Taguchi Approach for overcut minimization in WEDM for sub-zero treated SKD 11

IJSRD - International Journal for Scientific Research & Development| Vol. 3, Issue 08, 2015 | ISSN (online): 2321-0613

Taguchi Approach for Overcut Minimization in WEDM for Sub-Zero Treated SKD 11 Nishant Sharma1 Virender Kumar2 Department of Mechanical Engineering 1,2 Modern Institute of Engineering and Technology, Mohri, Kurukshetra 1,2

Abstract— Present research devotes the optimization of process parameters of wire-cut electric discharge machining (WEDM) for sub-zero treated SKD11. WEDM are successfully employed for machining of hard to cut materials and also for intricate shape which are impossible by conventional ones. Pulse-on-time, pulse-off-time, servo voltage and wire feed are the input parameter while overcut is the response variable. Confirmation experiments represent good agreement with the predicted results.

Key words: Sub-Zero Treated SKD 11, WEDM I. INTRODUCTION Wire cut Electrical-discharge machining (WEDM) has been an important manufacturing process for the tool, mould, automobile and die industries. Due to the ability to make intricate shape and machining of hard material with WEDM, its use is increasing. WEDM has capability to machine any kind of electrically conductive work in the present day. The mechanism of WEDM constitutes the erosion of material due to discrete spark discharge between wire tool and job immersed in a liquid dielectric medium. The microprocessor also constantly maintains the gap between the wire and the work piece, which varies from 0.025 to 0.05 mm. Very high frequency pulses are generated with the help of DC power supply. The electrical discharge melts or erodes the material in a very small amount which is flushed away by dielectric.

The work piece and wire electrode are separated by deionised water. The deionized water works as dielectric fluid and flushes out the eroded or melted material. WEDM is rarely able to achieve optimal performance due to large number of variables and their stochastic nature. This problem can be solved by determining the relationship between performance of the process and its input parameters using designed experiments.

In 1991 Williams and Rajurkar [1] observed that the complex and random nature of the erosion process in WEDM requires the application of deterministic as well as stochastic techniques. They present the results of their investigations into the characteristics of WEDM generated surfaces. Some industries like aerospace, automotive and electronic have used this process in their production line to improve wear resistance and dimentional stability of components [2]. Rajurkar and Wang [3] developed WEDM sparking frequency monitor to detect the thermal load for on-line control to prevent the wire from rupture. The wire rupture phenomenon is also analyzed with a thermal model.

Newman et al. [4] found that WEDM is a specialized thermal machining process capable of accurately machining parts with varying hardness or complex shapes. Beltrami et al. [5] work deals with a technical realization to improve the WEDM accuracy while cutting at full speed on virtually any contour. The system which is readily available on commercial WEDM is based on the on-line monitoring and control of the wire position by means of an optical sensor.

The deviation of the wire position relative to the

programmed wire path position is continuously measured and corrections are made during the machine cutting.

Mamalis et al. [6] reported in their research that requirements of the materials used for WEDM electrodes that will lead to the improvement of WEDM performance.

Liao et al. [7] proposed an approach of determining the parameter settings based on the Taguchi quality design method and the analysis of variance. The results showed that the MRR and SF are easily influenced by the table feed rate and pulse on-time, which can also be used to control the discharging frequency for the prevention of wire breakage.

Sharma et al. [2013a, 2013b] investigated the effect of process parameter on cutting rate in WEDM using response surface methodology (RSM). In this study the high strength low alloy steel is used as a work material. It was found that the cutting rate increases with increase in pulse on time and peak current. The cutting rate decreases with increase in pulse of time and servo voltage and the wire mechanical tension has no significant effect on cutting rate.

Gupta et al. (2012) investigated the effect of parameters on Overcut for WEDM using HSLA as work piece. They revealed that kerf width decreases with increase in pulse on time, pulse off time, spark gap voltage and peak current.

Khanna and Singh (2013) optimized the process parameters of WEDM while machining D-3 tool steel.

Sharma et al. (2015) investigated the recast layer and unmachined area of NiTi alloy on WEDM. It has been found that some un-machined area observed which can be minimized or eliminated by trim cutting. A number of researchers (Sharma et al., 2014a; 2014b; 2014c, Jangra et al., 2014) utilized planning of experimentation to reduce the experiments and analysis the process parameters.

