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Electric discharge drilling with gas-assisted multi-hole slotted tool

This paper proposes the use of a novel electrode containing modified design explicitly intended to promote gas-assisted tool rotation-induced debris removal. The proposed tool was observed to be efficient in dispensing the accumulation of eroded materials from the discharge gaps. In this work, the influence of process parameters like discharge current, tool speed, gas pressure, pulse duration, and duty cycle has been investigated on output responses: material removal rate (MRR), electrode wear ratio (EWR), and surface roughness (SR). A comparative study of the output responses was made with a solid rotary tool and the gas-aided multi-hole slotted tool. The outcome reveals that the application of the multi-hole slotted tool increased the MRR in Air assisted electric discharge machining (AAEDD) by 40-80%. Besides this, the EWR decreased in AAEDD by 17–25% compared to rotary electric discharge machining (REDD). Moreover, the SR of the AAEDD process was comparatively higher (9-15%) than that of the REDD process. It has been found less surface crack, micropores and recast layers on specimens machined by the AAEDD process in comparison to the REDD process. This work proposed a novel method for improving the machining performance by improving the flushing efficiency of the machining gap on the way to improve the material removal mechanism.


Introduction
Electrical discharge machining (EDM) is perhaps the most widely used novel method of machining. It is mainly used in the manufacture of molds, dies, and complicated shape profiles [1]. Flushing of debris from the electrode gap is one of the main problems faced in EDM machining. Arcing and short-circuit occurs when the eroded material collected in the interelectrode gap, which also decreases the material removal rate (MRR) and surface integrity [2]. In the electrical discharge drilling (EDD) process as the tool progresses further into the specimen, the impact of flushing continues to reduce. During deep drilling, the flushing winds up insufficient and the eroded materials difficult to removes from the inter-electrodes gap. The collection of debris in the electrodes gap brings about continuous short-circuiting and resulted in excess electrode wear and inferior machining [3].
Various researchers have addressed the issue of debris removal from a different perspective. To rectify these issues, various methodologies have been developed by the researchers. One of the remedies of such problems is the application of the rotary tool in conventional EDM.
Experimental exploration confirms that the rotary electrode significantly improves the EDM performance [4][5][6][7][8]. Yan et al. [4] studied the influence of the rotary ball burnishing electrode during EDM of Al 2 O 3 /6064Al composite. The result revealed that the rotary ball burnishing electrode produced a better surface finish with improved machining performance. The application of the proposed tool resulted in a decrease in SR from 55 to 92% under the different processing conditions. Singh et al. [5] explored the impact of electrode geometry on MRR during rotary EDM. They proposed that the use of a tapered shaped tool would lead to better material removal when contrasted with the use of conventional or chamfer-shaped electrodes. Aliakbari and Baseri [6] have studied the impact of multi-hole rotary tools on significant EDM process output such as MRR, SR and electrode wear ratio (EWR). Their results indicated that the multihole tool had a significant impact on the performance of the EDM process. To improve the performance of the EDM process, researchers investigated various approaches to the use of the rotary-tool magnetic field during EDM machining [7,8]. Their findings suggested that the combined action of the rotating magnetic field, as well as the, rotation of the electrode could improve the machining efficiency. Findings show that improved machining performance was due to the better flushing efficiency of the discharge gap.
Several studies [9][10][11] investigated the ability of liquid dielectric supply through the electrode to improve EDM performance. Gu et al. [9] carried out EDM machining using a bundled electrode.
Results have shown that the eroded materials are effectively removed from the spark gap with the help of the inward flushing of the bundled electrode resulting in improved material removal.
Li et al. [10] used an innovative flushing technique by using a bunched electrode for bulk material removal in the EDM. They explore the impact of flushing on EDM responses with the help of experiments and simulations. The authors found that the bunched electrode provided efficacious flushing that caused larger MRR and higher TWR. The reason for the improvement in flushing is probably because of the proposed tool effectiveness which reduces the concentration of debris in the electrodes gap. Xu et al. [11] analyzed the impact of flushing through a multi-hole electrode during EDM operation. Their experimental and simulation results indicated that machining performance was significantly influenced by flushing velocity.
