- Email: [email protected]
Wire electrochemical grinding of tungsten micro-rods using neutral electrolyte
Precision Engineering xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Wire electrochemical grinding of tungsten micro-rods using neutral electrolyte ⁎
Wei Han , Masanori Kunieda Department of Precision Engineering, The University of Tokyo, Tokyo 113-8656, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrochemical machining Wire electrochemical grinding Electrostatic induction feeding method Tungsten Micro-rods
A new method, wire electrochemical grinding(WECG), is proposed for machining tungsten micro rods with neutral electrolyte and bipolar current. The inevitable problem of tool wear due to the bipolar current can be eliminated by using a running wire as the tool electrode with this method. In this study, three kinds of wire guides were used for feeding the workpiece rod in the axial direction: cylindrical cemented tungsten carbide (WC) guide, cylindrical zirconium oxide (ZrO2) guide, and disk-shaped cemented WC guide, to investigate the influence of materials and configurations of the guide on the machining characteristics. The experimental results showed that the disk-shaped WC wire guide can realize higher machining accuracy because of less influence of stray current flowing through the gap formed between the machined micro-rod and wire guide. Even if the ZrO2 material has a much lower electrical conductivity than that of the cemented WC, the machining process was still influenced by the stray current between the micro-rod and wire guide. Furthermore, micro-rods were also machined by feeding the rod workpiece in the radial direction. Compared to when the workpiece is fed in the axial direction, the current was higher, which was clarified by the simulation of the current density distribution. However, the machining time was much longer than that with the workpiece fed in the axial direction because more machining steps were necessary to obtain a smooth side surface. With the workpiece fed in the axial direction method, a tungsten micro-rod of 35 μm in diameter was machined to the length of 163 μm. The results confirmed that the new method is capable of miniaturization equivalent to micro electrical discharge machining (EDM).
1. Introduction Demands for the fabrication of micro-rods are increasing, because micro-rods are widely used as tools for micro-drilling and micro-milling 3D structures [1] and probes for measuring system such as the atomic force microscopy (AFM). Electrochemical machining (ECM) is a promising method for machining micro-rods because it is an anodic dissolution process [2] which is free of residual stress and surface cracks inevitably generated in thermal processes such as electrical discharge machining (EDM) and laser machining. In the application, tungsten is widely used as micro-rod material because of the high erosion resistance, high electrical and thermal conductivity, and high stiffness, which are significantly important especially for micro-rods with high aspect ratios. However, the electrochemical machining of tungsten is difficult because tungsten oxide layer is generated on the anode tungsten surface, blocking the electrolytic current. Hence, Fan and Hourng [3] and Lim and Kim [4] used NaOH and KOH aqueous solution, respectively, as the electrolyte to fabricate tungsten rods. However, since these strongly alkaline electrolytes are highly harmful to the ⁎
environment, use of neutral electrolytes is desirable. Maeda et al. [5] reported that cemented tungsten carbide can be machined with a neutral electrolyte such as sodium nitrate (NaNO3) aqueous solution using a bipolar current, because NaOH is generated when the tungsten carbide electrode is in negative polarity, thereby removing the oxide layer from the surface of tungsten carbide. Thus, Natsu and Kurahata [6] machined cemented tungsten carbide (WC) rods successfully using a NaNO3 aqueous solution. However, the machining accuracy was not sufficiently high to obtain micro-rods with diameters equivalent to that which can be obtained by EDM, because the bipolar current resulted in a significantly large tool wear. Furthermore, since the pulse duration of the machining current was as long as several tens to several hundreds of ms, the electrochemical reaction could not be localized in a small gap width. Nevertheless, it is considered that pure tungsten can also be machined using a neutral electrolyte and bipolar current based on the same mechanism reported by Maeda et al. [5] and Natsu and Kurahara [6]. On the other hand, Schuster et al. [7] found that electrochemical micromachining can be performed when ultra-short pulse current of
Corresponding author at: Room 833, Engineering Building 14#, Department of Precision Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan. E-mail addresses: [email protected] (W. Han), [email protected] (M. Kunieda).
https://doi.org/10.1016/j.precisioneng.2018.02.006 Received 5 November 2017; Received in revised form 17 January 2018; Accepted 31 January 2018 0141-6359/ © 2018 Elsevier Inc. All rights reserved.
Please cite this article as: Han, W., Precision Engineering (2018), https://doi.org/10.1016/j.precisioneng.2018.02.006
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
in wire EDM, the wire is supported by a cylindrical guide which is circumferentially grooved on the edge. Since the present work aims to machine tungsten micro-rods electrochemically with a high accuracy equivalent to WEDG using a bipolar current, the same tool electrode system as WEDG was introduced and named as wire electrochemical grinding (WECG). This method is therefore able to solve both the problems of the tool electrode wear and wire vibration. Three kinds of wire guides were used in this study, namely the cylindrical cemented tungsten carbide (WC) guide, cylindrical zirconium oxide (ZrO2) guide, and disk-shaped WC guide. The ZrO2 was used as guide material because of its significantly low electrical conductivity, which may reduce the stray current during the electrochemical process, compared with cemented tungsten carbide. The cylindrical and disk-shaped wire guides form different gap areas during machining because of their different surface areas with different geometries. Micro-rods can be machined with the workpiece fed either in the axial or radial directions, and because the wire tool electrode is running during machining, tool wear which is inevitably generated by the bipolar current does not influence machining accuracy.
Fig. 1. Electrostatic induction feeding ECM.
Fig. 2. Principle of electrostatic induction feeding ECM.
2. Wire electrochemical grinding (WECG) method several tens of ns in duration is used. In order to easily obtain such an ultra-short pulse current in ECM without using an expensive ultra-short pulse generator, Koyano and Kunieda [8] developed the electrostatic induction feeding ECM as shown in Fig. 1. Fig. 2 shows the equivalent circuit of this method and gap current and voltage waveforms. When the pulse voltage is supplied across the working gap, the electric double layer is formed on the surface of electrodes, which can be expressed as Cdl. Then, the working gap can be modeled as the Faraday impedance Rf, resistance of electrolyte in the machining gap Rg and Cdl [9]. Since the pulse voltage with a constant pulse duration is coupled to the tool electrode by a feeding capacitance C1, current only flows at the instance when the pulse voltage changes to high or low as shown in Fig. 2. Hence, the current pulse duration is nearly equal to the rise and fall time regardless of the pulse on-time of the pulse voltage. In addition, the current is bipolar, and this is advantageous to tungsten machining using a neutral electrolyte. With this method, a pulse duration shorter than several tens ns can easily be obtained. Hence, Han and Kunieda [10] fabricated tungsten micro-rods successfully with a neutral electrolyte, NaNO3 or NaCl aqueous solution, and bipolar current. They used an ultra-short pulse current of 20 ns to 40 ns in their research, and thus successfully fabricated a micro-rod with a diameter of 7.1 μm and aspect ratio of 14 by localizing the electrochemical dissolution in a significantly small working gap. To obtain such a high accuracy, however, they used a platinum tool electrode to avoid tool electrode wear. Thus, this process was costly. A wire tool electrode was first used in micro EDM by Masuzawa et al. [11] to fabricate micro-rods without the influence of tool wear due to discharge, known as the wire electro-discharge grinding (WEDG) method. In WEDG, to avoid the wire vibration, which normally occurs
The WECG method is shown in Fig. 3. The micro EDM machine (Panasonic, MG-ED72W) was converted to the micro ECM machine by replacing the EDM pulse generator with the circuit in Fig. 1. The tool electrode, which is kept running during machining, is commercial brass wire with a diameter of 100 μm and held by a wire guide. Since the wire tool electrode is running during machining, the influence of tool wear caused by the bipolar current on machining accuracy can be eliminated. The rod workpiece is fed toward the wire tool electrode during machining and rotates with a speed of 3000 rpm. The feed distance in the axial direction determines the rod length while the cut depth in the radial direction determines the rod diameter. The electrolyte is jetted into the working gap by a nozzle 210 μm in inner diameter placed near the working gap. The ultra-short-pulse bipolar current is supplied by the electrostatic induction feeding method. The three kinds of wire guides used in this study are shown in Fig. 4: the cylindrical cemented WC guide, cylindrical ZrO2 guide, and diskshaped cemented WC guide. All their diameters were set to 10 mm. The guides in Fig. 4(a) and (c) are made of a material with a high electrical conductivity, but have different geometries, while (b) is made of ZrO2 which has a much lower electrical conductivity. Table 1 shows the properties of the materials used for the guides [12]. Fig. 5 shows the brass wire held by one of the wire guides. Since the brass wire is running off the edge of the wire guide by 75 μm, the cut depth in radial direction must be made smaller than 75 μm to avoid the wear of the wire guide due to electrochemical dissolution, because current can flow through the gap between the shoulder surface of workpiece and the top surface of the wire guide as shown in Fig. 3.
