal rate. It

is costlier then graphite and copper.

• The selection of proper tool material is influenced by

• Size of electrode and volume of material to be removed.

• Surface finish required.

• Tolerance required.

• Nature of coolant application etc.

• The basic requirements of any tool materials are

• It should have low erosion rate.

• It should be electrically conductive.

• It should have good machinability.




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• Melting point of the tool should be high.

• It should have high electron emission.

6

a)Discuss about the material removal rate and Surface finish in EDM. (May/Jun 2014)(7M)

BTL 4 Answer: Page 3.8 – Dr.S.Senthil.

Metal Removal Rate (MR) And Surface Finish:(7M)

The metal removal rate is generally described as the volume of metal removed per unit

time.

• Metal removal rate depends upon current density and it increases with current. But high

removal rate produce poor finish. Therefore, the usual practice in EDM is, a roughing cut

with heavy current followed by a finishing cut with less current.

• Metal removal rates upto 80mm3/s can be achieved and surface finishes of 0.25 µm can be

obtained at very low cutting rats.

• Tolerances of the order of ± 0.05 to 0.13mm are commonly achieved by EDM in normal

production and with extra care, tolerances of ± 0.003 to 0.013mm are possible.

• The material being cut will affect the metal removal rate. The experiments indicate that

the metal removal rate (MRR) varies inversely with melting point of the metal. The

approximate value is

𝑀𝑒𝑡𝑎𝑙 𝑅𝑒𝑚𝑜𝑣𝑎𝑙 𝑅𝑎𝑡𝑒 (𝑀𝑅𝑅) = 2.44

(𝑀𝑒𝑙𝑡𝑖𝑛𝑔 𝑝𝑜𝑖𝑛𝑡 ℃)1.25

b) Discuss about the factors affecting the material removal rate in EDM. (May/Jun 2014)

(6M) BTL 4 Answer: Page 3.9 – Dr.S.Senthil.

Factors Affecting The Metal Removal Rate (MRR):(6M)

• Metal removal rate increases with forced circulation of dielectric fluid.

• It increases with capacitance.

• It increases upto optimum value of work tool-gap, after that it drops suddenly.

• It increases upto optimum value of spark discharge time, after that it decreases.

• Metal removal rate is maximum when the pressure is below atmosphere

PART * C

1

Discuss any four power circuit used for EDM process? (May/June 2013) (15M) BTL 4


Power Generator: (3M)

• Power generator is one of the most important part of an electrical discharge machining

processes.

• Its primary function is to convert an alternating current (AC) into a pulse direct current

(DC) which is required to produce the unidirectional spark discharges between the gap of

the tool and workpiece. A rectifier is used to convert the AC into DC.

• The most commonly used spark generating circuits are give below

• Resistance-Capacitance circuit (RC circuit) or relaxation circuit.

• R-C-L circuit

• Rotary pulse generator circuit

• Controlled pulse generator circuit




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i) Relaxation circuit: (4M)

The operation of Resistance – Capacitance (R-C) generator circuit. This type of generators

are quite common because of its simplicity and lower cost. In this system, Direct Current

(DC) is flowing through a resistor (R) and it charges the capacitor (C). The charged

capacitor is connected to the machine. When the voltage across the capacitor is

sufficiently high (50 to 200V), dielectric medium breakdown occurs. So, the dielectric

medium between the tool and work is ionized and spark takes place. Millions of electrons

are developed in each spark. During sparking period, the voltage falls and it again rising

(since the capacitor is charged again).

Energy released per spark = E = ½ CVd

2

Where

C = Capacitor value

Vd = discharge voltage

= Vo [ 1- exp [-t/Rc] ]

Vo = DC source voltage

For maximum power delivery, the discharge voltage (Vd) should be 75% of the supply

voltage (Vo).

Drawbacks of Relaxation circuit:

1. Though the discharge current in a relaxation circuit reaches a high value, it is of very short

duration.

2. Since the time for charging the capacitor is high, the use of high frequencies is limited.

ii) R-C-L circuit: (4M)

In the relaxation circuit, metal removal rate increases as R is decreased. But R cannot be

decreased below a critical value. If R decreases below a critical value, arcing will take place

instead of sparking. Further, the capacitor charging time in R-C circuit is much higher than

discharging time. Therefore an inductance (L) is included in the charging circuit. This R-C-L

circuit is shown in fig:

R-Resistance C-Capacitance L-Inductance




6-42

iii) Rotary pulse generator: (4M)

The introduction of pulse generator has overcome the drawbacks of R-C and R-C-L

circuits.

R-C and R-C-L circuits yield low metal removal rate. Therefore, rotary pulse generator is used

for spark generation. It yields high metal removal rate, low tool wear and more precise control

parameters. The fig shows the schematic diagram of rotary impulse generator circuit. In this

circuit, the capacitor (C) is discharged through the diode during the first half cycle. During the

next half cycle, the sum of voltages generated by the generator and the charged capacitor is

applied to the work-tool gap. This arrangement gives metal removal rate, but surface finishing is

poor.

iv) Controlled pulse generator circuit: (4M)

R-C, R-C-L and rotary pulse generator circuits are not having automatic prevention of the

current flow incase a short circuit is developed. To obtain such an automatic control, a vacuum

tube or a transistor is used as switching device.




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2

Describe the process parameters to be considered during the EDM process. (Nov/Dec 2013)


Process Parameters:

The following factors influence the process parameters in EDM processes.

1. Operating parameters: (4M)

Operating process involves the removal of metal from the workpiece and tool as a

measure of electrical energy input.

Metal Removal Rate (MRR) = φ (E) f

𝑀𝑅𝑅 = ∅ ∫ 𝑣𝑖𝑑𝑡𝜏

0

Where,

Φ – Function

E – Electrical Energy

V – Voltage

I – Instantaneous current

dt – Time interval

f – Torque

τ – Pulse width

𝑇𝑜𝑜𝑙 𝑊𝑒𝑎𝑟 𝑅𝑎𝑡𝑒 (𝑇𝑊𝑅) = ∈ (𝐸)𝑓 = ∈ ∫ 𝑣𝑖𝑑𝑡𝜏

0

Based on the above considerations, it is well understood that the MRR, and TWR are governed

by the following

• Energy content of the pulses.

• The rate at which the energy supplied.

• Servo sensitivity.

• Gap width and

• Dielectric parameters.

Tool wear rate is required to calculate tooling cost, machining accuracy and estimate the time

of machining for a desired depth of cut.

There are three types of wear observed in the tool (electrode) which is shown in fig.

They are:

• End wear

• Corner wear




6-44

• Side wear

2. Taper: (4M)

Tapering effect is observed due to the side sparks which is shown in fig. Under high

dielectric pollution, side sparks are more pronounced as compared to frontal sparks. In

this case, over cut at any instant is given by

𝛿 = 𝐶3 (𝜋

4𝑑2) ℎ

At maximum,

𝛿𝑚𝑎𝑥 = 𝐶3 (𝜋

4𝑑2) ℎ

Where,

h – depth of machining at any instant.

𝑇𝑎𝑝𝑒𝑟, 𝑇𝑝 = 𝐷 − 𝑑

2ℎ=

𝛿𝑚𝑎𝑥

ℎ=

𝐶3𝜋

4𝐷2

ℎ




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3. Surface finish: (3M)

The surface finish of the material depends upon the following factors,

➢ Energy of the pulse and

➢ Frequency of operation

The roughness of the material is observed within a bandwidth depending upon single or

multispark conditions.

4. Current density: (4M)

The current density is the most important parameter which determines the material

removal rate and surface condition. Current density is affected by either changing the current or

changing the electrode (tool) – workpiece gap.

When the current is increased, each individual spark removes a larger crater of metal from

the workpiece. But it also increases surface roughness. Increasing spark frequency results in

decrease in surface roughness and reduces the removal of crater of metal from the workpiece.

The gap between the electrode (tool) and workpiece is determined by the spark voltage

and current. A small gap produces more accuracy with a better surface finish and slower metal

removal rate.

