- Email: [email protected]
The Effects of Graphite Powder on Tool Wear in Micro Electrical Discharge Machining
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 553 – 558
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
The Effects of Graphite Powder on Tool Wear in Micro Electrical Discharge Machining Yoo-Seok Kima, Chong-Nam Chua,* a
Seoul National University, 1 Kwanak-ro Kwanak-gu, Seoul, 08826, South Korea
* Corresponding author. Tel.: +82-2-880-7147; fax: +82-2-887-7259. E-mail address: [email protected]
Abstract This study proposes a new explanation of how tool wear is reduced during powder-mixed micro electrical discharge machining. Owing to its lower discharge energy than a transistor discharge circuit, an RC discharge circuit outperforms in micro EDM. Given that the energy distribution of a cathode is lower than that of an anode during discharge, a tool is negatively charged to reduce tool wear and increase the material removal rate. However, due to the stray inductance of an RC discharge circuit, the tool polarity and direction of the current fluctuate before the discharge plasma channel is extinguished, which increases the tool wear. In this study, it was found that graphite powder in a dielectric fluid disperses the charged energy into several small discharge plasma channels. These small channels were extinguished quickly before the tool polarity and direction of the current fluctuate; which decreases the amount of tool wear. The machining time and tool wear length in powder-mixed micro ED-drilling were evaluated with various particle diameters and concentrations of powder-mixed dielectric fluid. With the optimal tool feeding conditions, the machining time and tool wear length of graphite-powder-mixed kerosene were reduced by 30.9% and 28.3%, respectively, as compared to those in non-powder-mixed EDM. ©2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining Keywords: Micro EDM; graphite powder; tool wear; RC discharge circuit; mechanism
1. Introduction Tool wear is a chronic problem of micro electrical discharge machining (EDM). As a thermal and non-contact machining method, EDM can machine microstructures on any conductive material regardless of the hardness of the material because there is no breakdown of the tool. However, tool wear is inevitable because the discharge spark melts or evaporates not only the workpiece, but also the tool. This inherent problem lowers the efficiency and accuracy of micro EDM, which has been an obstacle preventing the broader use of micro EDM in actual industry applications. Many engineers have attempted to solve this tool wear problem over the past few decades. Marafona et al. explained the tool wear protection mechanism by means of the deposition of a carbonaceous layer onto a tool [1]. To build up this sacrificial layer, a pulse duration of tens of microseconds is needed. However, because micro EDM usually uses pulse
durations of tens or hundreds of nanoseconds to minimize the discharge energy and stabilize the machining condition, a long pulse duration may not be suitable for micro EDM. Tsai et al. showed that materials with a high melting point and low thermal conductivity could alleviate tool wear [2]. However, as tool size becomes smaller, only tungsten carbide should be selected as the tool material given that it does not bend during the high-speed rotation of a spindle. Song et al. used deionized water as a dielectric fluid to machine tungsten carbide [3]. Deionized water, however, causes electrolytic corrosion on the periphery of machined structures, and an ion filter is continuously necessary to maintain the dielectric strength of deionized water. Bleys et al. used a real-time tool wear compensation method in ED-milling [4]. Although this method could enhance the accuracy of micro EDM, low efficiency still remains as the method did not reduce tool wear. It is known that powder-mixed electrical discharge machining (PMEDM) can mitigate the tool wear problem in
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.121
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micro EDM. There were various studies of PMEDM, but the mechanism of tool wear reduction has not been clarified. Jeswani added graphite powder to kerosene to increase the material removal rate (MRR) and reduce the wear ratio [5]. The study showed that the addition of graphite powder lowers the insulating strength of a dielectric fluid, but the link to tool wear was not explained. Yeo proposed a hypothesis involving a crater-formation mechanism when a powder-mixed dielectric fluid was used [6]; the size of the crater is reduced, and a larger amount of resolidified materials forms because the increased viscosity and enhanced thermal conductivity of the mixed fluid decreases the plasma heat flux in the workpiece and raises the rate of heat dissipation away from the molten cavity. However, this study focused only on crater formation and not on the tool wear reduction relationships. Singh et al. explained that particles between a tool electrode and a workpiece become charged by electric fields and help to bridge the discharge gap via a process termed the ‘bridging effect’ [7]. This effect decreases the insulating