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Int. J. , Vol. x, No. x, xxxx 1
Copyright 200x Inderscience Enterprises Ltd.
Analysis on Fabrication of Micro-tools by Micro-
electrochemical Machining Process
ABSTRACT
The removal of material by pulse electrochemical machining process
increases the dimensional accuracy of the micro components because of
locally confined dissolution of the anode material. In the present context,
experiments are carried out to fabricate micro-electrodes (tools) of
diameter < 30 m from cylindrical copper bars of initial diameter 780 m
and to see the influence of different experimental parameters i.e. pulse on
time, frequency, applied voltage and concentration on the amount of
material dissolved during the fabrication of micro electrodes. The detailed
analysis has been presented in this paper.
Keywords: Electrochemical machining; Micro machining; Micro-ECM;
Pulse current, Micro-tools.
1. Introduction
Micromachining is the very basic technology for manufacturing the
miniaturized components and devices. In addition to tight tolerance limit,
the machining affected area and the induction of residual stresses in micro
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parts due to fabrication process should be limited strictly. This is the
reason for which conventional machining techniques are least competent
for micro-fabrication (Zhang et. al, 2007.). Micro-fabrication by
electrochemical dissolution is a promising and cutting edge technology in
the modern age. The material removal mechanism involves in this process
indicate high potential of the process to be utilized for micromachining of
almost all conductive materials regardless of their strength, hardness and
toughness (Debarr and Oliver, 1968). But the machining performance is
always governed by the anodic behavior of workpiece material in a
particular electrolyte (Bhattacharyya et al., 2004). Stress free, burr free
surface with low surface roughness can be obtained by this process as
there is no direct contact between tool and the job (Osenbruggen and.
Philips, 1985). Further more the tool wear rate is zero in pure
electrochemical machining process (Rajurkar et al. 2006, Sen and Shan,
2005, Masuzawa, 2000). Recently extensive researches have been reported
on the easy fabrication of extremely thin micro electrodes, micro hole
drilling and micro slot/profile cutting. For example, the control of micro
tool profile by controlling the current and voltage and the control of
diameter of cylindrical micro-tool by mathematical formulation method
has been reported by Lim and Kim (2001). Fabrication of tungsten carbide
cylindrical micro shafts with good surface finish has been reported by
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Choi et al (2005) using H2SO4 as electrolyte. Wang and Peng (2008)
suggested that the size and accuracy of the micro holes and micro slots are
greatly influenced by the machining parameters setting like voltage,
feeding speed and electrolyte concentration and frequency of supply in
micro electrochemical process. Kim et al. (2005 a) reported that micro
structures with good surface finish and less side cut can be produced on
stainless steel material by micro-ECM process using 0.1M H2SO4
electrolyte. Ahn et al. (2004) reported that the pulse on time plays major
role for the localization of the electrochemical reaction in micro
electrochemical machining. Machining of tungsten electrode with 5m
diameter and micro hole of diameter below 50 m can be possible with
micro-ECM by properly controlling the pulse on time of the power supply
( Lee et al. ,2007). Micro-ECM is very sensitive to the concentration of
electrolyte and the applied voltage, so for proper machining these
parameters setting is important (Kim et al., 2005 b).
However, most of the papers are focused on the effect of input parameters
such as pulse on time, pulse frequency, machining voltage, inter electrode
gap on the machined micro tool diameter or micro hole diameter, over cut
during the hole drilling process and the machining time. So, in the present
context, an experimental analysis has been carried out to determine the
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influence of different input process parameters on the material removal in
the micro electrochemical machining during the fabrication of copper
micro electrodes.
