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  2. Introduction  Electrical discharge machining (EDM) is an electro-thermal process.  It is one of the most extensively used material removal processes.  Its unique feature is to use thermal energy to machine electrically conductive parts regardless of hardness.  With no direct contact between the tool and workpiece, EDM supports smooth machining.  The main principle of EDM is to erode a material through the effect of electric discharge (spark). EDM Ref:
  3. History  The origin of EDM dates back to 1770 when English scientist Joseph Priestly discovered the erosive effect of electrical discharges.  Several attempts were made to achieve a controlled EDM process.  But, These processes were not very precise due to overheating of the machining area.  Pioneering work on EDM was carried out in 1943 by two Russian scientists, B.R. and N.I. Lazarenko at the Moscow University.  They developed a controlled process for machining materials by vaporizing and melting its surface.
  4. History (Cont…)  The RC (resistance–capacitance) relaxation circuit was introduced in 1950s.  Later, it served as the model for successive developments in EDM technology.  It provided the first consistent dependable control of pulse times. RC circuit Ref:
  5. Main Components in EDM  Workpiece and tool  Electrical power supply  Dielectric  Servo control unit  Fixture Ref: Schematic of EDM process
  6. Working Principle  A potential difference is applied between the tool and workpiece.  Depending upon the applied potential difference and the gap between the tool and workpiece, an electric field would be established.  As the electric field is established between the tool and the job, the free electrons on the tool are subjected to electrostatic forces.  If the bonding energy of the electrons is less, electrons would be emitted from the tool (cold emission).  They start moving towards the job and there would be collisions between the electrons and dielectric molecules (ionization). Ref:
  7. Working Principle (Cont…)  This cyclic process would increase the concentration of electrons and ions in the dielectric medium between the tool and the job at the spark gap.  Concentration would be so high that the matter existing in that channel could be characterized as “plasma”.  Electrical resistance of such plasma channel would be very less.  Thus all of a sudden, a large number of electrons will flow from the tool to the job and ions from the job to the tool.  This is called avalanche motion of electrons.  High speed electrons then impinge on the job and ions on the tool. Plasma channel Ref:
  8. Working Principle (Cont…)  Kinetic energy of the electrons and ions on impact with the surface of the job and tool respectively would be converted into thermal energy or heat flux.  Localized extreme rise in temperature leads to material removal.  Material removal occurs due to instant vaporization of the material as well as due to melting.  As the potential difference is withdrawn, the plasma channel is no longer sustained.  As the plasma channel collapse, it generates pressure or shock waves, which evacuates the molten material forming a crater of removed material around the site of the spark. Spark initiation Ref:
  9. EDM Process Parameters Discharge Voltage  Discharge voltage in EDM is related to the spark gap and break down strength of the dielectric.  Before current can flow, the open gap voltage increases until it has created an ionization path through the dielectric.  Once the current starts to flow voltage drops and stabilizes at the working gap level.  The preset voltage determines the width of the spark gap between the leading edge of the electrode and workpiece. Ref:
  10. EDM Process Parameters  It is most important machining parameter in EDM.  Higher current is directly related to the surface area of the cut.  It is used in roughing operations and cavities.  Higher current improves MRR, but it leads to poor surface finish and tool wear.  New improved electrode materials, especially graphite, can work on high currents without much damage. Peak Current Ref:
  11. EDM Process Parameters Pulse Duration and Pulse Interval  Pulse on-time is commonly referred to as pulse duration and pulse off-time is called pulse interval.  With longer pulse duration, more workpiece material will be melted away.  Metal removal is directly proportional to the amount of energy applied during the on-time.  Pulse interval will affect the speed and stability of the cut.  In theory, the shorter the interval, the faster will be the machining operation.  But if the interval is too short, the ejected workpiece material will not be swept away by the flow of the dielectric and the fluid will not be deionized.  This will cause the next spark to be unstable. Ref:
  12. EDM Process Parameters Polarity  The polarity of the electrode can be either positive or negative.  The plasma channel is composed of ion and electron flows.  As the electron processes (mass smaller than anions) show quicker reaction, the anode material is worn out predominantly. Ref:
  13. EDM Process Parameters Electrode Gap  The tool servo-mechanism is of considerable importance in the efficient working of EDM.  Its function is to control responsively the working gap to the set value.  The gap stability and the reaction speed of the system is important for good performance.  Gap width is not measurable directly, but can be inferred from the average gap voltage. Ref:
  14. Properties of Tool Materials A tool material need to have these properties:  High electrical conductivity  High thermal conductivity  Higher density  High melting point  Easy manufacturability  Graphite is the most commonly used material.  Other materials are brass, tellurium copper, electrolytic oxygen free copper etc.
  15. EDM Surface Layers  There is a top white layer which crystallizes from the liquid cooled at high speed.  The depth of this top melted zone depends on the pulse energy and duration.  Below the top layer there is a chemically affected layer with changes in the average chemical composition and phase changes.  There after, a plastically deformed zone is observed with strains. Surface layers after electrical discharge machining Ref:
  16. Surface Modifications  When mild steel is eroded in liquid medium paraffin using copper electrode, the workpiece is coated with a very hard layer.  Using Titanium powder compact electrodes with carbon steel results increase in hardness of steel.  Powder-mixed dielectric is used to facilitate ignition process and get good surface finish.  Some powder that is usually used in doping are Ni, Co, Ti, Cr etc.  Abrasive powders such as silicon carbide and alumina are mixed in the dielectric to improve the material removal rate.
  17. Product Quality Issues  surface roughness in EDM would increase with increase in spark energy.  Surface finish can be improved by decreasing working voltage, working current and pulse on time.  Taper cut can be prevented by suitable insulation of the tool.  Overcut cannot be prevented as it is inherent to the EDM process.  But the tool design can be done in such a way so that same gets compensated. Ref:
  18. Conclusions  EDM is a viable machining option of producing highly complex parts.  Extremely hard materials can also be machined to very close clearances by this.  Very small workpieces can be machined where conventional tools may damage the part from excess cutting tool pressure.  There is no direct contact between tool and work piece.  Therefore, delicate sections and weak materials can be machined without perceivable distortion.
  19. Conclusions (Cont…)  Material removal in EDM mainly occurs due to formation of shock waves as the plasma channel collapse.  Material removal depends mainly on thermal properties of the work material rather than its strength, hardness etc.  Hybrid processes enhance stability by influencing the flushing.  Material transfer from electrode bodies and material transfer from powders suspended dielectric is under research.
  20.  References: