Mech Vi Non Traditional Machining [10me665] Notes

Non Traditional Machining

10ME665 NON-TRADITIONAL MACHINING

Subject Code: 10ME665

IA Marks: 25

Hours/Week: 04

Exam Hours: 03

Total Hours: 52

Exam Marks: 100 PART – A A

UNIT - 1 Introduction: History, Classification, comparison between conventional and Non-

conventional machining process selection.

05 Hours

UNIT - 2 Ultrasonic Machining (USM): Introduction, equipment, tool materials & tool size, abrasive slurry, cutting tool system design:- Effect of parameter: Effect of amplitude and frequency and vibration, Effect of abrasive grain diameter, effect of applied static load, effect of slurry, tool & work material, USM process characteristics: Material removal rate, tool wear, Accuracy, surface finish, applications, advantages & Disadvan tages of USM. 08 Hours UNIT - 3 Abrasive Jet Machining (AJM): Introduction, Equipment, Variables in AJM: Carrier Gas, Type of abrasive, size of abrasive grain, velocity of the abrasive jet, mean number. abrasive particles per unit volume of the carrier gas, work material, standoff distance (SOD), nozzle design, shape of cut. Process characteristics-Material removal rate, Nozzle wear, Accuracy & surface finish. Applications, advantages & Disadvantages of AJM. Water Jet Machining: Principal, Equipment, Operation, Application, Advantages and limitations of water Jet machinery 07 Hours UNIT - 4 Electrochemical Machining (ECM): Introduction, study of ECM machine, elements of ECM process : Cathode tool, Anode work piece, source of DC power, Electrolyte, chemistry of the process, ECM Process characteristics – Material removal rate, Accuracy, surface finish, ECM Tooling: ECM tooling technique & example, Tool & insulation materials, Tool size Electrolyte flow arrangement, Handling of slug, Economics of ECM, Applications such as Electrochemical turning, Electrochemical Grinding, Electrochemical Honing, deburring, 06 Hours Advantages, Limitations. PART – B B UNIT - 5 Chemical Machining (CHM): Introduction, elements of process, chemical blanking process : Preparation of work piece, piec e, preparation of masters, masking with photo resists, etching for blanking, accuracy of chemical blanking, applications of chemical blanking, chemical milling (contour machining): process steps – steps – masking, masking, Etching, process characteristics of CHM: material removal rate, accuracy, surface finish, Hydrogen embrittlement, advantages & application of 06 Hours CHM.

DEPARTMENT OF MECHANICAL ENGG, SJBIT

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UNIT - 6 Electrical Discharge Machining (EDM): Introduction, mechanism of metal removal, dielectric fluid, spark generator, EDM tools (electrodes) Electrode feed control, Electrode manufacture, Electrode wear, EDM tool design, choice of machining operation, electrode material selection, under sizing and length of electrode, machining time. Flushing; pressure flushing, suction flushing, side flushing, pulsed flushing synchronized with electrode movement, EDM process characteristics: metal removal rate, accuracy, surface finish, Heat Affected Zone. Machine tool selection, Application, EDM accessories / applications, electrical discharge grinding, Traveling wire EDM. 08 Hours UNIT - 7 Plasma Arc Machining (PAM): Introduction, equipment, non-thermal generation of plasma, selection of gas, Mechanism of metal removal, PAM parameters, process p rocess characteristics. 05 Hours Safety precautions, Applications, Advantages and limitations. UNIT - 8 Laser Beam Machining (LBM): Introduction, equipment of LBM mechanism of metal removal, LBM parameters, Process characteristics, Applications, Advantages & limitations. Electron Beam Machining (Ebm): Principles, equipment, operations, applications, advantages and limitation of EBM. 07 Hours TEXT BOOKS:

1. Modern machining process, Pandey and Shan, Tata McGraw Hill 2000 2. New Technology, Bhattacharya 2000 REFERENCE BOOKS:

1. Production Technology, HMT Tata McGraw Hill. 2001 2. Modern Machining Process, Aditya. 2002 3. Non-Conventional Machining, P.K.Mishra, The Institution of Engineers (India) Test book series, Narosa Publishing House – House – 2005. 2005. 4. Metals Handbook: Machining Volume 16, Joseph R. Davis (Editor), American Society of Metals (ASM)


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UNIT - 6 Electrical Discharge Machining (EDM): Introduction, mechanism of metal removal, dielectric fluid, spark generator, EDM tools (electrodes) Electrode feed control, Electrode manufacture, Electrode wear, EDM tool design, choice of machining operation, electrode material selection, under sizing and length of electrode, machining time. Flushing; pressure flushing, suction flushing, side flushing, pulsed flushing synchronized with electrode movement, EDM process characteristics: metal removal rate, accuracy, surface finish, Heat Affected Zone. Machine tool selection, Application, EDM accessories / applications, electrical discharge grinding, Traveling wire EDM. 08 Hours UNIT - 7 Plasma Arc Machining (PAM): Introduction, equipment, non-thermal generation of plasma, selection of gas, Mechanism of metal removal, PAM parameters, process p rocess characteristics. 05 Hours Safety precautions, Applications, Advantages and limitations. UNIT - 8 Laser Beam Machining (LBM): Introduction, equipment of LBM mechanism of metal removal, LBM parameters, Process characteristics, Applications, Advantages & limitations. Electron Beam Machining (Ebm): Principles, equipment, operations, applications, advantages and limitation of EBM. 07 Hours TEXT BOOKS:

1. Modern machining process, Pandey and Shan, Tata McGraw Hill 2000 2. New Technology, Bhattacharya 2000 REFERENCE BOOKS:

1. Production Technology, HMT Tata McGraw Hill. 2001 2. Modern Machining Process, Aditya. 2002 3. Non-Conventional Machining, P.K.Mishra, The Institution of Engineers (India) Test book series, Narosa Publishing House – House – 2005. 2005. 4. Metals Handbook: Machining Volume 16, Joseph R. Davis (Editor), American Society of Metals (ASM)


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CONTENT

1. Introduction

04-07

2. Ultrasonic Machining (USM)

08-14

3. Abrasive Jet Machining (AJM)

15-21

4. Electrochemical Machining (ECM)

22-26

5. Chemical Machining (CHM)

27-32

6. Electrical Discharge Machining (EDM)

33-36

7. Plasma Arc Machining (PAM)

37-40

8. Laser Beam Machining (LBM)

41-45


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UNIT-1 1.1 Introduction

Manufacturing processes can be broadly divided into two groups and they are primary manufacturing processes and secondary manufacturing processes. The former ones provide basic shape and size to the material as per designer’s requirement. Casting, forming, forming, powder metallurgy are such processes to name a few. Secondary manufacturing processes provide the final shape and size with tighter control on dimension, surface characteristics etc. Material removal processes are mainly the secondary manufacturing processes. Material removal processes once again can be divided into mainly two groups and they are “Conventional Machining

Processes”

and

“Non-Traditional “Non-

Manufacturing

Processes”.

