24-11-2012, 03:47 PM
ELECTRIC DISCHARGE MACHINE(EDM)
ELECTRIC DISCHARGE MACHINE.pptx (Size: 398.38 KB / Downloads: 81)
INTRODUCTION
Electric Discharge Machine (EDM) is machine used for unconventional machining of work piece with a very high degree of accuracy and precision.
Here the process of metal removal is done by a controlled erosion through a series of electric sparks.
It was first started in the USSR around 1943.
Working Principle
When a discharge takes place between two points of the anode and the cathode, the intense heat generated near the zone melts and evaporates the materials in the sparking zone.
For improving the effectiveness, the workpiece and the tool are submerged in a dielectric fluid (hydrocarbon or mineral oil like kerosene).
A suitable gap, known as the spark gap, is maintained between the tool and the workpiece surfaces. The sparks are made to discharge at a high frequency with a suitable source.
Mechanism of EDM
Due to asperities and irregularities which are always there in between the surface, local gap varies and at a given point it is minimum at one point (let it be A)
When a suitable voltage is built up across the tool and the workpiece (cathode and anode), an electrostatic field of sufficient strength is established, causing cold emission of electrons from the cathode at point A to accelerate towards anode.
After gaining sufficient velocity these electrons collide with molecules of dielectric fluid, breaking them into electrons and positive ions, thus forming a narrow column of ionized dielectric fluid molecules at point A. Thus a avalanche of electrons is formed causing a clearly visible spark.