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Effect of machining fluid on the process performance of electric discharge milling of insulating Al2O3 ceramic
ARTICLE IN PRESS
International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035 www.elsevier.com/locate/ijmactool
Effect of machining fluid on the process performance of electric discharge milling of insulating Al2O3 ceramic Y.H. Liu, R.J. Ji, X.P. Li, L.L. Yu, H.F. Zhang, Q.Y. Li School of Mechanical and Electronic Engineering, China University of Petroleum, No. 271 Beier Road, Dongying, Shandong, 257061, China Received 14 August 2007; received in revised form 26 December 2007; accepted 31 December 2007 Available online 11 January 2008
Abstract Wire electric discharge machining (WEDM) and electrical discharge machining (EDM) promise to be effective and economical techniques for the production of tools and parts from conducting ceramic blanks. However, the manufacturing of insulating ceramic blanks with these processes is a long and costly process. This paper presents a new process of machining insulating ceramics using electrical discharge (ED) milling. ED milling uses a thin copper sheet fed to the tool electrode along the surface of the workpiece as the assisting electrode and uses a water-based emulsion as the machining fluid. This process is able to effectively machine a large surface area on insulating ceramics. Machining fluid is a primary factor that affects the material removal rate and surface quality of the ED milling. The effects of emulsion concentration, NaNO3 concentration, polyvinyl alcohol concentration and flow velocity of the machining fluid on the process performance have been investigated. r 2008 Elsevier Ltd. All rights reserved. Keywords: Insulating ceramics; Electric discharge milling; Electric discharge machining; Machining fluid
1. Introduction Engineering ceramics has been widely used in modern industry such as ballistic armor, ceramic composite automotive brakes, diesel particulate filters, a wide variety of prosthetic products, piezo-ceramic sensors and nextgeneration computer-memory products because of their higher hardness and wear resistance, lower thermal expansion coefficient and density, and chemical inertness [1,2]. However, most of the ceramic parts shaped by sintering processes cannot meet the requirements of accuracy and surface quality. Therefore the machining and surface finishing of parts become necessary [3]. Ceramics are known as very difficult-to-machine materials [4]. The main reasons for this are their high hardness, non-electrical conductivity and brittleness. Diamond grinding is one of the most commonly used techniques for insulating ceramics blank shaping but it is costly Corresponding author. Tel.: +86 0546 8932303; fax: +86 0546 8393914. E-mail address: [email protected] (Y.H. Liu).
0890-6955/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2007.12.011
and inefficient. The high hardness of ceramics induces higher grinding force and quick wear of diamond cutting edges [5–7]. Electromachining processes hold the promise of being an effective and economical technique for production of tool and parts from conducting ceramic blanks. Wire electric discharge machining (WEDM) is able to effectively slice conducting ceramics [8,9]. Electrical discharge machining (EDM) and electrical discharge grinding (EDG) shape conducting ceramic blanks at a low cost [10,11]. However, electromachining techniques cannot be directly used to machine insulating ceramics, since these materials are nonconducting. The development of the process for electromachining insulating ceramics is an important research topic in the field of non-traditional machining. Some researchers have used electrolyte as the machining fluid to achieve WEDM, EDM, EDG, arc discharge machining, gas-filled electrochemical discharge machining, and mechanical electrodischarge and electrochemical compound machining of insulating ceramics [12–16]. These processes generate harmful gas during machining. They show low efficiency, and their equipments can be easily
ARTICLE IN PRESS Y.H. Liu et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035
