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Development of new fabrication and measurement techniques of micro-electrodes with high aspect ratio for micro EDM using typical EDM machine
Accepted Manuscript Proposing New Fabrication and Measurement Techniques of Micro-electrodes with High Aspect Ratio for Micro EDM Drilling of Tungsten Carbide (WC) Using EDM Machine Mehdi Hourmand, Ahmed A.D. Sarhan, Noordin Mohd Yusof PII: DOI: Reference:
S0263-2241(16)30656-X http://dx.doi.org/10.1016/j.measurement.2016.11.020 MEASUR 4441
To appear in:
Measurement
Received Date: Accepted Date:
30 July 2015 8 November 2016
Please cite this article as: M. Hourmand, A.A.D. Sarhan, N. Mohd Yusof, Proposing New Fabrication and Measurement Techniques of Micro-electrodes with High Aspect Ratio for Micro EDM Drilling of Tungsten Carbide (WC) Using EDM Machine, Measurement (2016), doi: http://dx.doi.org/10.1016/j.measurement.2016.11.020
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Proposing New Fabrication and Measurement Techniques of Micro-electrodes with High Aspect Ratio for Micro EDM Drilling of Tungsten Carbide (WC) Using EDM Machine Mehdi Hourmand1*, Ahmed A. D. Sarhan1, Noordin Mohd Yusof2 1
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya (UM), 50603, Kuala Lumpur, Malaysia 2 Department of Materials, Manufacturing & Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia (UTM), 81310, Skudai, Malaysia *Corresponding author: Mehdi Hourmand Email address: [email protected] Telephone number: 0060187721401 Abstract. Micro-electrodes are used for micro holes EDM drilling in miniaturized products. This study represents novel fabricating and measuring processes of high aspect ratio micro-electrodes based on horizontal moving block electrical discharge grinding (horizontal moving BEDG [HMBEDG]) and gage block with a typical EDM machine, as well as solving the problems that occur during micro-electrode fabrication. The EDM machine has been used as a coordinate measuring machine (CMM) for measuring micro-electrode diameter, where the gage block acts as a probe and the micro-electrode as a workpiece. As a result, a micro-electrode with the highest aspect ratio (over 60.25) was fabricated successfully with 78.19 µm diameter and 4.70 mm length after producing a through micro hole with 134.5 µm diameter on WC-Co. As these processes are simple and do not require high investment for special equipment, micro-electrodes can be fabricated and measured for micro EDM drilling and milling in conventional workshops equipped with EDM machines. Keywords: Micro EDM, Micro-electrode, Fabrication process, Measurement, Tungsten Carbide (WC, WC-Co), Micro drilling, High aspect ratio
1 Introduction Micromachining is among the fundamental technologies for manufacturing and miniaturizing parts and products with dimensions of 1 to 999 µm based on the CRIP Committee of Physical and Chemical processes [1]. Miniaturized products are mainly used in information technology, biotechnology, environmental and medical industries, miniaturized machines, electric devices and so on [2, 3]. Recently, Micro-Electro Mechanical System (MEMS) advancements have popularized micromachining [4]. Micromachining processes are basically classified into three groups, including conventional, non-conventional, and hybrid machining processes. Conventional machining process requires mechanical force and energy to remove material by shear stress. Shearing entails simple machining by physical contact between the cutting tool and material [5, 6]. Conventional machining processes, such as micro turning, micro milling and grinding, function with recommended singlepoint diamond cutters or very fine grit grinding wheels. Other sources of energy, including spark energy, light energy, vibration energy, electrolysis energy, mechanical energy (based on erosion mechanisms), energy beams (laser, electron or ion beams), etc., are used to remove material in non-
conventional processes [6-9]. Hybrid machining process is a combination of conventional and nonconventional material removal methods to produce high-precision microstructures [3, 10]. A micro-mold cavity is required for the mass production of micro components [11]. Micro holes are used in micro dies and molds [12]. Moreover, hard-to-machine materials are applied for microinjection mold, which requires very accurate machining of complex shapes and three-dimensional forms in the micron range [11]. Tungsten carbide (WC) and its composite (WC-Co) are employed to produce cutting tools, dies and other special tools and components owing to their high hardness, strength, wear and corrosion resistance over a wide range of temperatures [13-16]. Thus, WC machining plays significant roles in manufacturing [17]. In the EDM method, conductive materials are removed based on thermal energy (melting and partial workpiece vaporization) by series of sparks that occur between the electrode and electrical workpiece [18-21] in the presence of the dielectric media. However, adding aluminum powder to the dielectric leads to increased material removal rate (MRR) and improved surface roughness (SR) during EDM [22, 23]. Electromagnetic detection, acoustic signal detection and electrical signal detection are the methods employed for detecting spark location in EDM machines [24, 25]. EDM is a useful process in industry for machining electrically conductive materials with accurate dimensions regardless of hardness, strength and toughness [26, 27]. Micro EDM and EDM have the same characteristics, with only the electrode size, discharge energy and axis movement resolution at the micron level in micro EDM [5]. Micro mold making, production of dies and cavities, and complex three-dimensional shapes at the micro level are the major functions of micro EDM [28]. In EDM and micro EDM, no direct contact and negligible force between the workpiece and tool cause the elimination of tool deformation, chatter, mechanical stress and vibration errors during machining [5, 29, 30]. Micro EDM is one of the most effective methods of machining WC [31, 32]. Moreover, micro EDM has a significant role in the field of micromachining. Very long and thin electrodes can be used in micro EDM due to its non-contact nature [11]. Micro EDM has disadvantages, among which electrode wear ratio and low material removal rate. Electrode wear can be compensated by changing the micro-electrode, preparing a longer micro-electrode from the beginning or fabricating the micro-electrode in situ for further machining [4]. Also, changing the micro-electrode during machining causes decreased accuracy due to changes in setup or microelectrode re-clamping. Thus, on-machine micro-electrode fabrication can eliminate clamping error in the micro EDM process [4]. The processes applicable for the fabrication of micro-electrodes from electrodes thicker than required electrodes, are wire electro-discharge grinding (WEDG), self-drilled holes, stationary sacrificial block electro discharge grinding (BEDG), rotating sacrificial disk, moving block electrodischarge grinding (moving BEDG), micro turning, and hybrid machining. In WEDG, a 0.07 mm diameter guided running wire with 3-5 mm/s speed is used as an electrode [11]. Various WEDG methods are compared in Fig. 1(a), (b) and (c). During conventional radial feed WEDG the microelectrode feeds along the central axis in the direction of the symmetry axis ON (Y-axis) to the wire, while in tangential feed WEDG (TF-WEDG), the micro-electrode’s position is fixed on the Y-axis and it feeds along the tangential direction of the wire guide arc (X-axis) [10]. Fig. 1(d) shows the principles of micro-electrode fabrication by self-drilled holes. Initially, a hole is produced with a rod electrode with negative polarity. Subsequently, the rod electrode polarity is positioned offcenter from the center of the hole and its polarity changes to positive. In the final step, the rod electrode either with or without rotation, is fed to the plate to fabricate the micro-electrode [33]. Fig. 1(e) depicts the BEDG method, which is the simplest means of on-machine micro-electrode fabrication. The rotating raw material of the micro-electrode is fed vertically to the stationary
sacrificial block electrode for machining [11]. The rotating sacrificial disk process is illustrated in Fig. 1(f). The rotating disk electrode is 5 mm thick and has 60 mm diameter and 90 rpm rotating speed [11]. Fig. 1(g) demonstrates the moving BEDG process for micro-electrode fabrication. In this process, the Z-axis controls the EDM gap and there is relative translational motion between the micro-electrode and sacrificial block electrode. Moreover, most sparks occur between the top surface of the block electrode and the un-machined part of the micro-electrode, and there is no spark between the sides of the micro-electrode and block electrode. Moreover, previous researchers have used WC as a block electrode [3, 34] and 500 to 2500 rpm rotating speed spindle [34] to fabricate micro-electrodes. It has been reported that very low electrode rotational speeds during micro-electrode fabrication can cause longer machining time and less dimensional accuracy. It has also been found that 1000 to 1500 rpm are the optimum spindle rotating speeds for micro-electrode fabrication with low levels of taper and good dimensional accuracy at higher material removal rates [34]. Combining conventional (turning, milling) and non-conventional (EDM, EDG) material removal processes makes a hybrid machining process [4] (Figs. 1(h) and (i)). The spindle (holder) can rotate during electrode fabrication [11]. Another process for the fabrication of micro-electrodes is a method of micro-double-staged laminated object manufacturing (micro-DLOM). It is a hybrid process of fabricating 3D microelectrodes that can be divided into two stages. First, as in Fig 2.(a), 100 µm-thick Cu foils were cut with WEDM to obtain multi-layer 2D microstructures under machining parameters of 10 µs pulse ON time, 40 µs pulse OFF time, 0.42 A wire cutting current, and 80 V voltage. Then multi-layer 2D the micro-structures were interconnected by vacuum pressure thermal diffusion welding to fit the 3D micro-electrode under the process parameters of 850°C thermal diffusion temperature, 10 h thermal diffusion time and 100 N force [35]. In the second process, (Fig. 2(b)) a femtosecond laser was used to cut 50-µm-thick Cu foils coated with Sn on both sides to produce multi-layer 2D microstructures. The multi-layer 2D microstructures were then laminated and bonded to fit composite 3D micro-electrodes through transient liquid phase (TLP) bonding [36]. Previous micro-electrode fabrication processes have some advantages. For instance, BEDG requires a simple setup and the rotating sacrificial electrode fabricates a micro-electrode with smooth surface [11]. The block electrode is capable of fabricating cylindrical, triangular or square electrodes at low cost [37]. Finally, the advantages of the rotating sacrificial disk and BEDG processes are combined in the moving BEDG method. Owing to the simple setup, moving BEDG facilitates the fabrication of non-tapered cylindrical or non-cylindrical micro-electrodes with smooth surface, shape accuracy and low cost [3, 4]. Beside, Micro-DLOM is a good process for fabricating 3D micro-electrodes. Although the described micro-electrode fabrication processes have some advantages, they also have a number of disadvantages. For example, during micro turning of thin micro-electrodes, deflection occurs [4]. The micro-electrode made with a micro turning machine needs to be transferred from this machine and clamped to the micro EDM or EDM machine. It cannot be aligned on the machine, thus accuracy is reduced. Micro-electrodes produced with self-drilling holes are usually tapered [10]. Moreover, tapered or uneven diameters of the micro-electrodes can emerge from the BEDG process [11]. The surface roughness of micro-electrodes fabricated by WEDG (guided running wire) is not as good as that of rotating sacrificial electrodes [11]. In addition, WEDG requires special equipment and high investment. Using a rotating sacrificial disk involves a rather complex setup [3]. Hybrid machining requires special equipment and high investment. Besides, it cannot be easily obtained from all workshops. A micro-electrode made by micro-DLOM needs to be transferred and clamped to the micro EDM or EDM machine. Micro-DLOM does not
have the capability to fabricate micro-electrodes for micro-EDM drilling and milling. Due to the moving BEDG concept, this process cannot be performed on an EDM machine (AG40L Sodick) to manufacture micro-electrodes. Therefore, some modifications are necessary. Hence, on-machine measurement is additionally required to attain the desired micro-electrode diameter and reduce clamping error. Micro-electrode diameter cannot be measured by a micrometer due to the small diameter as it may deform or break down even under low contact forces. Moreover, it is very difficult to estimate the diameter of micro-electrodes during the fabrication process, and microelectrode re-clamping for measurements outside the machine cause lower accuracy. Therefore, researchers have used optical measurement and charge coupled devices (CCD) to measure a thin electrode (Fig. 3) [3, 4, 10, 11]. Such equipment cannot be found everywhere and it is very expensive. Thus, the process of on-machine measurement using a gage block is proposed, because gage block measurement is an economic, easy and readily available technique. Previous researchers have used micro EDM machines, hybrid micro machine tools and different processes to fabricate micro-electrodes for micro EDM drilling [3, 4, 10, 11, 34]. They have selected 500 to 2500 rpm spindle rotating speed and WC as a block electrode to fabricate micro-electrodes in moving BEDG [3, 34]. Optical measurement and charge coupled devices (CCD) have also been applied for on-machine measurement of thin electrodes [3, 4, 10, 11]. Some of the processes necessitate expensive and special equipment to fabricate and measure the micro-electrodes [3, 4, 10, 11, 34]. In this research, an EDM machine with maximum 20 rpm spindle rotating speed is used instead of a micro EDM machine and other equipment. CuW is the micro-electrode used to produce micro holes in a WC-16% Co plate. Copper serves as a block electrode to produce micro-electrodes. However, there is little information on the process of fabricating straight micro-electrodes with an EDM machine and copper block electrode. This research is carried out to fabricate high aspect ratio CuW micro-electrodes with a horizontal moving BEDG (HM-BEDG). In addition, on-machine measurement technique using a gage block is proposed because gage block measurement technique is economic, simple as well as readily available.
