Class notes on EDM_Dr.Deepak.pdf

25-03-2015

1

ByDr.Deepak Lawrence.KMechanical Engineering DepartmentNIT Calict,Kerala,India

Electrical Discharge Machining( EDM)& Wire EDM

ME6324:Modern Machining Processes

Electrical Discharge Machining

Wire EDM Die sinking EDM Hole drilling EDM

2

25-03-2015

2

Electrical Discharge MachiningElectrical Discharge Machining (EDM) is theprocess of machining electrically conductivematerials by using precisely controlled sparksthat occur between an electrode and aworkpiece in the presence of a dielectric fluid

Close-up view of EDM machining

3

Electrical Discharge MachiningNo longer is EDM a "non-conventional"machining method.It is claimed that EDM is now the fourth mostpopular machining method. The first three aremilling, turning, and grinding.Three basic EDM processes:wire EDM,ram EDM, andsmall hole EDM drilling

4

25-03-2015

3

Basic EDM system.

5

Ram EDM

Ram EDM, also known as conventional EDM, sinker EDM, diesinker, vertical EDM, and plunge EDM is generally used toproduce blind cavities as shown in Figure.In ram EDM sparks jump from the electrode to the workpiece.This causes material to be removed from the work piece bymelting and vaporization.

6

25-03-2015

4

Wire EDM

In wire EDM, the spark jumps from the wire electrode tothe workpiece and erodes metal both from the wireelectrode and the workpiece. Wire EDM is used primarilyfor through hole machining as shown in Figure

7

Hole drilling EDMSmall hole EDM drilling, also known as fast hole EDM drilling, hole popperuses a hollow electrode to drill holes by means of electrical dischargemachining by eroding material from the workpieceHole drilling EDM uses low cost electrode tube (normally brass or coppermaterial) to drill holes on a electrically conductive material at a very highspeed, the hole depth diameter ratio can reach up to 200.The hole diameter is normally from 0.3mm to 3.0mm, with five axisconfiguration the machine can drill hole at any angles on a inclined surfaceworkpiece.This technology is widely used for hole machining in aerospace, energy,cutting tools, automotive, medical, mold and die industries.

8

25-03-2015

5

Electrical Discharge MachiningElectrical Discharge Machining (EDM) is theprocess of machining electrically conductivematerials by using precisely controlled sparksthat occur between an electrode and aworkpiece in the presence of a dielectric fluid

Close-up view of EDM machining

9

EDM-fundamentalsEDM can be used to machine conductive materials of any hardness(for example steel or titanium) to an accuracy of up to one-thousandthof a millimeter with no mechanical action.

EDM removes material by a series of rapidly recurring electric arcingdischarges between an electrode (the cutting tool) and the workpiece, in the presence of an energetic field.

The EDM cutting tool is guided along the desired path very close tothe work piece but it does not touch the piece.

Consecutive sparks produce a series of micro-craters on the workpiece and remove material along the cutting path by melting andvaporization.

By virtue of these properties, EDM is one of the key technologies inmold and tool making.

10

25-03-2015

6

EDM-fundamentalsEDM differs from most chip-making machiningoperations in that the electrode does not makephysical contact with the workpiece for materialremoval.Since the electrode does not contact theworkpiece, EDM has no tool force. The electrodemust always be spaced away from the workpiece bythe distance required for sparking, known as thesparking gap.Should the electrode contact the workpiece,sparking will cease and no material will be removed

11

EDM-fundamentals

Another basic fundamental of the process is that only one spark occurs at anyinstant. Sparking occurs in a frequency range from 2,000 to 500,000 sparks persecond causing it to appear that many sparks are occurring simultaneously.In normal EDM, the sparks move from one point on the electrode to another assparking takes placeThe spark removes material from both the electrode and workpiece, which increases thedistance between the electrode and the workpiece at that point. This causes the nextspark to occur at the next-closest points between the electrode and workpiece

12

25-03-2015

7

EDM-fundamentals

EDM is a thermal process; material is removed by heat.Heat is introduced by the flow of electricity between theelectrode and workpiece in the form of a spark.Material at the closest points between the electrode andworkpiece, where the spark originates and terminates,are heated to the point where the material vaporizes

13

EDM-fundamentalsWhile the electrode and workpiece should neverfeel more than warm to the touch during EDM,the area where each spark occurs is very hot.The area heated by each spark is very small sothe dielectric fluid quickly cools the vaporizedmaterial and the electrode and workpiecesurfaces.However, it is possible for metallurgical changesto occur from the spark heating the workpiecesurface.

14

25-03-2015

8

Dielectric fluidA dielectric material is required to maintain the sparkinggap between the electrode and workpiece.This dielectric material is normally a fluid.Die-sinker type EDM machines usually use hydrocarbonoil, while wire-cut EDM machines normally use deionizedwater.The main characteristic of dielectric fluid is that it is anelectrical insulator until enough electrical voltage isapplied to cause it to change into an electrical conductorWhen the spark is turned off, the dielectric fluiddeionizes and the fluid returns to being an electricalinsulator.

15

Dielectric fluidDielectric fluid used in EDM machines providesimportant functions in the EDM process.These are:controlling the sparking-gap spacing betweenthe electrode and workpiece;cooling the heated material to form the EDMchip; andremoving EDM chips from the sparking area.

16

25-03-2015

9

EDM-fundamentals

Spark occurs within a column of ionized dielectric fluid.

17

EDM-fundamentalsAs each spark occurs, a small amount of the electrodeand workpiece material is vaporized.The vaporized material is positioned in the spark-ing gapbetween the electrode and workpiece in what can bedescribed as a cloud.When the spark is turned off, the vaporized cloudsolidifies.Each spark then produces an EDM chip or a very tinyhollow sphere of material made up of the electrode andworkpiece material.For efficient machining, the EDM chip must be removedfrom the sparking area. Removal of this chip isaccomplished by flowing dielectric fluid through thesparking gap.

18

25-03-2015

10

EDM-fundamentals

Spark-OFFSpark-ON

19

EDM-fundamentals

Spark-OFF: vaporized cloud solidifies to formEDM chip. 20

25-03-2015

11

EDM FINISHThe finish that is created by the EDM process is theresult of tiny craters being formed by the randomimpacts of thousands of sparks.After each cycle the rim of the created crater formsa new high point, making it a likely target for a newcycle.The EDM spark generated chips is of the size 2monlyDue to this situation overlapping micro-craters areformed, accounting for the random nature of theElectrical Discharge Machined surface.This Surface Finish(SF) is one of the attractivefeatures of EDM in many industries.

