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Thank you for downloading this docum ment from the RMIT Research R R Repository 7KH50 0,75HVHDUFFK5HSRVLWR RU\LVDQRSH GDWDEDVHVK KRZFDVLQJWWKHUHVHDUF FK HQDFFHVVG RXWSXWVVRI50,78QLYHUVLW\UHV VHDUFKHUV 50,75 HSRVLWRU\KWWSUHVHDUFKEDQNUPLWHGXDX 5HVHDUFK5H Citatio on: Ye, S, Pan, W, Ding, S, Mo, J, Brandt, M and Mackie, A 2012, 'Electrical discharge characteristics of polycrystalline diamonds ', Advanced Materials Research, vol. 426, pp. 44-47. See this record in i the RMIIT Researcch Repository at: http://researchbank.rmit.edu.au/view/rmit:18003 Version n: Accepted Manuscript Copyright Statem ment: © 2012 Trans Tech Publications Link to o Published d Version: http://dx.doi.org/10.4028/www.scientific.net/AMR.426.44 PLEASE DO NOT REMOVE THIS PAGE Electrical Discharge Characteristics of Polycrystalline Diamonds Shulin Ye1, Wencheng Pan2, Songlin Ding2, John Mo2, Milan Brandt2, Andrew Mackie2 2 1 Department of Mechatronics Engineering, Foshan University, China School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Australia Abstract This paper introduces the electrical discharging characteristics of PCD based on experimental studies. The relationship between gap voltage, peak current and electrical resistance of various PCD materials is presented. The influence of peak current on the discharge waveforms is analyzed. The conclusion provides important information for the development of new generators specifically designed for the machining of PCD tools. Key Words: Electrical Discharge Machining, EDM, Polycrystalline Diamond, PCD 1 Introduction Since PCD was developed over 40 years ago the manufacturing processes of PCD discs has not progressed very much. PCD discs are produced by sintering together graded and selected diamond particles with binding materials at temperatures of up to 2,000 oC and pressures of 5 – 7 GPa [1]. The grain size of diamond particles is in the range of 1 – 50 µm. The thickness of the PCD layer is in the range of 0.3 – 1 mm. The PCD is supported on a substrate of cemented carbide of 0.3-5 mm in thickness. The binding material is usually cobalt, which is electrically conductive making the PCD also electrically conductive. The bulk electrical resistivity is not constant; it varies with the percentage of binder and the grain size of diamond particles. Since diamond is the hardest material in the world, to make PCD tools though traditional grinding and shaping processes is extremely difficult. Electrical Discharge Machining (EDM) is a non-contact machining method for the manufacturing of geometrically complex components and the shaping of hard-to-machine materials. It turns electrical power into thermal energy through a series of discrete electrical discharges occurring between an electrode and the workpiece. The thermal energy generates a channel of plasma between the two poles at temperature of up to 20,000 oC [2]. The high temparature melts and evaporates the material from the surface of the workpiece. A material removal rate (MRR) of 2 – 400 mm3/min can be achieved depending on applications [3,4]. Since PCD is electrically conductive, it is possible to grind PCD tools using EDM technology. As PCD is a relatively low bulk conductivity material, the discharging characteristics of PCD are different from conventional materials such as carbide and High Speed Steel (HSS) [5]. Theoretically high voltage, smaller current and high frequency (short On-Time) should be applied to achieve better surface quality. In practice the values of these parameters should be determined based the discharging characteristics of the PCD workpiece. This is particularly important in the tool manufacturing industry. Unfortunately little research has been done in this field and no detailed information is available in the public domain. 1 2 Experimental setup and procedure The experiment was carried out on a new EDG machine equipped with a rotating grinding wheel. The electrode was a copper bar (3 mm in diameter) installed in the axial direction of the grinding wheel. The test sample was brazed on the end face of a Tungsten carbide holder, which was clamped by the headstock spindle of the machine. The linear gap distance is adjustable in Z direction through the servo system. The EDM power supply is able to generate short pulses with a minimum On-Time of 1 µs. The output voltage is in the range of 90 – 260V. The current is in the range of 1 – 20A. Two types of PCD samples made by Element Six for metal cutting (CTM302 and CMX850) were tested. To analyze the relationship between gap voltage, gap distance, current, resistance and the physical characteristics of PCD materials such as grain size and binding materials, a total of nine groups of experiments were conducted by applying parameters shown in Table 1. Both normal and abnormal sparks have been recorded. In order to compare the discharging characteristic of PCD with that in ordinary EDM processes, experiments on Tungsten carbide samples were carried out before tests on PCD samples. Table 1 Experimental parameters Materials Tungsten carbide CMX850 CTM302 Experimental parameters Test Number Open voltage (V) Peak current (A) 120 1 B0001 120 1 A0013 160 1 A0001 160 5 A0003 160 10 A0004 120 1 A0019 160 1 A0027 160 5 A0028 160 10 A0029 3 Results and Discussion 3.1 Difference in waveforms caused by variation of materials Discharging waveforms of carbide, CMX850 and CTM302 under the same machining condition are shown in Fig. 3, 4 and 5. It can be seen from the figures that the discharging waveforms of