Journal of Engineering and Technology

ARTICLE
Year
: 2011  |  Volume : 1  |  Issue : 2  |  Page : 59--64

Parametric Optimization of Cryogenic-Treated D-3 for Cutting Rate in Wire Electrical Discharge Machining


Hari Singh1, Rajesh Khanna2,  
1 Department of Mechanical Engineering, NIT, Kurukshetra, Haryana, India
2 Department of Mechanical Engineering, M M Engg College Mullana, Ambala, India

Correspondence Address:
Hari Singh
Department of Mechanical Engineering, NIT, Kurukshetra, Haryana
India

Abstract

Cryogenic treatment (DQCryoDQ) is a supplementary process to improve the properties of metals like high carbon high chromium alloy tool steels (D-3) which are increasingly used in manufacturing high-performance cutting tools (dies and punches), blanking and punching tools, extrusion tools, parts of aerospace and automotive industries, etc. The purpose of this study is to investigate the effect of parameters like pulse width, time between two pulses, maximum feed rate, servo reference mean voltage, short pulse time, and wire mechanical tension, on cutting rate (cr) of cryogenic-treated D-3 in wire electrical discharge machining. An L27 orthogonal array has been used to conduct experiments and statistically evaluate the experimental data by analysis of variance (ANOVA). It is seen that cr decreases with increase in pulse width, time between two pulses, and servo reference mean voltage. cr first decreases and then increases with increase in wire mechanical tension. The confirmation experiments have also been conducted to validate the results obtained by Taguchi technique.



How to cite this article:
Singh H, Khanna R. Parametric Optimization of Cryogenic-Treated D-3 for Cutting Rate in Wire Electrical Discharge Machining.J Eng Technol 2011;1:59-64


How to cite this URL:
Singh H, Khanna R. Parametric Optimization of Cryogenic-Treated D-3 for Cutting Rate in Wire Electrical Discharge Machining. J Eng Technol [serial online] 2011 [cited 2019 Aug 21 ];1:59-64
Available from: http://www.onlinejet.net/text.asp?2011/1/2/59/86633


Full Text

 1. Introduction



Electrical discharge wire cutting, more commonly known as wire Electrical-discharge machining (WEDM), is a spark erosion process used to produce two- and three-dimensional shapes through electrically conductive workpieces by using a wire electrode. WEDM has been an important manufacturing process for the tool, mould, and die industries. It is now increasingly used owing to its ability to produce geometrically complex shapes, as well as its ability to machine hard materials that are extremely difficult to machine using conventional processes. In the 1930s and 1940s, it was shown that this treatment can improve the performance of the tool steel [1] . Several investigators [2],[3] have focused their attention on studying this process and trying to raise the efficiency of tool steels through cryogenics. Most researchers [4],[5] agree that cryogenic treatment can improve the performance of the tools. Improvement of the wear resistance of the tool steels was the most significant effect of this treatment. Some industries like aerospace, automotive, and electronic have used this process in their production line to improve wear resistance and dimensional stability of components [6] . Manna and Bhattacharyya [7] found open gap voltage and pulse on period as the most significant machining parameters for controlling the metal removal rate using Taguchi method-based analysis for WEDM on Al/SiCMMC. Ramakrishnan and Karunamoorthy [8] reported the development of artificial neural network (ANN) models and multiresponse optimization technique to predict and select the best cutting parameters in WEDM process.

Scott et al. [9] used a factorial design requiring a number of experiments to determine the most favorable combination of the WEDM parameters. They found that the discharge current, pulse duration, and pulse frequency are the significant control factors affecting the MRR and SF, while the wire speed, wire tension, and dielectric flow rate have the least effect. Liao et al. [10] 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.

Rajurkar and Wang [11] analyzed the wire rupture phenomenon with a thermal model. An extensive experimental investigation has been carried out to determine the variation of machining performance outputs, namely MRR and surface finish, with machining parameters in the study. Spedding and Wang [12] optimized the process parameter settings by using ANN modeling to characterize the WEDM workpiece surfaces.

Barron [13] concluded that cryogenic process uses sub zero temperature down to -196°C in a super-cooled bath containing liquid nitrogen and is used for treating wide range of metal components including hot die steel.