21

EDM Spark generatorEDM machines use different kinds of sparksdepending on the electronic circuitry provided.Sparking is normally produced by one of twotypes of EDM-power supplies: resistor-capacitor power supply, and pulse-power supply

22

25-03-2015

12

EDM -principles

23

RAM EDM

24

25-03-2015

13

Basic EDM system.

In a basic, ram -type EDM system , the ram head is driven up and down withextreme accuracy by a servo-driven system. The servo system is controlled bya microprocessor connected to the power supplyThe power supply is solid-state and is also microprocessor controlled. Onelead from the power supply is connected to the work piece , which isimmersed in a tank of dielectric oil.The dielectric tank is connected to a dielectric pump, an oil reservoir, and afilter system.The pump provides pressure for flushing the work area and moving the oilwhile the filter system removes and traps debris in the oil.The oil reservoir stores surplus oil and provides a container for draining theoil between operations.

25

Basic EDM system.

26

25-03-2015

14

Basic EDM system.

The other lead from the power supply is connected to the electrode. The power supplyprovides a pulsed DC output to the electrode /work piece system. On-times and off-times are set manually, along with voltage and current values.

When the EDMmachine is turned on, the servo microprocessor , sensing that the gapis too wide for cutting to take place, signals the servo mechanism to lower ram head

When the first spark jumps the gap, downward travel of the ram head stops. With thegap setting held constant, the process gradually erodes the surface.

When enough metal has been removed to change the gap distance, themicroprocessor senses this and signals the servo mechanism to advance the ramhead sufficiently to maintain the proper gap width and the process continues.

27

Basic EDM circuit

Lazarenko resistor-capacitor (R-C) EDM circuit28

25-03-2015

15

EDM

29

Spark-ON/OFF time for EDM RC circuit

Spark-ON/OFF time is determined using the followingT = R x Cwhere:T = time (seconds)R = resistance (ohms)C = capacitance (farads)

30

25-03-2015

16

EDM RC circuit

Variation of voltage with time using an RC generator

31

Pulse-power-supply waveform-Principle

32

25-03-2015

17

Pulse-power-supply waveform

ON and OFF times determine spark frequency 33

Basic transistor sparking circuit for EDM

When an electronic signal from the switch control turns the transistor ON and OFF, itcauses the transistor to act like an electronic switch that can be opened or closed.During spark-ON time, the transistor is closed to let electricity flow from the DC-powersource to the electrode, across the sparking gap to the workpiece, and then back tothe DC-power source.During the OFF time, the transistor is open, stopping the flow of electricity. Spark-ONand -OFF times are set by the EDM-power-supply controls, either manually or, if a CNC-controlled machine, by computer program. 34

25-03-2015

18

EDM-MechanismApplication of voltage pulses causes electricalbreakdown to the dielectric in a channel.The breakdown arises from the accelerationtoward the anode of both electrons emittedfrom the cathode by the applied field and thestray electrons present in the gap.These electrons collide with neutral atoms ofthe dielectric, thereby creating positive ions andfurther electrons, which in turn are acceleratedrespectively toward the cathode and anode.

35

EDM-Mechanism

When the electrons and the positive ions reach the anode and cathode,theygive up their kinetic energy in the form of heat.Temperatures of about 8000 to 12,000C and heat fluxes up to 1017 W/m2are attained.With a very short duration spark of typically between 0.1 to 2000 s thetemperature of the electrodes can be raised locally to more than their normalmelting points.Owing to the evaporation of the dielectric, the pressure on the plasmachannel rises rapidly to values as high as 200 atmospheres. Such greatpressures prevent the evaporation of the superheated metal.At the end of the pulse, the pressure drops suddenly and the superheatedmetal evaporates explosively. Metal is thus removed from the work . 36

25-03-2015

19

EDM-Mechanism

A charged electrode is brought near the work piece. Between them is an insulating oil, known in EDMas dielectric fluid

37

EDM-Mechanism

As the number of ionic (charged) particles increases, theinsulating properties of the dielectric fluid begin to decreasealong a narrow channel centered in the strongest part of thefield. Voltage has reached its peak, but current is still zero.38

25-03-2015

20

EDM-Mechanism

A current is established as the fluid becomes lessof an insulator . Voltage begins to decrease.39

EDM-Mechanism

Heat builds up rapidly as current increases and thevoltage continues to drop. The heat vaporizes some ofthe fluid, work piece , and electrode , and a dischargechannel begins to form between the electrode and workpiece40

25-03-2015

21

EDM-Mechanism

A vapor bubble tries to expand outward, but its expansion is limited by a rush of ionstowards the discharge channel l. These ions are attracted by the extremely intenseelectromagnetic field that has built up. Current continues to rise, voltage drops.41

EDM-Mechanism

Near the end of the Spark-ON time, current and voltage have stabilized, heat andpressure within the vapor bubble have reached their maximum, and some metal isbeing removed. The layer of metal directly under the discharge column is in a moltenstate, but is held in place by the pressure of the vapor bubble. The discharge channelnow consists of a superheated plasma made up of vaporized metal, dielectric oil, andcarbon with an intense current passing through it.42

25-03-2015

22

EDM-Mechanism

At the beginning of the off-time, current andvoltage drop to zero. The temperature decreasesrapidly, collapsing the vapor bubble and causingthe molten metal to be expelled from the workpiece43

EDM-Mechanism

Fresh dielectric fluid rushes in, flushing the debrisaway and quenching the surface of the work piece.Un expelled molten metal solidifies to form what isknown as the recast layer

44

25-03-2015

23

EDM-Mechanism

The expelled metal solidifies into tiny spheres dispersed in thedielectric oil along with bits of carbon from the electrode.Theremaining vapor rises to the surface. Without a sufficient off-time, debris would collect making the spark unstable. Thissituation could create a DC arc which can damage theelectrode and the work piece 45

CONTROLLING THE SPARK

Each cycle has an ON time and an off-time that are expressed in units ofmicroseconds (figure).Since all the work is done during ON time, the duration of these pulses and thenumber of cycles per second (frequency) are important.

46

25-03-2015

24

CONTROLLING THE SPARK

Once the open gap voltage creates an ionizationpath for current flow, the voltage decreases to theworking gap voltage . Current increases until itreaches peak current during the effective ON timeportion of the cycle.47

CONTROLLING THE SPARK

Metal removal is directly proportional to the amount of energy applied duringthe ON timeThis energy is controlled by the peak amperage and the length of the ONtimeThe longer the ON time pulse is sustained the more work piece material willbe melted away.The resulting crater will be broader and deeper than a crater produced by ashorter ON time. These large craters will create a rougher surface finish .Extended on-times also allow more heat to sink into the work piece andspread, which means the recast layer will be larger and the heat affectedzone will be deeper.

