Unit 3 electrical energy based process
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Electrical discharge machining (EDM) is a machining process that uses electrical sparks to remove material from a conductive workpiece. EDM works by producing sparks between an electrode tool and the workpiece submerged in a dielectric fluid, which erodes away tiny amounts of material. This allows EDM to machine very hard or brittle materials and create complex shapes regardless of material hardness. EDM provides a high-precision, burr-free finish and can machine thin walls and small features. Wire cut EDM is a similar process that uses a continuously moving thin wire as the electrode to cut intricate shapes.
Unit 3 electrical energy based process
- 1. UNCONVENTIONAL MACHINING PROCESS – UNIT 3 Electrical Energy based processes
- 2. Electrical Energy based processes • Electrical energy is directly used to cut the material to get the final shape and size – Electrical discharge machining (EDM) – Wire cut Electrical Discharge Machining (WC EDM)
- 3. Electrical Discharge Machining (EDM) • Principle – Metal is removed by producing powerful electric spark discharge between the tool (cathode) and the work material (anode) – Also known as Spark erosion machining or electro erosion machining
- 4. Why EDM? • EDM has the following advantages: • 1. Cavities with thin walls and fine features can be produced. • 2. Difficult geometry is possible. • 3. The use of EDM is not affected by the hardness of the work material. • 4. The process is burr-free.
- 7. EDM • Construction and Working
- 9. EDM • Dielectric Fluid – Fluid medium which doesn’t conduct electricity – Dielectric fluids generally used are paraffin, white spirit, kerosene, mineral oil – Must freely circulate between the work piece and tool which are submerged in it – Eroded particles must be flushed out easily – Should be available @ reasonable price – Dielectric fluid must be filtered before reuse so that chip contamination of fluid will not affect machining accuracy
- 10. EDM • Functions of dielectric fluid – Acts as an insulating medium – Cools the spark region & helps in keeping the tool and work piece cool – Carries away the eroded material along with it – Maintains a constant resistance across the gap – Remains electrically non-conductive
- 11. EDM • Tool materials and tool wear – Metallic materials • Copper, Brass, Copper-tungsten – Non metallic materials • graphite – Combination of metallic and non metallic • Copper – graphite – Three most commonly used tool materials are • Copper, graphite, copper-tungsten
- 12. EDM • Tool materials – Graphite • Non-metallic • Can be produced by molding, milling, grinding • Wide range of grades are available for wide applications • It is abrasive and gives better MRR and surface finish • But costlier than copper – Copper • Second choice for tool material after graphite • Can be produced by casting or machining • Cu tools with very complex features are formed by chemical etching or electroforming – Copper-tungsten • Difficult to machine and also has low MRR • Costlier than graphite and copper
- 13. EDM • Selection of cutting tool is influenced by – Size of electrode – Volume of material to be removed – Surface finish required – Tolerance allowable – Nature of coolant application • Basic requirement of any tool materials are – It should have low erosion rate – Should be electrically conductive – Should have good machinability – Melting point of tool should be high – Should have high electron emission
- 14. EDM • Tool wear – Tool does not comes in contact with the work – So, life of tool is long and less wear takes place Wear ratio = vol. of work material removed vol. of electrode consumed • Tool wear ratio for – Brass electrode is 1:1 – Copper of 2:1 – Copper tungsten is 8:1 – Graphite varies between 5 and 50:1
- 15. EDM • Metal Removal Rate (MRR) – Defined as volume of metal removed per unit time – Depends upon current intensity and it increases with current – Usually a rough cut with heavy current and finishing cut with a less current is performed – MRR up to 80Cu.mm/S, can be obtained – Surface finish of 0.25 microns is obtained – Tolerances of the order of ±0.05 to 0.13 mm are commonly achieved
- 16. EDM • Factors affecting MRR – Increases with forced circulation of dielectric fluid – Increases with capacitance – Increases up to an optimal value of work-tool gap, after that it drops suddenly – Increases up to an optimum value of spark discharge time, after that it decreases – MRR is maximum, when the pressure is below atmospheric pressure
- 17. EDM • Power generating circuits – Resistance capacitance circuit (RC Circuit) – R-C-L Circuit
- 18. EDM – Rotary pulse generator circuit – Controlled pulse generator circuit
- 19. EDM • Process Parameters – Operating parameters • Electrical energy • Voltage • Time interval • Instantaneous current • Torque • Pulse width – Taper – Surface finish • Energy of the pulse • Frequency of operation – Current density
- 20. EDM • Characteristics of EDM Metal removal technique By using powerful electric spark Work material Electrically conductive materials Tool material Copper, alloy of Zinc, yellow brass, Copper-Tungsten MRR 15 to 80 Cu.mm/S Spark gap 0.005 to 0.05 mm Spark frequency 200 to 500 KHz Volts 30 to 250 V Current 5 to 60 A Temperature 10,000 degree celcius Dielectric fluid Petroleum based HC fluids, Paraffin, White Spirit
- 21. EDM • Applications – Production of complicated and irregular profiles – Thread cutting in jobs – Drilling of micro holes – Helical profile drilling – Curved hole drilling – Re-sharpening of cutting tool and broaches – Re-machining of die cavities without annealing • Recent developments – EDM change from using relaxation circuit to faster and more efficient impulse circuits – Instead of using Cu; WC is used as electrode
- 22. EDM • Advantages – Can be used to machine various conductive materials – Gives good surface finish – Machining of very thin section is possible – Does not leaves any chips or burrs on the work piece – High accuracy is obtained – Fine holes can be easily drilled – Process once started does not need constant operators attention – It is a quicker process – Well suited to machine complicated components
- 23. EDM • Disadvantages – Used to machine only electrically conductive materials – Non-metallic compounds such as plastics, ceramics or glass can never be machined – Suitable for machining small work pieces – Electrode wear and overcut are serious problems – Perfect square corners can not be machined – MRR is slow – Power requirement is high – The surface machined has been found to have micro holes
- 24. Wire Cut Electro-Discharge Machining (WC EDM)
- 25. WC EDM
- 26. WC EDM • Applications – Best suited for production of gears, tools, dies, rotors, turbine blades and cams • Disadvantages – Capital cost is high – Cutting rate is slow – Not suitable for large work pieces
- 27. WC EDM • Features / Advantages of WC EDM – Manufacturing electrode – Electrode wear – Surface finishing – Complicated shapes – Time utilization – Straight holes – Rejection – Economical – Cycle time – Inspection time