Examples

of

conventional machining processes are turning, boring, milling, shaping, broaching, slotting, grinding etc. Similarly, Abrasive Jet Machining (AJM), Ultrasonic Machining (USM), Water Jet and Abrasive Water Jet Machining (WJM and AWJM), Electro discharge Machining (EDM) are some of the Non Traditional Machining (NTM) Processes. 1.2 History of Non Traditional processes

Although, the non conventional machining processes have created a revolution in the field of machining technology by the development of idea of various processes were initiated as early as in nineteen- twenties in USSR.

1920 The initiation was first made by Gussev towards the end of 1920 in USSR. He suggested a

method of machining by combination of Chemical and mechanical means. His work is basis for all Electro Chemical processes known today.

1941 Burgess, American Scientist had demonstrated the possibility of ECM process by drawing

a sharp contrast between the mechanical and electrolyte methods in metal removal

1942 The idea of Ultrasonic machining was invented by Balamuth at the He invented at the time

of investigation of dispersion of solids in Liquids with the help of a vibrating magne-tostrictive nickel tube However, the origination of the process was made by Rosenberg DEPARTMENT OF MECHANICAL ENGG, SJBIT

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1943 DM was developed by B R Lazarenko and N I Lazarenko in USSR. They first developed

the idea of spark erosion machining. In the early nineteen-sixties, the idea of Ultrasonic machining began to to develop widely in USSR and basis of this development was laid on extensive investigation that took place in the mechanism of ultrasonic machining and in the design of Magneto-strictive transducers, converters and wave guides.

1950 The basis of laser machining was established by the process which Which were developed

by Basov, Prokhorov and Fabrikanth in USSR in 1950.

1950 Electro chemical Grinding has practically been developed in about 1950.

1960 The concept of whirling jet machining was innovated.

1.3 Classification of NTM processes

Classification of NTM processes is carried out depending on the nature of energy used for material removal. 1. Mechanical Processes a) Abrasive Jet Machining (AJM) b) Ultrasonic Machining (USM) c) Water Jet Machining (WJM) d) Abrasive Water Jet Machining (AWJM) 2. Electrochemical Processes a) Electrochemical Machining (ECM) b) Electro Chemical Grinding (ECG) c) Electro Jet Drilling (EJD) 3. Electro-Thermal Processes a) Electro-discharge machining (EDM) b) Laser Jet Machining (LJM) DEPARTMENT OF MECHANICAL ENGG, SJBIT

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c) Electron Beam Machining (EBM) 4. Chemical Processes a. Chemical Milling (CHM) b. Photochemical Milling (PCM)

1.4 Comparison between conventional and Non-conventional machining process selection.

Sl Conventional Process No. 1 The cutting tool and work piece arealways in physical contact withrelative motion with each other,which results in friction and toolwear. 2 Material removal rate is limited bymechanical properties of workmaterial.

3

4

5

6 7 8

9 10

Non Conventional Process

There is no physical contact betweenthe tool and work piece, In some nontraditional process tool wear exists. NTM can machine difficult to cut andhard to cut materials liketitanium,ceramics,nimonics,SST,composites,se miconductingmaterials Relative motion between the tooland work is Many NTM are capable of producingcomplex 3D typically rotary orreciprocating. Thus the shape shapes and cavities ofwork is limited to circular or flatshapes. In spite of CNC systems,production of 3D surfaces is still adifficult task. Machining of small cavities , slits ,blind holes or Machining of small cavities, slits andProduction of through holes aredifficult non-circular, micro sized,large aspect ratio, shall entry angleholes are easy using NTM Use relative simple and inexpensivemachinery and Nontraditional processes requiresexpensive tools readily availablecutting tools and equipment as wellas skilled labour, which increase theproduction cost significantly Capital cost and maintenance cost islow Capital cost and maintenance cost ishigh Traditional processes are wellestablished and Mechanics of Material removal of Someof NTM physics of processis well understood process are still under research Conventional process mostly usesmechanical Most NTM uses energy in direct formFor example energy : laser, Electron beam inits direct forms are used in LBM andEBM respectively. Surface finish and tolerances arelimited by High surface finish(up to 0.1 micron)and machining inaccuracies tolerances (25 Microns)can beachieved High metal removal rate. Low material removal rate.


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1.5 Applications

Some of the applications of NTM are given below:


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10ME665 UNIT-2

2.1 Introduction

Ultrasonic Machining is a non-traditional process, in which abrasives contained in aslurry are driven against the work by a tool oscillating at low amplitude (25-100 microns) and high frequency (15-30kHz). 2.2 Equipment:

Ultrasonic Machining consists of : 1. High Power sine wave generator 2. Magneto-strictive Transducer 3. Tool Holder 4. Tool


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High power sine wave generator

This unit converts low frequency (60 Hz) electrical power to high frequency (20kHz) electrical power.

Transducer

The high frequency electrical signal is transmitted to traducer which converts it into high frequency low amplitude vibration. Essentially transducer converts electrical energy to mechanical vibration. There are two types of transducer used 1. Piezo electric transducer 2. Magneto-stricitve transducer. Piezo electric transducer : These transducer generate a small electric current when they are

compressed. Also when the electric current is passed though crystal it expands. When the current is removed, crystal attains its original size and shape. Such transducers are available up to 900 Watts. Piezo electric crystals have high conversion efficiency of 95%. Magneto-strictive transducer: These also change its length when subjected to strong magnetic

field. These transducers are made of nickel, nickel alloy sheets. Their conversion efficiency is


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about 20-30%. Such transducers are available up to 2000 Watts. The maximum change in length can be achieved is about 25 microns. Tool holder OR Horn.

The tool holder holds and connects the tool to the transducer. It virtually transmits the energy and in some cases, amplifies the amplitude of vibration. Material of tool should have good acoustic properties, high resistance to fatigue cracking. Due measures should be taken to avoid ultrasonic welding between transducer and tool holder. Commonly used tool holders are Monel, titanium, stainless steel. Tool holders are more expensive, demand h igher operating cost. Tool

Tools are made of relatively ductile materials like Brass, Stainless steel or Mild steel so that Tool wear rate (TWR) can be minimized. The value of ratio of TWR and MRR depends on kind of abrasive, work material and tool materials. 2.3 Process parameters

1. Amplitude of vibration ( 15 to 50 microns) 2. Frequency of vibration ( 19 to 25 kHz). 3. Feed force (F) related to tool dimensions 4. Feed pressure 5. Abrasive size 6. Abrasive material 

Al203, SiC, B4C, Boron silicarbide, Diamond.

