edm notes
DESCRIPTION
SHORT NOTES ON EDMTRANSCRIPT
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Electrical Discharge Machining
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• The process dates back to WW I & II when work as wellas substantial tool material was removed due to manualfeeding of electrode.
• Later vibratory electrodes were used to control interelectrode gap.
• Two Russian scientists developed R-C circuit and servocontroller.
• The Die sinking version of EDM was developedsometime in 1940s.
• The process modeling involves understanding ofcomplex hydrodynamic and thermodynamic behavior ofthe fluid.
Fundamentals of EDM
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Fundamentals of EDM
Preparation Phase
Phase of Discharge
Interval Phase
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Fundamentals of EDM
Voltage –Current curves (Free, Normal, Stationary located, and Short circuit discharges)
General observations Difficult to start the process with very clean
dielectric Firing of high current discharges at same voltage is
easy in contaminated dielectric New ignition opt to ignite in prior discharge regions Greater ignition preferences in more contaminated
regions
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• DC pulses of appropriate shape, frequency and duty cycle are used. This is used even for motor control now-a-days. Frequency is ~ 100,000 Hz.
• Spark is initiated at the peak between the contacting surfaces and exists only momentarily. Spark temp is 12,000 C. Metal as well as dielectric will evaporate at this intense localized heat. A crater is caused by both due to the local evaporation as well as the vapor action.
• Vapor quenches and next spark it at another narrow place. Thus, spark wanders throughout the surface making uniform metal removal for the desired finish.
Fundamentals of EDM
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• Material removal in EDM is based on erosion effect. • Several theories have been proposed:
– Electro-mechanical theory: electric field force exceeds the cohesive force of lattice.
– Thermo-mechanical theory: Melting of material by ‘flame-jets’. – Thermo-electric theory: Generation of extremely high
temperature due to high intensity discharge current.
Fundamentals of EDM
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Debris gathering at Bubble boundary
Debris and Bubble particles generated by single spark
Fundamentals of EDM
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Large number of Spherical particles with few non-spherical particles
Spherical particles are rich in workpiece materialand non-spherical particles are rich in tool material
Understanding of Erosion Mechanism and Oxidefree power production
Important parameters affecting Debris morphologyare
Current Voltage Pulse On-time Capacitance
Input Energy
Fundamentals of EDM
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Micro analysis reveals that there is movement of material from workpiece to cathode and vice-versa Normal distribution of particle size (Stochastic nature) Structures of Debris-
Large Size & Small Size Hollow & Solid Debris Satellite structure Hollow Spheres Dents Burnt Cores
Fundamentals of EDM
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a)Dendrite structure; b)Solidsphere; c)Satellite formation;d) Non-spherical particles
Microanalysis of Debris – Low EnergyDensely populated, Small diameter, solid particles
Fundamentals of EDM
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a)Debris structure, b)Hollow sphere, c)Dendrite structure, d)Satellite with dent formation, e)Dent formation
Larger population of hollow satellites with dents, surface cracks, and burnt core
Fundamentals of EDM
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Effect of Tool Rotation.Results in fine debris particles and improved process
stability. Effect of Ultrasonic Vibrations. Larger particles Large number of particles with spherical geometry More uniformity of spherical and non-spherical particles Uniform mixing of materials More collision between debris particles
Fundamentals of EDM
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Fundamentals of EDM
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A series of voltage pulses of magnitude about 20 to 120 V and frequency on the order of 5 kHz is applied between the two electrodes, which are separated by a small gap, typically 0.01 to 0.5 mm.
When using RC generators, the voltage pulses are responsible for material removal.
Fundamentals of EDM
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Breakdown of dielectric during one cycle
Temperatures of about 8000 to 12,000 C and heat fluxes up to 1017 W/m2 are attained during process
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Breakdown of dielectric during one cycle Explosion and
implosion action of dielectric
EDM performance measures such as material removal rate, electrode tool wear, and surface finish, for the same energy, depends on the shape of the current pulses.