2 Methodology 2.1 Experimental setup Fig. 4 illustrates the experimental design for the on-machine fabrication and measurement techniques of micro-electrodes and micro EDM drilling. In this research, an AG40L Sodick electrical discharge machine was used in place of a micro EDM machine. The resolution of movement of each axis was 0.0001mm (= 0.1 µm) as calibrated by Sodick company, and the EDM machine monitor showed the movement of each axis with 0.1 µm accuracy. The amount of discharge energy could be controlled toward lower discharge energy. CuW material (30Cu + 70W) with 1 mm diameter was selected as the raw material for the micro-electrode and WC-16%Co (HSUS16 plate with 0.331 mm thickness) was selected for the workpiece. Tables 1 and 2 list the CuW electrode and WC-16%Co properties. Copper was selected as a block electrode to produce the micro-electrode (Fig. 4). A WC electrode was used to dress the tapered part of the micro-electrode following micro hole EDM drilling in order to produce a straight electrode. The polarity of the micro-electrode during fabrication and electrode dressing was positive, while negative polarity helped produce the micro holes. Oil-based dielectric fluid mixed with aluminum powder (PGM WHIT 3) was also used as the exciting of the aluminum powder in dielectric could help increase the material removal rate (MRR) and improve the surface quality during EDM [22, 23]. A 10mm thick
gage block was utilized for measuring the diameter and cylindricity of the micro-electrode. The copper block electrode and gage block were aligned with a dial test indicator with 2 µm accuracy. Table 3 demonstrates the machining parameter values in each step (roughing, semi-finishing and finishing) for fabricating the micro-electrode using turn off Loran pattern technique. Loran operation is one of the machining conditions used by Sodick EDM machines to introduce electrode motion during the machining process. Loran operation indicates the orbiting motion of an electrode in a simple contour pattern, which is conducted by two axes that are not included in the servocontrolled electrode axis. Such electrode orbiting motion serves the purpose of stable discharge by accelerating chip expulsion and also for machining on the side face with regard to the electrode’s advancing direction. In addition to single-axis machining, Loran is used in two-axis and other multiaxis machining. Loran operation is classified into five types: Free Loran, HS Loran, Lock Loran, Think Loran, and 3-axis Loran, according to operation mode, and two more types, standard Loran and quadrant Loran, according to the Loran pattern selection method. Fig 5 shows the orbiting motion of an electrode in the Think Loran mode. This mode is the most frequently selected Loran type. The Think Loran mode allows simultaneous servo-machining of the side and bottom. The gaps at the side and bottom are always maintained in the best condition to achieve stable machining. This mode is used for machining shapes with a wide side area or for finishing deep-hole shapes [38].
2.2 Micro-electrode fabrication using EDM Fig. 6 illustrates the on-machine micro-electrode fabrication process, which is a modified version of the moving BEDG called horizontal moving BEDG (HM-BEDG) with the following changes: 1. The servo system controls the motion along the X-axis and the EDM gap between the ZY surface (side surface) of the block electrode and the micro-electrode. 2. The spindle rotates around the Z-axis. 3. The electrode’s initial position setting on the Y-axis direction determines the microelectrode diameter and amount of material removed from the micro-electrode, as shown in Fig. 6. The micro-electrode position on the Y-axis is fixed during machining in each step and it will change for the next step. 4. The electrode’s initial position setting on the Z-axis determines the micro-electrode’s length, as shown in Fig. 6. The micro-electrode’s location is fixed in all steps of the fabrication process. 5. The machine table is stationary while the tool head is rotating and moving in the X, Y and Z-axis directions. Most sparks take place between the block electrode’s side surface and the micro-electrode, and this creates erosion on the side surface of the micro-electrode and side surface of the block electrode as well. While the micro-electrode is moving along a +X-axis direction, the diameter becomes smaller until the desired micro-electrode diameter is completely achieved at the end of the process. The micro-electrode was fabricated in three steps using roughing, semi-finishing and finishing parameters in order to reduce machining time, as the amount of material removal rate (MRR) in the finishing step is lower than in other steps. Moreover, the amount of erosion in the finishing section of the block electrode is very low compared with the roughing and semi-finishing steps, since the diameter used is very small and the amount of material removed from the micro-electrode is very low. Hence, at the beginning roughing and semi-finishing parameters are used to decrease
machining time. However, the finishing parameters are used later, as the very low erosion in the finishing process helps improve accuracy. Fig. 4 shows the shape of the copper block electrode used to manufacture micro-electrodes at different machining levels (roughing, semi-finishing and finishing). This shape helps increase accuracy and decrease setting and machining time of the micro-electrode due to the length used for alignment in the block electrode; electrode roughing, semi-finishing and finishing were aligned simultaneously, therefore lead time was reduced by using this electrode. On the other hand, the maximum spindle rotating speed of this EDM machine was 20 rpm, which was considered very low. The optimum spindle rotating speed to fabricate micro-electrodes with low taper levels and good dimensional accuracy at higher material removal rates is from 1000 to 1500 rpm [34]. Low spindle rotational speeds during micro-electrode fabrication can cause longer machining time and less dimensional accuracy [34]. Hence, in this case where 20 rpm is used, the diameter and cylindricity of the micro-electrode have to be measured frequently after each step to investigate the accuracy. If the electrode is cylindrical, the next step should be run. Otherwise, the step should be repeated to remove the oval and non-cylindrical part until a cylindrical micro-electrode is achieved. Upon finishing, the micro-electrode diameter is measured. If the micro-electrode diameter is equal to the desired value, the micro-electrode is fabricated. Otherwise, the micro-electrode diameter should be modified with the finishing step until the desired value is attained.
2.3 Measuring the diameter and cylindricity of a micro-electrode on the EDM Machine with a gage block In this research, the EDM machine functions like a coordinate-measuring machine (CMM) to measure the micro-electrode diameter on the machine. Hence, the EDM machine is able to recognize the touching point between the micro-electrode and the product (wokpiece) through the electricity concept without contact force and micro-electrode bending. This process is known as an on-machine measurement technique, where the gage block clamped on the table of machine is used to measure the micro-electrode diameter. The gage block works like a prop of CMM and the microelectrode serves as a workpiece to measure the diameter. It is an entirely new technique and the gage block is cheaper and more widely available than other measurement processes. The process of electrode measurement using a gage block is shown in Figs. 4 and 7 and described below: 1. The gage block is fastened with a vise (Fig. 4). 2. The gage block is aligned with a dial test indicator with 2 µm accuracy. 3. The micro-electrode touches the gage block, and the X-axis position on the machine (point O1) is identified (Fig. 7). 4. The micro-electrode touches the other side of the gage block, and the X-axis position on the machine (point O2) is identified (Fig. 7). 5. According to the following equation, the electrode diameter can be measured. D=
O 2 - O1 - T
where D is the micro-electrode’s diameter, O1 is the first position where the micro-electrode touches the gage block, O2 is the second position where the micro-electrode touches the gage block, and T is the gage block thickness. The cylindricity of the micro-electrode is identified by measuring the micro-electrode’s diameter in different directions with a 90° phase shift.
2.4 Micro drilling of the WC workpiece, Micro-electrode dressing and Micro hole measurement The fabricated micro-electrode is used for micro EDM drilling of the WC workpiece. After micro EDM drilling of every hole, the end of the micro-electrode becomes tapered due to microelectrode wear. The dressing process serves to remove the taper part of the micro-electrode and produce a straight micro-electrode using a tungsten carbide (WC) electrode as shown in Figs. 4 and 8. The process is carried out with positive polarity for the micro-electrode, which causes more material removal from the micro-electrode and a faster dressing process [39]. The dressing process was performed at a voltage of 70 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs, and positive micro-electrode polarity. Field Emission Scanning Electron Microscopy (FESEM) and optical microscope were used to measure the micro-electrode and micro hole diameters in the WC workpiece.