7. Flow strength of the work material 8. Flow strength of the tool material 9. Contact area of the tool 10. Volume concentration of abrasive in water slurry 11. Tool a. Material of tool b. Shape c. Amplitude of vibration d. Frequency of vibration e. Strength developed in tool 12. Work material DEPARTMENT OF MECHANICAL ENGG, SJBIT

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a. Material b. Impact strength c. Surface fatigue strength 13. Slurry a. Abrasive – hardness, size, shape and quantity of abrasive flow b. Liquid – Chemical property, viscosity, flow rate c. Pressure d. Density 2.4 Process capability

1. Can Machine work piece harder than 40 HRC to 60 HRC like carbides, ceramics, tungsten glass that cannot be machined by conventional methods 2. Tolerance range 7 micron to 25 microns 3. Holes up to 76 micron have been drilledhole depth upto 51mm have been achieved easily. Hole depth of 152mm deep is achieved by special flushing techniques. 4. Aspect ratio 40:1 has been achieved 5. Linear material removal rate -0.025 to 25mm/min 6. Surface finish -0.25 micron to 0.75 micron 7. Non directional surface texture is possible compared to conventional grinding 8. Radial over cut may be as low as 1.5 to 4 times the mean abrasive grain size. 2.5 Applications

1. Machining of cavities in electrically non-conductive ceramics 2. Used to machine fragile components in which o therwise the scrap rate is high 3.

Used for multistep processing for fabricating silicon nitride (Si3N4) turbine blades

4. Large number of holes of small diameter. 930 holes with 0.32mm has been reported ( Benedict, 1973) using hypodermic needles 5. Used for machining hard, brittle metallic alloys, semiconductors, glass, ceramics, carbides etc. 6. Used for machining round, square, irregular shaped holes and surface impressions. 7. Used in machining of dies for wire drawing, punching and blanking operations 8. USM can perform machining operations like drilling, grinding and milling operations on all materials which can be treated suitably with abrasives. DEPARTMENT OF MECHANICAL ENGG, SJBIT

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9. USM has been used for piercing of dies and for parting off and blankingoperations. 10. USM enables a dentist to drill a hole of any shape on teeth without any pain 11. Ferrites and steel parts , precision mineral stones can be ma chined using USM 12. USM can be used to cut industrial diamonds 13. USM is used for grinding Quartz, Glass, and ceramics 14. Cutting holes with curved or spiral centre lines an d cutting threads in glass and mineral or metallo-ceramics 2.6 Advantages

1. It can be used machine hard, brittle, fragile and non-conductive material 2. No heat is generated in work, therefore no significant changes in physical structure of work material 3. Non-metal (because of the poor electrical conductivity) that cannot be machined by EDM and ECM can very well be machined by USM. 4. It is burr less and distortion less processes. 5.

It can be adopted in conjunction with other new technologies like EDM,ECG,ECM.

2.7 Disadvantages

1. Low Metal removal rate 2. It is difficult to drill deep holes, as slurry movement is restricted. 3. Tool wear rate is high due to abrasive particles. Tools made from brass, tungsten carbide, MS or tool steel will wear from the action of abrasive grit with a ratio that ranges from 1:1 to 200:1 4. USM can be used only when the hardness of work is more than 45 HRC. 2.8 Material removal models in USM

Theoretical analysis and experimental results have revealed that USM is a form of abrasion and material removal in the form of small grains by fou r mechanisms i.

Throwing of abrasive grains

ii.

Hammering of abrasive grains

iii.

Cavitations in the fluid medium arising out of ultrasonic vibration of tool.

iv.

Chemical erosion due to micro – agitation

Material removal due to throwing and hammering is significant and MR due to cavitations and chemical erosion can be ignored. DEPARTMENT OF MECHANICAL ENGG, SJBIT

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Abrasive particles are assumed to be spherical in shape having diameter dg. Abrasive particles move under high frequency vibrating tool. There are two possibilities whenthe tool hit the particle. 

If the size of the particle is small and gap between the tool and work is large, then particle will be thrown by tool to hit the work piece.



If the size of the particle is large and gap between tool and work is small, then particle is hammered over the work surface.

From the geometry


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UNIT-3 3.1 Definition

In abrasive jet machining, a focused stream of abrasive particles, carried by high pressure air or gas is made to impinge on the work surface through a nozzle and the work material is made to impinge on the work surface through a nozzle and work material is removed by erosion by high velocity abrasive particles. 3.2 Abrasive Jet Machining Equipment

In Abrasive Jet Machining (AJM), abrasive particles are made to impinge on the work material at a high velocity. The high velocity abrasive particles remove the material by micro-cutting action as well as brittle fracture of the work material.


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A schematic layout of AJM is shown above. The gas stream is then passed to thenozzle through a connecting hose. The velocity of the abrasive stream ejected through the nozzle is generally of the order of 330 m/sec. Abrasive jet Machining consists of 1. Gas propulsion system 2. Abrasive feeder 3. Machining Chamber 4. AJM Nozzle 5. Abrasives 3.3 Gas Propulsion System

Supplies clean and dry air. Air, Nitrogen and carbon dioxide to propel the abrasive particles. Gas may be supplied either from a compressor or a cylinder. In case of a compressor, air filter cum drier should be used to avoid water or oil contamination of abrasive powder. Gas should be nontoxic, cheap, easily available. It should not excessively spread when discharged from nozzle into atmosphere. The propellant consumption is of order of 0.008 m3/min at a nozzle pressure of 5bar


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and abrasiveflow rate varies from 2 to 4 gm/min for fine machining and 10 to 20 gm/min forcutting operation. Abrasive Feeder.