48

25-03-2015

25

AmperesThe amperes available for most EDM-power supplies(pulse wave form based) range from1400 A.EDM-power-supply output is rated in amperes. Ampereoutput indicates the material-removal capability of theunit.The ampere output indicates the material-removalcapabilities of the power supply.As amperes increase, material-removal rates alsoincrease and the surface finish becomes coarserThere are two types of amperes:1. peak amperesdetermined by the amplitude of theamperes as shown by the square-wave diagram and2. average amperesdetermined by peak amperes, withconsideration for the spark-ON and -OFF time.

49

DUTY CYCLERarely are spark-ON and -OFF times equal in actualEDM operationsTo determine average machining amperes,calculate the ratio of spark-ON to -OFF timeDuty cycle = ON/(ON + OFF)where:ON = spark-ON time in microsecondsOFF = spark-OFF time in microsecondsIa = Ip duty cyclewhere:Ia = average amperesIp = peak amperes

50

25-03-2015

26

DUTY CYCLE

average-ampere output based on a short dutycycle, with a peak output of 100 A

51

Average ampere output based on long duty cycle

52

25-03-2015

27

FrequencyFrequency is the number of cycles produced across thegap in one second. The higher the frequency , the finerthe surface finish that can be obtained. As the number ofcycles per second increases, the length of the ON timedecreasesShort on-times remove very little metal and createsmaller craters. This produces a smoother finish with lessthermal damage to the W/P.Frequency (in kiloHertz) is calculated by dividing 1000 bythe cycle time (on-time + off-time) in microseconds (s),

53

EDM Spark energySpark energy is determined by the amount ofelectrical power contained in each spark,multiplied by the amount of time the electricalpower is flowing.spark energy in EDM is

54

25-03-2015

28

Effect of duty cycle and frequency on surfacefinish

55

Electronic-switch ON/OFF and R-C-power suppliesIn comparing the two types of power supplies, it is importantto know that both electronic-switch ON/OFF and R-C-powersupplies are used in die-sinker- and wire-cut-machiningapplications.The electronic switch ON/OFF-power supply produces thegreatest efficiency for most EDM applications and it is themost commonly used type. The electronic-switch, ON/OFF-power supply has the capability of machining with eithergraphite or metallic electrodes at higher amperes.In general, the R-C-power supply is primarily considered forapplications that require a fine surface finish or for thedrilling of small, precise micro-holes.R-C-power supplies have an approximate 15A maximum limitfor machining. They are also used primarily with metallicelectrodes. R-C-power supplies, therefore, work well forapplications that require lower ampere-sparking output.Creating fine surfaces or drilling small precision holes aretypical applications.

56

25-03-2015

29

Profile of a single EDM pulse,

a, ionization time; b, discharge time;c, deionization time; d, rest time.

Most of the electrode wear occursduring the ionization time.

Three-dimensional schematic of the EDMspark. The spark energy available for materialremoval is proportional to the product ofeffective on-time, current, and spark voltage.

57

SparkEach sparking occurrence between the electrode and workpiece isdetermined by the strength of the dielectric fluid.Dielectric strength for a typical hydrocarbon-oil fluid is 170 volts per mil. A mil is equal to .001 in. (0.025mm)During the electrode advance time, 170 V is applied between theelectrode and the work piece.This voltage is called open-circuit voltage, since there is no electricityflowing between the electrode and the work piece.With the voltage equal to 170 V and the spacing equal to .001 in.(0.025 mm), the dielectric fluid ionizes and changes from anelectrical insulator into an electrical conductor.Electricity flows between the electrode and the work piece throughthe ionized dielectric fluid.

58

25-03-2015

30

Spark

59

SparkWhen the power supply is turned ON, but theelectrode is not close enough to the work piecefor sparking to occur, the voltmeter will indicateopen-circuit voltage.Voltage indicated during sparking is themachining voltage.Open-circuit voltage may be in a normal range

of 100300 V. Machining voltage is normally in arange of 2050 V.

60

25-03-2015

31

Dielectric strengthdielectric strength is based on a voltage and adimension; dielectric strength for dielectric fluid isexpressed as a particular number of volts per mil(dimension); the dielectric fluid changes from an electricalinsulator into an electrical conductor when thevoltage and dimension equal the fluids dielectric-strength rating; the point at which the dielectricfluid changes from an electrical insulator into anelectrical conductor is called the ionization point

61

Spark -basics

62

25-03-2015

32

Spark -basics

63

Spark -basics

64

25-03-2015

33

Spark -basics

65

Spark -basics

Dielectric-fluid atomic charge, without electrode-to-workpiece voltage

66

25-03-2015

34

Spark -basics

67

Spark -basics

68

25-03-2015

35

Spark -basics

69

Spark -basics

Positive vapor cloud attracted to negative electrode.70

25-03-2015

36

Spark -basics

Spark OFF, vapor cloud cools to form chip.71

Spark -basics

72

25-03-2015

37

Spark -basics

73

OVERCUT

Overcut is the gap distance between the electrode and the workpiecesmachined surface produced by sparking.. Overcut is expressed as a per-side dimension.This per-side dimension must be taken into consideration when designing theelectrode for die-sinker machines and the programmed path for wire-cutmachines.

74

25-03-2015

38

Overcut

75

Overcut

The width of the EDM cavity is always larger than the electrode andthe difference is called the overcut (see figure).The overcut gets larger as amperage and on-times are increased.These two parameters directly affect the size of the overcut and theroughness of the finish .The amount of overcut has to be known in order to properly undersizethe electrode . Most equipment manufacturers supply accurateovercut information. 76

25-03-2015

39

OvercutOvercut is determined by the dielectric strength ofthe dielectric fluid.Dielectric strength is specified as a voltage and adimension at which the dielectric fluid changesfrom an electrical insulator to an electricalconductor.A typical hydrocarbon oil may have a dielectricstrength of 200 V per mil (.001 in. or 0.025 mm).EDM machines often use an open circuit voltage of100 V.The spark-length distance for a machine using anopen-circuit voltage of 100 V may be calculated forthis particular dielectric fluid using a ratio formula

77

Overcut

78

25-03-2015

40

Overcut information

79

Ionization and Electrode Wear

The spark is the electricity flowing through the ionized column of dielectric fluid.

Within the ionized column, electrons separate from the dielectric-fluid atoms and flowfrom the negative-polarity electrode toward the positive-polarity work piece.

