Experimental characterization of material removal in dry electrical discharge drilling

ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 50 (2010) 431–443

Contents lists available at ScienceDirect

International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool

Experimental characterization of material removal in dry electrical discharge drilling P. Govindan, Suhas S. Joshi n Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai – 400 076, India

a r t i c l e in fo

abstract

Article history: Received 3 November 2009 Received in revised form 10 February 2010 Accepted 11 February 2010 Available online 21 February 2010

Dry electrical discharge machining is one of the novel EDM variants, which uses gas as dielectric fluid. Experimental characterization of material removal in dry electrical discharge drilling technique is presented in this paper. It is based on six-factor, three-level experiment using L27 orthogonal array. All the experiments were performed in a ‘quasi-explosion’ mode by controlling pulse ‘off-time’ so as to maximize the material removal rate (MRR). Furthermore, an enclosure was provided around the electrodes with the aim to create a back pressure thereby restricting expansion of the plasma in the dry EDM process. The main response variables analyzed in this work were MRR, tool wear rate (TWR), oversize and compositional variation across the machined cross-sections. Statistical analysis of the results show that discharge current (I), gap voltage (V) and rotational speed (N) significantly influence MRR. TWR was found close to zero in most of the experiments. A predominant deposition of melted and eroded work material on the electrode surface instead of tool wear was evident. Compositional variation in the machined surface has been analyzed using EDAX; it showed migration of tool and shielding material into the work material. The study also analyzed erosion characteristics of a singledischarge in the dry EDM process vis-a´-vis the conventional liquid dielectric EDM. It was observed that at low discharge energies, single-discharge in dry EDM could give larger MRR and crater radius as compared to that of the conventional liquid dielectric EDM. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Dry EDM Material removal rate Tool wear rate Dimensional accuracy Single discharge Taguchi methods

1. Introduction Dry EDM is one of the novel EDM techniques, which employs gas as a dielectric medium instead of liquid. It mainly involves supply of a gas through a rotating thin-walled pipe electrode, which also flushes out the debris from inter-electrode gap [1]. The process holds the potential to be a viable alternative to conventional liquid dielectric EDM for precision oriented machining applications. The major advantage of this method is its simplicity. In conventional EDM, in addition to the problems associated with a complex dielectric supply system, a kerosene or oil-based dielectric causes carbon deposition over the machined surfaces. On the other hand, use of water-based dielectric leads to formation of cracks, electrolysis and corrosion of the EDMed surface [2,3]. The force generated after the dielectric breakdown in EDM due to expansion and contraction of a bubble formed by evaporation, dissociation and ionization of dielectric liquid and electrode materials is higher in liquid as compared to gas, since expansion of the bubble is prevented by greater inertia and viscosity of the dielectric liquid. Since the electrical permittivity of the liquid

n

Corresponding author. Tel.: + 91 22 25767527. E-mail address: [email protected] (S.S. Joshi).

0890-6955/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2010.02.004

medium is higher than the gas medium, the electrostatic force is higher as well [4]. It is known that the liquid dielectric has difficulties in accessing the sparking region, besides its fumes are hazardous to the environment. Mist EDM, a new fine finishing EDM process [5], also causes a reduction in efficiency, high tool wear, pollution, difficulty in cleaning and piping required for mist circulation. Another emerging technology, viz. powder mixed EDM, increases the cost of machining and is environment-unfriendly [6]. Dry EDM technology overcomes some of these demerits and has a few additional advantages such as formation of thinner white layer [7,8], low dielectric constant resulting in easy breaking of dielectric and formation of plasma, lesser viscosity of gas and lower heat concentration causing better debris removal and flushing. At present, the investigations related to dry EDM process are mainly aimed at demonstrating the feasibility of the process and improving the basic process outputs. In this regard, Kunieda et al. [1] has demonstrated dry EDM process in machining of steel, where oxygen gas was used as a dielectric medium. Furthermore, a potential method to enhance MRR by controlling pulse-off-time, using ‘quasi-explosion mode’ was proposed in Ref. [7]. Yu et al. [9] belonging to the same research group applied dry EDM milling to machine ‘difficult-to-cut’ cemented carbides. They found that the process reduces machining time and is cost-effective. Zhang et al.

ARTICLE IN PRESS 432

P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

[10–12] employed ultrasonic vibrations to improve MRR of the process and introduced surface roughness as a response variable. In these investigations, wall thickness of the electrode and amplitude of ultrasonic vibration were identified as two additional input parameters. In addition, the material removal mechanisms in ultrasonic assisted dry EDM of cemented carbides were studied in Ref. [13]. An investigation involving control of gap length in dry EDM using piezoelectric actuator was conducted by Kunieda et al. [14], which improved MRR as well as the process stability. Kao et al. [5] developed near dry EDM using liquid–gas mixture as a dielectric fluid for machining aluminium. Moreover, Tao et al. [3] explored the capability of dry EDM for better MRR and near-dry EDM for fine surface finish, and found that among the few electrode material-dielectric medium combinations, copper–oxygen was the best for dry EDM and graphite with water–nitrogen mixture for the near-dry EDM. In a recent work by Saha and Choudhury [15], a set of central composite design (CCD) experiments were conducted. They found that discharge current, duty factor, air pressure and spindle speed significantly influenced MRR, and all the parameters except gap voltage influenced the average surface roughness (Ra). Nevertheless, one of the major unresolved issues related to dry EDM is the low MRR. Furthermore, low stability, arcing and micro-crack formation over the surface, poor surface finish and adherence of debris to the electrode are some other problems that are yet to be resolved adequately [2,3]. Some researchers have shown that gas pressure is a significant parameter that influences the process performance [1–3,6–15]. However, the gas is released into the atmosphere during the process. In spite of various research works, it appears that no attempts have been made to solve the problem of uncontrollable plasma expansion in a dry EDM process. It is felt that if in some way, the release of gas can be constrained to create a back pressure, and hence the plasma expansion could be controlled during the process. Therefore, the objective of this paper is to investigate dry EDM in drilling by employing a shield around the plasma. It is proposed to run the process in the ‘quasi-explosion mode’. The process analysis involves a study of effect of processing conditions on MRR, TWR, and accuracy. Pulse ‘off-time’ and clearance between the shield and the electrode at the sparking region were used in conjunction with the other regular dry EDM parameters. Also, analysis of the mechanism of material removal has also been carried out by performing singledischarge experiments in dry EDM. The details are elaborated in the following sections of the paper.

