Parametric Analysis of CNC EDM on OHNS Tool Steel

Transcription

1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 214) December 12 th 14 th, 214, IIT Guwahati, Assam, India Parametric Analysis of CNC EDM on OHNS Tool Steel 1 K.Saraswathamma *, 2 Madhu Durgam 1* Assistant Professor, Mechanical Engineering Department, Osmania University, Hyderabad-7 saraswathi.ouce@gmail.com 2 M.E. Student, Mechanical Engineering Department, Osmania University, Hyderabad-7 durgam.madhu@gmail.com Abstract Electrical Discharge Machining (EDM) is well known unconventional machining and capable of machining geometrically complex and hard material components, that are precise and difficult-to-machine such as heat treated tool steels, composites, super alloys, ceramics, carbides, heat resistant steels etc. In the present work, detailed study through statistical design of experiments were carried out to study the effect of machining parameters such as Pulse current, Pulse on time(ton) and Pulse pause time(toff) on responses variables such as Material Removal Rate(MRR) and Tool Wear Rate (TWR) on OHNS Tool Steel. The experiments were designed using Response surface methodology (RSM) - Central Composite Design (CCD) involving three variables with three levels. An attempt has been made to develop mathematical model for relating the MRR and TWR to the input parameters. Separate Analysis of Variance (ANOVA) are conducted and contribution of each parameter affecting improvement in MRR and TWR is calculated. Investigation results indicates that MRR and TWR increases with increase in pulse current and pulse on time. Key words: CNC EDM, OHNS, Pulse current, Pulse on time (Ton), MRR, TWR. 1. Introduction Electro Discharge Machining (EDM) is an electrothermal non-traditional machining Process, where electrical energy is used to generate electrical spark and material removal mainly occurs due to thermal energy of the spark(jain 29). EDM is mainly used to machine difficult-to-machine materials and high strength temperature resistant alloys. EDM has application capability in a large number of industrial areas like automotive industry, in die and mould making industries, aerospace, aeronautics and nuclear industries. The main advantage of EDM is that the tool and the workpiece do not come into contact with each other, hence eliminates chatter and vibration problems and also allows tiny or thin components to be machined without mechanical force. During machining, the discharge energy produces very high temperatures at the point of the spark on the surface of the workpiece removing the material by melting and vaporization. The top surface of the workpiece re-solidifies and subsequently cools extremely quickly. Although the metal removal is due to thermal effect, but there is no heating of bulk material. OHNS tool steel is very hard and used many application of automobile industry, aerospace industry specifically it is used in dies used for cutting, blanking, piercing, and extrusion operations. These steels are categorized as difficult to machine materials, possess greater strength and toughness are usually known to create major challenges during conventional and nonconventional machining. Shankar Singh et.al (Singh, Maheshwari et al. 24) focused their investigation on machining parameters such as pulsed current on material removal rate, diameter overcut, electrode wear, and surface roughness in electric discharge machining of En-31 tool steel hardened and tempered to 55 HRC copper, copper tungsten, brass and aluminium electrodes by varying the pulsed current at reverse polarity. M. Kiyak, et.al. (Kiyak and Çakır 27) investigated the influence of EDM parameters on surface roughness for machining of tool steel (AISI P2) which is widely used in the production of plastic mold and die. And their experimental study shows that high surface finish quality and machining outputs with the increase in the current and also surface roughness increased with increasing pulse current and pulse time. Mohammad Jafar Haddad et al(haddad and Tehrani 28) carried out a surface roughness (Ra), roundness and material removal rate (MRR) study on the cylindrical wire electrical discharge turning (CWEDT) on AISI D3 tool steel by varying power, voltage, pulse off time, and spindle rotational speed. Sanjeev Kapoor et al(kumar, Singh et al. 29) presented a review on surface modification by EDM which is novel application and 561-1