3 Results and discussion Figure 9 represents the first trial in micro-electrode fabrication. As seen in Fig. 9, the microelectrode was burned from using the roughing parameters shown in Table 3 - row No. 1 (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs, 20 rpm rotating speed, and standard Think Loran (circle type)). This result depicts that using only roughing parameters cannot achieve smaller micro-electrode diameters of down to 800 µm. To solve this problem, all roughing as well as semi-finishing and finishing parameters should be subsequently used until the smallest diameter without burning is reached. This is because to attain a smaller micro-electrode diameter, a machining condition with lower spark energy should be used otherwise the microelectrode will burn, as shown in Fig. 9. Fig. 10 shows the second fabrication trial. The tapered and non-cylindrical micro-electrode was fabricated using standard Think Loran (circle type) and the parameters are shown in Table 3 - row No. 1 (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs, and 20 rpm rotating speed). It seems that using Standard Think Loran (circle type) and 20 rpm spindle rotating speed are the main factors in producing tapered and non-cylindrical micro-electrode. The 20 rpm spindle rotating speed is very low for fabricating micro-electrodes and it causes the production of non-cylindrical micro-electrodes [34]; however, the maximum spindle rotating speed of this machine is 20 rpm. To mitigate the non-cylindrical problem caused by using low spindle speed to produce cylindrical micro-electrodes, each step of the fabrication process should be repeated to help remove the non-cylindrical part and produce cylindrical micro-electrodes. Fig. 1 shows the micro-electrodes produced with repeated fabrication process steps using standard Think Loran (circle type) and the parameters shown in Table 3 - row No. 1 (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed). Fig. 11 indicates that a tapered and cylindrical micro-electrode was achieved. To solve the taper problem and produce straight micro-electrodes, the Loran pattern was turned off to eliminate the micro-electrode’s orbiting motion in a contour pattern. Fig. 12 shows that non-tapered and cylindrical micro-electrodes with 376.4 and 363.7 µm diameters were successfully fabricated by turning off the Loran pattern and repeating each step of the fabrication process at voltage of 35 V,
current of 5 A, pulse ON time of 50 µs, pulse OFF time of 53 µs and 20 rpm rotating speed (roughing parameters in Table 3 row No. 2). Furthermore, to fabricate non-tapered and cylindrical micro-electrodes with smaller diameters down to 160.8 µm, the spark energy should be decreased more to be set in the lowest level. Fig. 13 demonstrates a thin, cylindrical and straight micro-electrode fabricated at the lowest level of spark energy using the finishing parameters as shown in Table 3 – row No. 6 (voltage of 35 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs and 20 rpm rotating speed). In this fabrication process, the Loran pattern was switched off and each step of the fabrication process at 20 rpm rotating speed was repeated. The next step after successfully fabricating the micro electrode is that the fabricated microelectrode should be used for micro EDM drilling on the WC workpiece. For this purpose, the reference point on each axis (X, Y, Z = 0) should be identified on the EDM machine by the touching of the micro-electrode and the workpiece. The EDM machine recognizes this touching point through an electrical connection between the workpiece and micro-electrode without any mechanical force. Fig. 14(a) illustrates that the micro-electrode bent after touching the WC plate to find a reference point on the Z-axis (Z=0). To investigate the reason for this problem, a FESEM picture was taken of the recast layer at the bottom of the bent micro-electrode in Fig. 14(b) and EDX spectrum analysis was performed to analyze the elemental composition on the recast layer at the bottom of the bent micro-electrode in Fig. 14(c). In addition, EDX spectrum analysis was done on the raw material of the micro-electrode to analyze the elemental composition shown in Fig. 15. According to Fig. 15, C, O, Cu, W elements are present in the micro-electrode’s raw material, while Fig. 14(c) shows that C, O, Al, Cu, W elements are present in the recast layer on the bottom of the bent micro-electrode. From the EDX analysis it could be recognized that the tungsten (W) reacted with carbon to produce tungsten carbide (WC), while aluminum (Al) reacted with oxygen (O) to produce Al2O3 composite. The recast layers of WC and Al2O3 composites decreased the electrical conductivity of the bottom micro-electrode surface, hence the EDM machine did not easily detect the touching point, so the machine applied greater feed motion of the micro-electrode that led to higher touching force and caused the micro-electrode to bend. In addition, the thickness of these recast layer composites increased with increasing the spark energy, leading to less electrical conductivity [40]. To solve the bend problem of the micro-electrode, the bottom of the microelectrode was dressed with a WC electrode and low spark energy (voltage of 70 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs, and positive micro-electrode polarity), causing a decreasing recast layer at the bottom of the micro-electrode, making it more electrically conductive. The dressing process was described in section 2.4. Fig. 16 displays a micro-electrode with 97.14 µm diameter after dressing. Upon mitigating the above-mentioned problems, straight, cylindrical micro-electrodes were fabricated. Fig. 17(a) shows a micro-electrode with 78.19 µm diameter and 4.70 mm length (aspect ratio of more than 60.25) after producing a through micro hole with 134.5 µm diameter in Fig. 17(b). Negative microelectrode polarity, voltage of 70 V, current of 0.4 A, pulse ON time of 1 µs, and pulse OFF time of 1 µs were used to micro hole EDM drilling. Figs. 18(a) and 18(c) show two micro-electrodes with 165.2 and 169.7 µm diameter and around 6.8 mm length (aspect ratio = 41.16) fabricated using the setup shown in Fig. 4. The smooth surfaces of these micro-electrodes captured by FESEM are shown in Figs. 18(b) and (d). Moreover, Fig. 18(e) and (f) depict the two micro holes produced with these electrodes. The hole in Fig. 18(e) was machined with 90 V voltage, 0.4 A current, pulse ON time of 1 µs, pulse OFF time of 1 µs, capacitor of 0.1 µF, and without spindle rotation. The hole
shown in Fig. 18(f) was machined with voltage of 90 V, current of 0.4 A, pulse ON time of 5 µs, pulse OFF time of 9 µs, capacitor of 0.1 µF, and without spindle rotation. Fig. 18(g) shows all holes produced by these micro-electrodes with various machining conditions. According to Fig 18, the two micro-electrodes were fabricated with approximately the same dimensions. As a result, it can be concluded that HM-BEDG enables economic fabrication of long and thin electrodes without taper and with smooth surface. In addition, the proposed micro-electrode fabrication and measurement processes are repeatable. As observed in Figs. 18(a) and (c), the end of micro-electrodes became tapered after machining of the micro hole due to electrode wear. The tapered part of the micro-electrode was removed via the dressing process as described in section 2.4. The dressing process produced a straight microelectrode for the subsequent micro EDM drilling as shown in Fig 19. As a result, a longer microelectrode can be used rather than several micro-electrodes. Table 4 illustrates the micro-electrode diameters from Figs. 17(a), 18(a) and 18(c) that were measured by FESEM and gage block in different directions. The gage block measurement results indicate that the maximum difference between the electrode diameters in two directions was 0.4 µm (Table 4 and Fig. 20). Thus, a cylindrical electrode was produced, signifying that the fabrication procedure was correct. Fig. 20 demonstrates that the electrode diameters measured by FESEM are less than by gage block. The amount of error (6.6 to 9.81 µm) between the gage block and FESEM measurements is shown in Fig. 21. The difference in measurement error happened because the temperature affected the gage block dimensions, and there was error during gage block alignment as well as EDM machine backlash.
4 Conclusion This study represented the steps for fabricating and measuring micro-electrodes based on horizontal moving BEDG (HM-BEDG) and gage block using a typical EDM machine, and solving the problems occurring during micro-electrode fabrication. The modifications included altering the movement directions and control gap and using a copper block electrode to manufacture a micro electrode at different machining levels (roughing, semi-finishing and finishing). These modifications resulted in a more simplified setup and shorter electrode fabrication time. The EDM machine worked like a CMM machine in measuring the micro-electrode diameter and used the gage block as a probe and the micro-electrode as a workpiece. Gage block measuring is an economic and simple technique besides being readily available. The fabrication and measurement steps are straightforward without the need for high investment in special equipment. In addition, these changes and developments enabled the use of EDM machining for the fabrication of straight and thin micro-electrodes (cylindrical, cubic and so on) with smooth surface and low cost. As a result, a micro-electrode with the highest aspect ratio (more than 60.25) was fabricated successfully with 78.19 µm diameter and 4.70 mm length after producing a through micro hole with 134.5 µm diameter. Since these procedures are simple and inexpensive in terms of special equipment, microelectrodes can be fabricated and measured for micro EDM drilling and milling in typical workshops equipped with EDM machines. Acknowledgments The authors would like to acknowledge the University of Malaya for providing the necessary facilities and resources for this research. This research was fully funded by the Ministry of Higher
Education, Malaysia with the high impact research (HIR) grant number HIR-MOHE-16001-00D000001.
Reference [1] T. Masuzawa, H. Tönshoff, Three-dimensional micromachining by machine tools, CIRP Ann. Manuf. Technol. 46 (1997) 621-628. [2] J. Goda, K. Mitsui, Development of an integrated apparatus of micro-EDM and micro-CMM, Measurement, 46 (2013) 552-562. [3] M. Rahman, A. Asad, T. Masaki, T. Saleh, Y. Wong, A. Senthil Kumar, A multiprocess machine tool for compound micromachining, Int. J. Mach. Tools Manuf. 50 (2010) 344-356. [4] A. Asad, T. Masaki, M. Rahman, H. Lim, Y. Wong, Tool-based micro-machining, J. Mater. Process. Technol. 192 (2007) 204-211. [5] T. Masuzawa, State of the art of micromachining, CIRP Ann. Manuf. Technol. 49 (2000) 473488. [6] E.J. Weller, Nontraditional machining process, Society of Manufacturing Engineers, Dearborn, Michigan, 1984. [7] R. Snoeys, F. Staelens, W. Dekeyser, Current trends in non-conventional material removal processes, CIRP Ann. Manuf. Technol., 35 (1986) 467-480. [8] S. Matsui, T. Kaito, J. Fujita, M. Komuro, K. Kanda, Y. Haruyama, Three-dimensional nanostructure fabrication by focused-ion-beam chemical vapor deposition, J. Vac. Sci. Technol. B: Microelectronics and Nanometer Structures, 18 (2000) 3181-3184. [9] H. Okuyama, H. Takada, Micromachining with SR and FEL, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 144 (1998) 5865. [10] L. Zhang, H. Tong, Y. Li, Precision machining of micro tool electrodes in micro EDM for drilling array micro holes, Precision Engineering, 39 (2015) 100-106. [11] H. Lim, Y. Wong, M. Rahman, M. Edwin Lee, A study on the machining of high-aspect ratio micro-structures using micro-EDM, J. Mater. Process. Technol. 140 (2003) 318-325. [12] B. Mark, M. Toshihiko, M. Teruaki, T. Kazushi, Micro-Hole Punching Using Multi-Point Punch and Die Made by EDM, Int. J. Adv. Manuf. Technol. 3 (2009) 73-86. [13] R. Mahdavinejad, A. Mahdavinejad, ED machining of WC–Co, J. Mater. Process. Technol. 162 (2005) 637-643. [14] A.K. Sahoo, B. Sahoo, A comparative study on performance of multilayer coated and uncoated carbide inserts when turning AISI D2 steel under dry environment, Measurement, 46 (2013) 2695-2704. [15] A.K. Sahoo, B. Sahoo, Performance studies of multilayer hard surface coatings (TiN/TiCN/Al 2 O 3/TiN) of indexable carbide inserts in hard machining: Part-II (RSM, grey relational and techno economical approach), Measurement, 46 (2013) 2868-2884. [16] A.K. Sahoo, B. Sahoo, Performance studies of multilayer hard surface coatings (TiN/TiCN/Al 2 O 3/TiN) of indexable carbide inserts in hard machining: Part-I (An experimental approach), Measurement, 46 (2013) 2854-2867. [17] K. Jangra, S. Grover, A. Aggarwal, Optimization of multi machining characteristics in WEDM of WC-5.3% Co composite using integrated approach of Taguchi, GRA and entropy method, Frontiers of Mechanical Engineering, (2012) 1-12. [18] S. Dewangan, S. Gangopadhyay, C. Biswas, Study of surface integrity and dimensional accuracy in EDM using Fuzzy TOPSIS and sensitivity analysis, Measurement, 63 (2015) 364376. [19] M. Hourmand, S. Farahany, A.A. Sarhan, M.Y. Noordin, Investigating the electrical discharge machining (EDM) parameter effects on Al-Mg2Si metal matrix composite (MMC) for high material removal rate (MRR) and less EWR–RSM approach, Int. J. Adv. Manuf. Technol. (2014) 1-8.
