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ELECTRIC DISCHARG

MACHINING.

PARIKH KRUTIK R.

ROLL NO.931

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Principles of EDM

Electrical Discharge Machining (EDM) is a controlled metal-removal process that is used

to remove metal by means of electric spark erosion. In this process an electric spark is used as

the cutting tool to cut (erode) the work-piece to produce the finished part to the desired shape.

The metal-removal process is performed by applying a pulsating (ON/OFF) electrical charge of

high-frequency current through the electrode to the work-piece. This removes (erodes) very tiny

pieces of metal from the work-piece at a controlled rate.

EDM Process

EDM spark erosion is the same as having an electrical short that burns a small hole in a

piece of metal it contacts. With the EDM process both the workpiece material and the electrode

material must be conductors of electricity.

The EDM process can be used in two different ways:

1. A pre shaped or formed electrode (tool),usually made from graphite or copper, is shaped

to the form of the cavity it is to reproduce. The formed electrode is fed vertically down

and the reverse shape of the electrode is eroded (burned) into the solid work piece.

2. A continuous-travelling vertical-wire electrode, the diameter of a small needle or less, is

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controlled by the computer to follow a programmed path to erode or cut a narrow slot

through the work piece to produce the required shape.

Process characteristics of EDM

The process can be used to machine any work material if it is electrically conductive Material removal depends on mainly thermal properties of the work material In EDM there is physical tool ,and geometry of the tool is positive impression of the hole

or geometric feature machined

The tool has to be electrically conductive as well. The tool wear once again depends onthe thermal properties of the tool material.

Though there is a possibility of taper cut and over cut in EDM ,they can be controlled andcompensated

DIELECTRIC MEDIUM

In EDM ,as has been discussed earlier ,material removal mainly occurs due to thermal

evaporation and melting .as thermal processing is required ,EDM process is to be carried out in

absence of oxygen, so that it can be controlled and oxidation often leads to poor surface

conductivity (electrical) of the work piece hindering further machining

Hence, dielectric fluid should provide an oxygen free machining environment. further it should

have enough strong dielectric resistance so that it does not breakdown electrically too easily but

at the same time ionize when electrons collide with its molecule. Moreover, during sparking it

should be thermally resistance as well.

Generally kerosene and de-ionized water is used as dielectric fluid in EDM.Tap water cannot be

used as it ionizes easily and thus breakdown occurs due to presence of salts as impurities.

Dielectric medium is generally flushed around the spark zone. It is also applied through the tool

to achieve efficient removal of molten metal.

Electrode material

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Electrode material should such that it would not undergo much tool wear when it is impinged by

positive ions.Thus the localised temperature rise has to be less by tailoring or properly choosing

its properties or even when temperature increases,there would be less melting.further,the tool

should be easily workable as intricate shape geometric features are machined in EDM. Thus the

basic charcteristics of materials are:

High electrical conductivity- electrons are cold emitted more easily and there is less bulkelectrical heating.

High thermal conductivity: for the same heat load,the local temperature rise would be lessdue to faster heat conducted to the bulk of the tool and thus less tool wear.

Higher density-for the same heat load and same tool wear by weight there would be lessvolume removal or tool wear and ths less dimensional loss or inaccuracy.

High melting point- high melting point leads to less tool wear due to less tool materialmelting for the same heat load

Easy manufacturabily Cost-cheap

The following are the different materials which are used commonly in the industry:

Electrolytic oxygen free copper Graphite Tellurium copper99% Cu+0.5% tellurium Brass

Equipments

Dielectric reservoir ,pump and circulation system Post generator and control unit Working tank with work holding device X-y table accommodating the working table The tool holder The servo system to feed the tool

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.

Advantages of EDM

Complex shapes that would otherwise be difficult to produce with conventional cuttingtools

Extremely hard material to very close tolerances Very small work pieces where conventional cutting tools may damage the part from

excess cutting tool pressure.

There is no direct contact between tool and work piece. Therefore delicate sections andweak materials can be machined without any distortion.

A good surface finish can be obtained. Very fine holes can be easily drilled.

Disadvantages of EDM

The slow rate of material removal. The additional time and cost used for creating electrodes for ram/sinker EDM.

Reproducing sharp corners on the workpiece is difficult due to electrode wear. Power consumption is high. "Overcut" is formed. Excessive tool wear occurs during machining. Electrically non-conductive materials can be machined only with specific set-up of the

process.[25]

Applications:

Cutting small holes. Blanking parts from sheets. Cutting off rods from materials with poor machinability Machining of cavities and dies. Flat or firm grinding.

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Sharpning of tools cutters and broaches.

REFERENCES :

An introduction to electrometallurgy : SHARAN & NARAIN. GOOGLE SEARCH & WIKIPEDIA