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Electric Discharge Machining Electric Discharge Machining Electric Discharge Machining Electric Discharge Machining

(EDM)(EDM)(EDM)(EDM)

BYBYBYBY---- MOHIT BHATIAMOHIT BHATIAMOHIT BHATIAMOHIT BHATIABYBYBYBY---- MOHIT BHATIAMOHIT BHATIAMOHIT BHATIAMOHIT BHATIA

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MAE, 8MAE, 8MAE, 8MAE, 8thththth SEMESTERSEMESTERSEMESTERSEMESTER

19-Sep-16Dr. Ravinder Kumar

Introduction � Electro Discharge Machining (EDM) is an electro-thermalnon-traditional machining process, where electrical energy isused to generate electrical spark and material removal mainlyoccurs due to thermal energy of the spark.

� EDM is mainly used to machine difficult-to-machinematerials and high strength temperature resistant alloys.EDM can be used to machine difficult geometries in smallbatches or even on job-shop basis. Work material to bemachined by EDM has to be electrically conductive.


Characteristics of EDM

� The process can be used to machine any work material if it iselectrically conductive

� Material removal depends on mainly thermal properties ofthe work material rather than its strength, hardness etc

In EDM there is a physical tool and geometry of the tool is� In EDM there is a physical tool and geometry of the tool isthe positive impression of the hole or geometric featuremachined

� The tool has to be electrically conductive as well. The toolwear once again depends on the thermal properties of thetool material


Characteristics of EDM

� Though the local temperature rise is rather high, still due tovery small pulse on time, there is not enough time for theheat to diffuse and thus almost no increase in bulktemperature takes place. Thus the heat affected zone islimited to 2 – 4 µm of the spark crater

� However rapid heating and cooling and local hightemperature leads to surface hardening which may bedesirable in some applications

� Though there is a possibility of taper cut and overcut inEDM, they can be controlled and compensated.


Equipment

� Dielectric reservoir, pump and circulation system

� Power generator and control unit

� Working tank with work � Working tank with work holding device

� X-y table accommodating the working table

� The tool holder

� The servo system to feed the tool


Power generator

� Resistance-capacitance type (RC type) Relaxation generator

� Rotary impulse type generator

� Electronic pulse generator

Hybrid EDM generator � Hybrid EDM generator


Schematic representation of the basic working

principle of EDM process.


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Cavities made with electricalCavities made with electricalCavities made with electricalCavities made with electrical----dischargedischargedischargedischarge----machining processmachining processmachining processmachining process

� (a) Examples of cavities produced by the electrical-discharge-machining process, using shapedelectrodes. The two round parts (rear) are the set of dies for extruding the aluminum piece shown infront. (b) A spiral cavity produced by a rotating electrode. (c) Holes in a fuel-injection nozzle madeby electrical-discharge machining. Material: Heat-treated steel.


Stepped CavitiesStepped CavitiesStepped CavitiesStepped Cavities

� Stepped cavities produced with a square electrode by EDM. The work piece moves in the two principal horizontal directions, and its motion is synchronized with the downward movement of the electrode to produce various cavities. Also shown is a round electrode capable of producing round or elliptical cavities.


Mechanics Of EDM

� In EDM, a potential difference is applied between the tool and work-piece. Both the tool and the work material are to be conductors ofelectricity.

� The tool and the work material are immersed in a dielectric medium.

� A gap is maintained between the tool and the work-piece. Known asspark gap. Spark occurs at the spot where the tool &W.P. surfaces arethe closest & since the spot changes after each spark.the closest & since the spot changes after each spark.

� To maintain the spark gap a servo-control unit is used . The gap issensed through the average voltage across it & this voltage is comparedwith preset value .The difference is used to control the servomotor.

� Depending upon the applied potential difference and the gap betweenthe tool and work-piece, an electric field would be established.


Mechanics Of EDM

� Generally the tool is connected to the negative terminal of thegenerator and the work-piece is connected to positive terminal.

� Due to electric field established between the tool and the job, thefree electrons on the tool are subjected to electrostatic forces.electrons would be emitted from the tool (assuming it to beconnected to the negative terminal). Such emission of electronsare called or termed as cold emission.are called or termed as cold emission.

� The “cold emitted” electrons are then accelerated towards the jobthrough the dielectric medium. As they gain velocity and energy,and start moving towards the job, there would be collisionsbetween the electrons and dielectric molecules.


Mechanics Of EDM

� Collision may result in ionisation of the dielectric molecule.As the electrons get accelerated, more positive ions andelectrons would get generated due to collisions. This cyclicprocess would increase the concentration of electrons andions in the dielectric medium between the tool and the job atthe spark gap.

The concentration would be so high that the matter existing� The concentration would be so high that the matter existingin that channel could be characterized as “plasma”.

� The electrical resistance of such plasma channel would bevery less. Thus all of a sudden, a large number of electronswill flow from the tool to the job and ions from the job tothe tool.This is called avalanche motion of electrons.


EDM Video


YouTube - EDM - Electrical Discharge Machining.flv

Mechanics Of EDM

� Such movement of electrons and ions can be visually seen as a spark.Thus the electrical energy is dissipated as the thermal energy of thespark.

� The high speed electrons then impinge on the job and ions on the tool.The kinetic energy of the electrons and ions on impact with thesurface of the job and tool respectively would be converted intothermal energy or heat flux. Such intense localised heat flux leads toextreme instantaneous confined rise in temperature which would be inextreme instantaneous confined rise in temperature which would be inexcess of 10,000oC.

� Such localised extreme rise in temperature leads to material removal.Material removal occurs due to instant vapourisation of the material aswell as due to melting. The molten metal is not removed completelybut only partially.

