Electrical Discharge Machining Unit 95

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

Controlled spark removes metal during electrical discharge machining Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Tool Rectifier Work 220-V AC Current Control Servo Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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)

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

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

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

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

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

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

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

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

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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

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

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

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 .0002 to .007 in. and dependent on amount of gap voltage

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

Surface Finish Low metal-removal rates, surface finishes of 2 to 4 µin. possible High metal-removal rates, finishes of 1000 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

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

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

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

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 .0001 in. in every .250 in.

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 .0002 in. but is easily removed Slight case hardening occurs However, may be classed as advantage in some instances

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

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

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

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

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

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

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

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