j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470

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On the mechanism of material removal inelectrochemical spark machining of quartzunder different polarity conditions

V.K. Jaina,∗, S. Adhikaryb

a Mechanical Engineering Department, I.I.T., Kanpur 208016, Indiab BHEL, R.C. Puram, Hyderabad 500032, India

a r t i c l e i n f o

Article history:

Received 4 April 2007

Received in revised form

19 July 2007

Accepted 22 August 2007

Keywords:

ECSM

a b s t r a c t

Electrochemical spark machining (ECSM) process has been successfully applied for cutting

of quartz using a controlled feed and a wedge edged tool. Contrary to the common belief that

only cathode works as a tool, both cathode and anode have been used as a tool, i.e. ECSM

with reverse polarity (ECSMWRP) as well as ECSM with direct polarity (ECSWDP) have been

used to machine quartz plates. In ECSMWRP, deep crater on the anode (as a tool) and work-

piece interface is formed because of chemical reaction. Chemical analysis of electrolyte

solution after the ECSM experiments, also agrees with the feasibility of dissolution of quartz

into solution due to chemical reaction. Reverse polarity cuts quartz plate at a faster rate

as compared to the direct polarity. But in reverse polarity overcut, tool wear and surface

Reverse polarity

Direct polarity

Machining at both electrodes

Quartz work-piece

Chemical analysis

roughness are higher as compared to the direct polarity machining. Magnified view of the

machined surface also shows a difference in the mode of material removal in ECSMWDP

and ECSMWRP. The cutting is possible even if we make auxiliary electrode of small size.

In conclusion, experiments have revealed that cutting can be performed simultaneously at

both the electrodes (anode and cathode) during ECSM.

machining electrically non-conductive advanced engineering

1. Introduction

Advanced ceramics and composites have high potential fortheir applications in various fields of engineering due tothe superior properties such as high compressive strength,good thermal shock resistance, high wear resistance, highhardness, high strength to weight ratio, etc. Such improvedmaterial properties, however, pose new challenges to man-ufacturing engineers to shape and size these electricallynon-conductive materials economically and efficiently.

Electrical discharge machining (EDM) and electrochemicalmachining (ECM) are the two electrically assisted and mostversatile advanced machining processes (Jain, 2002). They

∗ Corresponding author.E-mail address: vkjain@iitk.ac.in (V.K. Jain).

0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2007.08.071

© 2007 Elsevier B.V. All rights reserved.

are being successfully used in industries for machining elec-trically conductive materials. Diamond grinding, ultrasonicmachining, abrasive jet machining, abrasive water jet machin-ing, laser beam machining and ion beam machining are theprocesses which can be used for machining of electricallynon-conductive materials also. But these processes have theirown limitations. Electrochemical spark machining (ECSM) isa newly developed hybrid process that combines both ECMand EDM (ECM + EDM → ECSM). It can be successfully used for

materials (Withrich and Fascio, 2005; Kim et al., 2006; Tandonet al., 1990; Jain et al., 1990). However, it is not being used onthe shop floor due to its limitations hence it requires in-depth

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Fig. 1 – Schematic diagram of basic electrochemical cell inE

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CSM process.

esearch to make it viable to use on the shop floor. Some ofhe process limitations include comparatively low materialemoval rate, low depth of penetration and poor surfacentegrity.

In ECSM process, cathode (tool) of small size is dipped–3 mm in an electrolyte and anode is a large size dummy elec-rode that is kept at a distance of about 30–50 mm away fromhe cathode (Fig. 1). Application of potential (50–80 V) betweenhe electrodes causes the flow of electric current through thelectrolytic cell, resulting in the generation of hydrogen gasubbles at the cathode and oxygen gas bubbles at the anodeue to electrochemical reactions discussed elsewhere. Surfacerea of the cathode dipped in the electrolyte is very smallompared to anode hence high current density at the cathodeesults in rapid generation of hydrogen gas bubbles. Boilingf electrolyte near small electrode would occur due to Ohmiceating of the electrolyte. A high electric field of the order of07 V/m is developed across the H2 bubbles. This causes spark-ng across H2 gas bubbles between the cathode and electrolyteJain et al., 1999).

