CIRP Annals - Manufacturing Technology 60 (2011) 239–242

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CIRP Annals - Manufacturing Technology

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Experimental characterization of dry EDM performed in a pulsating magnetic field

S. Joshi a, P. Govindan a, A. Malshe (2)b,*, K. Rajurkar (1)c

a Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai, Indiab Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR, USAc Industrial and Management Systems Engineering, University of Nebraska-Lincoln, Lincoln, NE, USA

A R T I C L E I N F O

Keywords:

Dry EDM

Pulsating magnetic field

Material removal rate

A B S T R A C T

This paper presents an investigation of the hybrid dry EDM process performed in a pulsating magnetic

field for improving process performance. In this study, the pulsating magnetic field is applied tangential

to the electric field, for increasing the movement of electrons and degree of ionization in the plasma.

Experiments with parametric variations showed that this hybrid approach leads to productivity-

improvement by 130% and zero tool wear as compared to the dry EDM process without the magnetic

field. The improvement in surface quality is illustrated by scanning electron microscopy (SEM).

� 2011 CIRP.

1. Introduction

Dry EDM uses gas as dielectric medium, and is a potentialalternative for liquid dielectric based EDM. It is characterized bysimplicity, low viscosity of dielectric helping better debris evacua-tion, low wear of tool electrode, thin white layer on machinedsurfaces and eco-friendliness [1]. However, major challenges in dryEDM process are low stability of arc column, low material removalrate, arcing and poor surface quality as compared to conventionalEDM. A few researchers have attempted to improve processperformance with a limited success [2]. Their efforts involveoperating in a quasi-explosion mode [3], providing tool rotation/planetary motions [1], ultrasonically vibrating the workpiece [4],applying water and oil mist [5], and shielding spark [6].

Unlike the use of permanent magnetic field in an earlier work[7] for a limited objective of debris manipulation, this work uses apulsating magnetic field around the dry EDM plasma, like a jacket,to provide magnetic field tangential to the plasma. Also, thepulsation reduces magnetic flux losses over the continuous field. Itwas hypothesized that the field would assist in increasing theplasma ionization and allows control the plasma expansion.Incidentally, such requirements are not prevalent in liquiddielectric EDM as relatively higher density liquid dielectric ensuresfocused plasma. The objective of this paper therefore is to assessthe performance and mechanism of material removal in pulsedmagnetic field assisted dry EDM process (MFA dry EDM) vis-a-visthe process without magnetic field (WMFA dry EDM). In addition,the tool wear, dimensional accuracy and machined surfacecharacteristics are studied.

2. Experimental details

The pulsating magnetic field is provided using a triangularconfiguration of electromagnets energized by a 0–30 V DC variable

* Corresponding author.

0007-8506/$ – see front matter � 2011 CIRP.

doi:10.1016/j.cirp.2011.03.114

switch mode power supply, which actuates magnets sequentially.The magnetic field was varied from 0.1 T to 0.3 T. The magnetic fluxlines around each electromagnet results in a tangential magneticfield, which is in anticlockwise direction as seen from the top (seeFig. 1a and b). As the magnets are energized sequentially, theygenerate a rotating magnetic field around the spark. The fieldinfluences flow of major current carriers i.e. electrons in the dry EDMprocess. It is known that a vector addition of electrical force Fe due toelectric field, and the magnetic force Fm due to magnetic field BR,yields resultant force FR acting on the electrons (Fig. 1a). The force FR

deflects electrons by an angleu towards the sparking region (Fig. 1a).Consequently, the magnetic field helps generation of Lorentz forces[8] in the discharge region. It causes: (i) an increase in the electrondensity and consequently the energy of the dry EDM plasma, and (ii)a reduction in the mean free path of electrons by constraining theirmotion, thereby giving rise to an increase in ionization due tomultiple collisions between charged particles [8]. To assess theperformance of MFA dry EDM and WMFA dry EDM, a total of ninety-three experiments were performed using the parameters and theirlevels, and using design of experiments (DOE), see Table 1. Oxygenpressure refers to the input pressure of the gaseous medium andswitching frequency is the frequency of pulsation of the electro-magnets around the inter-electrode gap.

