IJSTE - International Journal of Science Technology & Engineering | Volume 4 | Issue 5 | November 2017 ISSN (online): 2349-784X
Predicting the Machining Performance of in-situ of Aluminium and Silicon Carbide Composite Using Wire EDM Technique N. Magesh Assistant Professor Department of Mechanical Engineering Kings College of Engineering, Punalkulam, Thanjavur
R. Arun Assistant Professor Department of Mechanical Engineering Kings College of Engineering, Punalkulam, Thanjavur
Abstract Wire-cut electrical discharge machining (WEDM) is one of the most emerging non-conventional manufacturing processes for machining hard to machine materials and intricate shapes which are not possible with conventional machining methods. This paper focuses on prediction and comparison of machining performance during Wire Electric Discharge Machining process of Aluminium- SiC in-situ composite to achieve minimum surface roughness. The control factors considered for this study are speed, wire feed, Pulse On-Time and Pulse Off-Time. The Universally known statistical software Minitab 16 is used for the research. The process parameters have been selected based on Taguchi’s L27 orthogonal array. The research reveals the better characteristics of Aluminium composite which a good scope in aerospace application. The predicted machining characteristics are in good agreement with the experimental values. Further the research can extend for different process parameters and various levels for the better optimal machining characteristics. Keywords: Wire EDM, Aluminium, Silicon carbide, Taguchi’s L27 array, Surface roughness ________________________________________________________________________________________________________ I.
INTRODUCTION
Electrical discharge machining (EDM) is a non-traditional, thermoelectric process which erodes material from the work piece by a series of discrete sparks between a work and tool electrode immersed in a liquid dielectric medium. These electrical discharges melt and vaporize minute amounts of the work material, which are then ejected and flushed away by the dielectric. The sparks occurring at high frequency continuously & effectively remove the work piece material by melting & evaporation. The dielectric acts as a deionising medium between 2 electrodes and its flow evacuates the resolidified material debris from the gap assuring optimal conditions for spark generation. In micro wire EDM metal is cut with a special metal wire electrode that is programmed to travel along a pre-programmed path. A wire EDM generates spark discharges between a small wire electrode (usually less than 0.5 mm diameter) and a work piece with deionized water as the dielectric medium and erodes the work piece to produce complex two- and three dimensional shapes according to a numerically controlled (NC) path. The wire cut EDM uses a very thin wire 0.02 to 0.3 mm in diameter as an electrode and machines a work piece with electrical discharge like a band saw by moving either the work piece or wire, erosion of the metal utilizing the phenomenon of spark discharge that is the very same as in conventional EDM . The prominent feature of a moving wire is that a complicated cut-out can be easily machined without using a forming electrode .Wire cut EDM machine basically consists of a machine proper composed of a work piece contour movement control unit ( NC unit or copying unit), work piece mounting table and wire driven section for accurately moving the wire at constant tension ; a machining power supply which applies electrical energy to the wire electrode and a unit which supplies a dielectric fluid ( distilled water) with constant specific resistance. The main goals of WEDM manufacturers and users are to achieve a better stability and higher productivity of the WEDM process, i.e., higher machining rate with desired accuracy and minimum surface damage. However, due to a large number of variables and the stochastic nature of the process, even a highly skilled operator working with a state-of-the-art WEDM is unable to achieve the optimal performance and avoid wire rupture and surface damage as the machining progresses. Although most of the WEDM machines available today have some kind of process control, still selecting and maintaining optimal settings is an extremely difficult job. The lack of machinability data on conventional as well as advanced materials, precise gap monitoring devices, and an adaptive control strategy that accounts for the time-variant and stochastic nature of the process are the main obstacles toward achieving the ultimate goal of unmanned WEDM operation. Many researches devised an approach to determine machining parameter settings for wire edm process .Based on the taguchi quality design and the analysis of variance (ANOVA), the significant factors affecting the machining performance such as MRR, gap width ,surface roughness ,sparking frequency ,average gap voltage, normal ratio(ratio of normal sparks to total sparks) are determined. By means of regression analysis, mathematical models relating the machining performance and various machining parameters are established. Based on the mathematical models developed, an objective function under the multi-constraint conditions is formulated. The optimization problem is solved by the feasible direction method, and the optimal machining parameters are obtained. Experimental
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Predicting the Machining Performance of in-situ of Aluminium and Silicon Carbide Composite Using Wire EDM Technique (IJSTE/ Volume 4 / Issue 5 / 018)
results demonstrate that the machining models are appropriate and the derived machining parameters satisfy the real requirements in practice. The present work evaluates the effect of wire feed (WF), wire speed (WS), pulse on time (T on), pulse off time (T off) MRR and Surface roughness. The Aluminium silicon carbide is made up of 80 % of aluminium and 20% of silicon carbide. As per Taguchi’s Method L27 orthogonal array have been used in the search of an optimal parametric combination to achieve desired yield of the process, the important parameters have been identified and reported. II. EXPERIMENTAL PREPARATION The experiments were conducted based on the standard experimentation developed using DOE with input parameters like speed, feed, Time on (Ton) and Time off (T off). The molybdenum wire as tool electrode with flushed type dielectric fluid pressure 0.2 kgf/cm2 (distilled water) bath between work piece and electrode. Electrical power and controlling system is controlled with servo controlled resistance capacitance (Rc) circuit which ensures low discharge current with high frequency to control input process parameters. The analysis is done to study the main effects and their interactions to explore the effect of the influence of parameters on the performances. In this study, Taguchi method, a powerful tool for parameter design of the performance characteristics has been used to determine optimal machining parameters for minimization of SF in wire EDM. Experiments have been carried out using Taguchi’s L27 Orthogonal Array (OA) experimental design which consists of 27 combinations of four process parameters. According to the design catalogue prepared by Taguchi, L27 Orthogonal Array design of experiment has been found suitable in the present work. It considers four process parameters (without interaction) to be varied in three discrete levels. Based on Taguchi‟s L27 Orthogonal Array design, based on three criteria. The characteristic that lower value represents better machining performance, such as surface roughness is called „lower-the-better‟, LB. Therefore, LB for the SF have been selected for obtaining optimum machining performance characteristics. The representation of the prepared composite plate 80% Al with 20% SiC is present in figure 1. The drilled holes using Wire EDM techniques is represented in figure 2.
