Available online at www.sciencedirect.com ScienceDirect Procedia CIRP 42 (2016 ) 709 714 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) Development of a Pulse Generator for Rough Cutting of Oil-based Micro Wire-EDM Mu-Tian Yan*, Tsung-Chien Lin Department of Mechatronic Engineering, Huafan University, No. 1, Huafan Rd., Shihtin District 223 New Taipei City, Taiwan, R.O.C. * Corresponding author. Tel.: 886-2-26632102 Ext. 4027; fax:886-2-26633173.e-mail address: mtyan@huafan.hfu.edu.tw. Abstract This paper aims to develop a specific pulse generator for rough cutting of oil-based micro wire electrical discharge machining (wire-edm). A two-stage current-limiting resistance power supply was presented to suppress thermal damage on the machined surface of polycrystalline diamond (PCD) while achieving stable machining by supplying high open voltage and low discharge current pulse waveforms. Tests revealed that the two-stage resistance power supply could achieve lower thermal damaged layer, better surface quality and surface finish than a conventional pulsed power supply since the former could provide smaller peak current and shorter discharge duration than the later. Experimental results demonstrate that boron-doped polycrystalline composite diamond (BD-PCD) is superior to polycrystalline diamond in the smoothness of cutting edge, surface quality and surface roughness since the former has lower specific resistance and thermal conductivity than the later. This study has indicated that the proposed power supply could enable a prototype micro wire-edm machine to achieve a surface finish as low as 0.45 μm R a and a thermal damaged layer as small as 6 μm on the machined surface of BD-PCD after one rough machining operation. 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license 2016 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review XVIII). under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) Keywords: Micro Wire-EDM; Polycrystalline Diamond; Pulse Generator; Thermal Damaged Layer; Surface Roughness 1. Introduction Micro wire-edm has emerged as one of the most important machining processes for the manufacture of micro and miniature parts and structure because of its excellence in machining intricate two dimensional shapes and varying tapers in all electrically conductive materials irrespective of their hardness and toughness. Deionized water has widely been used as a dielectric fluid in wire EDM, but an upsurge of the machining of hardened materials such as single-crystal silicon, tungsten carbides and polycrystalline diamonds (PCD) have urged for the use of oil dielectric instead. Some studies have reported that wire-edm in oil can obtain higher surface quality, smoother surface and smaller micro chips and cracks than those in water dielectric for cutting polished singlecrystal silicon [1]. Compared to water-based wire-edm operations, oil-based wire-edm not only could eliminate electrolysis and corrosion but also prevent cobal depletion for carbide cutting and therefore achieve better surface qulaity and surface finish [2]. Since 2010, some machine tool builders have introduced micro wire-edm machine tools using oil dielectric fluid to the market for the growing demands of micromachining and cutting of hard materials and PCD [3-5]. Recently, a new PCD (boron-doped polycrystalline composite diamond, BD-PCD) or called electrically conductive PCD (EC-PCD) that possesses lower specific resistance and thermal conductivity than a standard PCD (S-PCD) was developed and introduced to the market for enhancing the machinability of PCD by electrical discharge machining [6]. Suzuki et al. [7] investigated the machinability of EC-PCD in water and oil and made a comparison on machining properties between EC-PCD and standard PCD. Their study has shown that wire-edm with oil dielectric can obtain far better surface quality than wire- EDM with water dielectric and EC-PCD is superior to S-PCD in cutting speed and surface roughness. Chen and Chang [8] presented the fabrication of boron-doped PCD wheel by micro 2212-8271 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.306