Copper pipes of OD: 4.25 mm, ID: 3.25 mm with end facespolished were used as tool electrodes. Oxygen gas with 99.9% purity,that promotes exothermic oxidation [3] and the oxides depositionincreases electron emission from cathode [9], was used as a dielectricfluid. The gas was passed into the inter-electrode gap through thepipe electrodes (see Fig. 1b). Two Stainless steel split workpieces,each of size 27 mm� 14 mm� 10 mm, with their mating faceshighly polished were used as work specimens. All the drillingexperiments were performed at the intersection of the split workspecimens using an experimental set-up (Fig. 1b and c) on a CNC EDMmachine in a ‘quasi-explosion mode’ [3] to maximize MRR.Therefore, the pulse off-time was taken as one-sixth of pulse on-time.

The MRR and TWR were calculated on the ultrasonically cleanedwork and tool specimens, by measuring their weight difference

[()TD$FIG]

Fig. 1. a: Schematic of tangential magnetic field and forces. b: Schematic of

pulsating electromagnetic field application. c: Photograph of experimental set-up.

d: Reduced mean free path and more ionization in MFA dry EDM. e: Average

percentage improvement in MRR in MFA dry EDM and WMFA dry EDM.

Table 1Input parameters and their levels.

Parameter Levels

1 2 3

Gap voltage, V (V) 50 65 80

Discharge current, I (A) 12 15 18

Pulse off time, Toff (ms) 22 33 67

Oxygen pressure, P (MPa) 0.15 0.20 0.25

Electrode speed, N (rpm) 100 200 300

Magnetic field, B (T) 0.1 0.2 0.3

Switching frequency, fs (Hz) 100 10 1

S. Joshi et al. / CIRP Annals - Manufacturing Technology 60 (2011) 239–242240

before and after machining, respectively. The dimensions of the tooland machined holes were measured to evaluate oversize (%). Themachined surfaces morphology and the compositional variations onthem were analyzed using scanning electron microscopy (SEM) andenergy dispersive X-ray analysis (EDS), respectively.

3. Results and discussion

3.1. MRR improvement

A comparative evaluation of MRR in MFA dry EDM and WMFAdry EDM shows that a maximum improvement of 130% was[()TD$FIG]

Fig. 2. a–g: A comparative parametric plots for M

observed in trial #11 (65 V, 12 A, 33 ms, 0.25 MPa, 100 rpm, 0.3 Tand 100 Hz) over the corresponding experiment without usingmagnetic field. See Fig. 1e for the best five trials. The results werereplicated once. In general, a minimum improvement in MRR of41% was found in all the trials using the pulsating magnetic field.We propose that the increase in MRR due to the use of magneticfield is because of the following reasons: (i) In the MFA dry EDM,the magnetic force acts tangential to the plasma and prevents itsexpansion. It increases the plasma pressure (P), thereby reducesthe mean free path (l) and consequently increases the density ofthe plasma given by,

l ¼ RT

20:5pd2NA

(1)

where R is the universal gas constant, T is the temperature of thegas, d is the diameter of a gas molecule and NA is the Avogadro’snumber. See schematic in Fig. 1d. (ii) A decrease in the mean freepath of the ions increases the number of ionization events (a) inthe inter-electrode gap given by:

a ¼ PA exp�BP

E

� �(2)

where A and B are constants for a particular gas, P is plasmapressure and E is the electric field in the inter-electrode gap. (iii)The magnetic field increases the inter-molecular collisions andcontributes to the ionization, which helps faster dielectric break-down given by the breakdown criteria:

g ¼ 1

ea:VB=E � 1(3)

where g is number of electrons emitted from cathode due to initialionization and VB is the breakdown voltage. As a result, electroniccurrent density je and the energy (J) of the plasma increase as givenby [10]:

J ¼ pr2c jeV (4)

where rc is the radius of a micro-peak on cathode emittingelectrons, and V is the voltage across cathode and anode. Thus, theapplication of magnetic field controls the plasma expansion andincreases the energy at the anode thereby increasing MRR in theMFA dry EDM (see Fig. 1d). In MFA dry EDM, magnetic force onoxygen molecule and ions causes 40% higher oxidation reactionsthereby increasing MRR [11].