Different studies [12][13][14][15][16][17] suggested that the supply of compressed gas in the discharge gap causes the effective removal of eroded materials from the machining region resulting in improved EDM performance. Singh and Pandey [12] investigated the impact of the air-assisted multi-hole rotating tool during EDM of high carbon chromium steel. They perceived that the application of the proposed air-assisted tool results in a better MRR and a lower EWR compared to a solid rotary tool for the same parametric combinations. During the EDM process, Kunieda and Furuoya [13] provided air and oxygen gas via a tubular tool. Exothermic reactions were noted when oxygen and air were utilized in machining. The additional heat generated by the exothermic reaction attributable to oxygen and air allowed the workpiece materials to melt and evaporate further, resulting in better MRR. Beravala and Pandey [14] noted that the supply of air/argon gas through a multi-hole tool reduced the deposition of eroded materials on the machine surface and enhanced the MRR. Some studies have also suggested that the use of liquidgas dielectrics may help to improve significant EDM responses. Wang and Shen [15] use different combinations of liquid-gas dielectrics for high-speed EDM milling with a hollow electrode. Experimental findings indicated that the supply of compressed gas-liquid results in better MRR compared to the supply of dielectric gas. A groundbreaking approach to the study of the effects of gas-liquid mixing using tomography with electrical resistance is also explored [16].
Li and Wataru [17] conducted micro-EDM drilling with mist deionized water jets. Findings revealed that the employment of mist deionized water jet improves the rate of removal of eroded materials from the discharge gap during the deep micro-hole drilling process.
The geometry of the electrodes has been reported to be effective in reducing the collection of spark eroded materials in the gap between the electrodes, leading to improved efficiency of EDM. Nastasi and Koshy [18] used different tools with distinct geometrical features to know their effect on debris removal. Finding revel that the slotted tool out-performs the conventional solid tool in terms of MRR. Yan et al. [19] used a notched tool in their exploratory investigation and assumed that a notch tool could effectively improve the removal of debris. Kumar and Singh [20] proposed a tool with an inclined hole during an EDD operation. The findings show that the proposed tool has self-flushing characteristics and is more potent in the evacuation of eroded materials from the machining zone than the conventional solid tool. Kumar and Dhanbalan [21] investigated the machinability and tolerances of inconel-718 employing a multi-hole electrode with an engraved hole resulting in increased MRR and a lower tolerance discrepancy.
Researchers have also explored the use of different novel reference electrodes in various processes [22][23]. The finding indicated that novel electrodes have a positive impact on the performance of the process.
It is established from the above-mentioned literature review that the geometry of the tool is very effective in preventing the accumulation of eroded particles in the machining region. Besides this, the supply of gaseous dielectrics through multi-hole rotary electrodes reduces the possibility of arcing and short-circuiting and results in improved EDM performance. Moreover, small clearance for gas flow results in incomplete removal of eroded materials from the electrodes gap.
This shortcoming can be eliminated by manufacturing a multi-hole slotted electrode that can provide a mechanism for easy removal of debris from the inter-electrode gap and reduce arcing and short-circuiting resulting in better flushing action. It has been noted from the literature review that no attempt has been made to incorporate multi-hole slotted rotary electrode and to supply compressed air together in the EDD operation. In this research work, compressed air through a multi-hole slotted rotary electrode is provided in the discharge gap to study the influence of airflow on EDD functioning. Studies have been performed for comparing the MRR, EWR, and SR of air-aided electric discharge drilling (AAEDD) and rotary electric discharge drilling (REDD) processes. Current work aims to create statistical-based models for MRR, EWR, and SR by employing flow air through the multi-hole slotted electrode in EDD operation.
In this paper, the work is conducted in two phases. The first phase emphasized the establishment of statistical-based MRR, EWR, and SR models during the AAEDD process. The main effect and interactions among various process parameters on MRR, EWR, and SR which have been discussed to evaluate the effect of applying compressed air through the multi-hole slotted rotary electrode in the traditional EDD process. The second phase is a comparison between REDD and AAEDD processes relating to MRR, EWR, and SR.