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Voltage and Current characteristics
Types of pulses Effect of pulses Pulse classification systems Data acquisition and classification
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EDM Schematics
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Components of EDM
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Tool Wear and Tool MaterialsGraphite is suitable material with good electrical conductivity and machinabilityCopper WCu and WAgBrass
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Corner wear ratio
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Flushing
The main functions of the dielectric fluid are to1. Flush the eroded particles from the machining gap2. Provide insulation between the electrode and the workpiece3. Cool the section that was heated by the discharging effect
The main requirements of the EDM dielectric fluids are adequate viscosity, high flash point, good oxidation stability, minimum odor, low cost, and good electrical discharge efficiency
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Parameters affecting EDM performance
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Erosion Rate and Surface Finish
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Effect of Pulse Current and Pulse on time
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EDM hazards
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Indication of constantly moving spark Importance of Debris content in inter-electrode
gap Discharge conduction through debris chain Effect on surface cracks Process stability primarily depends on discharge
transitivity rather than breakdown strength Absence of Debris can be one of the causes of
arching
Process Stability
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Processing and Response parameters
• Electrode material
• Accuracy and finish of electrode manufacture
• Current/ voltage
• Frequency
• Pulse width
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• Current and voltage: As the voltage drops from A to B, the current increases because of the negative voltage-current relationship. At C, current is interrupted, and voltage goes to zero and reverses to D; but since there is no break down in opposite direction, no current reversal takes place. The voltage now returns to zero and waits for the next pulse.
Operating parameters
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• The energy dissipated in the system is voltage times current times time, it remains fairly constant.
• At ‘A’ energy is zero.• ‘B’ represents the power going to
the work.• ‘C’, ‘D’, ‘E’ and ‘F’ represent traces
at where there are either voltage or current is zero, hence no power.
• In section ‘B’ voltage times current is nearly constant, indicates a constant input of power during a current pulse.
Operating parameters
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• In the inter electrode gap, there isa mixture of electrons, ions, andneutral atoms in the gaseousform.
• Cathode supplies electrons for theflow of current so should beenough to emit the electrons, alsopositive ions in front of cathodeprovide a pulling force.
• Cathode material also matters –Cu is a low melting point alloy so itmelts (at 1083 C) and emitselectrons by heat and electricfield.
• Graphite, W, Mo emit electrons atthe temperatures below theremelting points hence are morestable as cathode.
Operating parameters
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• Resistance to the flow of current is higher near the electrodes.• The voltage drop near cathode is smaller as compared to that
of anode. It helps electrons in achieving high speed to ionizethe gases near cathode.
• Cathode voltage drop ranges from 12V for Cu to 25V forgraphite.
• The plasma generated is at 6000 to 10,0000 C.• (+) ions and electrons (-), due to the mass difference ions move
slowly therefore, 95% of the current is carried by electrons.• The electrons and ions provide major power input to the
cathode and anode surfaces.• When the current is high, evaporation of material from anode
occurs, the stream of atoms coming out of anode surfaceinterferes with the electrons going to the anode.
• Some ions get ionized at the near anode drop but the electronsget additional energy to cause more vaporization of anode.
Operating parameters
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• Straight polarity: in which electrode is usually a cathode (-). Here, work surface energy can be controlled by controlling the current so that anode drop energy provides proper wear and desired surface finish.
• Reverse polarity: in which electrode anode (+) and work (-), in which rough cut higher cutting rates can be obtained with virtually no electrode wear.
Operating parameters
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• Electrode rotating: Improves flushing difficulties with speed of about 200 rpm max. It provides better surface finish.
• Electrode orbiting: Electrode does not rotate but revolve in an orbit. Orbiting need not be restricted to round shape.
• Both actions reduce electrode wear as it gets distributed uniformly.
Operating parameters
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• No Wear EDM: It is defined as the condition when the electrodeto work wear ratio is 1% or less.
• Effect of arc duration: Melting depth is a function of arc durationfor a circular non expanding heat source.
• The maximum melting depth occurs at different durations fordifferent materials subjected to same energy. The melting depthreaches a peak value with an increase in arc duration, it reduceswith further increase in the arc duration.
• Thus, it should be possible to choose an arc duration whichmaximizes the work erosion while holding the electrode to somelesser value.
• In Cu and steel system, at the arc duration suitable for maximummelting of steel, the melting of Cu is at the minimum.
Operating parameters
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• Electrode polarity: The energy distribution between anode and cathode is a function of –– ratio of electron current to ion current at cathode– Physical constant (work function) of the cathode material.– In Cu as cathode current density decreases, the electron to ion
current ratio also decreases. As the arc duration increases, the energydelivered to the gap concentrates at the cathode. Therefore, theelectrode must be of positive duration if long arc durations are usedto achieve the no-wear condition.