3

Explain the function of dielectric fluid used in EDM with neat sketch (May/Jun 2014) (15M)


Dielectric Fluid:(4M)




6-46

• A dielectric fluid is a medium that does not conduct electricity. In electrical discharge

machining process, the tool and work piece are submerged in a dielectric fluid medium.

The dielectric fluid generally used are petroleum based hydrocarbon fluids, paraffin,

white spirit, transformer oil, kerosene, mineral oil or mixture of these.

• Dielectric fluids must not be hazardous to operators or corrosive to equipment.

• The choice of any dielectric fluid depends on the workpiece size, type of shape,

tolerance, metal removal rate and surface finish. White spirit is best suited for machining

tungsten carbide.

• The dielectric fluid should not be changed frequently on a machine, and it is chosen

according to the most frequent application to be carried out in the machine.

• The dielectric fluid must circulate freely between the tool and workpiece.

• The eroded particles should be flushed out at the earliest since it reduces the further metal




6-47

removal rate.

• The various methods of flushing are pressure flushing, suction flushing and side flushing.

• The dielectric fluid should be filtered before reuse so that chip formation contamination

of the fluid will not affect machining accuracy.

• The dielectric fluid should be easily available at reasonable price.(6M)

FUNCTIONS OF DIELECTRIC FLUID: (5M) The dielectric fluid has the following functions

• It acts as an insulating medium.

• It cools the spark region and helps in keeping the tool and workpiece cool.

• It carries away the eroded metal particles along with it.

• It maintains a constant resistance across the gap.

• It remains electrically non conducting until the required breakdown votage has been

reached.

• It breakdown electrically in the shortest possible time once the breakdown voltage has

been reached.




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UNIT IV - CHEMICAL AND ELECTRO-CHEMICAL ENERGY BASED PROCESSES

Chemical machining and Electro-Chemical machining (CHM and ECM)-Etchants – Maskant -

techniques of applying maskants - Process Parameters – Surface finish and MRR-Applications.

Principles of ECM- equipments-Surface Roughness and MRR Electrical circuit-Process Parameters-

ECG and ECH - Applications.

PART * A

Q.No. Questions

1.

Define ECM.(Dec/Nov 2010) BTL 1

It is the controlled removal of metals by the anodic dissolution in an electrolytic medium, where

the work piece (anode) and the tool (cathode) are connected to the electrolytic circuit, which is

kept, immersed in the electrolytic medium.

2

Write the Faraday’s first law of electrolysis? (May/June 2011) BTL 2

The amount of any material dissolved or deposited is proportional to the quantity of electrolyte

passed.

3

What are the main functions of electrolysis in the ECM? (May/June 2014) BTL 1

• For completing the electric circuit between the tool & the work piece & to allow the

reaction to proceed efficiently.

• To remove the products of machining from the cutting region.

• To carry away the heat generated during the chemical reaction.

• To avoid ion concentration at the work piece- tool gap.

4

What are the properties are expected from the electrolysis used in the ECM? (May/Jun

2016) BTL 2

• High thermal conductivity.

• Low viscosity and high specific heat.

• Should chemically stable even at high temperature.

• 4. Should be non-toxic and non-corrosive.

5

What are the electrolysis commonly used in ECM? (Dec/Nov 2011) BTL 3

15 -20 % NaClin water, sodium nitrate, potassium nitrate, sodium sulphate, sodium chromate and

potassium chloride.

6

What are the parameters that affect the MRR in ECM? (May/June 2012) (Nov/Dec 2016)

BTL 2

• Feed rate.

• Voltage.

• Concentration of the electrolyte.

• Temperature of the electrolyte.

• Current density.

• 6. Velocity of the electrolyte.

7

What are the advantages of ECM? (May/June 2016)(April/May 2017) BTL 1

• ECM is simple, fast and versatile method.

• Surface finish can be extremely good.

• 3. Fairly good tolerance can be obtained.

8 What are the limitations of ECM? (May/Jun 2016) BTL 2

• Large power consumption,




6-49

• Sharp internal corners cannot be answered

• Maintenances of higher tolerances require complicated contours.

9

What are the applications of ECM? (May/ June 2013) BTL 2

ECM is used for sinking, profiling and contouring, multi hole drilling, trepanning, broaching,

honing, steel mill applications, surfacing, sawing, contour machining of hand to hand machine

materials.

10

Which material is used to make the grinding wheel? (Dec/Nov 2013) BTL 2

• Metal bonded diamond

• Aluminum oxide.

11

Distinguish between electro-chemical machining and electro plating process. (May/June

2016) BTL 3

Electro-chemical Machining:

• it is used to make profile on the work material.

• It is a subtractive process.

Electro plating:

• It is used to make coating on the work material.

• 2. It is a additive process.

12

What is meant by Maskants? (April/May 2015)/ (May/June 2016) BTL 1

In chemical machining process the areas of the workpiece which are not to be machined are

covered with a resistant material called resist or maskant.

13

Mention the applications of ECH. (April/May 2015) BTL 2

• The process markedly reduces the errors in roundness through the rotary motion.

• Moreover, through tool reciprocation both taper and waviness errors are also reduced

Because of the light stone pressure used, heat distortion is avoided.

• ECH can be used for hard and conductive materials that are susceptible to heat and

distortion. The process can tackle pinion gears of high-alloy steel as well as holes in cast

tool steel components.

14

What is the advantage of ECG over conventional grinding? (Nov/Dec2016) BTL 2

ECG is a cold process thus prevents structural and grinding damage, grinding wheel life is

improved in ECG, Requires less frequent wheel dressing when compared to conventional

grinding.

15

What is self-adjusting feature in ECM? (April/May 2008) BTL 2

In a constant feed rate the ECM system machining process is inherently self regulated since the

MRR tend to approach the feed rate, to maintain the equilibrium the ECM adjust itself and gap

remains constant.

16

What do you understand by etch factor? (April/May 2008) BTL 2

During machining all the exposed surfaces to the etching medium are subjected to undesired

undercut which is known as etch factor. The etch factor restricts size of mask.

17

What are the various process parameters which the effectiveness of ECG process depend

upon? (May/Jun 2014) BTL 2

• Contact pressure,

• Grinding wheel speed,

• DC potential

• Abrasive grain size

18 Mention the specific applications of ECG. (Nov/Dec 2009) BTL 1

• Process is used to produce tungsten carbide tools,




6-50

• for grinding of fragile parts.

19

List the different steps of chemical machining process (April/May 2011) BTL 1

• Cleaning the work piece,

• Coating with masking material,

• Scribing of the maskants,

• Etching of required portion,

• Cleaning masking material.

20

Why there is no need to have short circuit protection device in ECG system? (May/June

2012) BTL 2

Always the abrasive particles in the grinding wheel maintain the minimum distance required to

avoid short circuit.

21

What is meant by maskant used in chemical machining process? What are the

characteristics of cut and peel type maskant used in the process? (Nov/Dec2009) BTL 2

• To protect the work piece from chemical etchant,

• Withstand handling,

• Easy to scribe,

• Withstand heat.

22

What are the factors that influence oxidation in ECM? BTL 2

• Nature Of Work Piece,

• Type Of Electrolyte,

• Current Density,

• Temperature Of Electrolyte.

23

What is the working principle behind ECM? (April/May 2011) BTL 3

Through the electrolyte the circuit gets closed & induces a chemical reaction which results in

material from anode. This flow of electrolyte removes the products of machining and unwanted

chemical effect from the working zone.

24

What are the advantages of chemical machining? (May/June 2006) BTL 2

• Low tooling cost,

• No burr formation,

• No stress in work piece.

25

Write the formula for finding the MRR in ECM process? (Nov/Dec 2015)

MRR=I.A/(FpV) ,

Where,

I-current,

p-density,

F-Faradays constant,

A-atomic weight,

V-voltage.

PART * B

1

Explain in detail the Electro Chemical Grinding (ECG) process with neat sketch describe

and also mention the advantages, disadvantages and application?

(May/June2009) (May/June 2016)(13M) BTL 4 Answer: Page 4.21 – Dr.S.Senthil.