Kamody [14] reported a process for the treatment of materials to improve stability, shockability and hardness, and extended wearability. Molinari et al., [15] in their study, reported that cryogenic treatment improves the surface hardness and thermal stability of the materials. Kamody [16] describes the effect of cryogenic treatment to minimize the instability effects of workpiece, which results in regulation and compensation of wire burning action.

The review of literature indicates that there is limited published work on the effect of machining parameters on cutting rate (cr) in WEDM in a cryogenic cutting environment. In this study, the effect of the machining parameters and their level of significance on the cr are statistically evaluated by using analysis of variance (ANOVA). Experiments were conducted to render best cr while machining cryogenically treated D-3 material.

 2. Experimental Methodology



Cryogenic processor (CP200LH), as shown in [Figure 1], was used for cryogenic treatment of workpiece. The different sets of experiments were performed using a Robofil 290 Charmilles Technologies WEDM machine, as shown in [Figure 2]. During the experiments, the cr of the workpiece was measured. The work material, electrode, and the other machining conditions are as follows:{Figure 1}{Figure 2}



Workpiece: High carbon high chromium alloy tool steel (D-3)Electrode (tool): 250 μm φ, CuZn37 Master Brass wire (900 N/mm 2 tensile strength)Workpiece height: 30 mmDielectric conductivity: 20 mhoCutting voltage (V): 80 VDielectric temperature: 22-25°CInjection pressure set point was at 4 (around 6.5 bars)Ignition pulse current (IAL) at 8 Amp.

2.1 Deep Cryogenic Treatment



D-3 material as shown in [Figure 3] used in WEDM is placed in cryogenic processor (CP200LH).{Figure 3}In ramp, down stage temperature is decreased at the rate 0.39°C per minute from room temperature (25°C Aprox.).Temperature is decreased up to -184°C in 9 hours in cryogenic processor.A typical soak segment will hold the temperature -184°C for a period of 18 hours.Temperature is increased at the rate of 0.39°C per minute in ramp up stage for a period of 9 hours.Temperature is bought to room temperature (25°C Aprox.).

An orthogonal array L27 (313 ) has been employed according to the Taguchi method based robust design philosophy to evaluate the main influencing factors that affect the cr. A set of six WEDM parameters with three levels for control factors, such as factor A (pulse width), factor B (time between two pulses), factor A j (servo reference mean voltage), factor T ac (short pulse time), factor S (maximum feed rate), and factor W b (wire mechanical tension), are considered as the controlling factors for optimal analysis during machining of D-3. [Table 1] shows the various control factors and their levels selected while experimenting.{Table 1}

 3. Results and Discussions



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 characteristic, cr, have been discussed in this section. The average value and S/N ratio of the response characteristic for each variable at different levels were calculated from experimental data. The analysis of variance (ANOVA) of raw data and S/N data is carried out to identify the significant variables and to quantify their effects on the response characteristic. The most favorable values (optimal settings) of process variables in terms of mean response characteristic were established by analyzing the response curves and the ANOVA Tables.

The experimental data are given in [Table 2]. The average values of cr for each parameter at levels 1, 2, and 3 for raw data and S/N ratios are plotted in [Figure 4] and [Figure 5]. It is seen that cr decreases with increase in pulse width, time between two pulses, and servo reference mean voltage. cr first decreases and then increases with increase in wire mechanical tension.{Figure 4}{Figure 5}{Table 2}

3.1 Selection of Optimal Levels

In order to study the significance of the process variables toward cr, analysis of variance (ANOVA) was performed as shown in [Table 3] and [Table 4]. It was found that maximum feed rate and wire mechanical tension are nonsignificant process parameters for cr. Nonsignificant parameters were pooled and the pooled versions of ANOVA of the S/N data and the raw data for cr are given in [Table 5] and [Table 6], respectively. From these tables, it is observed that pulse width, time between two pulses, servo reference mean voltage, and wire mechanical tension significantly affect both the mean and the variation in the cr values. Time between two pulses has the greatest effect on cr and is followed by servo reference mean voltage, pulse width, and wire mechanical tension in that order. As cr is the "larger the better" type quality characteristic, from the experiments, it can be seen from the [Table 7] that the first level of pulse width (A 1 ), first level of time between two pulses(B 1 ), first level of servo reference mean voltage (A j)1 , and first level of wire mechanical tension (W b ) 1 provide maximum value of cr. The S/N ratio analysis also suggests the same levels of the variables [A 1 , B 1 , (A j ) 1 and (W b ) 1 ] as the best levels for maximum cr in WEDM process.{Table 3}{Table 4}{Table 5}{Table 6}{Table 7}