• Electrode coating is observed in Cu-steel system.– Coating of electrodes with thin black film of carbon which has erosion
resistance and tend to reduce electrode wear.
Operating parameters
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As current increases, the depth and width of the crater becomes larger. So also the MRR. But this may result in rough surface. However, this can be used to our advantages to obtain mattysurface.
Processing and Response parameters
Effect of Current
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As frequency increases, the depth and width of the crater becomes smalleralthough the MRR may not be affected as there will be more craters per unittime. However, frequency has a limit since initiation of spark requires certainminimum time required for the breakdown of the dielectric. Similarly thespark needs some time to quench. In principle, one should operate as high afreq as possible.
Processing and Response parameters
Effect of Frequency
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Gap ↓ Voltage ↓Voltage ↓ Current ↓ Current ↓ MRR ↓Current ↓ Accuracy & finish ↑Gap ↓ Poor flow of dielectric.
Processing and Response parametersEffect of Voltage
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A layer of resolidified metal of 0.002– 0.050 mm thick remains on thesurface. This may flake off duringcyclic loading. When high fatigue lifeis required, this layer must beremoved on a subsequent operationsuch as chemical etching.
Processing and Response parametersEffect on fatigue Life
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Machine Construction
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EDM process Variations
0
10
20
30
40
50
1 2 3Group Number
Cont
ent P
erce
ntag
e
Normal DischargeOpen CircuitAbnormal Discharge
Group Number Group 1 Group 2 Group 3
Planetary Motion Yes No No
Debris Layer Yes Yes No
Input Voltage 15mV 15mV 15mV
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Modern controllers uses gap controlling strategy tocontrol debris
Dielectric flushing (injection, suction, & electrodejump)
Jet sweeping
Rotary Electrode/workpiece method.
Without Rotation
With Rotation
EDM process Variations
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Use of Magnetic field Magnetic force used to change path of debris motion.
Magnets attached on plates rotating under machiningzone
Magnetic force is useful not only at low energy but also athigh energy inputs
1(05A,20µs), 2( 20A,350µs)
Magnetic Assistance
EDM process Variations
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Condition of Adhesion Debris removal and Sparking
The combined process of EDM with USM had the potential to preventdebris accumulation, improve machining efficiency, and modify themachined surface.
Vibration Assistance EDM process Variations
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• Break down characteristic: Non-conducting until breakdown and very high conduction through rapid ionization just after breakdown.
• High latent heat
– to minimize evaporation
– to contain the spark in a narrow region for localized sparking
• Low viscosity for ease of flow
• Efficiency as coolant. It is kerosene or water.
Dielectric Fluid – Desirable properties
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• Functions of Dielectric FluidIt acts as an insulator until sufficiently high potential is
reached .
Acts as a coolant medium and reduces the extremely high temp. in the arc gap.
More importantly, the dielectric fluid is pumped through the arc gap to flush away the eroded particles between the work piece and the electrode which is critical to high metal removal rates and good machining conditions.
Dielectric Fluid
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Dielectric Fluid
Work Material Fluid Medium Application
Aluminum
Hydrocarbon oil or glycerin-water (90:10)
Submerged
BrassMild SteelStainless steelTool steelTungstenCarbide Mineral oil
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• Dielectric fluids: should have very high flash point and very low viscosity.– Petroleum based hydrocarbons– Silicon fluids mixture with petroleum oils for machining of titanium,
high MRR and good SF.– Kerosene, water-in oil emulsion, distilled water.
• Cooling of dielectric is required sometimes while cutting with high amperage can be done by using heat exchangers.
• Filtering of dielectric is necessary to filter out 2 – 5 µm particles.
Dielectric Fluid
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• Insulation and conduction: Insulating characteristic is measured by the maximum voltage that can be applied before ionization.
• Cooling: ability to resolidify vaporized material into chips , thermal transfer capability.
• Flushing: Sufficiently viscous to pass through a small gap &remove debris.