2. Experimental work In order to characterize the dry EDM process qualitatively and quantitatively through an assessment of the influence of input

parameters on the machining performance, the following three approaches have been used in this work: (i) To perform the experiments under well-known ‘quasiexplosion’ conditions [7], by controlling pulse ‘off-time’, so as to maximize the MRR. (ii) To provide an enclosure around the electrode, aiming at creating a back pressure thereby restricting expansion of the plasma. This action is expected to stabilize the process and help prevent spark column contamination. (iii) To analyze the radius of crater and MRR obtained in a singledischarge in dry EDM and compare it with that of the liquid dielectric EDM. It is felt that the analysis of the various characteristics of dry EDM process shown in Fig. 1 could reveal the performance of the process. Since MRR and TWR are the most commonly analyzed performance indicators in this work, the dimensional oversize and machined surface topography have also been included as response variables. 2.1. Experimental design In this experimentation, various independent parameters are selected (see Fig. 1) based on the authors’ preliminary experimentation and the past literature, as mentioned in Table 1. It was evident that MRR is higher with reverse polarity (work is positive). Therefore, all the experiments were performed using the reverse polarity. From the physics of the process, three twofactor interactions (i.e. voltage  current, voltage  pulse off-time and current  pulse off-time) which could influence energy of spark have been chosen for the experimentation. Accordingly, the DOF of this experiment is 26. Therefore, the L27 OA (orthogonal array) has been selected for the experimentation. The levels of parameters and allotment of various factors to various columns of L27 array based on the linear graph [16] are depicted in the right hand bottom corner of Table 2. 2.2. Experimental set-up and procedure 2.2.1. Experimental set-up A ELECTRA PS ZNC EDM machine having a programmable Z-axis control, with NC multi-step programming facility with a rotary attachment, has been used for conducting the dry EDM drilling experiments on SS304 (see Table 3 for material composition). A schematic diagram and a photograph of the dry EDM set-up are shown in Fig. 2a,b. Dry EDM set-up consisting of an oxygen cylinder with a regulator for flow control was connected to the copper electrode.

Input variables Gap voltage

Debris particle

Rotating Oxygen gas supply electrode

Discharge current

Output variables Approaches 1 and 2 MRR TWR

Shield

Oversize

Pulse off-time Machined surface Oxygen pressure Work piece

Electrode speed Dry EDM process Shielding clearance

Approach 3 MRR in single discharge Crater radius in single discharge

Fig. 1. Schematic of the process and theme of experimental work on dry EDM drilling.

ARTICLE IN PRESS P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

433

Table 1 Experimental parameters, levels and reason for selection. No. Parameter, units

Levels

Reason for selection

1

Discharge current (I), A

12, 15, 18

2 3

Gap voltage (V), V Pulse off-time (Toff), ms

4

Gas pressure (P), MPa

5

Electrode speed (N), rpm

6

Radial clearance of shield at bottom (Cb), mm

Higher levels of pulse current increases thermal loading and causes damage to the work, therefore lower current levels selected. 50, 65, 80 Based on the range available with the machine and within a range in one of the recent investigations [3]. 22, 33, 67 To satisfy ‘quasi-explosion’ condition. The three levels of pulse ‘off-time’ chosen are: one-third, one-sixth and oneninth of pulse ‘on-time’. Approximately one-sixth has been proposed as the range for quasi-explosion mode earlier [7]. 0.15, 0.20, Range available with the existing set-up. Similar machining conditions used in few investigations [7,9]. 0.25 100, 200, Rotation of the tool enhances dielectric flow and stability of the process [1]. However, very high speed also reduces 300 accuracy [17]. Therefore, the range selected is 100–300 rpm. 4.0, 4.5, Due to high mobility of gas molecules, which causes uncontrollable plasma expansion, a shield is provided. An 5.0 aluminium shield was provided, having lowest ‘‘rCp’’ factor (2.42) compared to copper (3.44) and iron (3.46).

Table 2 L27 array with levels of input parameters, their allocation (including selection of interactions), response variables (MRR, TWR and oversize) and linear graph for independent variables assignment. Trial No.

Input parameters and their levels V (Volts)

I (A)

Toff (ms)

P (MPa)

N (rpm)

Response variables (for original trials and replicated trials) Cb (mm)

MRR (mm3/min)

MRRR (mm3/min)

TWR (mm3/min)

TWRR (mm3/min)

Oversize at locations along depth of hole (%) Entry

1 50 2 50 3 50 4 50 5 50 6 50 7 50 8 50 9 50 10 65 11 65 12 65 13 65 14 65 15 65 16 65 17 65 18 65 19 80 20 80 21 80 22 80 23 80 24 80 25 80 26 80 27 80 Assignment of

12 22 0.15 100 4 0.376 0.416  0.0056 12 33 0.20 200 4.5 0.414  0.0093 12 67 0.25 300 5 0.441 0.458  0.0074 15 22 0.20 200 5 0.552  0.0018 15 33 0.25 300 4 0.620  0.0056 15 67 0.15 100 4.5 0.515 0.0000 18 22 0.25 300 4.5 0.794 0.811  0.0018 18 33 0.15 100 5 0.679 0.634  0.0005 18 67 0.20 200 4 0.691  0.0167 12 22 0.20 300 4.5 0.426  0.0074 12 33 0.25 100 5 0.380 0.0000 12 67 0.15 200 4 0.449 0.0130 15 22 0.25 100 4 0.517 0.523 0.0167 15 33 0.15 200 4.5 0.513 0.552  0.0204 15 67 0.20 300 5 0.578  0.0093 18 22 0.15 200 5 0.699 0.729  0.0037 18 33 0.20 300 4 0.755  0.0093 18 67 0.25 100 4.5 0.679 0.649 0.0093 12 22 0.25 200 5 0.303 0.357  0.0112 12 33 0.15 300 4 0.401 0.0056 12 67 0.20 100 4.5 0.315 0.347  0.0074 15 22 0.15 300 4.5 0.441 0.454  0.0074 15 33 0.20 100 5 0.437  0.0112 15 67 0.25 200 4 0.586 0.464 0.0093 18 22 0.20 100 4 0.580  0.0018 18 33 0.25 200 4.5 0.624  0.0130 18 67 0.15 300 5 0.605  0.0018 factors to columns and selection of interactions based on the linear graph

0.0242  0.0112

 0.0204  0.0037

0.0018  0.0056  0.0093 0.0018  0.0112  0.0056 0.0093  0.0018

50%

90%

OS

OSR

OS

OSR

OS

OSR

 29.29  32.33  29.94  36.23  38.69  34.60  44.01  43.96  39.86  32.79  31.06  37.92  36.57  37.11  37.81  40.2  44.74  41.44  29.54  35.58  32.16  38.84  35.62  43.38  40.88  40.71  38.32

 41.16

 16.74  22.66  17.70  23.83  23.70  20.82  23.89  27.00  24.85  21.08  16.54  26.15  14.66  19.59  22.41  28.42  24.19  21.61  16.39  21.68  20.51  21.68  20.40  24.13  26.25  25.52  23.96

 21.54

 2.67  6.64  8.15  8.75  8.53  2.76  5.02  6.31  7.17  5.26  5.91  3.82  8.88  3.72  3.63  2.39  2.80  1.79  2.81  1.48 0.81  4.84  6.59  4.94 0.31  7.74  5.14

 7.36

 36.83

 43.33  40.53

 40.39  39.85  45.89  44.74  32.37  37.13  37.03  41.78

 20.13

 24.34  25.22

 21.35  24.67  27.16  22.62  20.49  18.62  21.81  23.37

ASSIGNMENT OF FACTORS TO COLUMNS AND SELECTION OF INTERACTIONS BASED ON THE LINEAR GRAPH Col No.

