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EDM.pptx
- 1. Seminar ELECTRICAL DISCHARGE MACHINING (EDM): A REVIEW
- 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: https://www.makino.com
- 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: https://www.sciencedirect.com
- 5. Main Components in EDM Workpiece and tool Electrical power supply Dielectric Servo control unit Fixture Ref: https://www.researchgate.net 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: https://www.rajagiritech.ac.in
- 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: https://link.springer.com
- 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: https://www.sciencedirect.com
- 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: https://www.slideshare.net
- 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: https://www.sciencedirect.com
- 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: https://www.sciencedirect.com
- 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: https://www.researchgate.net
- 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: https://www.edm.kd-solution.com
- 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: https://www.sciencedirect.com
- 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: https://www.sciencedirect.com
- 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.