ELECTROCHEMICAL GRINDING (ECG)
Electrochemical grinding or electrolytic grinding or anodic machining (Fig.) consists of a metallic grinding wheel as cathode (-ve) and workpiece as anode (+ve) using DC power supply 2 to 30 V with automatic adjustment of voltage. An electro conductive electrolyte such as an aqueous solution of water glass is used in the process which works both as an electrolyte and a coolant. On the periphery of the metal bonded grinding wheel are attached the abrasive particles which protrude from the face of the grinding wheel and which acts as insulators preventing direct contact between the wheel and the workpiece.
The current flows from the cathode (grinding wheel) to the anode (workpiece) through the electrolyte. This leads to an electrochemical oxidation on the workpiece surface and the oxide film thus formed is removed by the protruded abrasives on the grinding wheel which results in the production of an accurate finished surface. Metal removal is thus by both, the electrochemical decomposition (about 90%) and abrasion of metal (about 10%).
A constant gap of 0.025 mm is maintained between the grinding wheel and the workpiece. The electrolyte is passed through this gap at a high speed by the rotary action of the wheel. The metal is removed from the workpiece by simultaneous action of electrolytic and abrasive action by the wheel. The abrasive grains protruding from wheel surface act as paddle and pick up electrolyte. The electrolyte is entrapped in small cavities or pores of semi-conductive oxide layer of the wheel (Fig.) forming electrolytic cells and when these cells come in contact with the workpiece, the current flows from the wheel to the workpiece resulting into electrochemical decomposition of the workpiece.
The protruding abrasives help in removing the inactive layers formed on the workpiece by abrasion and that way make the work surface more receptive. Major part of the metal removed is by electrolytic action. The grinding wheels used may be metal bonded or carbon bonded. The current maintained in the process is 50 to 3000 amp at 4 to 10 volts.
Any material which is electrically conductive may be ground by ECG. It is best suited for very precision grinding of high tensile strength and hard steels, tungsten, reconditioning and resharpening of carbide and other cutting tools such as milling cutters, carbide tipped tools and tool bits because of very less grinding pressure and the temperature is also low due to which grinding cracks, tempering of works, etc. are eliminated. It is also used for cutting thin sections of hard material.
Advantages and limitations:
Wear of tool is negligible; work does not suffer from distortion, etc. as heat is not generated; fairly good dimensional accuracy and surface finish are obtained: work material is not subjected to any structural change as very little cutting force is applied. ECG involves high initial cost and high power consumption. For grinding, metal removal rate is slower than conventional grinding. Only electrical conductive materials can be machined by ECG.
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