1. Electrical Discharge Machining
Unit 95

2. Objectives
Define electrical discharge machining and state its principle
Summarize the EDM process
Identify the advantages and the limitations of electrical discharge machining
Name the main operating systems of wire-cut electrical discharge machines

3. Electrical Discharge Machining
Commonly known as EDM
Proved valuable in machining of super touch new space-age alloys
Made it relatively simple to machine intricate shapes
Used extensively in plastics industry to produce cavities in steel molds

4. Controlled spark removes metal during electrical discharge machining
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5. Tool Rectifier Work 220-V AC Current Control Servo
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6. Principle of EDM
Controlled metal-removal technique where electric spark used to cut (erode) workpiece
Takes shape opposite to that of cutting tool
Electrode (cutting tool) made from electrically conductive material
Dielectric fluid surrounds both tool and work
Servo mechanism gives gap .005 to .001 in. between work and tool
Direct current of low voltage and high amperage

7. Types of EDM Circuits
Several types of electrical discharge power supply used for EDM
Two most common types of power supplies:
Resistance-capacitance power supply
Widely used on first EDM machines
Capacitor charge through resistance from direct-current voltage source
Pulse-type power supply

8. Resistance-Capacitance Circuits
Combination of low frequency, high voltage, high capacitance, and high amperage results:
Rather coarse surface finish
Large overcut around tool
Larger metal particles being removed and more space to flush out particles
Advantages of resistance-capacitance power
Circuit simple and reliable
Works well at low amperages

9. Pulse-Type Power Supply
Similar to resistance-capacitance type
Vacuum tubes or solid-state devices used to achieve extremely fast pulsing switch effect
More discharges per second produces finer surface finish
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10. Main Advantages of Pulse-Type Circuit
Versatile and can be accurately controlled for roughing and finishing cuts
Better surface finish produced as less metal removed per spark
Many sparks per unit of time
Less overcut around electrode (tool)

11. The Electrode Formed to shape of cavity desired
Characteristics of good electrode materials:
Be good conductors of electricity and heat
Be easily machined to shape at reasonable cost
Produce efficient metal removal from work
Resist deformation during erosion process
Exhibit low electrode (tool) wear rates

12. Electrode Common materials (not general-purpose)
Graphite, cooper, copper graphite, copper tungsten, brass, and steel
Yellow brass used for pulse-type circuits
Good machinability, electrical conductivity
Copper used in resistance-capacitance circuits with higher voltages
Graphite
Gaining acceptance, relatively inexpensive
Tool wear rate less and high metal-removal rate almost double of other materials

13. EDM Process Servo mechanism EDM power supply
Automatically maintains constant gap ~.0005 to .001 in. between electrode and work
Advance tool into workpiece, senses and corrects any shorted condition by rapidly retracting tool (vertical movement)
Feed control applied to table for horizontal moves
EDM power supply
Provides direct current electrical energy for electrical discharges between tool and work

14. Characteristics of Pulse-Type Circuits
Low voltages
Normally about 70 V, drops to 20 V after spark initiated
Low capacitance
About 50 mF or less
High frequencies
Usually 20,000 to 30,000 Hz
Low-energy spark levels

15. The Discharge Process
Dielectric fluid changes into gas when sufficient electrical energy applied
Allows heavy discharge of current to flow through ionized path and strike workpiece
Heat between electrode and work surface causes small pool of molten metal to form on work surface

16. Electrical discharges occur at rate of 20,000 to 30,000 Hz
Current stopped (microseconds), molten metal particles solidify and washed away
Electrical discharges occur at rate of 20,000 to 30,000 Hz
Each discharge removes minute amount of metal
Voltage constant so amount of metal removed will be proportional to amount of charge between electrode and work
Current maintained but frequency increased, results in smaller craters and better surface

17. Main Functions of Dielectric Fluid
Serves as insulator between tool and workpiece until required voltage reached
Vaporizes (ionizes) to initiate spark between electrode and workpiece
Confines spark path to narrow channel
Flushes away metal particles to prevent shorting
Acts as coolant for both electrode and workpiece

18. Types of Dielectrics
Must be able to ionize and deionize rapidly and have low viscosity
Allow them to be pumped through narrow machining gap
Most common have been various petroleum products
Light lubricating oils, transformer oils, silicon-base oils and kerosene
Selection of dielectric important since it affects metal-removal rate and electrode wear

19. Methods of Circulating Dielectrics
Must be circulated under constant pressure
Pressure used generally begins with 5 psi and increased until optimum cutting obtained
Four methods to circulate dielectric fluid
All must use fine filters in system to remove metal particles so they are not recirculated

20. Down Through the Electrode
Hole drilled through electrode and dielectric fluid forced through electrode and between it and work
Rapidly flushes away metal particles
Pressure
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21. Up Through the Workpiece
Pressure
Cause fluid to be circulated up through workpiece
This type limited to through-hole cutting applications and to cavities having holes for core or ejector pins
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22. Vacuum Flow
Negative pressure (vacuum) created in gap, which causes dielectric to flow through normal .001 in. clearance between electrode and workpiece
Improves machining efficiency, reduces smoke and fumes and helps to reduce or eliminate taper in work
Suction
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23. Vibration
Pumping and sucking action used to cause dielectric to disperse chips from spark gap
Valuable for very small holes, deep holes, or blind cavities
Vibration
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24. Metal-Removal Rates Rate dependent on following factors:
Amount of current in each discharge
Frequency of discharge
Electrode material
Workpiece material
Dielectric flushing conditions
Normal metal-removal rate ~1 in3 work material per hour for every 20 A of current