2.1.Details of workpiece and electrode material
The experiment was executed on D3 die steel as a workpiece using copper as the tool material.
The specimen consists of a rectangular shape (20X15X15mm). A multi-hole slotted tube as a tool was used to supply high-velocity air through the electrode. The selected specimen chemical composition as depicted in Table 1. The hardness of material has been kept constant (51HRC).Due to the good thermal and electrical conductivity of copper material, a rotary electrode and a multi-hole slotted electrode of copper were fabricated. A copper attachment was used for maintaining the conductivity of the electrode. The specific property of the copper electrode is shown in Table 2. Figure 1 demonstrates the setup used in the experiments of the present work.

2.2.Tool Design
To ensure smooth compressed air flow, appropriate tool geometry was selected. Appropriate tool dimensions were selected to remove maximum heat from the tooltip. The REDD process experiments were conducted with a conventional solid tool with a diameter of 8.35 mm and a length of 70 mm. The outer diameter and length of the tools used in REDD and AAEDD processes remain the same. The effect of the different number of 1 mm dia. and 1.5 mm dia.
holes (1, 3, 5 holes) were studied in the preliminary experimentation. It was found that the electrode with 3 holes gave higher MRR and lower tool wear rate relatively. The 1.5 mm diameter holes were penetrated on the tool surface at 4 mm, 5 mm, and 6 mm pitch circle diameter. Notwithstanding the way that the machining was upgraded with this design, recurring arcing and short-circuiting happened because of the ineffective expulsion of the eroded debris from electrode gaps. Thus, to rectify such types of problems three slots of size 2 mm × 2 mm at 120 0 apart were provided along the outline of the tool to rectify such issues. The use of a multihole slotted tool ensured the flow of compressed air through the opening of the arc. The multi-hole slotted tool has been shown in Figure 2 (a). The chip removal mechanism of slotted electrodes is shown in Figure 2 (b). Literature [14] shows that the multi-hole rotating electrode improves the material removal rate and reduces the electrode wear rate compared to the solid electrode in the air-aided EDM process. The gaseous dielectric flowing out of the electrode takes a substantial part of eroded matters and takes away them from the gap between the electrodes.
Small passage for this flow however leads to inadequate removal of eroded matter. The debris removal is even very crucial in the electric discharge drilling process. Hence, in order to improve the flushing efficiency, slotted electrode was designed in this study. The slots manufactured into cylindrical form having multi-holes which provided the shortest path for removal of the debris from the narrow inter-electrode gap. It resulted in an increase in flushing efficiency and reduces the arcing and short-circuiting events during the EDD process.

Experimental procedure
The air-aided EDD with the multi-hole slotted tool has been carry-out on an EDM machine. For each experiment, the machining time was suitably chosen which was considered as 25 minutes. It is important to select a rational set of factors to be varied in the experiment. The EDM efficiency is governed by electrical parameters as well as non-electrical parameters. The literature review suggested that of all the machining factors (electrical parameters such as discharge current, pulse duration, and duty cycle, and non-electrical parameters including tool speed, discharge gas pressure) have a notable impact on EDM performance. Based on the above observation for the exploratory work, five process parameters, i.e., pulse on time, discharge current, duty cycle, electrode speed, and discharge air pressure were chosen. Initially, four parameters were utilized during the REDD and five parameters were considered for the AAEDD process. The main objective of this research work is to analyze the effect of the air stream on the EDD performance, and therefore five parameters (the fifth parameter is air pressure) were used in the AAEDD process for the design of the experiments. Table 3 shows the machining factors considered with their levels used during experimentation. EDM oil as a liquid dielectric in REDD, while liquidair mixed dielectric oil was used in the AAEDD process. During the machining process, the electrode was immersed in the liquid dielectric. Moreover, during the AAEDD process, the air was supplied directly into the gap evenly through the multi-hole slotted electrode. An opencircuit voltage of 60V was maintained constant for all the experiments.
The electrode wear ratio was calculated as the ratio of the electrode wear to the workpiece erosion [24] and is expressed in percentage.
MRR is demonstrated as the ratio of the workpiece erosion weight to the machining time as stated below [25]: Every specimen considered for the experimental analysis was cleaned with acetone. To get an exact machining time, the electronic clock having the least count of 0.1 seconds was utilized.
Portable surface roughness tester (Surf-test-SJ-210) having cut-off length 0.8mm and tracing speed 1.00mm/sec was utilized to measure the Ra value of machined specimens.