If the work-piece is kept in the vicinity of the spark zone,aterial removal by melting and vaporization of the tool

cathode) and work-piece both takes place. There are variousheories (Jain et al., 1999; Basak and Ghosh, 1997; Withrich andof, 2006) proposed to explain the mechanism of spark gener-tion at the cathode. However, none of them has been verifiedxperimentally.

Jain et al. (1991) successfully demonstrated the use ofraveling wire electrochemical spark machining (TW-ECSM)f thick sheets of composite materials. They have reportedn increase in MRR, TWR, and overcut with an increase inoltage and electrolyte concentration (NaOH, up to 22.5% byeight). Beyond this concentration of NaOH, above stated

esponses decrease because specific conductance of NaOHtarts decreasing. The effects of artificially generated bub-les on the performance of ECSM process have also beentudied by these researchers. Introduction of artificial bubblesnto the machining zone reduces MRR. Majority of the artifi-ial bubbles are larger in size, hence discharge does not takelace across them at this potential gradient. Secondly, somef the small size bubbles merge into large size artificial bub-les across which sparking does not take place. As a resultf this, MRR decreases. However, it improves the machining

ccuracy (or lower average diametral overcut). Findings relatedo TW-ECSM experiments on partially electrically conductive

aterials viz, piezo-electric ceramics (PZT) and carbon fiberpoxy composites, have been reported (Singh et al., 1996). They

h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470 461

concluded that further investigations related to the effect ofmachining parameters on the surface integrity are requiredbefore this process becomes commercially viable. Experi-ments were performed (Gautam and Jain, 1998) on ceramicswith different tool kinematics such as stationary tool, rotatingtool with or without electrolyte flow, tool with orbital rotation,and controlled feeding of the work-piece. By using trepan-ning action and controlled feed to the work-piece, MRR anddrilling depth can be substantially increased. However, theresearchers who were using gravity feed to the tool could notget more than 1 mm depth of penetration and it resulted inpoor performance of the ECSM process. With a controlled feedto the tool in trepanning operation, more than 5 mm depthof penetration has been achieved. Theoretical analysis of themechanism of spark generation has been carried out (Basakand Ghosh, 1997) with the conclusion that the mechanismof sparking in ECSM process is similar to switching off phe-nomenon in electrical circuit. MRR is also found to increaseby 200% by introducing external inductance to the circuit.Jain et al. (1999) proposed the application of “valve theory”to the spark generation in ECSM process. From the V–I char-acteristics in a discharge tube they concluded that sparkingin ECSM process is similar to the arc discharge that occurs ina discharge tube. Limited machined depth obtainable in ECSdrilling operation can be explained by plotting the isotherms(Jain et al., 1999). There is the possibility of no sparking nearsome of the unmelted material that causes limited machiningdepth in ECSM process. Bhattacharya et al. (1999) conductedexperiments on alumina and concluded that the most effec-tive parametric combination for moderately higher machiningrate and dimensional accuracy are 80 V and 25% NaOH concen-tration. Tool tip geometry was also found to play an importantrole in a controlled spark generation. ECSM process for drillingof silicon nitride has also been applied (Doloi et al., 2001) andconcluded that interelectrode gap has insignificant effect onMRR and radial overcut. A rotating tool having impregnatedabrasive particles (ECSM + Grinding → Electrochemical sparkgrinding) has been used, and found (Jain et al., 2002) that MRRand machined depth increase because of a combined effect ofthermal erosion and grinding operation.

Researchers have been using tool as a cathode and adummy metallic piece as an anode. However, in the pre-liminary, slicing experiments on quartz show that if thepolarity is reversed (tool is anode) then MRR is higher thandirect polarity (tool is cathode), even though spark intensityis lower for a reverse polarity. Therefore, ECSM process canbe classified as electrochemical spark machining with directpolarity (ECSMWDP) and electrochemical spark machiningwith reverse polarity (ECSMWRP). This paper also reports theexperiments conducted by making both the electrodes smallin size so that machining takes place on both the electrodessimultaneously.

2. Experimentation

Experimental set-up consists of work holding and work feed-ing mechanism, tool and tool holder, mechanical switch forsensing the gap between wedge shaped tool (Fig. 2a) andwork-piece, stepper motor controller, temperature controller

462 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470

d m

ting. Details of the cutting conditions are listed in Table 1.Temperature of the electrolyte was maintained at 40 ◦C andit was monitored with the help of a contact thermometer.

