The Drilling of Al2O3 Using a Pulsed DC Supply With a Rotary Abrasive Electrode by the Electrochemical Discharge Process 2008

8/2/2019 The Drilling of Al2O3 Using a Pulsed DC Supply With a Rotary Abrasive Electrode by the Electrochemical Discharge

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ORIGINAL ARTICLE

The drilling of Al 2 O 3 using a pulsed DC supply with a rotaryabrasive electrode by the electrochemical discharge process

Sanjay K. Chak & P. Venkateswara Rao

Received: 20 July 2006 /Accepted: 1 October 2007 / Published online: 16 November 2007# Springer-Verlag London Limited 2007

Abstract The electrochemical discharge machining(ECDM) process has the potential to machine electricallynon-conductive high-strength, high-temperature-resistant (HSHTR) ceramics, such as aluminum oxide (Al 2 O3 ).However, the conventional tool configurations and machin-ing parameters show that the volume of material removeddecreases with increasing machining depth and, finally,restricts the machining after a certain depth. To overcomethis problem and to increase the volume of material removedduring drilling operations on Al 2 O3 , two different types of tool configurations, i.e., a spring-fed cylindrical hollow brass tool as a stationary electrode and a spring-fed cylin-drical abrasive tool as a rotary electrode, were considered.The volume of material removed by each electrode wasassessed under the influence of three parameters, namely, pulsed DC supply voltage, duty factor, and electrolyteconductivity, each at five different levels. The resultsrevealed that the machining ability of the abrasive rotaryelectrode was better than the hollow stationary electrode, asit would enhance the cutting ability due to the presence of abrasive grains during machining.

Keywords Electrochemical discharge machining (ECDM) .High-strength, high-temperature-resistant (HSHTR)materials . Central composite rotatable design (CCRD) .Abrasive rotary electrode

1 Introduction

Aluminum oxide (Al 2 O3 ) is an important ceramic materialsthat has wide applications in machine tools, aerospace, andelectrical and electronic fields in view of its typical properties, such as high strength and hardness at hightemperature, low thermal conductivity, density, and chem-ical inertness. The production of small through and blindholes, grooves, etc. by any conventional machining is verydifficult and uneconomical. To overcome these problems,researchers have found a novel method of machining calledelectrochemical discharge machining (ECDM) [ 1, 2]. Thisis a hybrid process which has combined characteristics of electrochemical machining (ECM) and electrical dischargemachining (EDM). This process has been successfullyapplied to machine electrically conductive materials withimproved productivity [ 3, 4], and has also been proven asthe most suitable process to machine electrically non-conductive materials, such as quartz, glass, composites,granite stone, etc.

Unlike ECM and EDM, the electrical discharge inECDM occurs between comparatively smaller sized toolelectrode and electrolyte interface. The heat generated dueto this discharge is used for machining electrically non-conductive materials, therefore, the machining efficiency of this process greatly depends on the properties of thematerial to be machined and on the type of parametersselected. The ECDM process shows low penetration depthon Al 2 O3 due to its low fracture toughness, while themachining of deep holes is still difficult, as it showssusceptibility for cracking due to thermal shocks at highvoltage, which is caused by the abrupt nature of discharge at greater tool depth inside the electrolyte. As a result, theradial overcut and the heat-affected zone widen and, finally,leads to microcracks on the machined surface. At low

Int J Adv Manuf Technol (2008) 39:633 641DOI 10.1007/s00170-007-1263-x

S. K. Chak : P. Venkateswara Rao ( * )Mechanical Engineering Department,Indian Institute of Technology (IIT),Delhi, New Delhi 110016, India e-mail: [email protected]


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voltages, the machining performance is poor and, after some period of time, the volume of material removed becomesnegligible. Thus, to reduce the probability of such kind of damage or poor performance, this process still needs better tool configurations with revised states of machining parameters.

Therefore, an attempt has been made in this work to drillholes on Al 2 O3 by using two different types of electrodeconfigurations, i.e., a spring-fed hollow brass tool as a stationary electrode that provides electrochemical dischargealong the internal surface exposed to the tool electrolyteinterface and a spring-fed cylindrical abrasive tool as a rotary electrode that provides additional electrochemicaldischarge at the bottom face of the tool electrolyteinterface, due to the gap maintained by abrasive grains

and also incorporates a better cutting action duringmachining due to abrasive action. The volumes of materialremoved by these tools were measured and compared at various levels of significant parameters.

