ELECTROCHEMICAL MACHINING (ECM)
In ECM, shaped tool (or electrode) forms the cathode; the workpiece forms the anode and a small gap is maintained between the tool and the workpiece which is filled with electrolyte. A voltage DC is used which in the presence of electrolyte, enables a controlled removal of metal from the workpiece by anodic dissolution. The electrolyte does not allow sticking of separated workpiece metal on to the tool face because of its certain chemical properties.
Most widely used electrolyte is sodium nitrate solution. Sodium chloride solution in water can used but it also be used it is corrosive. Other chemicals used for electrolyte include sodium hydroxide, sodium sulphate, potassium nitrate and potassium chloride.
General set-up for ECM is shown in (Fig.). The tool used should be a conductor of electricity, should be rigid to take load due electrolyte pressure, should be easily machinable. to the desired shape. Copper, brass and stainless steels are mostly used for the tool when electrolyte is made of salts of sodium or potassium. Other materials used for the tool are graphite, aluminium, platinum, etc.
DC power supply provides current 50 to 40,000 amperes at 5 to 30 volts for attaining current density of 20 to 300 amperes per cm² across a gap of 0.05 to 0.7 mm between the tool and the work. Electrolyte flows through this gap at a velocity of 30 to 60 m/s forced by an inlet pressure of about 20 kgf/cm². Temperature of electrolyte is maintained between 25 and 60°C. The pieces of metal cut from the workpiece and in suspension in electrolyte are removed from electrolyte by setting, centrifuging or filtering and filtered electrolyte is sent for reuse.
ECM is based on Faraday’s laws of electrolysis. The workpiece acts as anode and the tool acts as cathode. Since the tool and the workpiece are held very close to each other (0.5 mm) with electrolyte in between, when a mild DC voltage of 3 to 30 volts is applied between the tool and the workpiece, current flows through the electrolyte with +ively charged ions attracted towards the tool (cathode) and -ively charged ions attracted towards the workpiece (anode).
The electrochemical reaction, which takes place due to flow of ions, results in the removal of metal from the workpiece in the form of sludge which is taken away from the gap by the flowing electrolyte. The area where the tool and the workpiece are closer experiences flow of higher current (due to less resistance) and consequently metal removal rate in this area will be higher. This enables the reproduction of tool shape on the workpiece (Fig.).
Illustrating working principle of electrochemical machining (a) workpiece shape before machining and (b) workpiece shape after machining Note that the tool shape is reproduced on the workpiece after machining.
Accuracy of machining:
ECM provides on the workpiece dimension a tolerance of the order of ± 0.02 mm or less. Surface finish is 0.2 to 0.8 micron (CLA) depending on the work material. ECM results in internal radii greater than 0.2 mm and external radii of 0.05 mm on the machined workpieces.
ECM is mainly used in machining hard heat-resisting alloys, for cutting cavities in forging dies, drilling holes, machining of complex external shapes of turbine blades and aeroplane components, machining tungsten carbide parts and nozzles of alloy steel. Any conducting material can be machined by this process.
Advantages and limitations:
Intricate shapes machined cavity: metal removal rate is high: wear on tool is insignificant; machined surface is free of stress; very thin sections as sheer metal are easily machined without distortion; ECM can dispense with several machining operation such as grinding, milling, polishing, etc. ECM gives good surface finish, 0.2 to 0.8 micron, ECM is not applicable to machining non-conductors of electricity. It consumes very high power in the operation. Electrolytes used create problems of corrosion of machine components. Larger floor space is needed. It needs special fixture to hold the work in position during machining because it may be otherwise displaced due to the pressure of electrolyte.
Also, high velocity flow (30-60 m/s) of electrolyte over electrode surfaces is required to prevent crowding of hydrogen gas and debris of machining. If this is not done, then bubbles of hydrogen gas may fill the machining gap and machining will stop in that area.
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