strength of the dielectric fluid, causing more frequent discharges to occur and in turn increasing the MRR. Moreover, the surface roughness decreases because the plasma channels become enlarged and more open. This study, however, as well did not explain the mechanism causing the tool wear reduction. Singh et al. stated that the discharge gap enlarged by powder led to reductions of the discharge impact force and energy density between a tool and a workpiece [8]. This may explain the mechanism of tool wear reduction, but potential conflicts exist with regard to higher MRR results in PMEDM. The present study proposes a new explanation of the mechanism of tool wear reduction in micro PMEDM. An RC discharge circuit was used in our experiments because it can achieve a shorter pulse duration than a transistor circuit, which shows better machining performance outcomes [9]. The tool is negatively charged in micro EDM because the energy density is lower at the cathode [10] and a sacrificial carbonaceous layer cannot be deposited on an anode given the short pulse duration of micro EDM. However, the polarity of the tool fluctuated before the plasma channel is extinguished due to stray inductance, which aggravates the tool wear. It was found that adding graphite powder eliminated the fluctuation of the polarity. It dispersed the charged energy into several small plasma channels when discharges occurred, and these small plasma channels were extinguished quickly before the polarity fluctuated. In other words, the addition of graphite powder eventually prevented the current from oscillating, thus reducing the tool wear. Nomenclature C1,2 IL L R1,2 SW t V VC
Capacitance 1, 2 Current on stray inductance Stray inductance Resistance 1, 2 Switch Time Power source Voltage on capacitor
In the following sections, the mechanism of tool wear reduction is explained in more detail. Several experimental results are then presented to support our explanation. The particle diameter of the powder and concentration of a powder-mixed dielectric fluid are also optimized. Finally, the paper concludes with a discussion of tool wear reduction considering the experimental results presented here. 2. Experimental setup Fig. 1 shows the experimental setup used in this study. The WEDG system was used to fabricate the micro tools [11]. A custom-made micro EDM system was used, which consists of an RC discharge circuit, the precision three-axis stage (Parker, USA), and the controller (Delta Tau, USA). An overhead stirrer was installed in both the bottle of the powder-mixed dielectric fluid and in the machining tank to maintain the concentration of the mixed fluid. Two centrifugal pumps were used to circulate the mixed fluid. The specific machining conditions are tabulated in Table 1. Table 1. Machining Conditions Open voltage
100 V
Resistance (R1)
1 kΩ
Capacitance (C1)
5, 10 nF
Tool (diameter), polarity
WC-Co (300, 100, 50 μm), cathode
Workpiece, polarity
STS 304, anode
Dielectric fluid
Kerosene
Powder (average particle diameter)
Graphite (0.04, 0.3, 3 μm)
Concentration
0.1, 0.5, 1, 5 g/l
Machining depth
300 μm
3. The mechanism of tool wear reduction in PMEDM In this section, the RC discharge circuit is modeled and its simulation results are compared with the experimental results. By investigating the difference in the charging slope between pure kerosene and graphite-powder-mixed kerosene (GPMK), the role of graphite powder can be revealed.
Fig. 1 The experimental setup
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(a) (b) Fig. 3. The simulation results of (a) RLC discharging and charging, and (b) RC charging
Fig. 2. The RC discharge circuit modeling
3.1. RC and RLC discharge circuits The micro EDM system with the RC discharge circuit was modeled. As illustrated in Fig. 2, it consists of charging and discharging loops, with a switch used to connect these two loops. The charging loop is composed of a power source, a resistor and a capacitor, whereas the discharge loop consists of stray inductance, a resistor, and a capacitor. Stray inductance is unavoidable because it is inherent inductance caused by electrical wires. When the switch is closed, which means a breakdown of the dielectric fluid occurs or the discharge is initiated, the entire circuit becomes an RLC circuit. Its governing equations are as follows: 2
d I L (t ) dt
2
1
dI L (t )
R1C1
dt
I L (t )
LC1 dVC 2 (t )
1 dVC 2 (t ) L
dt
dt
VC 2 (t )
V
R1 LC1
R1LC1
VC 2 (t )
I L (t )
R2 C2
C2
(1)
(2)
Otherwise, when the switch is open, which means the plasma channels are extinguished or the discharge ends, only the RC charging loop remains as the entire circuit. Its governing equation is shown below. dVC1 (t ) dt
VC1 (t )
V
R1C1
R1C1
(a)
(3)
Graphite powder mitigates tool wear by eliminating the oscillation of the current. The charging characteristics of the RLC and RC circuit have different slopes, and they represent the charging aspects of kerosene and GPMK, respectively. Fig. 3(a) depicts a discharging and charging graph of a RLC circuit governed by (1) and (2) when V = 100 V, R1 = 1 kΩ, R2 = 10 Ω, C1 = 5 nF, C2 = 0.1 nF, and L = 2.3 μH. When the discharge starts, the voltage becomes negative, after which charging and discharging are repeated until the charged energy is dissipated. However, during the actual EDM process, voltage fluctuations continue until the plasma channels become extinct. When the plasma channels are eliminated, which corresponds to an open switch, the circuit converts from an RLC to an RC circuit and the voltage then starts to charge following the RC charging characteristics depicted in
