ISSN Page 39

Transcription

1 OPTIMIZATION OF MACHINING PARAMETERS OF WIRE CUT EDM PROCESS USING TAGUCHI METHOD Pawan Kumar 1, Randeep Singh 2 1 M.Tech Scholar, 2 Assistant Professor, Department of Mechanical Engineering, OITM, Hisar 1 pawan_mech@ymail.com, 2 randeepkundu@gmail.com Abstract-- Wire electrical discharge machining (WEDM) is extensively used in machining of conductive materials when precision is of prime importance to measures such as metal removal rate (MRR), surface roughness (R a ), and cutting speed (CS) are used to obtain a precision work. By using Taguchi s parameter design, significant machining parameters affecting the performance measures are identified as pulse on time (T -ON), pulse off time (T-OFF), peak current (IP), wire feed (WF), wire tension (WT). It can be seen that a combination of factors for optimization of each performance measure is different. In this study, the relationship between control factors and responses like MRR, R a, and Cutting speed are established by means of Taguchi s methodology design. The material used in this present work is die steel H13 as a work piece and soft brass wire as a tool and also the dielectric fluid used (de-ionized water) as a coolant. Key words: WEDM, H13, Material Removal Rate, Cutting Speed, Surface Roughness, Taguchi Method I. INTRODUCTIONS Wire electrical discharge machining (WEDM) is a specialized thermal machining process capable of accurately machining parts with varying hardness or complex shapes, which have sharp edges that are very difficult to be machined by the main stream machining processes. The WEDM process is based on the conventional EDM sparking phenomenon utilizing the widely accepted non-contact technique of material removal. Since the introduction of the process, WEDM has evolved from a simple means of making tools and dies to the best alternative of producing micro-scale parts with the highest degree of dimensional accuracy and surface finish quality. WEDM is a process which erodes and removes material by using the channel of plasma generated by electric sparks between two conductive materials (i.e. electrode and the work piece), this channel of plasma is converted into thermal energy at a temperature range of 000 to 12000º C at a pulsating direct current supply of to Hz. The electrode and work piece are separated by a small gap being immersed in dielectric fluid, an electric spark is produced in between this small gap and the work piece material is eroded, as the pulsating current is turned off, the plasma breaks down which leads to sudden reduction in the temperature and the eroded material is flushed away with the help of dielectric fluid in the form of microscopic debris. With each electric spark discharge a small crater is formed on both the work piece and the electrode which is a prime decider in the final surface quality. WEDM is considered as a unique adoption of the conventional EDM process which comprises of a main worktable, wire drive mechanism, a CNC controller, working fluid tank and attachments. Fig 1. Schematic Diagram of Wire EDM The work piece is placed on the fixture table and fixed securely by clamps and bolts. The table moves along X and Y-axis and it is driven by the DC servo motors. Wire electrode usually made of thin copper, brass, molybdenum or tungsten of diameter mm, which transforms electrical energy to thermal energy, is used for cutting materials. The wire is stored and wound on a wire drum which can rotate at 1500rpm. The wire is continuously fed from wire drum which moves though the work piece and is supported under tension between a pair of wire guides ISSN Page 39

