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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




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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REFERENCES

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chemical micro drilling using ultra short pulses, Precision

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Bhattacharyya, B., Munda, J., Malapati, M. (2004) Advancement in

electrochemical micro-machining, International Journal of Machine

Tools & Manufacture , No.44 , pp.15771589.

Choi. S. H., Ryu. S. H., Choi., D. K., Chu, . C. N. (2007) Fabrication of

WC micro-shaft by using electrochemical etching, Int J Adv Manuf

Technology, No.31(7-8), pp. 682-687.

Das, A.K., Saha, P. (2008) Experimental investigation into micro-tool

manufacturing with ECM process, Proc. of 2nd

International and 23nd

AIMTDR conference, IIT Madras, Dec.15th

-16th

, pp.609-614.

Debarr, A.E., Oliver, D.A. (1968) Electrochemical machining, first ed.

Macdonald & Co, London.

Kim, B. H., Na, C. W., Lee, Y. S., Choi, D.K., Chu, C.N. (2005), Micro

electrochemical machining of 3D micro structure using dilute

sulpfuric acid,Annals of the CIRP, No. 54(1),pp. 191-194.

Lim, Y., Kim, S. H.(2001) An electrochemical fabrication method for

extremely thin cylindrical micropin, International Journal of

Machine Tools & Manufacture, No.41, pp.22872296.

Masuzawa, T. (2000) State of the art micromachining,Annals of the CIRP

No.49(2), pp.473-488.

Masuzawa,T., Okajima,K.,Taguchi,T., (2002) EDM-Lathe for

Micromachining. Annals of the CIRP 51/1: 355~358.

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McGeough, J. A., Leu, M., Rajurkar, K. P., De Silva, A., Liu, Q., (2001)

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Microsystem Technology, No.13, pp.607612.

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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: 6V



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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

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1978264531010001000