Analysis of experimental outcome
Central Composite Rotatable Design (CCRD) is being used in the present work to design the experimental plan since it is fit for anticipating quadratic and interaction effects of different process parameters on the process response. The correlation of the deliberate estimations of each trial concerning notable EDD responses for AAEDD and REDD is presented in Figure 3.
According to the experimental plan, thirty-two experiments were conducted for AAEDD and thirty-one experiments were performed for REDD. In REDD, four procedure parameters at five levels have been utilized, nonetheless, in the AAEDD five parameters with five levels were used for the design of experiments.
Regression analysis of the experimental outcome was carry-out to instigate the models, which established the correlation between input parameters (discharge current, tool speed, gas pressure, pulse duration, and duty cycle) and output factors (MRR, EWR, and SR).After eliminating all the insignificant terms, requisite regression models based on the experimental outcomes of the AAEDD process are represented by the following equations.
Analysis of variance (ANOVA) was applied to explore the competency of the established model.
For a specific confidence interval (CI), the F-ratio of the model was estimated and contrasted with the tabulated F-value. In the ANOVA of the design of experiments, the measured value of the F-ratio tends to be less than one, and it is a trivial approximation that this estimated F-value is less than the 5% significance level of the table F-value, and it is assumed that the impact of this factor under investigation is insignificant. The ANOVA of the second-order model of the AAEDD process has been represented in Tables 4, 5, and 6. Equations 3, 4, and 5 appear for the statistical model of MRR, EWR, and SR individually. For the developed models, the estimation of "Prob>F" is under 0.05 considered with a 95% confidence level. Henceforth, it uncovers that the process factors (discharge current, tool speed, gas pressure, pulse duration, and duty cycle) in the model have a noteworthy impact on the responses considered. Figure 4, Figure 5, and Figure 6 shows the interaction effects between significant process parameters on MRR, EWR, and SR in the AAEDD process.

Response surface of MRR
The response surface of MRR for the current and discharge air pressure is manifested in Figure 4 (a). A high MRR was caused by an increase in discharge current. This is due to the enhancement of the discharge energy in the machining region which reason added evaporation and melting of the work material [22]. Figure 4(a) shows that the MRR improved with an increase in the air pressure discharge reaching its maximum value and then started to decrease at a low discharge current. This can be accounted for the reason that the flushing effects of the process improved with an increase in air pressure, which contributed to higher MRR [10]. In addition, plasma channel expansion occurred at higher discharge air pressure and as a result of this discharge energy was reduced, leading to a lower MRR up to a certain discharge current value [13]. Figure   4(b) presents the response surface of MRR for air pressure and duty cycle. From the figure, it is observed that with increment in discharge air pressure and duty cycle, MRR is increased. It was most likely due to the reason that the larger value of the duty cycle promoted large spark energy, so it increases the melting and removal of the workpiece material [26]. The increase in air pressure from 3 mmHg to 9 mmHg prompted to improve the flushing of eroded materials from the discharge gap, which resulted in higher MRR [12]. When compressed air was delivered through a multi-hole slotted tool, more turbulence and cavitation were produced, leading to the increased ejection of the molten material. This improved the removal rate and also reduced the amount of liquid material re-to the surface. Kunieda et al. [23] and Singh et al. [27] have also reported similar findings.

Response surface of EWR
The response surface of EWR for pulse duration and discharge current has been illustrated in Figure 5(a). It is evident from the graph that the effect of high current and low pulse on-time results in higher EWR. High current has resulted in an increase in discharge energy and thus a high temperature has been produced. Owing to this extreme heat, the rotating electrode became overheated and the electrode wear increased along with the MRR, thus increasing the EWR [6].
The black carbon released due to the cracking of hydrocarbon oil sticks as a layer on the surface of the electrode for longer pulse duration. Such a formation of layer resulted in resistance against the wear of electrode [4]. Figure 5(b) depicts the surface plots of EWR for tool rotation and air pressure. It has been observed that EWR increased with an increase in tool speed. As the rotational speed of the tool increases, the extent of the centrifugal force increases. Black carbon particles are expelled from the tool surface due to increased centrifugal force resulting in increased EWR. Nastasi et al. [24] and Teimouri and Baseri [28] have found similar findings.
From Figure 5(b), it was also perceived that the EWR decreased with increment in air pressure.
With a rise in discharge air pressure, the flushing action of debris got improved, because this MRR of the process improved. However, compressed air flowing through multi-hole slotted tool decreased the tooltip temperature radically and as an outcome size of a crater on tool reduced resulting in low tool wear rate and hence EWR reduced [3].

