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Micro-EDM processesand Application of micro-reverse
EDM processfor microscale fabrication
1
Presented bySachin A. Mastud
VJTI, Mumbai
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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
2
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Principle of EDM
3
Preparation Phase
Phase of Discharge
Interval Phase
10 MHz
Elecro-mechanical Theory
Thermo-mechanical Theory
Thermo-electric Theory
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Electrode gap monitoring and control
4
10 MHz
Mathematical adaptive control theory Advances in computer technology and advanced algorithms for machine control
(Artificial intelligence, ANN)
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Electrode gap monitoring and control
Debris gathering at Bubble boundary
Debris and Bubble particles generated
by single spark Difficult to understand the EDM gapphenomena Even in steady state
conditions
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Arc Machining and Spark Machining Any difference? Criteria of selection of dielectric
Air as a dielectric Dry EDM
Flushing Suction, pressurized, interrupted, two-way,
internal, oxidized dielectric
Forces during EDM
Expansion, dissociation, and contraction of the bubbles
formed during process
Variation from maximum positive to minimum negative,
negligible in case of sinking EDM however quantifiable in W-
EDM and Micro-EDM
6
Characteristicsof EDM
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Measurement of discharge location Factors affecting the discharge position
Discharge location Nano-powders (Mirror surface finish)
One voltage pulse for bundle of electrode
Multi-spark EDM process
7
Control ofdischargelocation
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Pulse conditions
Pulses with longer peak current and
duration achieves high MRR and
less tool wear Formation of
carbon layer Rough conditions
Low current and low pulse
duration brings high surface finish,
less tool wear and low MRR
High current and low pulse on time
results in higher MRR, and EDMefficiency; however high tool wear
8
Interaction of parameters
Removal
Rate
SurfaceRoughness
Tool Wear
D
ischarge
C
urrent
Time
No waveform which can satisfy allthe constraints
Factorsinfluencingthe tool wear Influence of polarity normal EDM tool-positive
micro-EDM Workpiece positive
Influence of thermal properties to tool and workpiece material
- Thermal conductivity
- Higher melting or boiling point
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MicroMicro--ManufacturingManufacturing - What is it?
9
Micro-structures manufactured by micro-SLA Japan
Klocke NanotechnikMicro-Motor
Zeiss- GermanyMicro-parts
Micro-EDMNTU - Taiwan
Micro-millingFanuc- Japan
70 m - Human Hair
25 m - Characters
Manufacture of productswith the followingfeatures:
about 100100 m to about 10 mm in sizem to about 10 mm in size
contain very complex 33--DD (free(free--form) surfacesform) surfaces
employ a wide range of engineeringmaterialswide range of engineeringmaterials
possess extremely high relative accuraciesextremely high relative accuraciesin 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 level
Micro/nano components and material removal is at micro/nano level
Unfortunately, the
present day notion is
Machining of highly miniature
components with miniature
features NOTCORRECT
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 -
machining
Micro-machining
Beam energy based
- machining
Chem. & EC -
machining
-nano finishing
USM
AJM
AWJM
WJM
EBM
LBM
EDM
IBM
PBM
PCMM
ECMM
Micro-fabrication
Classification of micromachining processes
HybridProcesses
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Micromachiningprocesses
13
Energy Used Principle Processesand Features
Mechanical
Force
Material removal via highly
concentrated force
Cutting, grinding, sandblasting.