2 Parametric Analysis of CNC EDM on OHNS Tool Steel added a new dimension to the conventional EDM. In surface modification by EDM significant amount of material is transferred from the powder suspended in dielectric medium to the machined surface under appropriate machining conditions which changes the surface composition and its properties. Klocke et al(klocke, Schwade et al. 213) investigated the specific wear behavior and material removal rate with different grades of graphite electrode in detail and linked to the physical characteristics of the graphite material. To reduce the tool wear in EDM, Yin et al (Yin, Wang et al. 214) investigated a new method simultaneous EDM and ECM (SEDCM). Many researchers have done experimental work on EDM of tool steel to find material removal rate and tool wear rate. From literature review, it was observed that very few work has been reported using the machining parameter i.e. Pulse pause time on oil hardened nonshrinkable tool steel (OHNS). Hence, this present work is focused to study the effect of individual machining parameters such as pulse current, pulse on time (T on ) and pulse pause time (T off ) on material removal rate(mrr) and tool wear rate(twr) in electric discharge machining of OHNS tool steel with copper electrode using design of experiments. 2. Experimentation In this experimental work, OHNS tool steel is selected as a workpiece material and it is very hard and used many application such as automobile industry, and aerospace industry. Specifically it is used in dies used for cutting, blanking, piercing, and extrusion operations. Chemical composition of the OHNS tool steel is given in Table 1. Experiments are carried out on CNC EDM CREATOR CR-6C (die-sinking type) with servo-head (constant gap) and straight polarity. Commercial grade Kerosene oil is used as a dielectric fluid. The photograph of the CNC EDM CREATOR CR-6C machine is given in Fig.1. Table 1: Chemical Composition of OHNS TOOL STEEL chemical test C S Ph Mn Si Cr Result Experiments were conducted as per the experimental plan in random order to eliminate experimental errors. Workpiece is prepared by setting the weight max 15gm. The work pieces are prepared by cutting into the size 15mm*4mm*16mm and then grounded in order to get good finish. The circular copper rod of 12mm diameter and 1mm length was taken as an electrode and the facing operation was done on lathe to get the good surface finish on the faces of the rod. Each workpiece and tool was weighted and recorded before machining. CNC EDM CREATOR CR- 6C is used for machining OHNS work pieces. The EDM tank is filled with kerosene oil which is used as a die electric fluid. Each work piece was machined for 2min and all experiments carried out according to the experimental plan. After machining each work piece and tool was weighted and recorded. The weight different before and after machining indicates the MRR on work piece and TWR on electrode. Figure 1 CNC EDM CREATOR CR-6C Machine Two level full factorial design with 6 central runs and 6 axial runs leading to central composite rotatable design was used to conduct experiment(montgomery 1997). Table 2 listed the coded and actual values of different parameters used in EDM of Tool steel. The experimental plan and summary of results was given in Table 3. Table 2: Coded levels and actual values of process parameters Machining parameters Pulse current(a) Pulse pause time (T off )(B) units Levels amp µs

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 214) December 12 th 14 th, 214, IIT Guwahati, Assam, India Run Pulse duration (T on )(C) µs Table 3 Experimental plan and summary of results Coded factors Actual factors A B C A B C MRR TWR selected. After dropping insignificant terms, the reduced model of ANOVA is shown in Table 4. The model F- value of 8.33 implies that the model is significant. There is only a.1 chance that this large Model F- Value could occur due to noise. Value of Prob>F less than.1 indicates model terms are significant. In this case A, B, C, AB, BC, AC, and B 2 are significant model terms. Depending on coefficients calculated, the final regression equation in term of coded and actual values, respectively are as follows: The final equation for MRR in terms of coded factors The final equation for MRR in terms of actual factor RSM Analysis Response surface regression analysis is done to evaluate the effect of individual parameter and their interactions on response parameters viz. MRR and TWR using Stat-Ease Design Expert software. The contribution of each term of the model in affecting improvement in response parameters are found out through the sum of square method. 3.1Material Removal Rate: Based on lack of fit test, quadratic model is Table 4. ANOVA for MRR Source Sum of Squares df Mean Square F Value Model A B C AB AC p-value Prob > F BC 1.1E B^2 6.1E Residua l 3.8E E-5 Lack of Fit 3.3E E Pure Error 4.9E E-6 Cor Total Remark Significant not significant

4 Parametric Analysis of CNC EDM on OHNS Tool Steel 3.2 Tool Wear Rate The analysis of variance (ANOVA) of this model for tool wear rate is conducted after neglecting contribution of all the insignificant model terms is shown in Table.5. The model F-Value of implies that the model is significant. In this TWR, A, B, C, AC, BC, A 2 and C 2 are significant model terms. The final equation in terms of coded factors and actual factors are given as The final regression equation for TWR in coded factors: Pulse current is the most imporatnt macining parameter in EDM and ANOVA Table 5 also highlighting that pulse current is the most significant factor. It is evident from the Fig.2 that MRR increases with increase in pulse current. This is due to the spark energy is increased to accelerate the action of melting and vaporization, and proceeding the large impulsive force in the spark gap, thereby increasing the MRR. Higher currents improves the MRR, but at the same time surface roughness and tool wear also increases. Source The final regression equation for TWR in actual factors: Sum of Squar es Table 5. ANOVA for TWR df Mean Square F Value Model A p-value Prob > F B C AC BC A C Residual Lack of Fit Pure Error Cor Total Rema rk Signif icant not signif icant MRR-gm/min At Toff -2µs Ton 2 µs Ton 4 µs Ton 6 µs Pulse current-amp Figure 2 Pulse current Vs MRR The MRR increases when pulse current increases and also the MRR increases with increase in pulse on time but the increase rate is lower with the individual parameters. The interaction of both pulse current and T on, on MRR is greater as shown in Fig.3. In actual also, discharge current amount and duration increases MRR increases, as well as gas bubbles in the discharge zone. High pulse current and pulse on time increases the MRR and also increases the surface roughness (Kiyak and Çakır 27). 4. Results and Discussions 4.1 Effect of Pulse current A.Metal Removal Rate ( MRR ) Figure 3 Pulse current and T on interaction effect on MRR 561-4

