*Corr. Author’s Address: Department of Mechanical Engineering, Sona College of Technology, Salem - 636005, India, maniprabujkpm@gmail.com 595 Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 Received for review: 2024-06-27 © 2024 The Authors. CC BY-NC 4.0 Int. Licensee: SV-JME Received revised form: 2024-10-04 DOI:10.5545/sv-jme.2024.1076 Original Scientific Paper Accepted for publication: 2024-10-21 Experimental Investigation on SS202 using Tubular and Double D Tubular Electrode Tool in Electrical Discharge Drilling Machining Manikandaprabu, P. – Saravanan, K.G. Manikandaprabu Pandiyan 1,* – Saravanan Kanthasamy Ganesan 2 Sona College of Technology, Department of Mechanical Engineering, India To meet the demands of the manufacturing industry, a more robust approach is needed for current manufacturing processes. Machining of very hard materials with excellent dimensional accuracy and surface integrity is a challenging task, Currently, the electrical discharge machining (EDM) process is the optimal solution for machining such materials. Electrical discharge drilling machines find applications in various sectors such as aerospace, automobile and medicine. In this research, two different brass electrode geometries are considered: tubular electrode tool (TET) and double D tubular electrode tool (DDTET) on an SS202 work-piece. The input parameters of peak current (PI), pulse time ON (PONT), and pulse time OFF (POFFT) were varied. The output response measured and compared were material removal rate (MRR) and over cut (OC). The Taguchi technique was used to design the l16 orthogonal array (OA) experiment, and analysis of variance (ANOVA) was employed for result analysis using Minitab20 statistical software package. The results showed that the DDTET method produced significantly better MRR and OC improvements of 21.66 % and 2.28 % respectively. Additionally, a study was conducted on the formation of the recast layer (RL) and the heat affected zone (HAZ). Keywords: modified electrode geometry, material removal rate, over cut, heat affected zone, recast layer, analysis of variance Highlights • A novel tool (DDTET) geometry was designed and utilized to perform drilling operations on SS 202 material. • The performance of the novel tool was compared to that of a conventional tool, based on response characteristics such as MRR, OC, HAZ, and RL. • Taguchi and ANOVA methods were employed to identify the optimal performance outcomes. • The PONT input parameter demonstrated superior performance in terms of MRR and OC for both TET and DDTET geometries. • A study on RL and HAZ was conducted using scanning electron microscope (SEM) imaging. 0 INTRODUCTION EDM is a non-conventional manufacturing process utilized for precision material removal. This technique involves immersing an electrically conductive work-piece and a tool, known as an electrode, in a dielectric fluid. Upon applying a potential difference between the work-piece and the electrode, controlled electrical discharges occur, leading to localized melting and vaporization of the work-piece material. This phenomenon enables precise shaping of intricate geometries in materials that are traditionally challenging to machine. EDM finds application in industries such as aerospace, automotive, and medicine, where high precision and complex component fabrication are required. R. Kirubagharanet et al. [1] conducted drilling operations using electrical discharge drilling (EDD) on Inconel X750 utilizing a modified square electrode tool. The results of the experiment demonstrated enhancements of 12 % in MRR, 9 % in SR, and 11 % in EWL. Pal et al. [2] conducted an experiment on SS310 and SS316 using EDD, considering input parameters such as V voltage [V], C capacitance [pF], PONT [µs], and POFFT [µs]. The findings of the study indicated that the optimum machining conditions for SS310 steel were