(b) Fig. 4. Discharging and charging graphs for (a) pure kerosene and (b) GPMK
Fig. 3(b). The slope of the RC charging graph is gentler compared to that of the RLC circuit. Figs. 4(a) and (b) show the discharging and charging graphs for kerosene and GPMK, respectively. Capacitor C1 was 5 nF, the concentration of the GPMK was 5 g/l, and the average particle diameter was 0.04 μm in both cases. The charging graph of kerosene shows a dramatic transition from steep to gentle, indicating that the circuit was converted from an RLC circuit to an RC circuit by the open switch. The plasma channels were dissipated and the RC circuit started to charge VC1 from the slope-changing point. Therefore, the current fluctuated until the plasma channels were extinguished or until the slope-changing point. On the other hand, the slope-changing point is not observed in the graph of the GPMK. This indicates that the switch opened as soon as discharge ended, or the plasma channels were extinguished quickly before VC1 started charging following the RLC charging characteristic. There were no fluctuations of the current, and thus tool wear is reduced in this case. The moment of dissipation of the plasma channels is related to the amount of discharge energy. The weaker the discharge energy is, the more quickly the moment of dissipation arrives. Fig. 5 shows the discharging and charging graphs of pure kerosene when C1 was 0.3 nF. As in the charging slope of GPMK, no slope-changing point is observed
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Fig. 5. Discharging and charging graph of pure kerosene when C 1 equals 0.3 nF
(a)
(a)
(b) Fig. 7. The comparison of tool wear length when 5 and 10 nF were used as C 1: (a) pure kerosene, and (b) GPMK
(b) Fig. 6. The 3D surface profiles of discharge craters: (a) pure kerosene, and (b) GPMK
despite the fact that pure kerosene was used as the dielectric fluid, which means that the plasma channels were extinguished quickly as soon as the discharge ended. 3.2. Energy dispersion in PMEDM Graphite powder disperses charged energy into small discharge channels, and these small channels are extinguished quickly owing to their small amounts of discharge energy. Figs. 6(a) and (b) show the 3D surface profiles of the initial discharge craters of pure kerosene and GPMK, respectively. The experiment was performed with identical machining times (less than 1 sec) and capacitance levels (5 nF) as the tool (300 μm diameter) approached the workpiece. The concentration of the GPMK was 5 g/l and the average particle diameter was 0.04 μm. The average crater diameter and depth of pure kerosene were 36 μm and 3 μm, whereas those of GPMK were 16 and 0.3 μm, respectively. There were more discharge craters on GPMK than on pure kerosene. Those on GPMK were more evenly distributed on the total tool area. Because each discharge crater corresponds to each plasma
channel, these findings indicate that identical amounts of charged energy are dispersed into smaller but more numerous discharge channels with GPMK than with pure kerosene. Energy dispersion by graphite powder also explains other experimental results. Figs. 7(a) and (b) are the tool wear lengths of pure kerosene and GPMK respectively, when C1 equaled 5 and 10 nF. As shown in Fig. 7(a), the tool wear length is greater when C1 is 10 nF than when C1 is 5 nF due to the larger discharge energy. However, in Fig. 7(b), the tool wear length does not show any significant differences with the two different C1 values. This occurs because graphite powder scattered the greater charged energy into additional small discharge channels, and each small discharge channel dissipated quickly before the current oscillated. Furthermore, as Klocke et al. showed with a high-speed framing camera in earlier work, discharge channels spread when powders are added to a dielectric fluid [12]. All of these results support the energy-dispersing effect of graphite powder. ډڏComparisons between ED and PMED drillingٻ ٻ In this section, the machining times and tool wear lengths for pure kerosene and GPMK are compared. The concentration of the mixed fluid and the average diameter of the graphite powder were varied to investigate the optimal machining conditions. Each experiment was repeated three times with identical machining conditions.
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(a)
(a)
(b) Fig. 8. Comparison between pure kerosene and GPMK with the five concentrations: (a) machining time and (b) tool wear length
(b) Fig. 9. Comparison between pure kerosene and GPMK with the three particle diameters: (a) machining time and (b) tool wear length
4.1. Identical tool diameter
μm. However, no significant differences were noted in the machining time or tool wear length between the three different average diameters of the graphite particles.