UR ~ 100 nm, edge radius
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Micromachiningprocesses
14
Energy Used Principle Processesand Features
Dissolution Chemical or electrochemicalreaction based ionic
dissolution
Chemical, PCM and ECM. Small UR,negligible force. Inter-electrode gap,
flow of electrolyte influences
accuracy
PlasticDeformation
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 orliquid 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, brittleness
dose not affect the process
Use of deionized water as dielectric
Absence ofSize Effect
15
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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= CV 2
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EDM Micro-EDM
Circuitry Elements
RC relaxation type Single spark process Forced process for constant voltageand current
User defined pulse on time
RC single pulse discharge Single spark process Single capacitance discharge, noconst V and I
No control gap characteristics
Scaling Effects
Interelectrode gap is 10s of m Low efficiency
Interelectrode gap is 1-5 m 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 and
non-spherical particles are rich in tool material
Understanding of Erosion Mechanism and Oxide free
power production
Important parameters affecting Debris morphology are Current
Voltage
Pulse On-time
Capacitance
Input Energy
Micro-analysisof Debris
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Structures of Debris
Large Size & Small Size
Hollow & Solid Debris
Satellite structure
Hollow Spheres
Dents
Burnt Cores
Micro-analysisof 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
HighEnergy
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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 processstability How will you measure? Ignition delay time
GroupNumber Group1 Group2 Group3
PlanetaryMotion Yes No NoExternal 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 transitivity
rather than breakdown strength Yo et al.
Absence of metallic particles can be one of the causes of arching
Micro-EDM processstability
1 Low energy2 High Energy
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Variants of micro-EDM
22
Figure : Micro rods machining processes
Process Capability Limitation
BEDG Min. 3 m diameter electrode, maximum10aspect ratio, 0.6 Ra surface finish
Only single electrodescanbe machined
Micro-WEDG Min. 5 mdiameter electrode, maximum10aspect ratio, 0.8 Ra surface finish
Cylindrical electrodesaswell asarrayedelectrodescant be machined
Micro-WEDM Best resultsobtainedare 10x10square array (23m width, 700 m height), minimum machiningsize achievable is20 m, surface finish 0.07-0.35m Ra, and maximumaspect ratio 100
Cylindrical arrayed structurescant bemachined
Diamond milling microtower of 1 mm in height and 25 m square Mechanicalprocessinvolvesmachiningstresses
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Research issuesin micro-EDMMicro-EDM Research Areas
Handling Electrode andworkpiece
preparation
Off-machine electrodepreparation
Drilling,threading
holes(WEDM)
Mfg. Micro 3Delectrode
On-machine electrode
Stationery
blockRotating Disk
Guided
running wire
MachiningProcess
ProcessParameters
SourcesofErrors
Machine
Electrode
Jigsand
Fixture
Electrodewear and
machiningstrategies
Multi
electrode Z-compensation
Wear
monitoringsystem
Uniform wear
method
Measurement
Surfacequality
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
24
Applications
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Reverse replication of
arrayed hole on theplate electrode to the
bulk material by change
in the polarity
Machined structures
have a dimensions
equal to the original
dimension of pocket
minus interelectrode
gap
Important operating
parameters are voltage ,
capacitance, threshold,
and the feed
25
Machining of arrayed micro-structuresby REDM
Figure : Working of micro and reverse micro EDM processes
aa) Normal EDM
ab) ReverseEDM
Figure : a) array of 4 microrod machined, b) plate used as
a tool during machining
Bulk Rod
Micro-rods
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Problem Statement : Machining of high aspect ratio arrayedmicrostructures by micro reverse EDM process.
26Figure : set up of the micro-REDM process
Machining of arrayed micro-structuresby REDM
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Applicationsof micro-REDM
Process is capable of achieving
Machining ofarrayed structureswith varying shapes(Replication of initial geometry)
High aspect ratio structures
(aspect ratio of 33 has been achieved)
Reduced errors due to tool position and tool wear
(Less error due to no need of positioning)
Increase in productivity Thin and long wall structures
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Applicationsof micro-REDM
Mechanical
Micromachining
As a electrode inarrayed hole/cavity
machining
Mask preparation
As a tool for generatingstable plasma
Heat ExchangingHexagonal and thin wall
structures
Automobile
Micronozzels
Biomedical
As a interface device for
capturing neural signals
Brain neural activity
recording
Arrayed microholes as a
spray nozzels in thebiotechnology applications
Microneedels- syringe
Holding sights for the
testing reagents
MEMS
Arrayed holes for passing
wires in MEMS devices
Thin wall structures as a
cooling devices in MEMS
system
Shaft for micro robotsmicro actuator
Applications
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Authors ResultsAchieved Methodology
Kim et. al.
