nontraditional machining techniques unit - 4
TRANSCRIPT
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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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