Required quantity of abrasive particles is supplied by abrasive feeder. The filleted propellant is fed into the mixing chamber where in abrasive particles are fed through a sieve. The sieve is made to vibrate at 50-60 Hz and mixing ratio is controlled by the amplitude of vibration of sieve. The particles are propelled by carrier gas to a mixing chamber. Air abrasive mixture moves further to nozzle. The nozzle imparts high velocity to mixture which is directed at work piece surface. Machining chamber

It is well closed so that concentration of abrasive particles around the working chamber does not reach to the harmful limits. Machining chamber is equipped with vacuum dust collector. Special consideration should be given to dust collection system if the toxic material (like beryllium) are being machined. AJM nozzle

AJM nozzle is usually made of tungsten carbide or sapphire ( usually life – 300 hours for sapphire , 20 to 30 hours for WC) which has resistance to wear. The nozzle is made of either circular or rectangular cross section and head can be head can be straight, or at a right angle. It is so designed that loss of pressure due to the bends, friction etc is minimum possible. With increase in wear of a nozzle, the divergence ofjet stream increases resulting in more stray cutting and high inaccuracy. 3.4 Process parameters

For successful utilization of AJM process, it is necessary to analyze the following process criteria. 1. Material removal rate 2. Geometry and surface finish of work piec e 3. Wear rate of the nozzle However, Process criteria are generally influenced by the process parameters as enumerated below: 

Abrasives

a) material – Al2CO3 SiC Glass beads Crushed glass Sodium bi carbonate DEPARTMENT OF MECHANICAL ENGG, SJBIT

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b) shape – irregular/regular c) Size – 10 to 50 microns d) Mass flow – 2-20 gm/min 

Carrier Gas

a) Composition – Air, CO2, N2 b) Density – 1.3 kg/m3 c) Velocity - 500 to 700 m/s d) Pressure - 2 to 10 bar e) Flow rate - 5 to 30 microns 

Abrasive Jet

a) Velocity - 100 to 300 m/s b) Mixing ratio – Volume flow rate of abrasives/Volume flow rate of gas



c)

Standoff distance – SOD- 0.5 to 15mm.

d)

Impingement angle – 60 to 90 deg

Nozzle

a) Material – WC/Sapphire b) Diameter – 0.2 to 0.8 mm c) Life – 300 hours for sapphire, 20 to 30 hours for WC 3.5 Applications

1. This is used for abrading and frosting glass more economically as compared to etching or grinding

2. Cleaning of metallic smears on ceramics, oxides on metals, resistive coating etc. 3. AJM is useful in manufacture of electronic devices , drilling of glass wafers, de burring of plastics, making of nylon and Teflon parts permanent marking on rubber stencils, cutting titanium foils

4. Deflating small castings, engraving registration numbers on toughened glass used for car windows

5. Used for cutting thin fragile components like germanium, silicon etc. 6. Register treaming can be done very easily and micro module fabrication for electrical contact, semiconductor processing can also be done effectively. DEPARTMENT OF MECHANICAL ENGG, SJBIT

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7. Used for drilling, cutting, deburring etching and polishing of h ard and brittle materials. 8. Most suitable for machining brittle and heat sensitive materials like glass, quartz, sapphire, mica, ceramics germanium , silicon and gallium.

9. It is also good method for deburring small hole like in hypodermic needles and for small milled slots in hard metallic components. 3.6 Advantages of AJM

1. High surface finish can be obtained depending upon the grain sizes 2. Depth of damage is low ( around2.5 microns) 3. It provides cool cutting action, so it can machine delicate and heat sensitive material 4. Process is free from chatter and vibration as there is no contact between the tool and work piece 5. Capital cost is low and it is easy to operate and maintain AJM. 6. Thin sections of hard brittle materials like germanium, mica, silicon, glass and ceramics can be machined. 7. It has the capability of cutting holes of intricate shape in hard materials. 3.7 Disadvantages of AJM

1. Limited capacity due to low MRR. MRR for glass is 40 gm/minute 2. Abrasives may get embedded in the work surface, especially while machining soft material like elastomers or soft plastics.

3. The accuracy of cutting is hampered by tapering of hole due to unavoidable flaring of abrasive jet.

4. Stray cutting is difficult to avoid 5. A dust collection system is a basic requirement to prevent atmospheric pollution and health hazards.

6. Nozzle life is limited (300 hours) 7. Abrasive powders cannot be reused as the sharp edges are worn and smaller particles can clog the nozzle.

8. Short standoff distances when used for cutting, damages the nozzle.


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3.8 Machining characteristics

Following are the AJM process criteria 1. Material removal rate 2. Geometry and surface finish of work piece 3. wear rate of the nozzle Process criteria are generally influenced by the process parameters The characteristics of above process parameters on process criteria are as follows 1. Effect of abrasive flow rate and grain size on MRR

It is clear from the figure that at aparticular pressure MRR increase with increase of abrasive flow rate and is influenced by size of abrasive particles. But after reaching optimum value, MRR decreases with further increase of abrasive flow rate. This is o wing to the fact that Mass flow rate of gas decreases with increase of abrasive flow rate and hence mixing ratio increases causing a decrease in material removal rate because of decreasing energy available for erosion.

2. Effect of exit gas velocity and abrasive particle density

The velocity of carrier gas conveying the abrasiveparticles changes considerably with the change of abrasive particle density as indicated in figure. The exit velocity of gas can be increased to critical velocity when the internal gas pressure is nearly twice the pressure at exit of nozzle for the abrasive particle density is zero. If the density ofabrasive particles is gradually increased exit velocity will go on decreasing for the same pressure condition. It is due to fact that Kinetic energy of gas is utilized for transporting the abrasive particle


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3. Effect of mixing ratio on MRR

Increased mass flow rate of abrasive will result in a decreased velocity of fluid and will therebydecreasesthe available energy for erosion and ultimately the MRR. It is convenient to explain to this fact by term MIXING RATIO.

4. Effect of Nozzle pressure on MRR The abrasive flow rate can be increased by increasing the flow rate of the carrier gas. This is only possible by increasing the internal gas pressure as shown in the figure. As the internal gas pressure increases abrasive mass flow rate increase and thus MRR increases. As a matter of fact, the material removal ratewill increase with the increase in gas pressureKinetic energy of the abrasive particles is responsible for the removal of material by erosion process. The abrasive must impinge on the work surface with minimum velocity for machining glass by SIC particle is found to be around 150m/s.


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10ME665 UNIT – 4

4.1 Introduction

Electrochemical Machining (ECM) is a non-traditional machining (NTM) process belonging to electrochemical category. ECM is opposite of electrochemical or galvanic coating or deposition process. Thus ECM can be thought of a controlled anodic dissolution at atomic level of the work piece that is electrically conductive by a shaped tool due to flow of high current at relatively low potential difference through an electrolyte which is quite often water based neutral salt solution. Fig. 1 schematically shows the basic principle of ECM.