710 Mu-Tian Yan and Tsung-Chien Lin / Procedia CIRP 42 ( 2016 ) 709 714 rotary wire-edm using both resistance-capictance (RC) and transistor-controlled discharge circuits. According to previous works [1-5], oil-based wire-edm could achieve smaller discharge energy and thus producing smaller thermal damaged layer and slit width than waterbased wire-edm. However, thermal damage on the machined surface of PCD is unavoidable after wire-edm. A larger damaged layer on the machined surface of PCD not only contributes to a decrease of fatigue strength but also leads to a longer time period of grinding process for final finishing and dressing operations of PCD cutting tools. Therefore, developing a pulse generator is crucial for the development of oil-based wire-edm technology. This paper presents a specially designed pulse generator for rough cutting of oilbased micro wire-edm. Experimental verification of the developed pulse generator on a prototype micro wire-edm machine for the cutting of PCD is shown. A comparison between micro wire electrical discharge machining mechanism of S-PCD and BD-PCD are also demonstrated in this work. generate an electro-hole in the crystal lattice and therefore, cause the electrical conductivity of the diamond lattice. As it can be seen from Figure 1(b), sparks occur on BD-PCD surface more easily due to both of the electrical conductibility of the cobalt and boron doped diamond grains. Diamond grains can be cut by spark erosion without difficulty, which contributes to the smoothes of cutting edge. Without the dislodgement of diamonds grains like S-PCD, only a small amount of debris remains in the spark gap, thereby reducing the occurrence of arc discharge and short circuit. As a result, smaller thermal damaged layer and no pitting can be obtained on the machined surface of BD-PCD after micro wire-edm. 2. Micro Wire-EDM Mechanism of PCD PCD consists of very small synthetic crystals (1 100 μm grain size). They are synthesized at very high temperature (1,400 1,600 C) and under great pressure (5.0 6.5 GPa) on top of a cemented tungsten carbide substrate or base by using cobalt as a catalyst. Both the electrical conductibility of the PCD layer, in virtue of its cobalt content, and the tungsten carbide substrate allows a PCD tool blank to be machined by micro wire-edm [9, 10]. Figure 1 shows micro wire-edm mechanisms of both S-PCD and BD-BCD. As depicted in Figure 1 (a), a pulse voltage ( 150 V) is applied between the wire electrode and workpiece in the oil dielectric fluid, and then sparks occur once the insulating property of oil dielectric is broken down by the high electric field. The conductive cobalt catalyst is eroded by the process of melting and evaporating. S-PCD contains nonconductive diamond particles which cannot be evaporated and eroded away by micro wire-edm and therefore there can produce a large amount of debris in the spark gap during the erosion process. As a result, the debris particles stagnated in the machined kerf could contaminate the spark gap. A lot of micro pits were generated due to the expulsion of diamond particles from the machined surface of S-PCD and a thermal damaged layer was also produced on the S-PCD surface after micro wire-edm. BD-PCD fabricated by the same process as S-PCD production but with different micro powder (boron doped diamond) possesses higher electrical conductivity than standard PCD. Boron doped diamond was sintered by adding pure boron, boron carbide and iron boron in the catalyst or graphite under a high-pressure and high-temperature condition [6]. Okano et al. [11] synthesized boron-doped diamond films by using a boron-doping technique and investigated the doping characteristics of diamond films. Chen and Chang [12] illustrated a doping process of boron impurities to replace carbon atoms in such a way as to (a) (b) Fig. 1. Micro wire-edm mechanisms of both (a) S-PCD and (b) BD-PCD.
Mu-Tian Yan and Tsung-Chien Lin / Procedia CIRP 42 ( 2016 ) 709 714 711 3. Pulse Generator Design for Oil-Based Micro Wire- EDM To achieve precision cutting for extremely hard materials such as PCD, machining performance of micro wire-edm must meet certain specifications( i.e. machining feed-rate 0.1 mm/min, surface roughness 0.7 μm R a and thickness of damaged layer 15 μm), which are provided by a cutting tool manufacturer in Taiwan. Therefore, a specific pulse generator should be designed to meet these specifications for the rough cutting of PCD by micro wire-edm. Because PCD has the physical properties of high melting point, thermal conductivity and specific heat, discharge duration of each spark should be controlled within several hundred nanoseconds to enhance high temperature and thermal explosive force for the removal of workpiece material during the spark erosion process. Since the electrical resistance of oil dielectric is higher than that of water dielectric, more power electrical field is required to break down the insulating properties between wire electrode and workpiece. In order to reduce thermal damage on machined surface as well as avoid wire breakage, peak current of each spark