2 Literature review

To study the influence of different machining parameterssuch as power supply, supply voltage, electrode polarity,electrolytes, and concentrations on the volume of materialremoved, researchers have conducted studies using differ-ent materials, such as borosilicate glass, composites, quartz,etc. An extensive study on the machining of glass showsthat the volume of material removed and the depth of cut

Fig. 1 a Machine tool setupfabricated for experimentation.b Schematic line diagram of the

machine tool setup fabricatedfor experimentation

634 Int J Adv Manuf Technol (2008) 39:633 641


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increases with increase in supply voltage, electrolyteconcentration, and temperature [ 5]. It was noticed that theincrease in pulse duration increases the material removalrate and surface finish when compared to the smooth DCvoltage, while the machining rate decreases gradually withtime. NaOH was found to be the best suited electrolytecompared to KOH, NaCl, and NaNO 3 and provides themaximum material removal rate at different concentrations.Kim et al. [ 6] reported that the material removal rateincreased with increasing duty ratio and decreasing pulsefrequency, while the heat-affected zone and surface rough-ness increased with increasing duty ratio and decreasingvoltage pulse frequency during the microdrilling process of Pyrex glass. The experimental verification of these factswas easier on glass due to its melting ability, whileceramics disintegrate instead of melting and, hence, aredifficult to machine.

Machining holes on alumina and silicon nitride was first reported by Tokura et al. [ 7]. With a stationary gravity-fed Ni electrode of 0.5-mm diameter as the cathode, a small pit formation was noticed under the influence of full-waverectified DC. The results obtained showed that a pit could be formed on ceramics, while its depth apparently variedwith the ceramic material. It was observed that the volumeof material removed, size, and depth of the pit increasedwith increasing voltage and electrolyte concentration. A20 wt% concentration of NaOH was found to be suitablefor the maximum volume of material removed. However,the rate of machining was found to be very low whenmachining ceramics.

In order to increase the volume of material removed inceramics, researchers have tried different machining possibilities, such as; a vibrating tool electrode [ 8], gas-filledECDM with a side-insulated tool electrode [ 9], copper tooltip with flat front-taper side wall/straight side wall [ 10],rotary and trepanning action of a copper electrode [ 11, 12],etc. Jain et al. [ 13] used an abrasive tool with rotary motionunder smooth DC voltage to drill holes on Al 2 O3 . Their results showed that the volume of material removed and thedepth of cut increased with increasing supply voltage andelectrolyte temperature. But the volume of material

removed could not exceed more than 20 mg at 70 V andthe material showed a tendency to cracking beyond theseoperating conditions. Though these methods have partiallyimproved the performance, there is still a lot of scope for further improvement. Recently, Bhondwe et al. [ 14 ]developed a model to investigate the effect of duty factor,electrolyte concentration, and energy partition on thematerial removal rate by the finite element method. It wasreported that the material removal rate increases signifi-cantly with increasing the electrolyte concentration from10% to 30%, increasing the duty factor, and increasing theenergy partition, especially in soda lime glass, due to itslow melting temperature, and a similar trend had beenfollowed while machining Al 2 O3 .

From the above observations, it was learnt that the effect of supply voltage under smooth DC, full-wave rectifiedDC, and pulsed DC were used to improve the volume of material removed during the machining of aluminum oxide.However, the results obtained were not as good asanticipated and need further improvement. Therefore, inthe present experimental study, an attempt has been made toimprove the volume of material removed by the use of pulsed DC at a fixed value of T-on, where the supplyvoltage, duty factor, and electrolyte conductivity were testedat five different levels to find their effect on the materialremoval process using a spring-fed stationary hollow brasselectrode and a spring-fed rotary abrasive electrode.

Table 1 Values of the three parameters at five different levels

Parameters Levels

1.682 1 0 +1 +1.682

Voltage (V), X 1 60 72 90 108 120Duty factor (%), X 2 0.48 0.58 0.72 0.86 0.96Electrolyte conductivity

(mmho/cm), X 3275 295 325 355 375

Table 2 Coded values of three machining parameters given by thecentral composite rotatable design (CCRD) for the 20 experiments

Experiment number

X 1 X 2 X 3 Y 1(mg)

Y 2(mg)

1 1 1 1 0.11 0.422 1 1 1 4.04 12.723 1 1 1 2.81 3.824 1 1 1 13.88 25.065 1 1 1 1.69 2.956 1 1 1 10.32 29.617 1 1 1 5.13 14.148 1 1 1 20.48 45.139 1.682 0 0 0.62 2.3610 1.682 0 0 17.95 42.7811 0 1.682 0 1.94 3.8712 0 1.682 0 12.35 23.0113 0 0 1.682 2.81 3.1114 0 0 1.682 9.86 25.4715 0 0 0 6.34 11.8816 0 0 0 6.58 12.40