Response surface of SR
The response surface of SR for duty cycle and pulse-on time has been presented in Figure 6(a). It can be inferred that high surface roughness was generated by the combination of high duty cycle and low pulse duration. At high duty cycle, large discharge energy and impetuous force were produced which resulted in larger sized carters on the workpiece, as a consequence, the surface finish decreased with a rise in duty cycle [21,29]. At the same time, for high pulse duration, the enlargement of the plasma channel took place in the gap of electrodes, because of which the peak current and magnitude of impulsive force decreased [22]. Thus a formation of shallower carter occurred at the surface of the workpiece, which reduces the surface roughness. Response surface of SR for the pulse on time and discharge air pressure has been illustrated in Figure 6(b). It is evident from a plot that a combined set of a higher pulse on time with higher discharge air pressure leads to an improvement in surface finish. Higher discharge air pressure facilitated the effective flushing of the eroded particles still at lower pulse off-time [3]. The response surface of SR for tool speed and duty cycle has been presented in Figure 6(c). It can be inferred that high surface roughness was generated with an increase in the tool speed and duty cycle. At the high duty cycle, large discharge energy resulted in larger sized carters on the workpiece [20,24]. At the point when the tool speed enhanced the flushing efficiency becomes better and brought about less recast layer deposition [30]. The carbon deposition is accordingly less which results in bigger size craters and subsequently, high surface roughness is acquired [26]. Figure 6(d) depicts the surface plots of SR for tool rotation and air pressure. The interaction plot revealed that an increase in tool speed and discharge air pressure caused the SR to increase. Because of an increase in tool rpm and air pressure, the plasma can be quickly moved and severed rapidly, elevating the molten workpiece material to be expelled and creating profound discharge crater on the surface of the specimen resulting in high surface roughness [12].

Effect of process factors on MRR
The impact of discharge current during AAEDD and REDD processes has been shown in Figure   7(a). MRR enhanced with an increase in discharge current. This was presumably due to the enhancement of discharge energy in the zone of machining, which causes increased melting and evaporation of material [24]. From plot (refer Figure 7(a)) it can be inferred that MRR during AAEDD was relatively higher concerning MRR of the REDD process. This is because the density of spark energy increased with an enhancement in discharge current which caused a large crater volume [25]. The electrical energy provided to generate a single spark in EDM is given by , where "r" is the plasma radius and "l" be the inter-electrode gap existed at applied gap voltage. It can be concluded from the above mentioned relationship that the density of the spark energy increased with an increase in the discharge current [32].The oxygen in the air induces an exothermic reaction that produces additional heat for material removal in AAEDD [27]. The molten metal of the steel workpiece reacts with oxygen from the air and releases heat.
The reaction is exothermic, which transfers heat to the discharge crater as seen by the chemical reaction (2Fe + 1. shown to improve. It was the reason that intense spark energy liberated at a high duty cycle, thus it stimulates further melting of specimen machined [24]. It could be seen from the outline that MRR during AAEDD was better in comparison to the REDD process. It was most likely due to the use of a multi-hole slotted tool which helped to extract eroded particles and heat from the machining area quickly. Thus, it improved the flushing action during operation which in turn resulted in high MRR.