Principle: (1M)

In electro-chemical grinding method, the work is machined by the combined action of




6-51

electrochemical effect and conventional grinding operation. But the major portion of the metal

(about 90%) is removed by electrochemical effect.

Construction: (3M)

• The schematic arrangement of electrochemical grinding process is shown in fig 4.6.

• It consist of work piece, work table, grinding wheel, spindle, D.C power source,

electrolyte, pump, motor for pump, nozzle, filter for incoming electrolyte, and reservoir

for electrolyte.

• The grinding wheel is mounted on a spindle, which rotates inside bearings.

• The work piece is held on the machine table in suitable fixture. The table can be moved

forward and backward to feed the work or to withdraw it.

• The grinding wheel and spindle are separate from the machine by using an insulation

sleeve as shown in fig 4. 6.

• Sodium nitrate, sodium chloride and potassium nitrate with a concentration of 0.150 top

0.300 kg/ litre of water are usually used as electrolyte.

• The electrolyte from the reservoir is pumped and passed through nozzle in the gap

between the wheel and work piece.

• A constant gap of 0.025 mm is maintained between the grinding wheel and work piece.

• The grind wheel is made of fine diamond particles. These particles are slightly projected

out from the surface and come in contact with work surface with very little pressure.

• The grinding wheel runs at a speed of 900 to 1800 m/ min

• The work piece is connected to positive terminal (anode) of battery and grinding wheel is




6-52

connected to negative terminal (cathode)

Working: (3M)

• A mild D.C voltage of about 3 to 30 V is applied between the grinding wheel and work

piece.

• Due to the applied voltage, the current flows through the electrolyte with positively

charged ions. The positively ions move towards the work piece (anode).

• The electrochemical reaction takes place due to this flow of ions and it causes the

removal of metal from the work piece.

• It can be seen that the work piece is fed against the rotation of grinding wheel and the

metal is removed from the work piece surface by the simultaneously abrasive action and

electrolyte reaction. In fact, 10% of the work piece metal removed by abrasive cutting,

and 90% by electrolyte reaction.

• Grinding wheel wear is negligible because the major part of the cutting action is

electrolytic, and little dressing of grinding wheel is needed.

• The short-circuit between the wheel and the work is prevented due to point contact made

by the fine diamond points.

Advantages Of ECG: (3M)

• Since the tool wear is negligible, the life of the grinding wheel is increased. This factor is

most valid in the grinding of hard metals such as tungsten carbide, where, costly diamond

grinding wheels are used. In ordinary grinding there are high wear rates on these

expensive diamond wheels.

• Work is free of surface cracks and distortion because heat is not generated in the process.

• As compared to conventional grinding, a very little cutting force is applied to the work

piece.

• Good surface finish is obtained.

• Work material is not subjected to any structural changes.

• Intricate parts can be machined without any distortion.

• The surface finish produced by this process is varied from 0.20 to 0.4µm.

• Accuracy of the order of 0.01 mm can be achieved by proper selection of wheelk grit size

and abrasive particles.

• Burr free and stress free components are produced.

• The wheel bonds wear very slowly. So, the grinding wheel need not be dressed frequently.

Disadvantages: (2M)

• Initial cost is high.

• Power consumption is high.

• Metal removal rate is lower than conventional grinding.

• Non-conducting materials cannot be machined.

• Preventive measures are needed against corrosion by the electrolyte.

• Maintenance cost is high.

• Since the tolerance achieved are slightly low, the workpiece needed final abrasive

machining.

Applications: (1M)

• Very precision grinding of hard metals like tungsten carbide tool tips, high speed steel

tools.

• Cutting thin sections of the hard material without any damage or distortion.




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2

Explain in detail the ECM process with neat sketch describe and also mention the

advantages and application? (Nov/Dec 2013, 2016) (May/June 2016)(13M) BTL 4 Answer:


Introduction: (1M)

Electro-chemical machining (ECM) is one of the recent and most useful machining

processes. In the process, electrolysis method is used to remove the metal from the work piece. It

is best suited for the metals alloys which are difficult to be machined by mechanical machining

processes.

Principle: (2M) This process is based on the principle of faraday’s laws of electrolysis which may be

stated as follows

• The first law states that the amount of any material dissolved or deposited, is proportional

to the quantity of electricity passed.

• The second law proposes that amount of change produced in the material is propotional to

its electrochemical equivalent of the material.

Basically in electroplating, the metal is deposited on the work piece, while in ECM. The

objective is to remove the metal from the work piece. So, the reverse of electroplating is

applied in the ECM process. Therefore, the work piece is connected to positive terminal when

the current is passed, the work piece loses metal and the dissolved metal is carried out by

circulating an electrolyte between the work and tool.

Construction: (4M)

• The schematic arrangement of ECM process is shown in the fig 4.2

• It consist of work piece, tool, servomotor for controlled tool feed, D>C power supply,

electrolyte and reservoir for electrolyte.

• A shaper tool (electrode) is used in this process, which is connected to negative terminal

(cathode) and the work piece is connected to positive terminal (anode).

• The tools used in this process should be made up of the material which have enough

thermal and electrical conductivity, high chemical résistance to electrolyte and adequate

stiffness and machinability.

• The widely used tool materials are stainless steel, titanium, brass and copper.

• The tool is of hollow tabular type as shown in the fig 4.2 and an electrolyte is circulated

between the work and tool.

• Most widely used electrolyte in this process is sodium nitrate solution. Sodium chloride

solution in water is a good alternative but it is more corrosive than the former. Some other

chemicals used in this process are sodium hydroxide, sodium sulphate, sodium fluoride,

potassium nitrate and potassium chloride.

• Servomotor is used for controlling the tool feed and the filter is used to remove the dust

particles from the electrolytic fluid.




6-54

Working: (2M)

• The tool and work piece are held close to each other with a very small gap (0.05 to

0.5mm) between them by using servomotor.

• The electrolyte from the reservoir is pumped at high pressure and the flows through the

gap between the work piece and tool at a velocity of 30 to 60 m/s

• A mild D.C voltage about 5 to 30 volts is applied between the tool and work piece.

• Due to the applied voltage, the current flows through the electrolyte with the positively

charged ions and negative charged ions. The positive ions move towards the tool

(cathode) while negative ions move towards work piece (anode)

• The electrochemical reaction takes place due to this flow of ions and it causes the removal

of metal from the work piece in the form of sludge.

Advantage Of ECM Process: (2M)

1. The metal removal rate by this process is quite high for high strength-temperature

resistant (HSTR) materials compared to conventional machining process.

2. Wear and tear of tool is negligible.

3. Machining is done at low voltage.

4. Intricate and complex shape can be machined easily through this process.

5. The machined work surface is free of stresses.

6. No cutting forces are involved in the process.

7. High surface finish, of the order of 0.2 to 0.8 microns, can be obtained.




6-55

8. Very thin sections such as sheet metal can be easily machined without any damage.

9. it is an accurate process and close tolerance of the order 0.005mm can be easily obtained.

10. No burrs are produced and this process can be easily automated.

11. Toughness and brittleness of a material has no effect on the machining process.

Application Of ECM: (2M)

It is used for

1. Machining complicated profile, such as jet engine blades, Turbine blades, turbine wheels

etc.,

2. Drilling small deep holes, such as in nozzles.

3. Machining of cavities of blind holes of irregular shapes.

4. Machining of blind holes and pockets, such as in forgoing dies.

5. Machining of hard materials and heat resistant materials.

3

a)During machining of iron using aqueous solution of NaCl what are the possible reactions

at electrodes (7M) BTL 4 Answer: Page 4.11 – Dr.S.Senthil.

Electrolysis process : (7M) A mild D.C voltage of about 5 to 30 volts is applied between the tool and work piece. So

the current in water flows through the electrolyte (solution of NaCl) with charge ions. Many

chemical reactions occur at the cathode and the anode.