2. Principle of the process
When a metal piece dipped inside an electrolytic solution the highly
energetic surface atom leaves the surface as metal ions and dispersed into
the solution. The simultaneous discharge of ions from the solution forms a
layer over the metal surface. The equilibrium is reached when the total
charge (electron) left in the metal contributes to the formation of layer of
ions whose cumulative charge is equal and opposite to that of the metal
surface. The layers of positive and negative charges at metal-electrolyte
interface leads to the formation of electrical double layer (McGgeough,
1974). This double layer behaves as an electrical capacitor, when low
voltage is applied across the electrodes which are in electrolytic bath. The
conventional ECM is carried out at very high current density that provides
high material removal capacity, but in micro-ECM the applied voltage is
very low which leads to charging of the double layer capacitor. The
double layer effect is feeble in conventional ECM.
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Figure 1 Principle of micro electrode machining
The charging and discharging of double layer capacitor both at cathode-
electrolyte and anode-electrolyte interface affect the electrochemical
dissolution process. In micro-ECM, if continuous DC supply is applied
across the electrodes then there will be no current flow through electrolyte
after full charging of capacitor, hence the material dissolution eventually
ceases. To overcome this problem, a pulsed DC power supply is used.
Figure 1 shows the principle of micro electrode machining by pulsed
electrochemical process. The anode can be rotated and translated along its
own axis. In fig. 1 V represents the applied pulse voltage, C dl is the
double layer capacitance and R is the resistance of electrolyte. The
Cathode
R
C dl
Anode
Pulse
voltage (V)
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charging current flows through the electrolyte along the least resistance
path which is proportional to the gap width (distance) between the
electrodes. So, for localized machining the gap between cathode and
anode is kept as minimum as possible. At minimum gap distance, a
portion of electrode is substantially charged where the time constant for
the formed capacitor does not exceed the pulse on time duration of the
power supply, for which localized material dissolution takes place
(Bhattacharyya et al., 2004). By controlling the gap and the pulse on time
of the DC power supply, micro electrodes of cylindrical shape can be
fabricated. Figure 2 shows the charging and discharging of the double
layer capacitor at two different gaps. Figure 2(a) shows the square pulse
supplied from power supply unit. Strong charging takes place when the
gap between the electrodes is less (fig.2 (b)). So the material dissolution
rate is higher when the gap is smaller (Kozak et al., 2004).
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Figure 2 (a) applied voltage pulse, (b) charging and discharging of double
layer capacitor
(a) (b)
3. Experimental setup
Figure 3 illustrates the indigenously developed micro-ECM setup. It
consists of high precision X, Y and Z CNC stages and a spindle which is
mounted over Z stage. A software based inter electrode gap controlling
system has been deployed to maintain constant inter electrode gap during
the machining. The job is mounted to the spindle by the help of self
centered cullet and can be rotated as well as traversed along its own axis.
The pulse generator supplies pulse voltage of different frequency and duty
factor. The electrolyte is kept inside the Perspex tank where both the
electrodes are kept submerged. An agitator is used to induce electrolyte
flow in the inter electrode gap so that the concentration of electrolyte does
not change during the machining time.
V
t
Strong charging
Weak charging
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Z
X
Y
X,Y, Z controller
Pulsed power
supply
+ve -ve
Spindle
Computer
Column
Agitator
Base
Figure 3 Schematic of experimental setup
4. Experimentation
For experimentation, straight copper bars (98% purity) of 0.78 mm initial
diameter were used as anode and a cylindrical copper block was made
cathode. Two phases of experiments were performed. In phase I,
experiments were carried out for fabricating copper micro electrodes with
the variation of machining voltage, using aqueous NaCl as electrolyte, to
see the capability of the developed setup to fabricate micro tools. In
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second phase, the experiments were conducted with the variation of
different experimental parameters to see their influence on the material
dissolution. The difference in weight of the copper electrode before and
after the machining gives the amount of material removal from the
electrode surface. The time duration for machining, inter electrode gap,
spindle speed, electrolyte agitation and its temperature for all the
experiments were maintained uniformly. Two sets of experiments were
performed for each input parameter one at higher setting and the other at
lower settings. The limits of different parameters were chosen according
to the results of trial experiments. Aqueous NaNO3 with low concentration
was used as electrolyte for all the experiments. The variations in material
removal with the variation of pulse on time, frequency, applied voltage,
electrolyte concentration, length of immersion of the anode in the
electrolyte and variation of inter electrode gap current with immersion
length of anode was analyzed from experimental results.