1

2

5

9

10

12 1- V

EMPTY COLUMNS

3, 4, 6, 7, 8, 11, 13

V×I

V × Toff 9-P

2-I

5-Toff

10-N 12- Cb

I × Toff

Linear graph and arrangement of independent variables to columns of L27 OA [16] R

R MRR - MRR for original trials , MRR - MRR for replicated trials, TWR- TWR for original trials, TWR - TWR for replicated trials, OS- Oversize for original trials, R OS -Oversize for replicated trials

 2.81

 3.78  7.05

 4.27  4.78  7.9  3.45  5.45  7.27  4.99  6.07

ARTICLE IN PRESS 434

P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

Table 3 Thermal and physical properties of work piece, tool, dielectric and shield. Element

Specific heat (J/g K)

Melting point (K)

SS 304

0.5

1455

Copper Aluminium Oxygen

0.4 0.9 0.92

1375 933.2 500.5

Thermal conductivity (W/m K) 16.2 401 237 0.0267

Density

Chemical composition (wt%)

8.03 g/cm3 3

8.9 g/cm 2.7 g/cm3 1.429 g/L

Cr Ni 18.0 8.0 100% Cu 100% Al 99.9% pure

Mn 2.0

Si 0.75

Dt

C 0.08

P 0.04

S 0.03

Fe Balance

Shield

H Clamping force

Db

Entry 10% L 50% L 90% L Exit

Fig. 2. (a)–(e) Experimental set-up for dry EDM: (a) schematic diagram, (b) photograph, (c) SS 304 split work piece in assembly and one-half of the split specimen, (d) specifications of a section of shield used in experiments (Dt =diameter of the shield at top= 9 mm, Db = diameter of the shield at the bottom= 13, 14 and 15 mm), (e) typical hole SEM photograph showing locations of the oversize measurement.

In this study, the work material was designed in the form of a split specimen [17,18], and dimensions of each part were: 27 mm  14 mm  10 mm (see Fig. 2c). The mating surfaces (interfaces) on the specimen were ground and polished using waterproof SiC papers of grit size varying from 600 to 1200 to achieve parallelism. Knowing that in dry EDM, oxygen gas and copper electrode provides the best option [13,15], thereby they were used in experiments. Electrodes were prepared by using round copper tubes (OD: 4.75 mm, ID: 3.25 mm) of 60 mm long with polished flat end faces. A hollow cylindrical aluminium shield having a height of 15 mm with three different levels of radial clearances (2, 2.5 and 3 mm) around the electrode, near the sparking zone, was used (Fig. 2d). Height H as shown in Fig. 2d is taken as 15 mm in all experiments, based on initial experiments carried out. Height was varied between 10 and 20 mm. It was observed that the optimum value of MRR was at a height of 15 mm. The properties and specifications of work, tool, dielectric and shield are also listed in Table 3.

2.2.2. Experimental procedure To make the holes exactly at the intersection of two parts of the split work piece, accurate alignment of the electrode with reference to the split workpiece intersection was done. Dry electrical discharge drilling experiments based on L27 orthogonal array were performed in a random order. Among the total 27 trials, 12 experiments were replicated. Each experiment was continued for an hour. After machining, specimens were cleaned using an ultrasonic cleaner to remove loose debris deposited around the work surfaces. Fig. 2e shows a scheme of typical measurements of oversize on drilled holes. The weight loss of workpiece and tool, during the dry EDM process, was measured using Sartorius CP 4235 precision scale. Knowing the density of stainless steel 304 (workpiece) and copper (electrode), MRR and TWR are estimated: MRR ðin mm3 =minÞ ¼

ðWi Þw ðWf Þw

rss



1  1000 Tm

ð1Þ

ARTICLE IN PRESS P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

where (Wi)w is the weight of the workpiece before machining, (Wf)w is the final weight of the workpiece after machining, rss is the density of SS304 in g/cm3 and Tm is the machining time in minutes, TWRðin mm3 =minÞ ¼

ðWi Þe ðWf Þe

rcu



1  1000 Tm

ð2Þ

where (Wi)e is the weight of the electrode before machining, (Wf)e is the weight of the electrode after machining, rcu is the density of copper in g/cm3 and Tm is the machining time in minutes. The same method has been followed for all the experiments. Depth achieved during the process and outside diameter of the drilled holes was measured using a Nikon MM-400 microscope. Surface morphology of the cross section of machined surfaces and compositional variations using EDAX were observed on a FEI Quanta200 SEM.

3. Results and discussion The results of the experiments including the values of response variables (MRR and TWR) are presented in Table 2. 3.1. Analysis of material removal rate The MRR values (in Table 2) show that the highest MRR (0.811 mm3/s) was achieved for 7th trial-replication (50 V, 18 A, 22 ms, 0.25 MPa, 300 rpm and 4.5 mm) and the lowest MRR (0.303 mm3/s) was for 19th trial (80 V, 12 A, 22 ms, 0.25 MPa, 200 rpm, 5 mm). In order to investigate the effect of machining parameters on MRR, statistical analysis using analysis of variance (ANOVA) has been performed (see Table 4). Table 4 ANOVA for MRR (mm3/min). ANOVA for material removal rate (MRR) Parameter

DOF

Seq SS

Adj SS

Adj MS

F

P

Statistical significance

V I Toff P N Cb VnI VnToff InToff Error Total

2 2 2 2 2 2 4 4 4 14 38

0.1173 0.5145 0.0003 0.0054 0.0278 0.0046 0.0037 0.0044 0.0056 0.0188 0.7028

0.0547 0.4679 0.0001 0.0041 0.0299 0.0050 0.0025 0.0044 0.0056 0.0188

0.02738 0.23395 0.00007 0.00205 0.01496 0.00250 0.00129 0.00224 0.00280 0.00094

29.09 248.5 0.08 1.64 11.94 2.66 1.38 2.38 2.98

0.00 0.00 0.92 0.13 0.00 0.09 0.27 0.11 0.07

O O X X O X X X X

V = Gap voltage; I =discharge current; Toff = pulse-off time; P = pressure; N = spindle speed; Cb = clearance of shield at bottom.

