CHAPTER-1
INTRODUCTION
Electrical Discharge Machining (EDM) is the process of machining electrically conductive material by using precisely controlled sparks that occurs between an electrode and a work piece in the presence of a dielectric fluid. The Electrode may be considered the cutting tool.
Electrical Discharge Machining (EDM) is non-conventional process based on removing of material through melting and evaporation. No physical cutting forces between the tool and workpiece,high precision metal removing process using thermal energy by generating a spark to erode the workpiece.The work piece must be electrically conductive which is submerged into the dielectric fluid for the better erosion. A voltage is attached to both electrodes and when the breakdown voltage of the medium is reached a plasma channel allowing for a current flow is established and a discharge takes place. On the basis of plasma channel the temperature can reach T>10000K, melting and evaporating electrode material. When the energy input is stopped the discharge ends, leading to an implosion of the plasma channel followed by a collapse of the surrounding gas bubble. The reflow of the dielectric medium flushes particles away and cools the electrodes.
Wire –Cut EDM machine uses a continuous wire as the electrode. Sparking occurs from the electrode wire-side surface to the work piece.In Die-sinking type EDM machine, it requires the electrode to be machined in the exact opposite shape as the one in the work piece.
Basic fundamental of the process is that only one spark occurs at any instant. Sparking occurs in a frequency range from 2000 to 500000 sparks per seconds causing it to appear that many sparks occurring simultaneously. The spark removes
the material from both electrode and work piece, which increases the distance between electrode and the work piece. This causes the next spark to occur at the next closest point between electrode and work piece.
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. Fig-1.1-Schematic of EDM Process
EDM is a thermal process .i.e. the material is removed by heat. Heat is introduced by the flow of electricity between the electrode and work piece in the form of spark. Material at the closest point between both i.e. electrode and work piece, where the spark originates and terminates are heated to the point where the material vaporizes.
Now the electrode and work piece should never feel more than warm to the touch during machining ,the area where each spark occurs is very hot. The area heated by the spark is very small so the dielectric fluid quickly cools the vaporised material and the electrode and the work piece surfaces. But it is possible for metallurgical changes to occur from the spark heating the work piece surface
A dielectric material is required to maintain the sparking gap between the electrode and work piece. This dielectric material is normally fluid. Die-sinker type EDM machine usually use hydrocarbon oil, while wire-cut type EDM machine normally use deionised water.
Fig.1.2 Basic Components of EDM
Dielectric fluid used in EDM machine provides important function in the EDM process, these are:
Cooling the heated material to form the EDM chip.
Introduction
Controlling the spark-gap spacing between the electrode and workpiece.
Removing EDM chip from the sparking area.
In EDM, for efficient machining, the chip must be removed from the sparking area. Removal of this chip is accomplished by flowing dielectric fluid through the sparking gap.
EDM development in the USSR: In 1941, the USSR was busy in World War-ll ,so critical materials needed to be conserved. Tungsten was widely used as electrical contact material for automotive-engine, distributor-breaker points. As pitting occurred, the engine required maintenance. It was probable that military vehicles would not be in service when needed..Even the replacement of the breaker points caused valuable tungsten to be discarded. So to address this issue, the government assigned Moscow University Professor Dr,Boris Lazaenko and Dr.Natalya Lazarenko at All Union Electro Technical Institute to investigate whether life of the components could be extended by suppressing sparking between breaker points. As the part of their experimentation, Dr Lazarenkos immersed the breaker points in oil. They observed that while the oil did not eliminate the sparking, it did create more uniform and predictable sparking and pitting as compared to operating the breaker points in air.