(2006)
V and C as parameters, array of 3 microrods each
of 35 m in diameter and 1.5 mm in length formed
on WC bulk rod
micro-EDM to fabricate the microholes
on a plate and then plate was used as a
tool
Yi et. al.
(2007)
3X3 and 4X4 array of 80m square, height 600 m,
wall thickness was 15 m.
Single electrode machined by micro-
WEDM, arrayed holes are machined on SS
plate by micro-EDM
Zeng et. al.
(2008)
AgW 5X5 electrode array, average diameter 35
m, length 250 m, 100 m hole spacing.
Use of ultrasonic vibrations at 20 KHz frequency.
Block micro-WEDG to fabricate single rod,
machining arrayed holes on plate by
machining individual hole
Review of work done in micro-REDM
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Cu plate thickness = 200 m
Bulk WC rod of 300 m initialdiameter
Operating parameters were 80 and
100 V voltage and 50, 650, 5000 pF
capacitance
30
Figure : Fabricated arrayed microrods (30 m
dia., 1.5 mm height) [2].
Figure : Effect of voltage at the constant
capacitance on machining time [2]
Figure : Effect of capacitance on
machining time [2]
Review of work done in micro-REDM
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Componentsfabricated by micro-REDM
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0
50
100
150
200
250
300
60 80 100 150
MachiningTime(Min)
Voltage
Effect of Voltage
0
200
400
600
800
1000
1200
1 2 3 4
MachiningTime(Min)
CapacitanceL evel
Effect of Cpacitance
0
10
20
30
40
50
60
70
80
0.2 0.4 0.6 0.8
MachiningTime(Min)
Amplitude
Eff ect of Amplitudeof vibration
a)
b) c)
Figure : 5x5 arrayed electrode
(35 m diameter, 8 aspect ratio) [13]
Figure : Effect of a) voltage, b) amplitude of vibration , and c) capacitanceon machining rate . [13]
V= 100 V C =4700 pF
Amplitude = 0.4 m
Review of work done in micro-REDM
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Machining of 3x3 and 4x4
microarrayed structure with
each square rod of 80 m
Micro-REDM is done at 100V
and 500 pF capacitance
No characterization of micro-
REDM process
33
Figure : a) hole array on plate prior to machining,
b) after the machining
Figure : a) machined electrode array , b) used electrode
array for micro-EDM of holes
Review of work done in micro-REDM
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Review of work in Reverse-micro Wire EDM
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No detailed study which characterize the micro-REDM process indicating
the process capability is done
Process responses like surface finish, erosion rate and the geometricalaccuracies are not quantified
No data on the process mechanicsdifference between the micro-EDM andthe micro-REDM process is reported
Only plausible reasons are given in the literature for process instability in
micro REDM process
No mathematical model is available which can predict the processresponses in arrayed structure machining via reverse EDM
35
Review of work done in micro-REDM
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Issuesin micro-REDM
Issuesin micro-REDM process
Issuesrelated to processstabilityand accuracy
Formation of short bridge
Less positional accuracy
compared with number of
pulses
Spark formation on singlerod even though it is
simultaneous processing
No tool rotation
Less MRR
Issuesrelated totool electrode
Tool wear
Thin plates, more
pocket enlargement
Accurate machining
of pockets on plate
Issuesrelated tofabricatedstructures
Tapered surface
Surface burning
Non uniform surface
morphology
High surface roughnessDelicate structures
Deflection of microrods
Redeposit layer
Gas entrapment
Deformation of electrode
due to power of arc
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Experimentsin micro-REDM
Level Voltage(V) Capacitance(nF) Threshold(%) Feed(m/ sec)
1 80 1 25 5
2 100 10 50 15
ColumnA B C D
Expt. run Voltage Capacitance Threshold Feed
1 1 1 1 1
2 1 1 2 2
3 1 2 1 2
4 1 2 2 1