[20] G. Mandaloi, S. Singh, P. Kumar, K. Pal, Effect on crystalline structure of AISI M2 steel using copper electrode through material removal rate, electrode wear rate and surface finish, Measurement, 61 (2015) 305-319. [21] D. Puhan, S.S. Mahapatra, J. Sahu, L. Das, A hybrid approach for multi-response optimization of non-conventional machining on AlSiC p MMC, Measurement, 46 (2013) 3581-3592. [22] S. Assarzadeh, M. Ghoreishi, A dual response surface-desirability approach to process modeling and optimization of Al2O3 powder-mixed electrical discharge machining (PMEDM) parameters, Int. J. Adv. Manuf. Technol. 64 (2013) 1459-1477. [23] K.-Y. Kung, J.-T. Horng, K.-T. Chiang, Material removal rate and electrode wear ratio study on the powder mixed electrical discharge machining of cobalt-bonded tungsten carbide, The Int. J. Adv. Manuf. Technol. 40 (2009) 95-104. [24] Y. Li, W. Zhao, X. Feng, H. Wei, Research advancement on on-line detection of EDM spark locations, Measurement, 22 (1997) 29-35. [25] H. Qiang, H. Yong, Z. Wansheng, Research of two-dimension EDM spark locations detection using electromagnetic method, Measurement, 31 (2002) 117-122. [26] S. Gopalakannan, T. Senthilvelan, Application of response surface method on machining of Al–SiC nano-composites, Measurement, 46 (2013) 2705-2715. [27] I. Puertas, C. Luis, L. Alvarez, Analysis of the influence of EDM parameters on surface quality, MRR and EW of WC–Co, J. Mater. Process. Technol. 153 (2004) 1026-1032. [28] L. Alting, F. Kimura, H.N. Hansen, G. Bissacco, Micro engineering, CIRP Ann. Manuf. Technol. 52 (2003) 635-657. [29] K. Ho, S. Newman, State of the art electrical discharge machining (EDM), Int. J. Machine Tools Manuf. 43 (2003) 1287-1300. [30] Y.Y. Tsai, T. Masuzawa, An index to evaluate the wear resistance of the electrode in microEDM, J. Mater. Process. Technol. 149 (2004) 304-309. [31] M. Jahan, M. Rahman, Y. Wong, A review on the conventional and micro-electrodischarge machining of tungsten carbide, Int. J. Machine Tools Manuf. 51 (2011) 837-858. [32] M.P. Jahan, Y.S. Wong, M. Rahman, A study on the fine-finish die-sinking micro-EDM of tungsten carbide using different electrode materials, J. Mater. Process. Technol. 209 (2009) 3956-3967. [33] M. Yamazaki, T. Suzuki, N. Mori, M. Kunieda, EDM of micro-rods by self-drilled holes, J. Mater. Process. Technol. 149 (2004) 134-138. [34] M. Jahan, M. Rahman, Y. Wong, L. Fuhua, On-machine fabrication of high-aspect-ratio microelectrodes and application in vibration-assisted micro-electrodischarge drilling of tungsten carbide, Proceedings of the Institution of Mechanical Engineers, Part B: J. Eng. Manuf. 224 (2010) 795-814. [35] B. Xu, X.-y. Wu, J.-g. Lei, R. Cheng, S.-c. Ruan, Z.-l. Wang, Laminated fabrication of 3D micro-electrode based on WEDM and thermal diffusion welding, J. Mater. Process. Technol. 221 (2015) 56-65. [36] J. Lei, X. Wu, B. Xu, Z. Zhao, S. Ruan, R. Cheng, Laminated fitting fabrication of Cu-Sn composite 3D microelectrodes and elimination of ridges on the machined surface of 3D microcavities, J. Mater. Process. Technol. (2015). [37] N. Ravi, S.X. Chuan, The effects of electro-discharge machining block electrode method for microelectrode machining, J. micromech. microeng. 12 (2002) 532. [38] Code instructions LN/LP/LN2/LP2, Sodick NC power supply unit for EDM, Version 6.0. [39] M.P. Jahan, Y. San Wong, M. Rahman, A comparative experimental investigation of deep-hole micro-EDM drilling capability for cemented carbide (WC-Co) against austenitic stainless steel (SUS 304), Int. J. Adv. Manuf. Technol. 46 (2010) 1145-1160. [40] S.H. Lee, X. Li, Study of the surface integrity of the machined workpiece in the EDM of tungsten carbide, J. Mater. Process. Technol. 139 (2003) 315-321.
Figure(s)
Fig. 1 Various kinds of on-machine micro-electrode fabrication process (a) WEDG construction [10] (b) radial feed WEDG [10] (c) tangential feed WEDG (TF-WEDG) [10] (d) self-drilled holes [33] (e) stationary sacrificial electrode (BEDG) [11] (f) rotating sacrificial disk [11] (g) moving BEDG [4] (h) hybrid process (micro turning and micro-EDM) [4] (i) hybrid process (self-drilled holes and TF-WEDG) [10]
Fig. 2 Processes sketch of fabricating 3D micro-electrode with two various micro-double-staged laminated object manufacturing (micro-DLOM) [35, 36].
Fig. 3 Online measurement (a) Laser light [3, 4, 11] (b) charge coupled device (CCD) [10]
WC electrode
Micro-electrode 1
2
3
4
Gage block
Cu block electrode
Workpiece (WC-Co plate)
1. Roughing section 2. Semi-finishing section 3. Finishing section 4. Alignment Section
Fig. 4 a) Experimental setup design for the on-machine fabrication and measurement techniques of micro-electrodes and micro EDM drilling.
Fig. 5 Think loran pattern method [38]
Fig. 6 Horizontal moving BEDG (HM-BEDG) process for on-machine micro-electrode fabrication
Fig. 7 Gage block measuring process for On-machine micro-electrode measurement a) The micro-electrode touches the gage block at first position point (O1) b) The micro-electrode touches the gage block at the second position Point (O2)
Fig. 8 Dressing process a) Taper part of micro-electrode after machining micro hole and before dressing process b) Straight micro-electrode after dressing process
Fig. 9 Burnt electrode in the multi angle view (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed)
Fig. 10 Taper and non-cylindrical micro-electrode in the multi angle view (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed)
Fig. 11 Taper micro-electrode (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed)
Fig. 12 Non-taper cylindrical micro-electrodes (microscope and FESEM pictures) were produced at voltage of 35 V, current of 5 A, pulse ON time of 50 µs, pulse OFF time of 53 µs and 20 rpm rotating speed
Fig. 13 Non-taper cylindrical micro-electrodes (microscope and FESEM pictures) was produced at voltage of 35 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs and 20 rpm rotating speed
Fig. 14 a) Bent micro-electrode after touching the WC plate b) FESEM picture from the bottom surface of the bent micro-electrode with recast layer c) EDX spectrum analysis of the bottom surface of the bent micro-electrode
Fig. 15 EDX spectrum analysis of the surface CuW before micro-electrode fabrication.
Fig. 16 Micro-electrode after dressing process (microscope and FESEM pictures)
Fig. 17 a) Micro-electrode with 78.19 µm diameter and 4.70 mm length (more than 60.13 high aspect ratio) after producing through micro-hole b) Micro-hole was fabricated with microelectrode which had 78.19 µm diameter.
Fig. 18 a) Micro-electrode with 165.2 µm diameter b) Surface of micro-electrode with 165.2 µm diameter c) Micro-electrode with 169.7 µm diameter d) Surface of micro-electrode with 169.7 µm diameter e) The micro hole was produced with micro-electrode that had 165.2 µm diameter. f) The micro hole was produced with micro-electrode that had 169.7 µm diameter. g) All of the micro holes were produced at various machining conditions with these micro-electrodes
Fig. 19 Straight micro-electrode was achieved after dressing process
Fig. 20 Measuring of the micro-electrode diameter
Fig. 21 Error of measurement between gage block and workpiece
Table 1. Table Properties of CuW micro-electrode
Kind
Ingredient
Specific Gravity
CuW
30Cu + 70W
14.3
Conductivity %IACS 28.0
Solidity HRB93
Table 2. Properties of WC-Co (Typical nominal properties)
Grade HC-US16
Percent Cobalt 16
Grain Structure SUB-MICRON
Rockwell Hardness 90.8
Transvers Rupture 500
Density Grams/C.C. 13.82
Table 3. Machining parameters for fabrication of micro-electrode
No.