eroded by electrolyte. Naotake Mohri et al. [17,18] have developed the techniques of WEDM and EDM nonconducting ceramics using kerosene as machining fluid. In this method, a metal plate or metal mesh is arranged on the surface of the insulating ceramics, as an assisting electrode. With the help of assisting electrode, insulating ceramics can be machined by sinking EDM or by WEDM in work oil. However, these processes of machining a large surface area on insulating ceramics show low efficiency. This paper proposes a new technique of machining insulating ceramics using electric discharge (ED) milling. The effects of emulsion concentration, NaNO3 concentration, polyvinyl alcohol concentration and flow velocity of the machining fluid on the process performance have been investigated. 2. Principle of ED milling The principle of the ED milling is shown in Fig. 1. The tool electrode and the assisting electrode are connected, respectively, to the positive and the negative poles of the pulse generator. The tool is a steel wheel and is mounted onto a rotary spindle, driven by an A.C. motor. The workpiece is an insulating ceramic blank and is mounted onto a numerically controlled (NC) table. The assisting electrode is a thin copper sheet. The stored sheet bobbin is mounted on to a rotary spindle, driven by a D.C. servomotor. The actuating wheel is driven by a D.C. servomotor with gearing. The machining fluid is a waterbased emulsion. During machining, the tool electrode rotates at a high speed; the assisting electrode is fed towards the tool electrode along the surface of the insulating ceramic workpiece driven by the actuating wheel. As short circuits or arcs are generated in the ED milling, the assisting electrode is fed back by the stored sheet bobbin. After the Tool electrode
Workpiece
Nozzle
short circuits or arcs are cleared up, the assisting electrode is fed on again. The machining fluid is flushed to the assisting electrode and discharge gap with the nozzle. The flushing force of the machining fluid presses the assisting electrode close to the workpiece. As the assisting electrode approaches the tool electrode and the distance between them reaches the discharge gap, electrical discharges are produced. Because the assisting copper sheet electrode is very thin and close to workpiece, most of electrical discharge energy acting on the assisting electrode and plasma energy can directly act on the surface of the workpiece. The instantaneous thermal energy of the electrical discharge causes the plasma temperature to research nearly 40,000 K, pressure of the plasma channel rises to 300 MPa. The instantaneous high temperature and pressure cause the insulating ceramics to be removed by ED milling. 3. Experiments and discussion In the following experiments, workpiece material was insulating Al2O3 ceramic, tool electrode material was steel, assisting electrode material was red copper, pulse time was 500 ms, pulse off-time was 400 ms, peak current was 25 A, tool electrode was positive polarity, and machining fluid was a water-based emulsion. 3.1. Effect of emulsion concentration of machining fluid on the process performance The effect of emulsion concentration on material removal rate (MRR) and surface roughness (SR) is illustrated in Figs. 2 and 3; the machining fluid was water+emulsion. As shown in Fig. 2, MRR initially increases fast with increase of emulsion concentration and then decreases Stored sheet bobbin D C servo motor NC table
Machining fluid
NC controller
+ Pulse generator Stepping motor
Stepping motor
Actuating wheel
1031
Assisting electrode
Fig. 1. Schematic illustration of ED milling.
ARTICLE IN PRESS Y.H. Liu et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035
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MRR (mg/min)
MRR (mg/min)
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200 150 100 50
200 150 100 50
0 0
10 2.5 5 7.5 Emulsion concertration (%)
12.5
Fig. 2. Effect of emulsion concentration on MRR.
0 0
0.25 0.5 0.75 NaNO3 concertration (%)
1
Fig. 4. Effect of NaNO3 concentration on MRR.
12 Voltage wave
Current wave
10
SR (µm)
8 6 4 2 0 0
7.5 2.5 5 10 Emulsion concertration (%)
12.5
Fig. 5. Discharge waves with 5% emulsion+water.
Fig. 3. Effect of emulsion concentration on SR.
Voltage wave
slowly with increase in emulsion concentration. There are many reasons for this. Dielectric strength, washing capability, density and viscosity of machining fluid increase with increase in emulsion concentration. Pinch-effect and energy density of discharge channel are enhanced. Ejection effect of the eroded material increases and, therefore, MRR rises. However, with a very high viscosity of the machining fluid it is difficult to flush away the eroded material, the stability of electrical discharges becomes unsatisfactory, and hence MRR falls. Fig. 3 shows the influence of emulsion concentration on SR. SR increases with increasing emulsion concentration. This is because energy density of the discharge channel increases with increase of emulsion concentration; the crater size generated by a single pulse becomes large and, therefore SR rises with increase in emulsion concentration. 3.2. Effect of NaNO3 concentration of machining fluid on process performance The effects of NaNO3 concentration on MRR and SR are illustrated in Figs. 4 and 7. The machining fluid was 5% emulsion+water+NaNO3. Fig. 4 shows the relationship between MRR and NaNO3 concentrations. The figure indicates that MRR initially
Current wave
Fig. 6. Discharge waves with 5% emulsion+0.5% NaNO3+water.
