Study of process parameters on wire electric discharge grinding

ISSN - 0973 - 1334, National Journal of Technology, Vol 12, Issue 3, September 2016 Study of process parameters on wire electric discharge grinding Krishnaraj V*, Parthiban M, Sindhumathi R PSG College of Technology, Coimbatore, India *Corresponding author: vkr@mec.psgtech.ac.in ABSTRACT This paper reports the effects of machining parameters on surface roughness in cylindrical wire electrical discharge grinding (WEDG) process. In this research tungsten carbide is used as workpiece. A rotary axis spindle is set up in the electric discharge machine in a submerged dielectric environment so as to machine cylindrical parts. The speed of the spindle is maintained constant at 200 rpm. Experiments have been under different process conditions like peak current, voltage, pulse off time and number of passes. Number of passes in the wire electric discharge grinding decreases the surface roughness. Keywords: WEDG, Rotary axis spindle unit, Tungsten carbide, Quality of the workpiece INTRODUCTION Manufacturing micro parts using tungsten carbide with tight tolerances is important to many applications. In order to produce cylindrical micro parts machining process like wire electric discharge grinding is preferred. Many researchers contributed to the development of wire electric discharge grinding [1-5]. The surface roughness and material removal rate of the workpiece during wire electric discharge grinding were reported by few researchers [1,4,5] and kerf width [2], cutting speed [3] were also considered as performance measure of the wire electric discharge grinding. From their study, it found that the significant machining parameters affecting the performance measures were discharge current, pulse duration, pulse frequency, wire speed, wire tension and dielectric flow. Krishnan and Samuel [4] analysed the generation of cylindrical form of difficult to machine materials in WEDG. They also tried to optimize the process parameters considering the MRR and surface roughness. Rees et al.[5] also studied the effects of various factors which affect the surface finish of the workpieces when machined by wire electrical discharge grinding process. The various parameters which are studied for surface finish optimization are spindle speed, open circuit voltage, flushing pressure of the dielectric, pulse on time and pulse off time. From the study, it is found that open 50 circuit voltage and pulse off time have a significant effect on the surface finish. Zhang et al [6] investigated using tangential feed WEDG method to improve the machining accuracy of micro electrodes. The accuracy of micro electrodes produced were less than 2 microns. Matoorian et al [7] carried out an investigation by considering the influence of six design factors such as, intensity supplied by the generator of the wire electric discharge grinding, pulse-on time, voltage, pulse-off time, servo, and rotational speeds are the influential parameters to be monitored by the WEDG process on MRR. Uhlmann et al [8] reported that the wire electric discharge grinding process can be influenced by the rotational movement of the workpiece. They concluded that the size of the discharge craters decreases with a growing circumferential speed and the removal rate can be increased by raising the circumferential speed with higher discharge energies. Material removal rate is important during roughing where as during finishing process surface finish and less diameter deviation are important. Alias et al.[9] analysed kerf width, MRR, Ra and surface topography based on different feed rates and current. Mohammadi et al.[10]investigated effect of ultrasonic vibration combined with wire electric discharge turning to identify the influence of power, pulse off time, spindle speed and ultrasonic vibration ISSN - 0973 - 1334, National Journal of Technology, Vol 12, Issue 3, September 2016 over MRR and cylindrical parts. Song et al. [11] fed a strip electrode continuously to increase the material removal rate. Yang et al. [12] analysed the surface integrity, MRR, roundness and corner deviation by considering the process parameter like wire feed rate and part rotational speed in brass and carbide workpiece during machining. The effects of rotational speed, pulse off time, pulse on time, wire speed and spark gap on material removal rate and surface roughness in the fields of manufacturing the cylindrical parts using wire EDM were studied and also the recast layer and heat affected zone (HAZ) with surface roughness and MRR [13,14]. Our work is focused on manufacture of single point tool of ø0.4 mm using tungsten carbide work material by wire electric discharge grinding by considering surface finish and diameter deviation. 