ELECTRICAL DISCHARGE MACHINE 

Electrical discharge machine also known as spark erosion, electro-erosion or spark machining is a process of metal removal based on the principle of erosion of metal by an interrupted electric spark discharge between the electrode tool  (usually cathode) and the work (anode)

Fundamentally, the electric erosion effect is understood by the breakdown. Of electrode material accompanying any from of electric discharge. The discharge is usually through a gas, liquid of in some cases through solids. A necessary condition for producing a discharge is ionization of the dielectric, splitting up of as molecules into ions and electrons.

Illustrates the schematic layout of the electric discharge machining system, The main components are the electric power supply, the electric medium, the work piece and the tool, and a servocontrol.

The workpiece and the tool are electrically connected to a dc electric power. The workpiece is connected to the positive terminal of the electric source, so that it becomes the anode. The tool is the cathode. A gap, known as the “spark gap” in the ranges of 0.05 to 0.05 mm is maintained between the work-piece and the tool, and suitable dielectric slurry, which is non-conductor of electricity is forced through this gap at a pressure of 2 Kgf/cm² or less. When a suitable voltage in the range of 50 to 450 V is applied, the dielectric breaks down and electrons are emitted  form the cathode and the gap is ionized. In fact, a small ionized fluid column is formed owing to formation of an avalanche of electrons in the spark gap where the process of ionizational collision takes place. When more electrons collect in the gap the resistance drops causing electric spark to jump between the workpiece surface and the tool. Each electric discharge or spark causes a focused stream of electrons to move with a very high velocity acceleration from the cathode towards the anode, and ultimately creates compression shock waves on both the electrode surface, particularly at high spots on the workpiece surface, which are closest to the tool. The generation of compression shock waves develops a local rise in temperature. The whole sequence of operation occurs within a few microseconds. However the temperature of spot hit by the electrons is of the order of 10,000 °C. This temperature is sufficient to melt a part of the metals. The forces of electric and magnetic fields caused by the spark produce a tensile force and tear off particles of molten and softened metal from this spot on the workpiece. A port of the metal may vaporize and fill up the gap. The metal is thus removed in this way from the workpiece. The electric and magnetic fields on the heated metal cause a compression force to act on the cathodic tool so that metal  removal from the tool is at a slower rate than that from the workpiece. Hence, the workpiece is connected to the positive terminal and tool to the negative terminal.

Advantage of EDM 

Extremely high popularity of the EDM process is due to the following advantage.

1. The process can be applied to all electrically conducting metals and alloys irrespective of their melting points, hardness, roughness or brittleness. 

2. Any complicated shape that can be made on the tool can be reproduced on the workpiece.

3. Highly complicated shapes can be made by fabricating the tool with split sectioned shapes, by welding, brazing or by applying quick setting conductive epoxy adhesives.

4. Time of machining is less than conventional machining processes.

5. EDM can be employed for extremely hardened work-piece. Hence the distortion of the work-piece arising out of the heat treatment process can be eliminated.

Disadvantage

1. Profile machining of complex contours is not possible at required tolerances.

2. Machining times are too long.

3. Machining heats the work-piece considerably and causes change in surface and metallurgical properties.

4. Excessive tool wear.

5. High specific power consumption.