rises with increase in NaNO3 concentration but decreases with further increase in NaNO3 concentration. The reason for this is that dielectric strength of the machining fluid and breakdown delay time decreases with increasing NaNO3 concentration. It can be seen from Figs. 5 and 6 that discharge time of a single pulse and effective pulse frequency for 0.5% NaNO3 concentration of the machining fluid are higher than that of the machining fluid without NaNO3. Therefore MRR is high. As NaNO3 concentration is higher than a suitable value, electrolysis becomes strong. The gap voltage drop is large and electric discharges become weak. So MRR is low. As shown in Fig. 7, SR initially increases with increase in NaNO3 concentration and then decreases slowly with an
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Y.H. Liu et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035
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0 0.25 0.5 0.75 NaNO3 concertration (%)
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Fig. 7. Effect of NaNO3 concentration on SR.
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Fig. 9. Effect of PVA concentration on SR.
400 350 MRR (mg/min)
300 250 200 150 100 50 0 0
0.25 0.5 0.75 PVA concentration (%)
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Fig. 8. Effect of PVA concentration on MRR.
increase in NaNO3 concentration. This is because under a suitable NaNO3 concentration, discharge energy of a single pulse increases. So the crater generated by a single pulse becomes large and SR rises. As NaNO3 concentration is higher than a suitable value, gap voltage drop is large and electric discharges become weak. The crater generated by a single pulse becomes small and therefore SR decreases. 3.3. Effect of polyvinyl alcohol concentration on process performance The effects of polyvinyl alcohol (PVA) concentration on MRR and SR are illustrated in Figs. 8 and 9. The machining fluid was 5% emulsion+water +PVA. As shown in Fig. 8, MRR initially increases with PVA concentration an increase of emulsion concentration and then decreases with an increase in PVA concentration. The reason for this is that dielectric strength and viscosity of the machining fluid increase with increase in PVA concentration, energy density of the discharge channel and discharge breakdown explosion force rise. Therefore MRR is high. As PVA concentration is higher than a suitable value, the eroded material is difficult to flush away. The stability of electrical discharges becomes unsatisfactory, and hence MRR falls.
Fig. 10. Surface photograph of the workpiece with 5% emulsion+water.
Fig. 9 shows the influence of PVA concentration on SR. SR increases with PVA concentration. This is because energy density of the discharge channel and discharge breakdown explosion force increase with increase in PVA concentration. Crater size generated by a single pulse becomes large. Figs. 10 and 11 show that crater size generated by a single pulse with 0.5% PVA concentration of the machining fluid are higher than that for the machining fluid without PVA. Therefore, SR rises with increase in PVA concentration. 3.4. Effect of flow velocity of machining fluid on process performance The effects of flow velocity on MRR and SR are illustrated in Figs. 12 and 13. The machining fluid was 5% emulsion+water. Fig. 12 shows the relationship between MRR and flow velocity of the machining fluid. MRR increases with flow velocity of the machining fluid. There are many reasons for this. Machining fluids are flushed into the gap between the workpiece and the tool electrode at
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discharge rises. Therefore, MRR increases with flow velocity of the machining fluid. As shown in Fig. 13, SR increases slowly with increase in flow velocity of the machining fluid. The reason for this is that high flow velocity of the machining fluid make the assisting electrode get closer to the surface of the workpiece. Electrical discharge energy acting on the surface of the workpiece increases. Crater size generated by electric discharge becomes large. So SR increases. 4. Conclusions
Fig. 11. Surface photograph of the workpiece with 5% emulsion+0.5% PVA+water.