Table 1 – Machining conditions

Work-piece material QuartzWork-piece thickness ≈ 2 mmElectrode material Copper

In ECSMWDPCathode size 4 mm × 0.5 mm × 20 mmAnode size 110 mm × 2 mm × 50 mm

In ECSMWRPAnode size 4 mm × 0.5 mm × 20 mmCathode size 110 mm × 2 mm × 50 mm

Electrolyte Sodium hydroxide (NaOH)Distance between anode and

cathode50 mm

Electrolyte temperature 40 ◦CFeed rate to work-piece 0.339 mm/minInitial gap between tool and 0.4 mm

Fig. 2 – (a) Tool used during experiments; (b) tool holding antemperature controller.

and power supply. Experimental set-up is designed and fabri-cated (Gautam and Jain, 1998) to feed work-piece towards thetool. For efficient machining, a gap between tool and work-piece is essential so that electrolyte is always present betweenthe bottom of the tool and top surface of the work-piece forgenerating sparks. Gravity feed mechanism does not providethis gap hence the total machined depth during ECSM is lim-ited. A stepper motor controller by using PC printer port hasbeen designed and fabricated to control the feed of the work-piece to maintain a certain minimum gap between tool andwork-piece. As work-piece is electrically non-conductive, gapvoltage or gap current sensing servomechanism for maintain-ing the gap between tool and work-piece cannot be used. Amechanical switch has been designed for sensing the con-tact between work-piece and tool. Mechanical switch consistsof an adjustable screw and a copper plate (Fig. 2b). They areconnected to status port of a PC printer. When feed rate ishigher than machining rate, tool touches the work-piece andit pushes tool holder assembly upwards. Therefore copperplate also starts moving in upward direction and it strikesthe adjusting screw. As a result, value of the status port ischanged from 127 to 255, and motor starts rotating in a reversedirection. Thus, the work table is lowered down for creat-ing microgap between the tool and work-piece. The microgapbetween copper plate and screw can be adjusted by screw-

ing on or screwing off the screw. When the motor rotates inthe reverse direction, power supply to the electrodes is auto-matically switched off to stop machining during that periodelse it will increase the overcut. Once the preset gap is created

echanical switch assembly; (c) schematic diagram of a

between the screw and the plate, the motor starts rotatingagain in the forward direction and the power supply againgets on automatically between the electrodes to start cut-

work-pieceGap between adjusting screw

and copper plate0.127–0.145 mm

Tool tip dipped into electrolyte ≈ 2 mm

t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470 463

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ig. 2c shows a schematic diagram of the temperature con-roller designed and fabricated for the ECSM process. With theelp of the contact thermometer, the desired temperature cane set and the relay through the immersion heater helps inchieving the same.

.1. Electrochemical spark machining with reverseolarity (ECSMWRP)

xperiments are conducted by using one small and one largelectrode, and small electrode is used as a tool. During elec-rochemical reaction, hydrogen gas is evolved at cathode andxygen gas is liberated at anode. Boiling of electrolyte solutionear the small electrode occurs due to Ohmic heating of elec-rolyte. In ECSMWDP, sparking occurs because of evolution ofydrogen gas at the cathode and in ECSMWRP, sparking occursecause of evolution of oxygen gas at the anode. Experimentalbservations reveal that spark intensity is higher at small elec-rode, when it is cathode (ECSMWDP) as compared to when its anode (ECSMWRP). But, material removal rate, penetrationate, overcut and tool wear rate are higher in ECSMWRP asompared to ECSMWDP, during machining of quartz (Table 2).herefore, it seems that material removal in ECSMWRP is notnly due to melting and vaporization of work-piece materialut also due to some other mechanism as well. Deep crater isbserved on the tool when it is acting as an anode (Fig. 3(a)) asompared to almost no crater when the tool is acting as theathode (Fig. 3(b)). It leads to the conclusion that some chem-cal reactions are taking place in the gap between the anodetool) and work-piece during reverse polarity because no other

echanism seems viable in the present configuration. Pos-ible chemical reactions that can occur at anode–electrolytenterface have been proposed from basic electrochemistry asollows.