should be controlled within several dozens of amperes. To fulfill these specifications for the rough cutting of PCD by micro wire- EDM, technical specifications for the design of a pulse generator were specified as: discharge duration 1 s, peak current 15 A, discharge frequency 100 KHz and open voltage = 200 V. As shown in Figure 1(a), a conventional pulsed power supply is composed of a discharge circuit, a snubber circuit, a pulse control circuit and a MOSEFET driver. Figure 1(b) shows timing charts of the pulse control signals for the conventional pulsed power supply. When the MOSFET M 1 is turned on through the pulse control signal P 1, the discharge circuit leads the gap to discharge by supplying an open voltage. As the wire electrode approaches the workpiece, an electrical field is formed and eventually breaks down the insulating properties of the oil dielectric and thus, a spark occurs while generating discharge current. After each discharge, the power MOSFET M 2 is turned on through the pulse control signal P 2, an excessive discharge energy stored in the spark gap is directed through to the dissipating resistor R 2. One time period of pulse on-time t i and one time period of pulse off-time t o constitutes one duty cycle. A period of off duty cycle followed by 10 successive duty cycles was provided to permits the wire electrode to be insulated the workpiece and as a result effectively allows reionization of the dielectric and aids in immediately cooling the material and flushing out the eroded particles, thereby avoiding the occurrence of continuous arc discharges or short circuits and preventing wire rupture. For this iso-frequency power supply, an open voltage of 200 V is required to break the insulation of polycrystalline diamond for discharge when using oil dielectric fluid. However, a higher open voltage contributes to an increase of peak current, thereby resulting in wider machining slit, poor surface quality and larger thermal damaged layer. Hence, maintaining a high open voltage during ignition delay time and providing short discharge duration and a low peak current during on-time period is a challenge for developing an oil-based pulse generator. (a) Circuit diagram (b) Timing charts of the pulse control signals Fig2. Conventional pulsed power supply. In order to cope with the problem of a conventional pulsed power supply, a two-stage resistance power supply is proposed as illustrated in Figure 3. The two-stage resistance power supply can provide a low resistance during ignition delay time and a high resistance during on-time period so as to maintain a high open voltage for breaking down the insulating property across the gap and provide low discharge energy in the spark gap, thereby achieving low thermal damaged layer and high surface quality. When the MOSFET M 1 is turned on through the pulse control signal P 1, the MOSFET M 2 is also turned on through the pulse control signal P 2, at this stage, resistor R 1 is in parallel with resistor R 2 and thus total current-limiting resistance in the discharge circuit is decreased. As a result, a high open voltage of 200 V is also kept across the sparking gap. After a short period of resistance on-time t ir, the MOSFET M 2 is turned off, the MOSFET M 1 is remained at a conducting stage. Once a spark occurs, total current-limiting resistance in the discharge circuit is increased, thereby achieving low discharge current. As depicted in Figure 3(b), open voltage can be divided into two levels by controlling the current-limiting resistance. The two-stage current-limiting resistance design serves the purpose of both breaking rapidly the insulation between the wire electrode and workpiece with a high open voltage of 200 V and providing a low discharge current for achieving high-
712 Mu-Tian Yan and Tsung-Chien Lin / Procedia CIRP 42 ( 2016 ) 709 714 precision machining. The two-stage resistance power supply can also provide a period of off duty cycle the same as that of the conventional pulsed power supply. A PCD blank contains nonconductive diamond particles which could not be evaporated and eroded away by micro wire-edm and therefore there could produce a large amount of debris in the spark gap during the erosion process. As a result, the debris particles stagnated in the machined kerf could contaminate the spark gap. The period of off duty cycle was devised intentionally to flush away the debris particles. (a) Circuit diagram was used as the wire electrode. S-PCD and BD-PCD were used as the workpiece materials. The S-PCD layer has a thickness of 0.6 mm and the tungsten carbide layer is 1 mm thick. The BD-PCD