435

The ANOVA results show that the gap voltage (V), discharge current (I) and rotational speed of the electrode (N) are the significant factors (at 95% confidence level) that influence the MRR (see Table 4). It is observed that none of the interactions are significant in influencing MRR. The trends of each factor in main effects plots are determined using analysis of means (AOM) plots in Fig. 3a–f. 3.1.1. Effect of discharge current on MRR Statistical analysis using ANOVA (see Table 4) reveals that discharge current (I) is the most significant parameter due to the highest F value. The main effects plot (see Fig. 3b) indicates that MRR increases linearly with current (I) at all levels. With a variation in current from 12 to15 A, and a further increase up to 18 A, a linear increase in average MRR has been observed. From ANOVA table for MRR, a very higher F value (248.5) indicates that discharge current I is more significant than gap voltage V, which is contrary to the usual findings. It is found that due to an increase in current, there is an increase in MRR at all levels of other parameters, which is attributed to an increase in pulse energy. From all previous investigations in EDM as well as dry EDM, it has been known that material removal increases with an increase in spark energy, a function directly proportional to discharge current (I) [23], due to greater transfer of thermal energy to the machining zone. 3.1.2. Effect of gap voltage on MRR Statistical analysis using ANOVA (see Table 4) shows that the gap voltage (V) is also a significant parameter at 95% confidence level. As can be observed from the main effects plot (see Fig. 3a), an increase in voltage appears to cause a decrease in MRR. An increase in gap voltage from 50 to 65 V causes a decrease in average MRR by 1.69%. As the voltage changes from 65 to 80 V, further reduction in MRR by 18.26% has been observed. Generally, it is anticipated that an increase in gap voltage increases discharge energy given by E¼

1 2 CV 2

ð3Þ

where C is the capacitance of the medium and V is the voltage across the gap. However, the relation does not appear to hold completely for dry EDM though the voltage has a significant influence (from statistical results). In conventional EDM, a reduction in MRR with an increase in gap voltage (V) as high as 43%, was evident [19]. This is due to an increase in electric field, which helps discharge to occur even at high gap widths and insufficient cooling of the work due to localized concentration of discharge [20]. A similar trend should be prevalent such that MRR also decreases beyond a certain voltage in dry EDM. A high voltage (above 80 V) increases gap of sparking, which reduces velocity of gas at the work surfaces consequently affecting flushing

MRR (mm3/min)

0.7 0.6 0.5 0.4 35 50 65 80 Voltage (V)

12 15 18 Current (A)

22 33 44 55 66 Off-time (

0.15 0.2 0.25 Pressure (MPa)

100 200 300 Speed (rpm)

Fig. 3. (a)–(f) Main effects plots of input parameters associated with MRR.

4 4.5 5 Clearance (mm)

ARTICLE IN PRESS 436

P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

and promoting arcing [15]. However, in this investigation, though a similar phenomena has been observed, even after a gap voltage of 65 V, increased MRR values were obtained under better flushing conditions, viz. increase in oxygen pressure and rotary speed, even at higher gap voltages.

conventional EDM, under similar machining conditions, the increase in MRR with an increase in speed of rotation (by 70 rpm) is relatively higher (up to 6%) than in dry EDM [21], due to better heat transfer from electrode surface and lesser reattachment of debris.

3.1.3. Effect of spindle speed on MRR The spindle speed (N) is also statistically significant (at 95% confidence level) for MRR (see Table 4). It appears from the main effects plot depicted in Fig. 3e that there is a slight overall increase in MRR when the spindle speed (N) increases. As the rotation speed changes from 100 to 200 rpm, an increase in MRR by 2.46% is observed. Similar observations were made by other researchers [21,22]. They have shown that MRR increases with speed because of centrifugal force and whirl which help flush debris out, and consequently improves MRR. Similarly, in dry EDM, a rotation of tool reduces short circuit [1]. In EDM process, under normal circumstances, spindle speed (N) is the third significant parameter after gap voltage (V) and current (I). When the effect of tool rotation on MRR in dry EDM is compared with that of the EDM, it is observed that spindle speed has a lower F value (11.94), compared to gap voltage and current. However, spindle speed appears to be more influential in the present investigation, with the same P value as that of mentioned two parameters. This could be due to the fact that a denser medium may not allow effective debris expulsion. However, in

3.1.4. Other factor effects It is clear that oxygen pressure (P), pulse ‘off-time’ (Toff) and shielding clearance (Cb) are not significant parameters (see ANOVA results in Table 4). The past investigations related to dry EDM show that most of the researchers have chosen gas pressure as an important parameter, but its statistical significance in the process was not evident. The main effects plot of the three variables (in Fig. 3c, d, f) also showed similar variation.

Table 5 ANOVA for TWR (mm3/min). ANOVA for tool wear rate (TWR) Parameter

DOF

Seq SS

Adj SS

Adj MS

F

P

Statistical significance

V I Toff P N Cb Vn I VnToff InToff Error Total

2 2 2 2 2 2 4 4 4 14 38

0.00005 0.00009 0.00013 0.00037 0.00034 0.00031 0.00017 0.00033 0.00028 0.00130 0.00341

0.00004 0.00008 0.00010 0.00041 0.00030 0.00031 0.00009 0.00016 0.00016 0.00130

0.000023 0.000040 0.000053 0.000205 0.000153 0.000156 0.000046 0.000081 0.000083 0.000065

0.36 0.62 0.82 3.15 2.35 2.41 0.71 1.25 1.29

0.704 0.548 0.454 0.065 0.121 0.116 0.502 0.308 0.298

X X X X X X X X X

V = Gap voltage; I = Discharge current; Toff = Pulse-off time; P= Pressure; N =Spindle speed; Cb = Clearance of shield at bottom.

TWR (mm3/min) 10-3

0

3.2. Analysis of tool wear TWR values were estimated for the original and replicated trials as given in Table 5. The absolute values of tool wear show that there are two main phenomena governing tool wear characteristics: erosion of electrode material and deposition of material on the electrode. The results of ANOVA and AOM on the data are shown in Table 5 and Fig. 4a–f, respectively. In spite that none of the parameters appear to be significant at 95% confidence level, some of them appear to be significant at about 90% confidence level: P—oxygen pressure, followed by Cb—radial clearance and N—spindle speed (P at 94.5%, Cb at 88.5%, N at 88%). As can be seen in main effects plots (Fig. 4a–f), central level of the relatively significant parameters (P, N and Cb) gives minimum tool wear (i.e. maximum material deposition on the tool). Considering the effect of oxygen pressure, the highest material deposition is observed at the central level (0.2 MPa), and it is found to be reduced further at the highest level (0.25 MPa). It is understood that in dry EDM, debris deposition is very high at low input pressures of oxygen due to inadequate cooling of electrode surface [3,15]. Therefore, an increase in deposition of material at the surfaces of inner walls of tool was observed, due to an increase in oxygen pressure from 0.15 to 0.20 MPa. It appears that pressure close to 0.20 MPa is sufficient to cool the electrode surface, but is not sufficient to blow away the debris particles. Furthermore, an increase in radial clearance of shield is found to increase the deposition effect on the tool. At larger clearances, cooling of electrode will be more effective, whereas for small clearances, it may not be so. Therefore, the deposition could be less at smaller clearances.