II. EXPERIMENTAL SET-UP A Wire EDM machine (Sprintcut ELPULS 40A DLX) was used as the experimental machine in this study. A brass Wire with a diameter of 250 µm was used as an electrode to erode a work piece of Sub-zero treated SKD11 steel (flat plate). Table 1 shows the Chemical Composition of SKD11 steel (Weight By %). The SKD11 steel plate 300 x 140 x 28 mm size has been used as a work piece material for the present experiments. This SKD11 steel plate is sub-zero treated before experiments. The sub-zero treatment is the process of treating work-pieces to sub-zero temperatures (i.e., below –100°C) to remove residual stresses and improve wear resistance on steels. Table 2 shows the ranges of significant process parameters. Apart from the parameters mentioned above following parameters were kept constant at a fixed value during the experimentation  Work piece: SKD11 STEEL  Electrode(tool) : 0.25mm Ø, Brass wire  Work piece height : 28mm  Cutting length : 7mm  Dielectric Conductivity: 20mho  Dielectric temperature : 20-240C

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Taguchi Approach for Overcut Minimization in WEDM for Sub-Zero Treated SKD 11 (IJSRD/Vol. 3/Issue 08/2015/102)

The sub-zero treated SKD11 is used for the treated SKD11 steel provides better mechanical properties experimentation. This is mounted on Electronica Sprintcut or greater resistance to wear. These are used to make mould, WEDM machine tool. Figure 1 shows the specimens cut dies etc. during experimentation work on WEDM. The Sub-zero C P S Si Mn Cr Mo Cu Ni V Fe 1.4-1.6 0.02 - 0.03 0.01-0.03 0.2-0.4 0.4-0.6 11-13 0.8-1.2 ≤0.25 ≤0.25 0.2-0.5 balance Table 1: Chemical Composition of SKD11 steel (Weight By %) Real Parameter Lower Upper Unit factor Name Limit Limit Pulse on 107 122 Ton µs Time Pulse off 30 45 Toff µs time Servo 30 60 SV V Voltage 5 11 WF Wire Feed m/min Table 2: Optimal ranges of significant process parameters Fig. 1: Specimens cut during Experimentation Main Effects Plot for Means Data Means

Ton

SV

0.25 0.20

Mean of Means

0.15 0.10 0.05 107

112

117

122

9

11

30

40

50

60

Wf 0.25 0.20 0.15 0.10 0.05 5

7

Fig. 2: Effects of Process Parameters on Overcut (Means) controlled parameter is on the quality characteristic of interest can be obtained. The total variation in the result is III. RESULTS AND DISCUSSIONS the sum of variation due to various controlled factors and The percentage contribution of various process parameters their interactions and variation due to experimental error. on the selected performance characteristic can be estimated by performing analysis of variance test (ANOVA). Thus, information about how significant the effect of each Residual Plots for Means Versus Fits

99

0.050

90

0.025

Residual

Percent

Normal Probability Plot

50 10 1

-0.050

-0.025

0.000 0.025 Residual

0.000 -0.025 -0.050

0.050

0.0

0.1 0.2 Fitted Value

Histogram

Versus Order 0.050

2

Residual

Frequency

3

1 0

0.3

-0.04

-0.02

0.00 0.02 Residual

0.04

0.025 0.000 -0.025 -0.050

1

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7

8

9 10 11 12 13 14 15 16

Observation Order

Fig. 3: Residual Plots for Means (Overcut)

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Taguchi Approach for Overcut Minimization in WEDM for Sub-Zero Treated SKD 11 (IJSRD/Vol. 3/Issue 08/2015/102)

Source Ton SV Wf Residual Error

DF 3 3 3

Seq SS 0.113693 0.007068 0.002763

Adj MS 0.037898 0.002356 0.000921

6

0.010510

0.001752

F 21.64 1.35 0.53

P 0.001 0.345 0.681

S = 0.04185 R-Sq = 92.2% R-Sq(adj) = 80.4% Table 3: Pooled Analysis of Variance for Means (Oc) Level Ton SV Wf 0.03042 0.16375 0.12542 1 0.07708 0.12417 0.14292 2 0.15250 0.11458 0.10833 3 0.25333 0.11083 0.13667 4 0.22292 0.05292 0.03458 Delta 1 2 3 Rank Table 4: Response Table for Means (Oc) The ANOVA for Means data are given in Tables 3.