� The material removal in EDM mainly occurs due to formation ofshock waves as the plasma channel collapse owing to discontinuationof applied potential difference. 19-Sep-16Dr. Ravinder Kumar

Dielectric

� In EDM material removal occurs due to thermal evaporation andmelting.

� Thermal processing is required to be carried out in absence ofoxygen so that the process can be controlled and oxidationavoided.

� Oxidation often leads to poor surface conductivity (electrical) ofthe work-piece hindering further machining. Hence, dielectricthe work-piece hindering further machining. Hence, dielectricfluid should provide an oxygen free machining environment.

� It should have enough strong dielectric resistance so that it doesnot breakdown electrically too easily but at the same time ionizewhen electrons collide with its molecule.

� During sparking it should be thermally resistant as well.


Dielectric

� Generally kerosene and deionized water is used as dielectricfluid in EDM.

� Tap water cannot be used as it ionizes too early and thusbreakdown due to presence of salts as impurities occur.

� Dielectric medium is generally flushed around the spark zone. Itis also applied through the tool to achieve efficient removal ofis also applied through the tool to achieve efficient removal ofmolten material.

� Mainly used dielectric mediums are Hydrocarbon(petroleumoil), Kerosene, liquid paraffin & silicon oil etc.

� It should have –low viscosity, absence of toxic vapour, chemicalneutrality , absence of inflaming tendency , low cost.


� The fluid serves the following three purposes:

1.Flushes material away

2. Serves as a coolant to minimize the heat affected zone (thereby preventing potential damage to the work piece)

3.Acts as a conductor for the current to pass between the electrode and the work piece.


Electrode Material Properties� Electrode material should be such that it would not undergo muchtool wear when it is impinged by positive ions.

� High electrical conductivity – electrons are cold emitted moreeasily and there is less bulk electrical heating

� High thermal conductivity – for the same heat load, the localtemperature rise would be less due to faster heat conducted to thebulk of the tool and thus less tool wearbulk of the tool and thus less tool wear

� Higher density – for the same heat load and same tool wear byweight there would be less volume removal or tool wear and thusless dimensional loss or inaccuracy

� High melting point – high melting point leads to less tool wear dueto less tool material melting for the same heat load

� Easy manufacturability & Cost – cheap19-Sep-16Dr. Ravinder Kumar

Electrode Material

The followings are the different electrode materials which areused commonly in the industry:

� Graphite

� Electrolytic oxygen free copper

� Tellurium copper – 99% Cu + 0.5% tellurium� Tellurium copper – 99% Cu + 0.5% tellurium

� Brass

� Copper-tungsten alloys

� Silver-tungsten alloys

� Al-alloys


EDM characteristics Summary

� Mechanics of metal removal: Melting and evaporation aided by cavitations' .

� Medium: Dielectric fluid

� Tool Materials: Cu, brass, Cu-W alloy, Ag-W alloy, graphite.

� Material removal rate: 5×10-³mm³/min.

� Gap between tool & W.P.: 10-125µm.� Gap between tool & W.P.: 10-125µm.

� Specific power consumption: 1.8W/mm³/min

� Critical parameters: voltage , capacitance , spark gap, dielectric circulation , melting temp.

� Material application: all conducting metals and alloys.

� Shape application: blind complex cavities , micro holes for nozzles , through cutting of non-circular holes , narrow slots.


Summary

� Limitations :high energy consumption, when forcedcirculation of dielectric is not possible , removal rate is quitelow, surface tends to be rough for larger removal rates ; notapplicable to non-conducting material.


TOOLS OF EDMTOOLS OF EDMTOOLS OF EDMTOOLS OF EDM

� Power supplies� Pulsed DC� 10 to 1000Amps

� Electrodes� High electrical conductivity� Ease of fabrication� Resistance to wear (high melting point)Copper, Graphite and Brass most common� Copper, Graphite and Brass most common

� Dielectric� Spark conductor� Coolant� Flushing medium� Petroleum Oil Base (EDM) & Ionized Water (EWDM) most popular

� Computer controlled servo


Wire EDM� In wire EDM a very thin wire serves as the electrode.Special brass wires are typically used; the wire is slowly fedthrough the material and the electrical discharges actuallycut the work piece. Wire EDM is usually performed in abath of water.

� The wire itself does not actually touch the metal to be cut;the electrical discharges actually remove small amounts ofmaterial and allow the wire to be moved through the work-piece.


Wire EDM Process

� FIGURE 9.35 Schematic illustration of the wire EDM process. As much as 50 hours of machining can be performed with one reel of wire, which is then discarded. 19-Sep-16Dr. Ravinder Kumar

Wire EDM

Fundamentals of wire EDM


To better understand the

wire EDM process,

visualize the wire EDM

machine as a super

precision band saw with

accuracies capable up

Super precision band saw

accuracies capable up

to +/-0.001’’

(0.0025mm), and under

certain circumstances

even closer.


The step by step Wire Cut EDM process

Deionized water surrounds the wire electrodes as the power supply generates

voltages and amps to produce the spark


The generated spark precisely melts and vaporizes the material


During the off cycle, the pressurized dielectric fluid immediately cools the

material and flushes out the eroded particles


New wire is constantly fed, while the eroded particles are removed and separated

by a filter system


EDM Machine


APPLICATIONSAPPLICATIONSAPPLICATIONSAPPLICATIONS

� Only conductive materials can be machined� Hardness/Toughness of material is not a concern� Popular in dies and molds� Well suited for machining difficult or intricate parts� Longer machining time limits use to prototyping or limited � Longer machining time limits use to prototyping or limited run parts


THANK YOU


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