The correct selection of manufacturing conditions and technique is one of the most important aspects to take into consideration in the majority of manufacturing processes and, particularly, in processes related to Electrical Discharge Machining (EDM).
This paper investigate wet and near-dry electrical discharge machining to achieve the high material removal rate (MRR) on oil hardened non shrinking steel. In near-dry EDM liquid and air mixture delivered through a tabular electrode instead of liquid dielectric used in wet EDM. L-9 orthogonal array is applied to investigate the effect of discharge current, pulse on time, gap voltage and pulse off time on material removal rate in wet and near-dry EDM. Pulse on time and discharge current are identified as a key factors for improving the MRR in wet and near-dry EDM. Comparative performance of wet and near-dry EDM has been made. It was experimentally found that, wet EDM exhibits the advantage of good machining stability at high discharge energy thus result in better MRR.
Keyword : Near-Dry EDM, Wet EDM and MRR
1. INTRODUCTION
Electric Discharge Machining (EDM) is a thermo–electric process in which material removal takes place through the process of controlled spark generation. It is one of the most popular non-traditional machining processes being used today. EDM has achieved a status of being nearly indispensable in the industry because of its ability to machine any
electrically conductive material irrespective of its mechanical strength. Despite its advantages, environmental concerns associated with the process have been a major drawback of EDM. Hydrocarbon oil based dielectric fluids used in EDM are the primary source of pollution from the process. Replacing liquid dielectric by mixture of gas and liquid is an emerging field in the environment-friendly EDM technology. Near dry EDM is a modification of the conventional EDM process in which the liquid dielectric is replaced by a mixture of gas and liquid medium. The flow of high velocity gas into the gap facilitates removal of debris and prevents excessive heating of the tool and work-piece at the discharge spots. Several experimental studies made in this field have brought out some of the essential features of the process. This study, the wet and near-dry EDM milling is investigated to understand the effect of discharge current, pulse on time, pulse off time and discharge voltage on response of material removal rate.
2. LITERATURE REVIEW
Shih et al [2]
investigated the dry and near-dry EDM milling to achieve the high MRR and fine surface finish for roughing and finishing operations, respectively. Oxygen gas and copper electrode was used in dry EDM, whereas nitrogen and de-ionized water mixture was used in near-dry EDM operation. A 25-1 fractional factorial design is applied to investigate the effect of discharge current, pulse duration, pulse interval and gap voltage on the MRR and surface finish in both process. The experiments were conducted on a CNC die-sinking EDM machine using AISI H13 tool steel as the work material. A rotary spindle, Rotobore RBS-1000, with through-spindle flushing capability is mounted on the EDM head. The input liquid flow is set at 5 ml/min. Negative polarity, i.e. electrode as cathode, is used in the experiment, due lower wear on cathode at low discharge pulse duration and smoother discharge crater on anode. It was found that, high MRR were achieved in dry EDM where as fine surface finish were obtained in near-dry EDM. Oxygen is to promote the MRR in dry and near-dry EDM due to exothermal oxidation. Near-dry was proven beneficial for the finishing operation. Because liquid phase is dispersed in gas medium is hypothesized to enhance the electrical field thus result is large discharge gap and stable discharge at low energy input.
Pradhan et al [6]
investigate three process parameters like discharge current, pulse duration and pulse off time. Response surface methodology was used to investigate the relationship and parametric interaction of variable and significant coefficient were obtained at 5% level by ANOVA. Experiment were conducted on Electronica Electroplus PS 50ZNC Die Shrinking Machine. Copper electrode of 30 mm diameter and 15x 15 mm2 AISI-D2 tool steel with 4 mm thickness work-piece was used.
It was found that, model for MRR were developed for three parameters namely, pulse current, discharge time and pulse off time for EDM process using RSM. It was also found that all three machining parameters and their interactions have significant effect on MRR
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
was investigate the influence of the machining parameters such as peak current, power supply voltage, pulse on time and pulse off time on out put responses Surface quality, Electrode wear and material removal rate. The result analyzed by using STATISTICA software. Design of experiments (DOE) technique and ANOVA analysis were used for experimental work Tungsten Carbide has been selected as the work-piece and copper tungsten as a electrode material.
It was found that, for surface roughness, the most influential factors were voltage followed by the pulse off time, while the peak current and pulse on time was not significant at the considered confidence level. For material removal rate, pulse on time factor was most influential followed by voltage, peak current, and pulse off time. For electrode wear, it was observed that the most influential factor were pulse off time, followed by the peak current factor. Ponappa et al
[8] focus on he effect of electrical machining parameters such as servo
speed, voltage gap, pulse on time and pulse off time on drilled hole quality such as taper and surface finish. Microwave-sintered magnesium nano composites as work material with rotary brass hollow tubular electrode of diameter 0.5 mm and de-ionized water circulated as a dielectric with pressure of 80 kg/cm2 were used. Experiment were conducted using ELECTRONICA small-hole super drill ED 32U machine. DOE is carried out by L9 orthogonal array and ANOVA were carried out to identify the significant factors that affects the hole accuracy and surface roughness. It was found that, high aspect ratios (0.5 mm dia. and 12 mm height) were drilled in magnesium nano composites by EDM. Servo speed and pulse on time were found out as a significant factors on surface roughness and taper.