identified as V = 150 V, C = 100 pF, PONT = 30 µs , and POFFT = 20 µs. Conversely, for SS316 steel, the optimal parametric conditions were found to be V = 200 V, C = 1000 pF, PONT = 20 µs, and POFFT = 25 µs using the Taguchi approach. Yadav et al. [3] conducted a study utilizing EDD on a copper work-piece, considered input parameters such as voltage, feed rate, and powder concentration in the dielectric fluid. The ANOVA results from this investigation revealed that voltage contributed 93.5 % of the MRR, 58.24 % of the tool wear rate (TWR), and 67.3 % of the aspect ratio (AR). Chaudhari et al. [4] conducted EDD on Ni55.8Ti alloy employing a graphite electrode tool. The results of this experiment showed enhancements of 76.91 %, 38.40 %, 34.36 %, and 44.54 % for MRR, TWR, SR, and dimensional deviation (DD), respectively. Manikandan et al. [5] performed EDD on a Si3N4–TiN ceramic composite with a copper electrode. The findings indicated enhancements in MRR by 0.0354 gm/min and TWR by 0.001035 gm/min Shabarinathan et al. [6] performed an EDM process on Inconel using an aluminum composite electrode. Their experiments identified the optimal process parameter for MRR at a Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 596 Manikandaprabu, P. – Saravanan, K.G. current of 12 A, beyond which the MRR declined due to plasma channel expansion. The optimal PONT was determined to be 8 μs, with a POFFT of 4 μs. Sawant et al. [7] performed EDD on Ti–6Al–4V alloy utilizing copper (Cu), tungsten copper (WCu), and tungsten carbide (WC) electrodes. The results revealed that the MRR achieved with the WCu electrode was 6.11 % higher than that with the Cu electrode and 21.92 % higher than that with the WC electrode. Selvarajan et al. [8] conducted an experiment involving EDD on molybdenum disilicide–silicon carbide composites, employing copper electrodes. The outcomes highlighted that the primary machining parameters were determined to be current (34.6749 %), PONT (25.6502 %), and gap voltage (12.0462 %). Rafaqat et al. [9] developed a modified triangular cross- section copper electrode for the EDD process on D2 steel. They observed notable outcomes, including a 32.6 % enhancement in MRR, a reduction of 20.6 % in overcut (OC), and a significant decrease of 72.5 % in taper angle (TA). Rafaqat et al. [10] fabricated copper tools with various cross-sectional geometries for use in the EDM process. Their results showed an improved MRR of 317.24×10 6 (µm 3 s -1 ) compared to traditional tool geometries. Kumar and Singh [11], Shah and Saha [12], and Saha et al. [13] conducted EDM experiments using a modified solid electrode tool, featuring slotted, cylindrical and conical shapes for the tool’s outer profile. Ji et al. [14] carried out EDM experiments using a helical-profiled TET. Yadav et al. [15] conducted EDM experiments with various straight grooved tool profiles and a helical-shaped tool in TET. From the literature reviewed, it is evident that global research on EDM has focused on enhancing MRR, SR, EWL, DD, and OC accuracy. Although various flushing methods and tool designs have been employed to enhance MRR, dimensional accuracy, and other factors, further in-depth studies are needed to fully understand the EDM process. This research investigates two different electrode geometries, TET and DDTET, focusing on their effects on MRR and AD. The findings reveal that the DDTET electrode has a significant impact on both MRR and AD. 1 METHODS A novel tool geometry was designed and utilized to perform drilling operations on SS 202 material using EDM. The performance of this novel tool was compared to a conventional tool, focusing on key response characteristics: MRR, OC, HAZ, and RL. Taguchi, surface response methodology, contour plots and ANOVA methods were employed to identify the optimal performance outcomes, ensuring a thorough statistical analysis