3.2. Parametric analysis of MRR

An improvement in MRR in dry EDM due to the application ofmagnetic field is shown in Fig. 2a–g. A linear increase in MRR withan increase in current (I) is due to increase in discharge energy (Ed):

Ed ¼Z Ton

0VðtÞIðtÞ dt (5)

where Ton is the pulse on-time. The relative increase in MRR in MFAdry EDM over the WMFA process is due to the effect of magneticfield and consequent increase in ionization. The MRR increaseswith electrode rotation speed (N) too (Fig. 2b). The electrode

RR in MFA dry EDM and WMFA dry EDM.

[()TD$FIG]

Fig. 3. a–g: A comparative parametric plots for TWR in MFA dry EDM and WMFA dry EDM.

S. Joshi et al. / CIRP Annals - Manufacturing Technology 60 (2011) 239–242 241

rotation helps improve uniformity of sparking and imparts acentrifugal force (Fc), given by

Fc ¼md4p2N2

3600(6)

on the molten droplets of mass md, thereby promoting moremelting of debris, which is another way of debris evacuation, asexplained in material transfer phenomena in the next section. Anincrease in magnetic field (B) from 0.1 T to 0.3 T, causes an increasein MRR (Fig. 2c) from 0.88 to 0.98 mm3/min. We believe, this is dueto confinement of plasma and increased ionization in plasma asexplained earlier. As the gap voltage increases from 50 V to 80 V,the MRR further increases by 3% and then decreases (Fig. 2d). Thiscould be because: (i) as the inter-electrode gap (d) increases, thevoltage increases, which causes an expansion of the plasma forsame V/d [12], thereby reducing MRR, also (ii) an increase involtage causes a decrease in discharge delay time, consequentlyde-ionization remains incomplete that leads to arcing [13].

3.3. Parametric analysis of TWR

In MFA dry EDM, the magnitude of negative tool wear (materialtransfer from work to tool electrode) is lower and close to one-fifththan that in WMFA dry EDM (see Fig. 3a–g). Zero electrode wearwas observed in five experiments (#1, #3, #4, #11, #12) in MFA dryEDM, where voltage and current used were low. However, in theWMFA dry EDM, the zero electrode wear was evident in only twocases (#6 and #11). The analysis of mechanism of material transferin dry EDM [10] has shown that for a molten droplet, the distancetraveled by the droplet away from the inter-electrode gap to avoiddeposition (s) is proportional to the square of the time (d) before itsolidifies, as given by

s ¼ Fd2

2md(7)

where F is the drag force on a debris particle. Since the anodetemperature is higher in MFA dry EDM, the molten particlesremain at higher temperature for longer duration (d) as comparedto WMFA dry EDM. Hence, the possibility of molten dropletsdepositing on electrode is less in MFA dry EDM. Therefore, there is

[()TD$FIG]

Fig. 4. a–g: A comparative parametric plots for oversiz

less negative tool wear in MFA dry EDM. As discharge current (I)increases, the deposition on the electrode increases. This effect ismuch lower (by 35–50%) in the case of MFA dry EDM (Fig. 3a). Thiswe believe is because of higher removal of molten droplets anddecreasing deposition. An increase in voltage (V) increases thedeposition on the electrode (Fig. 3b). As discussed in materialremoval mechanism earlier [13], a high voltage causes non-uniformtemperature distribution on molten droplets due to arcing, andreduces the distance of expulsion. An increase in pulse off-time (Toff),decreases the deposition on the electrode (Fig. 3c). Thus we believethat an increase in pulse off-time helps in sufficient deionization ofplasma and recovery of dielectric strength, thereby reducing arcing.As the pressure (P) increases, the deposition on the electrodeincreases (Fig. 3d). An increase in gas pressure causes more coolingeffect of the molten droplet, thereby reducing the distance (s) andpromoting their early deposition.

