CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS

Transcription

1 CHAPTER 8 CONCLUSIONS AND RECOMMENDATIONS 8.1 CONCLUSIONS In this investigation, modelling of some of the most important technological parameters is proposed for rotary electrical discharge machining process. Taguchi method is used to determine the main influencing factors affecting the selected technological variables such as MRR, SR and EWR as well dimensional characteristics such as deflection, circularity, cylindricity and taper along with increase in hardness. These outputs have been studied as a function of design factors such as peak current, pulse on time, diameter and depth. The other factors like pulse off time, duty factor, electrode rotation, flushing pressure of water, voltage etc. remained constant throughout the experiments. 1st order empirical models were developed for prediction of various output process parameters during rotary electrical discharge machining, using non-linear regression with logarithmic data transformation. The proposed models have been successfully applied to estimate the values of MRR, SR, EWR and I.H. under various machining conditions. Through this investigation of rotary electrical discharge machining of tool steel H11 with the brass electrode, the following conclusions are summarised: Through the Taguchi analysis for MRR it has been observed that current affects the MRR at the highest level and in order diameter, pulse on time and depth respectively are affecting the MRR. Even it highest MRR within the experimental data range optimized conditions are 25amp current, 10μs pulse on time, 3mm diameter and 10mm depth. Further based on the results of ANOVA analysis it has been seen that current and diameter are significant factors affecting MRR. Through the Taguchi analysis for SR it has been observed that current affects the SR at the highest level and in order depth, pulse on time and diameter respectively are affecting the SR. Even it has been seen, through the main effects plots of SN ratio, to achieve the best SR within the experimental data range optimized conditions are 25amp current, 9μs pulse on time, 1.5mm diameter and 15mm depth. Further based on the results of ANOVA analysis it has been seen that none of these input factors are significantly affecting the SR. Through the Taguchi analysis for EWR it has been observed that diameter affects the EWR at the highest level and in order current, 227

2 depth and pulse on time respectively are affecting the EWR. Even it the lowest EWR within the experimental data range optimized conditions are 15amp current, 8μs pulse on time, 1.5mm diameter and 10mm thickness. Further based on the results of ANOVA analysis it has been seen that current and diameter are significant factors affecting EWR. Through this investigation of rotary electrical discharge machining of brass material with the brass electrode, the following conclusions are summarised: Through the Taguchi analysis for MRR it has been observed that current affects the MRR at the highest level and in order diameter, depth and pulse on time respectively are affecting the MRR. Even it highest MRR within the experimental data range optimized conditions are 25amp current, 10μs pulse on time, 1.5mm diameter and 10mm depth. Further based on the results of ANOVA analysis it has been seen that current, diameter and depth are significant factors affecting MRR. Through the Taguchi analysis for SR it has been observed that current affects the SR at the highest level and in order diameter, depth and pulse on time respectively are affecting the SR. Even it has been seen, through the main effects plots of SN ratio, to achieve the best SR within the experimental data range optimized conditions are 25amp current, 10μs pulse on time, 2mm diameter and 25mm depth. Further based on the results of ANOVA analysis it has been seen that none of these input factors are significantly affecting the SR. Through the Taguchi analysis for EWR it has been observed that diameter affects the EWR at the highest level and in order current, depth and pulse on time respectively are affecting the EWR. Even it the lowest EWR within the experimental data range optimized conditions are 20amp current, 10μs pulse on time, 1.5mm diameter and 25mm thickness. Further based on the results of ANOVA analysis it has been seen that diameter is significant factor affecting EWR. Through this investigation of rotary electrical discharge machining of EN31 with the brass electrode, the following conclusions are summarised: Through the Taguchi analysis for MRR it has been observed that current affects the MRR at the highest level and in order depth, diameter and pulse on time respectively are affecting the MRR. Even it 228

3 highest MRR within the experimental data range optimized conditions are 25amp current, 10μs pulse on time, 1.5mm diameter and 25mm depth. Further based on the results of ANOVA analysis it has been seen that current is significant factor affecting MRR. Through the Taguchi analysis for SR it has been observed that diameter affects the SR at the highest level and in order depth, pulse on time and current respectively are affecting the SR. Even it has been seen, through the main effects plots of SN ratio, to achieve the best SR within the experimental data range optimized conditions are 25amp current, 10μs pulse on time, 2.5mm diameter and 20mm depth. Further based on the results of ANOVA analysis it has been seen that diameter is significant factor affecting the SR. Through the Taguchi analysis for EWR it has been observed that current affects the EWR at the highest level and in order diameter, depth and pulse on time respectively are affecting the EWR. Even it the lowest EWR within the experimental data range optimized conditions are 25amp current, 7μs pulse on time, 1.5mm diameter and 20mm thickness. Further based on the results of ANOVA analysis it has been seen that current and diameter are significant factors affecting EWR. There are some other common conclusions drawn like: Through the Taguchi analysis for deflection, circularity, cylindricity, taper and increase in hardness optimum conditions of parameters can be found out as table 8.1. Optimisation of the rotary electrical discharge machining process is concerned with maximizing the MRR while minimizing EWR, producing the optimum surface roughness for the best dimensional characteristics. But, these cannot be achieved simultaneously with a particular combination of control parameters settings. The developed empirical formulae can be used to evaluate MRR, SR, EWR and I.H., produced by rotary electrical discharge machining, with low prediction error. The empirical formulae in general used as follows: Y = A. (I) a. (T on ) b. (D) c. (D t ) d (8.1) The values of constants and exponents for various responses are given in Table