Schematic diagram of an electrolytic cell of NaOH solutionnd copper electrodes is shown in Fig. 4. External poten-ial is applied between the electrodes. In the external circuit,lectrons move towards the cathode–electrolyte interface,nd go to the solution. At the anode–electrolyte interfacequal number of electrons are discharged from the solu-ion to the anode. Electrochemical reactions that occur athe electrode–electrolyte interface continuously supply elec-rons from cathode to solution and solution to anode. Whichype of reactions will occur, depends on the characteristics oflectrodes, electrolyte and applied voltage (Mcgeough, 1984;eBarr and Oliver, 1967).

.1.1. Reactions at anode and electrolyte interfacehe electrochemical reactions at anode–electrolyte interfaceause generation of oxygen gas and dissolution of anode.

u → Cu2+ + 2e−

H2O → O2↑ + 4H+ + 4e−

ere, oxygen gas evolves at the anode and sparking occurscross the oxygen gas bubble at the interface of the anodend electrolyte. Tabl

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464 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470

e con

Fig. 3 – Tool shape after machining (voltage = 72 V, electrolytafter cutting by ECSMWDP.

2.1.2. Reactions at cathode and electrolyte interfaceFollowing electrochemical reactions take place atcathode–electrolyte interface, and also cause evolutionof hydrogen gas.

Cu2+ + 2e− → Cu

2H+ + 2e− → H2↑

Na+ + e− → Na

2Na + 2H2O → 2NaOH + H2↑

2H2O + 2e− → H2↑ + 2OH−

Here, hydrogen gas evolved at the cathode leads to sparkingacross the bubbles between the cathode and electrolyte.

2.2. Dissolution of quartz into NaOH solution inECSMWRP

Reduction of silicon dioxide (SiO2) occurs due to high electro-motive force generated by high potential gradient. Sparkingoccurs due to high charge density and high resistance tocurrent flow through oxygen gas bubbles (Jain et al., 1999).

Fig. 4 – Schematic diagram of the flow of electrons and ionsin electrolytic cell of NaOH solution and copper electrodes.

centration = 10 wt.%): (a) after cutting by ECSMWRP and (b)

Heat generated by sparking accelerates the reduction process.Freshly formed silicon is highly active and it is re-oxidized byfresh oxygen gas evolved at anode. This newly precipitatedSiO2 is highly amorphous in nature and prone to chemi-cal reactions (Windholtz et al., 1976). It reacts with sodiumhydroxide to form sodium silicate and so dissolves into theNaOH solution and also forms copper silicate. The possiblereactions are as follows:

SiO2 (quartz) + 4e− + 4H+ → Si + 2H2O

Si + O2 → SiO2

SiO2 (amorphous) + 2OH− → SiO32− + H2O

SiO2 + 4OH− → SiO44− + 2H2O

2Na+ + SiO32− → Na2SiO3

4Na+ + SiO44− → Na4SiO4

Cu2+ + SiO32− → CuSiO3

2.3. Chemical analysis of NaOH solution in ECSMWRP

After cutting quartz plate by ECSMWRP, NaOH solution wouldcontain silicon dioxide (SiO2), sodium silicate (Na2SiO3), cop-per silicate (CuSiO3), copper hydroxide (Cu(OH)2) and oxidesof copper (CuO). It has been observed that copper silicate inNaOH medium forms precipitate of copper oxide and silicaparts to go into the solution in the form of [Cu(OH)2(SiO3)x]n

(CuSiO3 + NaOH → [Cu(OH)2(SiO3)x]n + CuO). Standard coppersilicate is prepared from sodium silicate and copper chloride(Na2SiO3 + CuCl2 → CuSiO3 + 2NaCl). Copper silicate is put intowater and sodium hydroxide solution in two separate testtubes. In water, copper silicate precipitates, but in NaOH solu-tion black of copper oxide (CuO) precipitates and a light bluesolution is formed (Fig. 5). This indicates, in alkaline medium

from copper silicate, silicate ion goes into solution as a com-plex compound of copper ([Cu(OH)2(SiO3)x]n) that makes lightblue solution. Therefore if the residues are separated fromNaOH solution after cutting quartz, solution would contain

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470 465

Fig. 5 – Copper silicate in alkaline ((a) in NaOH solution)and in neutral solution ((b) in water).