17 0 0 0 6.28 12.6418 0 0 0 6.50 12.2319 0 0 0 6.40 12.0120 0 0 0 6.37 11.95

Int J Adv Manuf Technol (2008) 39:633 641 635


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3 Experimentation

In the present experimental study, machining in the form of drilling holes on sintered aluminum oxide inserts of dimensions 15157 mm has been attempted by two typesof electrode configurations, i.e., a spring-fed stationaryhollow brass tool, of 1.5-mm diameter, as a cathode in

which the tool tip touches the workpiece surface and a spring-fed cylindrical abrasive (diamond) tool, of 1.5-mmdiameter, as a cathode in which the tool is given a rotarymotion of 20 rpm. The volume of material removed by eachtool was measured under the influence of three parameters, pulsed DC power supply voltage, duty factor, and theelectrolytic conductivity. The machining was carried out for 30 min in each experiment.

A DC pulsed generator, specifically designed for theseexperiments that provides voltage ranges from 0 V to300 V, current from 0 A to 15 A, different steps of T-on/T-off timing ranges from 0.25 s to 1,000 s T-on and 8% to

96% duty factor, respectively, was used as the power supply. All of the experiments were conducted at theconstant value of T-on time of 1,000 s (maximumavailable with the DC pulse generator).

For the drilling process, a machine tool setup, as shownin Fig. 1, was fabricated which could provide precise verticalfeed motion (ranging from 2 m/min to 1.98 mm/min) to thetool in the Z axis with an option of controlled rotary motion(0.1 rpm to 99 rpm) with the help of a stepper motor and itscontroller. The workpiece can be moved in the X and Y axis

manually. Provision was also made for the tool to have(trepanning motion) orbital motion, if required. This was provided by a slider type tool assembly that could adjust theoffset between the tool axis and the spindle axis with thehelp of a nut and screw mechanism. An electrolyte tank wasfabricated in Perspex, which included a workpiece-holdingvice and a small submersible pump for flushing debris.

In order to increase the electrolytic conductivity at roomtemperature, experiments were conducted to compare theelectrolyte conductivity of the combined state (solutionhaving equal wt% concentration) of NaOH and KOH withthat of NaOH or KOH at the same concentration using thehelp of a digital conductivity meter. It was observed that a mixed solution of electrolyte provides a higher value of conductivity, which could only be obtained by heating theindividual electrolyte. However, due to the saturation of ions, a further rise in electrical conductivity could not beseen at higher concentrations. Therefore, solutions contain-ing equal wt% of NaOH and KOH (providing conductivity

in the range of 275 mmho/cm to 375 mmho/cm at roomtemperature) were used in this work.

Central composite rotatable design (CCRD) has beenused to conduct the experimentation at five different levelsof each of the three parameters shown in Table 1. Thisdesign is popularly used for fitting a second-order responsesurface which may be subdivided into three parts; 2 k

factorial (where k is the number of variables or parameters),six star points to form a central composite design (in order to make the design rotatable, their values are calculated by

Table 3 Analysis of variance for the volume of material removed during electrochemical discharge machining (ECDM) with a stationaryelectrode

Source of variation Degrees of freedom Sum of squares Mean of squares F calculated F 0.01 tabulated R2 (coefficient of

determination)

Regression 9 587.118 65.253 780.70 4.94 99.9%Residual error 10 0.836 0.084 12.82Lack of fit 5 0.775 0.155Pure error 5 0.060 0.012Total 19 587.953

Fig. 3 Schematic diagram of the ECDM process using a spring-fed cylindrical abrasiverotary tool



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2k /4 =1.682), and six center points to give roughly equal precision. The values of the parameters shown at minimumand maximum levels were set after conducting the trialexperiments, while intermediate values were calculatedaccording to the distribution ratio of the levels. This designhas reduced the total number of experiments to beconducted from 125 to 20; as a result, each experiment was repeated three times so as to find the volume of material removed. The combined effect of the three parameters, i.e., pulsed DC supply voltage ( X 1 ), duty factor ( X 2 ), and electrolyte conductivity ( X 3 ) on the total volumeof material removed from alumina by two types of toolelectrode configurations for 20 sets of experiments has beenshown as Y 1 and Y 2 in Table 2.