As discussed, an increase in temperature of electrode alsoreduces deposition. An increase in switching frequency (fs)decreases deposition on tool (Fig. 3f). The increased switchingfrequency causes more ionization and consequent release of moreelectrons from cathode. It increases the temperature of cathodecausing melting of droplets and increase in their distance traveledlowering possibility of deposition of droplets.

3.4. Parametric analysis of geometric oversize (%)

As explained in the mechanism of material removal, themagnetic field confines the plasma in MFA dry EDM, consequentlyreduces the geometric oversize in the process (Fig. 4a–g). Thecontrolling parameters in MFA dry EDM are: V, I, P, B and fs. Ascurrent (I) increases, the geometric oversize increases linearlybecause the effect of current is to widen the crater [12].

An increase in switching frequency (fs) causes a linear decreasein oversize (Fig. 4b), because of the frequent action of force (FR)around the plasma that helps confine the plasma. As voltage (V)increases, the oversize increases (Fig. 4c). The expansion of dryEDM plasma due to higher inter-electrode gap (d) also causes ahigher oversize. An increase in pressure (P) decreases the oversize(Fig. 4d). An increase in quantity of oxygen gas dielectric increasesthe number of free electrons and oxygen ions in the gap and higherconfinement of electrons occurs.

e at entry in MFA dry EDM and WMFA dry EDM.

[()TD$FIG]

Fig. 5. a–d: A comparison of surface topography in MFA dry EDM and WMFA dry

EDM.

S. Joshi et al. / CIRP Annals - Manufacturing Technology 60 (2011) 239–242242

3.5. Analysis of machined surface topography

The topographical analysis of the machined surfaces using SEMshows the presence of micro-cracks, blowholes and dimples,however, they are significantly smaller in number in MFA dry EDMas compared to the WMFA dry EDM. River lines indicate presenceof tensile stresses [14], which are very low in the case of MFA dryEDM surface (see Fig. 5a–d). The compositional analysis ofmachined surfaces however shows that there is an increase inalloying on surfaces in MFA dry EDM, with a reduction (in wt%) inFe and increase in (in wt%) C, Cu and O as compared to WMFA dryEDM. This indicates that magnetic field increases alloying in dryEDM. However, EDS results on MFA dry EDM machined surfacesshow a larger migration of tool element (Cu), oxygen and carbon tothe workpiece, unlike dry EDM without the field.

3.6. Process optimization

The experimental analysis shows that the maximum MRR of1.247 mm3/min was obtained at (V-80 V, I-18 A, Toff-67 ms, P-0.15 MPa, N-300 rpm, B-0.3 T, fs-10 Hz). The zero TWR wasobtained in other five trials, where the other parameters are atcentral level of experiments. Similarly, minimum oversize(�11.92%) at entry was obtained at (V-80 V, I-12 A, Toff-22 ms, P-0.25 MPa, N-200 rpm, B-0.3 T, fs-10 Hz).

4. Summary

A new hybrid approach using pulsating magnetic fieldassistance is introduced to confine dry EDM plasma and improveprocess performance. It is demonstrated that the magnetic field,due to higher ionization and plasma confinement, aids a highertransfer of thermal energy to the workpiece and helps to improvisematerial removal mechanism and melting in dry EDM. Animprovement in the geometric accuracy and the machined surfacequality were evident.

Acknowledgement

Authors (SJ & PG) thank DST-India (Project: 08DST058) forfinancial support.

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