09-02-2013, 02:20 PM
Electrical Discharge Machining
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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
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
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
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
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
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
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
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
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