4.2 Equipment

The electrochemical machining system has the following modules: • Power supply • Electrolyte filtration and delivery system • Tool feed system • Working tank


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4.3 Modelling of material removal rate

Material removal rate (MRR) is an important characteristic to evaluate efficiency of a nontraditional machining process. In ECM, material removal takes place due to atomic dissolution of work material. Electrochemical dissolution is governed by Faraday’s laws. The first law states that the amount of electrochemical dissolution o r deposition is proportional to amount of charge passed through the electrochemical cell, which may be expressed as: m∝Q, Where m = mass of material dissolved or deposited Q = amount of charge passed The second law states that the amount of material deposited or dissolved further depends on Electrochemical Equivalence (ECE) of the material that is again the ratio atomic weigh and valency. Thus


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4.4 Applications

1. ECM can be used to make disc for turbine rotor blades made up ofHSTR alloys 2. ECM can be used for slotting very thin walled collets 3. ECM can be used for copying of internal and external surfaces, cuttingof curvilinear slots, machining of intricate patterns, production of longcurved profiles, machining of gears and chain sprockets, production ofintegrally bladed nozzle for use in diesel locomotives, production ofsatellite rings and connecting rods, machining of thin large diameterdiaphragms. 4. ECM principle has be employed for performing a number of machiningoperations namely, turning, treplaning, broaching, grinding, fine holedrilling, die sinking, piercing, deburring,plunge cutting etc. 5. ECM can also be used to generate internal profile of internal cams. 4.5 Advantages

ECM offers impressive and long lasting advantages. 1. ECM can machine highly complicated and curved surfaces in a singlepass. 2. A single tool can be used to machine a large number of pieces withoutany loss in its shape and size. Theoretically tool life is high 3. Machinability of the work material is independent of its physical andmechanical properties. The process is capable of machining metals andalloys irrespective of their strength and hardness. 4. Machined surfaces are stress and burr free having good surface finish 5. It yields low scrap, almost automatic operation, low overall machiningtime, and reduced inventory expenses. 6. There is no thermal damage and burr free surface can be produced. 4.6 Disadvantages

1. High capital cost of equipment 2. Design and tooling system is complex 3. Hydrogen libration at the tool surface may cause hydrogen embrittlementof the surface. 4. Spark damage may become sometimes problematic 5. Fatigue properties of the machined surface may reduce as compared toconventional techniques (by 20%) DEPARTMENT OF MECHANICAL ENGG, SJBIT

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6. Non-conductive material cannot be machined. 7. Blind holes cannot be machined in solid block in one stage 8. Corrosion and rust of ECM machine can be hazard 9. Space and floor area requirement are also higher than for conventionalmachining methods. Some additional problems related to machine toolrequirements such as power supply, electrolyte handling and tool feed servo 4.7 Process Parameters

1. Power supply 

Type – DC



Voltage – 30V



Current - 40000A



Current Density – 500 A/Cm2

2. Electrolyte 

Type – Nacl, NaNo3, Proprietary mixtures.



Temperature – 26 to 50 deg.



Flow rate – 16 LPM to 20 LPM



Velocity – 1500 m/min to 3000 m/min



Inlet pressure – 2200 kPa.



Outlet Pressure- 300 kpa

3. Working Gap 

0.075 to 0.75mm

4. Side over cut 

0.125 to 1mm

5. Feed rate 

0.500 to 13 mm/min

6. Electrode material 

Copper , Brass, Bronze

7. Tolerance 

0.025mm (2D) and 0.050mm(3D)

8. Roughness 

1.5 microns


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10ME665 UNIT – 5

5.1 Introduction

Chemical machining is one of the non-conventional machining processes where material is removed by bringing it in contact of a strong chemical enchant. There are different chemical machining methods base on this like chemical milling, chemical blanking, photochemical machining, etc. 5.2 Working Principle of ECM

Electrochemical machining removes material of electrically conductor workpiece. The workpiece is made anode of the setup and material is removed by anodic dissolution. Tool is made cathode and kept in close proximity to the workpiece and current is passed through the circuit. Both electrodes are immersed into the electrolyte solution. The working principle and process details are shown in the Figure. This works on the basis of Faraday’s law of electrolysis. The cavity machined is the mirror image of the tool. MRR in this process can easily be calculated according to Faraday’s law. Process details of ECM are shown in Figure and described as below:

Workpiece

Workpiece is made anode, electrolyte is pumped between workpiece and the tool. Material of workpiece is removed by anodic dissolution. Only electrically conducting materials can be processed by ECM. Tool

A specially designed and shaped tool is used for ECM, which forms cathode in the ECM setup. The tool is usually made of copper, brass, stainless steel, and it is a mirror image of the desired machined cavity. Proper allowances are given in the tool size to get the dimensional accuracy of the machined surface.


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Power Supply

DC power source should be used to supply the current. Tool is connected with the negative terminal and workpiece with the positive terminal of the power source. Power supply supplies low voltage (3 to 4 volts) and high current to the circuit. Electrolyte

Water is used as base of electrolyte in ECM. Normally water soluble NaCl and NaNO

3

are used as

electrolyte. Electrolyte facilitates are carrier of dissolved workpiece material. It is recycled by a pump after filtration. Tool Feed Mechanism

Servo motor is used to feed the tool to the machining zone. It is necessary to maintain a constant gap between the workpiece and tool so tool feed rate is kept accordingly while machining. In addition to the above whole process is carried out in a tank filled with electrolyte. The tank is made of transparent plastic which should be non-reactive to the electrolyte. Connecting wires are required to connect electrodes to the power supply.

5.3 Tooling for CHM Tooling for CHM is relatively inexpensive and simple to modify. Four different types of tools are required: maskants, etchants, scribing templates, and accessories.

Maskants: Maskants are generally used to protect parts of theworkpiece where CD action is not

needed. Synthetic or rubber basematerials are frequently used. Table 3.1 shows the different maskantsand etchants for several materials together with the etch rate and etchfactor. Maskants should, however, possess the following properties: 1. Be tough enough to withstand handling DEPARTMENT OF MECHANICAL ENGG, SJBIT

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2. Adhere well to the workpiece surface 3. Scribe easily 4. Be inert to the chemical reagent used 5. Be able to withstand the heat generated by etching 6. Be removed easily and inexpensively after etching Etchants: Etchants (see below Table) are acid or alkaline solutionsmaintained within a controlled

range of chemical composition andtemperature. Their main technical goals are to achieve the following: 1. Good surface finish 2. Uniformity of metal removal 3. Control of selective and intergranular attack 4. Control of hydrogen absorption in the case of titanium alloys 5. Maintenance of personal safety 6. Best price and reliability for the materials to be used in the constructionof the process tank 7. Maintenance of air quality and avoidance of possible environmentalProblems