Since the dielectric-fluid atoms in the column are missing electrons, they arepositively charged and flow from the positive-polarity work piece toward the negativepolarity electrode.These positively charged atoms are known as positive ions.Within the column then, there are electrons flowing in one direction and positive ionsflowing in the other direction. 80

25-03-2015

41

Ionization and Electrode Wear

The nature of electricity as it flows through thecolumn 81

Ionization and Electrode WearThe weight of a positive ion, consisting of theatom nucleus and remaining electrons, isthousands of times greater than the weight ofan electron.Since the positive ion has such a heavy weight,it accelerates much slower than the electron.Fewer positive ions than electrons arrive at thebombardment surface during sparking, which iswhy electrons are considered the primarysource of energy for EDM material removal.

82

25-03-2015

42

Ionization and Electrode Wear

Most of the electrode wear occurs during the ionization time.83

EDM chip

Vapor clouds combine and cool to form EDM chip.84

25-03-2015

43

PolarityElectrical polarity of the electrode andworkpiece determines the direction of flow forelectrons and positive ions.Some EDM manufacturers describe electrodeand workpiece polarity as standard and reverse.This description is not acceptable since not allmanufacturers use the same polarity forstandard and reverse.It is now a practice to say the polarity ofelectrode ! (work is of opposite polarity)

85

PolarityIt is possible to use either negative or positive electrodepolarity for machining with most die-sinker machinesEDM researchers have determined that positiveelectrode polarity is useful for reducing electrode wear orproviding more stable servo operation when usingcertain electrode and workpiece materials.Positive electrode polarity usually removes workpiecematerial at a lower rate than negative electrode polarity.However, positive electrode polarity reduces wear ofcopper and graphite electrodes when settingsrecommended by the machine manufacturer are used forspark control.

86

25-03-2015

44

Polarity

Polarity refers to an electrical condition determining thedirection of the current flow relative to the electrode .The polarity of the electrode can be either positive ornegative. Depending on the application, some electrode/work metal combinations give better results when thepolarity is changed.Generally with graphite , a positive electrode givesbetter electrode wear characteristics and a negativeelectrode gives better speed(high MRR).

87

Polarity for various electrode-work combinations

88

25-03-2015

45

ELECTRODE WEARElectrode wear is a result of either electron orpositive-ion bombardment.When the electrode is negative, it is bombarded bypositive ions. When the electrode is positive, it isbombarded by electrons.As electrons or positive ions crash into the surfaceof the electrode, heat is generated.The heat vaporizes the electrode material and asmall amount of electrode material is removed witheach spark.This removal of material is electrode wear.

89

Electrode wear

Electrode wear is specified in one of four ways,

1. corner wear,2. end wear,3. side wear, and4. volumetric wear. 90

25-03-2015

46

CORNER WEAR

The number of sparks originating from a point on the electrode surface determines electrode wearElectromagnetic fields tend to concentrate at the electrode corners (figure ), subjecting the corners togreater wear.Many sparks must originate from the electrode corner to produce the machined shape in theworkpiece.By comparison, each spark on the flat surface of the electrode machines a corresponding point on theworkpiece. Since each spark removes material from the electrode, more material is removed from thecorner than the flat surface, causing electrode wear to be greater on the corner.

Illustrates the number of sparks required toproduce a flat surface, as compared to theamount required to produce a 90 corner.

91

CORNER WEARCorner wear is the difference between theoriginal electrode length and the point on theelectrode corner that still retains the originalcorner shape.Corner wear is the standard for determining thelength of the electrode or the number ofelectrodes required to complete the work pieceshape in die-sinking operations

92

25-03-2015

47

CORNER WEAR

Electrode with shape extending over full length 93

END WEAR

End wear is the difference between the original electrode length and the electrodelength after machining.For the illustration, the work piece has a hole, pre-drilled before EDM, which is usedfor dielectric-fluid flow to remove EDM chips.As the electrode machines the work piece, there are no sparks between the end ofthe electrode and the work piece in the area of the pre-drilled hole. The electrode endremains the original length of the electrodeAfter the EDM operation is completed, the electrodes end wear is noted bymeasuring the cylindrical extension of the electrode material that has passed throughthe pre-drilled hole.

94

25-03-2015

48

SIDE WEARSide wear is the comparison between theoriginal electrode length and the side surface ofthe electrode that shows the full electrodeshape after the machining operation iscomplete.Side wear is the wear used as a reference oncircular electrodes, since corner wear is not aconsideration.

95

VOLUMETRIC WEARVolumetric wear is the comparison of theelectrodes total volume prior to EDM, to theelectrodes volume upon completion of machining.There are instances when this type of wear is usedto compare the volume of electrode consumed tothe volume of workpiece machined.EDM-research engineers often use volumetric wearfor studying and analyzing the EDM process.Seldom is it used for actual EDM operations inindustry

96

25-03-2015

49

Electrode wear

Terms used in expressing wear of EDMelectrodes 97

Corner wear ratios for different electrode materials

98

25-03-2015

50

EDM-Electrodes

99

ELECTRODE MATERIALelectrode materials must be electrically conductiveShould have high melting point,an ability to be easily machined, anda low costNo single electrode material provides all of the desiredfeatures for any particular applicationIn the following discussion of electrode materials, costcomparisons are made by assigning copper a value of 1Electrode material costs are usually less than fabricationcosts and EDM machine time. Thus, the cheapestmaterial does not necessarily result in the lowest overallcost.

100

25-03-2015

51

ELECTRODE MATERIALSThee material should possess the essentialqualities of good metal removal,low wear, andthe ability to be accurately machined andfabricated at low cost.The five commonly used electrode materials arecopper, brass, zinc, tungsten, and Graphite.These materials can fall into two maincategories: metallics and graphite

101

GraphiteGraphite is the most commonly used electrode material because ofgood machinability and EDM wear characteristics.Small, flush holes are easy to drill. It is available in a great variety ofsize and form. Commercially available EDM grades range in grain sizesranging from 100 microns for a coarse grade, down to 1 micron forfine-grade material.the cost factor, compared to copper, was 1.3 to 24, with most gradesfalling between 2.6 and 10. A drawback of graphite is that it is dirty tomachine;Graphite has very good wear qualities. Although it is very machinable,graphite dust must be considered when machining the material.Graphite does not melt, but rather sublimes, meaning it goes from asolid directly into a gas, without melting and going through a liquidstate.Graphite may not be recommended for machining tungsten carbide.It may not be recommended for use with R-C-power-supply operations.

102

25-03-2015

52

Graphite Electrode

Graphites are divided into the following six classes according to theirparticlesize:Angstrofine Ultrafine 1-5 micronsSuperfine 6-10 micronsFine 11-20 micronsMedium 21-100 micronsCoarse>100 micronsGraphites in the Coarse classification are not suitable for EDM purposes.

Making- Graphite Electrode

103

Graphite Electrode

104

25-03-2015

53

Graphite Electrode

Isotropic structure.