With five different concentrations of the GPMK, the machining time and tool wear length were compared. Figs. 8(a) and (b) show the machining time and tool wear length with respect to the feed rate. The machining conditions were as follows: tool diameter = 100 μm, C1 = 5 nF, and average particle diameter = 0.04 μm. As evidenced by these figures, graphite powder can lower the machining time and tool wear length when the concentration of its mixed fluid exceeds 0.5 g/l. However, when the concentration is higher than 0.5 g/l, there were no noticeable differences between the mixed fluids at each feed rate. When the concentration is higher than 0.5 g/l, the machining time decreases and becomes saturated at approximately 60 seconds as the feed rate increases. With regard to the tool wear length, it increases and saturates at about 62 μm as the feed rate become faster. Compared with the pure kerosene, the maximum reduction rate of the machining time is 62.5% when the concentration is 5 g/l and the feed rate is 9 μm /s, and the maximum reduction rate of the tool wear length is 52.9% when the concentration is 0.5 g/l and the feed rate is 3 μm /s. The machining time and tool wear length were measured and compared using graphite particles with three different average diameters in the mixed fluid. The tool diameter was 100 μm, and the concentration was 5 g/l. The average particle diameters were 0.04, 0.3, 3 μm, and their results are illustrated in Fig. 9. As the feed rate increases, the machining time decreases and becomes saturated at approximately 55 seconds, while the tool wear length increases and saturates at about 65
4.2. Identical hole diameter Before machining holes with identical diameters in both pure kerosene and GPMK, it was necessary to determine the tool diameter. The discharge gap of GPMK is greater than that of pure kerosene; thus, the tool diameter of GPMK should be smaller to machine identical diameter holes. On the other hand, as the tool diameter becomes smaller, the tool wear length naturally increases even with the same amount of discharge energy due to the smaller machining area. Therefore, a machining condition which minimizes the discharge gap should be selected. Fig. 10 shows the discharge gaps with the concentrations of GPMK and the average particle diameters which were used in Section 4.1. The discharge gap was defined as half of the difference between the machined hole and the tool diameter. When the concentration was under 1 g/l, the discharge gap remained at 25 μm, and it increased slightly at 5 g/l. Therefore, the concentration should be below 1 g/l. The average particle diameter of 0.04 μm led to the minimum discharge gap; thus, 0.04 μm was selected as the average powder diameter. Under the optimal machining conditions, the machining times and tool wear lengths of both fluids were compared when 100 μm-diameter holes were machined with different tool diameters. The final machining conditions were as follows: C1 = 5 nF, tool diameter = 80 μm (pure kerosene), 50
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Fig. 10. Discharge gaps according to the concentrations of mixed fluids and the average particle diameter of graphite powder
With the dispersion of the energy into small discharge channels, the discharge craters are shallow and current oscillation does not occur. Therefore, the surface roughness and tool wear are reduced. With regard to the high MRR, Natsu et al. proposed the first-stage-expansion model to explain the expansion process associated with EDM arc plasma [13]. According to that study, machining occurs at the initial stage of discharge and the remaining discharge energy is used for arc plasma expansion until the temperature of the plasma reaches the melting point of the workpiece. Therefore, using more discharge channels could increase the MRR despite the fact that the discharge energy of each is lower. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1A2B2015774). References
Fig. 11. The reduction of the machining time and tool wear length when machining holes with identical diameters
μm (GPMK), feed rate = 5 μm/s (pure kerosene), 3 μm/s (GPMK), concentration = 1 g/l, and average particle diameter = 0.04 μm. These results are shown in Fig. 11. The machining time and tool wear length were reduced by 30.9% and 28.3%, respectively. GPMK still outperformed pure kerosene in terms of the machining time and tool wear length. 5. Conclusions This study proposed a new explanation for the tool wear reduction which occurs when graphite powder is mixed with a dielectric fluid. It was found that graphite powder dispersed charged energy into small discharge channels and that these small channels dissipated quickly before the current fluctuated. Because oscillation of the current can intensify the tool wear in micro EDM, the graphite powder decreases the tool wear via this energy dispersion mechanism. The discharge craters were also compared. The GPMK resulted in a much narrower crater diameter and shallower depth than those by pure kerosene, but the number and distribution of the craters were greater in the GPMK as compared to pure kerosene. Quantitative comparisons were also carried out between pure kerosene and GPMK. To find the optimal machining conditions, tools with identical diameters were used to measure the machining time, tool wear length, and discharge gap. Tools with different diameters were then used to machine holes with identical diameters. With graphite powder, the machining time and tool wear length were reduced by 30.9% and 28.3%, respectively. This study could explain the three advantages of PMEDM, i.e., low surface roughness, low tool wear, and a high MRR.