5 2 1 1 2
6 2 1 2 1
7 2 2 1 1
8 2 2 2 2
Measured Response
Geometrical accuracySurface roughness
Zero error length
Erosion rate
Surface finish
Surface morphology
Operating parameters and selected levels
Scheme of experimentation
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Imagesof the micro rods machined ineach run of experiment
Workpiece geometry :Machining of 400 m square
and 200 m cylindricalelectrodes, machined length 1
mm
Experimentsin micro-REDM
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Machining Time
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Main effect plot for dimensional accuracy- cylindrical rod
40
Meanof%ErrorinDiameter
100V080V
-30
-40
-50
-60
10nF01nF
50%25%
-30
-40
-50
-60
15micron/s05micron/s
GapVoltage Capacitance
Threshold Feed
MainEffectsPlot (data means) for%Error inDiameter CylindriacalRodat 0%L
OperatingParameter Voltage Capacitance Threshold Feed
Parameter Setting 80-100 V 1-10 nF 25-50 % 5-15 m/sec
Change in Response (at 0% L) 48% 16%
Change in Response (at 25% L) 36% 27%
Change in Response (at 50% L) 33% 30%
Change in Response (at 75% L) 48% 25%
Changein Response (at 100% L) Not Significant 110%
Meanof%ErrorinDiameter
100V080V
8
6
4
2
0
10nF01nF
50%25%
8
6
4
2
0
15micron/s05micron/s
GapVoltage Capacitance
Threshold Feed
MainEffectsPlot (data means) for%Error inDiameter at 100%L
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Preliminary experiments- Geometrical accuracy
41
Figure : % Error in diameter of Cylindrical Rod at varying a)
voltage, b) capacitance, c) threshold , and d) feed
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Main effect plot for geometrical accuracy- Square rod
42
OperatingParameter Voltage Capacitance Threshold Feed
Parameter Setting 80-100 V 1-10 nF 25-50 % 5-15 m/sec
Change in Response (at 0% L) 64% Not Significant -73% Not Significant
Change in Response (at 25% L) 28% 29% Not Significant Not Significant
Change in Response (at 50% L) 46% 31% Not Significant -25%
Change in Response (at 75% L) 49% 28% -40% -42%
Changein Response (at 100% L) Not Significant Not Significant Not Significant Not Significant
Meanof%ErrorinSideofRod
100V080V
-15
-20
-25
-30
10nF01nF
50%25%
-15
-20
-25
-30
15micron/s05micron/s
GapVoltage Capacitance
Threshold Feed
MainEffectsPlot (data means) for%Error inSideofRodat 0%L
Meanof%ErrorinSideofRod
100V080V
1
0
-1
-2
-3
10nF01nF
50%25%
1
0
-1
-2
-3
15micron/s05micron/s
GapVoltage Capacitance
Threshold Feed
MainEffectsPlot (data means) for%Error inSideofRodat 100%L
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%Error in the side of square rod
Figure : % Error in diameter of Square Rod at varying a) voltage,b) capacitance, c) threshold , and d) feed
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Main effect plot for Erosion Rate
Increase in the capacitance and voltage increases the pulse energy and
flushing strength
Increasing the energy content per pulse increases the crater size and thesurface roughness
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OperatingParameter Voltage Capacitance Threshold Feed
Parameter Setting 80-100 V 1-10 nF 25-50 % 5-15 m/sec
Change in Response (Erosion rate) 300% 250% Not Significant Not Significant
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Main effect plot for Zero Error Length- Cylindrical rod
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OperatingParameter Voltage Capacitance Threshold Feed
Parameter Setting 80-100 V 1-10 nF 25-50 % 5-15 m/sec
Change in Response (Zero error
length)
Not Significant -11% Not Significant 10%
None of the processingparameter isfound significant for zero error length of square rod
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Main effect plot for surface roughness
Value of surface roughness varies from 1.6 m Ra to as high as 6.3 m Ra
under different experimental conditions.