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ig. 6 – Flow chart for chemical analysis of solution afterxperiment.

odium silicate and complex compound of copper silicate thatre soluble in solution. Flow chart of the chemical analysis ishown in Fig. 6. Present chemical analysis also confirms theresence of copper ion and silica into the solution. Therefore

t can be concluded that the quartz is dissolved into NaOHolution by chemical reactions.

. Direct and reverse polarity machining ofuartz

xperiments were conducted independently for both directolarity and reverse polarity under the same machining condi-ions, while cutting quartz plate by sinking operation. Materialemoval rate, penetration rate, average surface roughness of

Fig. 7 – Different zones at tool tip.

the machined surface and tool wear were measured for theevaluation of process performance as shown in Table 2.

3.1. Material removal rate, overcut and tool wear

In direct polarity ECSM, material is removed by melting andvaporization of work-piece material while in reverse polarityECSM, it is by melting, vaporization and by chemical reac-tion. Material removal rate, penetration rate and overcut inreverse polarity are found to be higher as compared to straight(direct) polarity ECSM. Tool wear (difference in weight beforemachining and after machining) rate in ECSMWDP was negli-gible as compared to ECSMWRP mainly because of weight lossdue to melting and vaporization of cathode which is partiallycompensated by copper deposition due to electrochemicalreaction.

Cu2+ + 2e− → Cu

In reverse polarity, tool is anode which dissolves due toelectrochemical reaction (Cu → Cu2+ + 2e−). Material from theanode is also removed due to sparking which causes meltingand vaporization. Therefore removal of material from anodeby sparking and by anodic dissolution causes higher tool wearin ECSMWRP. A deep crater is also formed at the tool and work-piece interface in ECSMWRP (Fig. 3(a)). Different zones at thefront of the tool are shown in Fig. 7. At zone-a, kind of elec-trochemical reaction takes place is Cu2+ + 2OH− → Cu(OH)2. Atzone-b, due to the presence of silicate ions (formed from work-piece) two types of reactions occur (Cu2+ + OH− → Cu(OH)2 andCu2+ + SiO3

2− → CuSiO3). Therefore rate of formation of cop-per ion (i.e. dissolution of copper anode) at zone-b is more ascompared to that at zone-a. Hence, a deep crater is formedat zone-b. Average surface roughness value is lower in caseof direct polarity. It seems that chemical etching of the work-piece deteriorates surface finish in ECSMWRP.

3.2. Machined surface integrity

SEM photographs of the machined surface at high voltage aretaken (84 V, 16% electrolyte concentration) to study the surfaceintegrity of the machined surface. Cracks on the machinedsurface due to random thermal stress are clearly visible

for ECSMWDP (Fig. 8(a)). Magnified view of the machinedsurface clearly shows the difference in the surface integrityof the work-piece obtained after ECSMWDP and ECSMWRP(Fig. 8(b) and (c)). In direct polarity ECSM, surface is smooth

466 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470

Fig. 8 – (a) Cracks on the surface machined by ECSMWDP process (voltage = 84 V, electrolyte concentration = 16 wt.%).Magnified view of the machined surface (voltage = 80 V, electrolyte concentration = 16 wt.%.), (b) ECSMWDP (smooth surface

crac

Sparking occurs due to resistance to current flow at cath-ode tip and anode tip. It causes high voltage drop acrossthat zone. Therefore if sparking occurs at cathode tip, highvoltage drop occurs at cathode tip and sufficient voltage is

and cracks are visible) and (c) ECSMWRP (rough surface and

and microcracks on the surface are visible. In ECSMWRP, thecracks on the machined surface are not visible but the surfaceis rough. This indicates that chemical etching of the grainsis taking place in ECSMWRP. From the SEM photographs, itcan be concluded that there is a difference in the mode ofmaterial removal in ECSMWDP and ECSMWRP.

3.3. Machining by small auxiliary electrode

The concept of the ECSMWDP is that the size of the tool(cathode) is much smaller than the auxiliary electrode (anode)hence high current density at the tool generates more numberof hydrogen gas bubbles. Across a bubble, high potential gra-dient exists which develops a spark (Jain et al., 1999). To testthis hypothesis, auxiliary electrode (anode) is made as pivotedtype (Fig. 9a) so that the length of the anode dipped into theelectrolyte can be adjusted. Initially a small length of the tipof the anode is dipped into the electrolyte. But it is observedthat the spark intensity at small cathode is not affected by(small) size of the anode, and cutting takes place only at thecathode.