Anisotropic structure.105

Potential electrodes for EDM

Only graphite and tungsten composites remainsolid at temperatures remotely near gap conditions.Graphite does not change to a liquid when heated,but sublimes.

106

25-03-2015

54

COPPERIt often is used for R-C-power-supply operationsCopper has good EDM wear, good conductivity, and iseconomical.Copper is difficult to grind but has good no-wear-machiningcharacteristics.It does not machine as well as brass or graphite.Nonetheless, it is used almost as much as graphite, and isespecially good for machining tungsten carbide.Copper is preferred for finishes better than 0.5m RaCopper is readily available and normally specified aselectrolytic grade or tellurium-copper alloy. .Tellurium copper is copper with the element tellurium addedand it is equivalent in machinability to free-machining brass.It is only be slightly more expensive than copper (costfactor, 1.2), and it is nearly as machinable as brass.

107

BRASSBrass is readily available. The grade used isnormally specified as free-machining brass.is inexpensive and easy to machineIt has a fairly good wear ratio when machiningsteel, and a very high wear ratio when machiningtungsten carbide.Brass is not normally recommended for use with R-C-power supplies.It is often used for tubular electrodes in specialized

small-hole EDM drilling machines where high wear isacceptable.

108

25-03-2015

55

Tungsten and Tungsten CompositesTheoretically tungsten is the best of the metallicsfor use as an electrode .With its very high strength, density, hardness, anda melting point near 3400C, tungsten resists thedamaging effects of the EDM process very wellindeed.There are two main problems associated with usingpure tungsten as an electrode material.It is very difficult to machine and extremelyexpensive, which limits its usefulness as anelectrode material.Tungsten is used to make small holes (

25-03-2015

56

Electrodes of EDMCOPPER GRAPHITECopper graphite is fine-grain graphite that is infiltrated withcopper.It has the qualities of graphite, plus the electrical conductivityof copper.It is 1.5 to 2 times more expensive than the same graphitewithout copper, thus making it from 5 to 20 times moreexpensive than copperThe flexural strength is higher than the comparable grade ofgraphite, making it good for thin cross-section electrodes.Electrical conductivity is greatly improved, but corner wear isnot as good as it is for the same grade of pure graphite.This material works well on tungsten carbide.ZINC ALLOYSZinc alloys may be used as an electrode material, but thewear characteristics are very poor.

111

Claim by leading commercial EDM electorde OEM

USA 90 %Europe-75 %Asia-55 %

112

25-03-2015

57

EDM system components

113

Servo mechanism-EDM

When the EDM machine is turned on, the servo microprocessor , sensing thatthe gap is too wide for cutting to take place, signals the servo mechanism tolower ram head

When the first spark jumps the gap, downward travel of the ram head stops.With the gap setting held constant, the process gradually erodes the surface.

When enough metal has been removed to change the gap distance, themicroprocessor senses this and signals the servo mechanism to advance theram head sufficiently to maintain the proper gap width and the processcontinues.

114

25-03-2015

58

Servo mechanism-EDMIf the ram were allowed to move forward unchecked,there would be direct contact between electrode andworkpiece, causing an electrical short circuit.This is prevented by a servo mechanism in which thepotential is monitored and compared with a reference.If the potential is greater than the reference, the ramadvances; if it is less, the ram retracts. The movementmay be accomplished by a hydraulic cylinder or a direct-drive servomotor.As the work is machined by spark erosion, the distancebetween electrode and workpiece increases. Thepotential goes up, and the ram advances until thepotential matches the reference. Thus, the servomechanism maintains a constant gap.Erosion continues until a preset depth is reached. At thispoint, the electrode is retracted from the workpiece.

115

EDM-servo systemEDM machines require an automatic system for proper spacing of anelectrode from the workpiece.

This maintains efficient sparking. Such a system must be versatileenough to work with electrodes as small as (0.025 mm) in diameter,to very large electrodes that weigh several Kg.

This automatic operation is accomplished by the EDM-servo system.

Requirements for an EDM-servo system are:

the electrode must not touch the workpiece, and the electrode must advance toward and retract from the workpieceto maintain the voltage between the electrode and workpiece.

116

25-03-2015

59

SPARKING VOLTAGE

EDM-servo systems make use of the electrical characteristics of thedielectric fluid for their operationThe dielectric fluid during EDM changes from an insulator into anelectrical conductor, causing the voltage between the electrode andworkpiece to drop from the open circuit to sparking voltage. Thisnormally occurs in a range of 2050 VDC.

117

SERVO-REFERENCE VOLTAGE

As the machining-voltage range is constant for a particular dielectricfluid, a voltage in this range is selected as a reference for controllingthe servo systemThis reference voltage is compared to the actual machining voltagemeasured between the electrode and workpiece.The difference between the reference voltage and the actualelectrode-to-workpiece machining voltage is that the difference in thevoltages is used to command the electrode-servo system to advance,hold position, or retract from the workpiece. 118

25-03-2015

60

Metal and graphite servo-reference-voltage range

119

Electrical connections for sparking power andservo sensing.

The servo system is controlled by wires that connect the electrodeand work piece to the electronic assembly of the power supply.These sensing wires allow the electrode-to-workpiece voltage to becompared to the servo controls reference voltage

120

25-03-2015

61

Two servo systems in EDMThe system used primarily depends on the size andweight of the electrodes.Large machines using heavy electrodes normally use thehydraulic servo and those that use smaller electrodesgenerally use the electric motor- servo drive.Electric-motor-drive systems are preferred for wire-cutmachines because positioning- and data-feedbackdevices are readily availableMechanically, the electric-motor and hydraulic-drivesystems are simple structures. In both instances, theelectronic control commands an action to be taken.The mechanical drive assembly advances, holds position,or retracts the electrode.

121

Basic electric-servo system.

The electric motor is directly coupled here to a precision lead screw.The lead-screw nut is attached to the machine axis of movement.Any rotational movement of the motor will produce a correspondingmovement in the machine axis and electrode

122

25-03-2015

62

Basic hydraulic servo system.

123

Basic hydraulic servo system

When fluid pressure is applied to one side of the hydraulic-cylinder piston, fluid entersthat side of the cylinder and exits the opposite side.The piston and piston rod move in response to the fluid entering the cylinder.The fluid flowing to and from the hydraulic cylinder is controlled by the servovalve,which is electronically controlled by the servo-control unit in the power-supplycabinet. 124

25-03-2015

63

EDM Surface topography

When inspecting an EDM surface finish, the lack of surfacedirectionality needs to be considered.The surface resembles a cratered surface with all craters thesame size (Fig. ).There is no "lay" or directionality to the surface as inconventional machining. Because the crater size depends onspark energy, and spark energy varies widely, the EDMsurface finish surface can range from 0.2 to 12.5 m

125

Each discharge creates a crater

An AFM view

126

25-03-2015

64

EDM Surface-Metallurgical and Chemical Effects

Because of the rapid quenching by the dielectricand the heat sink effect of the workpiece, thesurface layer affected by the EDM process is quitethin--less than 0.1 mm for roughing settings and0.01 mm for finish settings

127

WORKPIECE METAL - Layers formed by EDM process.