Fig. 3. Wire electrochemical grinding method.
2
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 4. Wire guides used in this study.
and E0 is the total voltage amplitude of the pulse voltage, is constant [8] with the electrostatic induction feeding method. Fig. 7 shows the microrods machined with different pulse durations, demonstrating that the WECG method could machine tungsten with the neutral electrolyte and bipolar current. The diameter of micro-rod was larger and the side surface of micro-rod was straighter with the pulse duration of 20 ns. This result can be supported by the findings of Schuster et al. [7] who showed that the electrochemical dissolution can be localized in a smaller working gap using nanosecond pulse. In addition, it should be noted that the roots of both the micro-rods were rounded because the tool electrode was brass wire and the cross section shape of the wire was transferred to the workpiece at the root area. Fig. 8 shows the side surfaces and cross sectional shapes of the brass wire before and after machining with different wire running speeds. It was severely worn out with the wire running speed of 11.3 mm/s due to the bipolar current, while, the tool wear significantly decreased when the running speed was increased to 53.3 mm/min. This suggests that tool wear due to bipolar current can be eliminated by increasing the wire running speed. Fig. 9 shows the wear of a plate tool electrode made of stainless steel (SUS304) [10], which was seriously worn out by the bipolar current. Hence, compared with the plate tool electrode method, the influence of tool wear was eliminated with the proposed WECG method.
Table 1 Properties of cemented WC and ZrO2. Properties
Electric conductivity [S/m] Density [g/cm3] Melting point [K] Thermal conductivity [W/ m k]
Cemented WC
ZrO2
Minimum
Minimum
0.024 15.25 3000 28
Maximum 0.01 15.88 3193 88
3.16*10 5 2823 1.7
−14
Maximum 3.16*10−19 6.15 2973 2.7
3. Tungsten machining with workpiece fed in axial direction The workpiece rod can be machined when it is fed either in the axial or radial direction. The machining characteristics were first investigated with the workpiece fed in the axial direction. A commercial tungsten rod of 300 μm in diameter was used as the workpiece, and reshaped to a diameter of 200 μm with the wire electro discharge grinding (WEDG) method [11] to make it sufficiently straight. Table 2 shows the machining conditions used in WECG. The neutral electrolyte NaNO3 aqueous solution of 6 wt% in concentration was used as electrolyte. The bipolar current was generated by the electrostatic induction feeding method. In addition, the rise and fall times of pulse voltage, which determines the current pulse duration, were set as 20 ns and 40 ns. A ceramic capacitor of 350 pF was used as the feeding capacitance C1. The workpiece was positioned over the brass wire with an initial gap width of 3 μm before machining. The cut depth in the radial direction was 50 μm, which was kept constant during machining, and the feed distance in the axial direction was 100 μm. The feed speed of workpiece in the axial direction was 1.0 μm/s. The cylindrical cemented WC guide was used. Fig. 6 shows the waveforms of gap current and voltage with different pulse durations. It can be seen that the ultra-short bipolar current was easily obtained with the electrostatic induction feeding method. The current increased with decreasing pulse duration because the electric charge per pulse q = C1E0, where C1 is the feeding capacitance
4. Machining characteristics with different kinds of wire guide Three wire guides shown in Fig. 4 were used to machine tungsten micro-rods with the workpiece fed in the axial direction to investigate the differences in the machining characteristics. Table 3 shows the experimental conditions. The feeding capacitance was changed as 220 pF, 350 pF, 470 pF and 570 pF. The cut depth in the radial direction was 50 μm, and the feed distance in the axial direction was 150 μm, which is larger than the diameter 100 μm of the brass wire electrode. The feed speed of workpiece was 1.0 μm/s. Fig. 10 shows the waveforms of the gap current and voltage with the feeding capacitances of 470 pF and 570 pF when the cylindrical WC wire guide was used. The gap current and voltage increased with increasing feeding capacitance because the electric charge per pulse
Fig. 5. Top view of brass wire hooked on wire guide.
3
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
guides. This stray current could be decreased with the disk-shaped wire guide compared with the cylindrical wire guides by decreasing the radial gap area. Hence, a longer micro-rod was obtained with the diskshaped cemented WC wire guide. In addition, the length of the microrods was a little longer with the cylindrical ZrO2 guide than that with the cylindrical WC guide when the feeding capacitance was 220 pF and 570 pF, this is because the electric conductivity of the ZrO2 material is much lower than the WC material as shown in Table 1. Hence, the influence of the stray current flowing through the gap formed by the wire guide and micro-rod was decreased. However, the difference in the length of the micro-rods was insignificant between the WC guide and ZrO2 guide. In this experiment, the taper angle θ was defined as the included angle between the side surface of micro-rod and rod axis. In order to measure θ, diameters were measured along the axial direction with the same interval using SEM. Then least squares fitting was used to approximate the side surface of the micro-rod and calculate θ. θ with different voltage amplitudes and feed speeds is shown in Fig. 14. θ was smallest with the disk-shaped cemented WC guide because of less influence of the stray current in the radial gap. In addition, the taper angle increased with increasing feeding capacitance because the electric charge per pulse q = C1E0 increased, resulting in higher fraction of material dissolution from the side surface of the machined micro-rod due to the larger working gap. θ was significantly large with the cylindrical wire guides with the feeding capacitance of 220 pF. This is because the lengths of the micro-rods machined were shorter than the radius of the wire electrode as shown in Fig. 11.
Table 2 Experimental conditions used to machine micro-rod with workpiece fed in axial direction. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
90 500 50 20, 40 350 NaNO3 aq. 6 wt% Brass Φ100 μm 3000
q = C1E0 increased. Fig. 11 shows the micro-rods machined with different wire guides and feeding capacitances. With the small feeding capacitance of 220 pF, the machining processes were interrupted by collision between electrodes, resulting in the rod length becoming shorter than the preset feed distance of 150 μm in the axial direction. This is because the electric charge per pulse q = C1E0 decreases with decreasing feeding capacitance C1 resulting in a lower material removal rate. With the large feeding capacitance of 550 pF, however, the microrods were shortened due to the stray current flowing between the side surface of the guide and the micro-rods. The influence of the stray current in the radial gap increased because the axial gap width increased due to the larger feeding capacitance under the same feed speed of workpiece, resulting in more material dissolution from the side and end surfaces of the machined micro-rod. Fig. 12 shows the lengths of micro-rods machined with different kinds of wire guide. With the small feeding capacitance of 220 pF and large feeding capacitance of 570 pF, the lengths of micro-rod were smaller than the preset feed distance of 150 μm in the axial direction because of the influences of the collision between electrodes and stray current flowing through the side and end surfaces of the micro-rod. However, the length of the micro-rods with the disk-shaped cemented WC wire guide was longer than that with the cylindrical cemented WC guide and cylindrical ZrO2 guide. This is because there was a considerable stray current flowing through the gap formed by the wire guide and machined micro-rod as shown in Fig. 13 with the cylindrical
5. Tungsten machining with workpiece fed in radial direction When the workpiece was fed in the radial direction to machine a tungsten rod, the cut depth in the radial direction increased during machining and the feed distance in axial direction remained constant. To machine a longer micro-rod than the diameter of the wire tool electrode, the machining process should be repeated while changing the axial position of the workpiece electrode incrementally. Furthermore, some material is left un-machined when the axial position of the
Fig. 6. Waveforms of gap current and voltage with different pulse durations.