• Methods of fluidapplication
– Normal flow– Reverse flow– Jet flushing– Immersion flushing
Dielectric Fluid
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Workpiece and Tool Material Electrode Materials ApplicationsBrass High Accuracy for most metalsCopper Smooth finish
Low accuracy for holesZinc Alloys Commonly used for steel, forging
cavitiesCopper-Graphite General Purpose workSteel Used for nonferrous metalsCopper Tungsten High accuracy for detail workGraphite Large volume/fine details
Low wearExcellent machinability
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• Tool electrodes transport current to the work surface.
• Graphite– Coarse (for large volume) or fine (for fine finish).
– Normally used for steel provides large MRR/A as compared to other metallic electrodes.
– When used for WC, deposits of carbon on work leads to flow of current without ionization of dielectric and hence arcing. High density, fine particles preferred.
– Average surface finish using graphite electrodes:0.5 µm Ra.
• Copper Graphite– For rough and finish machining of WC.
Workpiece and Tool Material
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• Copper– When smoothest surface finish is required.
– In no-wear mode, copper works best under low ampere and long spark times.
– Tellurium increases the machinability of copper.
– Free machining brass is used for making complex shaped electrodes.
– Copper tungsten (70% W) for fine detail and high-precision EDM. High density, strength, thermal and electrical conductivity.
• Tungsten – Tungsten carbide is used for cutting steel and WC.
– Small holes of deeper dimensions.
Workpiece and Tool Material
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• Electrical conductivity
• Less wear due to the spark (Low rw)
• Good machinability
• Good surface finish on w/p
Loss of material from the toolWear ratio Loss of material from the work piecewr =
Tool W/P rw
Brass Brass 0.5
Brass Hard C.S. 1.0
Brass WC 3.0
rw increases with material hardness and decreases with the increase in melting point of the tool material.
Workpiece and Tool Material
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Any material that is electrically conductive can be cut Hardened work pieces can be machined eliminating the
deformation caused by heat treatment. Complex dies sections and molds can be produced
accurately, faster, and at lower costs. The EDM process is burr-free. Thin fragile sections such as webs or fins can be easily
machined without deforming the part.
Advantages
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High specific energy consumption (about 50 times that inconventional machining)
When force circulation of dielectric is not possible,removal rate is quite low
Surface tends to be rough for larger removal rates
EDM process is not applicable to non-conductingmaterials
Disadvantages
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Applications• Mold and die making, slowly becoming a production
process.• Machining of ‘difficult-to-machine’ materials.• Miniature and fragile parts that can not withstand the force
of conventional cutting. Holes of 0.05 mm, slots of 0.3 mm• As EDM is a very slow process, it can be justified only
where the hardness is too high or the features cannot berealized by other means.
• Tool making: sharp corners, small features, deep featuresetc. With the advent of hard cutting tools, full sinking is outof fashion.
• Removal of broken drills or fasteners
• Deep hole drilling of small holes. Eg.: turbine blades, fuelinjection nozzles, inkjet printer head etc.
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Wire-EDM
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Wire EDM• This process is similar to contour cutting with a band saw.
• Slow moving wire travels along a prescribed path, cutting the work piece with discharge sparks.
• Wire should have sufficient tensile strength and fracture toughness.
• Wire is made of brass, copper or tungsten. (about 0.25mm in diameter).
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Wire EDM
• Thin wire of as low as 0.03mm dia is used as the tool.
• For through features dies for punching, blanking and piercing; templates and profile gauges; extruder screws etc.
• Taper also possible
• Upto 4 axes available.
• Water is the common di-electric
Process
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WEDM machine classification
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• Machining of hard and complex shapes with Sharpcorners.
• Risk of wire breakage and bending has undermined thefull potential of the process drastically reducing theefficiency and accuracy of the WEDM operation
• WEDM utilizes a continuously travelling wire electrodemade of thin copper, brass or tungsten of diameter 0.05–0.3 mm, which is capable of achieving very small cornerradii
• The material is eroded ahead of the wire and there is nodirect contact between the workpiece and the wire,eliminating the mechanical stresses during machining
• Machining of EXOTIC and HSTR alloys
WEDM Process
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• The material removal mechanism of WEDM is very similarto the conventional EDM process involving the erosioneffect produced by the electrical discharges (sparks)
• The WEDM process makes use of electrical energygenerating a channel of plasma between the cathode andanode, and turns it into thermal energy at a temperature inthe range of 8000–12,000 C or as high as 20,000 C
• A varying degree of taper ranging from15 degree for a100 mm thick to 30 degree for a 400 mm thick workpiececan also be obtained on the cut surface.