layer has a thickness of 0.5 mm and the tungsten carbide layer is 1.5 mm thick. Both of the two PCD materials contain diamond grains with a diameter of 2 m. Specific resistance of S-PCD and BD-PCD are 1.4 10-4 Ω m and 1.6 10-5 Ω m, respectively. Thermal conductivity of S- PCD and BD-PCD are 500~600 W/m K and 440~580 W/m K, respectively. It should be noted that the diamond grain size of the S-PCD and BD-PCD used in the reference work has a diameter of 10 m [7]. Machining parameters used in the experiment is listed in Table 1. Analysis of pulse train data and measurement of discharge voltage and current waveforms were performed using a digital oscilloscope (Tektronix MOS- 3034) along with a specially designed current probe (Tektronix TCP0150). A laser scanning confocal microscope (KEYENCE VK-9710) was used to perform the analysis of 2D workpiece surface topography and measurement of surface roughness. An optical microscope (Nikon MM-11) fitted with a charge couple device (CCD) video camera was utilized for the analysis of thermal damaged layer and microstructural evaluation. Table 1 Machining settings used in the experiments. Items Value Open voltage (V) Pulse on-time (μs) 200 0.5 Resistance on-time (μs) 0.2 Pulse off-time (μs) 0.5 First-stage resistance (Ω) 14 Second-stage resistance (Ω) 21 Wire tension (gf) 200 Dissipating resistance (Ω) 200 Resistivity of water (KΩ cm) 100 5. Experimental Result (b) Timing charts of the pulse control signals 4. Experimental Method Fig3. Two-stage resistance power supply. A commercial wire-edm machine developed by local machine tool builder in Taiwan has been retrofitted with, a fine wire transport system with closed-loop wire tension control, a two-axis linear motor stage with sub-micro meter resolution of feed drive, a transistor-controlled power supply and an open architecture CNC system to achieve micro machining purpose and excellent machining results. The developed pulse generators including the conventional pulsed power supply and two-stage resistance power supply have been incorporated into this prototype machine to testify the effectiveness of this new technique in rough cutting of PCD by micro wire-edm. Tungsten wire with a diameter of 50 m Figure 4 shows open voltage of micro wire-edm under a rough cutting condition using both two power supplies. The conventional pulsed power supply can only provide an open voltage of 200 V. Whereas, the two-stage resistance power supply can provide a first-stage open voltage of 200 V by lowering current-limiting resistance into the discharge circuit and a second-stage open voltage of 90 V by increasing current-limiting resistance into the discharge circuit. Figure 5 shows gap voltage and current waveforms of micro wire- EDM using both two power supplies. The conventional pulsed power supply can achieve low energy pulses with a peak current of 10.2A and discharge duration of 853 ns compared with a lower peak current of 7 A and shorter discharge duration of 800 ns by the two-stage resistance power supply. This figure has verify the effectiveness of the proposed pulse generator in providing high open voltage and low energy pulses for rough-cutting of PCD by micro wire-edm. A comparison of BD-PCD slit and thermal damaged layer produced by micro wire-edm using both power supplies is illustrated in Figure 6. By supplying lower discharge energy
Mu-Tian Yan and Tsung-Chien Lin / Procedia CIRP 42 ( 2016 ) 709 714 713 pulses, the two-stage resistance power supply can achieve lower thermal damaged layer than the conventional pulsed power supply. Figure 7 demonstrates PCD cutting edge machined by micro wire-edm using two-stage resistance power supply. It can also be observed from the edge part of PCD surface, the cutting edge of BD-PCD appears much neater and smoother than that of S-PCD because the BD-PCD made up of electrically conductive diamond particles is prone to be machined more easily by the micro wire-edm process compared with the S-PCD. (a) (a) Convetional pulsed power supply (b) Fig. 6 BD-PCD slit and thermal damaged layer produced by micro wire-edm using (a) the conventional pulsed power supply and (b) the two-stage resistance power supply. (b) Two-stage resistance power supply Fig. 4 Open voltage of micro wire-edm under a rough cutting condition using two different power supplies. (a) Standard PCD (a) Convetional pulsed power supply (b) Two-stage resistance power supply Fig. 5 Gap voltage and current waveforms of micro wire-edm using both two power supplies. (b) Boron-doped PCD Fig. 7 Micrographs of PCD cutting edge machined by micro wire- EDM using two-stage resistance power supply.