5.4 Accuracy and surface finish

In CHM, the metal is dissolved by the CD action. This machining phase takes place both at the individual grain surfaces as well as at the grain boundaries. Fine grain size and homogenous metallurgical structure are, therefore, necessary, for fine surface quality of uniform appearance. Surfaces machined by CHM do not have a regular lay pattern. Based on the grain size, orientation, heat treatment, and previously induced stresses, every material has a basic surface DEPARTMENT OF MECHANICAL ENGG, SJBIT

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finish that results from CHM for a certain period of time. While surface imperfections will not be eliminated by CHM, any prior surface irregularities, waviness, dents, or scratches will be slightly altered and reproduced in the machined surface. The machining rate affects the surface roughness and hence the tolerance produced. Generally, slow etching will produce a surface finish similar to the original one. Figure 3.7 shows typical surface roughnesses for different materials. The orientation of the areas being etched withrespect to the rolling direction or the direction of the grain in the workpiece is also important for good CHM surfaces. The depth of cut tolerance increases when machining larger depths at high machining rates. Aluminum and magnesium alloys can be controlled more closely than steel, nickel, or titanium alloys. An etching rate of 0.025 mm/mm with tolerances of ±10 percent of the cut width can be achieved depending on the workpiece material and depth of cut. The surface roughness is also influenced by the initial workpiece roughness. It increases as the metal ion concentration rises in the etchant. For low machining depths, <200 μm, the roughness sharply increases with the depth of cut, while at higher depths a slight change in the roughness is evident. Figure 3.7 shows the dependence of the surface roughness and etch rate on the workpiece material. Typically, surface roughnesses of 0.1 to 0.8 μm, depending on the initial roughness, can be obtained. However, under special conditions, roughnesses of 0.025 to 0.05 μm become possible ( Machining Data Handbook , 1997). CHM can affect the mechanical properties of the machined parts when the surface layers have different mechanical properties from those of the base metal. The removal of such layers results in a change in the average mechanical properties of the finished parts. In this regard surface conditions such as a titanium oxide layer (alpha case), decarburized layer, and recast structure are easily removed by CHM, resulting in an improvement in the properties of the finished parts. Some loss of fatigue properties has been reported after CHM of aluminum; however, shot peening or grit blasting can restore it. 5.5 Material removal rate

The material removal or etch rate depends upon the chemical and metallurgicaluniformity of the workpiece and the uniformity of the solutiontemperature. As shown in Figs.and, castings, having thelargest grain size, show the roughest surface together with the lowest


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Figure 3.6 CHM average roughness of some alloys after removing 0.25 to 0.4 mm

Figure 3.7 surface roughnesses and etch rate of some alloys after removing 0.25 to 0.4 mm 5.6 Applications of ECM Process

There are large numbers of applications of ECMs some other related machining and finishing processes as described below: 1. Electrochemical Grinding: This can also be named as electrochemical debrruing. This is used for anodic dissolution of burrs or roughness a surface to make it smooth. Any conducting material can be machined by this process. The quality of finish largely depends on the quality of finish of the tool. 2. This is applied in internal finishing of surgical needles and also for their sharpening. 3. Machining of hard, brittle, heat resistant materials without any problem. 4. Drilling of small and deeper holes with very good quality of internal surface finish.


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5. Machining of cavities and holes of complicated and irregular shapes. 6. It is used for making inclined and blind holes and finishing of conventionally machined surfaces. 5.7 Advantages of ECM Process

Following are the advantages of ECM process: 1. Machining of hard and brittle material is possible with good quality of surface finish and dimensional accuracy. 2. Complex shapes can also be easily machined. 3. There is almost negligible tool wear so cost of tool making is only one time investment for mass production. 4. There is no application of force, no direct contact between tool and work and no application of heat so there is no scope of mechanical and thermal residual stresses in the workpiece. 5. Very close tolerances can be obtained. 5.8 Disadvantages and Limitations of ECM

There are some disadvantages and limitations of ECM process as listed below: 1. All electricity non-conducting materials cannot be machined. 2. Total material and workpiece material should be chemically stable with the electrolyte solution 3. Designing and making tool is difficult but its life is long so recommended only for mass production. 4. Accurate feed rate of tool is required to be maintained.


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10ME665 UNIT – 6

6.1 Introduction

The history of electro discharge machining (EDM) dates back to the daysof World Wars I and II when B. R. and N. I. Lazarenko invented the relaxation circuit (RC). Using a simple servo controller they maintainedthe gap width between the tool and the workpiece, reduced arcing, andmade EDM more profitable. Since 1940, die sinking by EDM has beenrefined using pulse generators, planetary and orbital motion techniques,computer numerical control (CNC), and the adaptive control systems. During the 1960s the extensive research led the progress of EDMwhen numerous problems related to mathematical modeling were tackled.The evolution of wire EDM in the 1970s was due to the powerful generators,new wire tool electrodes, improved machine intelligence, andbetter flushing. Recently, the machining speed has gone up by 20 times,which has decreased machining costs by at least 30 percent and improvedthe surface finish by a factor of 15. EDM has the following advantages: 1. Cavities with thin walls and fine features can be produced. 2. Difficult geometry is possible. 3. The use of EDM is not affected by the hardness of the work material. 4. The process is burr-free. 6.2 Principles of EDM

Electrical Discharge Machining (EDM) is a controlled metal-removal process that is used to remove metal by means of electric spark erosion. In this process an electric spark is used as the cutting tool to cut (erode) the workpiece to produce the finished part to the desired shape. The metal-removal process is performed by applying a pulsating (ON/OFF) electrical charge of highfrequency current through the electrode to the workpiece. This removes (erodes) very tiny pieces of metal from the workpiece at a controlled rate.


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6.3 EDM Process

EDM spark erosion is the same as having an electrical short that burns a small hole in a piece of metal it contacts. With the EDM process both the workpiece material and the electrode material must be conductors of electricity. The EDM process can be used in two different ways: I.

A pre shaped or formed electrode (tool), usually made from graphite or copper, is shaped to the form of the cavity it is to reproduce. The formed electrode is fed vertically down and the reverse shape of the electrode is eroded (burned) into the solid workpiece.

II.

A continuous-travelling vertical-wire electrode, the diameter of a small needle or less, is controlled by the computer to follow a programmed path to erode or cut a narrow slot through the workpiece to produce the required shape.

6.4 Conventional EDM

In the EDM process an electric spark is used to cut the workpiece, which takes the shape opposite to that of the cutting tool or electrode. The electrode and the workpiece are both submerged in a dielectric fluid, which is generally light lubricating oil. A servomechanism maintains a space of about the thickness of a human hair between the electrode and the work, preventing them from contacting each other.