of the results. Additionally, an in-depth study on the RL and HAZ was conducted using SEM imaging. The RL was generated in the sparking zone, where localized heating of a thin layer of the work-piece occurred. The high temperature caused the formation of a molten layer, which solidified during the subsequent quenching process, leading to the creation of the RL. Due to the low thermal conductivity properties of SS202 material, a tempered layer was formed below the RL, referred to as the HAZ [16]. This allowed for a detailed examination of the microstructural changes and quality of the drilled holes, providing insights into the efficacy of the novel tool geometry in minimizing defects and enhancing the overall drilling performance. 2 EXPERIMENTAL 2.1 Experimental Setup The experiments to drill the SS202 work-piece were performed using the Suzhou Baom electrical discharge drilling machine (EDDM) with various settings for PI, PONT, and POFFT. The machine has an adaptive Z-axis movement and automatically adjusts the arc gap between the work-piece and the electrode tool. The spindle rotational movement was maintained at 110 rpm, and the dielectric fluid pressure, supplied by an electrically assisted pump, was kept constant at 1 MP, while the electrode tool with a 3 mm outer diameter and 1 mm hole diameter was fixed as negative polarity and the work-piece as positive polarity. The EDDM is capable of adapting to various electrode diameters from 0.3 mm to 3 mm. The dielectric fluid, deionized water, is provided to the work area through a reservoir connected to a filter system, pump, and pressure adjustment valve. The ionization process in the work area produced etching at about 8,000 °C to 10,000 °C. The EDDM is illustrated in Fig. 1. The SS202 material, which is widely used in high-speed aerospace motor housing, automotive trim, railway carriages, trailers, fluid filters, and water clams, was chosen due to its high hardness, good corrosive resistance, and durability. Detailed specifications of the EDDM machine and SS202 material composition are given in Tables 1 and 2, respectively. Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 597 Experimental Investigation on SS202 using Tubular and Double D Tubular Electrode Tool in Electrical Discharge Drilling Machining Fig. 1. Experimental setup Table 1. EDDM machine specification No Specification Range 1 Supply power 3 Phase 2 Maximum voltage 415 V; AC 3 Frequency 45 Hz 4 Axis travel (X, Y , Z) 400 mm × 300 mm × 300 mm 5 Rated power 35 kVA 6 Weight 650 kg 7 Rated current 35 A Table 2. SS202 Material Composition [24] No Element Content [%] 1 Iron (Fe) 68 2 Chromium (Cr) 17 to 19 3 Manganese (Mn) 7.50 to 10 4 Nickel (Ni) 4 to 6 5 Silicon (Si) ≤ 1 6 Carbon (C) ≤ 0.15 7 Phosphorous (P) ≤ 0.060 8 Sulphur (S) ≤ 0.030 9 Nitrogen (N) ≤ 0.25 2.2 Construction of DDTET The brass electrodes used in the experiment were tubular in shape, with an outer diameter of 3 mm and a hole diameter of 1 mm. The hole allowed the pressurized dielectric fluid to enter the machining area. This standard electrode is called the TET geometry, as depicted in Fig. 2a. Brass was chosen as the material for the tool due to its high conductivity property. A modified version of the novel DDTET geometry was designed for the drilling process, as shown in Fig. 2b. The DDTET was fabricated using a wire electrical discharge machining machine (Concord Wire EDM, India). The modified tool dimension was reduced by 350 microns on one side, and the same amount was reduced in the opposite direction, which increased the gap between the electrode and work-piece. Lo et al. [17] and Malayath et al. [18] conducted an EDD process on carbon steel using a slotted copper electrode. This increased gap resulted in an improvement in MRR by 120 % to 153 %, as well as enhanced specimen precision and dimensional accuracy. Both the standard TET and DDTET