Electro discharge machining (EDM):
We use electrical energy to cut the material to final shape and size. The work piece (anode) and tool (cathode) are separated by a dielectric fluid medium. The workpiece is made anode (positive) and tool is made cathode (negative) in order to remove maximum metal and minimum wear on tool.
We pass high current between the tool and work piece. When we reduce the distance between tool and work piece, the dielectric medium ionizes and electric current flow between the two electrodes which causes the spark discharge. The spark will be developed into an arc if potential difference is maintained. This results in localized heating of workpiece which raises the local temperature of the order of 10,000°C. This results in melting of work surface and erosion. Some of the removed particles are vaporized and fine particles are carried away by dielectric medium. The surface is quenched by the surrounding fluid. A true replica is produced on the workpiece in this process.
The spark gap is generally maintained 0.01 to 0.5 mm by a Servomotor. Higher gap decreases the spark frequency. If the potential difference decreases, the fluid will de-ionize and discharge will cease. The gap between the tool and work piece is varied to match metal removal rate. The greatest erosion takes place upon positive electrode (anode) if both electrodes are made of same material.
The shape/profile of tool is produced on work piece so any complicated shape that can be produced on tool can be reproduced on the work piece.
All electrically conducted metals and alloys can be machined by this process so it is used in tool manufacturing.
Complicated jigs and fixtures are not required to hold the job.
Mainly paraffin, kerosene or transformer oil is used as dielectric fluid medium. It is pumped through tool or workpiece at a pressure of 2 kg/cm2 or less. An automatic controller ensures a constant level of dielectric in tank.
The tool used in EDM can be metallic, no-metallic, and combination of metallic and non-metallic.
Advantages:
– Any hard material can be machined by this process which can not be machined otherwise. Example – Ceramic carbide.
– Very thin section can be machined by this method.
– Very close tolerances of up to 0.04 μm can be achieved.