Effect of process factors on EWR
tool electrode should be electrically conductive with the low wear rate and should result in a better surface finish along the workpiece. The electrical erosion resistance is determined by the thermo-physical and mechanical attributes of the EDM electrode [31].
The impact of discharge current on EWR during REDD and AAEDD has been shown in Figure   8(a). For both processes, EWR was considered to be improved with a rise in the discharge current. This was presumably because high current produced additional discharge energy. This leads to high temperatures in the machining area, which increased the wear of the electrode alongside MRR, thereby increasing the wear ratio of the electrode [10]. Moreover, at high discharge current, a lot of disintegrated particles get amassed in the machining gap that in the end enhanced the electrode wear [21]. From the Figure 8(a), one can see that in AAEDD, EWR was lower than in the REDD. Such behavior was due to the reason that as the high-pressure air is passed through the multi-hole slotted electrode, it produces a cooling effect on the surface of the electrode and due to this the crater produced during the AAEDD process reduced on the tool surface [12]. Figure 8(b) presents the effect of pulse on time on EWR. It can be seen that EWR had an inverse correlation with a pulse on time. This can be represented the reason that the span of the plasma channel extended at high value of pulse on time, which at last cut down the high temperature of the tool surface [21]. In addition, the black carbon produced by cracking the hydrocarbon oil that is stuck on the electrode surface at a longer pulse duration. The deposited layers of carbon protect the tool against excess wear of the tool electrode [7]. Further, from the plot, it could be observed that EWR was lowered in AAEDD in comparison to the REDD process. It was probably due to the reason that the temperature of the tool decreased by compressed air which limited the additional melting of the tool subsequently decreasing its wear. and reduces the temperature of the tip of the electrode [13,14]. Hence, when compressed air was passed through tool the formation of carbon deposition increased.

Influence of process parameters on SR
The effect of discharge current on SR during REDD and AAEDD has been presented in Figure   9(a). It very well may be seen from the plot that SR of both the processes enhanced with an increase in discharge current. Discharge energy increased with increment in discharge current.
The available large discharge energy and impetuous force there created an intense and largesized carter on a workpiece, as a consequence, surface finish deteriorated with an increase in discharge current [29]. From Figure 9(a) it can also be seen that surface roughness was higher in AAEDD in contrast to REDD. This may be the reason that compressed air and rotation of electrode allowed more molten material to be dislodged by each discharge resulting in a deep crater, causing high surface roughness in the AAEDD process in comparison to REDD process. . It was perceived from the plot that surface roughness was higher during the AAEDD process in comparison to the REDD process. It was most likely due to the reason that with a decrease in impulsive force, the debris could not be extracted thoroughly from the electrode gap and the cooling effect caused by compressed air in case of the AAEDD process caused the emergence of recast layer, which ultimately degraded the surface quality [12]. For AAEDD and traditional REDD process Figure 9(c) shows the effect of the duty cycle on SR. It showed that SR increased in direct correlation with the duty cycle. This was because at low duty cycle material removal contemplated at a very low level. This provided adequate time for the removal of debris from the machining zone. Hence, fewer recast layers were formed at a low duty cycle and as a result surface roughness decreased [33]. Further, from Figure 9(c) it can be seen that SR was found more in AAEDD in comparison to REDD. This can be accounted for the reason that in AAEDD oxygen content of air caused big spark; large crater and it leads to poor surface quality [13]. The impact of tool rotational speed on SR for AAEDD and the conventional REDD has been presented in Figure 9(d). It can be inferred from a plot that, for both processes, surface roughness increased with increment in electrode rotation. Because of the rotation of the electrode, the air moved radially outwards from the core and removed eroded particles from the discharge gap under the influence of centrifugal force. Because of the inefficient removal of eroded work material from the machining gap and under the influence of tool rotation the eroded particles get deposited. This caused a higher surface roughness [20].
Further, SR was found more in the AAEDD as compared to the conventional REDD process (refer Figure 9(d)). This was possibly due to the fact that the recast layer increased due to highpressure air cooling and led to an increase in SR. It can be perceived from Figure 9 that SR was found more in AAEDD in comparison to REDD. This was possibly due to the reason that in AAEDD, the supplementary heat due to the exothermic reaction resulted in a large discharge crater [14]. This eventually enhanced the roughness of specimen even at the same discharge energy. In order to obtain the improved surface finish, instead of air, an inert gas can be used because it can prevent an exothermic reaction to take place. Therefore, it is suggested that the first air-aided multi-hole slotted tool should be used during the rough cutting operation and thereafter inert gas should be used for deep hole drilling thus providing the optimum surface finish.