were used to drill the SS202 work-piece material. Fig. 2. a) Model of TET, and b) model of DDTET 2.3 Experimental Design Using Taguchi Method The PI, PONT, and POFFT values were selected as 4 A, 5 A, 6 A, and 7 A; 3 µs, 4 µs, 5 µs, and 6 µs; and 5 µs, 6 µs, 7 µs, and 8 µs, respectively, through trial runs and prior experimental investigations. The experiments were designed using the Taguchi method [19]. Three variables, each at four levels, were utilized, resulting in a degree of freedom calculation of 3(4-1) = 9. Consequently, an L16 OA, which exceeds the Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 598 Manikandaprabu, P. – Saravanan, K.G. required 9 degrees of freedom, was selected and is presented in Table 3. Table 3. Machining parameters of L16 RO Run order (RO) PI [A] PONT [µs] POFFT [µs] 1 4 3 5 2 4 4 6 3 4 5 7 4 4 6 8 5 5 3 6 6 5 4 5 7 5 5 8 8 5 6 7 9 6 3 7 10 6 4 8 11 6 5 5 12 6 6 6 13 7 3 8 14 7 4 7 15 7 5 6 16 7 6 5 The Taguchi ratio is a transformed value derived by subtracting the experimental value from the target value. The signal/noise (SN) ratio is calculated by dividing the mean by the standard deviation and is used to measure the quality of the output. The SN ratio is categorized by Taguchi into three groups, namely ‘larger is better’, ‘nominal is better’, and ‘smaller is better’, depending on the nature of the response [20]. In the present experimental study, MRR were considered as ‘larger is better’ (Eq. (1)) response characteristic and OC considered as ‘smaller is better’ (Eq. (2)). The Minitab20 statistical software package was employed for the Taguchi experimental design, mean graph analysis, SN ratio graph analysis, analysis of variance (ANOVA) results, surface plots, and contour plots. SN y n               10 1 10 2 log, (1) SN yn        10 10 2 log/ . (2) 2.4 Experimental Analysis Using ANOVA Method ANOVA was employed to examine the statistical outcomes of the TET and DDTET geometries. It aimed to identify the input parameters that had a more substantial impact on the machining process and determine the extent of their influence. Input parameters with a p-value less than 0.05 were considered significant at a 95 % confidence level. 2.5 Measurement of MRR The 3 mm thick work-piece was drilled using EDDM, where three different drilling methods were typically available, as illustrated in Fig. 3. In this study, the normal drilling method, shown in Fig. 3b was selected. TET and DDTET electrode tools were used, each responsible for 16 holes, totaling 32 holes on the SS202 material. Drilled holes were represented by yellow and pink circles for TET and DDTET respectively as shown in Fig. 4. MRR was calculated using the weight reduction method (Eq. (3)), where initial and final weights of the work-piece (Wi and Wf), density (ρ), and machining time (t) are considered [21] and [22]. MRR WW t if          m 31 min. (3) a) b) c Fig. 3. Drilling types; a) approach drilling, b) normal drilling, and c) break-through drilling Fig. 4. Drilled holes on SS202 work-piece OC dddd          1 234 4 mm , (4) CoP VV VV               maxm in maxm in / %, 2 100 (5) where V max is maximum value and V max minimum value. Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 599 Experimental Investigation on SS202 using Tubular and Double D Tubular Electrode Tool in Electrical Discharge Drilling Machining a) b) Fig. 5. a) Depth of OC measurement, and b) measurement of OC using CMM 2.5 Measurement of OC The OC measurement was conducted using a co- ordinate measuring machine (CMM) (RENISHAW MCU lite-2, United Kingdom). A multiple point touching method was employed to determine the OC value as shown in Fig. 5b, with the OC measured at four different depths: 0.5 mm (d1), 1 mm (d2), 1.5 mm (d3), and 2 mm (d4) as shown in Fig. 5a. Eq. (4) was used to calculate the OC value. The change in percentage (MRR and OC) presented in Table 4 was determined using Eq. (5) [22]. 