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 214) December 12 th 14 th, 214, IIT Guwahati, Assam, India TWR-gm/min B. Tool Wear Rate (TWR) Tool wear is related to the melting point of the tool materials. Tool wear is affected by the precipitation of carbon from the hydrocarbon dielectric on the electrode surface during sparking. And also the rapid wear on the electrode edge was because of the failure of carbon to precipitate at difficult to reach regions of the electrode. As shown in Fig.4 TWR increases with increase in pulse current. At a higher pulse current a stronger spark is generated producing more heat. The size of the workpiece is massive and heat is easily dissipated through it. But since the electrode is a smaller one, heat is accumulated in it resulting in high temperature and consequently high EW. The pulse current alone shows high influence on TWR whereas pulse on time also has less effect on TWR. This may be due to the impaired evacuation of machining products, a portion of the discharge energy is spent on re-melting and evaporation of solidified metal particles. TWR increases with increasing in both the parameters such as pulse current and pulse on time and both showing considerable effect on TWR (Fig. 5) At T off -2µs Pulse current-amp Figure 4 Pulse current Vs TWR Ton 6 µs Ton 4 µs Ton 2 µs, Figure 5 Interaction effect of pulse current and T on on TWR 4.2 Effect of pulse pause time (T off ) A. Metal Removal Rate (MRR) From the Fig.6, it is evident that increasing in pulse pause time decreases the MRR. If increase in pulse pause time the spark contact time with work piece is decreases and hence MRR will decrease. Due to more time gap in between the successive spark will leads to less metal removal rate. At lower pulse currents even in increase in pulse pause time does not have significant effect. But at higher pulse currents the decrease is more with increase in the pulse pause time. Figure 6 Pulse pause time (T off ) Vs MRR B. Tool War Rate (TWR) As shown in Fig.7 TWR is constant at lower pulse pause time and decreases when T off increases. At higher pulse current this has significant effect. This may be because of more time gap between the successive sparks leads less tool wear. Even surface roughness increases with increased in pulse current and pulse on time (Kiyak and Çakır 27). TWR-gm/min MRR-gm/min At T on - 4µs Pulse pause time ( T off )- µs At Ton -4µs current 12 µs current 8 µs current 4 µs Pulse pause time( Toff)- µs Figure 7 Pulse pause time (Toff) Vs TWR current 12 µs current 8 µs current 4 µs 561-5

6 Parametric Analysis of CNC EDM on OHNS Tool Steel 4.3 Effect of Pulse on time (T on ) A. Metal Removal Rate (MRR) It was observed from Fig.8 that MRR increases with the increase Pulse on time. By the increase in pulse on-time, the discharge energy of the plasma channel and the period of transferring of this energy into the electrodes increases. This occurrence leads to a formation of a bigger molten material crater on the workpiece which results in a higher MRR. In increasing on time the spark get more time to contact with the work which results in increase the MRR. MRR - gm/min At pulse current - 8µs Toff 3 µs Toff 2 µs Toff 1 µs Pulse on time (Ton) - µs Figure: 8 Pulse on time (T on ) Vs MRR B. Tool Wear Rate (TWR) As shown in Fig.9 TWR increases slightly with increasing in pulse on time, but the effect is less significant. TWR increases with increase pulse on time alone, but decreases with increse in pulse pause time. When increae in pulse on time, the discharge time increases, hence there may be more material removal from the electrode. TWR - gm/min At Current - 8µs Toff 3 µs Toff 2 µs Toff 1 µs Pulse on time ( Ton) - µs After analyzing the results of the experiments of OHNS tool steel with copper electrode, the following conclusions are arrived at: The increase in pulse current leads to a sharp increase in the material removal rate. And also it was observed that, tool wear rate are also increases with increasing the current. So, current was the most significant factor in both MRR and TWR. The increase in pulse on time leads to an increase in material removal rate and there is slight increase was observed with tool wear rate. The increase in pulse pause times both material rate and tool wear was decreasing. REFERENCES Haddad, M. J. and A. F. Tehrani (28). "Investigation of cylindrical wire electrical discharge turning (CWEDT) of AISI D3 tool steel based on statistical analysis." Journal of Materials Processing Technology 198(1 3): Jain, V. K. (29). Advanced machining processes, Allied Publishers. Kiyak, M. and O. Çakır (27). "Examination of machining parameters on surface roughness in EDM of tool steel." Journal of Materials Processing Technology 191(1 3): Klocke, F., M. Schwade, A. Klink and D. Veselovac (213). "Analysis of Material Removal Rate and Electrode Wear in Sinking EDM Roughing Strategies using Different Graphite Grades." Procedia CIRP 6(): Kumar, S., R. Singh, T. P. Singh and B. L. Sethi (29). "Surface modification by electrical discharge machining: A review." Journal of Materials Processing Technology 29(8): Montgomery, D. C. (1997). Design and analysis of experiments, Wiley New York. Singh, S., S. Maheshwari and P. C. Pandey (24). "Some investigations into the electric discharge machining of hardened tool steel using different electrode materials." Journal of Materials Processing Technology 149(1 3): Yin, Q., B. Wang, Y. Zhang, F. Ji and G. Liu (214). "Research of lower tool electrode wear in simultaneous EDM and ECM." Journal of Materials Processing Technology 214(8): Figure 9 Pulse on time (Ton) Vs TWR 5 CONCLUSIONS 561-6