The rate of generation of hydrogen gas bubbles at cathodedepends on the rate of electron transfer from cathode to thesolution and it is not affected by the size of the anode. Hence,spark intensity at cathode (i.e. machining rate in ECSMWDP)is not affected by the size of the anode. Now, polarity of theset-up shown in Fig. 9a is reversed to see the effect of smallsize cathode on the machining rate in ECSMWRP. Initially asmall length of the cathode tip is dipped into the electrolyte,and potential is applied across the electrodes. Sparking occursat cathode tip due to high charge density on it. In this case,

at anode tip cutting of the quartz plate is not taking place.Now, length of the cathode tip dipped into the electrolyte isgradually increased. After a certain length of cathode beingdipped, sparking at cathode tip suddenly stops and sparking

ks are not visible).

starts at the anode (tool) and the quartz under the anode tipstarts getting machined.

Total potential drop between electrodes = potential drop atcathode tip + potential drop due to resistance of electrolytebetween electrodes + potential drop at anode tip.

Fig. 9 – (a) ECSMWDP with small electrodes and (b)machining at both the electrodes.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470 467

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flow due to evolution of gas bubbles and vapour bubbles atboth the electrodes leads to sparking at electrode tips. Whensparking occurs at one electrode, at that time shielding of

Fig. 10 – Shifting of sparking zone between the elect

ot available near the anode tip for machining. That is whyutting does not take place at anode when there was spark-ng at cathode. If there is no sparking at cathode tip, lessotential drop occurs near cathode. With increased sinkingf anode tip in the electrolyte, more number of oxygen gasubbles are formed at the anode resulting in intense spark-

ng at the anode. High potential is available near anode tiphat causes high rate of dissolution of quartz near anodeip.

. Machining at both the electrodes

n the preceding section, direct polarity and reverse polarityave been separately applied for cutting of quartz plate. It haslso been observed that machining is possible even at an aux-liary electrode if it is small in size. Therefore, in ECSM process,ool electrode (cathode in ECSMWDP and anode in ECSMWRP)hould be small and auxiliary electrode may be small or large.he question arises whether machining would take place atoth the electrodes if both of them are small in size. Schematiciagram of the set-up is shown in Fig. 9b. In this case both the

lectrodes work as tools and machining occurred under bothhe electrodes.

While attempting to machine at both the electrodes, itas been observed that sparking occurs only at one electrode

Table 3 – Cutting conditions used during machining atboth electrodes

Electrolyte concentration 22 wt.%Electrolyte temperature 30 ◦CDistance between anode and cathode 60 mmWidth of electrodes 5 mmThickness of electrodes 0.5 mmVoltage 60–80 VWork-piece thickness 2 mm

s: (a) sparking at cathode and (b) sparking at anode.

at a time and spark zone keeps shifting from cathode tipto anode tip and vice versa (Fig. 10). Resistance to current

Fig. 11 – Magnified view of the kerf and locations for themeasurements of kerf width.

n g t

468 j o u r n a l o f m a t e r i a l s p r o c e s s i

other electrode by gas and vapour bubbles takes place. Thisincreases the resistance to a high value near that electrode.When resistance becomes sufficiently high, sparking zoneshifts to other electrode. Thus shifting of sparking zone occursbetween the electrodes. When the sparking zone is at cathodetip, the process is ECSMWDP. When sparking occurs at anodetip, the process is ECSMWRP. Thus, machining alternatelyoccurs at both the electrodes.

Three experiments were conducted in voltage range of(60–80 V) to study the process performance during cutting atboth the electrodes. Details of the cutting conditions are listedin Table 3. After cutting a groove, magnified views of the pro-files were taken by a shadowgraph (Figs. 11 and 12). For themeasurement of kerf width and surface roughness along the

machined depth, whole length of the groove was divided intofive equal divisions. Width of the groove was measured atthe locations 1–4 (Fig. 11). Two extreme points (location 0 andlocation 5) were not considered because in some cases, kerf