Voltage (V)

Current (A)

Off time (µs)

Pressure (MPa)

35 50 65 80

12 15 18

22 33 44 55 66

0.1 0.15 0.2 0.25 1

Clearance (mm)

2

-2

-4

Speed (rpm) 0 100 200 300 0

-2 -2

-6

-5

-8

-8 Fig. 4. (a)–(f) Main effects plots of input parameters associated with TWR.

-4

-6

4

4.5

5

ARTICLE IN PRESS P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

It is known that in conventional EDM, with an increase in spindle speed, formation of a deposited layer is observed around the electrode periphery [21,24]. A further increase in speed leads to an increase in heat transfer rate from electrode surface and subsequently causes separation of a deposited layer. This effect appears to be similar to that in present investigation of dry EDM. It may be noted that other parameters especially, the energy related parameters, viz. voltage (V), current (I) and pulse off time (Toff), are not significant in influencing the tool wear of the process. In conclusion, the parametric conditions that minimize and maximize the response variables are summarized in Table 6.

437

the hole dimension in comparison with the original electrode dimension has been estimated as the outside diameter error (%). It was done at three locations, viz., entry, 50% depth and 90% depth of each hole machined. Fig. 5a shows the factors that are statistically significant in influencing the hole oversize at various depths. The analysis is based on the results of ANOVA, which is not presented here. The main effect plots indicating trends in factor effects at the entry are displayed (see Fig. 5b–g). Similar plots are available for other depths too, as shown in Fig. 6a–f (at 50% depth) and in Fig. 7a–f (at 90% depth). The ANOVA results in Fig. 5a show that at entry of the hole, current (I) and clearance of the shield at bottom (Cb) are the significant parameters (at 95% confidence level) that govern the oversize. However, as can be observed in main effects plots (see Fig. 5b–g), the trend in variation of OD error (%) due to these two parameters, appears to be opposite to each other. For example, a linear increase in I causes a corresponding increase in oversize, whereas a linear increase in clearance of the shield decreases the

3.3. Analysis of oversize Analysis of oversize shows that the holes machined using dry EDM are of U-shape specifically after 50% of length. Oversize in

Table 6 Parametric conditions corresponding to different phenomena related to tool wear. Governing phenomena

Tool wear

Condition

Maximum

Minimum

Trial no.

1 (replicated)

13 (replicated)

Parametric conditions V (V) I (A) Toff (ms) P (MPa) N (rpm) Cb (mm)

50 12 22 0.15 100 4

65 15 22 0.25 100 4

Debris deposition on electrode

Zero tool wear/deposition

Maximum

Minimum

Zero variation

18 (replicated)

14

7 (replicated)

8

6

11

65 18 67 0.25 100rpm 4.5

65 15 33 0.15 200 4.5

50 18 22 0.25 300 4.5

50 18 33 0.15 100 5

50 15 67 0.15 100 4.5

65 12 33 0.25 100 5

Hole surface Entry 50% 90%

Toff

V I V I V I

Toff

Toff

P

P

N Cb

P N Cb N Cb

V*I V*I

V*I

V*Toff

V*Toff

I*Toff

I*Toff

V*Toff I*Toff

Bold parameter labels indicates statistical significance at 95% confidence level

Voltagge (V) -34

-30

Oversize-entry (%)

35 50 65 80

Current (A)

Off-time (µs)

Pressure (Mpa)

11 22 33 44 55 66

0.15 0.2 0.25

Speed (rpm)

Clearance (mm)

-34 9 12 15 18

-36

-33

-36

-38

-36

-38

-40

-39

-40

-42

-42

-42

100

200 300

4

4.5

5

Fig. 5. (a) Statistical significance of parameters influencing oversize at locations (entry, 50% and 90%). (b)–(g) Main effects plots of input parameters associated with oversize at entry.

ARTICLE IN PRESS 438

P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

Voltage (V)

Oversize-50% depth (%)

-18

35 50 65

Current (A) 80

Off-time (

12 15 18

Pressure (MPa)

22 33 44 55 66

0.15 0.2 0.25

Speed (rpm) 100 200 300

Clearance (mm) 4

4.5

5

-20

-22

-24

-26 Fig. 6. (a)–(f) Main effects plots of input parameters associated with oversize at 50% depth.

-4 35 Oversize-90% depth (%)

Current (A)

Voltage (V) 50

65

80

12

15

18

Off time (µs)

Pressure (Mpa)

22 33 44 55 66

0.15 0.2 0.25

Speed (rpm) 100 200 300

Clearance (mm) 4

4.5

5

-4.5 -5 -5.5 -6 -6.5 Fig. 7. (a)–(f) Main effects plots of input parameters associated with oversize at 90% depth.

oversize. In conventional EDM, there is a small increase in oversize (4%), with an increase in current, even at low values (below 12 A) [25]. A similar pattern is observed from the main effects plots (see Fig. 5b–g). It has also been observed that neither of the other chosen factors (V, Toff, P, N) nor the corresponding interactions (V  I, V  Toff, I  Toff) statistically influence the oversize. At the centre of the hole, i.e. at 50% depth, it is evident that current (I), oxygen pressure (P) and spindle speed (N) are significant at 95% confidence level (Fig. 5a). It is observed from main effects plots (Fig. 6a–f) that an increase in current causes a linear increase in OD error (%). However, an increase in pressure (P) results in a decrease in OD error (%). The spindle speed corresponding to the intermediate level (200 rpm) yields maximum OD error (%). At 50% depth of hole, neither of other parameters (V, Toff and Cb) nor the chosen interactions are found to be statistically significant. At 90% depth of hole, spindle speed (N) is the only statistically significant parameter (Fig. 5a). Further, the main effect plots (Fig.7a–f) show that the oversize appears to be maximum at the middle level of speed (N). However, at the other two levels of spindle speed (N), OD error (%) is found to be minimum. In summary, the extreme values of oversize at entry, 50% depth and 90% depth are presented in Table 7.