4 Results and discussion

The effect of three significant process parameters, namely, pulsed DC supply voltage, duty factor, and electrolyteconductivity (each at five different levels), were assessed onthe volume of material removed during the drillingoperation of Al 2 O3 by two different types of electrodeconfigurations. In order to improve the performance of process parameters, a pulsed DC power supply was used,which provided better control of the discharge energyduring machining.

The results obtained from this study were used toformulate a mathematical model of second order, shown below, that contains an average value from the two sets of experiments. A regression equation was developed toanalyze the effect of each parameter on the volume of material removed (M. R.), as shown in Fig. 2:

Y M : R : 0 Xk

i1 iXi X

k

i1 iiX

2i XX

i< j

ijX iX j 1

The second-order response surface (Eq. 2), obtainedfrom the experimental values of the stationary hollowelectrode, was used to obtain the volume of material

removed from Al 2 O3 at various levels. Analysis of variancefor the volume of material removed during ECDM with a stationary electrode showing the adequacy of the model isindicated in Table 3:

Y1 6: 42 4: 99X 1 3: 20X 2 2: 10X 3 0: 941X 1 X1 0: 185X 2 X2 0: 101X 3 X3

1: 73X 1 X2 1: 12X 1 X3 0: 132X 2 X3

2

From the results shown in Fig. 2, it was observed that the volume of material removed increased with the increasein supply voltage, and this could be due to the generation of a large number of positively charged hydrogen bubbles at the higher value of supply voltage. These hydrogen bubblesact as insulating medium, which become enlarged with timeand envelope the tool. Due to ohmic heating, these bubblesare burst, providing a way for discharge near the tool

electrolyte interface and, hence, contributing to the improved machining. The magnitude of the discharge energyremains proportional to the supply voltage under thespecific conductance of electrolyte.

It was also found that the volume of material removedincreases with increasing duty factor. The increasedduration of release of discharge energy per pulse producesmore heat at the tool electrolyte interface, which could bethe possible reason for the increased volume of materialremoval from the workpiece.

A similar trend has also been observed while increasingthe electrolyte conductivity. With increasing electrolyteconductivity, the capacity of the electrolyte to draw current increases. In view of this, electrochemical reaction gen-erates a greater amount of positively charged ionic gas bubbles that insulate the tool, i.e., cathode, and facilitatesthe increased action of electrical discharge. This could bethe possible reason for the increased volume of materialremoval from the workpiece at higher electrical conductiv-ity. In addition to this, the pulsed DC power sources that dominates input voltage and current release ability providesa better control of spark stability and concentrated releaseof spark energy has shown an improved state of machining performance compared to smooth DC power supply. Thus,

Table 4 Analysis of variance for the volume of material removed during ECDM with a rotary electrode

Source of variation Degrees of freedom Sum of squares Mean of squares F calculated F 0.01 tabulated R2 (coefficient of

determination)

Regression 9 3,121.09 346.79 603.50 4.94 99.8%Residual error 10 5.75 0.57 12.24Lack of fit 5 5.31 1.06Pure error 5 0.43 0.09Total 19 3,126.83



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260 280 300 320 340 360 380

0

10

20

30

40

50

60

70

80

M a

t e r i a

l R e m o v e

d

m g

Electrolyte Conductivitymmho/cm

Duty Fac tor 0 .48Duty Fac tor 0 .58Duty Fac tor 0 .72Duty Fac tor 0 .86Duty Fac tor 0 .96

60 Vo l t s DC

2 60 280 30 0 320 340 360 3 80

0

10

20

30

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70

80

M a

t e r i a

l R e m o v e

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m g



72 Vo l t s DC

260 280 300 320 340 360 380

0

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60

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80

M a

t e r i a

l R e m o v e

d

m g



90 Vo l t s DC

260 280 300 320 340 360 380

0

10

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30

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70

80

M a

t e r i a

l R e m o v e

d

m g



108 Vo l t s D C

260 280 300 320 340 360 380

0

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M a

t e r i a

l R

e m o v e

d

m g



120 Vo l t s DC

Fig. 4 Effect of supply voltage on the volume of material removed by a spring-fed cylindrical abrasive rotary tool



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from the above observations, it has been established that thevolume of material removed increases with increasingsupply voltage, duty factor, and electrolyte conductivityunder the influence of pulsed DC. The combined effects of melting, erosion due to cavitations, or chemical etchingcould be other possibilities that have influenced the materialremoval process from the ceramic surface.

