Analysis of WEDM for the Machining of OHNS (Oil Hardened No Shrinkage) Steel Using Response Surface Methodology (RSM)

Abstract

OHNS (Oil-hardened shrinkage) steel is gradually becoming a significant material due to its wide scope of use in manufacturing industries. It is also revealed from the literature that very few efforts have been made to achieve the optimum parameter setting in the wire-EDM process for OHNS. The Wire Electric Discharge Machine (WEDM) appears to be a suitable option for machining intricate shapes in hardened materials. In this proposed work, the effects of various process parameters of WEDM such as pulse on time (PON), pulse off time (POFF), Wire feed rate (WF), and current (I) on the material removal rate (MRR), surface roughness (SR) and the overcut or Kerf width (KF). An L27 array was proposed for the experiment. Response Surface Methodology (RSM) was used to model and analyze the data. The pulse on time, pulse off time, pulse current, and voltage are the control parameters of EDM. RSM method was used to design the experiment using a rotatable central composite design as this is the most widely used experimental design for modeling a second–order response surface

Introduction

Electrical Discharge Machining (EDM) is a non-traditional machining process that removes material from a workpiece through controlled electrical sparks between a tool electrode and the workpiece submerged in a dielectric fluid. The sparks generate high temperatures that melt and vaporize small amounts of material, which are removed by the dielectric flow. In Wire Electrical Discharge Machining (WEDM), a thin moving wire electrode is used to cut conductive materials into complex two- and three-dimensional shapes with high accuracy. The wire acts like a continuously moving cutting tool, eliminating the need for specially shaped electrodes.

WEDM is widely used for machining difficult-to-cut materials such as alloy steels, aerospace materials, and conductive ceramics because it can produce precise components with good surface finish, corrosion resistance, and wear resistance. However, achieving optimal machining performance is challenging due to the large number of process variables, unstable discharge conditions, wire breakage, and difficulty in selecting suitable machining parameters.

The study focuses on machining OHNS (Oil Hardened Non-Shrinkage) tool steel using a molybdenum wire electrode. The objective is to investigate the effects of important input parameters—pulse on time, pulse off time, current, and wire feed rate—on output responses including kerf width, material removal rate (MRR), and surface roughness. Experiments are performed using a CNC-WEDM machine, where machining conditions are controlled through numerical settings.

The main input parameters are:

• Pulse On Time (Ton): The duration of current flow during each spark discharge. Increasing Ton increases discharge energy and cutting speed but may also increase surface roughness and risk of wire breakage.
• Pulse Off Time (Toff): The interval between two consecutive sparks. Lower Toff increases spark frequency and cutting rate but may cause instability and wire breakage.
• Current (IP): The maximum current during discharge. Higher current increases material removal rate but may reduce machining stability.
• Wire Feed Rate: The speed at which the wire electrode moves. Higher feed rates improve machining stability and reduce wire breakage.

The output parameters evaluated are:

• Kerf Width: The width of material removed during cutting, measured using a precision measuring instrument.
• Material Removal Rate (MRR): The amount of material removed per unit time, calculated from weight loss of the workpiece.
• Surface Roughness (Ra): A measure of surface quality based on deviations from the ideal surface profile.

For experimental planning, the Taguchi design method is applied using an L27 orthogonal array. Four machining parameters are studied at different levels to understand their influence on WEDM performance. Experiments are conducted with different combinations of parameters, and the resulting MRR, surface roughness, and kerf width are measured for 27 samples.

The study also applies Response Surface Methodology (RSM), a statistical technique used to develop mathematical models and optimize machining parameters. RSM helps identify the best combination of input variables for maximizing material removal rate while minimizing undesirable effects such as excessive kerf width and surface roughness.

Analysis of Variance (ANOVA) is used to determine the significance of machining parameters and validate the developed models. ANOVA evaluates how individual factors influence the responses and checks the accuracy of the regression models. The analysis shows that pulse on time, pulse off time, current, and wire feed rate significantly affect kerf width. The quadratic model provides a statistically significant prediction for kerf behaviour, with a high percentage of variation explained by the model.

Conclusion

In this investigational experiment on WEDM, the effect of machining outputs taken into consideration is material removal rate, Kerf, and surface roughness of the OHNS workpiece using the Molybdenum electrode. These outputs are important in industrial applications. The conduction of the experiment depends upon various parameter settings such as discharge current (Ip), pulse on time (Pon) pulse off time (Poff), and wire feed (WF) have been selected. Based on the Box-Behnken design by Response Surface Methodology was conducted and the MINITAB 16 software package was used for analysis of the experiment. The second-order response models have been validated with analysis of variance. Finally, an attempt has been made to estimate the optimum machining conditions to produce the best possible response within the experimental constraints. Parameter values for multiple responses using RSM give settings that PON (µs )25, POFF (µs) 4, CR (I) 4, and FR 70. The effect of WEDM working parameters on the output parameters is as follows: 1) Predicted optimum setting obtained for maximizing MRR and minimizing KF and Ra is Pulse on Time as 25 µs, Pulse off Time as 4 µs, Discharge Current as 4 A, Feed rate as 70 and predicted values of responses MRR, KF, and Ra are 3.0819, 0.3573, 3.5491, and experimental values are 3.158, 0.3628, 3.588 respectively. Errors obtained between experimental and predicted responses are very less and within an acceptable range. Hence it is concluded that models obtained for each response are more accurate hence it can be used for developing a robust, reliable knowledge base and early prediction of MRR, KF, and Ra without experimenting with the WEDM process for OHNS. 2) The percentage error for the Material removal rate is 2.41%, for the kerf 1.52% and for surface roughness is 1.11

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Copyright © 2026 Smita P. Bhise. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.