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Figure 4 Optical images of the fabricated micro-electrodes
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,Duty factor: 0.5 andVoltage: 3V
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,Duty factor: 0.5 andVoltage: 6V
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,Duty factor: 0.5 andVoltage: 6V
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,Duty factor: 0.5 andVoltage: 8V
a
b
c
d
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5. Results and discussions
After completing all the experiments of phase I, images of the fabricated
micro-electrodes were captured with stereo zoom microscope (make
Leica), which are represented in fig. 4. In fig.4a, the small droplet like
structures on the surface of the fabricated electrode indicates the
deposition of debris on the surface. At machining voltage of 6V, the
surface finish of the fabricated electrodes was of good and the deposition
reaction residue was very less (fig.4b). Moreover, the cylindricity of the
fabricated electrode was better (fig4 c). With the increase of machining
voltage, it was observed that the machining was unstable for which the
cone shape electrodes were fabricated (fig.4d). In the phase I experiments,
NaCl solution was used as electrolyte, due to which the residue was
observed on the fabricated tool surface. So, in phase II experiments
NaNO3 solution with low concentration has been used as electrolyte. At
the machining voltage of 6V, the fabricated electrodes had better
cylindrical shape than the other fabricated electrodes.
After conducting phase II experiments, the difference in weight of the
electrodes were recorded and characteristics graphs were drawn. Figure 5
to fig. 12 shows the variation of material removal in the pulse micro
electrochemical machining of copper micro-electrodes in NaNO3 aqueous
solution with different parameter settings. It is found that the material
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removal increases in each plot with increasing in competing parameter
except in fig.11. With the increase of individual parameter setting i.e.
pulse on time, frequency and pulse voltage keeping others constant, high
charging and discharging of electrical capacitor takes place. The higher
charging and discharging of capacitor accelerates the electrochemical
reaction and hence the material removal increases. As the concentration of
electrolyte increases the mobility of the ions decreases due to frequent
collisions between the ions which needs higher driving force to move ions
between the electrodes. That is the reason, for which the material removal
first increases and than decreases with rising concentration of electrolyte
(fig.11). Further more in fig. 12, although the concentration increases but
the ions are getting high driving force due to higher value of constant
parameters which interns increase the material removal.
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Figure 5 Plot of Material removal Vs pulse on time at lower setting
100 150 200 250 300
0.05
0.10
0.15
0.20
0.25
0.30
400kHz, 5V, 0.2M, 3min.
Materialremoval(mg)
Pulse on time (ns)
Figure 6 Plot of material removal Vs Pulse on time at higher setting
100 150 200 250 300
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
700kHz, 8V, 0.5M, 3min.
Materialremoval(mg)
Pulse on time (ns)
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Figure 7 Plot of material removal Vs frequency at lower setting
400 500 600 700 800
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
0.28
100ns, 5V, 0.2M, 3min.
Materialremoval(mg)
Frequency (kHz)
Figure 8 Plot of material removal Vs frequency at higher setting
400 450 500 550 600 650 700
1.0
1.5
2.0
2.5
3.0
3.5
4.0
250ns, 8V, 0.5M, 3min.
Materialremoval(mg)
Frequency (kHz)
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Figure 9 Plot of material removal Vs applied voltage at lower setting
5 6 7 8 9
0.05
0.10
0.15
0.20
0.25
0.30
400kHz, 100ns, 0.2M, 3min.
M
aterialremoval(mg)
Applied voltage (V)
Figure 10 Plot of material removal Vs applied voltage at higher setting
5 6 7 8 9
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
700kHz, 250ns, 0.5M, 3min.
Materialremoval(mg)
Applied voltage (V)
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Figure 11 Plot of material removal Vs concentration at lower setting
0.2 0.3 0.4 0.5 0.6
0.05
0.06
0.07
0.08
0.09
0.10
400kHz, 100ns, 5V, 3min.