[1] Marafona J, Wykes C. A new method of optimising material removal rate using EDM with copper-tungsten electrodes. International Journal of Machine Tools & Manufacture; 2000, 40, p. 153-164. [2] Tsai Y-Y, Masuzawa T. An index of evaluate the wear resistance of the electrode in micro-EDM. Journal of Materials Processing Technology; 2004, 149, p. 304-309. [3] Song KY, Chung DK, Park, MS, Chu, CN. Micro electrical discharge drilling of tungsten carbide using deionized water. Journal of Micromechanics and Microengineering; 2009, 19, p. 045006. [4] Bleys P, Kruth JP, Lauwers B, Zryd A, Delpretti R, Tricarico C. Real-time tool wear compensation in milling EDM. CIRP Annals-Manufacturing Technology; 2002, 51, p.157-160. [5] Jeswani ML. Effect of the addition of graphite powder to kerosene used as the dielectric fluid in electrical discharge machining. Wear; 1981, 70, p. 133-139. [6] Yeo SH, Tan PC, Kurnia W. Effects of powder additives suspended in dielectric on crater characteristics for micro electrical discharge machining. Journal of Micromechanics and Microenginerring; 2007, 17, p. N91-N98. [7] Singh A, Singh R. Effect of powder mixed electrical discharge machining (PMEDM) on various materials with different powders: A review. International Journal for Innovative Research in Science & Technology; 2015, 2, p. 164-169 [8] Singh B, Kumar J, Kumar S. Investigation of the tool wear rate in tungsten powder-mixed electrical discharge machining of AA6061/10%SiCp composite. Materials and Manufacturing Process; 2016, 31, p. 456-466. [9] Jahan MP, Wong YS, Rahman M. A study on the quality micro-hole machining of tungsten carbide by micro-EDM process using transistor and RC-type pulse generator. Journal of Material Processing Technology; 2009, 209, p. 1706-1716. [10] Kunieda M, Lauwers B, Rajukar K, Schumacher BM. Advancing EDM through fundamental insight into the process. CIRP AnnalsManufacturing Technology; 2005, 54, p.64-87. [11] Masuzawa T, Fujino M, Kobayashi K, Suzuki T, Kinoshita N. Wire electro-discharge grinding for micro-machining. CIRP AnnalsManufacturing Technology; 1985, 34, p.431-434. [12] Klocke F, Lung D, Antonoglou D, Thomaidis D. The effects of powder suspended dielectrics on the thermal influenced zone by electrodischarge machining with small discharge energies. Journal of Materials Processing Technology; 2004, 149, p. 191-197. [13] Natsu W, Shimoyamada M, Kunieda M. Study on expansion process of EDM arc plasma. JSME International Journal Series C; 2006, 49, p. 600605.
ScienceDirect Procedia CIRP 68 (2018) 553 – 558
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
The Effects of Graphite Powder on Tool Wear in Micro Electrical Discharge Machining Yoo-Seok Kima, Chong-Nam Chua,* a
Seoul National University, 1 Kwanak-ro Kwanak-gu, Seoul, 08826, South Korea
* Corresponding author. Tel.: +82-2-880-7147; fax: +82-2-887-7259. E-mail address: [email protected]
Abstract This study proposes a new explanation of how tool wear is reduced during powder-mixed micro electrical discharge machining. Owing to its lower discharge energy than a transistor discharge circuit, an RC discharge circuit outperforms in micro EDM. Given that the energy distribution of a cathode is lower than that of an anode during discharge, a tool is negatively charged to reduce tool wear and increase the material removal rate. However, due to the stray inductance of an RC discharge circuit, the tool polarity and direction of the current fluctuate before the discharge plasma channel is extinguished, which increases the tool wear. In this study, it was found that graphite powder in a dielectric fluid disperses the charged energy into several small discharge plasma channels. These small channels were extinguished quickly before the tool polarity and direction of the current fluctuate; which decreases the amount of tool wear. The machining time and tool wear length in powder-mixed micro ED-drilling were evaluated with various particle diameters and concentrations of powder-mixed dielectric fluid. With the optimal tool feeding conditions, the machining time and tool wear length of graphite-powder-mixed kerosene were reduced by 30.9% and 28.3%, respectively, as compared to those in non-powder-mixed EDM. ©2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining Keywords: Micro EDM; graphite powder; tool wear; RC discharge circuit; mechanism
1. Introduction Tool wear is a chronic problem of micro electrical discharge machining (EDM). As a thermal and non-contact machining method, EDM can machine microstructures on any conductive material regardless of the hardness of the material because there is no breakdown of the tool. However, tool wear is inevitable because the discharge spark melts or evaporates not only the workpiece, but also the tool. This inherent problem lowers the efficiency and accuracy of micro EDM, which has been an obstacle preventing the broader use of micro EDM in actual industry applications. Many engineers have attempted to solve this tool wear problem over the past few decades. Marafona et al. explained the tool wear protection mechanism by means of the deposition of a carbonaceous layer onto a tool [1]. To build up this sacrificial layer, a pulse duration of tens of microseconds is needed. However, because micro EDM usually uses pulse