3 RESULTS AND DISCUSSION 3.1 Comparison Study with TET and DDTET Geometry 3.1.1 MRR Comparison Study Table 4 highlights notable trends in MRR for both TET and DDTET geometries. The 9 th RO (*consider the following input arrangement for the forthcoming RO) (*PI = 6 A, PONT = : 3 µs, POFFT = 7 µs) with low PONT = 3 µs in the TET geometry resulted in decreased MRR due to reduced pulse live time. Even increasing PI with low PONT didn’t significantly improve MRR. In contrast, the DDTET geometry’s 1 st RO (*4 A, 3 µs, 5 µs) had minimized MRR due to lower experimental parameters, resulting in less energy distribution and sparking time. The 16 th RO (*7 A, 6 µs, 5 µs) for both geometries showed improved MRR due to parameters set in a peak trend, enhancing material removal with increased thermal energy distribution. The change of electrode gap between the tool and work-piece had minimal effect on change of MRR 2.26 % at 3 rd RO (*4 A, 5 µs, 7 µs), and the 9 th Table 4. MRR and OC of TET and DDTET geometry RO TET geometry MRR [mm 3 min –1 ] DDTET geometry MRR [mm 3 min –1 ] Change of MRR [%] OC of TET geometry [mm] OC of DDTET geometry [mm] Change of OC [%] 1 14.76135 15.1400 2.5326 3.0499 3.0083 1.37 2 17.93606 18.4803 2.9889 3.0627 3.0191 1.43 3 21.175 21.6594 2.2618 3.0811 3.0286 1.72 4 22.47238 23.1102 2.7985 3.0992 3.0349 2.10 5 17.03451 18.8571 10.1561 3.0635 3.0189 1.47 6 27.78822 30.1863 8.2729 3.0662 3.0209 1.49 7 26.4138 27.4010 3.6688 3.0860 3.0324 1.75 8 34.00262 36.4855 7.045 3.0965 3.0417 1.79 9 14.42659 17.9304 21.658 3.0662 3.0227 1.43 10 24.15692 26.1635 7.9752 3.0836 3.0257 1.90 11 42.26533 45.0064 6.2817 3.0784 3.0231 1.81 12 46.45681 50.1515 7.6488 3.0928 3.0332 1.95 13 16.78942 18.5702 10.0724 3.0763 3.0128 2.09 14 30.07439 32.1693 6.7313 3.0773 3.0183 1.94 15 44.81558 46.4048 3.4843 3.0836 3.0178 2.16 16 62.87964 66.8733 6.1558 3.0865 3.0168 2.28 Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 600 Manikandaprabu, P. – Saravanan, K.G. RO (*6 A, 3 µs, 7 µs) had the highest change of MRR 21.66 % percentage change between TET and DDTET geometries. This suggests that increasing the annular gap significantly improved MRR at the 9 th RO due to increased ionization potential. 3.1.2 OC Comparison Study Table 4 shows that the 1 st RO (*4 A, 3 µs, 5 µs) resulted in a reduced OC for both TET and DDTET geometries due to a downward trend in all experimental input parameters, indicating that low current intensity, low PONT, and low POFFT reduces ionization in the machining zone, resulting in a decreased OC. Conversely, the 4 th RO (*4 A, 6 µs, 8 µs) in the TET geometry and the 8 th RO (*5 A, 6 µs, 7 µs) in the DDTET geometry had an increased OC, indicating that higher PONT along with increased ideal timing (POFFT), increase the ionization potential at the machining zone and result in a larger hole size. Table 4. Showing that the 1 st RO (*4 A, 3 µs, 5 µs) had the least deviation percentage between TET and DDTET geometry, indicating that changing the annular gap had little effect on OC deviation. In contrast, the 16 th RO (*7 A, 6 µs, 5 µs) had the highest deviation of (2.28 %), indicating that the maximum distance between the tool and work-piece, significantly affected OC deviation at 16 th RO. 3.2 SN Ratio and Mean Graphs for TET and DDTET Geometries 3.2.1. MRR Comparison Study Using SN Ratio and Mean Graphs Figs. 6 and 7 depict graphs of the TET and DDTET geometries respectively which followed the same contour pattern. PI and PONT positively impact MRR, indicating that higher values result in more efficient material removal. Conversely, increasing POFFT reduces MRR due to longer intervals between electrical discharges, which decreases thermal distribution within the work-piece, hindering material removal. These findings emphasize the importance of optimizing input parameters for