3.2 Optimum Value of Cutting Rate

The optimum value of cr is predicted at the selected levels of significant variables: Pulse width (A 1), time between two pulses (B 1 ), servo reference mean voltage (A j ), and short pulse time (T ac) [Table 6]. The estimated mean of the response characteristic (cr) can be determined as:

[INLINE:1]

Where, [INSIDE:1] = overall mean of cutting rate = (ΣR 1 + ΣR 2 + ΣR 3 )/81 = 1.4975mm/min

Where, R 1 , R 2 , and R 3 values are taken from the [Table 2], and the values of [INSIDE:2], and (T ac)3 are taken from the Taguchi's experimental data.

[INSIDE:3] =average value of cutting rate at the first level of pulse width=1.578 mm/min

[INSIDE:4] = average value of cutting rate at the first level of time between two pulses=1.789 mm/min

[INSIDE:5] =average value of cutting rate at the first level of servo reference mean voltage=1.644 mm/min

(W b ) 1 =Average value of cutting rate at the third level of wire mechanical tension=1.604 mm/min

Substituting the values of various terms in the above equation,

μ cr =1.578+1.789 + 1.644 + 1.604-3 (1.4975) = 2.1225 mm/min

The 95% confidence intervals of confirmation experiments (CI CE) and population (CI POP) are calculated as below:

[INLINE:2]



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

[INLINE:3]

N = Total number of results=27 × 3=81, R=Sample size for confirmation experiments = 3

V e =Error variance=0.01523; f e =error DOF=18 [Table 6]

F 0.05 (1, 18) = 4.4139 (Tabulated F value (Ross, 1996))

So, CI CE = ± 0.1728, and CI POP=± 0.08642

Therefore, the predicted confidence interval for confirmation experiments is:

Mean μ cr -CI CE < μcr < Mean μcr +CI CE i.e. 1.9497 < μcr <2.2953

The 95% confidence interval of the population is:

Mean μ cr -CI POP< μ cr < Mean μ cr +CI POP i.e. 2.03608 < μ cr < 2.20892

The optimal values of process variables at their selected levels are as follows:

(A 1 ): 0.8 machine units; (B 1 ): 6.6 machine units; (A j ) 1 : 34; (W b ) 1 :0.8.

 4. Confirmation Experiment



Conducting a verification experiment is a crucial, final, and indispensable part of the Taguchi method-oriented robust design project. Its aim is to verify the optimum condition suggested by the matrix experiment estimating how close are the respective predictions with the real ones. However, if the observed S/N ratios under the optimum conditions differ drastically from their respective predictions, the additive model proves to be a failure eventually. The S/N values were predicted under the optimum condition for the above case study. Also, S/N values were estimated from the machining results under optimum parametric settings. The results are tabulated in [Table 8]. It is clear that the data agree very well with the predictions. Therefore, the optimum settings given in [Table 8] may be adopted and implemented accordingly.{Table 8}

 5. Conclusion



Influences of WEDM machining variables on cr of newly developed cryogenic-treated high carbon high chromium alloy tool steel (D-3) were investigated in this paper. The machining variables included pulse width, time between two pulses, servo reference mean voltage, short pulse time, maximum feed rate, and wire mechanical tension. The variables affecting the cr significantly were identified using ANOVA technique. Results showed that time between two pulses, servo reference mean voltage, pulse width, and wire mechanical tension were significant variables to the cr of wire-EDMed D-3, high carbon high chromium alloy tool steel. The cr decreases with increase in pulse width, time between two pulses, and servo reference mean voltage. cr first decreases and then increases with increase in wire mechanical tension. A 1 -0.8 units, B 1 -6.6 units, A j -34 V, and W b -0.8 is the optimized setting of machine tool for the present study.

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