1
Roughing
30
20
600
Pulse OFF time (µs) 257
2 3 4
Roughing Roughing Semifinishing Semifinishing Finishing
35 35 35
5 2 1.5
50 25 10
53 27 11
Rotating speed (rpm) 20 (maximum) 20 20 20
35
0.4
2
3
20
35
0.4
1
1
20
5 6
Steps
Voltage Current Pulse ON time (V) (A) (µs)
Table 4. Micro-electrode diameters
Equipment FESEM Gage block
Rotation of microelectrode ------0° 90°
Micro-electrode Diameter (µm) No. 1 No. 2 No. 3 78.19 88 87.6
165.2 173.3 173.5
169.7 176.3 176.7
Highlights 123456-
Micro-electrodes are used for drilling micro-hole in miniaturized products Proposing new fabrication and measurement techniques of micro-electrode are simple These processes do not require high investment for special equipments Micro-electrode can be fabricated and measured in workshop by usual EDM machine The highest aspect ratio (more than 60.25) of micro-electrode was fabricated It had 78.19 µm diameter and 4.70 mm length after producing hole on WC (134.5 µm)
S0263-2241(16)30656-X http://dx.doi.org/10.1016/j.measurement.2016.11.020 MEASUR 4441
To appear in:
Measurement
Received Date: Accepted Date:
30 July 2015 8 November 2016
Please cite this article as: M. Hourmand, A.A.D. Sarhan, N. Mohd Yusof, Proposing New Fabrication and Measurement Techniques of Micro-electrodes with High Aspect Ratio for Micro EDM Drilling of Tungsten Carbide (WC) Using EDM Machine, Measurement (2016), doi: http://dx.doi.org/10.1016/j.measurement.2016.11.020
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Proposing New Fabrication and Measurement Techniques of Micro-electrodes with High Aspect Ratio for Micro EDM Drilling of Tungsten Carbide (WC) Using EDM Machine Mehdi Hourmand1*, Ahmed A. D. Sarhan1, Noordin Mohd Yusof2 1
Department of Mechanical Engineering, Faculty of Engineering, University of Malaya (UM), 50603, Kuala Lumpur, Malaysia 2 Department of Materials, Manufacturing & Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia (UTM), 81310, Skudai, Malaysia *Corresponding author: Mehdi Hourmand Email address: [email protected] Telephone number: 0060187721401 Abstract. Micro-electrodes are used for micro holes EDM drilling in miniaturized products. This study represents novel fabricating and measuring processes of high aspect ratio micro-electrodes based on horizontal moving block electrical discharge grinding (horizontal moving BEDG [HMBEDG]) and gage block with a typical EDM machine, as well as solving the problems that occur during micro-electrode fabrication. The EDM machine has been used as a coordinate measuring machine (CMM) for measuring micro-electrode diameter, where the gage block acts as a probe and the micro-electrode as a workpiece. As a result, a micro-electrode with the highest aspect ratio (over 60.25) was fabricated successfully with 78.19 µm diameter and 4.70 mm length after producing a through micro hole with 134.5 µm diameter on WC-Co. As these processes are simple and do not require high investment for special equipment, micro-electrodes can be fabricated and measured for micro EDM drilling and milling in conventional workshops equipped with EDM machines. Keywords: Micro EDM, Micro-electrode, Fabrication process, Measurement, Tungsten Carbide (WC, WC-Co), Micro drilling, High aspect ratio
1 Introduction Micromachining is among the fundamental technologies for manufacturing and miniaturizing parts and products with dimensions of 1 to 999 µm based on the CRIP Committee of Physical and Chemical processes [1]. Miniaturized products are mainly used in information technology, biotechnology, environmental and medical industries, miniaturized machines, electric devices and so on [2, 3]. Recently, Micro-Electro Mechanical System (MEMS) advancements have popularized micromachining [4]. Micromachining processes are basically classified into three groups, including conventional, non-conventional, and hybrid machining processes. Conventional machining process requires mechanical force and energy to remove material by shear stress. Shearing entails simple machining by physical contact between the cutting tool and material [5, 6]. Conventional machining processes, such as micro turning, micro milling and grinding, function with recommended singlepoint diamond cutters or very fine grit grinding wheels. Other sources of energy, including spark energy, light energy, vibration energy, electrolysis energy, mechanical energy (based on erosion mechanisms), energy beams (laser, electron or ion beams), etc., are used to remove material in non-
conventional processes [6-9]. Hybrid machining process is a combination of conventional and nonconventional material removal methods to produce high-precision microstructures [3, 10]. A micro-mold cavity is required for the mass production of micro components [11]. Micro holes are used in micro dies and molds [12]. Moreover, hard-to-machine materials are applied for microinjection mold, which requires very accurate machining of complex shapes and three-dimensional forms in the micron range [11]. Tungsten carbide (WC) and its composite (WC-Co) are employed to produce cutting tools, dies and other special tools and components owing to their high hardness, strength, wear and corrosion resistance over a wide range of temperatures [13-16]. Thus, WC machining plays significant roles in manufacturing [17]. In the EDM method, conductive materials are removed based on thermal energy (melting and partial workpiece vaporization) by series of sparks that occur between the electrode and electrical workpiece [18-21] in the presence of the dielectric media. However, adding aluminum powder to the dielectric leads to increased material removal rate (MRR) and improved surface roughness (SR) during EDM [22, 23]. Electromagnetic detection, acoustic signal detection and electrical signal detection are the methods employed for detecting spark location in EDM machines [24, 25]. EDM is a useful process in industry for machining electrically conductive materials with accurate dimensions regardless of hardness, strength and toughness [26, 27]. Micro EDM and EDM have the same characteristics, with only the electrode size, discharge energy and axis movement resolution at the micron level in micro EDM [5]. Micro mold making, production of dies and cavities, and complex three-dimensional shapes at the micro level are the major functions of micro EDM [28]. In EDM and micro EDM, no direct contact and negligible force between the workpiece and tool cause the elimination of tool deformation, chatter, mechanical stress and vibration errors during machining [5, 29, 30]. Micro EDM is one of the most effective methods of machining WC [31, 32]. Moreover, micro EDM has a significant role in the field of micromachining. Very long and thin electrodes can be used in micro EDM due to its non-contact nature [11]. Micro EDM has disadvantages, among which electrode wear ratio and low material removal rate. Electrode wear can be compensated by changing the micro-electrode, preparing a longer micro-electrode from the beginning or fabricating the micro-electrode in situ for further machining [4]. Also, changing the micro-electrode during machining causes decreased accuracy due to changes in setup or microelectrode re-clamping. Thus, on-machine micro-electrode fabrication can eliminate clamping error in the micro EDM process [4]. The processes applicable for the fabrication of micro-electrodes from electrodes thicker than required electrodes, are wire electro-discharge grinding (WEDG), self-drilled holes, stationary sacrificial block electro discharge grinding (BEDG), rotating sacrificial disk, moving block electrodischarge grinding (moving BEDG), micro turning, and hybrid machining. In WEDG, a 0.07 mm diameter guided running wire with 3-5 mm/s speed is used as an electrode [11]. Various WEDG methods are compared in Fig. 1(a), (b) and (c). During conventional radial feed WEDG the microelectrode feeds along the central axis in the direction of the symmetry axis ON (Y-axis) to the wire, while in tangential feed WEDG (TF-WEDG), the micro-electrode’s position is fixed on the Y-axis and it feeds along the tangential direction of the wire guide arc (X-axis) [10]. Fig. 1(d) shows the principles of micro-electrode fabrication by self-drilled holes. Initially, a hole is produced with a rod electrode with negative polarity. Subsequently, the rod electrode polarity is positioned offcenter from the center of the hole and its polarity changes to positive. In the final step, the rod electrode either with or without rotation, is fed to the plate to fabricate the micro-electrode [33]. Fig. 1(e) depicts the BEDG method, which is the simplest means of on-machine micro-electrode fabrication. The rotating raw material of the micro-electrode is fed vertically to the stationary
sacrificial block electrode for machining [11]. The rotating sacrificial disk process is illustrated in Fig. 1(f). The rotating disk electrode is 5 mm thick and has 60 mm diameter and 90 rpm rotating speed [11]. Fig. 1(g) demonstrates the moving BEDG process for micro-electrode fabrication. In this process, the Z-axis controls the EDM gap and there is relative translational motion between the micro-electrode and sacrificial block electrode. Moreover, most sparks occur between the top surface of the block electrode and the un-machined part of the micro-electrode, and there is no spark between the sides of the micro-electrode and block electrode. Moreover, previous researchers have used WC as a block electrode [3, 34] and 500 to 2500 rpm rotating speed spindle [34] to fabricate micro-electrodes. It has been reported that very low electrode rotational speeds during micro-electrode fabrication can cause longer machining time and less dimensional accuracy. It has also been found that 1000 to 1500 rpm are the optimum spindle rotating speeds for micro-electrode fabrication with low levels of taper and good dimensional accuracy at higher material removal rates [34]. Combining conventional (turning, milling) and non-conventional (EDM, EDG) material removal processes makes a hybrid machining process [4] (Figs. 1(h) and (i)). The spindle (holder) can rotate during electrode fabrication [11]. Another process for the fabrication of micro-electrodes is a method of micro-double-staged laminated object manufacturing (micro-DLOM). It is a hybrid process of fabricating 3D microelectrodes that can be divided into two stages. First, as in Fig 2.(a), 100 µm-thick Cu foils were cut with WEDM to obtain multi-layer 2D microstructures under machining parameters of 10 µs pulse ON time, 40 µs pulse OFF time, 0.42 A wire cutting current, and 80 V voltage. Then multi-layer 2D the micro-structures were interconnected by vacuum pressure thermal diffusion welding to fit the 3D micro-electrode under the process parameters of 850°C thermal diffusion temperature, 10 h thermal diffusion time and 100 N force [35]. In the second process, (Fig. 2(b)) a femtosecond laser was used to cut 50-µm-thick Cu foils coated with Sn on both sides to produce multi-layer 2D microstructures. The multi-layer 2D microstructures were then laminated and bonded to fit composite 3D micro-electrodes through transient liquid phase (TLP) bonding [36]. Previous micro-electrode fabrication processes have some advantages. For instance, BEDG requires a simple setup and the rotating sacrificial electrode fabricates a micro-electrode with smooth surface [11]. The block electrode is capable of fabricating cylindrical, triangular or square electrodes at low cost [37]. Finally, the advantages of the rotating sacrificial disk and BEDG processes are combined in the moving BEDG method. Owing to the simple setup, moving BEDG facilitates the fabrication of non-tapered cylindrical or non-cylindrical micro-electrodes with smooth surface, shape accuracy and low cost [3, 4]. Beside, Micro-DLOM is a good process for fabricating 3D micro-electrodes. Although the described micro-electrode fabrication processes have some advantages, they also have a number of disadvantages. For example, during micro turning of thin micro-electrodes, deflection occurs [4]. The micro-electrode made with a micro turning machine needs to be transferred from this machine and clamped to the micro EDM or EDM machine. It cannot be aligned on the machine, thus accuracy is reduced. Micro-electrodes produced with self-drilling holes are usually tapered [10]. Moreover, tapered or uneven diameters of the micro-electrodes can emerge from the BEDG process [11]. The surface roughness of micro-electrodes fabricated by WEDG (guided running wire) is not as good as that of rotating sacrificial electrodes [11]. In addition, WEDG requires special equipment and high investment. Using a rotating sacrificial disk involves a rather complex setup [3]. Hybrid machining requires special equipment and high investment. Besides, it cannot be easily obtained from all workshops. A micro-electrode made by micro-DLOM needs to be transferred and clamped to the micro EDM or EDM machine. Micro-DLOM does not
have the capability to fabricate micro-electrodes for micro-EDM drilling and milling. Due to the moving BEDG concept, this process cannot be performed on an EDM machine (AG40L Sodick) to manufacture micro-electrodes. Therefore, some modifications are necessary. Hence, on-machine measurement is additionally required to attain the desired micro-electrode diameter and reduce clamping error. Micro-electrode diameter cannot be measured by a micrometer due to the small diameter as it may deform or break down even under low contact forces. Moreover, it is very difficult to estimate the diameter of micro-electrodes during the fabrication process, and microelectrode re-clamping for measurements outside the machine cause lower accuracy. Therefore, researchers have used optical measurement and charge coupled devices (CCD) to measure a thin electrode (Fig. 3) [3, 4, 10, 11]. Such equipment cannot be found everywhere and it is very expensive. Thus, the process of on-machine measurement using a gage block is proposed, because gage block measurement is an economic, easy and readily available technique. Previous researchers have used micro EDM machines, hybrid micro machine tools and different processes to fabricate micro-electrodes for micro EDM drilling [3, 4, 10, 11, 34]. They have selected 500 to 2500 rpm spindle rotating speed and WC as a block electrode to fabricate micro-electrodes in moving BEDG [3, 34]. Optical measurement and charge coupled devices (CCD) have also been applied for on-machine measurement of thin electrodes [3, 4, 10, 11]. Some of the processes necessitate expensive and special equipment to fabricate and measure the micro-electrodes [3, 4, 10, 11, 34]. In this research, an EDM machine with maximum 20 rpm spindle rotating speed is used instead of a micro EDM machine and other equipment. CuW is the micro-electrode used to produce micro holes in a WC-16% Co plate. Copper serves as a block electrode to produce micro-electrodes. However, there is little information on the process of fabricating straight micro-electrodes with an EDM machine and copper block electrode. This research is carried out to fabricate high aspect ratio CuW micro-electrodes with a horizontal moving BEDG (HM-BEDG). In addition, on-machine measurement technique using a gage block is proposed because gage block measurement technique is economic, simple as well as readily available.