durations of tens or hundreds of nanoseconds to minimize the discharge energy and stabilize the machining condition, a long pulse duration may not be suitable for micro EDM. Tsai et al. showed that materials with a high melting point and low thermal conductivity could alleviate tool wear [2]. However, as tool size becomes smaller, only tungsten carbide should be selected as the tool material given that it does not bend during the high-speed rotation of a spindle. Song et al. used deionized water as a dielectric fluid to machine tungsten carbide [3]. Deionized water, however, causes electrolytic corrosion on the periphery of machined structures, and an ion filter is continuously necessary to maintain the dielectric strength of deionized water. Bleys et al. used a real-time tool wear compensation method in ED-milling [4]. Although this method could enhance the accuracy of micro EDM, low efficiency still remains as the method did not reduce tool wear. It is known that powder-mixed electrical discharge machining (PMEDM) can mitigate the tool wear problem in
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.121
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micro EDM. There were various studies of PMEDM, but the mechanism of tool wear reduction has not been clarified. Jeswani added graphite powder to kerosene to increase the material removal rate (MRR) and reduce the wear ratio [5]. The study showed that the addition of graphite powder lowers the insulating strength of a dielectric fluid, but the link to tool wear was not explained. Yeo proposed a hypothesis involving a crater-formation mechanism when a powder-mixed dielectric fluid was used [6]; the size of the crater is reduced, and a larger amount of resolidified materials forms because the increased viscosity and enhanced thermal conductivity of the mixed fluid decreases the plasma heat flux in the workpiece and raises the rate of heat dissipation away from the molten cavity. However, this study focused only on crater formation and not on the tool wear reduction relationships. Singh et al. explained that particles between a tool electrode and a workpiece become charged by electric fields and help to bridge the discharge gap via a process termed the ‘bridging effect’ [7]. This effect decreases the insulating strength of the dielectric fluid, causing more frequent discharges to occur and in turn increasing the MRR. Moreover, the surface roughness decreases because the plasma channels become enlarged and more open. This study, however, as well did not explain the mechanism causing the tool wear reduction. Singh et al. stated that the discharge gap enlarged by powder led to reductions of the discharge impact force and energy density between a tool and a workpiece [8]. This may explain the mechanism of tool wear reduction, but potential conflicts exist with regard to higher MRR results in PMEDM. The present study proposes a new explanation of the mechanism of tool wear reduction in micro PMEDM. An RC discharge circuit was used in our experiments because it can achieve a shorter pulse duration than a transistor circuit, which shows better machining performance outcomes [9]. The tool is negatively charged in micro EDM because the energy density is lower at the cathode [10] and a sacrificial carbonaceous layer cannot be deposited on an anode given the short pulse duration of micro EDM. However, the polarity of the tool fluctuated before the plasma channel is extinguished due to stray inductance, which aggravates the tool wear. It was found that adding graphite powder eliminated the fluctuation of the polarity. It dispersed the charged energy into several small plasma channels when discharges occurred, and these small plasma channels were extinguished quickly before the polarity fluctuated. In other words, the addition of graphite powder eventually prevented the current from oscillating, thus reducing the tool wear. Nomenclature C1,2 IL L R1,2 SW t V VC
Capacitance 1, 2 Current on stray inductance Stray inductance Resistance 1, 2 Switch Time Power source Voltage on capacitor
In the following sections, the mechanism of tool wear reduction is explained in more detail. Several experimental results are then presented to support our explanation. The particle diameter of the powder and concentration of a powder-mixed dielectric fluid are also optimized. Finally, the paper concludes with a discussion of tool wear reduction considering the experimental results presented here. 2. Experimental setup Fig. 1 shows the experimental setup used in this study. The WEDG system was used to fabricate the micro tools [11]. A custom-made micro EDM system was used, which consists of an RC discharge circuit, the precision three-axis stage (Parker, USA), and the controller (Delta Tau, USA). An overhead stirrer was installed in both the bottle of the powder-mixed dielectric fluid and in the machining tank to maintain the concentration of the mixed fluid. Two centrifugal pumps were used to circulate the mixed fluid. The specific machining conditions are tabulated in Table 1. Table 1. Machining Conditions Open voltage
100 V
Resistance (R1)
1 kΩ
Capacitance (C1)
5, 10 nF
Tool (diameter), polarity
WC-Co (300, 100, 50 μm), cathode
Workpiece, polarity
STS 304, anode
Dielectric fluid
Kerosene
Powder (average particle diameter)
Graphite (0.04, 0.3, 3 μm)
Concentration
0.1, 0.5, 1, 5 g/l
Machining depth
300 μm
3. The mechanism of tool wear reduction in PMEDM In this section, the RC discharge circuit is modeled and its simulation results are compared with the experimental results. By investigating the difference in the charging slope between pure kerosene and graphite-powder-mixed kerosene (GPMK), the role of graphite powder can be revealed.