3.4. EDAX analysis of dry ED machined surfaces of hole Energy dispersive X-ray (EDAX) method of analysis has been used to identify the elemental composition in the machined surfaces generated after dry EDM. EDAX analysis of the samples corresponding to the parametric conditions that give the best and the worst MRR and TWR has been presented in Table 8. An area at the middle of the cross section of specimen was selected (see Fig. 8a, b), and average percentage composition of elements at various points covering the region has been estimated. EDAX analysis showed that the prominent elements detected in the samples other than iron (Fe) were nickel (Ni), followed by oxygen (O) and chromium (Cr). The results also indicated that oxygen (1.08–11.36%), copper (0.85–4.7%), and aluminium (0.227–0.395%) were the additional elements observed. A typical central region on a dry EDMed surface corresponding to trial #19 (80 V, 12 A, 22 ms, 0.25 MPa, 200 rpm and 5 mm) has been chosen for EDAX analysis, and the corresponding EDS pattern indicating intensities of various elements is shown (see Fig. 8b). A study of literature shows that during a rotary EDM process, migration of material from tool to work piece occurs, which increases with an increase in spindle speed [26]. EDAX analysis indicated that some quantity of copper (tool material) and aluminium (shield material) occurred on the work surface. In addition, an increase

ARTICLE IN PRESS P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

439

Table 7 Extreme values of oversize at various locations of hole and their parametric conditions. Location

Entry

50% depth

Condition

Maximum

Minimum

Maximum

Minimum

Maximum

Minimum

OD error (%) Trial no. Parametric conditions V (V) I (A) Toff (ms) P (MPa) N (rpm) Cb (mm)

 45.89% 16th (replicated)

 29.29% 1st

 28.42% 16th

 14.66% 13th

 8.88% 13th

0.31% 25th

50 12 22 0.15 100 4

65 18 22 0.15 200 5

65 15 22 0.25 100 4

65 15 22 0.25 100 4

80 18 22 0.20 100 4

65 18 22 0.15 200 5

90% depth

Table 8 Extreme parametric conditions for MRR and TWR and corresponding EDAX results. Element

Iron (Fe) Nickel (Ni) Chromium (Cr) Oxygen (O) Manganese (Mn) Aluminium (Al) Silicon (Si) Copper (Cu) Carbon (C)

Original composition, wt%

MRR

TWR Deposition

Average composition, wt%

Highest (Trial no.1 (replicated)) Average composition, wt%

Highest (Trial no.14), wt%

Lowest (Trial no.8), wt%

69.83 22.36 3.822

59.89 18.14 8.447

68.76 18.26 3.027

69.14 25.63 1.79

60.61 32.097 2.1

– 2.0

2.127 0.247

11.36 0.647

2.985 0.845

1.087 0.082

2.88 0.175

–

0.395

0.232

0.387

0.395

0.227

0.75 – 0.08

0.142 0.937 0.125

0.43 0.85 –

0.217 4.707 0.745

0.185 1.22 0.467

0.185 3.097 0.547

71.17% 8.0 18.0

Highest (Trial no.7 (replicated)) Average composition, wt%

Lowest (Trial no. 19)

Cursor: 10.192 keV 20 counts

Full scale counts: 2234 5000 4000

O Fe Cr Mn Ni Cu

3000 2000 1000 0

0

Fe Cr Mn Cr

Si Al 2

4

6

Ni Fe Ni Cu Cu 8

10

keV Fig. 8. (a), (b) EDAX analysis of dry EDMed surfaces: (a) typical central location for analysis: experiment #19 (80 V, 12 A, 22 ms, 0.25 MPa, 200 rpm, 5 mm) and (b) EDS spectrum (at 1000  magnification).

in carbon content on machined surface was found, which is similar to conventional EDM [29] and micro-EDM [18] processes.

3.5. Machined surface topography analysis The dry EDM machined samples that correspond to the severest and the mildest conditions of MRR, TWR and oversize were investigated using scanning electron microscopy (SEM) at various regions of the surface and at various magnifications (80  , 300  , 600  and 1200  ). It is envisaged that an interaction between spark generated and work surface causes a

change in the surface. A close examination of the images shows that in most of the cases, micro-cracks have been developed. An analysis of morphology of surfaces has been presented in Table 9. There have been a few attempts to characterize morphological features in an EDM process in the past [27–30]. In general, in an EDM process, crack formation is expected, where its density increases with an increase in discharge energy and carbon content [27]. Furthermore, in an EDM process, orientation of the cracks can be horizontal or vertical. Phenomena such as overlapping of craters, formation of features such as global appendages [28], spherical particles due to solidification of expelled molten metal, and pock-marks formed due to entrapped gases [30] are also

ARTICLE IN PRESS 440

P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

Table 9 Morphological features of dry EDMed surfaces, corresponding to extreme conditions.

Condition/Trial no

SEM micrograph

Comments on features

Maximum MRR (Trial no.7 (replicated))

Few micro-cracks and large blowholes found. Black patches indicating some carbon deposition. Large blow-holes observed.

Minimum MRR (Trial no.19)

Very few micro-cracks Solidified features (globular solidification)

Maximum TWR (Trial no.1 (replicated))

Larger and deeper micro-cracks Blow holes and porous structures observed in some region

Zero TWR (Trial no.6)

Dimples, river lines and few microcracks observed Few blowholes and folding indicating localized melting and solidification

Maximum Debris deposition (Trial no.14)

Relatively smooth surface Only very few continuous microcracks

Minimum Debris deposition (Trial no.8)

Few dimples observed Few micro-cracks due to void coalescence Projections over work surface due to tool material deposition

Maximum oversize (Trial no.16 (replicated))

Discontinuous micro-cracks Very few continuous micro-cracks

Minimum oversize (Trial no.25)

River lines in few region Continuous micro-cracks

found. Similar morphological features were observed in this study. However, the orientation of the cracks was highly random, and only few deep, continuous cracks were seen. Low discharge

current and high pulse duration were identified as reasons for crack formation [31], which are also evident in the case of dry EDM (see Table 9).

ARTICLE IN PRESS P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

441

Table 10 Optimum parametric conditions for MRR, TWR and oversize in dry EDM process. Responses

Voltage (V)

Current (I)

Pulse-off-time (Toff)

P (pressure)

N (speed)

Clearance at bottom (Cb)

Maximum depth Maximum MRR Zero TWR

50 50 50 65

18 18 15 12

22 22 67 33

0.25 0.25 0.15 0.25

300 300 100 100

4.5 4.5 4.5 5

Minimum OD error Entry 50 50% 65 90% 80

12 12 18

22 22 22

0.15 0.25 0.20

100 100 100

4 4 4

Values

Trial no.

2.2 mm 0.811 mm3/min 0

 29.29%  14.66% 0.31%

7 7(R) 6 11 1 13 25

3.6. Optimum parametric conditions This experimental work has enabled to obtain a number of factors significantly affecting dry EDM process. The optimum parametric conditions to maximize MRR, and minimize TWR and hole oversize are presented in Table 10.

3.7. Single-spark analysis of dry EDM Single-spark dry EDM experiments were conducted at the parametric conditions similar to that in Ref. [32] knowing that they are important to understand the material removal mechanism [32]. Fig. 9 illustrates schematic of the set-up. The experiments were conducted on a S50CNC EDM machine using a single-pulse generator. The workpiece material was a SS304 block of dimensions 27 mm  14 mm  10 mm. Copper rods of Ø5 mm with pointed tip were used as electrodes. Oxygen gas is supplied at a low pressure of 0.05 MPa. A minimum gap distance is ensured so as to apply only a single-spark and generate a single crater on the work surface. The programming for the single-spark operation was done in MDI mode. Six single-spark dry EDM experiments were performed at various I, Ton, Toff combinations as depicted in Fig. 12c.Voltage has been kept constant as 25 V so that results are comparable with that of Patel et al. [32].