enhanced drilling efficiency. The Taguchi research showed a mean R-Squared (R-Sq) value of 98.06 % and an adjusted R-Squared (R-Sq(adj)) value of 95.16 % for the TET geometry. For the DDTET geometry, the R-Sq and R-Sq(adj) values were 98.11 % and 95.26 %, respectively. These values indicate that the results closely followed the regression line. a) b) Fig. 6. a) TET geometry’s SN ratio and b) means graph a) b) Fig. 7. a) DDTET geometry’s SN ratio and b) means graph Based on the Taguchi analysis PONT was the most significant factor for both TET and DDTET geometries, indicating that increasing sparking time improves MRR. PI was the second most significant factor, with increased input resulting in peak thermal energy in the work zone and an increased MRR value due to the maximized etching process. POFFT was found to be the least significant factor, with a peak Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 601 Experimental Investigation on SS202 using Tubular and Double D Tubular Electrode Tool in Electrical Discharge Drilling Machining POFFT value reducing energy distribution time and leading to decreased material etching from the machining zone. 3.2.2. OC Comparison Study Using SN Ratio and Mean Graphs Fig. 8 shows that in TET geometry a decreased PI leads to a lower OC value due to lower electron intensity limiting ionization, resulting in minimal heat generation and material etching. Lower PONT minimizes current flow time and etching, creating a smaller working area diameter. A lower POFFT further reduces material removal and hole diameter. Fig. 9 indicates that in DDTET geometry a maximum PI reduces OC due to increased distance between the tool and work-piece, regulating ionization. Initially, reducing PI decreases OC, but further increases in PI first raise and then reduce OC again. In TET geometry, the R-Sq and R-Sq(adj) values of 97.92 % and 94.79 % show a good fit of the regression line using the Taguchi technique. For the means, R-Sq is 99.04 % and R-Sq(adj) is 97.61 %, indicating a close match to experimental values in DDTET geometry. a) b) Fig. 8. a) TET geometry’s SN ratio and b) means graph Based on the Taguchi analysis, PONT is the most significant input parameter affecting OC values for both TET and DDTET geometries. In TET geometry, POFFT is the second most significant factor. In contrast, in DDTET geometry, PI is the second most significant factor due to the additional gap between the tool and work-piece, which requires higher current intensity to achieve the desired hole diameter. PI is the least significant factor in TET geometry, while in DDTET geometry, the least significant input is POFFT. a) b) Fig. 9. a) DDTET geometry’s SN ratio and b) means graph 3.3 Comparison Study of MRR and OC Using Bar Chart Fig. 10 shows that the DDTET geometry of the modified tool achieved better MRR than the TET geometry. The increased annular gap in the DDTET geometry allowed for better debris evacuation, leading to faster electron movement from the electrode tool to the work-piece and improved thermal energy distribution, resulting in increased MRR. In contrast, the TET geometry had a high volume of debris in the machining zone, which increased the likelihood of electron restriction and decreased etching from the tool approaching zone, resulting in reduced MRR. Fig. 11 revealed that the DDTET model achieved that lower OC than the TET model, with the minimal distance between tool and work-piece (TET) increasing ionization and thermal energy in the machining zone. Malayath et al. [18] conducted EDM micro end milling using both the electrode and work-piece as WC. They found that a reduced annular gap increases debris concentration, dielectric fluid conductivity, secondary sparking, and material Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 602 Manikandaprabu, P. – Saravanan, K.G. PONT to OC decreased by 24.88 % in DDTET