Percentage of harmful discharges for surface current density monitoring in electrical discharge machining process

Transcription

1 1677 Percentage of harmful discharges for surface current density monitoring in electrical discharge machining process O Blatnik*, J Valentincic, and M Junkar Faculty of Mechanical Engineering, University of Ljubljana, Slovenia The manuscript was received on 8 January 2007 and was accepted after revision for publication on 15 August DOI: / JEM816 Abstract: Electrical discharge machining (EDM) is performed in the gap between the workpiece (usually the anode) and the electrode (usually the cathode). The gap is filled with a dielectric and the pulses generated by a special generator cause an electrical breakdown in the gap. A breakdown discharge occurs which removes material from the workpiece. The EDM process has been used for more than 50 years, but the influence of the surface current density on the process stability is not well understood. The surface current density is very important in the case of the machining of rough surfaces where the highest material removal rate (MRR) is preferred. For a given eroding surface there exists an optimal electrical current at which the highest MRR is achieved. It is reported in the literature that the percentage of the discharges that are harmful to the EDM process depends on the surface current density in the gap. Thus, experiments were performed to find out the possibility of using the percentage of harmful discharges to monitor the surface current density of the EDM process. Keywords: spark erosion, pulse discrimination, surface current density, rough machining, machining surface, process stability, tool wear 1 INTRODUCTION Sinking electrical discharge machining (EDM) and wire electrical discharge machining (WEDM) are commonly used technologies in tool production. EDM is a relatively slow machining process and it usually requires an electrode that has been specifically produced for the machining of a given product. The shape of the electrode is mapped into the shape of the cavity in the tool. An advantage of EDM is its ability to produce small, even micro-sized features. The EDM process is used mostly for making moulds whereas WEDM is used for precise contour cutting. Current research on the EDM process is mainly focused on micro-machining [1]. However, there is still interest in improving the EDM process by tuning the large number of processing parameters to create optimal machining conditions [2]. Most leading EDM machine producers have solved the problem of gap *Corresponding author: Faculty of Mechanical Engineering, University of Ljubljana, Askerceva 6, Ljubljana 1000, Slovenia. oki.blatnik@fs.uni-lj.si contamination; however, only a few of them have solved the problem of controlling the machining parameters when the machining surface becomes rough during the machining process. It is necessary to obtain the optimal average electric current (in future referred to as the optimal current) in the gap between the electrode and the workpiece in order to achieve the highest material removal rate (MRR). The optimal current depends on the size of the eroding surface (Fig. 1), which is a projection of the machining surface, i.e. the surface between the workpiece and the electrode, to the plane perpendicular to the machining direction [3]. Many approaches to the online selection of machining parameters for EDM and WEDM processing of rough surfaces have been described in the literature and in patents. Systems based on the capacitance or the conductivity of the gap [4] are very sensitive to contamination of the gap between the electrode and the workpiece. The main drawback, however, is that both the capacitance and the conductivity reflect the size of the machining surface and not the size of the eroding surface. JEM816 Ó IMechE 2007 Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture

2 1678 O Blatnik, J Valentincic, and M Junkar Fig. 1 The eroding surface is a projection of the machining surface of the electrode on to a plane perpendicular to the machining direction The size of the eroding surface can be determined by considering the machining parameters and the progress of the electrode into the workpiece. The machining parameters define the discharge energy, and thus, the MRR. The larger the eroding surface the slower the progress of the electrode into the workpiece, and vice versa [5 8]. In this paper, a different approach is presented. It is reported in the literature that the voltage and current flow across the electrode workpiece gap can provide sufficient information to allow online selection of an appropriate set of machining parameters for a surface [3]. It was reported that the percentage of harmful arc discharges calculated on the basis of the voltage and current flow across the electrode workpiece gap can be used to determine a set of machining parameters able to produce the optimal surface current. An algorithm for the selection of the appropriate set of machining parameters that only requires the percentage of harmful arc discharges as input data is presented in this paper. 2 OVERVIEW EDM is a machining technique in which the surface of a metal workpiece is altered by discharges that occur in the gap between the tool, which serves as an electrode, and the workpiece. The electrode workpiece gap is lubricated using a dielectric fluid. The process consists of numerous randomly ignited monodischarges from the electrode. During a discharge a plasma is formed which acts as a current conductor and heat generator. A crater is formed at the point where the plasma hits the workpiece. The size of the crater is determined by the discharge energy, which can be controlled by the discharge current and discharge duration settings. The MRR is determined by the crater size and the frequency of crater generation, i.e. discharge energy and the frequency of discharges. The latter is influenced by the discharge duration, the pulse interval between two discharges, and the ignition delay time. For a given discharge current there exists an optimal discharge duration [9] and the pulse interval should be long enough that complete deionization is achieved in the electrode workpiece gap. Thus, there exists an optimal combination of the machining parameter values to achieve, for instance, a certain surface roughness of the workpiece. The gap distance between the electrode and the workpiece is controlled by a servo system. In a closed loop, the average voltage across the electrode workpiece gap is compared to a reference voltage set on the machine: if the average voltage is greater than the reference voltage then the gap is reduced by moving the electrode closer to the workpiece and vice versa. In the case of stable EDM machining, the average voltage is equal to the reference voltage during machining. In other words, the gap distance is approximately constant during the machining process: the electrode slowly moves into the workpiece as material is removed from the workpiece. When conditions in the electrode workpiece gap cause variations in the average voltage value, the servo system acts to change the gap distance so as to equalize the average and reference voltages and the machining process is unstable. Thus, the required but not satisfactory condition for a stable machining process is for an appropriate reference voltage value to be set on the machine. The machining parameters define the process performance. Incomplete deionization of the lubricant resulting from a too short pulse interval, for example, results in the occurrence of stationary arc discharges, which degrade to the workpiece surface. Consequently, the process becomes unstable. Other parameters involved in maintaining appropriate gap conditions are beyond the scope of this paper. Gaining information on the process performance during machining requires the online monitoring of the machining process. Most often, the voltage across the electrode workpiece gap is used to monitor the process. The discharges can be identified in terms of the voltage signal. Many classification systems were have been presented in the literature [10 12]. In our case, five discharge types are distinguished (Fig. 2). Open-circuit discharges (type A) are voltage pulses with out a discharge occurring in the electrode workpiece gap (the current is equal zero). Normal discharges (type B) are the most desirable type since they achieve the highest MRR at the lowest electrode wear rate. Stationary arc discharges (type C1) reflect high levels of removed particles remaining in the electrode workpiece gap. They are not wanted since they damage the workpiece surface. Interrupted arcs (type C2) appear when the machine controller switches off the electric current to prevent damage to the workpiece surface when high levels of type C1 discharges occurs. Short-circuit (type D) discharges occur when Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture JEM816 Ó IMechE 2007

3 Surface current density monitoring in EDM process 1679 Fig. 2 The five discharge types the electrode suffers high wear rates. Arc (type C1), interrupted arc (type C2) and short-circuit (type D) discharges are considered to be harmful discharges, since they increase the electrode wear and decrease the MRR [13, 14]. Type C2 discharges are machinespecific effects and the ones shown in Fig. 2 are taken from an IT E 200M-E EDM machine. 3 PROBLEM DESCRIPTION The EDM process stability is measured in terms of the proportion of harmful discharges in the gap between the electrode and the workpiece. The process is considered to be stable at low levels of harmful discharges (less than 15 per cent). If an appropriate reference voltage is used for the servo system, then there are two causes of unstable EDM process, namely contamination of the electrode workpiece gap with discharge products, and current levels in the electrode workpiece that are too high for the size of the eroding surface. The latter is the topic of the present research. The power in the gap is defined by the equation P ¼ 1 t Z t¼te t¼0 uðtþ iðtþdt where u is the voltage across the electrode workpiece gap and i is current flowing across the ð1þ JEM816 Ó IMechE 2007 Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture

4 1680 O Blatnik, J Valentincic, and M Junkar electrode workpiece gap. The higher the power level in the gap, the higher the MRR and surface roughness. When machining of a rough surface is performed, the MRR should be as high as possible, the achieved surface roughness is of secondary importance. The highest MRR is achieved at the highest current flow across the electrode workpiece gap for which the machining process is still stable. This is the optimal current flow across the electrode workpiece gap for the machining of rough surfaces. Since the current across the electrode workpiece gap depends on the parameters set on the EDM machine, these parameters should be tuned to the size of the eroding surface to obtain the optimal current across the electrode workpiece gap [3, 13]. The optimal current in the gap I opt is calculated by the equation I opt ¼ i b A where i b is the boundary average surface current density (in future simply referred to as the boundary current density) and A is the size of the eroding surface. The boundary current density is the current density at which the machining process is on the verge of instability. At a constant eroding surface size A1, the MRR is increased by increasing the surface current density until the boundary current density is reached (Fig. 3). Unstable machining occurs at higher current densities and the MRR decreases. When a larger eroding surface is considered (A1), a higher current is needed to reach the boundary current density, thus the MRR is higher than the MRR for a smaller eroding surface (A2). However, in the literature, the boundary average surface current density is given rather than the boundary average current and it is stated that a stable EDM process is achieved when the average surface current density is less than 0.1 A/mm 2 [14]. This can be explained as follows. The voltage and current flow across the electrode workpiece gap define the electric ð2þ power in the gap (equation (1)). Since the discharge voltage is nearly constant at all machining parameter levels, the power in the gap depends only on the level of the discharge current across the electrode workpiece gap. Note that during the pulse interval both the current and power are equal to zero. 4 EXPERIMENTAL ARRANGEMENT The experiments were performed using an IT E 200M-E machine with an isoenergetic generator. The workpiece material was hardened cold work steel 210CR12 with a hardness HRc ¼ 60. The electrode material was electrolytic copper and the dielectric lubricant was Erozol 25, which is suitable for all machining regimes. The electric currents across the electrode workpiece gap were acquired using the measuring system presented in Fig. 4 at a sampling rate of khz. The experiments were undertaken for two different roughness regimes. Each of the regimes is suitable for a certain range of eroding surface sizes A r. The setup parameters for the r08 and r12 regimes are the ones suggested by the machine manufacturer (Table 1) and are suitable to machine a lubricated workpiece and to obtain only small levels of electrode wear. The output of each experiment is the electric current and voltage signal of more than 500 discharges. During the experiments the signals were saved into files and kept for further processing. Fig. 4 Experimental arrangement Table 1 Setup parameters for the r08 and r12 regimes Fig. 3 MRR versus the current density i for two eroding surfaces Regime r08 r12 Max. working current (A) 5 29 Ignition current (A) Ignition voltage (V) Impulse time (ms) Pause time (ms) Side gap (two sided) (mm) Front gap (mm) Roughness (R_max) (mm) Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture JEM816 Ó IMechE 2007