4
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 7. Micro-rods machined with different pulse durations.
Fig. 8. Brass wires in different conditions of (1) before machining, (2) wire speed 11.3 mm/s and (3) 53.3 mm/s.
workpiece is changed by a large distance because the cross-sectional shape of the wire tool electrode is circular, hence, the machining process of the workpiece fed in the radial direction method consists of two machining steps: rough and finish machining. In finish machining, the workpiece is fed continuously in the axial direction with the radial position fixed to remove the remained material and obtain a straight side surface. 5.1. Rough machining with workpiece fed in radial direction Fig. 15 shows the tungsten micro-rod machining method with the workpiece fed in the radial direction. The workpiece is positioned beside the brass wire electrode with an initial gap width of 3 μm, and fed in the radial direction as shown in Fig. 15(b). To machine a longer micro-rod than the diameter of the wire tool electrode, the workpiece is fed stepwise in the axial direction to change the axial position of the workpiece as shown in Fig. 15(c), and fed in the radial direction again. This machining process should be repeated to obtain a long micro-rod. Table 4 shows the experimental conditions used for rough machining with the workpiece fed in the radial direction. The feed distance in the radial direction was 50 μm and the feed interval in the axial direction was 100 μm, 50 μm and 25 μm. The rod length finally machined was 200 μm. The feed speed in the radial direction was 1.0 μm/s. Fig. 16 shows micro-rods machined with different feed steps of 100 μm, 50 μm and 25 μm in the axial direction when the voltage amplitude was 90 V. With the feed step of 100 μm in the axial direction, some material remained as shown in Fig. 16(a). Moreover, the remaining material decreased when the feed interval was decreased in the
Fig. 9. Tool wear with plate tool electrode and material of stainless steel (SUS304) [10].
Table 3 Experimental conditions used to investigate machining characteristics of different wire guides. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
90 500 50 40 220, 350, 470, 570 NaNO3 aq. 6 wt% Brass Φ100 μm 3000
5
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 10. Waveforms of gap current and voltage with different feeding capacitances.
Fig. 11. Micro-rods machined with different kinds of wire guide and feeding capacitances (1) 220 pF, (2) 350 pF, (3) 470 pF and (4) 570 pF.
time due to the increase of the feeding steps leads to more removal by the stray current. Fig. 18 shows the micro-rod machined with the axial feed interval of 25 μm when the voltage amplitude was decreased to 80 V. The feed distance in the radial direction was decreased to 35 μm from the previous experiment performed at 50 μm, because collision between electrodes occurred frequently when the feed distance in the radial direction was larger than 35 μm due to the increase in the gap area, which caused a lower current density. Compared with Fig. 16(c), a micro-rod was fabricated successfully with the feed interval of 25 μm when the voltage amplitude E0 decreased to 80 V. This is because the influence of stray current shown in Fig. 17 decreased with the decrease in the gap width due to the smaller electric charge per pulse q = C1E0. Fig. 12. Lengths of micro-rods with different kinds of wire guide.
5.2. Finish machining with workpiece fed in radial direction method
axial direction to 50 μm. The micro-rod disappeared totally when the feed interval was further decreased to 25 μm as shown in Fig. 16(c). It is thought that the micro-rod was shortened by the stray current flowing through the side surface which was fabricated in the previous step. Fig. 17 shows the stray current in the second feeding step of workpiece in the radial direction when the feed interval was 25 μm. Since the working area is smaller, the gap width is larger under the same feed speed, resulting in higher stray current. In addition, longer machining
Rough machining was performed with the feed interval of 50 μm under the machining conditions in Table 4 to obtain the micro-rod shown in Fig. 16(b). Next, in the finish machining process, the workpiece was positioned at 3 μm over the top surface of the wire electrode and the cut depth in the radial direction was equal to the feed distance in radial direction in the rough machining. Then, the workpiece was fed in the axial direction to remove the remaining material and obtain a
6
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
experimentally obtained waveforms. Hence, the total current flowing through the workpiece surfaces was set at 900 mA and the potential of the wire guide and brass wire surfaces was set as ground. Other surfaces were assumed as electrically insulative. The COMSOL Multiphysics simulation tool was used, and the mesh size was a user-controlled mesh with normal element size. The electrolyte was sodium nitrate (NaNO3) aqueous solution with concentration of 2 wt%. Thus, the electrical conductivity of the electrolyte was 2.05 S/m, which was measured using an electrical conductivity meter (LAQUAact ES-71). Fig. 21 shows the simulation results of the current density distributions with different machining methods, in which both machining methods are influenced by stray current. First, it can be noted that the influence of stray current with the workpiece fed in axial direction is less than that with the workpiece fed in radial direction. Hence, the current efficiency is higher with the workpiece fed in the axial direction than that with the workpiece fed in the radial direction. Furthermore, the influence of stray current is constant with increasing feed distance in the axial direction, as shown in Fig. 21(a). However, the influence of stray current increases with decreasing feed distance in radial direction, as shown in Fig. 21(b). This is because the machining area increases with increasing feed distance in the radial direction, resulting in more current flowing through the machining area. Since the electric charge per pulse q = C1E0 is constant with the electrostatic induction feeding method, the stray current decreases.
Fig. 13. Stray current in gap formed between micro-rod and wire guide.
6. Micro-rods machining with high aspect ratio 6.1. Influence of wire guide Table 6 shows the experimental conditions used to machine microrods with high aspect ratios. The workpiece was fed in axial direction with a simple machining process than the workpiece fed in the radial direction. According to the experimental results in Section 4, the feeding capacitance was set to 350 pF. Three kinds of wire guides were used to compare the machinability of micro-rods with high aspect ratio. The cut depth in the radial direction was set as 50 μm and feed distance in the axial direction was 850 μm. The feed speed was initially set at 1.0 μm/s and it was decreased from 1.0 μm/s to 0.5 μm/s when the feed distance in the axial direction increased to 400 μm. This is because the influence of the stray current shown in Fig. 13 increased with increasing feed distance in the axial direction, resulting in the decrease in the machining current in the axial working gap. Fig. 22 shows the micro-rods machined with different kinds of wire guides. The length of the micro-rod machined with the cylindrical cemented WC guide was much shorter than the preset feed distance of 850 μm because the machining process was interrupted by the collision between electrodes when the feed distance in axial direction reached 450 μm. The machining processes with the cylindrical ZrO2 guide and disk-shaped cemented WC guide completed the preset feed distance of 850 μm in the axial direction. However, it is noted that the taper of the micro-rod machined with the cylindrical ZrO2 guide was much larger
Fig. 14. Taper angles of micro-rod with different kinds of wire guides.
straight side surface. Table 5 shows the experimental conditions used for finish machining. The rise/fall time was decreased to 20 ns to localize the electrochemical dissolution in a smaller working gap. The feed distance in the axial direction was 200 μm which is equivalent to the length of the rod obtained from the rough machining process. Fig. 19 shows the micro-rod obtained after finish machining. 5.3. Comparison between workpiece fed in axial and radial direction methods To compare the workpiece fed in axial and radial direction methods, the 2D models in Fig. 20 were built to calculate the current density distribution. Since the electric charge per pulse q = C1E0 is constant with the electrostatic induction feeding method, the constant current of 900 mA during the pulse duration of 40 ns was assumed based on the
Fig. 15. Micro-rod machining method with workpiece fed in radial direction.
7
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 16. Micro-rods machined with different feed intervals (a) 100 μm, (b) 50 μm, (c) 25 μm.