2 sides of the work piece. The located at the opposite surface quality and material removal rate (MRR) of the machined surface by wire EDM will depend on different machining parameters such as applied peak current, and wire materials. WEDM process is commonly conducted on submerged condition in a tank fully filled with dielectric fluid; nevertheless it also can be conducted in dry condition. This method is used due to temperature stabilization and efficient flushing in cases where the work piece has varying thickness. As the machining operation is performed, conductivity of water rises and it is again recirculated through the resin tank. The purpose of de ionized water is to stabilize the spark erosion path and to act as the dielectric medium which is forced into the cutting gap to flush out the eroded metal. There is no wear and tear, as the machine does not make any type of the physical contact between the work piece and machine part itself. Many research work have been done to find optimization cutting parameters. Tzeng and Chiu (2003) have conducted experiments on castek-03 for medium carbon steel material having excellent wear resistance. The most important factors affecting the EDM process robustness were pulse on time, applied electric current in low voltage and 2 sparking current in high voltage. The most important factors affecting the machining speed were pulse on time and applied electric current in low voltage. The gain of db was able to decrease the variation range to 21.4%, which improved process robustness by 4. times. Juhr (2004) developed a new needle-pulse energy source for WEDM process for cemented carbide work piece. Performance parameters chosen for WEDM process were Pulse duration, current magnitude and pulse energy. Experiments were conducted and it has been observed that correct parameters should be used for main cut and post cuts. Material properties of cemented carbide improve after WEDM process and it is proved by an increase in the bending fraction stress. It has also been proposed that post cuts increasing the total productivity of the machining and significantly lowering the costs. Ramasawmy (2005) described the multi objective optimization of the WEDM process using parametric design of Taguchi methodology. The effect of various machining parameters such as pulse on time, wire tension, delay time, wire feed speed, and ignition current intensity has been studied in machining of Table 1: Process parameters and their levels heat-treated tool steel. It was identified that the pulse on time and ignition current intensity has influence more than the other parameters. Moreover, the multiple performance characteristics such as material removal rate, surface roughness, and wire wear ratio for the WEDM process could be improved by setting the various process parameters at their optimal levels. Mahapatra and Patnaik (2007) developed relationships between various process parameters and responses like MRR, SR and kerf by means of nonlinear regression analysis and then employed genetic algorithm to optimize the WEDM process with multiple objectives. Parashar (2009) describe the optimization of surface roughness using Taguchi s dynamic design of experiments is proposed for WEDM operations. Experimentation was planned as per Taguchi s L 32 (21 X 44) mixed orthogonal array. Each experiment has been performed under different cutting conditions of gap voltage, pulse ON time and pulse OFF time, wire feed and dielectric flushing pressure. Stainless Steel grade 304L was selected as a work material to conduct the experiments. Finally, experimental confirmation was carried out to identify the effectiveness of this proposed method. Singh and Singh (2012) proposed the wear behavior AISI D3 Die steel in EDM and compare the tool wear rate of cryogenic treated cooper and brass electrode with simple copper and brass electrode on machining of AISI D3 die steel using current setting as 4A and A. The electrolyte is used kerosene oil. Copper electrode is best electrode for high material removal rate. But cryogenic treated copper electrode has very low tool wear as compared to cooper electrode. II. METHODOLOGY In the present research work, Taguchi method using L 1 orthogonal arrays has been employed for the design of experiment. The various control factors along with their symbols shown in Table 1. Pulse on time, pulse off time, peak current, wire tension and wire feed are set as control factors (independent variable) which vary during the experiment. With the help of machine manual, many trail experiments were done to findout the operating range of each factor for selection.design of experiment includes determining controllable factors and the levels to be investigate. From the available literature on WEDM, total five numbers of input parameters were finally selected and range of levels of control factors is shown in Table 1 ISSN Page 40

3 Machining parameters Level Pulse ON time, (µs) Pulse OFF Time, (µs) Peak Current, (IP) Wire Feed, (WF) 2 4 Wire Tension, E (WT) 7 9 Table:2 Parameter according to L1 Orthogional Array Machining Parameter Exp. No T-on (µs) T-off(µs) IP (amp) WT (g) WF (mm/min) III. EXPERIMENTAL SET UP The experiments were carried out on a wire-cut EDM machine ELEKTRA SPRINTCUT 734 WEDM machine of Electronic Machine Tools Ltd. AISI H13 die steel is an important material for tool and dies mostly because of its high hardness, strength and wears resistance over a wide range of temperature. The die steel H13 plate of 200mm x 40mm x 1mm size has been used as a work piece material for the present experiments. Table 3: Chemical Composition of die steel H13 Elements Carbon Silicon Chromium Vanadium Molybdenum Sulphor Iron ISSN Page 41