PCD are similar to that of Tungsten carbide. At the point of ionization, the sparking gap voltage is reduced to the machining voltage. The flow of amperes was instantaneous with the ionization point and it continued for the complete On-time. The gap voltage is Ue and peak current is ip. In an EDM process the maintaining voltage is equal to the sum of the voltage in the discharging channel (Ug) and the voltage on the workpiece (Ur) caused by the resistance of the material. Value of Ug is dependent on the workpiece material, property of dielectric and the value of gap distance. Ur is the ratio of workpiece resistance and discharging current. Ue can be expressed as follows: 2 Ue=Ug + ip×Rc (1) Where Rc is the value of workpiece resistance. Because the resistance of Tungsten carbide is very small, Ur can be neglected. Therefore, in the machining of Tungsten carbide, Ue is equal to Ug. Fig. 3 B0001 discharging waveform Fig. 4 A0013 discharging waveform Fig. 5 A0019 discharging waveform 3 3.2 The influence of discharging current Experiments with different electric current were conducted to analyze the influence of resistance of PCD workpiece. The average voltage in the first 30 or 35 µs from ionization is taken as maintaining voltage. Table 2 shows the maintaining voltage and the corresponding current. Substitute the data into Equation 1, Ug and Rc of CMX850 and CMX302 can be found. Therefore, the maintaining voltage of CMX850 (Ue1) and CTM302 (Ue2) can be described as follows: Ue1=24.9+0.31×ip (2) Ue2=39.5+0.59×ip (3) Based on above calculation, the resistance of CTM302 (0.59Ω) is found to be bigger than the resistance of CMX850 which is 0.31Ω. This is because the thickness of CTM302 (0.7mm) is bigger than CMX850 (0.5mm) although the resistivity of CMX850 is smaller than CTM302. Table 2 Values of maintaining voltage and discharging current Sample 1 Sample 2 Discharging current(A) 1.82 5.428 13.36 maintaining voltage(V) 25.5 26.74 29.07 Discharging current(A) 1.635 4.797 11.75 maintaining voltage(V) 40.48 44.02 46.4 Fig. 6 Discharging waveform of zero gap distance 3.3 The impact of gap distance Short-circuit is an abnormal working condition in EDM machining when the gap distance is reduced to zero. In the experiment with PCD samples, a controversial phenomenon which is totally different from Tungsten carbide was found: when the electrode contacted the surface of PCD workpiece (gap distance became zero), a normal discharging waveform rather than the waveform of short circuit was captured (Fig. 6). The experiments were repeated for five times to eliminate the possibility of measuring errors or mis-operations. The non short-circuit sparking appeared 3 times in the 5 experiments. It can be concluded that this was not a random phenomenon. On the contrary, it was the unique electrical discharging characteristic of PCD. 4 The low conductivity of PCD relies on the conductive network formed by the conductive binding metals and non conductive diamond particles. When the electrode contacted the PCD workpiece in the experiment, it was very possible that it touched only the diamond particles protruded from the binder. This protrusion prevented the contact of electrode with the conductive binder. Thus, it prevented the short circuit. When the high voltage was applied between the electrode and the workpiece, the normal sparking was observed between the electrode and the binder. By chance, what contacted the electrode may also be the binder which protruded from the PCD sample. In this case, short-circuit were observed as expected. 4 Conclusions This paper introduces the electrical discharging characteristics of PCD. The relationship between gap voltage, peak current and electric resistance of various PCD materials is presented. The following results are found: 1. Discharging waveform in machining PCD is similar to that of machining ordinary materials such as Tungsten carbide. 2. The maintaining voltage in machining PCD is higher than that of conventional EDM machining of ordinary materials. The difference in the maintain voltage is significant with the variation of PCD materials. 3. Short-circuit arcing may not exist in the machining of PCD materials. However, it is difficult to predict if the non short-circuit will occur when the electrode contacts the workpiece surface. These conclusions are very important for the design of EDG machines specifically for the machining of PCD tools. The method for detecting short-circuit during the machining become invalid, new approaches are required using the discharging characteristics of PCD. References 1 Shane A. Catledge, Yogesh K. Vohra, Ram Ladi, Ghanshyam Rai, “Micro-Raman stress investigations and X-ray diffraction analysis of polycrystalline diamond (PCD) tools”, Diamond and Related Materials, 1996 Vol. 5, p. 1159-1165. 2 K.H.Ho and S.T. Newman “ State of the art electrical discharge machining (EDM)”, International Journal of Machine Tools & Manufacture, 2003, Vol. 23, p.1287-1300 3 N.M. Abas, D.G. Solomon, M.F.Bahari “ A review on current research tgrends in electrical discharge machining (EDM)”, International Journal of Machine Tools & Manufacture, 2007, Vol. 47, p.1214-1228 4 M.P. Jahan, Y.S. Wong, M. Rahman, “ A study on the fine-finish die-sinking micro-EDM of tungsten carbide using different electrode materials”, Journal of materials processing technology, 2009, Vol. 209 , p.3956–3967. 5 D.T. Pham, S.S. Dimov, S. Bigot, A. Ivanov, K. Popov, “Micro-EDM—recent developments and research issues”, Journal of Materials Processing Technology, 149 (2004) 50–57. 5