Electric Discharge Machining

It is also known as spark erosion machining orspark machining. Material of work piece removed is due to erosion caused byelectric spark. Working principle is described below:

Working Principle of Electric Discharge Machining: Electric discharge machining process is carried out in presence of dielectric fluid which creates path for discharge. When potential difference is created across the two surfaces of die electric fluid,it gets ionized. An electric spark/discharge is generated across the two terminals. The potential difference is developed by a pulsating direct current power supply connected across the two terminals. One of the terminal is positive terminal given to work piece and tool is made negative terminal. Two third of the total heat generated is generated at positive terminal, so work piece is generally given positive polarity. The discharge develops at the location where two terminals are very close. So tool helps in focusing the discharge or intensity of generated heat at the point of metal removal.Application of focused heat raises the temperature of work piece locally at a point, this way two metals is melted and evaporated.

Base and Container: A container of non-conducting, transparent material is used for carrying out EDM. The container is filled with dielectric solution. A base to keep work piece is installed at the bottom of container. The base is made of conducting material and given positive polarity.

Tool: Tool is given negative polarity. It is made of electricallyconducting material like brass, copper or tungsten. The tool material selectedshould be easy to machine and highly wear resistant. Tool is made slightlyunder size for inside machining and over size for out side machining. Tool isdesigned and manufactured according to the geometry to be machined.

Dielectric Solution: Dielectric solution is a liquid which should beelectrically conductive. This solution provides two main functions, firstly itdrives away the chips and prevents their sticking to work piece and tool.Secondly, It enhance the intensity of discharge after getting ionized and soaccelerates metal removal rate.

Power Supply: A DC power supply is used, 50 V to 450 V is applied. Due to ionization of dielectric solution, an electrical breakdown occurs. The electricdischarge so caused directly impinges on the surface of work piece. It takesonly a few micro seconds to complete the cycle and remove the material. Thecircuit can be adjusted for auto off after pre decided time interval.

Tool Feed Mechanism: In case of EDM, feeding the tool means controlling gap between work piece and the tool. This gap is maintained and controlled with the help of servo mechanism. To maintain a constant gap throughout the operation, tool is moved towards the machining zone very slowly.The movement speed is maintained by the help of gear and rack and pinion arrangement. The servo system senses the change in gap due to metal removal and immediately corrects it by moving the tool accordingly. The spark gap normally varies from 0.005 mm to 0.50 mm.

Work piece and Machined Geometry: The important point for work piece is that any material which is electrical conductor can be machined through this process,whatever be the hardness of the same. The geometry which is to be machined into the work piece decides the shape and size of the tool.

Applications of Electric Discharge Machining: This process is highly economical for machining of very hard material as tool wear is independent of hardness of work piece material. It is very useful in tool manufacturing. It is also used for broach making, making holes with straight or curved axes, and for making complicated cavities which cannot be produced by conventional machining operations. EDM is widely used for die making as complex cavities are to be made in the die making.

Advantages of EDM:

(a) This process is very much economical for machining very hard material.

(b) Maintains high degree of dimensional accuracy so it isrecommended for tool and die making. (c) Complicated geometries can beproduced which are very difficult otherwise.

(d) Highly delicate sections and weak materials can also beprocessed without any risk of their distortion, because in this process toolnever applies direct pressure on the work piece.

(e) Fine holes can be drilled easily and accurately.

(f) Appreciably high value of MRR can be achieved as compared to othernon-conventional machining processes.

Disadvantages and Limitations of EDM Process: There are some limitations of EDM process aslisted below:

(a) This process cannot be applied on very large sized work pieces as sizeof work piece is constrained by the size of set up.

(b) Electrically non-conducting materials cannot be processed by EDM.

(c) Due to the application of very high temperature at themachining zone, there are chances of distortion of work piece in case of thinsections.

(d) EDM process is not capable to produce sharp corners.

(e) MRR achieved in EDM process is considerably lower than the MRR in case of conventional machining process, so it cannot be taken as an alternative to conventional machining processes at all.

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