Table 4 Experimental conditions used for rough machining with workpiece fed in radial direction. Pulse voltage
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm]
80, 90 500 50 40 350 NaNO3 aq. 6 wt% Brass Φ100 μm 3000
Fig. 19. Micro-rod obtained by finishing machining.
than that with the disk-shaped WC guide. This is mainly because the stray current flowing through the gap between the wire guide and rod surface was negligibly low with the disk-shaped WC guide. There is also a significant difference in the length of micro-rod machined between the cylindrical WC and ZrO2 wire guides as shown in Fig. 22(a) and (b), although Fig. 12 shows a slight difference. Since the machined lengths of micro-rods machined were short in Fig. 12, the influence of stray current between the wire guide and machined micro-rod was not obvious, especially with a significantly small and large feeding capacitance with which the lengths of micro-rod were significantly small. Fig. 23 shows the waveforms of gap current and voltage with different wire guides at the feeding distance of 400 μm in the axial direction. The current with the cylindrical WC wire guide was much higher than that with the cylindrical ZrO2 wire guide because of the higher electrical conductivity of cemented WC than the ZrO2, resulting in a higher influence of the stray current in the gap between the wire guide and machined micro-rod as shown in Fig. 13. It should also be noted that the gap current is lowest with the disk-shaped guide.
Fig. 17. Current in working gap with feed interval of 25 μm.
6.2. Micro-rod machining with smallest diameter comparable to micro EDM Micro-rods with the smallest diameter equivalent to that which can be obtained in micro EDM were machined using the conditions shown in Table 7. The initial diameter of the workpiece rod prepared using WEDG was 200 μm. The depth of cut in the radial direction should be smaller than 75 μm to avoid the wear of the wire guide. Hence, to machine micro rods with a diameter smaller than 50 μm, a two-step machining process was used. In the first machining step, the voltage amplitude was 90 V and the rise/fall time was 40 ns. The feed speed of workpiece was 1.0 μm/s. The cut depth in the radial direction and feed distance in the axial direction were 50 μm and 200 μm, respectively. In the second machining step, the voltage amplitude was 40 V or 50 V and the rise/fall time was 20 ns. The feed speed of workpiece was 0.6 μm/s. The cut depth in the radial direction and feed distance in axial direction were 30 μm and 160 μm, respectively. Fig. 24 shows the micro-rods obtained with different voltage amplitudes in the second machining step. With the voltage amplitude of
Fig. 18. Micro-rods machined with feed interval of 25 μm and voltage amplitude of 80 V.
Table 5 Experimental conditions used for finishing machining. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm] Feed speed [μm/s]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
50 500 50 20 350 NaNO3 aq. 6 wt% Brass Φ100 μm 3000 1.0
8
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 20. Models of current density distribution calculation with (a) Feeding of workpiece in axial direction and (b) Feeding of workpiece in radial direction.
Fig. 21. Simulations of current density distribution with different machining methods.
50 V, the micro-rod was shortened because of the large working gap width resulting in excessive material dissolution. With the voltage amplitude of 40 V, the micro-rod was machined successfully to a diameter of 35 μm and length of 163 μm. Although the diameter is still larger than the smallest diameter achievable with micro EDM [1], the micro ECM method we proposed has the advantages of no crack or residual stress on the machined surface.
Table 6 Experimental conditions used for micro-rods machining with cut depth of 50 μm in radial direction. Pulse voltage
7. Conclusions
Feeding capacitance C1 [pF] Electrolyte Tool electrode rotation [rpm]
A wire electrochemical grinding (WECG) method was proposed in this study to machine tungsten micro-rods with neutral electrolyte 9
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
90 500 50 40 350 NaNO3 aq. 6 wt% 3000
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 22. Long micro-rod machined with different kinds of wire guides (a) Cylindrical cemented WC guide, (b) Cylindrical ZrO2 guide, (c) Disk-shaped cemented WC guide.
Fig. 23. Waveforms of gap current and voltage with different wire guides at feed distance of 400 μm in axial direction.
2) With the workpiece fed in the axial direction, the machining characteristics of three kinds of wire guides: cylindrical cemented WC guide, cylindrical ZrO2 guide and disk-shaped cemented WC guide, were investigated. The disk-shaped WC wire guide showed the best machining accuracy because the influence of the stray current in the gap formed by the wire guide and micro-rod was significantly decreased. 3) With the workpiece fed in the radial direction, micro-rods were machined successfully. Compared with the workpiece fed in the axial direction method, the current efficiency was lower due to stronger influence of the stray current flowing through the side and end surface of the machined micro-rod, which was confirmed by the simulation of current density distribution. 4) With the workpiece fed in the axial direction, micro-rods with a diameter of 100 μm could be machined to a length of 850 μm by a single machining step. Smaller rod diameters of 35 μm with microrod length of 163 μm could be obtained from two machining steps, realizing micromachining ability equivalent to micro EDM
Table 7 Experimental conditions used for micro-rod machining with two steps. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode rotation [rpm] Feed speed [μm/s]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
40, 50, 90 500 50 20, 40 350 NaNO3 aq. 6 wt% 3000 0.6, 1.0
NaNO3 aqueous solution and bipolar current supplied by the electrostatic induction feeding method. The following conclusions were obtained. 1) Tungsten micro-rods were successfully machined with the proposed WECG method which applies neutral electrolyte NaNO3 aqueous solution and bipolar current. The influence of tool wear due to bipolar current was significantly small when the running speed of wire tool electrode was 53.3 mm/min.
Fig. 24. Micro-rods obtained with different voltage amplitudes in second machining step.
10
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Acknowledgments
cylindrical micropin. Int J Mach Tools Manuf 2001;41(15):2287–96. [5] Maeda S, Saito N, Haishi Y. Principle and characteristics of electro-chemical machining. Tech Rep Mitsubishi Electr 1967;41(10):1267–79. in Japanese. [6] Natsu W, Kurahata D. Influence of ECM pulse conditions on WC alloy micro-pin fabrication. Proc. of the 17th International Symposium on Electromachining 2013:675–9. [7] Schuster R, Kirchner V, Allongue P, Ertl G. Electrochemical micromachining. Science 2000;289(5476):98–101. [8] Koyano T, Kunieda M. Micro electrochemical machining using electrostatic induction feeding method. CIRP Ann-Manuf Technol 2013;62(1):175–8. [9] Bard AJ, Faulkner LR, Leddy J, Zoski CG. Electrochemical methods: fundamentals and applications Vol. 2. New York: Wiley; 1980. [10] Han W, Kunieda M. Fabrication of tungsten micro-rods by ECM using ultra-shortpulse bipolar current. CIRP Ann-Manuf Technol 2017;66(1):193–6. [11] Masuzawa T, Fujino M, Kobayashi K, Suzuki T, Kinoshita N. Wire electro-discharge Grinding for micro-machining. CIRP Ann-Manuf Technol 1985;34(1):431–4. [12] Properties of ZrO2, August, 2017. https://www.azom.com/properties.aspx? ArticleID=133; Properties of WC, August, 2017 https://www.azom.com/ properties.aspx?ArticleID=1203.