• The microprocessor also constantly maintains the gapbetween the wire and the workpiece, which variesfrom0.025 to 0.05 mm
WEDM Process
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• Number of passes are required to achieve the requireddegree of accuracy and surface finish
• Dry WEDM (in gas) to achieve the high degree of surfacefinish
• The typical WEDM cutting rates (CRs) are 300 mm2/min fora 50 mm thick D2 tool steel and 750 mm2/min for a 150 mmthick aluminium , and SF quality is as fine as 0.04–0.25µRa
• The deionised water is not suitable for conventional EDMas it causes rapid electrode wear, but its low viscosity andrapid cooling rate make it ideal for WEDM
WEDM Process
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• WEDG – machining of fine rods used in electronic circuits;machining of electrodes as small as 5 micron in diameteradvantages of WEDG include the ability to machine a rodwith a large aspect ratio, maintaining the concentricity ofthe rod and providing a wider choice of complex shapessuch as tapered and stepped shapes at various sections.
• Ultrasonic Vibrations to wire to improve surface finish andcutting ratios
• Wire electrochemical grinding
Hybrid WEDM Process
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• Modern tooling applications - wafering of silicon and machiningof compacting dies made of sintered carbide
• For dressing a rotating metal bond diamond wheel used for theprecision form grinding of ceramics
• Advanced ceramic materials – other common machiningprocesses for machining ceramics are diamond grinding andlapping.
• Machining of boron carbide and silicon carbide• MRR and surface roughness depends on processing parameters
as well as workpiece material• Machining of naturally non-conductor by doping with
conducting material• Machining of modern composite materials• MMC and carbon fiber polymers
WEDM Applications
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• WEDM process optimizationFactors affecting performance measures – pulse duration, dischargefrequency and discharge current intensityCutting ratio – Factors affecting CR are properties of the workpiecematerial and dielectric fluid, machine characteristics, adjustablemachining parameters, and component geometry. Use of DOE, ANN.It was found that the machining parameters such as the pulse on/offduration, peak current, open circuit voltage, servo reference voltage,electrical capacitance and table speed are the critical parameters for theestimation of the CR and SF.MRR - discharge current, pulse duration and pulse frequency are thesignificant control factors affecting the MRR and SF, while the wirespeed, wire tension and dielectric flow rate have the least effectSurface finish – all the electrical parameters have a significant effect onthe surface finish
Major Research issues
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• Wire EDM process monitoring and control Fuzzy control system - proportional controls were used traditionally
control the gap. Conventional control algorithms based on explicitmathematical and statistical models have been developed for EDM orWEDM operations
Pulse discrimination system
Knowledge system
Ignition delay based system
Wire breakage - rapid rise in frequency is observed before wirebreaks; control strategy to switch off the generator at high frequency,localized high temperature causes wire breakage, excessive thermalforce
Wire material breakage and fracture
Wire lag and wire vibrations- plasma and material erosion forces,hydraulic forces due to dielectric flow
Major research issues
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Applications
The common applications of WEDM include the fabrication of thestamping and extrusion tools and dies, fixtures and gauges,prototypes, aircraft and medical parts, and grinding wheel formtools.
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END
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“Micro-EDM processes”
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Outline
Principle of EDM process Characteristics of EDM process Control of Discharge location Micro-manufacturing Scope of micromachining Classification of micromachining processes Role of micro-EDM in micromachining Micro-reverse EDM Research issues in micro-EDM related processes Experiments I micro-reverse EDM Future of micromachining
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Electrode gap monitoring and control
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10 MHz
• Mathematical adaptive control theory• Advances in computer technology and advanced algorithms for machine control (Artificial intelligence, ANN)
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Micro-Manufacturing - What is it?
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Micro-structures manufactured by micro-SLA Japan
Klocke NanotechnikMicro-Motor
Zeiss - GermanyMicro-parts
Micro-EDMNTU - Taiwan
Micro-millingFanuc - Japan
70 µm - Human Hair25 µm - Characters
Manufacture of products with the following features:
about 100 µm to about 10 mm in size contain very complex 3-D (free-form) surfaces employ a wide range of engineering materials possess extremely high relative accuracies in the 10-3 to 10-5 range
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• Minimizing energy and materials used for the manufacture of devices
• Integration with electronics; simplifying systems• Cost/performance advantages• Faster devices• Increased selectivity and sensitivity• Drawback-Size effect in mechanical micromachining
Why Miniaturization?
