Relationship of Surface Roughness with Current and Voltage During Wire EDM

Khan, Ahsan Ali; , Munira Bt. Mohd Ali; , Norhashimah Bt. Mohd Shaffiar

ABSTRACT

Wire EDM is in use for a long time for cutting punches and dies, shaped pockets and other machine parts. Surface finish of the machined surface mainly depends on current and voltage used during machining. In the present research experimental investigations have been conducted to establish relationships of job surface finish with current and voltage. Brass wires of diameters 0.3, 0.25, 0.20 and 0.15 mm were used. Work materials tested were mild steel, aluminum, cemented carbide, copper and stainless steel. After machining each material with specific current and voltage the hardness and the job surface roughness were measured and their surfaces were observed under an electron-scanning microscope. Results of the experiments show that in general the machined surface becomes rougher with increase in current and voltage. Microstructures of the specimens also show that craters on the finished surface become larger as a result of using higher current and voltage. It was also found that wires of smaller diameters give smoother surface than those cut with larger diameters. It has been established that machining of carbides should be limited to wires with diameter equal to or less than 0.15 mm. Use of wires of greater diameters causes frequent wire breakage. A statistical analysis was done to pick up the most probabilistic data from the bank of data obtained from the experiments. Finally, mathematical relationships have been developed between job surface finish with current and voltage within the specified ranges for a few work materials.

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INTRODUCTION

Wire EDM, a modification of EDM, is widely used for cutting complex profiles on conductive work materials. For Wire EDM consumable electrode is mostly used since non-consumable wires are expensive. Precision dies and profiles require smooth surface finish. As a result of each spark during cutting process a tiny volume of material is vaporized, which leaves a cavity on the machined surface. Thus smoothness of the machined surface depends on the depth of cavities produced during a series of sparks. The depth and size of these cavities depend on the intensity of the sparks and the intensity of the sparks mainly depends on current and voltage used during cutting process. Surface smoothness also depends on the characteristics of the work materials, wire tension, electrode wire material, dielectric fluid, etc. Brown (1998), Kalpakjian and Schmid (2003), Panday (1993), Tlusty (2002), Trend and Wright (2000) have analyzed the wire EDM process and have shown the relationship between surface smoothness and different cutting parameters. They have also given recommendations on cutting parameters to be used during cutting different work materials. But they didn’t give any mathematical relationship between surface smoothness and cutting parameters. In the present study mathematical relationships have been developed between surface roughness and current and voltage within certain ranges for a few work materials.

MATERIALS AND METHODS

The machine used for cutting different work materials was model FX-K with a maximum capacity of work dimension 800x575x215 mm. The dielectric material used during machining was ionized water. The wire material used in the present work was brass with tensile strength of 931 N/mm². Work materials used were mild steel, aluminum, carbide, copper and stainless steel. Each material was cut with brass wires of diameter 0.15, 0.2, 0.25 and 0.3 mm. During a set of experiments current was kept constant while the voltage was varied within a certain range in order to investigate the effect of voltage on surface smoothness. Then experiments were repeated with constant voltage and while current were varied within certain ranges. After cutting each specimen, its surface condition was observed under a scanning electron microscope model JEO JSM-5600. Roughness of each machined surface was measured by a surface roughness measuring equipment. The software NMTOOL was used to find the mathematical relationship between current and voltage used with surface roughness obtained after machining.

RESULTS AND DISCUSSION

Effect of current (I_p) on surface roughness (R_a): It was observed from the experimental results that in general the roughness of the machined surface increases with increase in current. One of such relationships is shown in Fig. 1. This trend is also supported by the SEM photographs of the microstructures of the specimen, shown in Fig. 2-4. Puertas et al. (2004) also found similar results during machining of 94WC-6Co.

Each experiment was conducted several times and each time the curve of current I_p vs. surface roughness R_a was plotted. Slope of each curve at the midpoint of the current range was determined. Then a frequency distribution analysis was done to find out the most probable curve.