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In EDM ram or sinker machining, a relatively soft graphite or metallic electrode can be used to cut hardened steel, or even carbide. The EDM process produces a cavity slightly larger than the electrode because of the overcut. 6.5 Wire-Cut EDM

The wire-cut EDM is a discharge machine that uses CNC movement to produce the desired contour or shape. It does not require a special shaped electrode, instead it uses a continuoustraveling vertical wire under tension as the electrode. The electrode in wire-cut EDM is about as thick as a small diameter needle whose path is controlled by the machine computer to produce the shape required. 6.6 Dielectric Fluids –

Conventional EDM During the EDM process the workpiece and the electrode are submerged in the dielectric oil, which is an electrical insulator that helps to control the arc discharge. The dielectric oil, that provides a means of flushing, is pumped through the arc gap. This removes suspended particles of workpiece material and electrode from the work cavity. 6.7 Flushing

One of the most important factors in a successful EDM operation is the removal of the metal particles (chips) from the working gap. Flushing these particles out of the gap between the workpiece to prevent them from forming bridges that c ause short circuits. 6.8 Flushing Ram Type EDM

Flushing is the most important function in any electrical discharge machining operation. Flushing is the process of introducing clean filtered dielectric fluid into the spark gap. Flushing applied incorrectly can result in erratic cutting and poor machining conditions. There are a number of flushing methods used to remove the metal particles efficiently while assisting in the machining process. Too much fluid pressure will remove the chips before they can assist in the cutting action, resulting in slower metal removal. Too little pressure will not remove the chips quickly enough and may result in short-circuiting the erosion process


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6.9 Wire EDM Dielectric Fluids

The dielectric fluid must be circulated under constant pressure to flush (wash) away the metal particles and assist in the machining or erosion process. If red sparks occur during the cutting operation, the water supply is inadequate. To overcome this problem, increase the flow of water until blue sparks appear. 6.10 The Servo Mechanism

Both wire and vertical EDM machines are equipped with a servo control mechanism that automatically maintains a constant gap of about the thickness of a human hair between the electrode and the workpiece. It is important for both machine types that there is no physical contact between the electrode and the workpiece, otherwise arcing could damage the workpiece and break the wire. The servomechanism advances the electrode into the workpiece as the operation progresses and senses the work-wire spacing and controls it to maintain the proper arc gap which is essential to a successful machining operation. 6.11 Advantages of EDM

Conventional EDM machines can be programmed for vertical machining, orbital, vectorial, directional, helical, conical, rotational, spin and indexing machining cycles. This versatility gives Electrical Discharge Machines many advantages over conventional machine tools. i. ii.

Any material that is electrically conductive can be cut using the EDM process. Hardened workpieces can be machined eliminating the deformation caused by heat treatment.

iii.

X, Y, and Z axes movements allow for the programming of complex profiles using simple electrodes.

iv. v. vi.

Complex dies sections and molds can be produced accurately, faster, and at lower costs. The EDM process is burr-free. Thin fragile sections such as webs or fins can be easily machined without deforming the part.


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10ME665 UNIT – 7

7.1 Introduction

When the temperature of a gas is raised to about 2000°C, the gas moleculesbecome dissociated into separate atoms. At higher temperatures,30,000°C, these atoms become ionized. The gas in this stage is termed plasma. Machining by plasma was adopted in the early 1950s as an alternativemethod for oxy-gas flame cutting of stainless steel, aluminum,and other nonferrous metals. During that time the process limitationsregarding the low cutting speed, poor machining quality, and the unreliableequipment were clear. Recently machining of both metallic and nonconductivematerials has become much more attractive. An importantfeature of plasma beam machining (PBM) is that it is the only fabricatingmethod that works faster in stainless steel than it does in mild steel. 7.2 Working Principle of PAM

In this process gases are heated and charged to plasma state. Plasma state is the superheated and electrically ionized gases at approximately 5000oC. These gases are directed on the workpiece in the form of high velocity stream. Working principle and process details are shown in below Figure.


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Process Details of PAM

Details of PAM are described below. Plasma Gun

Gases are used to create plasma like, nitrogen, argon, hydrogen or mixture of these gases. The plasma gun consists of a tungsten electrode fitted in the chamber. The electrode is given negative polarity and nozzle of the gun is given positive polarity. Supply of gases is maintained into the gun. A strong arc is established between the two terminals anode and cathode. There is a collision between molecules of gas and electrons of the established arc. As a result of this collision gas molecules get ionized and heat is evolved. This hot and ionized gas called plasma is directed to the workpiece with high velocity. The established arc is controlled by the supply rate of gases. Power Supply and Terminals

Power supply (DC) is used to develop two terminals in the plasma gun. A tungsten electrode is inserted to the gun and made cathode and nozzle of the gun is made anode. Heavy potential difference is applied across the electrodes to develop plasma state of gases. Cooling Mechanism

As we know that hot gases continuously comes out of nozzle so there are chances of its overheating. A water jacket is used to surround the nozzle to avoid its overheating. Tooling

There is no direct visible tool used in PAM. Focused spray of ho0t, plasma state gases works as a cutting tool. Workpiece

Workpiece of different materials can be processed by PAM process. These materials are aluminium, magnesium, stainless steels and carbon and alloy steels. All those material which can be processed by LBM can also be processed by PAM process. 7.3 Material removal rate

During PBM absorbing the heat energy from the plasma jet directed to theworkpiece activates metal removal. The plasma torch blows the molten andevaporated metal away as a fine spray or vapor. The resulting cuttingrates and hence the machinability depend on the workpiece beingmachined as well as the type of the cutting and shielding gases that determinethe maximum DEPARTMENT OF MECHANICAL ENGG, SJBIT

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temperature transfer rates. The maximum machiningspeed, as an index of machinability for dual gas plasma of carbonsteel, stainless steel, and aluminum, is shown in below Figuresshows the power consumption factor needed in plasma beam rough turningof some alloys. A low factor indicates either low energy required orhigh removal rates. The machining speed is found to decrease withincreasing the thickness of the metal or the cut width in case of beveling.As the power is increased, the efficient removal of melted metal isfound to need a corresponding rise in the gas flow rate. During plasmamachining of 12-mm-thick steel plate using 220 kW the machining speedis 2500 mm/min, which is 5 times greater than that for oxy-gas cutting.