The EDM process changes not only the surface of the work metal, butalso the subsurface.Three layers (total thickness may be 0.05-0.1 mm mm only) arecreated on top of the unaffected work metal (figure).The spattered surface layerThe recast (white) layer (2 to 50 micrometer)heat affected zone(25 micrometer)

128

25-03-2015

65

Layers formed by EDM process.

The spattered EDM surface layer is created when expelled molten metal and smallamounts of electrode material form spheres and spatter the surface of the workmetal.This spattered material is easily removed by polishining.The next layer is the recast (white) layer. The action of EDMing has actually alteredthe workmetal's metallurgical structure and characteristics in the recast layer.This layer is formed by the unexpelled molten metal solidifying in the crater. Themolten metal is rapidly quenched by the dielectric. Microcracks can form in this veryhard, brittle layer.If this layer is too thick or is not reduced or removed by polishing, the effects of thislayer can cause premature failure of the part in some applications. Microcracks occurin the recast layer and can act as initiation points for failure(reduced fatigue strength)The recast layer is characterized by a rapidly quenched structure. The structure isusually brittle and extremely hard(65 HRC) It may be porous and contain micro cracks.This can be removed by abrasive methods or shot peening operations

129

Layers formed by EDM process-heat affected zone

The last layer is the heat affected zone which has only been heated,not melted. Heating, cooling and diffused material are responsible forthis zone. HAZ may contain thermal residual stress and grainboundary cracks.The depth of the recast layer and the heat affected zone is determinedby the heat sinking ability of the material and the power used for thecut. This altered metal zone influences the quality of the surfaceintegrityThe recast layer is characterized by a rapidly quenched structure,while the heat-affected zone has an annealed or tempered structure.

130

25-03-2015

66

Schematic diagram of layers on an EDMdcomponent

131

A typical specification forEDM application could include the following items

total thickness of the re-deposited and re-solidified layers:0.025 mm; total thickness of the re-deposited, re-solidified, andchanged material characteristic layers: 0.05 mm); electrode material present in re-deposited layer: none; cracks in re-deposited and re-solidified layers: acceptable(depending on the end use); and cracks in parent material: not acceptable.

In many instances the EDM surface finish may be used without additionalfinishing operations, such as in plastic-injection molds or in press tooling.But for structural components, it is a common practice to do polishing (abrasivemethods) to remove recast layer. Shot peening is also used to remove a small partof the recast layer and to improve the fatigue properties.

132

25-03-2015

67

Process capability of EDMAccuracy-0.025 to 0.1mm (Best possible:2.5m)Taper in deep holes: 0.005 to 0.05mm/cmAspect ratio:100:1 (special care of flushing)Crater diameter: 5 to 12 m and craterdepth:0.25 to 1.25 mSurface finish (usual case):08 to 3.5m (may goup to 12 microns)Best Finish using ideal CNC EDMsetting:0.18 to0.25m

133

MRR and Finish for EDM

134

25-03-2015

68

Dielectric FluidsThe dielectric fluid performs several functions:It is a spark conductor that must ionize underan applied voltage.It is a coolant for work and electrode.cooling for the vaporized material that becomesthe EDM chip upon solidification;It is a flushing medium that carries away theEDM-spark debris (EDM chips) resulting fromthe process.

135

Desirable properties of dielectriclow viscosity,High flash point,generation of stable spark (spark gap control byacceptable dielectric strength)low cost.Because of small working gaps at finish spark settings, a low-viscositydielectric is especially important.Low viscosity also helps in settling of the swarf, thus keeping thedielectric fluid clean.The most common dielectric fluid is petroleum-base oil (hydrocarbon oils dedicated for EDMprocess) . Also used are kerosene, silicone oils, andwater-base dielectrics.

136

25-03-2015

69

Dielectric FluidsSpark-ON time determines how long sparkelectricity will flow after the ionization point isreached. When spark electricity is turned OFF,electricity stops flowing.The spark is then extinguished and the dielectricfluid is once again an insulator. This characteristicis most important, since the dielectric-fluid-ionization point controls each spark.These changes, from insulator, to conductor, toinsulator, take place for each spark. It is requiredfor this action to occur as often as 500,000 timesper second (500 kHz).

137

Dielectric FluidsDeionized water has some desirablecharacteristics

fire safety,low cost,low viscosity, andabsence of carbon to react with the work.

138

25-03-2015

70

Dielectric FluidsPetroleum products are often referred to ashydrocarbon fluids, since they break down intohydrogen, carbon, and other by-productswhen they are heated during sparking.Deionized water has the impurities removed thatwould make it electrically conductive. The heat ofsparking breaks down this water into hydrogen andoxygen.Usually, die-sinker machines use hydrocarbon fluidsas dielectric fluids, and wire-cut machines usedeionized water.

139

Deionized waterDeionized water absorbs materials that makethe water electrically conductive during thesparking process.As water absorbs materials, the dielectriccharacteristics of the water change.This also changes the waters ionization pointand it affects the reliability and repeatabilityof the sparking processThus deionized water is not an acceptabledielectric fluid for Die sinking EDM process

140

25-03-2015

71

Deionized water-Wire EDMIn most instances, wire-cut-machining operations are notperformed with the workpiece submerged.

Instead, a high-velocity flow of fresh deionized watersurrounds the electrode and covers the workpiece in thesparking area.

It then returns immediately to the collection system forreprocessing.

This process ensures that the deionized water passingthrough the sparking area will stay within the acceptablerange of electrical characteristics required for preciseEDM operations.

141

HYDROCARBON FLUIDSHydrocarbon fluids maintain their dielectriccharacteristics during the sparking processwhen sparking heat breaks the fluid down,and the machining process adds debris.This electrical integrity under such conditionsmakes hydrocarbon fluids the dielectric fluid ofchoice for submerged machining (RAM EDM).

142

25-03-2015

72

RESULTS OF CHIPS REMAINING IN SPARKINGGAP

If chips are not removed from the sparking gap, the spark electricity is forcedto pass through the chips on the workpiece surface.As it does so, the electricity re-machines the chips into smaller ones, whichrequires spark energy and reduces the size of chips being removed from theworkpiece surface.This smaller than normal amount of workpiece material being removedcreates inefficient EDM operations.As chips are free to move about the workpiece surface, variances are causedin the electrode- to-workpiece voltage that cause unstable servo operations.