geometry, indicating that the reduced annular gap enhances PONT’s influence in TET geometry. The PI contribution decreased by 23.04 % in the TET model, suggesting that a larger gap increases PI’s contribution to OC in DDTET geometry. The percentage of deviation (PoD) for POFFT was 2.97 %, indicating that the change in the annular gap does minor impact to POFFT’ s PoD for OC. Table 5. Contribution of input factors using ANOVA Response Geometry PONT [%] PI [%] POFFT [%] MRR TET 51.35 28.77 17.95 DDTET 48.58 31.67 17.85 % of deviation 2.77 –2.90 0.10 OC TET 72.25 5.33 20.33 DDTET 47.37 28.37 23.30 % of deviation 24.88 –23.04 –2.97 3.4.3 DDTET Regression Equation MRR = – 4.9 + 13.80 PI + 8.99 PONT – 13.67 POFFT – 0.611 PI×PI – 0.022 PONT×PONT + 0.645 POFFT×POFFT [mm 3 min –1 ], (6) OC = 2.8166+ 0.04051 PI+ 0.00333 PONT + 0.02395 POFFT – 0.003875 PI×PI + 0.000213 PONT×PONT – 0.001587 POFFT×POFFT [mm] . (7) 3.5 Surface and Contour Plots Analysis 3.5.1 Comparison Study of MRR Using Surface and Contour Plots Figs. 12 and 13 illustrate that higher values of PI = 7 A and PONT = 6 µs lead to an enhanced MRR in EDD. In contrast, lower values of these parameters and an increase in spark idling time result in a reduction in MRR. Moreover, it is observed that a combination of high PONT = 6 µs and low POFFT = 5 µs values also contribute to an increase in MRR. This is likely due to the extended pulse generation time, leading to heightened ionization in the machining zone, thus facilitating material removal. Notably, both the TET and DDTET geometries exhibit consistent responses to variations in these input parameters, reinforcing the observed patterns in MRR. removal at the side wall. Conversely, an increased distance between electrode and work-piece (DDTET) limits ionization and reduces material etching at the side wall compared to the TET method. Fig. 10. MRR chart comparison between TET and DDTET geometry Fig. 11. OC chart comparison between TET and DDTET geometry 3.4 ANOVA Analysis 3.4.1 MRR Comparison Study Using ANOVA Analysis Table 5 shows, in the ANOVA result, that PONT was the most significant factor in both TET and DDTET geometries, with a contribution of 51.35 % and 48.58 % respectively. Increased sparking time led to improved etching and increased MRR. PI contributed 28.77 % in the TET geometry and 31.67 % in the DDTET geometry, while POFFT had a contribution of 17.95 % and 17.85 % in TET and DDTET respectively for MRR. Table 5 shows that the contribution percentages of each input factor for MRR in both the TET and DDTET geometries had minimal deviations, indicating that the impact of each factor remained consistent throughout the experiments. 3.4.2. OC Comparison Study Using ANOVA Analysis Table 5 shows that in TET geometry, PONT, POFFT, and PI contributed 72.25 %, 20.33 %, and 5.33 %, respectively, to OC. In DDTET geometry, PONT, PI, and POFFT contributed 47.37 %, 28.37 %, and 23.30 %, respectively, to OC. The contribution of Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 603 Experimental Investigation on SS202 using Tubular and Double D Tubular Electrode Tool in Electrical Discharge Drilling Machining Fig. 12. MRR Surface plot of TET geometry Fig. 13. MRR Surface plot of DDTET geometry 3.5.2 Comparison Study of OC Using Surface and Contour Plots Fig. 14 illustrate that the most significant combinations for achieving smaller OC values were lower PONT and low-level PI. The second most significant combination involved low POFFT and low-level PONT. Additionally, it revealed that the optimal input values for minimizing OC in TET geometry were PONT at 3 µs, POFFT at 5 µs, and PI at 4 A, supporting the results in Table 4. Fig. 15 shows that in DDTET geometry, the smallest OC was achieved with a combination of low POFFT and high PI. Further reduction in PI and POFFT also resulted in smaller OC values. Additionally, a combination