2 Methodology 2.1 Experimental setup Fig. 4 illustrates the experimental design for the on-machine fabrication and measurement techniques of micro-electrodes and micro EDM drilling. In this research, an AG40L Sodick electrical discharge machine was used in place of a micro EDM machine. The resolution of movement of each axis was 0.0001mm (= 0.1 µm) as calibrated by Sodick company, and the EDM machine monitor showed the movement of each axis with 0.1 µm accuracy. The amount of discharge energy could be controlled toward lower discharge energy. CuW material (30Cu + 70W) with 1 mm diameter was selected as the raw material for the micro-electrode and WC-16%Co (HSUS16 plate with 0.331 mm thickness) was selected for the workpiece. Tables 1 and 2 list the CuW electrode and WC-16%Co properties. Copper was selected as a block electrode to produce the micro-electrode (Fig. 4). A WC electrode was used to dress the tapered part of the micro-electrode following micro hole EDM drilling in order to produce a straight electrode. The polarity of the micro-electrode during fabrication and electrode dressing was positive, while negative polarity helped produce the micro holes. Oil-based dielectric fluid mixed with aluminum powder (PGM WHIT 3) was also used as the exciting of the aluminum powder in dielectric could help increase the material removal rate (MRR) and improve the surface quality during EDM [22, 23]. A 10mm thick
gage block was utilized for measuring the diameter and cylindricity of the micro-electrode. The copper block electrode and gage block were aligned with a dial test indicator with 2 µm accuracy. Table 3 demonstrates the machining parameter values in each step (roughing, semi-finishing and finishing) for fabricating the micro-electrode using turn off Loran pattern technique. Loran operation is one of the machining conditions used by Sodick EDM machines to introduce electrode motion during the machining process. Loran operation indicates the orbiting motion of an electrode in a simple contour pattern, which is conducted by two axes that are not included in the servocontrolled electrode axis. Such electrode orbiting motion serves the purpose of stable discharge by accelerating chip expulsion and also for machining on the side face with regard to the electrode’s advancing direction. In addition to single-axis machining, Loran is used in two-axis and other multiaxis machining. Loran operation is classified into five types: Free Loran, HS Loran, Lock Loran, Think Loran, and 3-axis Loran, according to operation mode, and two more types, standard Loran and quadrant Loran, according to the Loran pattern selection method. Fig 5 shows the orbiting motion of an electrode in the Think Loran mode. This mode is the most frequently selected Loran type. The Think Loran mode allows simultaneous servo-machining of the side and bottom. The gaps at the side and bottom are always maintained in the best condition to achieve stable machining. This mode is used for machining shapes with a wide side area or for finishing deep-hole shapes [38].
2.2 Micro-electrode fabrication using EDM Fig. 6 illustrates the on-machine micro-electrode fabrication process, which is a modified version of the moving BEDG called horizontal moving BEDG (HM-BEDG) with the following changes: 1. The servo system controls the motion along the X-axis and the EDM gap between the ZY surface (side surface) of the block electrode and the micro-electrode. 2. The spindle rotates around the Z-axis. 3. The electrode’s initial position setting on the Y-axis direction determines the microelectrode diameter and amount of material removed from the micro-electrode, as shown in Fig. 6. The micro-electrode position on the Y-axis is fixed during machining in each step and it will change for the next step. 4. The electrode’s initial position setting on the Z-axis determines the micro-electrode’s length, as shown in Fig. 6. The micro-electrode’s location is fixed in all steps of the fabrication process. 5. The machine table is stationary while the tool head is rotating and moving in the X, Y and Z-axis directions. Most sparks take place between the block electrode’s side surface and the micro-electrode, and this creates erosion on the side surface of the micro-electrode and side surface of the block electrode as well. While the micro-electrode is moving along a +X-axis direction, the diameter becomes smaller until the desired micro-electrode diameter is completely achieved at the end of the process. The micro-electrode was fabricated in three steps using roughing, semi-finishing and finishing parameters in order to reduce machining time, as the amount of material removal rate (MRR) in the finishing step is lower than in other steps. Moreover, the amount of erosion in the finishing section of the block electrode is very low compared with the roughing and semi-finishing steps, since the diameter used is very small and the amount of material removed from the micro-electrode is very low. Hence, at the beginning roughing and semi-finishing parameters are used to decrease
machining time. However, the finishing parameters are used later, as the very low erosion in the finishing process helps improve accuracy. Fig. 4 shows the shape of the copper block electrode used to manufacture micro-electrodes at different machining levels (roughing, semi-finishing and finishing). This shape helps increase accuracy and decrease setting and machining time of the micro-electrode due to the length used for alignment in the block electrode; electrode roughing, semi-finishing and finishing were aligned simultaneously, therefore lead time was reduced by using this electrode. On the other hand, the maximum spindle rotating speed of this EDM machine was 20 rpm, which was considered very low. The optimum spindle rotating speed to fabricate micro-electrodes with low taper levels and good dimensional accuracy at higher material removal rates is from 1000 to 1500 rpm [34]. Low spindle rotational speeds during micro-electrode fabrication can cause longer machining time and less dimensional accuracy [34]. Hence, in this case where 20 rpm is used, the diameter and cylindricity of the micro-electrode have to be measured frequently after each step to investigate the accuracy. If the electrode is cylindrical, the next step should be run. Otherwise, the step should be repeated to remove the oval and non-cylindrical part until a cylindrical micro-electrode is achieved. Upon finishing, the micro-electrode diameter is measured. If the micro-electrode diameter is equal to the desired value, the micro-electrode is fabricated. Otherwise, the micro-electrode diameter should be modified with the finishing step until the desired value is attained.
2.3 Measuring the diameter and cylindricity of a micro-electrode on the EDM Machine with a gage block In this research, the EDM machine functions like a coordinate-measuring machine (CMM) to measure the micro-electrode diameter on the machine. Hence, the EDM machine is able to recognize the touching point between the micro-electrode and the product (wokpiece) through the electricity concept without contact force and micro-electrode bending. This process is known as an on-machine measurement technique, where the gage block clamped on the table of machine is used to measure the micro-electrode diameter. The gage block works like a prop of CMM and the microelectrode serves as a workpiece to measure the diameter. It is an entirely new technique and the gage block is cheaper and more widely available than other measurement processes. The process of electrode measurement using a gage block is shown in Figs. 4 and 7 and described below: 1. The gage block is fastened with a vise (Fig. 4). 2. The gage block is aligned with a dial test indicator with 2 µm accuracy. 3. The micro-electrode touches the gage block, and the X-axis position on the machine (point O1) is identified (Fig. 7). 4. The micro-electrode touches the other side of the gage block, and the X-axis position on the machine (point O2) is identified (Fig. 7). 5. According to the following equation, the electrode diameter can be measured. D=
O 2 - O1 - T
where D is the micro-electrode’s diameter, O1 is the first position where the micro-electrode touches the gage block, O2 is the second position where the micro-electrode touches the gage block, and T is the gage block thickness. The cylindricity of the micro-electrode is identified by measuring the micro-electrode’s diameter in different directions with a 90° phase shift.
2.4 Micro drilling of the WC workpiece, Micro-electrode dressing and Micro hole measurement The fabricated micro-electrode is used for micro EDM drilling of the WC workpiece. After micro EDM drilling of every hole, the end of the micro-electrode becomes tapered due to microelectrode wear. The dressing process serves to remove the taper part of the micro-electrode and produce a straight micro-electrode using a tungsten carbide (WC) electrode as shown in Figs. 4 and 8. The process is carried out with positive polarity for the micro-electrode, which causes more material removal from the micro-electrode and a faster dressing process [39]. The dressing process was performed at a voltage of 70 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs, and positive micro-electrode polarity. Field Emission Scanning Electron Microscopy (FESEM) and optical microscope were used to measure the micro-electrode and micro hole diameters in the WC workpiece.
