Electric discharge machining EDM 2 3 4 5

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Electric discharge machining (EDM)

Electric discharge machining (EDM)

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Electric discharge machining (EDM) • Started in USSR, 1943 • Controlled erosion • Series

Electric discharge machining (EDM) • Started in USSR, 1943 • Controlled erosion • Series of electric sparks • Discharge takes place between anode and cathode • – Intense heat generated – In sparking zone Melts material • Evaporates material To improve effectiveness – • • Submerged in dielectric fluid • Hydrocarbon • Mineral oils etc Anode erodes faster – Work , positive terminal 8

Cont………. • Suitable gap, spark gap • High frequency • Spark appear at the

Cont………. • Suitable gap, spark gap • High frequency • Spark appear at the spot where work and tool are the closest (as shown in the figure) • Sparks travel all over the surface • – Results uniform material removal – Work conforms to the tool Servo-control unit to control a uniform gap – Sense the voltage across it – Compares with preset value – Difference is used to control the servomotor • Or stepper motor • Solenoid control 9

Mechanics of EDM Tool A B C (-) D Work E (+) 10

Mechanics of EDM Tool A B C (-) D Work E (+) 10

Cont………. • Spark frequency: 200 to 500, 000 Hz • Spark gap: • Peak

Cont………. • Spark frequency: 200 to 500, 000 Hz • Spark gap: • Peak voltage across the gap: 30 V to 250 V • Metal removal rate: 300 mm 3/min with specific power of 10 W/mm 3/min • Efficiency and accuracy: improved with forced circulation of dielectric • Common dielectric: • General tool material: brass or copper alloy 0. 025 mm to 0. 05 mm kerosene 11

Cont………. • Asperities and irregularities are always present on the surfaces • Local gap

Cont………. • Asperities and irregularities are always present on the surfaces • Local gap varies • Say, minimum at C • Suitable voltage builds up • Emission of electron from cathode at C • Electron accelerated towards anode • Collides molecules of dielectric at high velocity • Breaks them into electrons and positive ions • These electrons collides with other • Avalanche of electrons • Seen as spark 12

Cont. . . • A very high temperature rise 10, 000 – 12, 000

Cont. . . • A very high temperature rise 10, 000 – 12, 000 o. C – – – Evaporation Melting Development of Small crater Increase in gap Next location with the shortest gap Cycle is repeated • Material removal rate – More at anode – Less at cathode 13

Reasons for mrr at anode • Momentum of electrons striking anode > momentum of

Reasons for mrr at anode • Momentum of electrons striking anode > momentum of heavier +ve ions striking cathode • Pyrolysis (breaking at high temperature) of dielectric fluid (normally hydrocarbons) creates a thin film on cathode • A compressive force is developed on cathode surface 14

Material removal due to a single discharge • Assumptions: – Spark is circular heat

Material removal due to a single discharge • Assumptions: – Spark is circular heat source, diameter = 2 a – Electrode surface is semi-infinite – The electrode surface is insulated except the portion of the heat source – Rate of heat input is constant through out the duration – Properties of electrode material remains constant – Vaporization of electrodes is negligible 15

a r f heat input, q z Time, t z td (a) Constant rate

a r f heat input, q z Time, t z td (a) Constant rate of heat input (b) Uniform heat flux (c) Circular heat source 16

Cont. . . H = heat input (calories) θ = temperature (o. C) T

Cont. . . H = heat input (calories) θ = temperature (o. C) T = Time (seconds) K = thermal conductivity (cal/cm-sec-o. C) α = thermal diffusivity (cm 2/sec) td = discharge duration (seconds) θm = melting temperature (o. C) There is a circular symmetry: 17

Equation of heat conduction Temperature is maximum at the centre i. e. r =

Equation of heat conduction Temperature is maximum at the centre i. e. r = 0 18

At the end of the spark, the temperature at a point on the axis

At the end of the spark, the temperature at a point on the axis Assuming maximum temperature is at t = td Where ξ is a dummy variable. If at depth Z melting temperature is reached then the equation is: 19

To take care of the heat of molten material, the actual heat input rate

To take care of the heat of molten material, the actual heat input rate can be found out by the heat used to melt from the heat supplied by the spark. Where Htotal = total heat released (cal) Hm = latent heat (cal/g) ρ = density of material (c/cm 2) Where W = total pulse energy (in joules) n 1, n 2, K = constants characterising electrodes and dielectric vc = crater volume if hc = crater depth (cm) 20