714 Mu-Tian Yan and Tsung-Chien Lin / Procedia CIRP 42 ( 2016 ) 709 714 Figure 8 shows a comparison between the machined surface of S-PCD and BD-PCD using two different power supplies. As depicted in Figure 8(a), machined surface of S- PCD obtained by using the conventional pulsed power supply has some micro pits because diamond grains are poor conductors of electricity and thus they were dislodged from the S-PCD surface after the cobalt binders have been eroded by electrical discharges. While, the appearance of the S-PCD and the BD-PCD surface obtained by using the two stageresistance power supply look flatter and exhibit no micro pit. Compared with the machined surface of S-PCD, the appearance of the BD-PCD surface looks flatter and smoother since the electrically conductive diamond particles of the BD- PCD can be easily cut or broken by spark erosion. Figure 9 shows a comparison of surface roughness for different PCDs between using two different power supplies. The two-stage resistance power supply can achieve better surface finish for both PCDs than the conventional pulsed power supply because the former can provide lower peak current and shorter discharge duration than the later. It can be visibly seen from this figure that the surface roughness of the BD-PCD is better than that of the S-PCD in rough cutting of micro wire-edm for two different power supplies. 6. Conclusion This study proposes the two-stage resistance power supply to maintain high open voltage for breaking down the insulating property of oil dielectric as well as provide lower discharge energy for achieving high surface quality on machined surface of PCD. Compared with the conventional pulsed power supply, the developed two-stage resistance power supply can provide lower peak current and shorter discharge duration, thereby achieving lower damaged layer and better surface finish on machined surface of both S-PCD and BD-PCD. Experimental results also demonstrate the superiority of the BD-PCD over the S-PCD in better cutting edge quality, surface quality and surface roughness since the former has lower specific resistance and thermal conductivity than the later, which indicate the improvement in machining performance of PCD by oil-based micro wire-edm. Acknowledgements This research was supported by the Ministry of Science and Technology, Taiwan under grant no. MOST 104-2221-E- 211-003-MY2. References (a) Conventional pulsed power supply (b) Two-stage resistance power.supply Fig. 8 Micrographs of PCD surface machined by micro wire-edm using two different power supplies. Fig. 9 Surface roughness of two PCDs using two different power supplies. [1] Takino, H., Ichinohe, T., Tanimoto, K., Nomura, K., and Kunieda, M., 2004, Cutting of Polished Single-Crystal Silicon by Wire Electrical Discharge Machining, Precision Engineering 28, p. 314. [2] Storr, M., Speth, J., and Rehbein, W., 2007, A new Dielectric for Wire-EDM, Proceedings of the 15th International Symposium on Electromachining (ISEM XV), Pittsburgh, USA, p. 195-199. [3] Information on http://www.agie.com/english/index_e.html [4] Information on http://www.makino.co.jp/en/index.html. [5] Information on http://www.mcmachinery.com/ [6] Information on http://www.factdiamond.com/index.htm [7] Suzuki, K., Shiraishi, Y., Nakajima, N., Iwai, M., Ninomiya, S., Tanaka, Y. and Uematsu, T., 2009, Development of New PCD Made Up of Boron Doped Diamond Particles and Its Machinability by EDM, Advanced Materials Research, 76-78, p.684. [8] Chen, S. T., Chang, C. H., 2012. Study on Thinning of a Boron-doped Polycrystalline Diamond Wheel-Tool by Micro Rotary W-EDM Approach, Applied Mechanics and Materials 217-219, p. 2167. [9] Klocke, F., Lung, D., Thomaidis, D., and Antonoglou, G., 2004, Using Ultra Thin Electrodes to Produce Micro-parts with Wire-EDM, J. of Mater. Process. Technol., 149, 1-3, p. 579. [10] Yan, M. T., Fang, G. R., Liu, Y. T., 2013, An Experimental Study on Micro Wire-EDM of Polycrystalline Diamond Using a Novel Pulse Generator, The International Journal of Advanced Manufacturing Technology, 66, 9-12, p. 1633. [11] Okano, K., Akiba, Y., Kurosu, T., Iida, M., Nakamura, T., 1990, Synthesis of b-doped diamond film, Journal of Crystal Growth, 99, p. 1192. [12] Chen, S. T., Chang, C. H., 2013, Development of an Ultrathin BD- PCD Wheel-tool for in Situ Microgroove Generation on NAK80 Mold Steel, J. of Mater. Process. Technol. 213/5, p. 740.