Fig. 12 – Enlarged view of the machined grooves, obt

e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470

width at the top is much higher and at the bottom is muchlower compared to the average kerf width. Surface rough-ness was measured near these points, on surface of the kerf.The kerf width and surface roughness values are listed inTable 4. Experimental results show that penetration rate isalmost same at both the electrodes for all the experiments.Negative tool in general produces better quality kerf (less over-cut and low surface roughness) compared to positive tool, butmaterial removal rate is higher for positive tool than nega-tive tool. Fig. 13a and b show that the penetration and kerfwidth increase with increased voltage for both reverse andstraight polarities, because of high discharge energy for spark-ing and high rate of chemical reaction at high voltage whichresults in higher material removal. Average surface roughness

value also increases with increase in voltage (Fig. 13c). How-ever, total number of experimental points should have beenmore to have a reliable trend. Because at higher voltage higherenergy discharge takes place during sparking, which causes

ained from a shadowgraph (magnification 20×).

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 0 ( 2 0 0 8 ) 460–470 469

Table 4 – Kerf width and surface roughness at different locations (Fig. 11)

Locationon groove

Kerf width (mm) Surface roughness, Ra (�m)

60 V 70 V 80 V 60 V 70 V 80 V

Tool (−) Tool (+) Tool (−) Tool (+) Tool (−) Tool (+) Tool (−) Tool (+) Tool (−) Tool (+) Tool (−) Tool (+)

1 0.663 0.963 0.917 1.175 0.930 0.944 7.7 10.1 9.0 6.4 14.1 11.12 0.565 0.715 0.725 0.875 0.797 0.890 9.2 8.4 9.9 13.0 9.7 9.83 0.575 0.540 0.675 0.720 0.759 0.925 5.7 7.7 3.2 7.2 11.9 9.44 0.585 0.510 0.760 0.515 0.718 0.971 3.4 6.4 8.3 14.4 4.0 13.9

Average 0.597 0.682 0.769 0.821 0.801 0.932 6.5 8.15 7.6 10.25 9.925 11.05

F verage kerf width, (c) average surface roughness (Ra), forr

dncdir

4

Iebs

ig. 13 – Effect of applied voltage on (a) penetration rate, (b) aeverse polarity and straight polarity.

eeper crater on the machined surface. Hence, surface rough-ess increases with voltage for the machined cavity at theathode. For anode at high voltage, higher rate of chemicalissolution of quartz occurs and energy discharged by spark-

ng is also more, and it leads to the increased material removalate and higher surface roughness value.

.1. Simultaneous machining at both electrodes

t has been observed that at a time, sparking occurs only at onelectrode for the range of 60–80 V. If the voltage is increasedeyond 80 V and both the electrodes are small in size then,parking occurs at both the electrodes rather than shifting

Fig. 14 – Melting of tool tip at high voltage (110 V, electrolyteconcentration 22 wt.%).

n g t

roverview. Int. J. Mach. Tool Manuf. 45, 1095–1108.

470 j o u r n a l o f m a t e r i a l s p r o c e s s i

between the electrodes. However, work-piece may crack dueto high thermal stresses and also melting of the tool may occurdue to high heat generation (Fig. 14). However, it requires fur-ther work in optimization of the process parameters so thatthe cracking can be avoided while machining at both the elec-trodes.

5. Conclusions

From this study following conclusions have been derived:

• Electrochemical spark machining can be successfully usedfor cutting ceramic plates. ECMWRP cuts quartz plate atfaster rate as compared to ECSMWDP, but produces higherovercut, higher tool wear and higher surface roughness.

• Material removal in ECSMWRP is due to melting, vaporiza-tion and chemical reaction on quartz. Chemical analysis ofreaction products confirms the dissolution of quartz intoNaOH solution. Magnified view of the machined surfacealso shows the difference in the mode of material removalbetween ECSMWDP and ECSMWRP.

• Auxiliary electrode can also be made small in ECSM pro-cess. If we want to use only one electrode at a time as atool, size of the other electrode should be such that spark-ing zone always confines near the small electrode, so thatsparking zone (high potential) is always available at thetool. Both the electrodes can be simultaneously used astool (ECSMWDP + ECSMWRP) under the machining condi-tions explained in the text.

Acknowledgements

The authors are thankful to Prof. S. Sarkar of ChemistryDepartment for his valuable suggestion and guidance duringchemical analysis of the residues. Thanks are also due to Dr.Sunil Jha (from I.I.T. Delhi), Mr. P. Dubey and Mrs. Reena Mittalfor their help extended during this work (I.I.T., Kanpur India).

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