4 Table 8.1 Optimised condition for various responses of different material Sr. No. Workpiece Material Responses Input Parameters I T on D D t 1 H11 2 Brass 3 EN31 Deflection Circularity Cylindricity Taper Increase in Hardness Deflection Circularity Cylindricity Taper Increase in Hardness Deflection Circularity Cylindricity Taper Increase in Hardness The proposed empirical models are validated and are fit for predictions of machining outputs such as MRR, SR, EWR and I.H. with low prediction error. For Brass, EN31 and H11 the average values of MRR are mm 3 /min, 15.1 mm 3 /min and 10.1 mm 3 /min respectively as well thermal conductivity is 159 W/mK, 46.6W/mK and 24.6W/mK respectively. That means as thermal conductivity increases higher MRR can be achieved. MRR is higher for higher thermal conductive materials. In other way for the same machining parameters MRR is higher for higher thermal conductive material. For Brass, H11 and EN31 the average values of SR are.5μ, 3.73μ and 4.09μ respectively as well specific heat is 0.38 J/g 0 C, 0.46 J/g 0 C and J/g 0 C respectively. That means as specific heat decreases better SR can be achieved. SR is higher (finishing deteriorates) for the 230

5 material with higher specific heat. In other way for the same machining parameters SR is higher for material with higher specific heat. Table 8.2 Values of constants and exponents for various responses Sr. No. Material Response (Y) Constant (A) Exponents a b c d 1 H11 2 Brass 3 EN31 MRR SR EWR Increase in Hardness MRR SR EWR Increase in Hardness MRR SR EWR Increase in Hardness For Brass, EN31 and H11 the average values of EWR are 65.29%, 62.63% and 60.11% respectively as well melting point is 1025 C, 1200 C and 1425 C respectively. That means as melting point increases lower EWR can be achieved. EWR is lower for the material with higher melting point. In other way for the same machining parameters EWR is lower for material with higher melting point. It is well known fact that geometric output can not only be co-related to these studied input parameters. But through the study carried out specifically parametric some observations can be made. These observations are only relates the respective material outputs only and not values. The material in which lesser deflection is achieve, SR is higher(finishing deteriorates). In other way material for which deflection is less SR in overall way would be higher. The minimum value of deflection achieved is mm in H11 material. 231

6 The material for the better values of circularity and cylindricity MRR would be lesser. We also can conclude this vice versa i.e. material in which higher MRR achieved circularity and cylindricity of the hole would be higher. The highest MRR among all values of all material is mm 3 /min achieved in Brasss material. The minimum circularity and cylindricity is 0.002mm and 0.006mm respectively achieved in H11 material. The material for lesser values of taper EWR would be higher. We also can conclude this vice versa i.e. material in which lesser EWR is achieved taper of the hole would be higher. Even for lesser taper MRR would be lesser. We also can conclude this vice versa i.e. to achieve higher MRR taper of the hole would be higher. The minimum EWR among all values of all material is 24% achieved in H11 material. The minimum taper is achieved in H11 material. If Lesser increase in hardness occurs MRR would be higher for particular material. We also can conclude this vice versa i.e. material in which higher MRR achieved increase in hardness of the hole would be higher. Least increase in hardness observed is 15HRC in EN31 & Brass both. 8.2 RECOMMENDATINS FOR FUTURE WORK The above results could find their application in the industry. After completing this research work, it is felt that the parametric study of EDM drilling opens a vast number of avenues for further exploration. Future work may be in the direction of: Three important materials were chosen for this experimental work. Other die and tool steel materials such as Tungsten hot work die steels (H2x-series), Molybdenum high speed tool steels (M-series), conductive carbon-carbon composites, Shock resisting steels etc. can be tried for the parametric study. Some other organic liquids (such as ethylene glycol) or deionized water (already used in wire-edm) may be tried as dielectric medium. In such a medium, graphite electrode can be used as a source for providing carbon. One of the most relevant extensions would be conducting further experimental investigations to identify the practical constraints (such as flushing efficiency) as a function of the input parameters in order to improve the empirical process models and the process optimization. 232

7 In this work, the process parameters have been optimised using Taguchi orthogonal array design, which reduces the number of experiments to a greater extent as compared to conventional parametric design. It is suggested that a comparison between the results obtained from Taguchi orthogonal array design with conventional parametric design be carried out to establish a correlation among them. The design of experiment can be further optimised with well described objectives, pin-pointing the predominant performance parameters with their significance statistically defined. Some parameters like polarity were kept constant, these can also be varied and their effect studied. For this particular experimental set-up, Workpiece is kept positive, whereas tool act as cathode (negative) i.e. straight polarity has been used. It can be appreciated that the main problem faced in improving the aspect ratio as well as reducing the diameter oversize is the secondary sparking which most of the times cannot be controlled. Some measures are necessary for efficient working and ultimately improve the aspect ratio and reduce the oversize. The waveform can be studied using cathode ray oscilloscope (CRO) to sense and control the secondary sparking. 233