3 Results and discussion Figure 9 represents the first trial in micro-electrode fabrication. As seen in Fig. 9, the microelectrode was burned from using the roughing parameters shown in Table 3 - row No. 1 (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs, 20 rpm rotating speed, and standard Think Loran (circle type)). This result depicts that using only roughing parameters cannot achieve smaller micro-electrode diameters of down to 800 µm. To solve this problem, all roughing as well as semi-finishing and finishing parameters should be subsequently used until the smallest diameter without burning is reached. This is because to attain a smaller micro-electrode diameter, a machining condition with lower spark energy should be used otherwise the microelectrode will burn, as shown in Fig. 9. Fig. 10 shows the second fabrication trial. The tapered and non-cylindrical micro-electrode was fabricated using standard Think Loran (circle type) and the parameters are shown in Table 3 - row No. 1 (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs, and 20 rpm rotating speed). It seems that using Standard Think Loran (circle type) and 20 rpm spindle rotating speed are the main factors in producing tapered and non-cylindrical micro-electrode. The 20 rpm spindle rotating speed is very low for fabricating micro-electrodes and it causes the production of non-cylindrical micro-electrodes [34]; however, the maximum spindle rotating speed of this machine is 20 rpm. To mitigate the non-cylindrical problem caused by using low spindle speed to produce cylindrical micro-electrodes, each step of the fabrication process should be repeated to help remove the non-cylindrical part and produce cylindrical micro-electrodes. Fig. 1 shows the micro-electrodes produced with repeated fabrication process steps using standard Think Loran (circle type) and the parameters shown in Table 3 - row No. 1 (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed). Fig. 11 indicates that a tapered and cylindrical micro-electrode was achieved. To solve the taper problem and produce straight micro-electrodes, the Loran pattern was turned off to eliminate the micro-electrode’s orbiting motion in a contour pattern. Fig. 12 shows that non-tapered and cylindrical micro-electrodes with 376.4 and 363.7 µm diameters were successfully fabricated by turning off the Loran pattern and repeating each step of the fabrication process at voltage of 35 V,
current of 5 A, pulse ON time of 50 µs, pulse OFF time of 53 µs and 20 rpm rotating speed (roughing parameters in Table 3 row No. 2). Furthermore, to fabricate non-tapered and cylindrical micro-electrodes with smaller diameters down to 160.8 µm, the spark energy should be decreased more to be set in the lowest level. Fig. 13 demonstrates a thin, cylindrical and straight micro-electrode fabricated at the lowest level of spark energy using the finishing parameters as shown in Table 3 – row No. 6 (voltage of 35 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs and 20 rpm rotating speed). In this fabrication process, the Loran pattern was switched off and each step of the fabrication process at 20 rpm rotating speed was repeated. The next step after successfully fabricating the micro electrode is that the fabricated microelectrode should be used for micro EDM drilling on the WC workpiece. For this purpose, the reference point on each axis (X, Y, Z = 0) should be identified on the EDM machine by the touching of the micro-electrode and the workpiece. The EDM machine recognizes this touching point through an electrical connection between the workpiece and micro-electrode without any mechanical force. Fig. 14(a) illustrates that the micro-electrode bent after touching the WC plate to find a reference point on the Z-axis (Z=0). To investigate the reason for this problem, a FESEM picture was taken of the recast layer at the bottom of the bent micro-electrode in Fig. 14(b) and EDX spectrum analysis was performed to analyze the elemental composition on the recast layer at the bottom of the bent micro-electrode in Fig. 14(c). In addition, EDX spectrum analysis was done on the raw material of the micro-electrode to analyze the elemental composition shown in Fig. 15. According to Fig. 15, C, O, Cu, W elements are present in the micro-electrode’s raw material, while Fig. 14(c) shows that C, O, Al, Cu, W elements are present in the recast layer on the bottom of the bent micro-electrode. From the EDX analysis it could be recognized that the tungsten (W) reacted with carbon to produce tungsten carbide (WC), while aluminum (Al) reacted with oxygen (O) to produce Al2O3 composite. The recast layers of WC and Al2O3 composites decreased the electrical conductivity of the bottom micro-electrode surface, hence the EDM machine did not easily detect the touching point, so the machine applied greater feed motion of the micro-electrode that led to higher touching force and caused the micro-electrode to bend. In addition, the thickness of these recast layer composites increased with increasing the spark energy, leading to less electrical conductivity [40]. To solve the bend problem of the micro-electrode, the bottom of the microelectrode was dressed with a WC electrode and low spark energy (voltage of 70 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs, and positive micro-electrode polarity), causing a decreasing recast layer at the bottom of the micro-electrode, making it more electrically conductive. The dressing process was described in section 2.4. Fig. 16 displays a micro-electrode with 97.14 µm diameter after dressing. Upon mitigating the above-mentioned problems, straight, cylindrical micro-electrodes were fabricated. Fig. 17(a) shows a micro-electrode with 78.19 µm diameter and 4.70 mm length (aspect ratio of more than 60.25) after producing a through micro hole with 134.5 µm diameter in Fig. 17(b). Negative microelectrode polarity, voltage of 70 V, current of 0.4 A, pulse ON time of 1 µs, and pulse OFF time of 1 µs were used to micro hole EDM drilling. Figs. 18(a) and 18(c) show two micro-electrodes with 165.2 and 169.7 µm diameter and around 6.8 mm length (aspect ratio = 41.16) fabricated using the setup shown in Fig. 4. The smooth surfaces of these micro-electrodes captured by FESEM are shown in Figs. 18(b) and (d). Moreover, Fig. 18(e) and (f) depict the two micro holes produced with these electrodes. The hole in Fig. 18(e) was machined with 90 V voltage, 0.4 A current, pulse ON time of 1 µs, pulse OFF time of 1 µs, capacitor of 0.1 µF, and without spindle rotation. The hole
shown in Fig. 18(f) was machined with voltage of 90 V, current of 0.4 A, pulse ON time of 5 µs, pulse OFF time of 9 µs, capacitor of 0.1 µF, and without spindle rotation. Fig. 18(g) shows all holes produced by these micro-electrodes with various machining conditions. According to Fig 18, the two micro-electrodes were fabricated with approximately the same dimensions. As a result, it can be concluded that HM-BEDG enables economic fabrication of long and thin electrodes without taper and with smooth surface. In addition, the proposed micro-electrode fabrication and measurement processes are repeatable. As observed in Figs. 18(a) and (c), the end of micro-electrodes became tapered after machining of the micro hole due to electrode wear. The tapered part of the micro-electrode was removed via the dressing process as described in section 2.4. The dressing process produced a straight microelectrode for the subsequent micro EDM drilling as shown in Fig 19. As a result, a longer microelectrode can be used rather than several micro-electrodes. Table 4 illustrates the micro-electrode diameters from Figs. 17(a), 18(a) and 18(c) that were measured by FESEM and gage block in different directions. The gage block measurement results indicate that the maximum difference between the electrode diameters in two directions was 0.4 µm (Table 4 and Fig. 20). Thus, a cylindrical electrode was produced, signifying that the fabrication procedure was correct. Fig. 20 demonstrates that the electrode diameters measured by FESEM are less than by gage block. The amount of error (6.6 to 9.81 µm) between the gage block and FESEM measurements is shown in Fig. 21. The difference in measurement error happened because the temperature affected the gage block dimensions, and there was error during gage block alignment as well as EDM machine backlash.
4 Conclusion This study represented the steps for fabricating and measuring micro-electrodes based on horizontal moving BEDG (HM-BEDG) and gage block using a typical EDM machine, and solving the problems occurring during micro-electrode fabrication. The modifications included altering the movement directions and control gap and using a copper block electrode to manufacture a micro electrode at different machining levels (roughing, semi-finishing and finishing). These modifications resulted in a more simplified setup and shorter electrode fabrication time. The EDM machine worked like a CMM machine in measuring the micro-electrode diameter and used the gage block as a probe and the micro-electrode as a workpiece. Gage block measuring is an economic and simple technique besides being readily available. The fabrication and measurement steps are straightforward without the need for high investment in special equipment. In addition, these changes and developments enabled the use of EDM machining for the fabrication of straight and thin micro-electrodes (cylindrical, cubic and so on) with smooth surface and low cost. As a result, a micro-electrode with the highest aspect ratio (more than 60.25) was fabricated successfully with 78.19 µm diameter and 4.70 mm length after producing a through micro hole with 134.5 µm diameter. Since these procedures are simple and inexpensive in terms of special equipment, microelectrodes can be fabricated and measured for micro EDM drilling and milling in typical workshops equipped with EDM machines. Acknowledgments The authors would like to acknowledge the University of Malaya for providing the necessary facilities and resources for this research. This research was fully funded by the Ministry of Higher
Education, Malaysia with the high impact research (HIR) grant number HIR-MOHE-16001-00D000001.
Reference [1] T. Masuzawa, H. Tönshoff, Three-dimensional micromachining by machine tools, CIRP Ann. Manuf. Technol. 46 (1997) 621-628. [2] J. Goda, K. Mitsui, Development of an integrated apparatus of micro-EDM and micro-CMM, Measurement, 46 (2013) 552-562. [3] M. Rahman, A. Asad, T. Masaki, T. Saleh, Y. Wong, A. Senthil Kumar, A multiprocess machine tool for compound micromachining, Int. J. Mach. Tools Manuf. 50 (2010) 344-356. [4] A. Asad, T. Masaki, M. Rahman, H. Lim, Y. Wong, Tool-based micro-machining, J. Mater. Process. Technol. 192 (2007) 204-211. [5] T. Masuzawa, State of the art of micromachining, CIRP Ann. Manuf. Technol. 49 (2000) 473488. [6] E.J. Weller, Nontraditional machining process, Society of Manufacturing Engineers, Dearborn, Michigan, 1984. [7] R. Snoeys, F. Staelens, W. Dekeyser, Current trends in non-conventional material removal processes, CIRP Ann. Manuf. Technol., 35 (1986) 467-480. [8] S. Matsui, T. Kaito, J. Fujita, M. Komuro, K. Kanda, Y. Haruyama, Three-dimensional nanostructure fabrication by focused-ion-beam chemical vapor deposition, J. Vac. Sci. Technol. B: Microelectronics and Nanometer Structures, 18 (2000) 3181-3184. [9] H. Okuyama, H. Takada, Micromachining with SR and FEL, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 144 (1998) 5865. [10] L. Zhang, H. Tong, Y. Li, Precision machining of micro tool electrodes in micro EDM for drilling array micro holes, Precision Engineering, 39 (2015) 100-106. [11] H. Lim, Y. Wong, M. Rahman, M. Edwin Lee, A study on the machining of high-aspect ratio micro-structures using micro-EDM, J. Mater. Process. Technol. 140 (2003) 318-325. [12] B. Mark, M. Toshihiko, M. Teruaki, T. Kazushi, Micro-Hole Punching Using Multi-Point Punch and Die Made by EDM, Int. J. Adv. Manuf. Technol. 3 (2009) 73-86. [13] R. Mahdavinejad, A. Mahdavinejad, ED machining of WC–Co, J. Mater. Process. Technol. 162 (2005) 637-643. [14] A.K. Sahoo, B. Sahoo, A comparative study on performance of multilayer coated and uncoated carbide inserts when turning AISI D2 steel under dry environment, Measurement, 46 (2013) 2695-2704. [15] A.K. Sahoo, B. Sahoo, Performance studies of multilayer hard surface coatings (TiN/TiCN/Al 2 O 3/TiN) of indexable carbide inserts in hard machining: Part-II (RSM, grey relational and techno economical approach), Measurement, 46 (2013) 2868-2884. [16] A.K. Sahoo, B. Sahoo, Performance studies of multilayer hard surface coatings (TiN/TiCN/Al 2 O 3/TiN) of indexable carbide inserts in hard machining: Part-I (An experimental approach), Measurement, 46 (2013) 2854-2867. [17] K. Jangra, S. Grover, A. Aggarwal, Optimization of multi machining characteristics in WEDM of WC-5.3% Co composite using integrated approach of Taguchi, GRA and entropy method, Frontiers of Mechanical Engineering, (2012) 1-12. [18] S. Dewangan, S. Gangopadhyay, C. Biswas, Study of surface integrity and dimensional accuracy in EDM using Fuzzy TOPSIS and sensitivity analysis, Measurement, 63 (2015) 364376. [19] M. Hourmand, S. Farahany, A.A. Sarhan, M.Y. Noordin, Investigating the electrical discharge machining (EDM) parameter effects on Al-Mg2Si metal matrix composite (MMC) for high material removal rate (MRR) and less EWR–RSM approach, Int. J. Adv. Manuf. Technol. (2014) 1-8.