4 Percentage of elements 0.40% 1.00% 5.30% 1.00% 1.40% 0.30 Balance All the measurements were conducted using Mitutoyo Form tracercs-5000 with5µm stylus tip and 40º of tip angle. The measurement length for each specimen was 20mm and it was divided into three sections. Each section was measured for three times before the average results were obtained. The time consumed for each run in this study was recorded by the machine time indicator. Fig.2 Wire EDM Machine Fig. 3 Specimen cutting under process on WEDM Machine IV. RESULT AND ANALYSIS Exp. No. Table4: Response table corresponded to each run of soft wire electrode T-on (µs) Machining Parameter T- off IP WF WT (mm/ (µs) (amp) min) (gram) R a (µm) Response CS (mm/ min) MRR (g/min) ISSN Page 42

5 The WEDM experiments were conducted by using the parametric approach of the Taguchi s method. The effects of individual WEDM process parameters, on the selected quality characteristics - surface roughness, cutting speed and MRR Studied. The average value and S/N ratio of the response characteristics for each variable at different levels were calculated from experimental data. The main effects of process variables both for raw data and S/N data were plotted. S/N data is carried out to identify the significant variables and to quantify their effects on the response characteristics. The most favorable values (optimal settings) of process variables in terms of mean response characteristics are established by analyzing the response curves. M a i n E f f e c t s P l o t f o r S N r a t i o s D a t a M e a n s - 9 T O N T O F F I P M e a n o f S N r a t io s W F W T S i g n a l - t o - n o i s e : S m a l l e r i s b e t t e r Fig. 4: Main Effect Plot for surface roughness (S/N) for for soft wire electrode Effect on Surface Roughness The S/N ratios for surface roughness are calculated as given in equation. Taguchi method is used to analysis the result of response of machining parameter for smaller is better criteria. S/N ratio for SF= -10 log 10 (LLB) ISSN Page 43

6 fig. 4 shows that the surface roughness increases factor wire feed ( WF). Surface roughness firstly with the increase of pulse on time, and peak current decrease and then increase with increase in factor and decreases with increase in pulse off time. It wire tension (WT). Wire tension (WT) is also shows that the factor is significant. Surface roughness depends upon work piece thickness. firstly increases and then decrease with increase in M a i n E f f e c ts P l o t f o r S N r a ti o s D a ta M e a n s T O N T O F F I P M e a n o f S N r a t io s W F W T S ig n a l- to - n o is e : La r g e r is b e tte r Fig. 5: Main Effect Plot for cutting speed (S/N) for soft wire electrode Effect on cutting speed The S/N ratios for Cutting Speed are calculated as given in equation. Taguchi method is used to analysis the result of response of machining parameter for higher is better criteria. The S/N ratio can be calculated as a logarithmic transformation of the loss function as shown below. S/N ratio for Cutting Speed = -10 log 10 (LHB) Fig.5 shows that the cutting speed increases with the increase of Ton and peak current. It shows that the factor is significant and decreases with increase in pulse off time, wire feed. The wire tension firstly increase then decrease and it is also depends upon work piece thickness. Cutting speed increases with the increase of Ton In this case, T on and wire tension were significant to the cutting speed. For the other factors namely pulse off time, peak current, wire feed, they are less significant. ISSN Page 44

7 M a i n E f f e c ts P l o t f o r S N r a ti o s Da ta M e a n s - 10 T O N T O F F I P M e a n o f S N r a t io s W F W T S ig n a l- to - n o is e : La r g e r is b e tte r Fig.: Main Effect Plot for MRR (S/N) for soft wireelectrode Effect on MRR The S/N ratios for material removal rate (MRR) are calculated as given in equation. Taguchi method is used to analysis the result of response of machining parameter for higher is better criteria Fig. shows that the MRR increases with the increase of T-on, it shows that the factor is significant and decreases with increase in pulse off time, wire tension firstly increase then decrease. In this case, pulse off, pulse on and Wire Feed were significant to the MRR. For the other factors namely Wire Tension and peak current they are less significant. ISSN Page 45

8 Table 5: Response table of each run of soft wire electrode in using Taguchi Method Ex p. N O T- on Machining Parameter T- off IP WF WT R a R a (S/N) CS Response CS (S/N) MRR MRR (S/N) Performance Measures/ Responses Table : Optimal results for soft wire electrode Optimal Set of Parameters Predicated Optimal Value Actual Value (Experiments Error R a Ton-10,TOFF- 50,IP-200,WF-,WT CS Ton-131,TOFF- 50,IP-230,WF-,WT MRR Ton-131,TOFF- 30,IP-220,WF-,WT The optimal sets of process parameters were obtained for various performance measures using Taguchi s design of experiment methodology MINITAB 15. Table 7: Optimal setting of parameters for soft wire electrode ISSN Page 4