This work was supported by the Cross-Ministerial Strategic Innovation Promotion Program (SIP): Innovative Design/ Manufacturing Technologies, funded by NEDO. The authors would also like to thank Takeshi Masaki for his kind help in installing the wire running system for this research. References [1] Masuzawa T. State of the art of micromachining. CIRP Ann-Manuf Technol 2000;49(2):473–88. [2] Rajurkar KP, Zhu D, McGeough JA, Kozak J, De Silva A. New developments in eletro-chemical machining. CIRP Ann-Manuf Technol 1999;48(2):567–79. [3] Fan ZW, Hourng LW. The analysis and investigation on the microelectrode fabrication by electrochemical machining. Int J Mach Tools Manuf 2009;49(7):659–66. [4] Lim YM, Kim SH. An electrochemical fabrication method for extremely thin
11
Contents lists available at ScienceDirect
Precision Engineering journal homepage: www.elsevier.com/locate/precision
Wire electrochemical grinding of tungsten micro-rods using neutral electrolyte ⁎
Wei Han , Masanori Kunieda Department of Precision Engineering, The University of Tokyo, Tokyo 113-8656, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrochemical machining Wire electrochemical grinding Electrostatic induction feeding method Tungsten Micro-rods
A new method, wire electrochemical grinding(WECG), is proposed for machining tungsten micro rods with neutral electrolyte and bipolar current. The inevitable problem of tool wear due to the bipolar current can be eliminated by using a running wire as the tool electrode with this method. In this study, three kinds of wire guides were used for feeding the workpiece rod in the axial direction: cylindrical cemented tungsten carbide (WC) guide, cylindrical zirconium oxide (ZrO2) guide, and disk-shaped cemented WC guide, to investigate the influence of materials and configurations of the guide on the machining characteristics. The experimental results showed that the disk-shaped WC wire guide can realize higher machining accuracy because of less influence of stray current flowing through the gap formed between the machined micro-rod and wire guide. Even if the ZrO2 material has a much lower electrical conductivity than that of the cemented WC, the machining process was still influenced by the stray current between the micro-rod and wire guide. Furthermore, micro-rods were also machined by feeding the rod workpiece in the radial direction. Compared to when the workpiece is fed in the axial direction, the current was higher, which was clarified by the simulation of the current density distribution. However, the machining time was much longer than that with the workpiece fed in the axial direction because more machining steps were necessary to obtain a smooth side surface. With the workpiece fed in the axial direction method, a tungsten micro-rod of 35 μm in diameter was machined to the length of 163 μm. The results confirmed that the new method is capable of miniaturization equivalent to micro electrical discharge machining (EDM).
1. Introduction Demands for the fabrication of micro-rods are increasing, because micro-rods are widely used as tools for micro-drilling and micro-milling 3D structures [1] and probes for measuring system such as the atomic force microscopy (AFM). Electrochemical machining (ECM) is a promising method for machining micro-rods because it is an anodic dissolution process [2] which is free of residual stress and surface cracks inevitably generated in thermal processes such as electrical discharge machining (EDM) and laser machining. In the application, tungsten is widely used as micro-rod material because of the high erosion resistance, high electrical and thermal conductivity, and high stiffness, which are significantly important especially for micro-rods with high aspect ratios. However, the electrochemical machining of tungsten is difficult because tungsten oxide layer is generated on the anode tungsten surface, blocking the electrolytic current. Hence, Fan and Hourng [3] and Lim and Kim [4] used NaOH and KOH aqueous solution, respectively, as the electrolyte to fabricate tungsten rods. However, since these strongly alkaline electrolytes are highly harmful to the ⁎
environment, use of neutral electrolytes is desirable. Maeda et al. [5] reported that cemented tungsten carbide can be machined with a neutral electrolyte such as sodium nitrate (NaNO3) aqueous solution using a bipolar current, because NaOH is generated when the tungsten carbide electrode is in negative polarity, thereby removing the oxide layer from the surface of tungsten carbide. Thus, Natsu and Kurahata [6] machined cemented tungsten carbide (WC) rods successfully using a NaNO3 aqueous solution. However, the machining accuracy was not sufficiently high to obtain micro-rods with diameters equivalent to that which can be obtained by EDM, because the bipolar current resulted in a significantly large tool wear. Furthermore, since the pulse duration of the machining current was as long as several tens to several hundreds of ms, the electrochemical reaction could not be localized in a small gap width. Nevertheless, it is considered that pure tungsten can also be machined using a neutral electrolyte and bipolar current based on the same mechanism reported by Maeda et al. [5] and Natsu and Kurahara [6]. On the other hand, Schuster et al. [7] found that electrochemical micromachining can be performed when ultra-short pulse current of
Corresponding author at: Room 833, Engineering Building 14#, Department of Precision Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan. E-mail addresses: [email protected] (W. Han), [email protected] (M. Kunieda).
https://doi.org/10.1016/j.precisioneng.2018.02.006 Received 5 November 2017; Received in revised form 17 January 2018; Accepted 31 January 2018 0141-6359/ © 2018 Elsevier Inc. All rights reserved.
Please cite this article as: Han, W., Precision Engineering (2018), https://doi.org/10.1016/j.precisioneng.2018.02.006
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
in wire EDM, the wire is supported by a cylindrical guide which is circumferentially grooved on the edge. Since the present work aims to machine tungsten micro-rods electrochemically with a high accuracy equivalent to WEDG using a bipolar current, the same tool electrode system as WEDG was introduced and named as wire electrochemical grinding (WECG). This method is therefore able to solve both the problems of the tool electrode wear and wire vibration. Three kinds of wire guides were used in this study, namely the cylindrical cemented tungsten carbide (WC) guide, cylindrical zirconium oxide (ZrO2) guide, and disk-shaped WC guide. The ZrO2 was used as guide material because of its significantly low electrical conductivity, which may reduce the stray current during the electrochemical process, compared with cemented tungsten carbide. The cylindrical and disk-shaped wire guides form different gap areas during machining because of their different surface areas with different geometries. Micro-rods can be machined with the workpiece fed either in the axial or radial directions, and because the wire tool electrode is running during machining, tool wear which is inevitably generated by the bipolar current does not influence machining accuracy.
Fig. 1. Electrostatic induction feeding ECM.
Fig. 2. Principle of electrostatic induction feeding ECM.
2. Wire electrochemical grinding (WECG) method several tens of ns in duration is used. In order to easily obtain such an ultra-short pulse current in ECM without using an expensive ultra-short pulse generator, Koyano and Kunieda [8] developed the electrostatic induction feeding ECM as shown in Fig. 1. Fig. 2 shows the equivalent circuit of this method and gap current and voltage waveforms. When the pulse voltage is supplied across the working gap, the electric double layer is formed on the surface of electrodes, which can be expressed as Cdl. Then, the working gap can be modeled as the Faraday impedance Rf, resistance of electrolyte in the machining gap Rg and Cdl [9]. Since the pulse voltage with a constant pulse duration is coupled to the tool electrode by a feeding capacitance C1, current only flows at the instance when the pulse voltage changes to high or low as shown in Fig. 2. Hence, the current pulse duration is nearly equal to the rise and fall time regardless of the pulse on-time of the pulse voltage. In addition, the current is bipolar, and this is advantageous to tungsten machining using a neutral electrolyte. With this method, a pulse duration shorter than several tens ns can easily be obtained. Hence, Han and Kunieda [10] fabricated tungsten micro-rods successfully with a neutral electrolyte, NaNO3 or NaCl aqueous solution, and bipolar current. They used an ultra-short pulse current of 20 ns to 40 ns in their research, and thus successfully fabricated a micro-rod with a diameter of 7.1 μm and aspect ratio of 14 by localizing the electrochemical dissolution in a significantly small working gap. To obtain such a high accuracy, however, they used a platinum tool electrode to avoid tool electrode wear. Thus, this process was costly. A wire tool electrode was first used in micro EDM by Masuzawa et al. [11] to fabricate micro-rods without the influence of tool wear due to discharge, known as the wire electro-discharge grinding (WEDG) method. In WEDG, to avoid the wire vibration, which normally occurs
The WECG method is shown in Fig. 3. The micro EDM machine (Panasonic, MG-ED72W) was converted to the micro ECM machine by replacing the EDM pulse generator with the circuit in Fig. 1. The tool electrode, which is kept running during machining, is commercial brass wire with a diameter of 100 μm and held by a wire guide. Since the wire tool electrode is running during machining, the influence of tool wear caused by the bipolar current on machining accuracy can be eliminated. The rod workpiece is fed toward the wire tool electrode during machining and rotates with a speed of 3000 rpm. The feed distance in the axial direction determines the rod length while the cut depth in the radial direction determines the rod diameter. The electrolyte is jetted into the working gap by a nozzle 210 μm in inner diameter placed near the working gap. The ultra-short-pulse bipolar current is supplied by the electrostatic induction feeding method. The three kinds of wire guides used in this study are shown in Fig. 4: the cylindrical cemented WC guide, cylindrical ZrO2 guide, and diskshaped cemented WC guide. All their diameters were set to 10 mm. The guides in Fig. 4(a) and (c) are made of a material with a high electrical conductivity, but have different geometries, while (b) is made of ZrO2 which has a much lower electrical conductivity. Table 1 shows the properties of the materials used for the guides [12]. Fig. 5 shows the brass wire held by one of the wire guides. Since the brass wire is running off the edge of the wire guide by 75 μm, the cut depth in radial direction must be made smaller than 75 μm to avoid the wear of the wire guide due to electrochemical dissolution, because current can flow through the gap between the shoulder surface of workpiece and the top surface of the wire guide as shown in Fig. 3.