M
aterialremoval(mg)
Concentration (M)
Figure 12 Plot of material removal Vs concentration at higher setting
0.2 0.3 0.4 0.5 0.6
1.5
2.0
2.5
3.0
3.5
4.0
700kHz, 250ns, 8V, 3min.
Materialremoval(mg)
Concentration (M)
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6. Conclusions and future scope
The experiments were performed successfully with copper electrodes
using aqueous NaNO3 solution as electrolyte. The variation of material
removal, according to the variation of input parameters, was studied. From
the analysis, following conclusions are drawn:
Always the electrochemical micromachining should be performed
at lower parameter settings to avoid spark machining.
However, the spark machining can potentially be utilized in
combination with the micro-electrochemical process in special
cases to improve the efficiency of the process.
The rotation of one electrode in electrochemical micro-machining
gives better results as it enhances the electrolyte flashing in the
inter electrode gap and to fabricate the cylindrical micro
electrodes.
However, the experimental results may not follow the same trend
if the electrode material/electrolyte changes. To confirm this
further research is required.
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Fig.1. Principle of micro electrode machining
Fig.2. (a) applied voltage pulse, (b) charging and discharging of double
layer capacitor
Fig.3. Schematic of experimental setup
Fig.4. Optical images of the fabricated micro electrodes
Fig.5. Plot of Material removal Vs pulse on time at lower setting
Fig. 6. Plot of material removal Vs Pulse on time at higher setting
Fig. 7. Plot of material removal Vs frequency at lower setting
Fig. 8. Plot of material removal Vs frequency at higher setting
Fig. 9. Plot of material removal Vs applied voltage at lower setting
Fig. 11. Plot of material removal Vs applied voltage at higher setting
Fig. 12. Plot of material removal Vs concentration at higher setting
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Figure 1
Figure 2
Cathode
R
C dlAnode
Pulse voltage (V)
V
t
Strong charging
Weak charging
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Z
X
Y
X,Y, Z controller
Pulsed power
supply
+ve -ve
Spindle
Computer
Column
Agitator
Base
(a) (b)
Figure 3
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Figure 4
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,Duty factor: 0.5 andVoltage: 3V
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,
Duty factor: 0.5 andVoltage: 6V
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,Duty factor: 0.5 andVoltage: 6V
Electrolyte: 4M NaCl,Pulse frequency: 20 kHz,Duty factor: 0.5 andVoltage: 8V
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Figure 5
100 150 200 250 3000.05
0.10
0.15
0.20
0.25
0.30
400kHz, 5V, 0.2M, 3min.
Materialremov
al(mg)
Pulse on time (ns)
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Figure 6
100 150 200 250 300
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
700kHz, 8V, 0.5M, 3min.
Materialremoval(mg)
Pulse on time (ns)
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Figure 7
400 500 600 700 8000.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
0.28
100ns, 5V, 0.2M, 3min.
Materialremov
al(mg)
Frequency (kHz)
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Figure 8
400 450 500 550 600 650 700
1.0
1.5
2.0
2.5
3.0
3.5
4.0
250ns, 8V, 0.5M, 3min.
Materialremoval(mg)
Frequency (kHz)
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Figure 9
5 6 7 8 9
0.05
0.10
0.15
0.20
0.25
0.30
400kHz, 100ns, 0.2M, 3min.
Materialremoval(mg)
Applied voltage (V)
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Figure 10
5 6 7 8 9
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
700kHz, 250ns, 0.5M, 3min.
Materialremoval(mg)
Applied voltage (V)
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Figure 11
0.2 0.3 0.4 0.5 0.6
0.05
0.06
0.07
0.08
0.09
0.10
400kHz, 100ns, 5V, 3min.
aterar
emova
mg
Concentration (M)
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Figure 12
0.2 0.3 0.4 0.5 0.6
1.5
2.0
2.5
3.0
3.5
4.0
700kHz, 250ns, 8V, 3min.
Materialremoval(mg)
Concentration M