unit3

Electric Discharge Machining Dr.S.Satish Kumar, Professor, Dept of Mech. Engineering

References 1. Modern machining Processes - P.C. Pandey & H.S Shan 2. Unconventional machining - P.K. Mishra

UNIT III

Syllabus

Electric Discharge Machining (EDM)- working Principles-equipments-Process Parameters-MRR-electrode / Tool –Power circuits- Tool wear – dielectric- Flushing – Wire cut EDM - applications

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Electric Discharge Machining

History of EDM

• In 1970 , the English scientist Priestley first detected the erosive effect of

electrical discharges on metals.

• EDM has found ready in the machining of hard metals or alloys which cannot be

machined easily by conventional methods.

• Application seen in machining of dies, tools made of tungsten carbides, Stellites

or hard steels.

Popularity of EDM

• Most suitable for electrically conductive materials. Physical and metallurgical

properties of work material, such as strength, toughness, microstructure etc are no

barrier

• Work piece is not subjected to mechanical deformation. As a result slender and

most fragile jobs cab be machined more easily

• Although metal removal in this case is due to thermal effects, there is no heating

in the bulk of the material.

• Complicated die contours in hard materials can be produced to a high degree of

accuracy and surface finish

Spark erosion machining process

• Consider the case of a discharge between two electrodes (tool cathode and work

anode) through a gaseous or liquid medium

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• Suitable voltage is applied. The potential intensity of the electric field between

them build up , until a predetermined value

• Individual electrons break loose from the surface of the cathode and impelled

towards the anode under the influence of field forces.

• While moving in inter electrode space, the electrons collide with the neutral

molecules of the dielectric detaching electrons from them and causing ionization.

• Ionization becomes such that a narrow channel of continuous conductivity is

formed

• This results in momentary current impulse or discharge.

• This leads to generation of extremely high temperature between 8000 C and

12000 C causing fusion or partial vaporization of the metal and dielectric fluid at

the point if discharge.

• This results in the formation of tiny crater at the point of discharge in the work

piece.

Mechanism of metal removal

• Fundamentally the electro-sparking method of metal working involves an electric

erosion effect which connotes the breakdown of electrode material accompanying

any form of electric discharge, (The discharge is usually through a gas, liquid or

in some cases solids)

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• A necessary condition for producing a discharge is the ionization of the dielectric

, that is , splitting up of its molecules into ions and electrons

• This leads to generation of extremely high temperature between 8000 C and

12000 C causing fusion or partial vaporization of the metal and dielectric fluid at

the point if discharge.

• This results in the formation of tiny crater at the point of discharge in the work

piece.

Spark erosion machining

Less tool wear (cathode)

• The momentum with which positive ions strike the cathode (tool) is much less

than the momentum with which the electron stream impinges on the anode surface

• A compressive force is generated on the cathode surface by the spark which helps

reduce tool wear

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Classification of Spark erosion machining process

Sinking by EDM

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Cutting by EDM

Grinding by EDM

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Electrode Feed control

• Both work piece and electrode is eroded

• Working gap is maintained hence spark voltage remain unaltered

• Overshooting may close the gap and hence short circuit may take place

• Rapid reversing speed is essential

• Servo mechanism affecting the movement may be electric motor driven, solenoid

operated or hydraulic operated

Di-electric fluids

• Remain electrically non conductive until required breakdown voltage is reached.

They should have high dielectric strength

• Breakdown electrically in the shortest possible time once the breakdown voltage

is reached

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• Quench the spark rapidly or de-ionise the spark gap after the discharge had

occurred

• Be capable of Carrying swarf particles.

• Good degree of fluidity & be cheap and easily available

• Transformer oil, paraffin oil , kerosene, lubricating oil and other petroleum

distillate can be used as dielectric fluids

Flushing

• Correct circulation of dielectric fluid between electrode and work piece to obtain

highest machining accuracy

• Flushing in EDM can be achieved by anyone of the following methods

(i) Injection flushing

(ii) Suction flushing

(iii) side flushing

(iv) Flushing by dielectric pumping

Pressure or injection flushing

Dielectric fluid is injected either through the work piece or tool

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Vacuum flushing / suction flushing

Dielectric fluid is sucked either through the work piece or tool

Side flushing

When flushing holes cannot be drilled either in the work piece or the tool side flushing is

employed

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Dielectric pumping /reciprocating electrode

• Flushing is done by electrode pulsation movement.