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MICRO MACHINING
Micro Machining
Removal of material at micro level
Macro components but material removal is at micro/nano levelMicro/nano components and material removal is at micro/nano level
Unfortunately, the present day notion is
Machining of highly miniature components with miniature
features – NOT CORRECT
DefinitionMaterial removal is micro/nano level with no constraint on the size of the
component
Scope of micromachining processes
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FABRICATION
Macro-fabrication
Mechanical - µ machining
Micro-machining
Beam energy based - µ machining
Chem. & EC -µ machining
µ-nano finishing
USM
AJM
AWJM
WJM
EBM
LBM
EDM
IBM
PBM
PCMM
ECMM
Micro-fabrication
Classification of micromachining processes
Hybrid Processes
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Micromachining processes
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Energy Used Principle Processes and Features
Mechanical Force
Material removal via highly concentrated force
Cutting, grinding, sandblasting.
UR ~ 100 nm, edge radius<1 µm
Melting and vaporization
Material removal via melting and/or vaporization and debris by high pressure gas
EDM, LBM, EBM. Small UR by reduced the pulse energy, concentration of energy via ultra short pulse duration and/or sharply focused beam by optics
Ablation Decomposition of atoms using incident photon energy or direct vaporization of material via high energy pulses
Excimer/Femto second laser. High dimensional accuracy, less HAZ but low machining speed and high cost of equipment
Solidification Liquid or paste is solidified in a mold and shape of the mold is replicated
Injection molding, die casting, etc.
curing may be required after molding and porosity
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Micromachining processes
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Energy Used Principle Processes and Features
Dissolution Chemical or electrochemical reaction based ionic dissolution
Chemical, PCM and ECM. Small UR, negligible force. Inter-electrode gap, flow of electrolyte influences accuracy
Plastic Deformation
Shape of the product specified by die/punch/mold
Micro-punching, extrusion, etc.
No UR is involved, high speed, spring-back and difficulties in die or mold making
Lamination Material in solid powder or liquid form is solidified layer-by-layer.
Stereolithography, internal as well as external profiles can be formed easily.
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Role of EDM in micromachining
Non-contact machining
3D machining
Physical characteristics such as hardness, brittlenessdose not affect the process
Use of deionized water as dielectric
Absence of Size Effect
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Comparison of EDM and micro-EDM The Resistance Capacitance Relaxation (RC-
relaxation) circuit used in EDM is replaced by the RC-
pulse circuit in micro-EDM.
In the RC-relaxation circuit, current and gap voltage
are controlled at a pre-defined level throughout the
pulse on-time but in modeling attempts in micro-
EDM based on RC pulse circuits, the current and
voltage are frequently assumed to be constant.
On the other hand, in a single discharge of RC-pulse
generator, the voltage and current are not
maintained to any pre-defined level but depend
upon the capacitor charge state at any instant.
E = V I Duty cycle
E = ½ CV^2
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EDM Micro-EDM
Circuitry Elements
• RC relaxation type• Single spark process• Forced process for constant voltage and current• User defined pulse on time
• RC single pulse discharge• Single spark process• Single capacitance discharge, no const V and I• No control – gap characteristics
Scaling Effects
• Interelectrode gap is 10’s of µm• Low efficiency
• Interelectrode gap is 1-5 µm • High efficiency
Typical single spark crater
Comparison of EDM and micro-EDM
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Large number of Spherical particles with few non-spherical particles
Spherical particles are rich in workpiece material andnon-spherical particles are rich in tool material
Understanding of Erosion Mechanism and Oxide freepower production
Important parameters affecting Debris morphology are Current
Voltage
Pulse On-time
Capacitance
Input Energy
Micro-analysis of Debris
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Structures of Debris
Large Size & Small Size
Hollow & Solid Debris
Satellite structure
Hollow Spheres
Dents
Burnt Cores
Micro-analysis of Debris Micro analysis reveals that there is movement of material from
workpiece to cathode and vice-versa
Normal distribution of particle size (Stochastic nature)
Low Energy
High Energy
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Effect of Tool Rotation
Effect of Ultrasonic Vibrations
Effect of workpiece-tool material combination
Effect of polarity
PMEDM
Effect of dielectric
EDM process stability How will you measure? Ignition delay time
Group Number Group 1 Group 2 Group 3
Planetary Motion Yes No No
External material layer Yes Yes No
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Indication of constantly moving spark
Importance of eroded material in inter-electrode gap
Discharge conduction through debris chain
Effect on surface cracks
Process stability primarily depends on discharge transitivityrather than breakdown strength – Yo et al.