The most favorable conditions or optimal levels of process parameters have been established by analyzing response curves of S/N ratio associated with the raw data.

It can be observed from Figure 2 that the most effective parameter with respect to overcut is pulse on time.

The most significant factor is pulse on time where as the effect of servo voltage and wire feed on the overcut was also significant. Moreover, the different input parameters used in the experimentation can be ranked in the order of increasing overcut as servo voltage, pulse off time, wire feed and pulse on time. From the Figure 1, the optimal overcut has been recorded with pulse on time (at level 1), servo voltage (at level 4), and wire feed (at level 3). In wire electric discharge machining of SKD11, the pulse on time is more significant factor for optimizing the overcut as compared to the other parameter i.e. servo voltage and wire feed with regard to overcut.

In order to estimate the contribution of each factor towards the variation of machining performance in terms of Overcut for wire electrical discharge machining of SKD11, Analysis of Variance test was conducted on the results obtained from the experimentation. The ANOVA test summary for Overcut has been recorded for both the average response (Table 4). Hence the Overcut is influenced by the pulse on time, servo voltage and wire feed which has also been suggested by many other researchers. It can also be concluded that the Overcut increases with the increase in Wire feed and Pulse on time.

The analysis of results showed that "A1C4D3" is the optimal parameter setting for the optimization of cutting rate. Hence, it can be concluded from this discussion that "input parameters settings Pulse on time at 107, servo voltage at 60 Vand Wire Feed at 9m/min have given the optimum results for Overcut; when SKD11 was machined with WEDM".

Total

15

0.134033

A. Overcut (Oc) The optimum value of Oc is predicted at the optimal levels of significant variables which have already been selected as pulse on time (A1), servo votage (C4) and wire feed (D3).

The estimated mean of the response characteristic (Oc) can be determined as µOc = A1 + C4 + D4– 2T

Where, T = overall mean of overcut = (ΣR1+ΣR2+ΣR3)/48 =0.128 mm.

Where, R1, R2 and R3 values are taken from the Table 5.4, and the values of A1, C4 and D3 are taken from the Table 5.20.

A1 = average value of overcut at the first level of pulse on time = 0.03042 mm.

C4 = average value of overcut at the fourth level of servo voltage = 0.11083 mm.

D3 = average value of overcut at the third level of wire feed = 0.10833 mm.

Substituting the values of various terms in the above equation, µOc = 0.03042 + 0.11083 + 0.10833 – 2 (0.128) = 0.007 mm.

The value 95 % confidence intervals of confirmation experiments (CICE) are Zero, Due to Zero Residual Errors.

Where, Fα (1, fe) = The F ratio at the confidence level of (1α) against DOF 1 and error degree of freedom fe.

= 48/ (1+6) = 6.85 Ve = Error variance= 0.001752; fe = error DOF = 6; F0.05 (1, 6) = 5.99 (Tabulated F value) CICE = ±0.07; and CIPOP = ± 0.039 The optimal values of process variables at their selected levels are as follows:  First level of pulse on time (A1) : 107 µs  Fourth level of spark gap set voltage (C4) : 60 V  Third level of wire feed (D3): 170 m/min B. Confirmation Experiment In order to validate the results obtained, confirmation experiments were conducted for each of the response characteristics (Oc) at optimal levels of the process variables. The average values of the characteristics were obtained and compared with the predicted values. The results are given in Table 5. It is to be pointed out that these optimal values are within the specified range of process variables. Any extrapolation should be confirmed through additional experiments.

Predicted Confiden Performan Optimal Predicte Actu ce ce Set of d al Intervals Measures/ Paramete Optimal Value at 95% Responses rs Value confidenc e level

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Taguchi Approach for Overcut Minimization in WEDM for Sub-Zero Treated SKD 11 (IJSRD/Vol. 3/Issue 08/2015/102)

CIPOP : 0.031 < μOc <0.109; 0.009 0.007 Overcut A1C4D3 mm CICE mm :0.063 < μOc <0.077 Table 5: Predicted Optimal Values and Results of Confirmation Experiments IV. CONCLUSIONS Pulse on time had the maximum effect on overcut followed by servo voltage and wire feed. Optimized setting suggest that first level of pulse on time, forth level of servo voltage and third level of wire feed gives minimum overcut equivalent to 0.007mm, while after setting the process parameter during the confirmation experiment the actual value found to be 0.009mm. So, with the help of Taguchi approach good results reproduced.

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