A spring-fed cylindrical abrasive (diamond) rotary toolwas used as the electrode for drilling holes on Al 2 O3 withan intention of improvement over previous tool config-urations. The tool was given a rotary motion to abrade thework material in addition to the ECDM process, as shownin Fig. 3. This concept has significantly improved thevolume of material removed compared to a hollow brasstool at the same parametric values. Due to the protrudedshape of abrasive grains, the electrode maintains a constant gap of a few m between the tool and workpiece duringmachining. In view of this gap, the generation of additionalelectrical discharge beneath the bottom face of the tool andelectrolyte interface occur, enabling the removal of extra material from the work surface, which, otherwise, was not possible by the use of conventional tools.

The second-order response (Eq. 3), obtained from theexperimental values of the rotary abrasive electrode, wasused to obtain the volumes of material removed from Al 2 O3at various levels. Analysis of variance for the volume of material removed during ECDM with a rotary electrodeshowing the adequacy of the model is indicated in Table 4:

Y2 12 : 2 11 : 7X 1 5: 47X 2 6: 40X 3

3: 61X 1 X1 0: 378X 2 X2 0: 678X 3 X3

1: 66X 1 X2 3: 01X 1 X3 1: 37X 2 X3

3

The mechanism of material removal in the ECDM process is a complex phenomenon that involves manyfactors which can cause changes to the volume of materialremoved. In addition to this, the inherent problemsassociated with this process, like the variation in the rate

of change of tool wear/workpiece properties, electrolyteconductivity, variation in heat loss with rise in temperature,etc., may also influence the result. In view of this, theanalysis of variance shown in Tables 3 and 4 indicates that the lack of a fit term is significant and a part of the physical phenomenon has not been captured, while the coefficient of determination ( R2 ) and F value (regression) are goodenough to justify the adequacy of the proposed empiricalmodel.

Figure 4 shows similar trends as observed in Fig. 2.However, the volume of material removed by the rotaryaction of the abrasive tool (as shown in Fig. 3) was higher than that of the stationary hollow brass tool. Thisdifference in the volume of material removed could be partly due to the abrasive action of the active grains,ensuring the removal of the recast layer in addition to theelectrochemical discharge, as well as due to additionaldischarge that takes place beneath the tool surface becauseof the gap created by the projected length of active abrasive particles.

Further, it was observed that, with increasing tool depthinside the electrolyte, the nature of discharge was not asabrupt as observed with a brass tool. Due to this effect, thetaper was found to be reduced from 13.14 to 2.65(measured by a tool maker s microscope). Another obser-vation was that radial overcut and circularity error obtained by the rotary abrasive tool was also comparatively less thanthat obtained by a brass tool and, hence, there is an overallimprovement compared to the stationary electrode config-uration. The sectioned views of the samples are shown inFig. 5 in order to illustrate the performance improvement due to the use of an abrasive electrode.

5 Conclusion

This study has established that the drilling of deep holes inceramics is possible by using pulsed DC and with the helpof an abrasive rotary electrode. It can also be concluded that the pulsed DC has reduced the tendency of cracking at higher supply voltages and improved the volume of

Fig. 5 a Sectional view of thehole produced by a spring-fedstationary hollow brass electrodeshowing the taper. b Sectionalview of the hole produced by a spring-fed rotary abrasive elec-trode showing the taper



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material removed to a greater extent during the machiningof Al 2 O3 . The use of an abrasive tool with rotary motionhas been proved to be a better electrode configuration for drilling holes in ceramics because it provided additionalelectrical discharge and cutting action to improve themachining rate. These techniques made the process of material removal stable and, hence, resulted in an overallimprovement in the quality of holes produced.

Acknowledgment The authors acknowledge the financial support provided by the Department of Science and Technology, New Delhi,for the project entitled Influence of process parameters in electro-chemical spark machining.

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4. Crichton IM, McGeough JA (1985) Studies of the dischargemechanisms in electrochemical arc machining. J Appl Electrochem15:113 119

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10. Bhattacharyya B, Doloi BN, Sorkhel SK (1999) Experimentalinvestigations into electrochemical discharge machining (ECDM)of non-conductive ceramic materials. J Mater Process Technol95:145 154

11. Gautam N, Jain VK (1998) Experimental Investigations intoECSD process using various tool kinematics. Int J Mach ToolsManuf 38(1):15 27

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The Drilling of Al2O3 Using a Pulsed DC Supply With a Rotary Abrasive Electrode by the Electrochemical Discharge Process 2008