The following reactions are possible at the cathode (tool)

𝑁𝑎+ + 𝑒− = Na

Na + 𝐻2O = Na(OH) + 𝐻+

2𝐻++ 2 𝑒− =𝐻2 ↑ (hydrogen evaluation)

It shows that there is no deposition on tool; but only gas (𝐻2) is formed

The following reaction occurs at the cathode (iron (Fe) work piece)

Fe ↔ Fe+++ 2 𝑒−

Fe+++ 2 𝐶𝑙− ↔ Fe𝐶𝑙2

Fe+++ 2 (𝑂𝐻)− ↔ Fe(𝑂𝐻)2

Fe𝐶𝑙2+ 2(OH) ↔ Fe(𝑂𝐻)2 +2 𝐶𝑙−

b) Explain the principle of electro chemical honing process. (April/May 2017) (6M) BTL 4


Construction: (3M)

• The schematic arrangement of electrochemical honing machine is shown in fig 4.7

• The work piece is connected to positive terminal (anode) of battery and tool is connected

to negative terminal (cathode).

• The gap between the tool and the work piece is usually maintained between .075 to 0.125

mm at the start of the cycle. It increases by the amount of stock removal per cycle up to

0.50mm.

• Electrolyte is passed between the tool and the work piece through several rows of small

holes in the tool body as shown in fig 4.7




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• Electrolyte is supplied about 112 lit/min under a pressure of upto 1.05 N/𝑚𝑚2 depending

upon the work piece size.

• Bonded-abrasive honing stones are inserted in slots in the tool and these stones are fed out

with equal pressure in all directions, so that their cutting faces are in constant contact with

the cylinder surface.

Working: (3M)

• A mild D.C voltage of about 25V is applied between the honing tool and the work

piece.

• Due to the applied voltage, the current flows through the electrolyte with positively

charged ions and the negatively charged ions. The positively charged ions move

towards honing tool (cathode) and the negatively charged ions move towards the work

piece (anode).

• The electrochemical reaction takes place due to this flow of the ions and it causes the

removal of metal from the work piece.

• It can be seen that work piece (cylinder) is fed against the rotation of honing tool and

the metal is removed from the work piece by simultaneous abrasive action and

electrolyte reaction.

• Automatic gauging devices designed into the system which initiates a signal and when

the cylinder is of the desired diameter size, the cycle is automatically terminated.

• It is mostly used for internal cylindrical grinding, with a size tolerance of 0.012 mm

on diameter and 0.005 m on roundness.

4

a)Discuss about the types of maskants used in chemical machining. (4M) BTL 4 Answer:


Types of Maskant:(4M)

In chemical machining process, the areas of the work piece which are not to be machined are

covered with a resistant material, called a resist or maskant.

The following table shows the maskants for different materials

S.NO MATERIAL ETCHANT

1. Aluminum Butyl rubber, neoprene rubber

2. Magnesium Polymers




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3. Titanium Translucent chlorinated polymers

4. Nickel Neoprene

5. Ferrous metals Polyvinyl chloride, polyethylene

b) Explain about the different types of masking available in CHM. (April/May 2017) (9M)


The usual methods of masking are:

i. Scribed and peeling maskants.

ii. Photoresists maskants.

i) Scribed and peeled maskants: (4M)

In this method, a maskant (like paint) is applied to the entire surface of the work

piece by dip, spray, brush or stencil. After the maskant hardens, it is removed from those

surfaces where metal removed is desired. The maskant is removed by scribing with knife

and peeling away the desired surface. Template can be used to assist in scribing. This

method is used when critical dimensional tolerances are not required.

ii) Photoresists Maskant: (5M)

• It is an excellent method of masking, especially for complex work, this method is used for

thin sections and components requiring closed dimensional tolerances.

• The workpiece to be machined is thoroughly cleaned and decreased by acid or alkalis. The

cleaned metal is dried and photoresist material is applied to the workpiece by dipping,

spraying, brushing or rolling coating.

• The coating is then dried and hardened by heating in an oven upto about 125⁰C.

• The design of the part to be machined is prepared at a magnification of up to 100x. the master

drawing is photographed and reduced to the size of the finished part.

• The master photographic negative is placed over the dried photo resist coated surface of the

work piece and exposed to ultraviolent light, which hardens the exposed areas.

• After exposure, the work piece is then developed by immersing it into a tank which contains

an organic solvent bath solution. The unexposed portions are dissolved out during the

developing process, while the exposed portions remain on the workpiece.

• Finally the treated workpiece is dipped into the etching solution. After 5 to 15 minutes. The

unwanted metal is removed from the workpiece and the finished part is washed thoroughly to

eliminate all chemical residues.

5

List the application, advantages and limitation of CHM process. (Nov/Dec 2015)(April/May

2017) (13M) BTL 4 Answer: Page 4.6 – Dr.S.Senthil.

Application Of Chemical Machinig Process (CHM): (4M)

• Chemical machining process is applied in great number of usage where the depth of metal

removal is critical to a few microns and the tolerances are close.

• The major application of chemical machining is in the manufacture of burr free

components.

Advantages Of CHM: (5M)

• Burr-free components are produced.

• Most different to machine materials can be processed.

• High surface finish is obtained.

• Any metal can be machined.

• Stress free components are produced.




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• Since the process is comparatively simple, there is no need of highly skilled labour.

• Both faces of the work piece can be machined

• Hard and brittle materials can be machined.

• Tooling cost is very low.

• Complex contours can be easily machined.

DisadvantagesOF CHM: (4M)

• Since the process is slow, metal removal rate is low.

• Manufacturing cost is high.

• Work piece thickness, that can be machined, is limited.

• Large floor area is needed.

• It is not possible to produce sharp corners.

6

Discuss about the tool material, tool design and tool insulation parameters of the tool used

in ECM process.(Nov/Dec 2013)(13M) BTL 4 Answer: Page 4.14 – Dr.S.Senthil.

Tool Material, Tool Design And Insulation: (5M)

• Since the tool has no contact with the work piece, there is no tool wear. So any material that is

good conductor of electricity can be tool material.

• The general requirements of the tool material in ECM process are

1. It must be a conductor of electricity.

2. It must be chemically inert to the electrolyte.

3. It must be easily machinable.

4. It must be rigid enough to take up the load due to fluid pressure.

• Most commonly used took material are copper, brass, titanium, copper-tungsten and stainless

steel when the electrolyte is made of sodium or potassium. The other materials which can be

used as the tool material are aluminum, graphite, bronze, platinum and tungsten carbide.

• The following table shows the properties of some of the tool materials

ECM tool material properties: (8M)

Properties Material

Copper Brass Stainless steel Copper tungsten

Electrical resistivity 1.00 4.00 53.00 8.00

Stiffness 1.60 1.00 1.90 2.20

Machinability 6.00 8.00 2.50 1.80

Thermal conductivity 25.00 7.50 1.00 10.00

• Electroforming and cold forging are used for tool shaping.

• As the surface finish of the bottom face of the tool, that portion of tool should be polished.

• The tool is made hallow for drilling holes so that electrolyte can passalong the bore in tool.

• Fig 4.5 shows the shape of tool to produce holes with minimum taper and excellent finish.

The outer surface of tool is insulated by vinyl, Teflon, enamels or high temperature varnish as

shown in fig 4.5




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Fig: Shape of Tool

The following features may be noted

1. Insulation on the sides of tool permits machining at bottom surface only. So, there is no ECM

effect on side and thus no tapering effect.

2. Provision of radii at bottom avoids turbulence and break of flow. It also encourages uniform

electrolyte flow around the corner.

3. The electrolyte is made to flow from the inside of the tool cut, around the cutting under edges

and up through the hole.

• While designing the tool, the following two aspects are important

1. Determine the tool shape, so that the desired shape of the job is obtained under existing

machining conditions.

2. Design the tool by considering the electrolyte flow, insulation strength and fixing

arrangements.

7

Describe the working principle and elements of CHM. What are the factors on which the

resists is selected for CHM? (Nov/Dec 2013)(13M) BTL 4 Answer: Page 4.1 – Dr.S.Senthil.

Construction: (7M)

• In this process, material is removed from the work piece through a controlled etching or

chemical attack of the work piece material.

• Material can be removed from selected area of a area of the work piece or from the entire

surface of the work piece, according to requirement.

• If selective machining is desired, the areas of the work piece which are not to be machined

are covered with a resistant material, called a resist or maskant.

