Electrical Discharge Machining. Optimization of chromium powder mixed EDM parameters during machining of H13 tool steel

In the present study, optimization of chromium powder mixed EDM parameters is studied during machining of H13 tool steel. Four input parameters of powder mixed EDM, namely peak current, pulse on time, duty cycle and powder concentration, are varied, each at three levels, to get the optimum responses. Material removal rate (MRR), Tool wear rate (TWR) and Surface Roughness (Ra) are considered as performance measures. Copper electrode of 16 mm is used as the tool. Response Surface Methodology is used to correlate input and output parameters. The variation of responses due to variation in input parameters has been studied and shown in the form of surface plots and contour plots.

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CHAPTER 3
OPTIMIZATION TECHNIQUE USED
This chapter provides the brief introduction about the various techniques used for optimization of input parameters in the present study. Response Surface Methodology (RSM) was used to design the experiment in this research work. Response Surface Methodology was implemented by using Minitab 16 software for constructing the experimental design. The optimization of input parameters was done by using Desirability approach. Minitab 16 was used to optimize PMEDM input parameters. A brief description of these methods is highlighted below:
3.1 Response Surface Methodology
Response surface methodology is a collection of mathematical and statistical techniques for empirical model building. By careful design of experiments, the objective is to optimize the response (output variable) which is influenced by several independent variables (input variables). An experiment is a series of tests, called runs, in which changes are made in the input variables in order to identify the reasons for changes in the output response. Suppose there are two input variables having different levels influence a response variable y, the relationship between input and output variable can be shown as
y = f(x1, x2) + e (3.1)
Where e is the error observed in response y. If we donate the expected response as per following:
E(y) = f(x1, x2) = µ (3.2)
then the surface represented by equation (3.2) is called response surface
µ = f(x1, x2) (3.3)
In most of the RSM problems the form of relationship between the response and the independent variable is unknown. Thus the first step in RSM is to find out the suitable approximation for the true functional relationship between y and set of independent variables.
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Almost all problems use these models for mathematical equation formation. The method of least square is used to estimate the parameters in approximating polynomials.
In Response Surface Methodology, normally surface plots and contour plots are used for evaluating the effect of various input parameters on the responses.
3.2 Surface Plot
By using Response Surface Methodology, response surface plot can be generated either for a single pair of input variable or for all possible pair of variables. Surface plots show how the response related to the two input variables. These plots are very fine representation of effect of various input variables on the response variables. A surface plot displays the three-dimensional relationship in two dimensions, with the variables on the x- and y-scales, and the response (z) variable represented by a smooth surface.
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4.2 Experimental Setup for Powder Mixed in the Dielectric
For carrying out the experiment by mixing the powder in the dielectric following points should be considered:
a) Entry of the powder to the main tank and filter unit of the machine should be prevented.
b) To prevent the settling of the powder at the bottom of the tank, proper stirring to the powder should be provided.
c) Proper electrical conductivity between the tool and the workpiece should be maintained.
The working tank of the machine is very large having dimensions of 850mm long 550mm wide and 300mm high. Therefore in order to prevent the mixing of powder with the
whole dielectric fluid and to avoid the clogging of main filter unit of the machine, a separate tank of 300mm X 150mm X 145 mm is fabricated from the mild steel sheet of 1.5 mm thick and this tank is placed in the working tank of EDM and the machining is performed in this tank. Fixture is placed in the tank to hold the workpiece. Tank is filled with the EDM oil. Small circulation pump is installed in the tank for proper circulation of powder mixed dielectric at the discharge gap. Two permanent magnets are used at the bottom of machine tank to hold the fixture during machining and to separate the debris from the fluid.
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4.3 Preparing Workpiece for Experimentation
A H13 steel plate of dimension 105 mm × 105 m