Fig. 3. Wire electrochemical grinding method.
2
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 4. Wire guides used in this study.
and E0 is the total voltage amplitude of the pulse voltage, is constant [8] with the electrostatic induction feeding method. Fig. 7 shows the microrods machined with different pulse durations, demonstrating that the WECG method could machine tungsten with the neutral electrolyte and bipolar current. The diameter of micro-rod was larger and the side surface of micro-rod was straighter with the pulse duration of 20 ns. This result can be supported by the findings of Schuster et al. [7] who showed that the electrochemical dissolution can be localized in a smaller working gap using nanosecond pulse. In addition, it should be noted that the roots of both the micro-rods were rounded because the tool electrode was brass wire and the cross section shape of the wire was transferred to the workpiece at the root area. Fig. 8 shows the side surfaces and cross sectional shapes of the brass wire before and after machining with different wire running speeds. It was severely worn out with the wire running speed of 11.3 mm/s due to the bipolar current, while, the tool wear significantly decreased when the running speed was increased to 53.3 mm/min. This suggests that tool wear due to bipolar current can be eliminated by increasing the wire running speed. Fig. 9 shows the wear of a plate tool electrode made of stainless steel (SUS304) [10], which was seriously worn out by the bipolar current. Hence, compared with the plate tool electrode method, the influence of tool wear was eliminated with the proposed WECG method.
Table 1 Properties of cemented WC and ZrO2. Properties
Electric conductivity [S/m] Density [g/cm3] Melting point [K] Thermal conductivity [W/ m k]
Cemented WC
ZrO2
Minimum
Minimum
0.024 15.25 3000 28
Maximum 0.01 15.88 3193 88
3.16*10 5 2823 1.7
−14
Maximum 3.16*10−19 6.15 2973 2.7
3. Tungsten machining with workpiece fed in axial direction The workpiece rod can be machined when it is fed either in the axial or radial direction. The machining characteristics were first investigated with the workpiece fed in the axial direction. A commercial tungsten rod of 300 μm in diameter was used as the workpiece, and reshaped to a diameter of 200 μm with the wire electro discharge grinding (WEDG) method [11] to make it sufficiently straight. Table 2 shows the machining conditions used in WECG. The neutral electrolyte NaNO3 aqueous solution of 6 wt% in concentration was used as electrolyte. The bipolar current was generated by the electrostatic induction feeding method. In addition, the rise and fall times of pulse voltage, which determines the current pulse duration, were set as 20 ns and 40 ns. A ceramic capacitor of 350 pF was used as the feeding capacitance C1. The workpiece was positioned over the brass wire with an initial gap width of 3 μm before machining. The cut depth in the radial direction was 50 μm, which was kept constant during machining, and the feed distance in the axial direction was 100 μm. The feed speed of workpiece in the axial direction was 1.0 μm/s. The cylindrical cemented WC guide was used. Fig. 6 shows the waveforms of gap current and voltage with different pulse durations. It can be seen that the ultra-short bipolar current was easily obtained with the electrostatic induction feeding method. The current increased with decreasing pulse duration because the electric charge per pulse q = C1E0, where C1 is the feeding capacitance
4. Machining characteristics with different kinds of wire guide Three wire guides shown in Fig. 4 were used to machine tungsten micro-rods with the workpiece fed in the axial direction to investigate the differences in the machining characteristics. Table 3 shows the experimental conditions. The feeding capacitance was changed as 220 pF, 350 pF, 470 pF and 570 pF. The cut depth in the radial direction was 50 μm, and the feed distance in the axial direction was 150 μm, which is larger than the diameter 100 μm of the brass wire electrode. The feed speed of workpiece was 1.0 μm/s. Fig. 10 shows the waveforms of the gap current and voltage with the feeding capacitances of 470 pF and 570 pF when the cylindrical WC wire guide was used. The gap current and voltage increased with increasing feeding capacitance because the electric charge per pulse
Fig. 5. Top view of brass wire hooked on wire guide.
3
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
guides. This stray current could be decreased with the disk-shaped wire guide compared with the cylindrical wire guides by decreasing the radial gap area. Hence, a longer micro-rod was obtained with the diskshaped cemented WC wire guide. In addition, the length of the microrods was a little longer with the cylindrical ZrO2 guide than that with the cylindrical WC guide when the feeding capacitance was 220 pF and 570 pF, this is because the electric conductivity of the ZrO2 material is much lower than the WC material as shown in Table 1. Hence, the influence of the stray current flowing through the gap formed by the wire guide and micro-rod was decreased. However, the difference in the length of the micro-rods was insignificant between the WC guide and ZrO2 guide. In this experiment, the taper angle θ was defined as the included angle between the side surface of micro-rod and rod axis. In order to measure θ, diameters were measured along the axial direction with the same interval using SEM. Then least squares fitting was used to approximate the side surface of the micro-rod and calculate θ. θ with different voltage amplitudes and feed speeds is shown in Fig. 14. θ was smallest with the disk-shaped cemented WC guide because of less influence of the stray current in the radial gap. In addition, the taper angle increased with increasing feeding capacitance because the electric charge per pulse q = C1E0 increased, resulting in higher fraction of material dissolution from the side surface of the machined micro-rod due to the larger working gap. θ was significantly large with the cylindrical wire guides with the feeding capacitance of 220 pF. This is because the lengths of the micro-rods machined were shorter than the radius of the wire electrode as shown in Fig. 11.
Table 2 Experimental conditions used to machine micro-rod with workpiece fed in axial direction. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
90 500 50 20, 40 350 NaNO3 aq. 6 wt% Brass Φ100 μm 3000
q = C1E0 increased. Fig. 11 shows the micro-rods machined with different wire guides and feeding capacitances. With the small feeding capacitance of 220 pF, the machining processes were interrupted by collision between electrodes, resulting in the rod length becoming shorter than the preset feed distance of 150 μm in the axial direction. This is because the electric charge per pulse q = C1E0 decreases with decreasing feeding capacitance C1 resulting in a lower material removal rate. With the large feeding capacitance of 550 pF, however, the microrods were shortened due to the stray current flowing between the side surface of the guide and the micro-rods. The influence of the stray current in the radial gap increased because the axial gap width increased due to the larger feeding capacitance under the same feed speed of workpiece, resulting in more material dissolution from the side and end surfaces of the machined micro-rod. Fig. 12 shows the lengths of micro-rods machined with different kinds of wire guide. With the small feeding capacitance of 220 pF and large feeding capacitance of 570 pF, the lengths of micro-rod were smaller than the preset feed distance of 150 μm in the axial direction because of the influences of the collision between electrodes and stray current flowing through the side and end surfaces of the micro-rod. However, the length of the micro-rods with the disk-shaped cemented WC wire guide was longer than that with the cylindrical cemented WC guide and cylindrical ZrO2 guide. This is because there was a considerable stray current flowing through the gap formed by the wire guide and machined micro-rod as shown in Fig. 13 with the cylindrical
5. Tungsten machining with workpiece fed in radial direction When the workpiece was fed in the radial direction to machine a tungsten rod, the cut depth in the radial direction increased during machining and the feed distance in axial direction remained constant. To machine a longer micro-rod than the diameter of the wire tool electrode, the machining process should be repeated while changing the axial position of the workpiece electrode incrementally. Furthermore, some material is left un-machined when the axial position of the
Fig. 6. Waveforms of gap current and voltage with different pulse durations.