Fig. 9. Scheme of a single discharge EDM method.

Crater radius, Rc

ANODE (+) Depth of Crater, z Conical shaped crater (approximated)

Anode melt cavity

Fig. 10. Simplified shape of the anode erosion cavity for MRR and crater radius evaluation.

3.7.1. Crater radius estimation and volume estimation using theoretical relations The values of crater depths obtained after simulation (zsim) corresponding to each parametric condition for a single-spark liquid EDM process were used in this work [32]. Approximating the shape of the anode crater as conical (Fig. 10), the volume removal rate (Vc) and the radius of the crater formed in singledischarge (Rc) have been evaluated for dry as well as liquid dielectric EDM. These are compared with the corresponding experimental values obtained in this work. Considering the geometry of a crater (in Fig. 10), volume of the crater (in mm3) is given by

No: of sparks in 1 min ¼ N ¼

60  106 sparks=min ðTon þ Toff Þ

ð7Þ

Experimental volume of the crater in liquid EDM per spark ¼

1 VC ¼ pR2c z 3

ð4Þ

Knowing the depth of erosion from the simulation of a single discharge in liquid dielectric EDM [32], the volume of crater can be calculated theoretically and experimentally in liquid dielectric EDM as VCðtheoreticalliquidÞ ¼

VCðexpliquidÞ ¼

where Rc(exp) is the experimental value of crater radius and Rc(th) is the theoretical value of crater radius. zsim is obtained from the process simulation. Similarly, knowing the pulse on-time (Ton), the number of sparks per minute and the volume of a crater formed in liquid dielectric EDM can also be calculated in two ways:

1 pR 2 z 3 cðthÞ sim

1 pRcðexpÞ 2 zsim 3

ð5Þ

ð6Þ

VCðexp-liquidÞ mm3 N

ð8Þ Theoretical volume of the crater in liquid EDM per spark ¼

VCðtheo-liquidÞ mm3 N

ð9Þ A similar method was followed for finding erosion rate in dry EDM. Depth measurement and diameter measurement were done at 20 locations and their average values have been taken. After conducting depth and diameter measurements on the experimental cavities shown in Fig. 11a,b and approximating it as conical, the experimental volume of the crater per spark in dry

ARTICLE IN PRESS 442

P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

Fig. 11. (a)–(b) Craters generated in single spark dry EDM, parametric conditions: (a) 5th (I = 25 A, Ton = 100 ms and Toff = 4.2 ms), (b) 6th (I = 36 A, Ton = 180 ms and Toff = 4.2 ms).

30

Rc-liquid EDM-experimental

35

Rc-liquid EDM-simulation

30

Rc-dry EDM

MRR (mm3/spark) X 10-6

Crater radius (µm)

40

25 20 15 10

MRR-liquid EDMexperimental

25

MRR-liquid EDM-simulation

20

MRR-dry EDM 15 10 5 0

5

-5

0 0

5

Expt no. I (A) Ton (µs) Toff (µs)

10

15 20 25 30 Discharge current (A) 1 5 18 2.4

35

2 10 32 2.4

40

0

3 13 42 3.2

10 20 30 Discharge current (A) 4 20 56 3.2

5 25 100 4.2

40

6 36 180 4.2

Fig. 12. (a)–(c) A comparison of (a) crater radius and (b) erosion rates for single spark liquid EDM and single spark dry EDM. (c) parametric conditions.

EDM is obtained as VCðdry-expÞ ¼

1 2 pR z mm3 =spark 3 c dry-exp dry-exp

ð10Þ

3.7.2. Radius of single-spark dry EDM A comparison between crater radii obtained after a single-spark operation in a liquid EDM and a dry EDM is presented in Fig. 12a. It is observed that for the experiments conducted at relatively low input energies (experiments from 1 to 4), crater radius for a singlespark in dry EDM is higher than that of in the liquid EDM. However, for the liquid EDM, a higher radius of crater is found at the higher discharge energies (see experiments 5 and 6). 3.7.3. MRR for single-spark dry and liquid EDM A comparison between the erosion rates for a single-spark in dry and liquid dielectric EDM is shown in Fig. 12b. It is observed that at lower discharge energies (in experiments 1, 2 and 3), the erosion rate in a single-spark in dry EDM is slightly higher than that of in a single-spark liquid dielectric EDM. Further, at the high discharge energies, it is observed that the liquid dielectric EDM

yields a very high MRR than the dry EDM (in experiments 4, 5 and 6). It appears that a greater material melting and solidification occurred at the machined surface in experiment 6 (see Fig. 12b) than that in experiment 5 (see Fig. 12a).

4. Conclusions In this work, experimental evaluation of dry electrical discharge drilling was carried out. It has been shown that dry EDM can be performed by providing an enclosure (shielding) to the sparking region. This study has revealed various characteristics of dry EDM process by measurement of oversize, examination of machined surface morphology and EDAX analysis, in addition to MRR and TWR study. The following remarks have been determined:

 It was evident that gap voltage, discharge current and electrode rotational speed significantly (with 95% confidence) influence MRR. However, none of the two-factor interactions were statistically significant.

ARTICLE IN PRESS P. Govindan, S.S. Joshi / International Journal of Machine Tools & Manufacture 50 (2010) 431–443

 TWR was found close to zero in most of the experiments. A











predominant deposition of melted and eroded work material on the electrode surface instead of tool wear was evident. None of the input parameters and their interactions were statistically significant (at 95% confidence level) in influencing TWR. Dimensional measurement of diameter on holes was performed at five different locations: entry, 10% depth of hole, 50% depth of hole, 90% depth of hole and at exit. The maximum oversize value ( 45.89%) was observed at the entry that corresponds to the 16th trial (replication), and the minimum value (0.31%) at 90% depth of the hole corresponding to the 25th trial. EDAX analysis of the central region of machined cross-section surfaces showed that there was a migration of tool (copper) and shield (aluminium) material as well as enrichment of carbon composition at work surface. Observation of morphology of the dry EDMed surfaces revealed micro-crack formation due to thermal stresses, deposition of spherical particles and marks due to entrapped gases on the machined surface. Though, most of the cracks were discontinuous, a few shallow and continuous cracks were also evident. In addition, dimples and river lines were observed in most of the cases. Optimum processing conditions for maximizing the MRR and depth were achieved; these corresponds to the trial #7 (50 V, 18 A, 22 ms, 0.25 MPa, 300 rpm and 4.5 mm). In experiments #6 and #11, zero TWR was observed. Similarly, corresponding experimental conditions for minimum oversize were in experiment #1 at entry, #13 at 50% depth and #25 at 90% depth. Single-spark analysis of dry EDM showed that at low input energies, there is an increase of crater radius (Rc) as well as MRR in dry EDM as compared to the liquid dielectric EDM. However, a larger crater radius (Rc) and MRR were observed at higher discharge energies.