Fig. 1: Pure 80%Al+20% SiC Plate
Fig. 2: Wire EDM drilled holes
Input Parameters In the present study according to the Taguchi method, a robust design and an L27 orthogonal array are employed for the experimentation. Based on the machine tool, cutting tool (electrode) and work piece capability, four machining parameters are considered as controlling factors – namely, speed, feed, Pulse on time, Pulse off time, and each parameter has three levels – namely low, medium and high. The factors and their respective levels have been selected on the based on pilot experiments. The input parameters are represented in table 1. Table – 1 Input parameters Process Parameters Unit Level 1 Speed rpm 500 Feed mm/min 0.5 Pulse-On Time microns 100 Pulse- Off Time microns 40
Level 2 1000 0.7 102 42
Level 3 1500 0.9 104 44
Output Response Taguchi method with L27 orthogonal array is used for this study. Taguchi method is applied were reduce the number of trial so that cost and time minimized. In Taguchi method only perform few experiments instead of all possible setting of experiments
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Predicting the Machining Performance of in-situ of Aluminium and Silicon Carbide Composite Using Wire EDM Technique (IJSTE/ Volume 4 / Issue 5 / 018)
which required in full factorial design. Each parameters at three levels are selected. The levels are taken from the previous study of literature review. The levels of selected parameters and the corresponding responses are tabulated in table 2. Trial 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Speed 500 500 500 500 500 500 500 500 500 1000 1000 1000 1000 1000 1000 1000 1000 1000 1500 1500 1500 1500 1500 1500 1500 1500 1500
Feed 0.5 0.5 0.5 0.7 0.7 0.7 0.9 0.9 0.9 0.5 0.5 0.5 0.7 0.7 0.7 0.9 0.9 0.9 0.5 0.5 0.5 0.7 0.7 0.7 0.9 0.9 0.9
Table – 2 L27 Array output response Time ON Time OFF Surface Roughness in Microns 100 40 2.35 100 40 2.402 100 40 2.37 102 42 2.645 102 42 2.721 102 42 2.82 104 44 3.094 104 44 3.265 104 44 3.354 102 44 1.45 102 44 1.526 102 44 1.582 104 40 1.752 104 40 1.79 104 40 1.808 100 42 3.203 100 42 3.012 100 42 3.208 104 42 1.257 104 42 1.283 104 42 1.265 100 44 1.468 100 44 1.457 100 44 1.365 102 40 2.308 102 40 2.45 102 40 2.22
III. RESULT AND DISCUSSION Taguchi method stresses the importance of studying the response variation in minimization of quality characteristics variation due to uncontrollable parameter. The roughness was considered as the quality characteristics with the concept of “smaller-the-better”. From the response tale of surface roughness. It is observed that from the figure 3 (obtained from MINITAB software) when pulse on time increase surface roughness also increases. This is due to increase in the discharge energy with increase in pulse on time so more strong explosion and the deeper crater created on the machined surface resulting rougher surface. It is found that with increase in pulse off time surface roughness decreases and wire feed have no longer effect on the surface roughness.
Fig. 3: Main effect plot for the means of surface roughness
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Predicting the Machining Performance of in-situ of Aluminium and Silicon Carbide Composite Using Wire EDM Technique (IJSTE/ Volume 4 / Issue 5 / 018)
IV. CONCLUSION The following conclusions were arrived from the research as follows. 1) The machining parameters are pulse on time, pulse off time, speed and feed. After successful application of Taguchi method the results reveals that the good agreement between the experimental and predicted results. 2) Surface roughness increases with increase on pulse on time and decrease with increase in pulse off time 3) Factors like speed, feed, Time on and Time off have been found to play a significant role for surface roughness. Taguchi‟s method is used to obtain optimum parameters combination for minimization of surface roughness. 4) From the mean effect plot it is denoted that at 1500 rpm, 0.5 wire feed, 104 Time-On and 44 Time-Off the surface roughness is in optimal condition. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
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