ELECTRIC DISCHARGE MACHINING (EDM) - PowerPoint Presentation

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ELECTRIC DISCHARGE MACHINING (EDM)

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INTRODUCTION TO EDM

EDM process involves a

controlled erosion of electrically conductive materials

by the initiation of

rapid and repetitive spark discharge

between electrode tool (cathode) and

workpiece

(anode) separated by a

dielectric fluid medium

and a small gap called

spark gap

.

Process of cratering out of metals using a spark

.

Electrical discharge machining

is

also known as

spark

machining or

spark

eroding machining

.

Different types of EDM include

Wire EDM, Die sinking EDM, Hole drilling EDM

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MECHANISM OF MATERIAL REMOVAL

Thermal principle of material removal mechanism is

melting and vaporization

When a voltage is applied between the two electrodes separated by a properly chosen spark gap, the

breakage of dielectric fluid medium

takes place

The strong electrostatic field produces

emission of electrons

from cathode and accelerates towards the anode.

These electrons will

collide with the dielectric fluid molecules

inside the dielectric fluid medium

The electrons will bounce back if it does not have the energy to eject another electron from dielectric medium or if it have the energy, then continuous collisions take place between electrons

.

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Due to this collisions, dielectric fluid molecules will be broken into electrons and positive ions

and hence form an ionized medium with low resistance and an

electrical discharge is initiated

with the resulting flow of electrons

Each electric discharge causes a focused stream of electrons from the cathode towards the anode and ultimately

creates compression shock waves

on both the electrode surfaces

The generation of shock waves develops a

huge local rise in temperature within microseconds

and the surface layer gets deformed

Pulse energy is ultimately released as heat and causes

pyrolysis of the dielectric medium

.

Erosion takes place both at anode and cathode

Striking velocity of electrons is much greater than positive ions. Hence there will be very

less wear of tool electrode and more wear on work piece

.

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MECHANICS OF CUTTING IN EDM

MRR depends on the current density and it increases with current.

High MRR produces less surface finish

Rough cut is taken with high current and finish cut with low current

Total energy discharged through the spark gap

Substituting,

=

and

and integrating,

Usually discharge time is very small and RC values are quite highTherefore energy discharged through the gap is

 

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MRR Model 1

MRR is

propotional

to total energy delivered per second

Frequency of operation, f is given by f =

C = capacitance

Therefore MRR =

Effect of on MRR should be decreased to increase MRR

The minimum value of resistance that will prevent arching is known as

critical

resistanceTheoretically Experimentally

 

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MRR Model 2

Material removal takes place due to intense localized heating almost by point heat source for a small frame of time

Molten metal can be assumed to be hemispherical in nature with a radius which forms due to single pulse or spark

But radius is unknown and hence Volume of material removal

cannot be found

Energy content of the spark

A part of spark energy gets lost in heating dielectric and rest is distributed between impinging electron and ions

 

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Material removal in a single spark is

propotional to spark energy

MRR = Volume of material removed by single spark

=

=

=

 

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SURFACE FINISH IN EDM

The surface finish of the material depends on

Energy of the pulse

Frequency of operation

In EDM, each spark results approximately spherical crater formation on the surface on the

workpiece

Model 1

Centre line average value of surface roughness will be a function of crater depth and sparking frequency

Total volume of the material deformed by crack

 

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Therefore

Substituting the value for frequency

Theoretically,

Experimentally,

 

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MODEL 2

Depth of penetration is the roughness value and occurs side by side

ie

, H = r

Volume of the material

But

Energy

content of the spark

Surface roughness increases with increase in spark energy

Surface roughness can be decreased by decreasing working voltage, working current and pulse on time

 

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SURFACE ROUGHNESS IN EDM

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BASIC REQUIREMENTS OF EDM PROCESS

Both the

workpiece

and tool material must be electrically conductive

Workpiece

must be held firmly and in exact position with respect to tool

A precise gap (usually .025 mm) must be maintained between tool and the

workpiece

Flowrate

and pressure of dielectric fluid must be properly chosen to avoid collection of debris between tool and

workpiece

, which otherwise causes short circuiting

A power supply must supply pulse of required characteristics like pulse energy, duration, frequency of pulses.Proper filtration system must be used to filter the debris of dielectricProper safety precautions are to be taken to prevent the damage of operator, equipment, tool and workpiece.

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EQUIPMENT

Power generators

To convert an alternating current (AC) to a pulsed Direct current (DC)

DC is used to produce the unidirectional spark discharges between the gap of the

tool and the work

Dielectric system

It consists of

dielectric fluid, reservoir, filters, pump and delivery devices

Tool electrode and

workpiece It should have high electrical and thermal conductivity, density and melting point. Graphite, Electrolytic oxygen free copper, Brass etc are common electrode materialsServosystem To maintain a predetermined gap between tool and the workpieceFrame It consists of a base, column and head

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PROCESS PARAMETERS

Open circuit voltage

Working Voltage

Maximum current

Pulse on time

Pulse off time

Gap between tool and

workpiece

Polarity

Dielectric medium

Flushing through spark gap

Capacitor discharge rate

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EDM PROCESS CHARACTERISTICS

Metal removal rate

Volume of the material removed per unit time

Increases with current value

Accuracy

Tolerance of .05mm could be easily achieved by EDM

EDM produces taper, over cut and corner radii

Surface finish

Depth of the crater formed due to spark discharge determines the surface finish

It also increases with current and the frequency

Heat affected zone

Resolidification

of melted material on the machined surface of the workpieceThermal stresses, plastic deformation and fine cracks may form in this region

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APPLICATIONS OF EDM

Any hard, tough, brittle carbides and steels can be machined

Hardened steel die cavities and miniature holes can be manufactured

Dies for

moulding

, casting, forging, extrusion, wire drawing process

Tiny holes, orifices and micro sized slots

Extremely difficult shapes like squares, splines, slots, grooves

etc

Deep holes in fuel injecting nozzles

Hydraulic valve spools

Thread cutting, helical profile drilling, trepanning, broaching

Delicate workpieces such as copper parts for fitting into vacuum tubesRepetitive partsTrepanning of rectangular holes in thin valve sleeves on curved surfaces

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ADVANTAGES

The process can be applied for all electrically conductive metals and alloys. Physical and metallurgical properties are no barrier to its application.

No physical contact between tool and work and therefore no mechanical deformation

No cutting forces are involved so that thin fragile materials can be machined

Complicated shapes made on tool can be reproduced on

workpiece

No heating in the bulk of the material

Complicated contours in hard materials can be produced to a high degree of accuracy and surface finish with less machining time

Process can be automated easily

Produces chip, burr free work surfaces

Soft material electrode can be used to make hard material die ( Copper tool can be used to make hardened steel dies

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DISADVANTAGES

High power is required for machining

Not suitable for non conductive materials

Non metallic materials like Ceramics, Plastics, Glass

etc

cannot be machined using EDM

Electrode wear and overcut are serious problems

Microcracks

are found deeper in the surface

Low volumetric material removal rate

Tool wear is high so that sometimes two or more tools are required to complete operation

Distortion of surface microstructure happens and thus requires subsequent etching

Difficult to employ more than one tool as it requires separate power generators and servo mechanismsGap cleaning is highly essential

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THANK YOU