ELECTRIC DISCHARGE MACHINING (EDM)

The process of electric discharge machining also known as spark erosion, electro-erosion or spark machining involves controlled erosion of electrically conducting materials by an interrupted, repetitive electric spark, discharge between the tool (cathode) and the work (anode) separated by a dielectric fluid medium. It is the most versatile of all the electrical machining methods. Metal removal takes place due to erosion caused by the electric spark. The process can be used for machining any material, irrespective of its hardness, but it should be an electrical conductor.

Metal removal rate and resulting surface finish on work can be controlled by proper variation in energy and duration of spark discharge. The liquid dielectric such as paraffin or transformer oil or kerosene oil is used. In some cases, gaseous or even solid dielectrics are also used. The schematic layout of the electric discharge machining system is given in Fig.

Illustrating the working principle of electric discharge machining. Tool is maintained cathode (-ve) and workpiece anode (+ve). Electro-hydraulic servo-controlled feed is given to the tool for maintaining correct gap between the tool and the workpiece. Electric spark is pulsed at frequencies ranging from few hundreds to several hundred thousand kilo-hertz.

Characteristics of EDM:

The workpiece and electrode (tool) are separated by a gap, called spark gap (0.005 to 0.05 mm) and a suitable dielectric slurry, which is non-conductor of electricity, is forced through this gap at a pressure of about 2 kgf/cm². When a proper voltage (50 to 450 V) is applied, the dielectric breaks down and electrons are emitted from cathode (tool) and the gap is ionized. Avalanche of electrons takes place with collection of more electrons in the gap; consequently the resistance drops causing electric spark to jump between the workpiece and the tool. Each electric discharge (or spark) causes a focused stream of electrons to move with a very high velocity from the cathode (tool) towards the anode (work) and their collision with the work results in the generation of compression shock waves on high spots of workpiece closest to the tool which consequently develops local rise in temperature to the tune of 10,000°C sufficient enough to melt a part of the workpiece metal.

The forces of electric and magnetic fields caused by the sparking produce a tensile force which tears off the particles of molten and softened metal from these high spots on the workpiece. A part of the metal may be vaporized also. These vaporized or melted particles of the workpiece metal are thrown into the gap by the electrostatic and electromagnetic forces, from where they are driven away by the flowing dielectric liquid. It may be mentioned that erosion takes place on both the tool and the workpiece but the tool is eroded much less in comparison to the workpiece. Also, the machining speed (or metal removal rate) cannot be increased by simply using multiple electrodes or tools since machining in EDM is confined to a particular spot only and if more than one electrode is used, separate power generator and servo mechanism will be needed for each electrode.

Gap control in EDM is affected through a servo system which correctly locates the tool in relation to the workpiece surface, maintains constant gap throughout the operation, senses the changes in the gap and corrects them immediately. A short-circuit across the gap causes the servo to reverse the motion of the tool until the correct gap is established. Servo system may be electrical or hydraulic type. Electro hydraulic servo control is preferred.

Tool or electrode in EDM is made from brass or copper although electrodes (tool) of tungsten, graphite, steel and silver alloys are also used. The shape of the tool is kept basically the same as that of the product desired except that an allowance is made for side clearance and over cut (the distance the spark will penetrate the work from the tool and remove metal. from the work). Tool is made slightly undersize for inside machining and oversize on outside machining. Low-wear tools are made from copper-tungsten or silver-tungsten. Work material influences the tool wear. Although any material which is an electrical conductor can be machined by EDM, harder materials are more adaptable to this process for economic reasons.

Spark generators may be of (a) relaxation type or (b) pulse-generator type. In the relaxation type generator, the generator supplies current to a condenser, the discharge from which produces spark but tool wear is more in this system. The pulse-generators are available in which reverse pulses (main cause of tool wear) are eliminated. The electronic switching units pass the current periodically. Tool wear is greatly reduced.

Metal removal rate (MRR) in EDM is proportional to the working current value. MRR also depends on the material being cut and varies inversely as the melting point of the metal.

This shows that in EDM, aluminium will be cut faster than steel. Tolerance value of ± 0.05 mm is easily attainable.

Application:

The process is used mainly for the manufacture of tools made of carbide and other hard materials and having complicated profiles such as dies used for molding, forging. extrusion, wire drawing, etc. in as hard condition (i.e. without annealing), re-sharpening of cutting tools, broaches removal of broken tools embedded in workpieces, making of fine slits, holes in nozzles and engraving on hard materials..

Advantages:

EDM is very popular because of the following advantages:

Process can be applied to all electrically conducting metals irrespective of their melting point, hardness or brittleness.
Any complicated shape that can be made on tool can be reproduced on workpiece.
Very complicated shapes can be machined by making tools with split-sectioned shapes.
Faster in metal removal than conventional machining.
Process can be employed for machining very hard metals without creating distortion effect on the work.
No mechanical stress is present in the process and hence slender and fragile workpiece can be machined without distortion.
The surface finish on the work is cratering type and hence accommodation lubricants is possible.
This is very useful for making dies which are made from hard and corrosion-resistant materials.

Disadvantages:

The following are the disadvantages of EDM:

Machining time is much longer.
Machining heats the workpiece and hence causes change in surface and metallurgical properties.
There is excessive tool wear.
There is high specific power consumption.
Profile machining of complex contours is not possible at required tolerance.

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