Lecture 2 edm wedm and medm

Electrical Discharge Machining

• 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

Fundamentals of EDM

Preparation Phase

Phase of Discharge

Interval Phase

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

• 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

• 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

Debris gathering at Bubble boundary

Debris and Bubble particles generated by single spark

Fundamentals of EDM

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

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

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

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

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

Fundamentals of EDM

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

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

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.

Voltage and Current characteristics

Types of pulses Effect of pulses Pulse classification systems Data acquisition and classification

EDM Schematics

Components of EDM

Tool Wear and Tool MaterialsGraphite is suitable material with good electrical conductivity and machinabilityCopper WCu and WAgBrass

Corner wear ratio

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

Parameters affecting EDM performance

Erosion Rate and Surface Finish

Effect of Pulse Current and Pulse on time

EDM hazards

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

Processing and Response parameters

• Electrode material

• Accuracy and finish of electrode manufacture

• Current/ voltage

• Frequency

• Pulse width

• 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

• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


• 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.


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.


Effect of Current

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.


Effect of Frequency

Gap ↓ Voltage ↓Voltage ↓ Current ↓ Current ↓ MRR ↓Current ↓ Accuracy & finish ↑Gap ↓ Poor flow of dielectric.

Processing and Response parametersEffect of Voltage

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

Machine Construction

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

Modern controllers uses gap controlling strategy tocontrol debris

Dielectric flushing (injection, suction, & electrodejump)

Jet sweeping

Rotary Electrode/workpiece method.

Without Rotation

With Rotation


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


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

• 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

• 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

Dielectric Fluid

Work Material Fluid Medium Application

Aluminum

Hydrocarbon oil or glycerin-water (90:10)

Submerged

BrassMild SteelStainless steelTool steelTungstenCarbide Mineral oil

• 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

• 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

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

• 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

• 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.


• 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.


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

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

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.

Wire-EDM

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).

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

WEDM machine classification

• 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

• 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

• 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

• 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

• 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

• 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

• 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

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.

END

“Micro-EDM processes”

71

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

72

Electrode gap monitoring and control

73

10 MHz

• Mathematical adaptive control theory• Advances in computer technology and advanced algorithms for machine control (Artificial intelligence, ANN)

Micro-Manufacturing - What is it?

74

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

75

• 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?

76

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

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

Micromachining processes

78

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

Micromachining processes

79

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.

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

80

81

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

82

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

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

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

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

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

87

Variants of micro-EDM

87

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

88

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

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

89

Applications

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

90

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

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

92

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

93

Components fabricated by micro-REDM

94

Reverse-micro Wire EDM

95

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

Surface Morphology

96

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

Arrayed structures machined at MTL IIT Bombay

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Education

Lecture 2 edm wedm and medm