• The workpiece to be machined is first cleaned in trichloroethylene vapour or in a solution

of mild alkaline at 85 to 90⁰C, followed by washing in a clean water. This removes dust

and oil from the work piece

• After cleaning, the work piece is dried and coated with the maskant material.

• The work piece is then immersed in a chemical reagent as shown in the fig 4.1. so,

chemical reaction takes place and the metal is removed from the work piece. The metal is

removed by the chemical conversion of the metal into metallic salt.

• The time of immersion of the work piece depend on the amount of material removed by

chemical action.

• The chemical etching agent depends upon the work piece material caustic soda is used as

etching reagent for aluminum, solution of the hydrochloric and nitric acids for the steel

and iron chloride for stainless steels.




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• In order to obtain a uniform depth of metal removal, temperature control and stirring of

chemical reagent is important.

• After machining , the work piece should be washed thoroughly to prevent reaction with

etching reagent residues.

(6M)

PART * C

1

What are the functions of an electrolyte? What are the factors need to be considered while

selecting it? (15M) BTL 4 Answer: Page 4.16 – Dr.S.Senthil.

Electrolyte: (7M)

• In ECM process the electrolyte is used for the following purposes

1. It carries the current between tool and the work piece.

2. It cools the cutting zone which becomes hot due to flow of high current.

3. It removes products of machining from the cutting zone.

• Electrolyte can be selected on the basis of material to be machined. The sodium and the

potassium salts are most commonly used as electrolytes.

S. No Material Electrolyte

1. Iron based alloys 20% of NaCl solution in water

2. Ni based alloys Mixture of brine and 𝐻2𝑆𝑂4

3. Ti based alloys 10%HF + 10%HCl + 10%HN𝑂3

4. Co-Cr based alloys NaCl

5. WC based alloys Strong alkaline solutions

Factors for selection: (8M)

• The electrolyte solution is pumped between the tool and work piece gap at a pressure of 2.5

N/mm2and at a velocity of about 30-60 m/s

• It may be noted that low electrolyte temperature results in low metal removal rate and high

temperature leads to vaporization of the electrolyte. So, it is maintained around 35⁰ to 65⁰C

• Since some electrolyte are naturally corrosive, the ECM equipment should be made of

stainless steel or plastics




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• The essential characteristics of electrolyte are as follows

1. It should be good conductor of electricity.

2. It should have non corrosive property.

3. It should be non-toxicity and chemical stability.

4. It should have low viscosity and high specific heat.

2

a) Briefly discuss about the various characteristics of Electro Chemical Machining

(ECM).(May/Jun 2016)(8M) BTL 4 Answer: Page 4.20 – Dr.S.Senthil.

Characteristics Of ECM: (8M)

1. Metal removal technique – based on faraday’s laws of electrolysis and reverse

electroplating.

2. Work material – difficult to machine.

3. Tool material – copper, brass or steel

4. Voltage – 5-30 V

5. Current – 50 – 40000A

6. Metal removal rate – 27 𝑚𝑚3/s

7. Electrolyte – 20% NaCl solution in water, mixture of brine and 𝐻2𝑆𝑂4

8. Surface finish – 0.2 to 0.8 µm

9. Tolerance – 0.005 mm

10. Specific power consumption- 7 W /𝑚𝑚3/min

b) Explain the difference between Electrical Discharge Machining (EDM) And Electro

Chemical Machining (ECM).(May/Jun 2016)(7M) BTL 4 Answer: Page 5.2 – Dr.S.Senthil.

S. NO EDM ECM

1. Work piece is submerged in

dielectric fluid.

Work piece needed not to be

submerged in electrolyte.

2. Tool wear takes place. Tool wear does not take place.

3.

Control system is required to

maintain constant gap between tool

and work piece.

Control system is not required

4. Machining cannot be done at low

voltage.

Machining can be done at low

voltage.

5. Metal removal rate is slow

compared to ECM

Metal removal rate is high

compared to EDM

6. Energy consumed is less compared

to ECM Energy consumed is large

(7M)

3

Describe the process parameters used in the selection of ECG process. (April/May

2015)(May/Jun 2016)(15M) BTL 4 Answer: Page 4.24– Dr.S.Senthil.

Process Parameters:

The following process parameters are involved in the effectiveness of electro chemical

grinding process.

(i) Current density: (4M) The metal is removed from the work piece based on the current density. It is of the order

100 to 200 A/cm2. The power supply is DC voltage of 3 to 30 V.

It is clear that the material removal rate increases with current density which leads to




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better surface finish.

(ii) Electrolyte:(5M)

The surface finish, precision and metal removal rate are influenced by the composition of

the electrolyte. Sodium nitrate, sodium chloride and potassium nitrate with a concentration of

0.150 to 0.300 kg/litre of water are usually used as electrolyte. It is passed through nazzle in the

gap (0.25mm) between the wheel and workpiece. Electrolyte is maintained at a temperature

between 15˚C to 30˚C.

(iii) Feed rate: (3M)

If the applied feed rate is very slow, it results in poor surface finish and tolerance. If the

feed rate is very fast, the abrasive particles will be forced into the workpiece, resulting in

excessive wheel wear. The maximum depth of cut for grinding wheel is 2.5mm.

(iv) Grinding wheel speed: (3M)

The grinding wheel runs at a speed of 900 to 1800 m/min. Higher speed of wheel leads to

wear and tear. Accuracy of wheel running and wheel pressure also influences the effectiveness of

electro grinding process.




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UNIT V - THERMAL ENERGY BASED PROCESSES

Laser Beam machining and drilling (LBM), plasma Arc machining (PAM) and Electron Beam Machining

(EBM). Principles – Equipment –Types - Beam control techniques – Applications.

PART * A

Q.No. Questions

1.

What is the principle of plasma arc machining? What are the two stages in which the

process of material removal is affected? (May/June 2009/16) BTL 2

In plasma arc machining process, material is removed by directing a high velocity jet of high

temperature (11,0000C to 28,0000C) ionized gas on the work piece. This high temperature plasma

jet melts the material of the work piece. Plasma formation and melting of work piece are the two

stages in which the process of materials removed is affected.

2

In electron beam machining, why is a high vacuum created in the apparatus? BTL 3

In electron beam machining process, high velocity focused beam of electrons are used to remove

the metal from the work piece. These electrons are travelling at half the velocity of light i.e.,

1.6×108 m/s.

3

What are the characteristics of laser used in laser machining? (Nov/Dec 2013) BTL 1

• It can be focused to maximum intensity or to minimum intensity as needed.

• It can be moved rapidly on the work piece.

• It is projected on the work piece at particular distance from the lens.

• Dedicated to an online process.

• (v) Power is shared on a job.

4

What are basic requirements of laser welding? BTL 1

• The focus of the beam should be adjusted to the thickness of the material.

• (ii) The wave length of the laser beam must be compatible with the material being welded.

5

What is the main industrial application of plasma cutting systems? (Nov/Dec 2010) BTL 1

• It is used for cutting alloy steels, stainless steel, cast iron, copper, nickel, titanium, aluminum

and alloy of copper and nickel, etc.

• It is used for profile cutting.

6

State any four limitations of EBM? (Nov/Dec 2012) BTL 1

• The metal removal rate is very slow.

• It is not suitable for large work pieces.

• Cost of equipment is very high.

• 4. A little taper produced on holes.

7

How is the work table protected from getting damaged by EBM while machining the work

piece? BTL 3

The running and guidance rail of the table in particular against dirt, spatter and metal condensate.

Hoods and ductwork should be constructed for fire resistant materials. Since x-rays are produced

as a by product the work table should be enclosed and shielded with lead or other materials suitable

for preventing x-ray exposure.

8

What are the important process parameters of electron beam machining processes? BTL 1

• Control of current,

• Control of spot diameter

• Control of focal distance of magnetic lens.




6-64

9

What is the use of servo control system in spark erosion machines? (Nov/Dec 2011) BTL 3

The servo drive system is automatically protected from mechanical overload by an electronic fail-

safe alarm system. A range of low energy discharges for machining fine defects in non-destructive

testing standards and notching fracture test specimens

10

What are the gases used in PAM? BTL 2

The commonly used gases are nitrogen, hydrogen, air, mixture of nitrogen-hydrogen and argon –

hydrogen,etc.