[20] G. Mandaloi, S. Singh, P. Kumar, K. Pal, Effect on crystalline structure of AISI M2 steel using copper electrode through material removal rate, electrode wear rate and surface finish, Measurement, 61 (2015) 305-319. [21] D. Puhan, S.S. Mahapatra, J. Sahu, L. Das, A hybrid approach for multi-response optimization of non-conventional machining on AlSiC p MMC, Measurement, 46 (2013) 3581-3592. [22] S. Assarzadeh, M. Ghoreishi, A dual response surface-desirability approach to process modeling and optimization of Al2O3 powder-mixed electrical discharge machining (PMEDM) parameters, Int. J. Adv. Manuf. Technol. 64 (2013) 1459-1477. [23] K.-Y. Kung, J.-T. Horng, K.-T. Chiang, Material removal rate and electrode wear ratio study on the powder mixed electrical discharge machining of cobalt-bonded tungsten carbide, The Int. J. Adv. Manuf. Technol. 40 (2009) 95-104. [24] Y. Li, W. Zhao, X. Feng, H. Wei, Research advancement on on-line detection of EDM spark locations, Measurement, 22 (1997) 29-35. [25] H. Qiang, H. Yong, Z. Wansheng, Research of two-dimension EDM spark locations detection using electromagnetic method, Measurement, 31 (2002) 117-122. [26] S. Gopalakannan, T. Senthilvelan, Application of response surface method on machining of Al–SiC nano-composites, Measurement, 46 (2013) 2705-2715. [27] I. Puertas, C. Luis, L. Alvarez, Analysis of the influence of EDM parameters on surface quality, MRR and EW of WC–Co, J. Mater. Process. Technol. 153 (2004) 1026-1032. [28] L. Alting, F. Kimura, H.N. Hansen, G. Bissacco, Micro engineering, CIRP Ann. Manuf. Technol. 52 (2003) 635-657. [29] K. Ho, S. Newman, State of the art electrical discharge machining (EDM), Int. J. Machine Tools Manuf. 43 (2003) 1287-1300. [30] Y.Y. Tsai, T. Masuzawa, An index to evaluate the wear resistance of the electrode in microEDM, J. Mater. Process. Technol. 149 (2004) 304-309. [31] M. Jahan, M. Rahman, Y. Wong, A review on the conventional and micro-electrodischarge machining of tungsten carbide, Int. J. Machine Tools Manuf. 51 (2011) 837-858. [32] M.P. Jahan, Y.S. Wong, M. Rahman, A study on the fine-finish die-sinking micro-EDM of tungsten carbide using different electrode materials, J. Mater. Process. Technol. 209 (2009) 3956-3967. [33] M. Yamazaki, T. Suzuki, N. Mori, M. Kunieda, EDM of micro-rods by self-drilled holes, J. Mater. Process. Technol. 149 (2004) 134-138. [34] M. Jahan, M. Rahman, Y. Wong, L. Fuhua, On-machine fabrication of high-aspect-ratio microelectrodes and application in vibration-assisted micro-electrodischarge drilling of tungsten carbide, Proceedings of the Institution of Mechanical Engineers, Part B: J. Eng. Manuf. 224 (2010) 795-814. [35] B. Xu, X.-y. Wu, J.-g. Lei, R. Cheng, S.-c. Ruan, Z.-l. Wang, Laminated fabrication of 3D micro-electrode based on WEDM and thermal diffusion welding, J. Mater. Process. Technol. 221 (2015) 56-65. [36] J. Lei, X. Wu, B. Xu, Z. Zhao, S. Ruan, R. Cheng, Laminated fitting fabrication of Cu-Sn composite 3D microelectrodes and elimination of ridges on the machined surface of 3D microcavities, J. Mater. Process. Technol. (2015). [37] N. Ravi, S.X. Chuan, The effects of electro-discharge machining block electrode method for microelectrode machining, J. micromech. microeng. 12 (2002) 532. [38] Code instructions LN/LP/LN2/LP2, Sodick NC power supply unit for EDM, Version 6.0. [39] M.P. Jahan, Y. San Wong, M. Rahman, A comparative experimental investigation of deep-hole micro-EDM drilling capability for cemented carbide (WC-Co) against austenitic stainless steel (SUS 304), Int. J. Adv. Manuf. Technol. 46 (2010) 1145-1160. [40] S.H. Lee, X. Li, Study of the surface integrity of the machined workpiece in the EDM of tungsten carbide, J. Mater. Process. Technol. 139 (2003) 315-321.
Figure(s)
Fig. 1 Various kinds of on-machine micro-electrode fabrication process (a) WEDG construction [10] (b) radial feed WEDG [10] (c) tangential feed WEDG (TF-WEDG) [10] (d) self-drilled holes [33] (e) stationary sacrificial electrode (BEDG) [11] (f) rotating sacrificial disk [11] (g) moving BEDG [4] (h) hybrid process (micro turning and micro-EDM) [4] (i) hybrid process (self-drilled holes and TF-WEDG) [10]
Fig. 2 Processes sketch of fabricating 3D micro-electrode with two various micro-double-staged laminated object manufacturing (micro-DLOM) [35, 36].
Fig. 3 Online measurement (a) Laser light [3, 4, 11] (b) charge coupled device (CCD) [10]
WC electrode
Micro-electrode 1
2
3
4
Gage block
Cu block electrode
Workpiece (WC-Co plate)
1. Roughing section 2. Semi-finishing section 3. Finishing section 4. Alignment Section
Fig. 4 a) Experimental setup design for the on-machine fabrication and measurement techniques of micro-electrodes and micro EDM drilling.
Fig. 5 Think loran pattern method [38]
Fig. 6 Horizontal moving BEDG (HM-BEDG) process for on-machine micro-electrode fabrication
Fig. 7 Gage block measuring process for On-machine micro-electrode measurement a) The micro-electrode touches the gage block at first position point (O1) b) The micro-electrode touches the gage block at the second position Point (O2)
Fig. 8 Dressing process a) Taper part of micro-electrode after machining micro hole and before dressing process b) Straight micro-electrode after dressing process
Fig. 9 Burnt electrode in the multi angle view (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed)
Fig. 10 Taper and non-cylindrical micro-electrode in the multi angle view (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed)
Fig. 11 Taper micro-electrode (voltage of 30 V, current of 20 A, pulse ON time of 600 µs, pulse OFF time of 257 µs and 20 rpm rotating speed)
Fig. 12 Non-taper cylindrical micro-electrodes (microscope and FESEM pictures) were produced at voltage of 35 V, current of 5 A, pulse ON time of 50 µs, pulse OFF time of 53 µs and 20 rpm rotating speed
Fig. 13 Non-taper cylindrical micro-electrodes (microscope and FESEM pictures) was produced at voltage of 35 V, current of 0.4 A, pulse ON time of 1 µs, pulse OFF time of 1 µs and 20 rpm rotating speed
Fig. 14 a) Bent micro-electrode after touching the WC plate b) FESEM picture from the bottom surface of the bent micro-electrode with recast layer c) EDX spectrum analysis of the bottom surface of the bent micro-electrode
Fig. 15 EDX spectrum analysis of the surface CuW before micro-electrode fabrication.
Fig. 16 Micro-electrode after dressing process (microscope and FESEM pictures)
Fig. 17 a) Micro-electrode with 78.19 µm diameter and 4.70 mm length (more than 60.13 high aspect ratio) after producing through micro-hole b) Micro-hole was fabricated with microelectrode which had 78.19 µm diameter.
Fig. 18 a) Micro-electrode with 165.2 µm diameter b) Surface of micro-electrode with 165.2 µm diameter c) Micro-electrode with 169.7 µm diameter d) Surface of micro-electrode with 169.7 µm diameter e) The micro hole was produced with micro-electrode that had 165.2 µm diameter. f) The micro hole was produced with micro-electrode that had 169.7 µm diameter. g) All of the micro holes were produced at various machining conditions with these micro-electrodes
Fig. 19 Straight micro-electrode was achieved after dressing process
Fig. 20 Measuring of the micro-electrode diameter
Fig. 21 Error of measurement between gage block and workpiece
Table 1. Table Properties of CuW micro-electrode
Kind
Ingredient
Specific Gravity
CuW
30Cu + 70W
14.3
Conductivity %IACS 28.0
Solidity HRB93
Table 2. Properties of WC-Co (Typical nominal properties)
Grade HC-US16
Percent Cobalt 16
Grain Structure SUB-MICRON
Rockwell Hardness 90.8
Transvers Rupture 500
Density Grams/C.C. 13.82
Table 3. Machining parameters for fabrication of micro-electrode
No.
1
Roughing
30
20
600
Pulse OFF time (µs) 257
2 3 4
Roughing Roughing Semifinishing Semifinishing Finishing
35 35 35
5 2 1.5
50 25 10
53 27 11
Rotating speed (rpm) 20 (maximum) 20 20 20
35
0.4
2
3
20
35
0.4
1
1
20
5 6
Steps
Voltage Current Pulse ON time (V) (A) (µs)
Table 4. Micro-electrode diameters
Equipment FESEM Gage block
Rotation of microelectrode ------0° 90°
Micro-electrode Diameter (µm) No. 1 No. 2 No. 3 78.19 88 87.6
165.2 173.3 173.5
169.7 176.3 176.7
Highlights 123456-
Micro-electrodes are used for drilling micro-hole in miniaturized products Proposing new fabrication and measurement techniques of micro-electrode are simple These processes do not require high investment for special equipments Micro-electrode can be fabricated and measured in workshop by usual EDM machine The highest aspect ratio (more than 60.25) of micro-electrode was fabricated It had 78.19 µm diameter and 4.70 mm length after producing hole on WC (134.5 µm)