Estimation of material removal rate (mrr) • Under normal working conditions And assuming average

Estimation of material removal rate (mrr) • Under normal working conditions And assuming average sparking condition. Where θm = mp (o. C) K 1 ~ 0. 4 for copper electrode K 2 ~ 0. 045 for kerosene used as dielectric 21

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Fig. 6. Effect of pulse on time and Si. C percentage on the GS

Fig. 6. Effect of pulse on time and Si. C percentage on the GS at peak current of 10 Amp. and gap voltage of 30 V. Sameh S. Habib Study of the parameters in electrical discharge machining through response surface methodology approach Applied Mathematical Modelling, Volume 33, Issue 12, 2009, 4397– 4407 http: //dx. doi. org/10. 1016/j. apm. 2009. 03. 021 23

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H = 0. 5 J 2 a = 0. 08 cm 0. 016 Z

H = 0. 5 J 2 a = 0. 08 cm 0. 016 Z (cm) 0. 024 10 -8 0. 032 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 Discharge time td (sec) (a) Variation of melting temperature depth Z with discharge time 25

c(cm 0 -4 H = 0. 5 J 2 a = 0. 08 cm

c(cm 0 -4 H = 0. 5 J 2 a = 0. 08 cm 3) x 103 10 -2 10 -8 10 -1 10 -7 10 -6 1 10 -5 10 -4 10 -3 10 -2 Discharge time td (sec) (b) Variation of crater volume with discharge time 26

Effect of fluid circulation • Material removal rate – Strongly depend upon circulation of

Effect of fluid circulation • Material removal rate – Strongly depend upon circulation of dielectric fluid – Without circulation wear particles • Melt • Reunite with electrode 27

Material removal rate With force d die lectri c circ ulatio n Wi tho

Material removal rate With force d die lectri c circ ulatio n Wi tho ut for ced die lec tric circ ula tio n Advance of electrode Figure. Effect of forced circulation of dielectric fluid 28

After the completion of discharge • Dielectric medium – Should be deionized • By

After the completion of discharge • Dielectric medium – Should be deionized • By keeping the voltage across the gap below – Discharge voltage • Otherwise – The current again starts flowing • The de-ionisation time depends up on – Energy release during the preceding discharge 29

Tool wear • Eroded during spark • Principle tool material – Graphite • Vaporises

Tool wear • Eroded during spark • Principle tool material – Graphite • Vaporises directly • Wear ratio (rmrr) • Ratio of – – Material removed from the work to Material removed from the tool – Related to • Ratio (rθ) – – Melting point of work to Melting point of tool 30

Tool material • Depends up on – – Material removal rate Wear ratio Ease

Tool material • Depends up on – – Material removal rate Wear ratio Ease of shaping the tool Cost • Common electrode material – – – Brass Copper graphite Aluminium alloy Copper-tungsten alloy Silver-tungsten alloy 31

Fabrication of tool • • Conventional machining – Copper – Brass – Cu-W alloys

Fabrication of tool • • Conventional machining – Copper – Brass – Cu-W alloys – Ag-W alloys – Graphite Casting – Zn base die casting alloys – Zn-Sn alloys – Aluminium alloys • Metal spray • Press forming • Flow holes are provided for dielectric circulation – Large for rough cuts to allow large flow at low pressure 32

Dielectric fluids • Basic requirements – Low viscosity – Absence of toxic vapours –

Dielectric fluids • Basic requirements – Low viscosity – Absence of toxic vapours – Chemical neutrality – Absence of inflaming tendency – Low cost 33

Cont. . . . • Ordinary water – Possesses all properties – Causes rusting

Cont. . . . • Ordinary water – Possesses all properties – Causes rusting • Work • machine – Electrodes are constantly under some potential difference • Starts distorting the work • Wastage of power • Hydrocarbon (petroleum) oil • Kerosene • Liquid paraffin • Silicon oils • etc 34

Surface integrity • High temperature – Melting – Vaporization – Shallow layers • 2.