11

What is the drawback of electron beam machining? BTL 2

One major diameter of electron beam welding has been the requirement of high degree of vacuum

essential or satisfactory operation of this process because of degassing.

12

Write the application of electron beam.BTL 1

• Thin film machining.

• Surface treatment.

• Engraving metals and non-metals.

• Cutting of materials.

13

What are the fundamentals of photons used in Laser? BTL 2

In the Laser the photons are in ground state at 0oC they are brought to the excited state by means

of absorption of energy by temperature change, collision etc

14

What is Maser? BTL 2

Laser can be melt diamond when focused by lens system. The energy density being of two orders

100,000 KW/cm2 this energy is due to atoms that have light energy level. When such an atom

impinge with electromagnetic waves having resonant frequency.

15

Write any four application of EBM. (Nov/Dec 2015) BTL 1

• Micro machining application on materials.

• Drilling of apertures for electron microscope.

• Drilling of holes in ruby and diamond crystal.

16

How basic plasma is generated? BTL 2

The basic plasma is generated by subjecting a stream of gas to the electron bombardment of the

electric arc.

17

What are the metals that can't be machined by plasma arc machining? BTL 1

• Stainless steel

• Monel

• Super alloys

18

Why vacuum is needed in EBM? (May/June 2016) BTL 2

• To reduce corrosion

• To get correct focusing

19

What is meant by laser beam drilling? (April/May 2017) BTL 2

Laser beam drilling is the process in which the laser source is focused on the spot where the drilling

is to be made. It is the hole drilling process by laser beam where the process of melting and

vaporizing the unwanted materials by means of narrow pulsed laser operating at 3 to 95 pulses per

second.

20

Write the advantages of PAM? (May/Jun 2014) BTL 1

• Clean smooth surface finish,

• Minimum energy to operate

• No need complicated maintenance

21 What is the principle of plasma arc machining? BTL 2




6-65

Material is removed by directing a high velocity jet of high temperature gas which is in ionized

state.

22

Compare LBM and EBM. (May/Jun 2014) BTL 3

LBM: focused on melting and vaporizing the unwanted material from the parent work piece

material, no need of vacuum chamber

EBM: focused on melting and vaporizing the unwanted material from the parent work piece

material, high range of vacuum chamber is maintained.

23

What are the techniques used for controlling beam in EBM process? (May/Jun

2016)(April/May 2017) BTL 2

Magnetic lens is used to shape the electron beam into a converging beam & a variable aperture is

used to reduce the diameter of the focused beam.

24

What is the principle of LBM process? (Nov/Dec 2015) BTL 2

It is a thermal process in which a laser beam is focused for melting and vaporizing the unwanted

material from the parent work piece material.

25

What do you mean by plasma? (Nov/Dec 2016) BTL 2

It is the fourth state of matter, looks and behaves like a high temperature gas and also conduct

electricity.

PART * B

1

Explain the production of laser beam and working principle of LBM? (Nov/Dec

2011)(April/May 2017) (13M) BTL 4 Answer: Page 5.14 – Dr.S.Senthil.

Principle Of LASER Beam Production: (4M)

Laser works on the principle of quantum theory of radiation consider an atom in the

ground state or lower energy state (E1) when the light radiation falls on the atom, it absorbs a

photon of energy ℎ𝜈 and goes to the excited state (E2)

Normally the atoms in the excited state will not stay there for a long time it comes out the

ground state by emitting a photon of energy Eℎ𝜈 such an emission takes place by one of the

following two methods.

Spontaneous Emission: (4M)




6-66

The atom in the excited state (E2) returns to the ground state (E1) by emitting their excess

energy (ℎ𝜈spontaneously. This process is independent of external radiation it is show in fig

Stimulated Emission:(4M)

In stimulated emission a photon having energy having E, equal to the difference in energy

between two levels E2 and E1 stimulate an atom in the higher state to make a transition to the

lower state with the creation of second photon as show in fig

Principle Of LASER Beam Machining: (1M)

In laser beam machining process, laser beam is focused on the workpiece by means of

lens to give extremely high energy density to melt and vaporize the work material.

2

What is plasmatron? Explain various types of plasmatron. (May/June2013) (13M) BTL 4


Types Of Plasma Arc Torches (PLASMATRON): (2M)

There are two types of plasma arc torches. They are,

• Direct arc plasma torches (or) transferred arc type.

• In-Direct arc plasma torches (or) Non- transferred arc type.

Dierct Arc Plasma Torches: (5M)

In direct arc plasma torches, electrode is connected to the negative terminal (cathode) of a

D.C power supply and work piece is connected to the positive terminal (anode) of D.C power

supply. So, more electrical energy is transferred to the work, thus giving more heat to the work.




6-67

Since it is difficult to strike an arc between the electrode and work piece directly through

the narrow torch passage, first an auxiliary arc is commonly produced between the electrode and

the nozzle. When the arc flame reaches the work piece, it automatically strikes the main arc between

the electrode and the work piece and the auxiliary arc is switched off.

Direct arc torches has the higher efficiency and this type of arc is preferred for cutting,

welding, depositing,etc.

In-Direct Arc Plasma Torches (Or) Non- Transferred Arc Type: (6M)

In these types of torches, electrode is connected to the negative terminal (cathode) of a D.C

power supply and nozzle is connected to the positive terminal (anode) of a D.C power supply.

When the working gas passing through the nozzle, a part of the working gas becomes

heated, ionized and emerges from the torch as the plasma jet. This plasma feeds the heat to the

work piece. This type of torches is used for non- conducting materials.




6-68

In many cases, plasma torches with a double or combined gas flow are used for welding

and cutting. Primary and secondary gases can differ in the designation, composition and flow rate.

In the cutting process the primary gas protects the tungsten electrode from the environment. The

secondary gas is used for forming plasma.

3

Explain the principle, construction and working of electron beam machining. Also how a

complex shape can be cut using EBM process. (Nov/Dec 2015) (May/June 2016) (13M) BTL

4 Answer: Page 5.1 – Dr.S.Senthil.

Introduction: (1M)

In Electron Beam machining process, high velocity focused beam of electrons are used to

remove the metal form the work piece. These electrons are travelling at half the velocity of light

i,e.,1.6*𝟏𝟎𝟖m/s This process is best suited for micro-cutting of materials.

Principle: (2M)

When the high velocity beam of electrons strike the work piece, its kinetic energy is

converted into heat, this concentrated heat raises the temperature of work piece material and

vaporizes a small amount of it, resulting in removal of material from the work piece.

Construction: (5M)

• The schematic arrangement of Electron Beam Machining is show in fig

• It consists of electron gun, diaphragm, focusing lens, deflector coil, work table

• In order to avoid collision of accelerated electrons with air molecules, vacuum is required. So,

the entire EBM setup is enclosed in a vacuum chamber, which carries vacuum of the order

𝟏𝟎−𝟓to 𝟏𝟎−𝟔mm of mercury. This chamber carries a door, is them closed and sealed.

• The electron gun is responsible for the emission of electrons, which consists of the following

three main parts.

1. Tungsten Filament – which is connected to the negative terminal of the DC power supply and

acts as cathode.




6-69

2. Grid cup – which is negatively based with respect to the filament.

3. Anode – which is connected to positive terminal of the DC power supply.

• The focusing lens is used to focus the electrons at a point and to 0.02 mm diameter.

• The electromagnetic deflector coil is used to deflect the electron beam to different spot on the

work piece. It can also be used to control the path of the cut.

Working: (5M)

• When the high voltage DC source is given to the electron gun , tungsten filament wire gets

heated and the temperature raises upto 2500 c

• Due to this high temperature, electrons are emitted from tungsten filament. These electrons are

directed by grid cup to travel towards downwards and attracted by anode.

• The electros passing through the anode are accelerated to achieve high velocity as half the

velocity of light

• The high velocity of these electrons is maintained till they strike the work piece It becomes

possible because the electrons travel through the vacuum.