143

Flushing -INSIDE THE CAVITY

Flushing - Flowing dielectric through the gap to removethe debris caused by machining with EDM.As the cut progresses through the work metal a cavitystarts to form. The deeper this cavity becomes, theharder it is for fresh dieletric fluid to get into the cavityto remove debris and quench the work piece andelectrode .In order to get smooth, even flow of dielectric throughthe gap flushing becomes an essential part of the EDMprocess. 144

25-03-2015

73

Pressure flow through the electrode andworkpiece

145

Flushing- Fluid-pressure flow through electrode

146

25-03-2015

74

Workpiece pressure flow for chip removal.

147

Flushing -INSIDE THE CAVITY

Good flushing allows the work piece particles and eroded electrode particles to beremoved from the gap. Flushing also allows fresh dielectric into the gap.

It is the volume of oil moving through the gap that performs particle removal.

Flushing at higher pressure may actually prevent the flow of particles out of the gapand the dielectric renewal in the gap. High pressure also tends to wear the electrode.The ideal pressure is usually between 0.2 to 0.3 bar .

The balance of volume and pressure is important. Roughing operations where the gapis large would require high volume and low pressure for good oil flow. Finishingoperations where the gap is smaller may necessitate higher pressure to improve theoil flow.

148

25-03-2015

75

Various methods for dielectric flushing

(a) suction through electrode, (b) suction through workpiece, (c)pressure throughelectrode, (d) pressure through workpiece, (e) jetflushing, (f) periodic cycling of electrode 149

Flushing-general rulesThere are three rules for good EDM ing(1)Flush ! (2)Flush !! and (3) Flush !!!Changing from very poor to very good flushing conditionscan improve efficiency and thus reduce machining timeby a factor of six (6) !

Through-the-tool flushing is preferred to side flushingMany small flush holes are preferable to a few largeones. Besides giving better fluid distribution, smaller andmore-easily-removed spikes (the column of metal leftfrom a flush hole) resultA steady flow of dielectric fluid over the entire electrode-workpiece interface is desirable

150

25-03-2015

76

MAKING EDM WORKThe EDMer's job is to control the machiningparameters and predict the results.Varying the on-time and/or off time will changethe duty cycle and the frequency .These changes plus varying the peak currentwill affect the metal removal, electrode wearand finish.

151

Effect of pulse current (energy)

Effect of pulse current (energy) on removal rate andsurface roughness.

152

25-03-2015

77

Effect of pulse on-time (energy)

Effect of pulse on-time (energy) on removal rate and surface roughness.

153

Basic die-sinker-EDM machine.

154

25-03-2015

78

Die-sinker major assemblies.

155

C-frame, die-sinker machine.

156

25-03-2015

79

Bridge-style, die-sinker machine

157

EDM applications

158

25-03-2015

80

Materials That Can Be EDMedAny material that conducts electricity can beEDMed, either hard or soft.

EDM is particularly used to cut Stellite, Inconel, Hastelloy, Nitralloy, Waspaloy,Nimonic, Udimet, tool steels, tungsten carbide and titanium alloys.

159

EDM applicationsAlthough the application of electrical dischargemachining is limited to the machining ofelectrically conductive workpiece materials, theprocess has the capability of cutting thesematerials regardless of their hardness ortoughness.Nonconductors such as glass, ceramics, or plasticscannot be machined using EDM techniques, butthe machining of hardened steel using EDMeliminates the need for subsequent heat treatmentwith possible distortion.

160

25-03-2015

81

EDM applicationsThe EDM process is most widely used by themold-making and tool and die industries, but itis increasingly being applied to make prototypeand production parts, especially in theaerospace and electronics industries, in whichproduction requirements are relatively low.Stamping ,extruding, heading, drawing, forging,and die casting dies, as well as molds forplastics, can be done with EDM techniques.

161

EDM applicationsElectrical discharge machining is particularly well suitedfor parts that are made from materials that are difficultto machine and/or contain small or odd-shaped holes, alarge number of holes, or holes having shallow entranceangles, intricate cavities, or intricate contours.Miniature parts and parts made from material too thin orfragile to withstand conventional, mechanical cuttingforces are also good applications.Round or irregular-shape holes as small as 0.05 mm (indiameter can be produced with length-to-diameter ratiosof about 20:1.Narrow slots as small as 0.05 to 0.30 mm wide are cutroutinely.

162

25-03-2015

82

Electric Discharge Machining-Applications

163

Electric Discharge Machining-Applications

WHY?NO CONTACTNO FORCENO DEFORMATION

Wire + Sinker EXAMPLES?- SURGICAL TOOLS- SATELLITE COMPONENTS- INERTIAL GUIDANCE- MICROWAVE HORNS- HONEYCOMB

This satellite structural component waswirecut from solid CAL-4V titanium

WHEN?VERY THIN WALLS

164

25-03-2015

83

Electric Discharge Machining-Applications

WHEN?RECESSED CUTSKEYWAYSBOTTLING INDUSTRY 165

Electric Discharge Machining-Applications

166

25-03-2015

84

Electric Discharge Machining-Applications

WHEN?FRAIL/FRAGILE CANT TAKE STRESS OF MACHININGWATCH PARTSLEAD FRAME DIEPRINTER HAMMER

167

Electric Discharge Machining-Applications

Medical applicationsDENTAL FIXTURES-MEDICAL CLAWS

168

25-03-2015

85

Electric Discharge Machining-Applications

169

Wire electrical discharge machining (Wire EDM)

170

25-03-2015

86

Wire EDM schematic.

Wire EDM schematic.171

Wire EDMed components

172

25-03-2015

87

Wire electric discharge machining (Wire EDM)In wire electric discharge machining (wire EDM), awire (about 0.05-0.30 mm diameter) is used as anelectrode and deionized water as dielectric.A nozzle is employed to inject the dielectirc in themachining area in wire EDM.Electrodes (wire and workpiece) are connected to apulsed DC supply.Heat generated due to sparking results in themelting of workpiece and wire material, andsometimes part of the material may even vaporizelike in conventional EDM.

173

Wire electric discharge machining (Wire EDM)A constant gap between tool (wire) and workpiece is maintained with the help of a computercontrolled positioning system.This system is used to cut through complicatedcontours specially in difficult-to-machinematerials.This process gives a high degree of accuracyand a good surface finish.

174

25-03-2015

88

Open-style wire-cut machine

175

wire EDM machine

176

25-03-2015

89

Enclosed-style wire-cut machine.