Surface Morphology
The potential difference between the tool electrode and the workpiece in pulse form is enforced to EDM. As the potential difference has adhered, electrons from the electrode begin to move towards the workpiece [29]. In EDM operation, the workpiece surface attains a high temperature due to a rapid and regular strike of highly frisky electrons. Because of high-temperature material easily melted and cavities were formed on the surface of electrodes. In EDM activity, the crack development begins, when the extent of thermal stresses surpasses the fracture strength of the specimen [4]. The SEM images are shown in Figure 10 which illustrates the formation of crack on the surface of the machined specimens. From Figure 10 it can be observed that the workpiece machined through the REDD has more micro pores, blow holes and surface cracks in

Process optimization and accuracy of models
The various calculated responses and parameters are often uncertain because of experimental error. Although the accuracy of process responses were determined through estimation of the confidence interval, determined using relation Here the process responses, EWR, MRR and SR are denoted by . The level of the CI is indicated by and its value is selected as 0.05. The variance of error of the foreseen procedure output is indicated by e V .The effectiveness of EDM experiments can be increased to a large scale by the optimization of machining factors. The machining factors include both electrical and nonelectrical parameters. The electrical parameters such as discharge current, pulse duration, and duty cycle, and non-electrical parameters including tool speed, discharge gas pressure have a major influence on EDM performance [1,2]. To obtain optimum values of process factors for minimum electrode wear, maximum MRR, and better surface finish; fmincon a toolbox of MATLAB 12.0 was used to get optimum levels of parameters, which are listed in Table 7.

Conclusions
In the present work, an air-aided multi-hole slotted electrode was successfully used for the machining of the D3 die steel workpiece. This work investigated the effectiveness of the airaided multi-hole slotted tool to increase the flushing efficiency of the discharge gap on the way to improving the material removal mechanism during EDM drilling.
 A comparative analysis of output responses in the case of a solid rotary electrode and the air-aided multi-hole slotted rotary electrode has been presented. High MRR and low EWR were achieved through AAEDD in comparison to the REDD. Moreover, the SR of the AAEDD process was relatively higher than in the REDD process.
 It was probably the use of the multi-hole slotted tool that helped in the rapid removal of eroded materials and heat from the machining area that the MRR was increased by 40-80%. The EWR was reduced by 17-25%, presumably because the tool temperature decreased by compressed air, which subsequently limited the additional melting of the tool and reduced its wear.
 Moreover, in the AAEDD process, due to extra heat other than the heat generated by the spark, the effect is a larger discharge crater on the machined surface, which gradually increases the roughness of the workpiece by approximately 9-15% compared to the REDD process.
 Statistical models have also been developed to predict responses such as EWR, MRR, and SR during the AAEDD process. Experimental inference suggested that the discharge current, pulse duration, and duty cycle, especially affected the MRR, while the discharge current, pulse duration and tool rotation significantly affected the EWR. SR was notably influenced by discharge current, pulse duration, and discharge air pressure.
 The surface analysis showed that the recast layers on the AAEDD machined samples were less pronounced compared to the REDD process. In addition, it was also noted that the surface cracks caused during the AAEDD process were less compared to the REDD process resulting in improved surface integrity.
 The optimal value of EDD parameters was observed to yield MRR, EWR, and SR comparable to 28.3125 mg/min, 0.8431% and 0.8861µ , respectively.  Since air is cheaper and readily available gas than the commercially available inert gases.
It is suggested that first of all air-aided multi-hole slotted tool should be used during rough cutting operation thereafter inert gas should be used for deep hole drilling in the case when optimum surface finish is needed.

Future work:
The findings discussed in this paper can be further applied in the following ways:        Table 1 Chemical composition of the workpiece Table 2 Specific properties of the copper electrode Table 3 Parameters in the process with their levels Table 4 ANOVA for MRR in AAEDD Table 5 ANOVA for EWR in AAEDD Table 6 ANOVA for SR in AAEDD Table 7 Optimum process parameters       Figure 9: Influence of (a) discharge current (b) pulse duration (c) duty cycle (d) tool speed on surface roughness (SR).