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Unit 4. THERMO ELECTRICAL ENERGY

TECHNIQUES

ME0028 NON TRADITIONAL MACHINING

TECHNIQUES

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Electrical Discharge Machining (EDM):

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Functions of Dielectric Fluids

The functions of a dielectric fluid in EDM are asfollows:

To serve as a spark conductor in the spark gap between

the tool and work material. To act as a coolant to quench the spark and to cool the tool

and work piece.

To carry away the condensed metal particles and to

maintain the gap for continuous and smooth operation.

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WIRECUT EDM:

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This process is similar to contour cutting with a band saw.

A slow moving wire travels along a prescribed path, cutting the workpiece with discharge sparks.

Material removal is affected as a result of spark erosion as the wire

electrode is fed through the work piece. In most of the cases, horizontal movement of the worktable, controlled by

CNC on modern machines, determines the path of cut.

However, some machines move the wire horizontally to define the path ofcut, leaving the part stationary.

On both types of machining configurations, the wire electrode moves

vertically over sapphire or diamond wire guides, one above and onebelow the work piece.

The electrode wire is used only once, then discarded because the wireloses its form after one pass through the work piece.

A steady stream of deionized water or other fluid is used to cool the workpiece and electrode wire and to flush the cut area.

Viewed from above, the electrode wire cuts a slot or kerf. The width of the kerf is the wire diameter plus EDM overcut as illustrated

in figure.

Strater or threading holes are required. In steel or other material a drilledhole suffices for carbide; the hole may have to be produced by EDM ormicro EDM.

Wire should have sufficient tensile strength and fracture toughness.

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Electrical Discharge Grinding

(EDG):

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THEORIES OF MATERIAL REMOVAL CONCEPTS:

1. High pressure theory.

2. Static field theory.

3. High temperature theory.

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a) High pressure theory:

Due to sudden stoppage of electro dynamic waves, high

impulsive pressure responsible for the erosion of electrodes

is released on the electrode surface. Pressure of electrical discharge reported might be as high as

1000 kgf/mm2, but expected plastic deformation was not

found on the surface.

From energy distribution of discharge spectrum, the pressurein the arc column remains between 10 and 100 kgf/cm2.

Actually the pressure is less than that reported because the

acting area of discharge pressure is thought to be wider than

the crater area. It is obtained that duration during which pressure acts is

longer than discharge periods.

To conclude, in smaller energy discharges, the discharge

pressure alone would not be sufficient to erode the

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b) Static field theory:

Two charged electrodes experience an electrostatic

force according to Coulombs law.

Accordingly, the force between the electrodes produce

stress on the electrodes which, when the gap is very

small, may cross the ultimate stress limit of the

electrode material resulting in a tensile rupture.

The force involved in tensile rupture erosion arises

because the extremely high current densities beneath

the surface of the anode produce a strong electric field

gradient, which acts on the positive ions of the crystallattice.

When this force reaches the tensile strength of the

material, a tensile fracture occurs removing one or

several particles.

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c) High temperature theory:

According to this theory, due to the bombardment of

high energetic electrons on the electrode surface, thespot attains high temperature about 10,000C especiallywith materials of low thermal conductivity.

At this high temperature, material at that spot

instantaneously melts and vaporises leaving a crater onthe surface.

This high temperature is not generated by electronbombardments alone.

The Joule heating by high density current is alsoconsidered to contribute.

Gradually high temperature theory became promising.Many experiments were carried out for exploring thistheory.

It was established that the energy given to anode per

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Where d = the gap between electrodes,

= mean free path of electron,

b = l/ ,

l = cathode fall area,

m = mass of electron,mg = mass of gas molecule,

Fa, Fc = anode and cathode work functions,

Vc = cathode fall.

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FLUSHING TECHNIQUES:

The circulation of dielectric fluid between the electrode

and the workpiece is called flushing. The effective

flushing removes waste products from the gap whereas

the bad flushing results in low MRR and poor surface

finish. The good flushing system is one that shoots the

dielectric to the place where the sparking occurs. It is

observed that flushing in blind cavities is difficult. So,

flushing does not perform good in blind cavities.

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Pressure (or Injection) Flushing

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Vacuum (or Suction) Flushing

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Side Flushing

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Reciprocating Electrode Flushing

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