Acknowledgement The authors wish to acknowledge financial support for this work from Department of Science and Technology, Government of India. The authors also wish to acknowledge the technical support from Electronica Machine Tools Limited, Pune (India). References [1] M. Kunieda, M. Yoshida, N. Taniguchi, Electrical discharge machining in gas, CIRP Annals—Manufacturing Technology 46 (1) (1997) 143–146. [2] N.M. Abbas, D.G. Solomon, M.F. Bahari, A review on current research trends in electrical discharge machining (EDM), International Journal of Machine Tools and Manufacture 47 (7–8) (2007) 1214–1228. [3] J. Tao, A.J. Shih, J. Ni, Experimental study of the dry and near dry electrical discharge milling processes, Journal of Manufacturing Science and Engineering 130 (1) (2008) 1–8. [4] T. Wang, M. Kunieda, Dry WEDM for finish cut, Key Engineering Materials 259–260 (1) (2003) 562–566. [5] C.C. Kao, J. Tao, A.J. Shih, Near dry electrical discharge machining, International Journal of Machine Tools and Manufacture 47 (15) (2007) 2273–2281. [6] J. Tao, A.J. Shih, J. Ni, Near-dry EDM milling of mirror-like surface finish, International Journal of Electrical Machining 13 (1) (2008) 29–33. [7] M. Kunieda, Y. Miyoshi, T. Takaya, N. Nakajima, Y. ZhanBo, M. Yoshida, High speed 3D milling by dry EDM, CIRP Annals—Manufacturing Technology 52 (1) (2003) 147–150.

443

[8] M. Kunieda, B. Lauwers, K.P. Rajurkar, B.M. Schumacher, Advancing EDM through fundamental insight into the process, CIRP Annals—Manufacturing Technology 54 (2) (2005) 64–87. [9] Z. Yu, T. Jun, M. Kunieda, Dry electrical discharge machining of cemented carbide, Journal of Materials Processing Technology 149 (1–3) (2004) 353–357. [10] Q.H. Zhang, J.H. Zhang, J.X. Deng, Y. Qin, Z.W. Niu, Ultrasonic vibration electrical discharge machining in gas, Journal of Materials Processing Technology 129 (1–3) (2002) 135–138. [11] Q.H. Zhang, J.H. Zhang, S.F. Ren, J.X. Deng, X. Ai, Study on technology of ultrasonic vibration aided electrical discharge machining in gas, Journal of Materials Processing Technology 149 (1–3) (2004) 640–644. [12] Q.H. Zhang, R. Du, J.H. Zhang, Q.B. Zhang, An Investigation of ultrasonic assisted electrical discharge machining in gas, International Journal of Machine Tools and Manufacture 46 (12–13) (2006) 1582–1588. [13] M.G. Xu, J.H. Zhang, Y. Li, J.H. Zhang, S.F. Ren, Material removal mechanisms of Cemented carbides machined by ultrasonic Vibration assisted EDM in gas medium, Journal of Materials Processing Technology 209 (4) (2008) 1742–1746. [14] M. Kunieda, T. Takaya, S. Nakano, Improvement of dry EDM characteristics using piezoelectric actuator, CIRP Annals—Manufacturing Technology 53 (1) (2004) 183–186. [15] S.K. Saha, S.K. Choudhury, Experimental investigation and empirical modeling of the dry electric discharge machining process, International Journal of Machine Tools and Manufacture 49 (3–4) (2009) 297–308. [16] M.S. Phadke, in: Quality Engineering using Robust Design, Prentice-Hall Publication, New Jersey, 1989. [17] M.S. Puranik, S.S. Joshi, Analysis of accuracy of high-aspect ratio holes generated using micro-EDM Drilling, Proceedings of the Institution of Mechanical Engineers, Part B, Journal of Engineering Manufacture 222 (11) (2008) 1453–1464. [18] O. Thao, S.S. Joshi, Analysis of heat affected zone in the micro-electric discharge machining, International Journal of Manufacturing Technology and Management 13 (2–4) (2008) 201–213. [19] S.H. Lee, X.P. Li, Study of the effect of machining parameters on the machining characteristics in electrical discharge machining of tungsten carbide, Journal of Materials Processing Technology 115 (3) (2001) 344–358. [20] J. Simao, H.G. Lee, D.K. Aspinwall, R.C. Dewes, E.M. Aspinwall, Workpiece surface modification using electrical discharge machining, International Journal of Machine Tools and Manufacture 43 (2) (2003) 121–128. [21] B. Mohan, A. Rajadurai, K.G. Satyanarayana, Effect of SiC and rotation of electrode on electrical discharge machining of Al–SiC composite, Journal of Materials Processing Technology 124 (3) (2002) 297–304. [22] J.S. Soni, G. Chakravarti, Machining characteristics of titanium with rotary electro-discharge machining, Wear 171 (1–2) (1994) 51–58. [23] K.D. Chattopadhyay, S. Verma, P.S. Satsangi, P.C. Sharma, Development of empirical model for different process parameters during rotary electrical discharge machining of copper–steel (EN-8) system, Journal of Materials Processing Technology 209 (3) (2009) 1454–1465. [24] P. Koshy, V.K. Jain, G.K. Lal, Experimental investigations into electrical discharge machining with a rotary disk electrode, Precision Engineering 15 (1) (1993) 6–15. [25] S. Singh, S. Maheshwari, P.C. Pandey, Some investigations into the electrical discharge machining of hardened tool steel using different electrode materials, Journal of Materials Processing Technology 149 (1–3) (2004) 272–277. [26] J.S. Soni, G. Chakraverti, Experimental investigation on migration of material during EDM of die steel (T215Cr12), Journal of Materials Processing Technology 56 (1–4) (1996) 439–451. [27] L.C. Lee, L.C. Lim, Y.S. Wong, Towards crack minimization of EDMed surfaces, Journal of Materials Processing Technology 32 (1–2) (1992) 45–54. [28] L.C. Lee, L.C. Lim, V. Narayanan, V.C. Venkatesh, Quantification of surface damage of tool steels after EDM, International Journal of Machine Tools and Manufacture 28 (4) (1988) 359–372. [29] F. Ghanem, C. Braham, H. Sidhom, Influence of steel type on electrical discharge machined surface integrity, Journal of Materials Processing Technology 142 (1) (2003) 163–173. [30] A.G. Mamalis, G.C. Vosniakos, N.M. Vaxevanidis, Macroscopic and microscopic phenomena of electro-discharge machined steel surfaces: an experimental investigation, Journal of Mechanical working Technology 15 (1) (1987) 335–356. [31] H. Lee, F. Hsu, T. Tai, Study of surface integrity using the small area EDM process with a copper–tungsten electrode, Material Science and Engineering A364 (1) (2004) 346–356. [32] M.R. Patel, M.A. Barrufet, P.T. Eubank, Theoretical models of the electrical discharge machining process. II. The anode erosion model, Journal of Applied Physics 66 (9) (1989) 4104–4111.