EDM machining of carbon-carbon composite- a Taguchi approach (1)

The objective of this paper is to determine the optimal setting of the process of the process parameters on the electro-diacharge machining (EDM) machine while machining carbon-carbon composites. The parameters considered are pulse current, gap voltage and pulse-on-time; whereas the responses are electrode wear rate (EWR) and material removal rate (MRR). The optimal setting of the parameters are determined through experiments planned, conducted and analysed using the Taguchi method. It is found that the electrode wear rate reduces substantially, within the region of experimentation, if the parameters are set at their lowest values, while the parameters set at their highest values increase the MRR drastically.

Carbon-carbon composites are highly potential materials in aeronautical and aerospace industries because of their favourable properties like high strength, high service temperature, high stiffness and low density. However, they are difficult to machine materials by the conventional machining methods, which limits their use. It is attributed to their non-homogeneity, anisotropy, hardness and intrinsic brittleness. In order to make parts of the required specifications an appropriate machining method is to be selected. Among the various non-conventional machining methods available, EDM is the most widely used and successfully applied one for the difficult to machine materials. Lau et al. analysed EDM machining of carbon fiber composite materials. The feasibility of EDM machining of C-C composites is analysed by Hocheng et al. GUu et al. studied the effect of EDM machining on the characteristics of carbon fiber reinforced carbon composites considering gap voltage and pulse current as proces variables while the response functions taken are material removal rate (MRR), surface finish and de-lamination factor. The literature idensifies that EDM machining is a promissing machining technique to machine C-C composite. The eletrode wear rate and MRR are the two important machining ability criteria with cconomical implications. Hence, their optimisation is significant and it needs further investigation using more process variables. Though, there are many EDM variables that influence the machinability criteria, this investigation deals with only three parameters namely pulse current, pulse-on-time and gap voltage.

Experimental work

The material used for this work is carbon-carbon composite plate of size 50mm×20mm×2mm (density: 1.65g/cm3) developed from carbon fiber cloth laminated using thermosetting phenolic resin and cured under appropriate pressure and temperature. The resistivity of the specimen is 0.096 Ωcm. The specimen is polished to get a plane surface. The electrode used is electrolytic copper of 8930kg/m3 density with a melting point of 1083°C. These electrodes are cylindrical in shape with a nominal diameter of 1.6mm.

The machine used is Electronica-EZMC machine with NC control in the Z-direction. The dielectric fluid used is IPOL Spark Erosion 450 produced by Sah Petrolium. Polarity of the electrode is negative and that of the composite is positive.

This paper uses Taguchi method, which is very effective to deal with responses influenced by multi-variables. This method is a powerful Design of Experiments tool, which provides a simple, efficient and systematic approach to determine optimal machining parameters. Compared to the conventional approach to experimentation, this method reduces drastically the number of experiments that are required to model the response functions. Traditional experimentation involves one-factor-at-a-time experiments, wherein one variable is changed while the rest are held constant. The major disadvantage of this strategy is that if fails to consider any possible interactions between the parameters. An interaction is the failure of one factor to produce the same effect on the response at different levels of another factor. It is also impossible to study all the factors and determine their main effects in a single experiment.

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