• This high velocity electron beam after leaving the anode passes through the tungsten diaphragm

and them through the electromagnetic focusing lens.

• Focusing lens is used to focus the electron beam on the desired spot of the work piece.

• When the electron beam impacts on the work piece surface, the kinetic energy of high velocity

electrons is immediately converted into the heat energy. This high intensity heat melts and

vaporizes the work material at the spot of beam impact.

• Since the power density is very high (about 6500 billion W/𝑚𝑚2) it takes a few micro seconds

to melt and vaporize the material on impact

• This process is carried out in repeated pulses of short duration may range from 4 to 65,000

microseconds.




6-70

• By alternately focusing and truing off the electron beam the cutting process can be continued

as long as it is needed.

• A suitable viewing device is always incorporated with the machine. So it becomes easy for the

operator to observe the progress of machining operation.

4

Explain the process of PAM with a neat sketch. With respect to principle, construction,

advantages, disadvantages and applications. (Nov/Dec 2015) (13M) BTL 4 Answer: Page 5.25

– Dr.S.Senthil.

Introduction: (3M)

Solids, liquids and gases are the three familiar state of matter. In general when solid is heated,

it turns to liquids and the liquids eventually become gases. When a gas is heated to sufficiency

high temperature, the atoms are split into free electrons ad icons are sufficiently different from the

normal unionized gas. So, it can be considered a fourth state of matter, and is given a new name,

‘PLASMA’. In other words, when a following gas is heated to a sufficiently high temperature of

the order of 11,000ºc to 28,000ºc, it becomes partially ionized and it is known as ‘PLASMA’. This

is a mixture of free electrons, positively charged ions and neutral atoms. This plasma is used metal

removing process. Plasma arc machining process is used for cutting alloy steels, stainless steel, cast

iron, copper, nickel ad aluminum, etc.

Working Priniciple: (2M)

In plasma arc machining process, material is removed by directing a high velocity jet of high

temperature (11,000ºc to 28,000ºc) ionized gas on the work piece. This high temperature plasma

jet melts the material of the work piece.

Construction: (4M)

• The schematic arrangement of plasma arc machining is shown in fig.

• The plasma arc cutting touch carries a tungsten electrode fitted in a small chamber.

• This electrode is connected to the negative terminal of DC power supply. So it acts as a

cathode.

• The positive terminal of D.C power supply is connected to the nozzle formed near the bottom

of the chamber. So, nozzle acts as an anode.

• A small passage is provided o one side of the torch, the electrode and the nozzle remains water

cooled.




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Working: (4M)

• When a D.C power is given to the circuit, a strong arc is produced between the electrode

(cathode) and the nozzle (anode).

• A gas usually hydrogen (H2) or Nitrogen (N2) is passed into the chamber.

• This gas is heated to a sufficiently high temperature of the order of 11,000ºc to 28,000ºc by

using an electric arc produced between the electrode and the nozzle.

• In this high temperature, the gases are ionized ad large amount of thermal energy is liberated.

• This high velocity ad high temperature ionized gas (plasma) is directed on the work piece

surface through nozzle.

• This plasma jet melts the metal of the work piece and the high velocity gas stream effectively

blows the molten metal away.

• The heating of work piece materials is not due to any chemical reaction, but due to the

continuous attack of plasma on the work piece material. So, it can be safely used for

machining of ay metal including those which can be subjected to chemical reaction.

5

a)With the neat sketch explain the mechanics of EBM.(April/May2015, 2017)(May/Jun

2016)(7M) BTL 4 Answer: Page 5.5 – Dr.S.Senthil.

Mechanics Of EBM: (7M)

Electrons are the smallest stable elementary particles with a mass of 9.109*𝟏𝟎−𝟑𝟏 kg with

a negative charge of 1.602*𝟏𝟎−𝟏𝟗coulomb. If it is assumed that the initial velocity of emitting

electrons to be negligible then the electron velocity at the striking is given as,

𝑽𝒔= 600 √𝑬𝒔 km/ s …………. (1)




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Where 𝑬𝒔 - Voltage of the electric field, volt

The power of the electron beam is given by,

𝑷𝒃=𝑬𝒔𝑰𝒑, watts …………….. (2)

𝑰𝒑– Beam current, amp

The electron beam pressure is given by ,

𝑭𝒑= 0.34 * 𝑰𝒅√𝑬𝒔, dyne /𝒄𝒎𝟐

𝑰𝒅 - current density, A/𝒄𝒎𝟐

The thermal velocity is acquired by an electron is given by,

𝑽𝒂= √𝟐𝒌𝜽

𝑴𝒂 m/s ………….. (4)

Where, k – Boltzmann’s constant = 1.38 * 𝟏𝟎−𝟐𝟑J/K/atom

Θ - Temperature raised, K

𝑴𝒂- mass of one atom of the work piece, kg

b) Discuss about the characteristics of the EBM process.(6M) BTL 4 Answer: Page 5.9 –

Dr.S.Senthil.

Accelerating voltage : 50 to 200kV

Beam current : 100 to 1000 µA

Electron velocity : 1.6 * 108m/s

Power density : 6500 billon W/𝑚𝑚2

Medium : vacuum (10−5𝑡𝑜10−6𝑚𝑚𝑜𝑓𝐻𝑔)

Work piece material : all materials

Depth of cut : up to 6.5 mm

Material removal rate : up to 40 𝑚𝑚3/s

Specific power consumption : 0.5 to 50 kW(6M)

6

Explain in detail about the process parameters of Electron Beam Machining(EBM).

(April/May 2017)(13M) BTL 4 Answer: Page 5.6 – Dr.S.Senthil.

Process Parameters: (2M)

The parameters which have significant influence on the beam intensity and metal removal

rate are given below.

1. Control of current.

2. Control of spot diameter.

3. Control of focal distance of magnetic lens.

1. Control Of Current: (4M)

The heated tungsten filament cathode emits electrons depending upon the thermionic

emission capability of the filament; it is given by Richardson-Dustman equation

J = A 𝑻𝟐𝒆−(𝑬𝑾

𝑲𝑻)

Where,

J- current density of the emitted current (amp/𝒄𝒎𝟐),

W- work function of the material of the filament (volts),

T- absolute temperature of the filament (K),

E- electronic charge (coulomb),

K- Boltzmann constant (1.38 * 𝟏𝟎−𝟐𝟑J/K),

A- Constant (120 amp/𝒄𝒎𝟐(𝐝𝐞𝐠𝐫𝐞𝐞)𝟐.

The above mentioned equation is valid only when the tungsten filament is in free space, But

in the presence of electric field around the filament, alters this current density very much.




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The grid bias voltage is use to control the beam current. The more negative grid with respect

to the cathode, the restriction of electron emission will be more.

Control Of Spot Diamerter: (4M)

The diameter of the spot depends upon beam current, accelerating voltage, magnetic lens,

distance between gun and workpiece, etc. the most important three factors which contribute to

change in spot diameter are given below

• Effect of thermal velocities:we know that different electrons converging at different points

along the longitudinal axis of the beam. So, the spot size will get spread out and the minimum

spot diameter is given by,

δ𝑫𝒕= 𝟐𝒓𝒄𝒙

𝒓𝒊√𝑲𝑻

𝑬𝑽

where,

δ𝑫𝒕- minimum spot diameter,

𝒓𝒄- cathode (tungsten filament) spot radius,

𝒓𝒊- radius of beam at magnetic lens,

𝒙 - distance between gun and work piece,

E- electronic charge,

V- anode voltage,

K- Boltzmann constant (1.38 * 𝟏𝟎−𝟐𝟑J/K),

T- absolute temperature of cathode.

• Spherical deviation of the focusing lens; the spherical deviation result in the marginal rays

causing the axis at a different position from the rays. So, it leads an ideal point image to be

confused in a disc whose diameter is given by,

δ𝑫𝒔= 2.5 𝒓𝒊𝟑[

𝒙

𝒇(𝑺+𝑫)𝟐]

where,

S- lens pole piece separation of the magnetic lens ,

D- bore diameter of the magnetic lens

f- focal length of the magnetic lens.

