Spark Description

COMPONENTS IN EDMPOWER SUPPLY

DIELECTRIC SYSTEM

ELECTRODES

SERVO SYSTEM

R-C PULSE GENERATORCapacitor charged from the DC supply and as soon as potential across the electrodes reaches to breakdown voltage spark takes place.Spark takes place at a point of narrowest IEG.

CharactersticsLow MRR because of long Idle time and short spark time.Improved surface finishing results in further decrease in MRR.High Tool wear rate.High peak current is attained which results in high temperature which is not required and may result in thermal damage of electrodes.

Controlled pulse generator Give low Peak current.

Short idle time.

Desired length of pulse.

Enabling us to select either ROUGH MACHINING or FINISHED MACHINING.Current Vs Time relationship

Power Supply

Power supply converts alternating current (AC) into pulsed direct current (DC) which is used to produce sparks between the workpiece and tool.Solid state rectifier is used to convert AC into DC .A Fraction of DC power is used to generate Square wave signal with the help of Multi -vibrator oscillator .These signals triggers bank of power transistors that act as high speed switches to control the flow of remaining DC power.For a given input voltage, certain IEG is required to generate spark between the electrodes .EDM power supply are also equipped with CUT-OFF PROTECTION circuit.

Power Supply

Power supply also control the PARAMETERS like Voltage , Current , Duration and Frequency of Pulse , Duty cycle & Electrode polarity.When using RC generators, the voltage pulses, shown in Fig. are responsible for material removal.A series of voltage pulses (Fig.) of magnitude about 20 to 120 V and frequency on the order of 5 kHz is applied between the two electrodes.

Dielectric SystemIt consists of Dielectric fluid , Reservoir , Filters, Pump and Delivery devices.In EDM, material removal mainly occurs due to thermal evaporation and melting.As thermal processing is required to be carried out in absence of oxygen so that the process can be controlled and oxidation avoided.Oxidation often leads to poor surface conductivity (electrical) of the workpiece hindering further machining. Hence, dielectric fluid should provide an oxygen free machining environment.

Dielectric FluidThree important functions of a dielectric medium in EDM:Insulates the gap between the tool and work, thus preventing a spark to form until the gap voltage are correct.Cools the electrode, workpiece and solidifies the molten metal particles.Flushes the metal particles out of the working gap to maintain ideal cutting conditions, increase metal removal rate.Requirements of dielectric fluidAdequate viscosity

High flash and fire point

Good oxidation stability

Minimum odor

Low cost

Properties of dielectric fluidHigh dielectric strength ( i.e remain electrically non -conductive until the required breakdown voltage is attained ).

During sparking it should be thermally resistant as well.

Deionize the IEG immediately after the spark occurred.

Minimum ignition delay.

High degree of fluidity.

Dielectric fluidKerosene Oil and Deionized water is generally used.

Transformer oil , parrafin oil are also used.

Deionized water gives High MRR and functions as more effective Cooling medium but it causes high Electrode wear and corrossion .

Dielectric is Filtered to remove DEBRIS as filteration is highly essential before re-circulation.

Pollution of fluid decreases the breakdown intensity of the field and Reproduction accuracy.

ELECTRODE MATERIALElectrode material should be such that it would not undergo much tool wear when it is impinged by positive ions.Further, the tool should be easily workable as intricate shaped geometric features are machined in EDM.Basic characteristics of electrode:1.High electrical conductivity electrons are emitted more easily and there is less bulk electrical heating2.High thermal conductivity faster heat conducted to the bulk of the tool and thus less tool wear. 3.Higher density for less tool wear and thus less dimensional loss or inaccuracy of tool.

Electrode Material

4. High melting point high melting point leads to less tool wear due to less tool material melting for the same heat load. 5.Cost cheap

The followings are the different electrode materials which are used commonly in the industry:

GraphiteElectrolytic oxygen free copperTellurium copper 99% Cu + 0.5% telluriumBrass

Tool gets tapered and its corners round off.

As it is seen that the major part of tool erosion occurs during first few microseconds of the pulse.

So , we use pulse with low initial current which reduce tool wear.

Increase in pulse duration also result in less tool wear.

Tool wear