25. Electrode (Tool) Wear
During discharge process, tool subject to wear or erosion
Difficult to hold close tolerances as tool gradually loses its shape during machining operation
Average wear ratio of workpiece to electrode is 3:1 for metallic tools
Graphite electrodes wear ratio 10:1

26. Reverse-Polarity Machining
New development that promises to be major breakthrough in reducing electrode wear
Molten metal from workpiece deposited on graphite electrode about as fast as electrode worn away
Operates best on low spark-discharge frequencies and high amperage
Improves metal-removal rates and reduces electrode wear

27. Overcut
Amount the cavity in the workpiece is cut larger than the size of electrode used in machining process
Distance between surface of work and surface of electrode (overcut) is equal to length of sparks discharged
Constant over all areas of electrode
Amount ranges from to .007 in. and dependent on amount of gap voltage

28. Overcut
Amount varied to suit metal-removal rate and surface finish required
Determines size of chip removal
Size of chip removed important factor in setting amount of overcut because:
Chip in space between electrode and work serve as conductors for electrical discharges
Large chips produced with higher amperages require larger gap to enable them to be flushed out effectively

29. Surface Finish
Low metal-removal rates, surface finishes of 2 to 4 µin. possible
High metal-removal rates, finishes of µin. produced
Fast metal removal (roughing cuts)
High amperage, low frequency, high capacitance and minimum gap voltage required
Slow metal removal (finish cut)
Low amperage, high frequency, low capacitance and highest gap voltage required

30. Advantages of EDM
Any material that is electrically conductive can be cut, regardless of its hardness
Work can be machined in hardened state, thereby overcoming deformation caused by hardening process
Broken taps or drills can readily be removed from workpieces

31. Does not create stresses in work material, since tool never comes into contact with work
Process is burr-free
Thin, fragile sections easily machined without deforming
Process is automatic – servo mechanism advances electrode into work as metal removed
One person can operate several EDM machines at one time

32. Intricate shapes, impossible to produce by conventional means, are cut out of a solid with relative ease
Better dies and molds can be produced at lower cost
A die punch can be used as electrode to reproduce its shape in matching die plate, complete with necessary clearance

33. Limitations of EDM Metal-removal rates are low
Material to be machined must be electrically conductive
Cavities produced are slightly tapered but can be controlled for most applications to as little as in. in every .250 in.

34. Rapid electrode wear can be come costly in some types of EDM equipment
Electrodes smaller than .003 in. in diameter are impractical
Work surface is damaged to depth of in. but is easily removed
Slight case hardening occurs
However, may be classed as advantage in some instances

35. Wire-Cut EDM Machine Uses thin brass or copper wire as electrode
Makes possible cutting most shapes and contours from flat plate materials
Complex shapes: tapers, involutes, parabolas, and ellipses
Process commonly used for:
Machining tungsten carbide, polycrystalline diamond, polycrystalline cubic boron nitride, pure molybdenum, difficult-to-machine material

36. The Process
Uses CNC to move workpiece along X and Y axes in horizontal plane toward vertically moving wire electrode
Electrode does not contact workpiece but operates in stream of dielectric fluid
Directed to spark area between work and electrode
When in operation, dielectric fluid in spark area breaks down, forming gas that permits spark to jump between workpiece and electrode
Eroded material caused by spark washed away

37. Operating Systems
Four main operating systems of wire-cut electrical discharge machines
Servo mechanism
Dielectric fluid
Electrode
Machine control unit

38. Servo Mechanism
Controls cutting current levels, feed rate of drive motors, and traveling speed of wire
Automatically maintains constant gap of .001 to .002 in. between wire and workpiece
Important there be no physical contact
Advances workpiece into wire, senses work-wire spacing, and slows or speeds up drive motors to maintain proper arc gap

39. Dielectric Fluid Usually deionized water Serves several functions:
Helps initiate spark between wire and work
Serves as insulator between wire and work
Flushes away particles of disintegrated wire and work from gap to prevent shorting
Acts as coolant for both wire and workpiece

40. Electrode
Spool of brass, copper, tungsten, molybdenum, or zinc wire ranging from .002 to .012 in. in diameter (2 to 100 lb)
Continuously travels from supply spool to takeup spool so new wire always in spark area
Both electrode wear and material-removal rate from workpiece depend on:
Material's electrical and thermal conductivity, its melting point and duration and intensity of electrical pulses

41. Characteristics of Electrode Materials
Be good conductor of electricity
Have high melting point
Have high tensile strength
Have good thermal conductivity
Produce efficient metal removal from workpiece

42. Machine Control Unit Separated into three individual operator panels
Control panel for setting cutting conditions (servo mechanism)
Control panel for machine setup and data required to produce part (numerical control)
Control panel for manual data input (MDI) and cathode ray tube display