Surface integrity • High temperature – Melting – Vaporization – Shallow layers • 2. 5 – 150 μm – Outmost layer rapidly chilled • Very hard • Below it is somewhat tempered – Better wear resistance – In finishing operation • Less hard 35

Cont. . . – Reduces fatigue resistance • Due to micro-cracks – Due to

Cont. . . – Reduces fatigue resistance • Due to micro-cracks – Due to chilling – Tensile strength is less affected – Chemical structure may get transformed – Reduces erosion resistance 36

Operating principles • Require – Pulsating dc current • Classification of dc supply –

Operating principles • Require – Pulsating dc current • Classification of dc supply – Resistance-capacitance relaxation circuit with a constant dc source – Rotary impulse generator – Controlled pulse circuit 37

Resistance-capacitance relaxation circuit • Used with the first EDM machine • Figure shows a

Resistance-capacitance relaxation circuit • Used with the first EDM machine • Figure shows a simple RC circuit R Vo Tool C Work Figure. Relaxation circuit 38

Vo Instantaneous voltage across gap Vd 0 tc Time, t 2 tc 3 tc

Vo Instantaneous voltage across gap Vd 0 tc Time, t 2 tc 3 tc 4 tc Figure. Variation of gap voltage 39

Cont. . . Voltage, V across the gap varies according to time, t as

Cont. . . Voltage, V across the gap varies according to time, t as : me, t starts at the instant V 0 is applied s the variable capacitance s the variable resistance ence, V approaches V 0 asymptotically, if allowed to do so. ark take place when V = Vd, commonly called discharge voltage pacitor is completely discharged scharge time is ≈ 10% of charging time, tc 40

Sparking frequency, ϑ 41

Sparking frequency, ϑ 41

Average power, Wav Let, E = energy released/ spark And tc = 1/ ϑ,

Average power, Wav Let, E = energy released/ spark And tc = 1/ ϑ, then Where ζ = tc/RC 42

For maximum power delivery Which yields: ζopt = 1. 26 If ζ ( =

For maximum power delivery Which yields: ζopt = 1. 26 If ζ ( = tc/RC) is known Optimum value of (Vd/V 0)opt can be fixed For maximum power, Vd = 72% of V 0 43

Material removal / spark Assuming Material removal per spark is proportional to energy released/spark

Material removal / spark Assuming Material removal per spark is proportional to energy released/spark Thus MRR is inversely proportional to resistance, R 44

Critical value of R • Cannot be decreased below a critical value • Otherwise,

Critical value of R • Cannot be decreased below a critical value • Otherwise, arcing will occur • R depends upon – Inductance, L of the circuit – Rmin = √(L/C) – Discharge circuit is seldom purely inductive – R < 30 √(L/C) • Hence, MRR (mm 3/min) 45

Problem • During an electric discharge drilling of a 10 -mm square hole in

Problem • During an electric discharge drilling of a 10 -mm square hole in a low carbon steel plate of 5 mm thickness, brass tool and kerosene are used. The resistance and the capacitance in the relaxation circuit are 50 Ω and 10 μF respectively. The supply voltage is 200 volts and the gap is maintained at such a value that the discharge (sparking) takes place at 150 volts. Estimate the time required to complete the drilling operation. • Answer: 306 min 46

Rotary impulse generator • Increase metal removal rate with respect to R-C relaxation circuit

Rotary impulse generator • Increase metal removal rate with respect to R-C relaxation circuit • Used for spark generation • Capacitor is charged through diode • Sum of voltage generated + voltage of the charged capacitor is applied • Operating frequency is the frequency of the sine wave – depends upon motor speed • Surface finish is deteriorated 47

Figure. Rotary impulse generator for EDM 48

Figure. Rotary impulse generator for EDM 48

Controlled pulse circuit • Automatic prevention of current flow when short circuit is developed

Controlled pulse circuit • Automatic prevention of current flow when short circuit is developed – No provision of prevention in • R-C relaxation circuit • Rotary impulse generator • A transmitter is used as switching device • Current during sparking comes through capacitor • Transmitter cut off and behaves as infinite resistance 49

Figure. Controlled pulse circuit for EDM 50

Figure. Controlled pulse circuit for EDM 50

Figure. Controlled pulse circuit with capacitor for EDM 51

Figure. Controlled pulse circuit with capacitor for EDM 51

Thanks 52

Thanks 52