Fig. 1:	Relationship of surface roughness and current


Fig. 2:	Surface machined at I_p = 4A, Vg = 44, material-carbide, wire dia -0.15 mm

The nature of the curve within the selected range of current shows that the relationship of current I_p with surface roughness R_a can be expressed by a quadratic equation of the form y = ax² + bx = c, where y is the value of surface roughness and x is the value of current. A software called MNTOOL (Numerical method tools) was used to find out the coefficients a, b and c. The final relationship of current with surface roughness for a few materials and wire diameters are shown in the Table 1.

Effect of voltage V_g on surface roughness R_a: Similar results were obtained during investigation on the effect of voltage V_g on surface roughness of the machined surface R_a. In general the machined surface becomes rougher as a result of using higher voltage. One of the results is shown in the Fig. 5. It is also clear from the microstructures of the specimens (Fig. 6-8) that a higher voltage produces a rougher surface.


Fig. 3:	Surface machined at I_p = 5A, Vg = 44, material-carbide, wire dia -0.15 mm


Fig. 4:	Surface machined at I_p = 7A, Vg = 44, material-carbide, wire dia -0.15 mm

Table 1:	Relationship of surface roughness with voltage

Table 2:	Relationship of surface roughness with current


Fig. 5:	Relationship of surface roughness and voltage


Fig. 6:	Surface machined at I_p = 13 A, Vg = 30 V, material-stainless steel, wire dia -0.2 mm

Experiments were conducted with different work materials and wires of different diameters. During the experiments the current and other parameters were kept constant and the voltage was varied within certain range. Each experiment was conducted several times. The slope of each curve (voltage V_g vs. R_a) was determined at the midpoint of the voltage range.

A frequency distribution analysis was done to find out the most probable curve. The nature of the curve sows that voltage and surface roughness maintain relationship that can be expressed by a quadratic equation of the form y = ax² + bx + c, where y is the value of surface roughness and x is the value of voltage. The software MNTOOL was used to find out the values of the coefficients a, b and c. The mathematical relationships found for different work materials and wire diameters within the specified range of voltage are shown in the Table 2.


Fig. 7:	Surface machined at I_p = 13 A, Vg = 36 V, material- stainless steel, wire dia -0.2 mm


Fig. 8:	Surface machined at I_p = 13 A, Vg = 40 V, material-stainless steel, wire dia -0.2 mm


Fig. 9:	Relationship of surface roughness and wire diameter

Effect of wire diameter D_w on surface roughness R_a: It was found that with the increase in wire diameter roughness of the machined surface increases. This is because of larger area of heat affected zone due to each spark while using a wire of larger diameter. One of the graphs illustrating the fact is shown in Fig. 9.

CONCLUSIONS

From the analysis of the experimental investigation, the following conclusion can be drawn:

•	In general the machined surface becomes rougher with increase in current and voltage.
•	Microstructures of the specimens show that craters on the finished surface become larger as a result of using higher current and voltage.
•	Wires of smaller diameters give smoother finished surface than those cut with wires of larger diameters.
•	Mathematical relationships have been developed between job surface finish with current and voltage for a few work materials within range specified.

REFERENCES

Brown, J., 1998. Advanced Machining Technology Handbook. McGraw Hill, New York.
Kalpakjian, S. and S. Schmid, 2003. Manufacturing Engineering and Technology. 4th Edn., Prentice Hall International, London.
Puertas, I., C.J. Luis and L. Alvarez, 2004. Analysis of the influence of edm parameters on surface quality. MRR and EW of WC-Co. J. Mater. Proces. Technol., 153-154: 1026-1032.
CrossRef Direct Link
Tlusty, J., 2002. Manufacturing Processes and Equipment. Prentice Hall, New Jersey.
Trend, E.M. and P.K. Wright, 2000. Metal Cutting. 4th Edn., Butterworth-Heineman, Boston.

Journal of Applied Sciences

Research Article