• Space charge spreading of target: The minimum spot size in limited when the electrons

converging in a conical beam to a point as the target is subjected to mutual repulsion and the

equation is given as,

The combined effect of these three gives the reduced formula for minimum spot diameter.

Contorl At Focal Distance Of Magnetic Lens: (3M)

The focal distance at magnetic lens is given by. 𝑓

𝑆+𝐷 =

25𝑉

(𝑁𝑇)2

Where,

V – electron accelerating voltage ,

NT- ampere turns in the lens winding.

PART * C

1

List down the advantages, disadvantages and applications of EBM process. (May/June2013)


Advantage Of EBM Process: (5M)

Electron beam machining has the following advantages:




6-74

• It is an excellent process for micro finishing (milligram/s).

• Very small holes can be machining in any type of material to high accuracy.

• Holes of different size and shape can be machined.

• There is no mechanical contact between the tool and the work piece.

• It is a quick process. Harder materials can also be machined at a faster rate than conventional

machining.

• Electrical conductor materials can be machined.

• The physical and metallurgical damage to the work piece are very les.

• This process can be easily automated.

• Extremely close tolerance are obtained.

• Brittle and fragile material can be machined.

Disadvantages(Limitations): (5M)

• The metal removal rate is very slow.

• Cost of equipment is very high.

• It is not suitable for large work piece.

• High skilled operators are required to operate this machine.

• High specific energy consumption.

• A little taper produced on holes.

• Vacuum requirement limits the size of work piece.

• It is applicable only for thin materials.

• At the spot where the electron beam strikes the materials , a small amount of recasting and

metal splash can occur on the surface. It has to be removed afterwards by abrasive cleaning.

• It is not suitable for the producing perfectly cylindrical deep holes.

Applications: (5M)

• EBM is mainly used for micro-machining operations on thin, materials.

• Drilling of holes in pressure differential device used in nuclear reactors, air craft engines,

etc.

• It is used for removing small broken taps from holes.

• Micro-drilling operation for thin orifices, dies for wires drawing, parts of electron

microscopes, injector nozzles for diesel engines, etc.

• A micromachining technique known as “electron beam lithography” is being used in the

manufacturing of field emission cathodes, integrated circuits and computer memories.

• It is particularly useful for machining of materials of low thermal conductivity and high

melting point.

2

Briefly discuss about the factors affecting the cutting process of PAM. (Nov/Dec

2010)(May/June 2014)(15M) BTL 4 Answer: Page 5.31 – Dr.S.Senthil.

Factors Affecting The Cutting Process (Or)Process Parameters Of PAM:(10M)

The metal removal rate mainly depends on thermo-physical and metallurgical properties of

the plasma-forming gases. The most commonly used gases are argon. Nitrogen, hydrogen and

oxygen.

Since hydrogen has high heat conductivity, it is possible to achieve the best conditions for

the transfer of plasma power to the metal. Due to high cutting speed of hydrogen, smooth surface

is obtained. Hydrogen containing mixtures are used for cutting thick, high alloy steel plates and

good heat conductors such as copper and aluminum.




6-75

Gas mixture containing hydrogen and argon (Maximum of 20%) is also used for forming

plasma. Argon gas is used to protect the tungsten electrode from the environment. But the

protection is not sufficiently reliable, since even the small deviation on the column from the axis

of the nozzle causes the damage of tungsten electrode. Besides, argon is a scare and expensive gas.

Carbon and alloy steels, cast iron, stainless steel, and aluminum are machined by using

nitrogen. The quality of plasma machining by using nitrogen is poor and the cutting speed is

considerably less compared to hydrogen-containing gases.

Air plasma is simple and most economical method for machining. Air contains nitrogen and

oxygen. The heat conductivity of air is higher than that of hydrogen. The speed of cutting steels

with the air plasma is 1.5 to 2 times greater than the use of nitrogen as the cutting gas. Non-ferrous

alloys can be machined by using air plasma. But the quality of the surface finish is poor.

Stand Off Distance: (5M)

Stand-off distance is the distance between the nozzle tip and work piece. When the stand-

off distance increases, depth of penetration is reduced. With an excessive reduction of the stand-

off distance, the plasma torch can be damaged by the metal spatter. The optimum stand-off distance

depends on the thickness of the metal being machined and varies from 6 to 10 mm.

3

Explain in detail about the machining applications of LASER . (Nov/Dec 2016)(15M) BTL 4


Machining Applications Of LASER:

A laser has a wide range of machining applications.

Laser in Metal cutting: (4M)

A laser beam can be used for cutting metals, plastics, ceramics, textile, cloth and even glass,

when its surface is coated with radiation absorbing material such as carbon. Normally, laser cutting

starts by drilling a hole through the work piece, then moving along a predetermined path of the

shape to be cut. Steel, titanium, nickel and plastics can be cut easily by using laser beam. But cutting

of aluminium metal and copper is very difficult, since these metals tend to absorb the applied heat.

The cutting speed of laser depends on the material being cut, its thickness, physical characteristics

and the output power of the laser beam. Laser has an additional advantage in cutting complex

shapes with corners and slots.

Laser in drilling: (4M)

Laser drilling was one of the first practical application of laser technology in industry and

the demand for laser drilling in increasing.

Hole drilling by laser is a process of melting and vaporising unwanted materials by means

of narrow pulsed laser operating at 3 to 95 pulses/s. Due to melting and vaporization process, high

accuracy is not possible in laser drilling. So, laser drilling is not suited for deep hole drilling and

for producing perfectly cylindrical holes.

Laser drilling is used in watch jewels, diamond dies and other machine parts for various

industries where a particularly high level of precision is not demanded.

Laser drilling is used in aircraft-turbine industry to make holes for air bleeds, air cooling or

the passage of other fluids. It is also used for making holes in hypodermic needles, automotive fuel

plates, various lubrication devices, holes in tungsten carbide tool plate, holes in baby bottle nipples,

relief holes in pressure plugs etc.

Laser in Welding: (4M)

In the process, a laser beam is focused on spot where the two parts are to be welded.

Laser beam welding requires more precise control of the input laser power than in the case of

drilling.




6-76

Laser welding is especially useful when it is essential to control the size of the heat affected zone,

to reduce the roughness of the welded surface ad to eliminate mechanical effects. It is generally

used for welding multilayer materials.

There are two different types of laser welding. They are:

1. Conduction limited welding

2. Deep penetration welding

Conduction Limited Welding

In this method, the metal absorbs the laser beam at the work surface, and the area below the surface

is heated by conduction. It is used for welding thin components.

Deep Penetration Welding

In this method, the metal absorbs the laser beam from top to bottom of the work surface. Thermal

conduction does not limit the penetration. This type of welding requires greater power and the CO2

laser is used for this purpose.

Basic Requirements for Laser Welding

1. The focus of the beam should be adjusted to the thickness of the materials.

2. The wavelength of the laser beam must be compatible with the material being

Welded.

3. Pulse waves are normally better than continuous waves.

4. A pulse shape of the laser beam should be controlled precisely from weld to weld.

Many metals and alloys can be laser welded. Some of the most readily processed are: low

carbon steel, titanium, zirconium, silicon bronze and some nickel alloys.

One of the major factors for laser welding is the proper joint preparation. The two surfaces

being welded should remain in close contact with each other. Since a filler material is not used I

laser welding. There should not be ay gap in the joint.

The advantage of the laser weld is the elimination of grinding from the entire process. I

conventional welding process, electron beam welding process and plasma welding process excess

filler material is removed by grinding.

Laser for Surface Treatment:(3M)

Gears saw teeth, valve wear pads, and cylinder liners can be strengthened by using laser

beam. The laser is used to deposit a thin layer of cobalt alloy on the turbine blade shroud-contact

areas. Argon gas is used for shielding during deposition of the cobalt alloy ad for cooling purposes.

By using laser, a thin ceramic coating is applied on metal surface for heat and wears resistance.

Laser can also be used to seal micro cracks which are usually preset in hard-chromium electroplates.

me6004 unconventional machining processes l t p …...• wire cut electrical discharge machining. 8...

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