Iqbal et al
[9] investigate machining parameter like voltage, rotational speed of electrode
and feed rate over a responses MRR, EWR and surface roughness on AISI 304 stainless steel as workpiece material and copper was used as a electrode material. RSM was used for interactions between the three controllable variables on MRR, EWR and Ra. Significant coefficients were obtained by performing ANOVA at 95% significance level. It was found that voltage and rpm have effect on MRR, EWR and surface roughness.
3 PLANNING OF EXPERIMENT
Oil hardened non shrinking steel material having size 5 x 50 x 50 mm with hollow copper alloy electrode with10 mm diameter were used. Separate workpiece used for each experiment. PS 50ZNC (die-sinking type) of EDM machine were used for experimentations. Commercial grade EDM oil (specific gravity= 0.763, freezing point= 94˚C) and air through a hallow tube was used as dielectric fluid with a pressure of 0.1 kgf/cm2 and 0.5 kgf/cm2 respectively. Circular shaped hollow copper tool with internal flushing of air was used to flush away the eroded materials from the sparking zone. In this process, machining time and duty cycle is kept constant 7 min and 0.75. Four factors were tackled with a total number of 9 experiments were performed on wet sinking EDM and near-dry EDM for roughing condition separately. The surface roughness measurement using contact type C3A Mahr Perthen Perthometer (stylus radius of 5 µm )
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
were done. The design scheme and machining parameters and their levels is shown in table1. Design matrix and results for MRR for wet and near-dry EDM as shown in table 2. Machine setup for wet and near-dry EDM as shown in figure1 & 2.
Table 1 Design Scheme of machining parameters and their levels
Parameters Unit Levels
Level-I Level-II Level-III
Discharge current ( Ip ) amp 10 15 20
Pulse on time ( Ton ) µs 300 400 500
Pulse off time ( Toff ) position 10 11 12
Gap voltage ( Vg ) volt 40 45 50
Table2. Experimental design matrix & results for MRR for wet & near-dry EDM.
Exp.
No. Ip Ton Toff Vg MRR for wet
EDM ( g/min )
MRR for near-dry
EDM ( g/min )
1 1 1 1 1 0.2829 0.2386
2 1 2 2 2 0.2671 0.2257
3 1 3 3 3 0.2600 0.2214
4 2 1 2 3 0.4229 0.3214
5 2 2 3 1 0.4029 0.3614
6 2 3 1 2 0.4329 0.3657
7 3 1 3 2 0.4871 0.3971
8 3 2 1 3 0.5571 0.4371
9 3 3 2 1 0.5671 0.4343
Figure1: Machine setup for wet EDM Figure2: Machine setup for near-dry EDM
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
The analysis was made using the MINITAB 16 software specifically used for design of experiment applications. This study uses four factor three level L9 Orthogonal Array for design of experiment as shown in table2.
The analysis of variances and response of signal to noise ratio for the factors is shown in Table 3 and 4 respectively. Which indicates that the Ip has greatest influence on material removal rate (MRR), followed by Toff, Vg and Ton. The delta values for Ip, Ton, Toff and Vg are 5.96, 0.264, 0.841 and 0.399 respectively, depicted in Table 4. Figure 3 shows that to achieve the maximum material removal rate in wet EDM, employ high discharge current of 20 ampere, high pulse on time of 500 µs with low pulse off time of 10 position and gap voltage of 40 volts. During the process of Electrical discharge machining, the influence of various machining parameter like Ip, Toff, Ton and Vg has significant effect on MRR, as shown in main effect plot for S/N ratio of MRR in Figure 3. The discharge current (Ip) is directly proportional to MRR in the range of 10 to 15A. This is expected because an increase in pulse current produces strong spark, which produces the higher temperature, causing more material to melt and erode from the work piece. Besides, it is clearly evident that the other factor does not influence much as compared to Ip.
The coefficients of model for MRR are shown in Table5. The parameter R2 describes the amount of variation observed in MRR is explained by the input factors. R2 = 99.62 % indicate that the model is able to predict the response with high accuracy. Adjusted R2 is a modified R2 that has been adjusted for the number of terms in the model. If unnecessary terms are included in the model, R2 can be artificially high, but adjusted R2 (=98.48 %.) may get smaller. The standard deviation of errors in the modeling, S= 0.0145907 Comparing the p-value to a commonly used α-level = 0.05, it is found that if the p-value is less than or equal to α, it can be concluded that the effect is significant, otherwise it is not significant.