of low PONT and low PI produced smaller OC values, but an increase in PI with low PONT led to significant response outcomes. A combination of low POFFT and low PONT also minimized OC values. In summary, the significant input values for achieving smaller OC values in DDTET geometry are low-level POFFT at 5 µs, low PONT at 3 µs, and either high 7 µs or low 4 µs PI values. Fig. 14. OC Surface plot of TET geometry Fig. 15. OC Surface plot of DDTET geometry Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 604 Manikandaprabu, P. – Saravanan, K.G. 3.6 Selective Electron Microscope (SEM) Image Analysis SEM images captured with ZEISS SEM equipment analysed the RL and HAZ in TET and DDTET geometries. In this instance, the boundary of the RL was indicated in red, while the HAZ was marked in yellow. Fig. 16 shows that the 4 th and 5 th RO in TET geometry have a high RL thickness and significant HAZ compared to the RL and HAZ in DDTET geometry. Fig. 17b, indicates that in RO 5 of the DDTET geometry, a wider gap between the tool and work-piece results in no RL formation but a notable HAZ, due to effective electrolyte discharge and quenching effect [23] and [24]. Fig. 17a, shows that in RO 4 of the DDTET geometry, there is minimal RL compared to TET geometry, with some patches of HAZ caused by high thermal energy from a smaller gap. 4 CONCLUSIONS • The experimental results demonstrated the significant impact of machining parameters on MRR and OC for both TET and DDTET geometries. According to the ANOVA analysis, PONT was identified as the most influential factor, contributing 51.35 % and 48.58 % to MRR in TET and DDTET geometries, respectively. PI contributed 28.77 % to MRR in TET and 31.67 % in DDTET, while POFFT accounted for 17.95 % in TET and 17.85 % in DDTET. These findings highlight the importance of fine-tuning the pulse parameters to maximize MRR in different geometries. • In the 16 th experimental run (*7 A, 6 µs, 5 µs), both geometries exhibited improved MRR due to the optimal distribution of thermal energy. The greatest variation in MRR, with a difference of 21.66 %, was observed in the 9th run (*6 A, 3 µs, 7 µs), indicating that the electrode gap between the tool and work-piece plays a crucial role in Fig. 17. a) DDTET geometry’s RL thickness and b) HAZ Fig. 16. a) TET geometry’s RL thickness and b) HAZ Strojniški vestnik - Journal of Mechanical Engineering 70(2024)11-12, 595-606 605 Experimental Investigation on SS202 using Tubular and Double D Tubular Electrode Tool in Electrical Discharge Drilling Machining MRR differences between TET and DDTET geometries. • With respect to OC, the ANOVA results revealed that PONT, POFFT, and PI contributed 72.25 %, 20.33 %, and 5.33 % in TET, and 47.37 %, 28.37 %, and 23.30 % in DDTET, respectively. The 1 st run (*4 A, 3 µs, 5 µs) showed a reduction in OC for both geometries, attributed to low PI, PONT, and POFFT, which led to decreased ionization in the machining zone. Moreover, the parameters (*7 A, 3 µs, 5 µs) significantly reduced OC values in DDTET geometry, showcasing its potential in achieving minimal dimensional deviation. • Notably, the 16 th run (*7 A, 6 µs, 5 µs) had the highest OC deviation of 2.28 %, which can be attributed to the maximum distance between the tool and work-piece. Furthermore, DDTET geometry exhibited a comparatively thinner RL and a smaller HAZ than TET geometry, underlining its effectiveness in reducing thermal damage during the machining process. 5 REFERENCES [1] Kirubagharan, R., Dhanabalan, S., Karthikeyan, T. (2024). The effect of electrode size on performance measures of Inconel X750 using nano-SiC powder mixing electrical discharge machining. Journal of Materials Engineering and Performance vol. 33, p. 1283-1303, DOI:10.1007/s11665-023-08835-z. [2] Pal, M.R., Debnath, K., Mahapatra, R.N. (2024). 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