Fig. 1 The experimental setup
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(a) (b) Fig. 3. The simulation results of (a) RLC discharging and charging, and (b) RC charging
Fig. 2. The RC discharge circuit modeling
3.1. RC and RLC discharge circuits The micro EDM system with the RC discharge circuit was modeled. As illustrated in Fig. 2, it consists of charging and discharging loops, with a switch used to connect these two loops. The charging loop is composed of a power source, a resistor and a capacitor, whereas the discharge loop consists of stray inductance, a resistor, and a capacitor. Stray inductance is unavoidable because it is inherent inductance caused by electrical wires. When the switch is closed, which means a breakdown of the dielectric fluid occurs or the discharge is initiated, the entire circuit becomes an RLC circuit. Its governing equations are as follows: 2
d I L (t ) dt
2
1
dI L (t )
R1C1
dt
I L (t )
LC1 dVC 2 (t )
1 dVC 2 (t ) L
dt
dt
VC 2 (t )
V
R1 LC1
R1LC1
VC 2 (t )
I L (t )
R2 C2
C2
(1)
(2)
Otherwise, when the switch is open, which means the plasma channels are extinguished or the discharge ends, only the RC charging loop remains as the entire circuit. Its governing equation is shown below. dVC1 (t ) dt
VC1 (t )
V
R1C1
R1C1
(a)
(3)
Graphite powder mitigates tool wear by eliminating the oscillation of the current. The charging characteristics of the RLC and RC circuit have different slopes, and they represent the charging aspects of kerosene and GPMK, respectively. Fig. 3(a) depicts a discharging and charging graph of a RLC circuit governed by (1) and (2) when V = 100 V, R1 = 1 kΩ, R2 = 10 Ω, C1 = 5 nF, C2 = 0.1 nF, and L = 2.3 μH. When the discharge starts, the voltage becomes negative, after which charging and discharging are repeated until the charged energy is dissipated. However, during the actual EDM process, voltage fluctuations continue until the plasma channels become extinct. When the plasma channels are eliminated, which corresponds to an open switch, the circuit converts from an RLC to an RC circuit and the voltage then starts to charge following the RC charging characteristics depicted in
(b) Fig. 4. Discharging and charging graphs for (a) pure kerosene and (b) GPMK
Fig. 3(b). The slope of the RC charging graph is gentler compared to that of the RLC circuit. Figs. 4(a) and (b) show the discharging and charging graphs for kerosene and GPMK, respectively. Capacitor C1 was 5 nF, the concentration of the GPMK was 5 g/l, and the average particle diameter was 0.04 μm in both cases. The charging graph of kerosene shows a dramatic transition from steep to gentle, indicating that the circuit was converted from an RLC circuit to an RC circuit by the open switch. The plasma channels were dissipated and the RC circuit started to charge VC1 from the slope-changing point. Therefore, the current fluctuated until the plasma channels were extinguished or until the slope-changing point. On the other hand, the slope-changing point is not observed in the graph of the GPMK. This indicates that the switch opened as soon as discharge ended, or the plasma channels were extinguished quickly before VC1 started charging following the RLC charging characteristic. There were no fluctuations of the current, and thus tool wear is reduced in this case. The moment of dissipation of the plasma channels is related to the amount of discharge energy. The weaker the discharge energy is, the more quickly the moment of dissipation arrives. Fig. 5 shows the discharging and charging graphs of pure kerosene when C1 was 0.3 nF. As in the charging slope of GPMK, no slope-changing point is observed
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Fig. 5. Discharging and charging graph of pure kerosene when C 1 equals 0.3 nF
(a)
(a)
(b) Fig. 7. The comparison of tool wear length when 5 and 10 nF were used as C 1: (a) pure kerosene, and (b) GPMK
(b) Fig. 6. The 3D surface profiles of discharge craters: (a) pure kerosene, and (b) GPMK
despite the fact that pure kerosene was used as the dielectric fluid, which means that the plasma channels were extinguished quickly as soon as the discharge ended. 3.2. Energy dispersion in PMEDM Graphite powder disperses charged energy into small discharge channels, and these small channels are extinguished quickly owing to their small amounts of discharge energy. Figs. 6(a) and (b) show the 3D surface profiles of the initial discharge craters of pure kerosene and GPMK, respectively. The experiment was performed with identical machining times (less than 1 sec) and capacitance levels (5 nF) as the tool (300 μm diameter) approached the workpiece. The concentration of the GPMK was 5 g/l and the average particle diameter was 0.04 μm. The average crater diameter and depth of pure kerosene were 36 μm and 3 μm, whereas those of GPMK were 16 and 0.3 μm, respectively. There were more discharge craters on GPMK than on pure kerosene. Those on GPMK were more evenly distributed on the total tool area. Because each discharge crater corresponds to each plasma
channel, these findings indicate that identical amounts of charged energy are dispersed into smaller but more numerous discharge channels with GPMK than with pure kerosene. Energy dispersion by graphite powder also explains other experimental results. Figs. 7(a) and (b) are the tool wear lengths of pure kerosene and GPMK respectively, when C1 equaled 5 and 10 nF. As shown in Fig. 7(a), the tool wear length is greater when C1 is 10 nF than when C1 is 5 nF due to the larger discharge energy. However, in Fig. 7(b), the tool wear length does not show any significant differences with the two different C1 values. This occurs because graphite powder scattered the greater charged energy into additional small discharge channels, and each small discharge channel dissipated quickly before the current oscillated. Furthermore, as Klocke et al. showed with a high-speed framing camera in earlier work, discharge channels spread when powders are added to a dielectric fluid [12]. All of these results support the energy-dispersing effect of graphite powder. ډڏComparisons between ED and PMED drillingٻ ٻ In this section, the machining times and tool wear lengths for pure kerosene and GPMK are compared. The concentration of the mixed fluid and the average diameter of the graphite powder were varied to investigate the optimal machining conditions. Each experiment was repeated three times with identical machining conditions.