4.2 Influences on MRR in near-dry EDM
The analysis of variances and response of signal to noise ratio for the factors is shown in Table 6 and 7 respectively. Which indicates that the Ip has greatest influence on material removal rate (MRR), followed by Toff, Vg and Ton. The delta values for Ip, Ton, Toff and Vg are 5.339, 0.457, 0.553 and 0.538 respectively, depicted in Table 4. Figure 4 shows that to achieve the maximum material removal rate in wet EDM, employ high discharge current of 20 ampere, high pulse on time of 400 µs with low pulse off time of 10 position and gap voltage of 40 volts. During the process of Electrical discharge machining, the influence of various machining parameter like Ip, Toff, Ton and Vg has significant effect on MRR, as shown in main effect plot for S/N ratio of MRR in Figure 4. The discharge current (Ip) is directly proportional to MRR in the range of 10 to 15A. This is expected because an increase in pulse current produces strong spark, which produces the higher temperature, causing more material to melt and erode from the work piece. Besides, it is clearly evident that the other factor does not influence much as compared to Ip.
The coefficients of model for MRR are shown in Table8. The parameter R2 describes the amount of variation observed in MRR is explained by the input factors. R2 = 98.72 %
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
indicate that the model is able to predict the response with high accuracy. Adjusted R2 is a modified R2 that has been adjusted for the number of terms in the model. If unnecessary terms are included in the model, R2 can be artificially high, but adjusted R2 (=94.87 %.) may get smaller. The standard deviation of errors in the modeling, S= 0.0196488 Comparing the p-value to a commonly used α-level = 0.05, it is found that if the p-value is less than or equal to α, it can be concluded that the effect is significant, otherwise it is not significant.
For validation, the predicted values obtained by regression equations 1 and 2 for surface roughness and material removal rate respectively are compared with the experimental values.
Regression Equation (wet)
MRR ( g/ min ) = -0.363378 - 0.001948 Ip - 0.000938167 Ton +.0562367 Toff + 0.017546
Confirmation experiments were conducted at the optimum set of the process parameters. The value of material removal rate at the optimum set of the process parameters was 0.6002 gms/min and 0.4571 gms/min and it fall near the predicted value of 0.5891 gms/min and 0.4476 gms/min in wet and near-dry EDM respectively.
6. COMPARISION OF WET AND NEAR-DRY EDM
Bar chart for MRR in wet & near-dry EDM
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
1 2 3 4 5 6 7 8 9
Exp. No.
MRR in kg/m
in
Wet MRR
Near-dry MRR
Fig. 5 Comparison of SF in wet EDM & near-dry EDM
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
Experiments were conducted for the same input parameters for w e t & n e a r - dry EDM.
The results are shown in Figure 3 It is found that the MRR in near-dry EDM is less than that
of the wet for same input parameters in each experiment. A comparison of process performance
of near-dry EDM with wet EDM has proven less beneficial for roughing process in a given
range of input variable because stable machining conditions were established at low discharge
energy (When Ip is in the range of 2 to 4 ampere) resulting in high spark frequency which is
beneficial for finishing operation . At high energy input (When Ip is in the range of 10 - 20
Ampere) low spark frequency were obtained with wet EDM which improve the flushing
condition resulting higher MRR and rough surface as compared to near-dry EDM. Thus wet
EDM proven to be beneficial for roughing operation.
7. CONCLUSION
On the basis of the experimental results, the calculated signal to noise ratio (dB), the analysis of variance (ANOVA), the developed mathematical models and confirmation test results, the following conclusions are drawn for the effective machining of oil hardened non shrinking steel in wet and dry EDM process: (1). For wet and near-dry EDM process, discharge current (Ip) was the most influencing factor for in wet EDM followed by pulse off time(Toff), discharge voltage(Vg) and pulse on time(Ton). So, to achieve the maximum material removal rate in wet and near dry EDM, employ high discharge current of 20 ampere, moderate pulse on time of 400 µs with low pulse off time of 10 position and high gap voltage of 40 volts.
(2). A comparison of process performance of near-dry EDM with wet EDM has proven less beneficial for roughing process in a given range of input variable because stable machining conditions were established at low discharge energy (When Ip is in the range of 2 to 4 ampere) resulting in high spark frequency which is beneficial for finishing operation . At high energy input (When Ip is in the range of 10 - 20 Ampere) low spark frequency were obtained with wet EDM which improve the flushing condition resulting higher MRR and rough surface as compared to near-dry EDM. Thus wet EDM proven to be beneficial for roughing operation.
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