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7.4 Accuracy and surface quality

The edges of the workpieces cut by PBM are often beveled. McGeough (1988) reported that the right side of the plasma arc relative to the cutting direction produces a square edge to within ±3°. The left-handedge is beveled to about 15° due to the clockwise swirling of the machininggas. Owing to the high rate of heat transfer the depth of fused metalextends to about 0.18 mm below the cut surface. The high machiningspeed does not allow the heat to penetrate more than a few microns from the edges of the cut which produces little or no distortion in the cut workpiece. The cut edge of the material tends to be harder than the base material. Afurther heat-affected zone (HAZ) of thickness 0.25 to 1.12 mm has been reported. Additionally due to the rapid cooling, cracks may arise beyond the heat-affected zone to 1.6 mm. A clean, smooth surface is produced by PBM. Large tolerances of ±1.6 mm can be achieved. Finish cuts are, therefore, required when narrow tolerances are required.

7.5 Applications of PAM

The chief application of this process is profile cutting as controlling movement of spray focus point is easy in case of PAM process. This is also recommended for smaller machining of difficult to machining materials.

7.6 Advantages of PAM Process

Advantages of PAM are given below: i.

It gives faster production rate.

ii.

Very hard and brittle metals can be machined.

iii.

Small cavities can be machined with good dimensional accuracy.

7.7 Disadvantages of PAM Process

i. ii.

Its initial cost is very high. The process requires over safety precautions which further enhance the initial cost of the setup.

iii.

Some of the workpiece materials are very much prone to metallurgical changes on excessive heating so this fact imposes limitations to this process.

iv.

It is uneconomical for bigger cavities to be machined.


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10ME665 UNIT – 8

8.1 Introduction Laser-beam machining is a thermal material-removal process that utilizes a high-energy, coherent light beam to melt and vaporize particles on the surface of metallic and non-metallic workpieces. Lasers can be used to cut, drill, weld and mark. LBM is particularly suitable for making accurately placed holes. A schematic of laser beam machining is shown in below Figure. Different types of lasers are available for manufacturing operations which are as follows: 

CO2 (pulsed or continuous wave): It is a gas laser that emits light in the infrared region. It can provide up to 25 kW in continuous-wave mode.



Nd:YAG: Neodymium-doped Yttrium-Aluminum-Garnet (Y3Al5O12) laser is a solidstate laser which can deliver light through a fibre-optic cable. It can provide up to 50 kW power in pulsed mode and 1 kW in continuous-wave mode.

8.2 Working Principle of LBM

LBM uses the light energy of a laser beam to remove material by vaporization and ablation. The working principle and the process details (setup) are indicated in Figure 5.6. In this process the energy of coherent light beam is focused optically for predecided longer period of time. The beam is pulsed so that the released energy results in an impulse against the work surface that does melting and evaporation. Here the way of metal removing is same as that of EDM process


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but method of generation of heat is different. The application of heat is very finely focused in case of LBM as compared to EDM. Laser Tube and Lamp Assembly

This is the main part of LBM setup. It consists of a laser tube, a pair of reflectors, one at each end of the tube, a flash tube or lamp, an amplification source, a power supply unit and a cooling system. This whole setup is fitted inside a enclosure, which carries good quality reflecting surfaces inside. In this setup the flash lamp goes to laser tube, that excites the atoms of the inside media, which absorb the radiation of incoming light energy. This enables the light to travel to and fro between two reflecting mirrors. The partial reflecting mirror does not reflect the total light back and apart of it goes out in the form of a coherent stream of monochromatic light. This highly amplified stream of light is focused on the workpiece with the help of converging lense. The converging lense is also the part of this assembly. Workpiece

The range of workpiece material that can be machined by LBM includes high hardness and strength materials like ceramics, glass to softer materials like plastics, rubber wood, etc. A good workpiece material high light energy absorption power, poor reflectivity, poor thermal conductivity, low specific heat, low melting point and low latent heat. Cooling Mechanism

A cooling mechanism circulates coolant in the laser tube assembly to avoid its overheatingin long continuous operation. Tool Feed Mechanism

There is no tool used in the LBM process. Focusing laser beam at a pre-decided point in the workpiece serve the purpose of tool. As the requirement of being focused shifts during the operation, its focus point can also be shifted gradually and accordingly by moving the converging lense in a controlled manner. This movement of the converging lense is the tool feed mechanism in LBM process. 8.3 Applications of LBM

LBM is used to perform different machining operations like drilling, slitting, slotting, scribing operations. It is used for drilling holes of small diameter of the order of 0.025 mm. It is used for very thin stocks. Other applications are listed below:


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Non Traditional Machining i.

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Making complex profiles in thin and hard materials like integrated circuits and printed circuit boards (PCBS).

ii.

Machining of mechanical components of watches.

iii.

Smaller machining of very hard material parts.

8.4 Advantage of laser cutting

1. No limit to cutting path as the laser point can move any path. 2. The process is stress less allowing very fragile materials to be laser cut without any support. 3. Very hard and abrasive material can be cut. 4. Sticky materials are also can be cut by this process. 5. It is a cost effective and flexible process. 6. High accuracy parts can be machined. 7. No cutting lubricants required 8. No tool wear 9. Narrow heat effected zone

8.5 Limitations of laser cutting

1

Uneconomic on high volumes compared to stamping

2

Limitations on thickness due to taper

3

High capital cost

4

High maintenance cost

5

Assist or cover gas required


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Electron Beam Machining 8.6 Introduction

The earliest work of material removal utilizing an electron beam wasattributed to Steigerwald who designed a prototype machine in 1947.Electron beam machining (EBM) has been used in industry since the1960s, initially in nuclear and aerospace welding applications. Drillingsmall holes, cutting, engraving, and heat treatment are a set of modernapplications used in semiconductor manufacturing as well as micromachiningareas. 8.7 Principles, equipment and operation ofElectron Beam Machining

The main components of EBM installation, shown in Fig. 5.43 arehoused in a vacuum chamber, evacuated to about 10 – 4 torr. The tungsten filament cathode is heated to about 2500 to 3000°C in order to emit electrons. A measure of this effect is the emission current, the magnitude of which varies between 20 and 100 mA. Corresponding current densities lie between 5 and 15 A/cm2. Emission current depends on the cathode material, temperature, and the high voltage that is usually about 150 kV. Such a high voltage accelerates a stream of electrons in the direction of the workpiece. After acceleration, electrons, focused by the field, travel through a hole in the anode. The electron beam isthen refocused by a magnetic or electronic lens system so that the beam is directed under control toward the workpiece. The electrons the velocity (228 × 103 km/s) imparted by the acceleration voltage until they strike the workpiece, over a well-defined area, typically 0.25 mm in diameter.


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Mech Vi Non Traditional Machining [10me665] Notes

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