2.0 EXPERIMENTAION The workpiece used for the experimentation is tungsten carbide rod of dimensions ø2.2 mm x 30 mm (DxL) to produce micro tool of ø 0.4 mm. The WEDG were carried out on a MITSHIBISHI ADVANCE FA10S machine. The rotary axis spindle is submerged in dielectric environment (deionised water). The photograph of the experimental setup is shown in Fig.1 2.1 Effect of process parameters on surface roughness 3 levels each). The Experiments were conducted using the process parameters such as input current (Ip), voltage (V), pulse off time (Toff), and number of passes (n). The process parameters have been selected to attain better finish[15,16]. Table 1 presents the process parameters and surface roughness (Ra) measured after the experimentation. Table 1. Effect of process parameter on surface roughness Exp. No 1 2 3 4 5 6 7 8 9 Input current (Ip) Amps 6 6 6 3 3 3 2 2 2 Voltage (V) Volts 3 5 6 3 5 6 3 5 6 Pulse off time (Toff) µs 8 11 7 11 7 8 7 8 11 No. of passes (n) Ra (µm) 1 2 3 3 1 2 2 3 1 1.87 1.52 1.84 1.32 1.68 1.31 1.42 1.08 1.52 To determine the optimal combination for Ra, the mean S/N ratios for each level of the parameters are calculated and a graph is produced using the mean S/ N ratio. The greater S/N ratio represents the optimal level of the related parameter in the graph [17]. The strategy is to minimize the response, the S/N ratio is found as follows. To obtain the optimal cutting performance, the smaller the better quality characteristic (Eq.1) for surface roughness were adopted and estimated using MINITAB 16 software. = S/N = -10 log (1/n n i=1 yi 2 ) (1) Fig.2 shows that input current (Ip), voltage(V), pulse off time (Toff) and number of passes (n) have effects on surface roughness. Few researchers [18,19,20] reported the most influencing factors which yield the minimum surface roughness during machining. Table 2 presents the optimum level of each factor for getting higher surface finish. The L9 orthogonal array chosen has 9 rows corresponding to the number of experiments (4 factors with 51 ISSN - 0973 - 1334, National Journal of Technology, Vol 12, Issue 3, September 2016 Fig.2. Effects of control factors on surface roughness Table 2. optimum cutting condition for surface roughness Input current (Ip) Amps Voltage (V) Volts 2 5 Pulse of time (Toff) µs 8 No.of passes (n) 3 With the results of ANOVA (Table 3) the input current (Ip) is identified as the most significant parameter with the P value of 0.095 followed by number of passes and pulse off time. 52 Table 3. Analysis of variance for surface roughness Factors Input current (Ip) Amps Voltage (V) Volts Pulse off time (Toff) µs No.of passes Error Total (Error) DoF Sum of square Mean square F ratio P value 2 0.266 0.133 4.46 0.095 2 *0.029 0.015 0.49 0.643 2 *0.089 0.045 1.50 0.325 2 0.151 0.078 2.63 0.186 0 8 (4) 0 0.536 (0.119) (0.0298) ISSN - 0973 - 1334, National Journal of Technology, Vol 12, Issue 3, September 2016 Figure 3 presents the factor and their contribution for the better surface finish. 2.1 Diameter deviation Fig.5. Diameter measurement points Fig.3 Factors and their contribution for surface roughness The carbide rod of ø2.2 mm machined with single pass and the parts machined with three passes are presented in Fig.4. Small input current and three passes yield better surface finish. To identify the diameter deviations, the number of passes up to three (1,2,3) have been selected. Fig.5 shows the measured diameter points from start to end position of the machined carbide pin. The carbide tool of ø 0.4mm has been machined using the WEM process and the machined tool has been measured at four places. Fig.6. Diameter deviation of each experiment Fig.4 a) minimum and b) maximum number of passes The measured values are subtracted from the nominal diameter and its deviation has been shown in Fig. 6. The experimental condition 4 gives less diameter deviations compare to all experiments. The diameter deviation could be based on the control factors. The parameters which give less deviation are input current (3amps), voltage (3V), pulse off time (11µs) and number of passes (3). 53 ISSN - 0973 - 1334, National Journal of Technology, Vol 12, Issue 3, September 2016 2.1 Manufacturing of micro single point cutting tool • The Figure 7 shows the micro single point cutting tool manufactured using the optimum parameter settings on the carbide tool of ø 0.4 mm. REFERENCES Based on the optimum conditions, the input current (3amps) and Voltage (3Volts), highest pulse off time (11µs), and maximum number of passes (3), a single point cutting tool has been fabricated. [1] M.J. Haddad and A.F. Tehrani, “Investigation of cylindrical wire electrical discharge turning (CWEDT) of AISI D3 tool steel based on statistical analysis,” Journal of material processing technology, vol.198, pp.77-85, 2008. [2] S.S. Mahapatra and A. Patnaik, “Optimization of wire electric discharge machining (WEDM) process parameters using Taguchi method,” International journal of advanced manufacturing technology, vol.34, pp.911-925, 2006. [3] N. Tosun, “The effect of the cutting parameters on performance of WEDM,” KSME international journal, vol.17, pp.816-824, 2003. [4] A. Krishnan and G L. 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Micro single point cutting tool 3.0 CONCLUSIONS From the experimental study, the following conclusions are drawn. • The effects of input current (Ip), voltage (V), pulse off time (Toff), servo voltage (Sv) and number passes were experimentally investigated to obtain better surface finish and minimum diameter deviation. • The better surface finish has been achieved based on optimum cutting conditions are input current (2amps), voltage (5V), pulse off time (8µs) and number of passes (3). • The parameters for getting less diameter deviations are, input current (3amps), voltage (3V), pulse off time (11µs) and number of passes (3). • The process parameters such as input current (23amps), number of passes (3), and pulse off time (8-11µs) yield better quality parts. 54 A micro single point cutting tool is manufactured using optimum conditions. ISSN - 0973 - 1334, National Journal of Technology, Vol 12, Issue 3, September 2016 [9] A. Alias, B. Abdullah and N. M. 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