4
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 7. Micro-rods machined with different pulse durations.
Fig. 8. Brass wires in different conditions of (1) before machining, (2) wire speed 11.3 mm/s and (3) 53.3 mm/s.
workpiece is changed by a large distance because the cross-sectional shape of the wire tool electrode is circular, hence, the machining process of the workpiece fed in the radial direction method consists of two machining steps: rough and finish machining. In finish machining, the workpiece is fed continuously in the axial direction with the radial position fixed to remove the remained material and obtain a straight side surface. 5.1. Rough machining with workpiece fed in radial direction Fig. 15 shows the tungsten micro-rod machining method with the workpiece fed in the radial direction. The workpiece is positioned beside the brass wire electrode with an initial gap width of 3 μm, and fed in the radial direction as shown in Fig. 15(b). To machine a longer micro-rod than the diameter of the wire tool electrode, the workpiece is fed stepwise in the axial direction to change the axial position of the workpiece as shown in Fig. 15(c), and fed in the radial direction again. This machining process should be repeated to obtain a long micro-rod. Table 4 shows the experimental conditions used for rough machining with the workpiece fed in the radial direction. The feed distance in the radial direction was 50 μm and the feed interval in the axial direction was 100 μm, 50 μm and 25 μm. The rod length finally machined was 200 μm. The feed speed in the radial direction was 1.0 μm/s. Fig. 16 shows micro-rods machined with different feed steps of 100 μm, 50 μm and 25 μm in the axial direction when the voltage amplitude was 90 V. With the feed step of 100 μm in the axial direction, some material remained as shown in Fig. 16(a). Moreover, the remaining material decreased when the feed interval was decreased in the
Fig. 9. Tool wear with plate tool electrode and material of stainless steel (SUS304) [10].
Table 3 Experimental conditions used to investigate machining characteristics of different wire guides. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
90 500 50 40 220, 350, 470, 570 NaNO3 aq. 6 wt% Brass Φ100 μm 3000
5
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 10. Waveforms of gap current and voltage with different feeding capacitances.
Fig. 11. Micro-rods machined with different kinds of wire guide and feeding capacitances (1) 220 pF, (2) 350 pF, (3) 470 pF and (4) 570 pF.
time due to the increase of the feeding steps leads to more removal by the stray current. Fig. 18 shows the micro-rod machined with the axial feed interval of 25 μm when the voltage amplitude was decreased to 80 V. The feed distance in the radial direction was decreased to 35 μm from the previous experiment performed at 50 μm, because collision between electrodes occurred frequently when the feed distance in the radial direction was larger than 35 μm due to the increase in the gap area, which caused a lower current density. Compared with Fig. 16(c), a micro-rod was fabricated successfully with the feed interval of 25 μm when the voltage amplitude E0 decreased to 80 V. This is because the influence of stray current shown in Fig. 17 decreased with the decrease in the gap width due to the smaller electric charge per pulse q = C1E0. Fig. 12. Lengths of micro-rods with different kinds of wire guide.
5.2. Finish machining with workpiece fed in radial direction method
axial direction to 50 μm. The micro-rod disappeared totally when the feed interval was further decreased to 25 μm as shown in Fig. 16(c). It is thought that the micro-rod was shortened by the stray current flowing through the side surface which was fabricated in the previous step. Fig. 17 shows the stray current in the second feeding step of workpiece in the radial direction when the feed interval was 25 μm. Since the working area is smaller, the gap width is larger under the same feed speed, resulting in higher stray current. In addition, longer machining
Rough machining was performed with the feed interval of 50 μm under the machining conditions in Table 4 to obtain the micro-rod shown in Fig. 16(b). Next, in the finish machining process, the workpiece was positioned at 3 μm over the top surface of the wire electrode and the cut depth in the radial direction was equal to the feed distance in radial direction in the rough machining. Then, the workpiece was fed in the axial direction to remove the remaining material and obtain a
6
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
experimentally obtained waveforms. Hence, the total current flowing through the workpiece surfaces was set at 900 mA and the potential of the wire guide and brass wire surfaces was set as ground. Other surfaces were assumed as electrically insulative. The COMSOL Multiphysics simulation tool was used, and the mesh size was a user-controlled mesh with normal element size. The electrolyte was sodium nitrate (NaNO3) aqueous solution with concentration of 2 wt%. Thus, the electrical conductivity of the electrolyte was 2.05 S/m, which was measured using an electrical conductivity meter (LAQUAact ES-71). Fig. 21 shows the simulation results of the current density distributions with different machining methods, in which both machining methods are influenced by stray current. First, it can be noted that the influence of stray current with the workpiece fed in axial direction is less than that with the workpiece fed in radial direction. Hence, the current efficiency is higher with the workpiece fed in the axial direction than that with the workpiece fed in the radial direction. Furthermore, the influence of stray current is constant with increasing feed distance in the axial direction, as shown in Fig. 21(a). However, the influence of stray current increases with decreasing feed distance in radial direction, as shown in Fig. 21(b). This is because the machining area increases with increasing feed distance in the radial direction, resulting in more current flowing through the machining area. Since the electric charge per pulse q = C1E0 is constant with the electrostatic induction feeding method, the stray current decreases.
Fig. 13. Stray current in gap formed between micro-rod and wire guide.
6. Micro-rods machining with high aspect ratio 6.1. Influence of wire guide Table 6 shows the experimental conditions used to machine microrods with high aspect ratios. The workpiece was fed in axial direction with a simple machining process than the workpiece fed in the radial direction. According to the experimental results in Section 4, the feeding capacitance was set to 350 pF. Three kinds of wire guides were used to compare the machinability of micro-rods with high aspect ratio. The cut depth in the radial direction was set as 50 μm and feed distance in the axial direction was 850 μm. The feed speed was initially set at 1.0 μm/s and it was decreased from 1.0 μm/s to 0.5 μm/s when the feed distance in the axial direction increased to 400 μm. This is because the influence of the stray current shown in Fig. 13 increased with increasing feed distance in the axial direction, resulting in the decrease in the machining current in the axial working gap. Fig. 22 shows the micro-rods machined with different kinds of wire guides. The length of the micro-rod machined with the cylindrical cemented WC guide was much shorter than the preset feed distance of 850 μm because the machining process was interrupted by the collision between electrodes when the feed distance in axial direction reached 450 μm. The machining processes with the cylindrical ZrO2 guide and disk-shaped cemented WC guide completed the preset feed distance of 850 μm in the axial direction. However, it is noted that the taper of the micro-rod machined with the cylindrical ZrO2 guide was much larger
Fig. 14. Taper angles of micro-rod with different kinds of wire guides.
straight side surface. Table 5 shows the experimental conditions used for finish machining. The rise/fall time was decreased to 20 ns to localize the electrochemical dissolution in a smaller working gap. The feed distance in the axial direction was 200 μm which is equivalent to the length of the rod obtained from the rough machining process. Fig. 19 shows the micro-rod obtained after finish machining. 5.3. Comparison between workpiece fed in axial and radial direction methods To compare the workpiece fed in axial and radial direction methods, the 2D models in Fig. 20 were built to calculate the current density distribution. Since the electric charge per pulse q = C1E0 is constant with the electrostatic induction feeding method, the constant current of 900 mA during the pulse duration of 40 ns was assumed based on the
Fig. 15. Micro-rod machining method with workpiece fed in radial direction.
7
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 16. Micro-rods machined with different feed intervals (a) 100 μm, (b) 50 μm, (c) 25 μm.