9 Exp. no. Performance Measures/ Responses Optimal Set of Parameters 1. Surface roughness(r a ) 2. Cutting Speed (CS) 3. MRR Ton-10,TOFF-50,IP-200,WF-,WT-7 Ton-131,TOFF-50,IP-230,WF-,WT-7 Ton-131,TOFF-30,IP-220,WF-,WT-7 V. CONCLUSION In this study, the relationship between control factors and responses like MRR, R a, and Cutting speed, is established by means of Taguchi s methodology. The material used in this present work is die steel H13 as a work piece and soft brass wire as a tool and also the dielectric fluid used (de-ionized water) as a coolant. Finally, the Analysis of variance, a popular evolutionary approach, is employed to optimize the wire electrical discharge machining process with multiple objectives. The study demonstrates that the WEDM process parameters can be adjusted to achieve better metal removal rate, surface roughness and cutting speed simultaneously. No. 9, pp [9] Parashar, V., Bhagoria, J.L., andpuri,y.m. (2010), Kerfs width analysis for wire cut electro discharge machining of SS 304L using design of experiments, Indian Journal of Science and Technology, Vol. 3, No. 4, pp [10] Pandey and Anand. (2010), Current research trends in variants of Electrical Discharge Machining, International Journal of Engineering Science and Technology, Vol. 2, No., pp [11] Datta, S., Mahapatra, S. (2010), Modeling, simulation and parametric optimization of wire EDM process using response surface methodology coupled with grey-taguchi technique, International Journal of Engineering, Science and Technology,Vol. 2, No. 5, pp [12] Pasam, V., Krishna, B., Surendra. B., and Valli, P. M. (2010), Optimizing surface finish in WEDM using the taguchi parameter design method, International Journal of the Brazilian Society of Mechanical Sciences and Engineering,Vol. 2, No. 5, pp REFERENCE [1] Mahapatra, S. and Patnaik, A. (200), Optimization of wire electrical discharge machining (WEDM) process parameters using Taguchi method, Journal of Adv. Manufacturing. Technology, Vol. 42, No. 2, pp [2] Chiang, K., Chang, F. and Tsai D. (2007), Modeling and analysis of the rapidly re-solidified layer of SG cast iron in the EDM process through the response surface methodology, Journal of Materials Processing Technology,Vol. 12, No. 1, pp [3] Kanlayasiri, K. and Boonmung, S. (2007), An investigation on effects of wire-edm machining parameters on surface roughness of newly developed DC53 die steel, Journal of Materials Processing Technology,Vol , No. 1, pp [4] Chiang, K., Chang, F.and Chang, D. (2007), Tsai Modeling and analysis of the rapidly re-solidified layer of SG cast iron in the EDM process through the response surface methodology, Journal of Materials Processing Technology,Vol. 12,No. 2, pp [5] Bhattacharya, B., Gangopadhyayb, S. andsarkara, B.R. (2007), Modeling and analysis of EDMED job surface integrity, Journal of Materials Processing Technology, Vol. 19, No. 1, pp [] Yan, Mu-Tian. and Fang, C. (200), Application of genetic algorithm-based fuzzy logic control in wire transport system of wire-edm machine, Journal of Materials Processing Technolog, Vol. 12, No. 2, pp [7] Saha, S. K., Srinivasan, K. and Dutta, P. (200), Studies on Optimum Distribution of Fins in Heat Sinks Filled with Phase Change Materials, Journal of Materials Processing Technology, Vol. 13, No.3, pp [].Parashar,V., Rehman,A, andbhagoria, J.L. (2009), Investigation and Optimization of Surface Roughness for Wire Cut Electro Discharge Machining of SS 304L using Taguchi Dynamic Experiments,International Journal of Engineering Studies, Vol. 1, ISSN Page 47