The enclosed wire-cut machine is a self-contained module, designed toprotect electronic /electrical components from exposure to dielectric fluid.Access to the work area and wire-feed unit is gained through sealed doors. 177

Wire-cut machine major assemblies

178

25-03-2015

90

wire electric discharge machining

Schematic illustration of wire electric dischargemachining

179

Wire-cut machine major assembliesThere are four basic elements of this machinetool,the power supply system,the dielectric system,the positioning system,and the wire drive system.

180

25-03-2015

91

Power Supply SystemEDWC machines are equipped only with pulsegenerators, where peak current and on-time are themajor variables controlling spark energy.Wire EDM power supply uses pulse frequency whichis about 1 MHz.It results in reduced crater size or better surfacefinish.The wire has a limited current capacity, so that thecurrent rating rarely exceeds 30 A. The potentialdifference between the wire electrode and the WPis usually set between 50 and 60 V.However, because of very small wire size, it usuallycannot carry current more than 20A.

181

Dielectric SystemWater is a likely substitute for hydrocarbon oilsas dielectric in EDM.It is an attractive proposition because of itsavailability, desirable thermal properties, lowviscosity and pollution-free working.It gives higher MRR and better surface finishunder the identical machining conditions.Deionized water has low viscosity, no firehazard, high cooling rate and high MRR. That iswhy water is used as dielectric in most of thewire EDM systems.

182

25-03-2015

92

Dielectric-fluid and Filtration Unit

The wire-cut dielectric system must filter the sparking by-productsfrom the water after it returns to the storage reservoir.Additives are sometimes used as antirust compounds or ethyleneglycol-based compounds to make the dielectric slippery.In addition to the filtration, the water must be processed to removeany dissolved materials before it is usable as an EDM dielectric.

183

Deionized-water dielectric assembly.

Used water from the machine tool is returned to the dielectric-unfiltered reservoir.A partition in the dielectric tank separates the unfiltered water from the filtered anddeionized water.Water is pumped from the unfiltered reservoir through the filter to remove the solidEDM debris.The water is then pumped through the resin tank and into the filtered and deionizedwater reservoir.The filtered and deionized storage tank includes a sensor to monitor the electricalconductivity of the deionized water 184

25-03-2015

93

Positioning SystemUsually positioning system is a computerizednumerical control (CNC) two-axes table.However, it operates in an adaptive controlmode so that in case wire approaches very nearto the workpiece, or the gap is bridged by debrisand causes a short circuit, the positioningsystem should be capable to sense it.Instantaneously, it should move back to re-establish proper cutting conditions in the gap

185

multi-axis, wire-cut servo system.

186

25-03-2015

94

Multi-axis, wire-cut servo system

The axes are identified as X axis, Y axis, U axis, V axis, and Z axis.In operation, the X and U axes are parallel in the direction ofoperation, the Y and V axes are parallel in their operation, while the Zaxis is perpendicular to the X-U and Y-V axes.The U and V axes offset the electrode wire from the vertical position.Z-axis operation may be manually operated or computer controlled.

187

Moving-workpiece-and-electrode wire-cut design.

188

25-03-2015

95

Wire Drive System

This system serves two purposes, viz continuously delivers fresh wire, and alwayskeeps the wire under appropriate tension so that it moves in the machining zone as astraight wire.The latter requirement is important from the point of view of quality of the machinedsurface. For example, it helps to minimize taper, streaks as well as vibration marks.It also minimizes the wire breaks during machining.On the way while moving to the machining zone, wire is guided by sapphire ordiamond wire guides .As it moves towards the take up spool, the wire passes through a series of tensioningrollers 189

Wire Drive System

The tensioned wire is used only once, traveling from a take-off spool toa take-up spool while being guided to provide an accurate narrow kerf.

190

25-03-2015

96

Wire Drive System

Large diameter (0.15-0.30 mm) wires, used in wire EDM, are made of copper or brass while smalldiameter wires are usually made of molybdenum steel.

The most widely used wire is brass wire. It has most of the qualities needed for wire EDM, that is, hightensile strength, high electrical conductivity, and good wiredrawing ability to close tolerances.

Layered wires are also recommended, but are more expensive; however, they cut faster than brass.

One example is steel/copper/graphite wire, with a steel core for tensile strength, a copper layer forelectrical conductivity, and graphite on the surface for attaining high machining speeds. Zinc-coatedbrass, with molybdnum-core, is also available

Wire is discarded after it has been used once because the sparking takes place at its leading surface,hence, it no longer remains round

Wire diameters range from 5 to 300 m. It travels at a constant velocity ranging from 0.2 to 9 m/min.

Stratified wire used in wire EDM.

191

Cutting SpeedIn EDWC, the cutting speed is generally given interms of cross-sectional area cut per unit time.

Typical examples are 18,000 mm2/h for 50 mmthick tool steel and 45,000 mm2/h for 150 mmthick aluminum block.

This rate indicates a linear cutting speed of 6mm/min and 5 mm/min, respectively.

192

25-03-2015

97

PROCESS CHARACTERISTICSThis process produces accurate matte finish. Thousands oftiny craters on the machined surface help in retaining thelubricating oil and result in increased die life.Surface finishes range from to 0.2 Im to 1.25 m Ra Surfacefinish of the order of 0.1m can be achieved in finish passNormal accuracy is about 0.013 mmSpecial measures such as m0ultiple passes and precisetemperature control are used for a higher accuracy of 0.005 mm

Work thickness capacity of 150 mm is average with somemachines capable of up to 420 mmWith todays systems, machining rate for definite materialshas gone up from 12.50 cm2/hr to about 40 cm2/hr.

193

Flushing and Dielectrics.Good flushing is as important in wire EDM as invertical EDM.Nozzles should be as close as possible to thework.Workpieces with large variations in thicknessprevent this and are especially troublesome.

194

25-03-2015

98

Overcut

As in EDM, an overcut exists in wire EDM thatmakes the kerf larger than the wire diameterThis overcut is in the range 0.020 to 0.050mmOnce cutting conditions have been established fora given cut, the overcut remains fairly constant andpredictable

195

Typical products cut by EDWC

The special features of wireEDMmake it ideal for making components forstamping ing dies, tools for lathes, electrodes for vertical EDM, broaches,and extrusion diesBecause the kerf is so narrow, it is often possible to fabricate punch and diein a single cutWire EDM also has many applications in metallurgy, such as the removal ofcore samples from castings to determine variation in chemistry; thesectioning of welds for metallography, and the making of mechanicalproperty specimensOther tools and parts with intricate outline shapes, such as lathe form tools,extrusion dies, and flat templates, are made with electric discharge wirecutting.

. (From AGIE Charmilles Group, Charmilles)

196

25-03-2015

99

Thank You.

197