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(a)
(a)
(b) Fig. 8. Comparison between pure kerosene and GPMK with the five concentrations: (a) machining time and (b) tool wear length
(b) Fig. 9. Comparison between pure kerosene and GPMK with the three particle diameters: (a) machining time and (b) tool wear length
4.1. Identical tool diameter
μm. However, no significant differences were noted in the machining time or tool wear length between the three different average diameters of the graphite particles.
With five different concentrations of the GPMK, the machining time and tool wear length were compared. Figs. 8(a) and (b) show the machining time and tool wear length with respect to the feed rate. The machining conditions were as follows: tool diameter = 100 μm, C1 = 5 nF, and average particle diameter = 0.04 μm. As evidenced by these figures, graphite powder can lower the machining time and tool wear length when the concentration of its mixed fluid exceeds 0.5 g/l. However, when the concentration is higher than 0.5 g/l, there were no noticeable differences between the mixed fluids at each feed rate. When the concentration is higher than 0.5 g/l, the machining time decreases and becomes saturated at approximately 60 seconds as the feed rate increases. With regard to the tool wear length, it increases and saturates at about 62 μm as the feed rate become faster. Compared with the pure kerosene, the maximum reduction rate of the machining time is 62.5% when the concentration is 5 g/l and the feed rate is 9 μm /s, and the maximum reduction rate of the tool wear length is 52.9% when the concentration is 0.5 g/l and the feed rate is 3 μm /s. The machining time and tool wear length were measured and compared using graphite particles with three different average diameters in the mixed fluid. The tool diameter was 100 μm, and the concentration was 5 g/l. The average particle diameters were 0.04, 0.3, 3 μm, and their results are illustrated in Fig. 9. As the feed rate increases, the machining time decreases and becomes saturated at approximately 55 seconds, while the tool wear length increases and saturates at about 65
4.2. Identical hole diameter Before machining holes with identical diameters in both pure kerosene and GPMK, it was necessary to determine the tool diameter. The discharge gap of GPMK is greater than that of pure kerosene; thus, the tool diameter of GPMK should be smaller to machine identical diameter holes. On the other hand, as the tool diameter becomes smaller, the tool wear length naturally increases even with the same amount of discharge energy due to the smaller machining area. Therefore, a machining condition which minimizes the discharge gap should be selected. Fig. 10 shows the discharge gaps with the concentrations of GPMK and the average particle diameters which were used in Section 4.1. The discharge gap was defined as half of the difference between the machined hole and the tool diameter. When the concentration was under 1 g/l, the discharge gap remained at 25 μm, and it increased slightly at 5 g/l. Therefore, the concentration should be below 1 g/l. The average particle diameter of 0.04 μm led to the minimum discharge gap; thus, 0.04 μm was selected as the average powder diameter. Under the optimal machining conditions, the machining times and tool wear lengths of both fluids were compared when 100 μm-diameter holes were machined with different tool diameters. The final machining conditions were as follows: C1 = 5 nF, tool diameter = 80 μm (pure kerosene), 50
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Fig. 10. Discharge gaps according to the concentrations of mixed fluids and the average particle diameter of graphite powder
With the dispersion of the energy into small discharge channels, the discharge craters are shallow and current oscillation does not occur. Therefore, the surface roughness and tool wear are reduced. With regard to the high MRR, Natsu et al. proposed the first-stage-expansion model to explain the expansion process associated with EDM arc plasma [13]. According to that study, machining occurs at the initial stage of discharge and the remaining discharge energy is used for arc plasma expansion until the temperature of the plasma reaches the melting point of the workpiece. Therefore, using more discharge channels could increase the MRR despite the fact that the discharge energy of each is lower. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1A2B2015774). References
Fig. 11. The reduction of the machining time and tool wear length when machining holes with identical diameters
μm (GPMK), feed rate = 5 μm/s (pure kerosene), 3 μm/s (GPMK), concentration = 1 g/l, and average particle diameter = 0.04 μm. These results are shown in Fig. 11. The machining time and tool wear length were reduced by 30.9% and 28.3%, respectively. GPMK still outperformed pure kerosene in terms of the machining time and tool wear length. 5. Conclusions This study proposed a new explanation for the tool wear reduction which occurs when graphite powder is mixed with a dielectric fluid. It was found that graphite powder dispersed charged energy into small discharge channels and that these small channels dissipated quickly before the current fluctuated. Because oscillation of the current can intensify the tool wear in micro EDM, the graphite powder decreases the tool wear via this energy dispersion mechanism. The discharge craters were also compared. The GPMK resulted in a much narrower crater diameter and shallower depth than those by pure kerosene, but the number and distribution of the craters were greater in the GPMK as compared to pure kerosene. Quantitative comparisons were also carried out between pure kerosene and GPMK. To find the optimal machining conditions, tools with identical diameters were used to measure the machining time, tool wear length, and discharge gap. Tools with different diameters were then used to machine holes with identical diameters. With graphite powder, the machining time and tool wear length were reduced by 30.9% and 28.3%, respectively. This study could explain the three advantages of PMEDM, i.e., low surface roughness, low tool wear, and a high MRR.
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