Absence of metallic particles can be one of the causes of arching
Micro-EDM process stability
1 –Low energy2 – High Energy
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Variants of micro-EDM
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Figure : Micro rods machining processes
Process Capability Limitation
BEDG Min. 3 µm diameter electrode, maximum 10 aspect ratio, 0.6 µRa surface finish
Only single electrodes can be machined
Micro-WEDG Min. 5 µm diameter electrode, maximum 10 aspect ratio, 0.8 µRa surface finish
Cylindrical electrodes as well as arrayed electrodes can’t be machined
Micro-WEDM Best results obtained are 10x10 square array (23 µm width, 700 µm height), minimum machining size achievable is 20 µm, surface finish 0.07-0.35 µm Ra, and maximum aspect ratio 100
Cylindrical arrayed structures can’t be machined
Diamond milling micro tower of 1 mm in height and 25 μm square Mechanical process involves machining stresses
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Research issues in micro-EDM Micro-EDM Research Areas
Handling Electrode and workpiece
preparation
Off-machine electrode preparation
Drilling, threading
holes (WEDM)
Mfg. Micro 3D electrode
On-machine electrode
Stationery block
Rotating DiskGuided
running wire
Machining Process
Process Parameters
Sources of Errors
Machine
Electrode
Jigs and Fixture
Electrode wear and machining strategies
Multi electrode
Z-compensationWear
monitoring system
Uniform wear method
Measurement
Surface quality
Dimensions
Electrode
Parts
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Machining of mould and die in high strength materials (Carbides,die steel, conducting ceramics) – Recently replaced by high speedmilling process
Chemical aspects of EDM– Production of fine particle powders
– RESA (for ultrafine powders)- Reactive Electrode Submerged Arc EDM
– Diamond like carbon and nano-tubes (solidification of evaporatedmaterial)
– Large amount of energy is consumed in the chemical action during EDM
– Supplying oxygen can enhance the MRR during the process
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Applications
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Reverse replication ofarrayed hole on theplate electrode to thebulk material by changein the polarity
Machined structureshave a dimensionsequal to the originaldimension of pocketminus interelectrodegap
Important operatingparameters are voltage ,capacitance, threshold,and the feed
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Machining of arrayed micro-structures by REDM
Figure : Working of micro and reverse micro EDM processes
aa) Normal EDM
ab) Reverse EDM
Figure : a) array of 4 microrod machined, b) plate used asa tool during machining
Bulk Rod
Micro-rods
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Problem Statement : Machining of high aspect ratio arrayedmicrostructures by micro reverse EDM process.
91Figure : set up of the micro-REDM process
Machining of arrayed micro-structures by REDM
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Applications of micro-REDM
Mechanical
MicromachiningAs a electrode in
arrayed hole/cavitymachining
Mask preparationAs a tool for generating
stable plasma
Heat Exchanging Hexagonal and thin wallstructures
AutomobileMicronozzels
Biomedical
As a interface device forcapturing neural signals
Brain neural activityrecording
Arrayed microholes as aspray nozzels in thebiotechnology applications
Microneedels- syringeHolding sights for the
testing reagents
MEMS
Arrayed holes for passingwires in MEMS devices
Thin wall structures as acooling devices in MEMSsystem
Shaft for micro robotsmicro actuator
Applications
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Components fabricated by micro-REDM
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Reverse-micro Wire EDM
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Images of the micro rods machined in each run of experiment
Workpiece geometry :Machining of 400 µm square and 200 µm cylindrical electrodes, machined length 1 mm
Experiments in micro-REDM
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Surface Morphology
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Surface near tip exhibits numberof craters , whereas the surface atthe root is relatively smooth.
Smooth surface with almost nopits is observed near the root inthe magnified image of fabricatedstructure
Root Surface
Tip Surface
A
A
Sample 3
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Arrayed structures machined at MTL IIT Bombay
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