Voltage and machining time affects the surface roughness, at 80V
machining time is more and at the 100V pulse energy is more
46
Sample 3
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Main effect plot for surface roughness-cylindrical rod
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OperatingParameter Voltage Capacitance Threshold Feed
Parameter Setting 80-100 V 1-10 nF 25-50 % 5-15 m/sec
Change in Response
(Surface Roughness)
Not Significant -28% 38% -20%
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Main effect plot for surface roughness- square rod
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OperatingParameter Voltage Capacitance Threshold Feed
Parameter Setting 80-100 V 1-10 nF 25-50 % 5-15 m/sec
Change in Response (Surface Roughness) -50% -85% Not significant Not significant
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Surface Morphology
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Surface near tip exhibits number
of craters , whereas the surface at
the root is relatively smooth.
Smooth surface with almost no
pits is observed near the root in
the magnified image of fabricated
structure
Root Surface
Tip Surface
A
A
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0
5
10
15
20
25
1 2 3 4
Oxygen
Percenatge
SampleNumber
SqaureRodAtTip
AtRoot
0
5
10
15
20
25
30
1 2 3 4
Oxygen
Percentage
SampleNumber
CylindricalRodAtTip
AtRoot
0
5
10
15
20
25
30
35
40
45
1 2 3 4
CarbonPercentage
SampleNumber
SquareRodAtTip
AtRoot
0
5
10
15
20
25
30
35
40
45
1 2 3 4
CarbonPercentage
Sample Number
CylindricalRodAtTip
AtRoot
(a) (b)
(c) (d)
EDSanalysis Oand Ccontent
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Element Carbon Oxygen Copper Zinc
Percentage 12.55 4.40 54.30 23.68
Element Oxygen Carbon
Square Cylinder Square Cylinder
At Tip At Root At Tip At Root At Tip At Root At Tip At Root
Maximum 18% 7% 27% 12% 42% 22% 41% 15%
Minimum 4% 4% 5% 3% 17% 0% 0% 0%
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0
10
20
3040
50
60
70
1 2 3 4
CopperPercentage
SampleNumber
SquareRodAtTip
AtRoot
0
10
20
30
4050
60
70
80
90
1 2 3 4
CopperPercentage
Sample Number
Cylindrical RodAt Tip
At Root
0
5
10
15
20
25
30
35
40
1 2 3 4
ZincPercentage
Sample Number
SquareRodAt Tip
At Root
0
5
10
15
20
25
30
35
40
1 2 3 4
ZincPercentage
SampleNumber
CylindricalRodAt Tip
At Root
(a) (b)
(c) (d)
EDSanalysis Cuand Zncontent
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Element Carbon Oxygen Copper Zinc
Percentage 12.55 4.40 54.30 23.68
Element Copper Zinc
Square Cylindrical Square Cylindrical
At Tip At Root At Tip At Root At Tip At Root At Tip At Root
Maximum 58% 65% 62% 82% 30% 34% 22% 34%
Minimum 25% 45% 18% 50% 5% 22% 0% 0%
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Experiment Results
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Maximum % error in dimensions are more at 80V compared with
100 V; 60%(max) error in the diameterof machined rod observed
at the tip
With change in voltage from 80 V to 100 V erosion rate
increases 3 times while increasing capacitance from 1 nF to
10 nF, erosion rate increases by 2.5 times.
Increasing the capacitance and decreasing the feed rate
achieves improvement in zero error length
Surface roughness (Ra ) varies between 1.6-6.3 mIn general less variation in the response parameters are
observed with the change in threshold and feed values
Maximum oxygen content (27%) is observed at the tip of
cylindrical rod, while maximum carbon (42%) is observed at the tip
of square rod.
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Conclusion
Understanding of micro-EDM machining characteristics are
important to achieve important response variables
Voltage and capacitance emerges as a most influential process
parameters in micro REDM
Quantification of the roles played by debris during the micro-
EDM/micro-REDM process are not reported widely
Tool (plate) wear and frequent arcing are the main causes for
tapered surface
Need to model the process for predicting geometrical accuracies and
bridge formation mechanism
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Arrayed structures machined at MTLIIT Bombay
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Future of micromachining
From Photonics Spectra, 1998
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Assembly
Machine
Robot
Punching
Machine
Electrical
Discharge
Machine
MillingMachine
Future of micromachining
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