• When the electrode is raised the gap increases, resulting in clean dielectric fluid

sucked into mix with contaminated fluid.

• Particle flushes out when electrode is lowered

• Suitable for deep hole drilling

ELECTRODES

• Spark erosion is basically a copying process

• the shape and accuracy of the machined part will therefore primarily depend on

the shape and accuracy of the tool or cutting electrode

Selection of electrodes Four main factors determine the suitability of a material for use as an electrode

• Maximum possible material removal rate

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• Wear ratio

• ease with which it can be shaped or fabricated to the desired shape

• Cost

Selection of electrode material

Tool electrode design

• Electrode is the mirror image of the work

• Clearance should be provided between tool and work cavity produced

• Magnitude of the clearance varies with the rate of metal removal , the material

and work

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Operating conditions on side clearance during boring

Process parameters

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Pulse energy at a constant voltage is varied by changing the size of the capacitors

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Machine tool selection A variety of EDM machines ranging from small machines to large machines are available

The factors that have to be considered are

The number of parts to be machined

• Accuracy required

• Size of the work piece

• Depth of the cavity

• Orientation of the cavity

Spark erosion generators Discharge may be produced in a controlled manner by ‘natural’ ignition and relaxation ,

or by means of a controllable switching element for eg, electronic valve, thyristor,

transistor etc

Generators for EDM may be classified to

Relaxation generators

Rotary pulse generators

Static pulse generators

. Relaxation Generators (R-C circuit)

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Rotary pulse generators

The capacitor is charged using a diode. The circuit permits high metal removal rates.

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Static pulse generator (Controlled pulse circuit)

These circuits offer the advantage of faster rate of metal removal and low tool electrode

wear; The RC circuits are replaced with vacuum tubes and oscillators.

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Wire Cut EDM

“Saw” is a wire electrode of small diameter Horizontal movement of the work table is controlled by CNC modern machines The electrode wire is used only once, then discarded because the wire looses its form after one pass through the workpiece Kerf in Wire Cut EDM

The electrode wire cuts a slot or “kerf” in EDM

The width of the kerf is wire diameter plus overcut illustrated in fig

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Operational summary

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Review Questions

Part A

1. What is the working principle of EDM process?

2. What is the purpose of dielectric in EDM?

3. What are the factors affecting Metal removal rate in EDM ?

4. What are the basic requirements of tool material in EDM?

5. Name the common tool materials

6. Define wear ratio in EDM

7. List the advantages of wirecut EDM

8. Name the materials that cannot be machined in EDM?

9. List the properties of dielectric fluids?

10. Why a tool feed mechanism is necessary in EDM?

11. What is overcut in EDM?

12. What is the mechanism of metal removal in EDM process?

13. Why tool has less wear when compared to work piece in EDM?

14. How do you classify spark erosion machining?

15. What is Electrode feed control?

16. List the properties of dielectric fluid.

17. What is flushing?

18. What are the factors that determine the suitability of a material for use as an

electrode?

19. How do you classify the generators for EDM?

20. What is Kerf in wirecut EDM ?

Part B

1. How gas, stacks and slugs and debris are formed during machining by EDM?

What are the provisions to be made for their removal?

2. What are the ways of gap flushing used in EDM? Explain in detail.

3. What are the functions of dielectric used in the machine? List its properties.

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4. In an EDM operation employing relaxation circuit, discuss the effects of

(i) charging resistance (ii) gap setting (iii) capacitance on the rate of metal

removal How does this type of machine compared with pulse generator

5. Discuss the factors influencing the choice of electrode material in EDM. Name

the best electrode material for finish machining a small die made of WC by EDM

process

6. Discuss the advantages of EDM as compared to other non traditional methods

with regard to (i) material removal rate (ii) accuracy (iii) surface finish

7. Explain with a neat sketch the working of Wire cut EDM.