Whenever sparking takes place between two electric contacts, a small amount of material is removed from each of the contact. this fact is used to produce sparks between the tool and work piece in electric discharge machine.
It is also known as spark erosion, electro-erosion or spark machining.
Principle of working
- Electric discharge machine works on the principle that heat energy generated by a spark is used to remove material from the work piece.
- The tool and work piece are separated by a small gap called as spark gap. The gap varies from 0.01mm to 0.5mm. The tool and work piece both are immersed in the dielectric fluid.
- When supply is made on thousand of sparks are produced per second. the duration of each sparks is very short.
- when sparks comes in contact with the dielectric fluid in the spark gap, the fluid get ionized. it allows current to flow between the tool and work piece as in fig.
- a very high temperature of around 10000 degree celsius is generated in the spark region. As result, the material gets melted and is removed from the work piece.
- These melted particles of the metal are then driven away by the dielectric fluid.
Set-up of electric discharge machine
- It consist of the four major component.
- Tool and work piece.
- Dielectric fluid system.
- R-C circuit and power supply unit.
- tool feed mechanism.
Tool and work piece
- The tool and the work piece is kept in a reservoir and is connected to a DC power supply.
- The tool is connected to a negative terminal, so that it becomes cathode, while work piece is connected to positive terminal and becomes anode.
- Work piece can be mounted on the machine table with any suitable word holding device like magnetic table, chuck, vice etc.
A dielectric system
- The reservoir is fit with dielectric fluid and the spark gap is immerse in it.
- Dielectric is continuously flushed into the spark gap. the used dielectric is filtered and recirculated in to the reservoir.
- the dielectric fluid is pumped at a pressure of 2kg/cm2 of less.
A R-C circuit of electric discharge machine
- A R-C circuit as shown below in figure. A resistor R is connected in series and a capacitor C is connected in parallel.
- When the supply is made ON the capacitor voltage starts rising continuously.
- when the capacitor voltage is equal to breakdown voltage; ( of dielectric fluid ) a spark discharge will occur in the spark gap.
- The spark persist until the capacitor voltage fail below that which is require to maintain sparking.
- After the capacitor discharge, sparking cases and the dielectric fluid in the spark gap get deionize. the capacitor is then recharge and cycle repeat itself.
- The time taken by the capacitor to recharge up to breakdown voltage should be sufficient to allow the dielectric to deionize.
Tool feed mechanism
- Since during operation both the tool and work piece is erode, it is necessary to feed the tool continuously towards the wok piece so as to maintain the spark gap.
- This can be achieve as suitable tool feed control mechanism along with servo mechanism system.
Dielectric fluid used in electric discharge machine
- A dielectric fluid is a liquid which acts as a insulator between consecutive spark discharge and a conductor during the spark discharge.
- It has electric property, to act as an insulator at one time and to act as a conductor at other time.
- The dielectric fluid used in paraffin oil, lubricating oils, kerosene, distilled water etc.
Function of dielectric fluid
- It should act as an insulator until the required breakdown voltage is attained.
- it should behaves a conductor once the breakdown voltage is reached.
- clean the spark gap by carrying away the molten metal.
- deionize the spark gap rapidly after the discharge has occured.
- it should cool the tool, work piece, and the spark region.
Characteristics of Dielectric fluid
- Low viscosity
- High flash point
- controlled level of toxicity
- Freedom from acid and alkaline products
- high dielectric strength
- cheap and easily available
- high fluidity
Tool used in electric discharge machine
- The tool used in electric discharge machine is in the form of an electrode.
- tool is made as slightly under size for inside machining and oversize for outside machining.
- The tool has reverse profile ( mirror image ) of what that is produce on the work piece.
- the electrode material is broadly classified into two groups metallic and Non-metallic group.
- The metallic group includes materials like copper, brass, aluminium, steel, zinc, etc..
- The most popular non-metallic used in EDM is graphite electrode with finer grains results in lower tool wear, better surface finish and higher metal removal rate.
Requirement of Tool Material
- Good conductor of electricity
- High thermal conductivity
- cheap and readily available
- low wear ( erosion ) rate.
- Low electric resistance
- Good machinability
- high melting point
Process parameter for electric discharge machine
- It is the voltage which is provided by the power supply system.
- It ranges between 50V to 400V DC supply.
Break down Voltage
- It is the voltage at which the dielectric breakdown.
- Increase in breakdown voltage results in increase in spark energy.
- consequently the metal removal rate increases which results in poor surface finish.
Resistance and Capacitance
- increase in capacitance will results in increase of metal removal rate.
- Decrease in resistance will result in increase of metal removal rate.
Spark gap setting
- Decrease in spark gap results in lower metal removal rate.
- This results in better surface finish and high accuracy.
- The pulse duration ranges from 2 to 2000 mili sec.
- Decrease in pulse duration will results in high tool wear.
- Increase in pulse duration results in lower metal removal rate.
Advantages of Electric Discharge Machine
- Any material is machine irrespective of its hardness.
- Any complicated shape is produce on the work piece.
- There is no physical contact between the tool and the work piece therefore no stresses is produce in the work piece.
- Very fine holes are easily drill.
- A very good surface finish is obtain.
Disadvantages of Electric Discharge Machine
- Metal removal rate is slow.
- Taper Holes produced.
- Overcut formed.
- Can not produced very sharp corners.
- Excessive tool wear are occurs during machining.
- Power consumption is high.
- Unable to machine electrically non-conductive material.
Application of Electric Discharge machine
- Used in making stamping tool.
- wire drawing
- extrusion dies
- header dies, forging
- internal thread
- machining of exotic material
- used in aerospace industries.