Table 4 Experimental conditions used for rough machining with workpiece fed in radial direction. Pulse voltage
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm]
80, 90 500 50 40 350 NaNO3 aq. 6 wt% Brass Φ100 μm 3000
Fig. 19. Micro-rod obtained by finishing machining.
than that with the disk-shaped WC guide. This is mainly because the stray current flowing through the gap between the wire guide and rod surface was negligibly low with the disk-shaped WC guide. There is also a significant difference in the length of micro-rod machined between the cylindrical WC and ZrO2 wire guides as shown in Fig. 22(a) and (b), although Fig. 12 shows a slight difference. Since the machined lengths of micro-rods machined were short in Fig. 12, the influence of stray current between the wire guide and machined micro-rod was not obvious, especially with a significantly small and large feeding capacitance with which the lengths of micro-rod were significantly small. Fig. 23 shows the waveforms of gap current and voltage with different wire guides at the feeding distance of 400 μm in the axial direction. The current with the cylindrical WC wire guide was much higher than that with the cylindrical ZrO2 wire guide because of the higher electrical conductivity of cemented WC than the ZrO2, resulting in a higher influence of the stray current in the gap between the wire guide and machined micro-rod as shown in Fig. 13. It should also be noted that the gap current is lowest with the disk-shaped guide.
Fig. 17. Current in working gap with feed interval of 25 μm.
6.2. Micro-rod machining with smallest diameter comparable to micro EDM Micro-rods with the smallest diameter equivalent to that which can be obtained in micro EDM were machined using the conditions shown in Table 7. The initial diameter of the workpiece rod prepared using WEDG was 200 μm. The depth of cut in the radial direction should be smaller than 75 μm to avoid the wear of the wire guide. Hence, to machine micro rods with a diameter smaller than 50 μm, a two-step machining process was used. In the first machining step, the voltage amplitude was 90 V and the rise/fall time was 40 ns. The feed speed of workpiece was 1.0 μm/s. The cut depth in the radial direction and feed distance in the axial direction were 50 μm and 200 μm, respectively. In the second machining step, the voltage amplitude was 40 V or 50 V and the rise/fall time was 20 ns. The feed speed of workpiece was 0.6 μm/s. The cut depth in the radial direction and feed distance in axial direction were 30 μm and 160 μm, respectively. Fig. 24 shows the micro-rods obtained with different voltage amplitudes in the second machining step. With the voltage amplitude of
Fig. 18. Micro-rods machined with feed interval of 25 μm and voltage amplitude of 80 V.
Table 5 Experimental conditions used for finishing machining. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode Tool electrode rotation [rpm] Feed speed [μm/s]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
50 500 50 20 350 NaNO3 aq. 6 wt% Brass Φ100 μm 3000 1.0
8
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 20. Models of current density distribution calculation with (a) Feeding of workpiece in axial direction and (b) Feeding of workpiece in radial direction.
Fig. 21. Simulations of current density distribution with different machining methods.
50 V, the micro-rod was shortened because of the large working gap width resulting in excessive material dissolution. With the voltage amplitude of 40 V, the micro-rod was machined successfully to a diameter of 35 μm and length of 163 μm. Although the diameter is still larger than the smallest diameter achievable with micro EDM [1], the micro ECM method we proposed has the advantages of no crack or residual stress on the machined surface.
Table 6 Experimental conditions used for micro-rods machining with cut depth of 50 μm in radial direction. Pulse voltage
7. Conclusions
Feeding capacitance C1 [pF] Electrolyte Tool electrode rotation [rpm]
A wire electrochemical grinding (WECG) method was proposed in this study to machine tungsten micro-rods with neutral electrolyte 9
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
90 500 50 40 350 NaNO3 aq. 6 wt% 3000
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Fig. 22. Long micro-rod machined with different kinds of wire guides (a) Cylindrical cemented WC guide, (b) Cylindrical ZrO2 guide, (c) Disk-shaped cemented WC guide.
Fig. 23. Waveforms of gap current and voltage with different wire guides at feed distance of 400 μm in axial direction.
2) With the workpiece fed in the axial direction, the machining characteristics of three kinds of wire guides: cylindrical cemented WC guide, cylindrical ZrO2 guide and disk-shaped cemented WC guide, were investigated. The disk-shaped WC wire guide showed the best machining accuracy because the influence of the stray current in the gap formed by the wire guide and micro-rod was significantly decreased. 3) With the workpiece fed in the radial direction, micro-rods were machined successfully. Compared with the workpiece fed in the axial direction method, the current efficiency was lower due to stronger influence of the stray current flowing through the side and end surface of the machined micro-rod, which was confirmed by the simulation of current density distribution. 4) With the workpiece fed in the axial direction, micro-rods with a diameter of 100 μm could be machined to a length of 850 μm by a single machining step. Smaller rod diameters of 35 μm with microrod length of 163 μm could be obtained from two machining steps, realizing micromachining ability equivalent to micro EDM
Table 7 Experimental conditions used for micro-rod machining with two steps. Pulse voltage
Feeding capacitance C1 [pF] Electrolyte Tool electrode rotation [rpm] Feed speed [μm/s]
Amplitude [V] Frequency [kHz] Duty factor [%] Rise/fall time [ns]
40, 50, 90 500 50 20, 40 350 NaNO3 aq. 6 wt% 3000 0.6, 1.0
NaNO3 aqueous solution and bipolar current supplied by the electrostatic induction feeding method. The following conclusions were obtained. 1) Tungsten micro-rods were successfully machined with the proposed WECG method which applies neutral electrolyte NaNO3 aqueous solution and bipolar current. The influence of tool wear due to bipolar current was significantly small when the running speed of wire tool electrode was 53.3 mm/min.
Fig. 24. Micro-rods obtained with different voltage amplitudes in second machining step.
10
Precision Engineering xxx (xxxx) xxx–xxx
W. Han, M. Kunieda
Acknowledgments
cylindrical micropin. Int J Mach Tools Manuf 2001;41(15):2287–96. [5] Maeda S, Saito N, Haishi Y. Principle and characteristics of electro-chemical machining. Tech Rep Mitsubishi Electr 1967;41(10):1267–79. in Japanese. [6] Natsu W, Kurahata D. Influence of ECM pulse conditions on WC alloy micro-pin fabrication. Proc. of the 17th International Symposium on Electromachining 2013:675–9. [7] Schuster R, Kirchner V, Allongue P, Ertl G. Electrochemical micromachining. Science 2000;289(5476):98–101. [8] Koyano T, Kunieda M. Micro electrochemical machining using electrostatic induction feeding method. CIRP Ann-Manuf Technol 2013;62(1):175–8. [9] Bard AJ, Faulkner LR, Leddy J, Zoski CG. Electrochemical methods: fundamentals and applications Vol. 2. New York: Wiley; 1980. [10] Han W, Kunieda M. Fabrication of tungsten micro-rods by ECM using ultra-shortpulse bipolar current. CIRP Ann-Manuf Technol 2017;66(1):193–6. [11] Masuzawa T, Fujino M, Kobayashi K, Suzuki T, Kinoshita N. Wire electro-discharge Grinding for micro-machining. CIRP Ann-Manuf Technol 1985;34(1):431–4. [12] Properties of ZrO2, August, 2017. https://www.azom.com/properties.aspx? ArticleID=133; Properties of WC, August, 2017 https://www.azom.com/ properties.aspx?ArticleID=1203.
This work was supported by the Cross-Ministerial Strategic Innovation Promotion Program (SIP): Innovative Design/ Manufacturing Technologies, funded by NEDO. The authors would also like to thank Takeshi Masaki for his kind help in installing the wire running system for this research. References [1] Masuzawa T. State of the art of micromachining. CIRP Ann-Manuf Technol 2000;49(2):473–88. [2] Rajurkar KP, Zhu D, McGeough JA, Kozak J, De Silva A. New developments in eletro-chemical machining. CIRP Ann-Manuf Technol 1999;48(2):567–79. [3] Fan ZW, Hourng LW. The analysis and investigation on the microelectrode fabrication by electrochemical machining. Int J Mach Tools Manuf 2009;49(7):659–66. [4] Lim YM, Kim SH. An electrochemical fabrication method for extremely thin
11