5 Surface current density monitoring in EDM process EXPERIMENTS The EDM machine used in the experiments produces an average current of 5.5 and 13.5 A, when machining in the r08 and r12 regimes respectively. According to Blatnik [15], the r08 regime is suitable for eroding surfaces larger than 55 mm 2 and the r12 regime for surfaces larger than 135 mm 2 since the surface current density should not be higher than 0.1 A/mm 2. Since the current generation of EDM machines can obtain a clear electrode workpiece gap owing to sophisticated movement options (quick rising and lowering of the electrode, orbiting, etc.), all the experiments were done in a clear gap and under very good flushing conditions. Numerous experiments were performed, thus only the most important ones are discussed. In the first set of experiments cylindrical electrodes with eroding surface sizes of 1, 10, 30, 50, 70, 90, 110, 190, and 500 mm 2 were used when machining under both the r08 and r12 regimes. These results are not further discussed since they were simply used to determine the smallest surface area which can be used when machining under either the r08 or the r12 regimes. Experiments using a conical electrode with a cone tip angle 45 were then performed under both regimes. Then combined eroding studies were performed. First the r08 regime was used, which was then switched to the r12 regime at the appropriate eroding surface size found in the earlier experiments. In the final set of experiments online selection of r08 and r12 regimes was performed. Altogether 22 experiments were performed, each lasting up to 4 h. In Fig. 5 the percentages of each discharge type are plotted against the size of the eroding surface when machining a conical shape under the r08 machining regime is shown. It can be seen that the r08 regime is not suitable for eroding surfaces smaller than 50 mm 2 due to the high percentages of harmful discharges (over 15 per cent), which results in high electrode wear and a low MRR. For eroding surfaces larger than 50 mm 2, the r08 regime can be successfully applied. Thus, obtaining a surface current density of around 0.1 A/mm 2. When an eroding surface size which is suitable for the higher regime is reached, this regime should be used. Similarly, the r12 regime is not suitable for eroding surfaces smaller than 110 mm 2 owing to the over 15 per cent of harmful discharges (Fig. 6). For surfaces larger than 110 mm 2, the machining regime that obtains the highest average current should be used; however, in this paper the aim was only to search for the boundary eroding surface size for the r08 and r12 regimes. Based on these two results a reference experiment was performed (Fig. 7). As previously mentioned, the switch between regimes was made at the boundary surface area of 110 mm 2. It can be seen that there are no harmful discharges when switching regimes at that exact eroding surface size. As stated in the Introduction, the machining parameters have to be tuned to the size of the eroding surface. When the size of the eroding surface varies during the machining, the machining parameters have to be determined online. Thus, it is necessary to monitor the appropriate process quantities z. The evaluation of the process quantities is a key step in the process of gaining suitable process attributes x to enable the selection of machine parameters for the processing of rough surfaces. The process attributes Fig. 5 Discharge-type percentage versus eroding surface size when machining a conical shape under the r08 machining regime JEM816 Ó IMechE 2007 Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture

6 1682 O Blatnik, J Valentincic, and M Junkar Fig. 6 Discharge-type percentage versus eroding surface size when machining a conical shape under the r12 machining regime Fig. 7 Discharge-type percentage versus eroding surface size when machining a conical shape: the experiment first used the r08 regime and then switched to the r12 regime (combined eroding) when a surface area of 110 mm 2 was reached. are used as the inputs for the model for the selection of the optimal machining parameters for rough surfaces (Fig. 8). In order to test the proposed model, an online selection of the r08 and r12 regimes was performed and the results are shown in Fig. 9. For this experiment the model shown in Fig. 8 was used. The experiment began with the r08 regime and was performed until stable eroding conditions were established at an eroding surface size of 50 mm 2, at which point the r12 regime was used. Three eroding cycles (about 10 s) were performed under the r12 regime and the Fig. 8 Online selection of the machining parameters to enable EDM processing of rough surfaces Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture JEM816 Ó IMechE 2007

7 Surface current density monitoring in EDM process 1683 Fig. 9 Discharge-type percentage versus eroding surface size when machining a conical shape with online selection of the r08 and r12 machining regimes (all eroding surface sizes marked with a are performed under the r08 regime all the others are done under the r12 regime) MRR [mm 3 /min] r08_45 r12_45 r0812_45 online_45 Regime Electrode edge wear [mm] r08_45 r12_45 r0812_45 online_45 Regime Fig. 10 (a) Volume removal rate versus eroding regime when machining a conical shape, and (b) electrode edge wear versus eroding regime when machining a conical shape data were analysed online. If more than 15 per cent of harmful discharges were found the machining was switched back to run under the r08 regime. After a delay of a minute, the cycle was repeated. When machining using an eroding surface size of 110 mm 2 or more with the r12 regime, the process becomes stable. In Figs 10(a) and (b) the MRR and electrode edge wear as a function of the applied regime are shown, respectively. It can be clearly seen that the r12 regime has a much higher MRR (almost 3.5 times higher) than the r08 regime as is expected owing to the higher average current in the electrode workpiece gap. If the combined regime (shown in Fig. 7 and noted as r0812 in Fig. 10) is used there is only a 15 per cent loss in the MRR compared to the r12 regime; however, the electrode edge wear decreases by more than ten times. This is due to the fact that the eroding process was not performed for a long period of time with more than 15 per cent of harmful discharges. If the online selection of the eroding regimes was used (shown in Fig. 9 and noted as online_45 in Fig. 10) the MRR is not much lower than the MRR obtained when using the combined regimes. The electrode edge wear is a little higher than that obtained when using the combined r0812 regime. Thus, if one of the other low eroding regimes is used (r06, r04, r02, etc.) the electrode edge wear can be as small as when the combined regime is used. JEM816 Ó IMechE 2007 Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture

8 1684 O Blatnik, J Valentincic, and M Junkar 6 CONCLUSIONS The following results obtained in this study are worth highlighting. 1. The theoretical border of 0.1 A/mm 2 for changing the regimes proposed by Liao et al. [14] was verified in the experiments with the conical electrode. 2. A model for the selection of the regimes based on the percentage of harmful arc discharges was proposed. 3. The machining results obtained using online selected parameter values for the machining of rough surfaces are comparable to those of the reference experiment. In future work the proposed model should be upgraded for application to other regimes (with currents of 1 A and higher) and programmed into a controller, which can be connected to conventional EDM machines. This controller will then control conventional EDM machines in such a way that the machining always takes place under optimal conditions even for rough surfaces. It is envisaged that the operator will input the starting eroding surface size and then the controller will always try to use a high regime (higher current) but if more than 15 per cent of harmful arc discharges are observed the system will be switched back, otherwise it will stay in the higher regime until the 15 per cent threshold is reached. The proposed system would allow operators with less EDM machining knowledge to be employed. Also the eroding process would take less time and in many cases also with lower electrode wear due to the fact that a lot of work, especially on conventional EDM machines, is not done at optimal eroding parameters. REFERENCES 1 Yan, B. H., Wang, A. C., Huang, C. Y., and Huang, F. Y. Study of precision micro-holes in borosilicate glass using micro EDM combined with micro ultrasonic vibration machining. Int. J. Mach. Tools Mf, 2002, 42 (10), Junkar, M. and Valentinčič, J. Towards intelligent control of electrical discharge machining. In Proceedings of the Conference on Manufacturing systems, Aachen, 1999, pp Valentinčič, J. A model of EDM process parameter selection upon the size of eroding surface. PhD Thesis (in Slovene), University of Ljubljana, Faculty of Mechanical Engineering, Kyoshi, I. Feeder for electrical discharge machining. Patent JP A, Wollenberg, G., Shulze, H. P., and Goerish, A. Online determination of the working area on sinking EDM. EDM Technology, EDM Technology Transfer, 1996, Rajurkar, K. P., Wang, W. M., and Zhao, W. S. WEDM adaptive control for variable height components. Ann. CIRP, 1994, DiBitonto, D. D., Eubank, P. T., Patel, M. R., and Barrufet, M. A. Theoretical models of the electrical discharge machining process. I. A simple cathode erosion model. J. Appl. Phys., 1989, 66(9), Crookall, J. R. An analysis of EDM utilisation in industrial tooling manufacture. Ann. CIRP, 27(1), 1978, CIRP, Scientific technical committee E: Summary specifications of pulse analysers for spark-erosion machining, Tarng, Y. S., Tseng, C. M., and Chung, L. K. A fuzzy pulse discriminating system for electrical discharge machining. Int. J. Mach. Tools Mf, 1997, 37(4), Garbajs, V. Statistical model for an adaptive control of EDM-process. Ann. CIRP, 34(1), 1985, Dauw, D. F. Advanced pulse discriminating system for EDM process analysis and control. Ann. CIRP, 32(1), 1983, Weck, M. and Dehmer, J. M. Analysis and adaptive control of EDM sinking process using the ignition delay time and fall time as parameter. Ann. CIRP, 41(1), 1992, Liao, Y. S., Yan, M. T., and Chang, C. C. A neural network approach for the online estimation of workpiece height in WEDM. J. Mater. Process. Technol., 121(2 3), 2002, Blatnik, O. Percentage of the short circuit discharges for surface current density monitoring in EDM process. Diploma work, University of Ljubljana, APPENDIX Notation i current in the gap (A) p average surface current density (A/mm 2 ) P power in the gap (W) i b boundary current surface current density I opt optimal current in the gap (A) u voltage in the gap (V) Proc. IMechE Vol. 221 Part B: J. Engineering Manufacture JEM816 Ó IMechE 2007