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300 250 200 150 100 50
(a) Using a thin copper sheet close to the surface of the insulating ceramics as the assisting electrode, insulating ceramics can be easily machined by ED milling. (b) Using a water-based emulsion as the machining fluid, harmful gas is not generated during ED milling, and its equipment is not corroded. (c) With a suitable emulsion concentration, high machining rate and good surface quality of the machining insulating Al2O3 ceramic can be easily obtained by ED milling. (d) Using a suitable chemical additive and its dosage for water-based emulsion, dielectric strength, washing capability and viscosity of the machining fluid can be modified in order to increase MRR of the ED milling. (e) Using high flow velocity of the machining fluid, MRR of ED milling increases and its SR changes less.
0 3
7.5 4.5 6 Flow velocity (m/s)
9
Acknowledgments The work is partially supported by a grant from Chinese National Natural Science Foundation (Grant no. 50675225) and a grant from Ministry of Education of the People’s Republic of China (Grant no. 20040425504).
Fig. 12. Effect of flow velocity on MRR.
16 14
SR (µm)
12
References
10 8 6 4 2 0 3
4.5 6 7.5 Flow velocity (m/s)
9
Fig. 13. Effect of flow velocity on SR.
high speed. The eroded material is easily flushed out. Stability of electrical discharges improves. High flow velocity of the machining fluid makes the assisting electrode move close to the surface of the workpiece. Electrical discharge energy acting on the surface of the workpiece increases. The material removed by the electrical
[1] Anon, USA advanced ceramics demand, Materials Technology 20 (2005) 168. [2] S.W. Yao, H. Wang, S.L. Wang, The study and application of the special ceramics, Industrial Heating 35 (2006) 1–4. [3] J.H. Du, Y.H. Liu, X.P. Li, C.Y. Miao, N.G. Hong, Z.M. Xiao, The grinding technology of engineering ceramics, Materials for Mechanical Engineering 29 (2005) 1–6. [4] N. Mohri, Y.T. Fukuzawa, T. Sata, Some considerations to machining characteristics of insulating ceramics—towards practical use in industry, Annals of the CIRP 51 (2002) 161–164. [5] S. Agarwal, P. Venkateswara Rao, A new surface roughness prediction model for ceramic grinding, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 219 (2005) 811–821. [6] X.P. Li, Y.H. Liu, R.J. Ji, L.L. Yu, Non-traditional machining technique for insulating engineering ceramics, Electromachining & Mould 40 (2006) 6–9. [7] A.V. Gopal, P.V. Rao, Modeling of grinding of silicon carbide with diamond wheels, Mineral Processing and Extractive Metallurgy Review 23 (2002) 51–63.
ARTICLE IN PRESS Y.H. Liu et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035 [8] Z.X. Jia, Z.Y. Wang, Z.J. Guo, Study on the surface quality of Si3N4 ceramics machined by wire EDM, Metallurgical Equipment 25 (2005) 1–7. [9] J. Kozak, K.P. Rajurkar, N. Chandarana, Machining of low electrical conductive materials by wire electrical discharge machining (WEDM), Journal of Materials Processing Technology 149 (2004) 266–271. [10] B. Lauwers, J.P. Kruth, K. Brans, Development of technology and strategies for the machining of ceramic components by sinking and milling EDM, CIRP Annals—Manufacturing Technology 56 (2007) 225–228. [11] Y.H. Liu, Y.F. Guo, J.C. Liu, Electric discharge milling of polycrystalline diamond, Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture 211 (1997) 643–647. [12] Y.F. Guo, J.C. Bai, G.Q. Deng, Z.S. Lu, High-speed wire electrical discharge machining (HS-WEDM) phenomena of insulating Si3N4 ceramics with assisting electrode, Key Engineering Materials 339 (2007) 281–285. [13] Y.H. Liu, Z.X. Jia, J.C. Liu, Study on hole machining of non-conducting ceramics by gas-filled electrodischarge and electrochemical
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compound machining, Journal of Materials Processing Technology 69 (1997) 198–202. Y. Fukuzawa, H. Gotoh, N. Mohri, T. Tani, Line swept surface generation on insulating ceramics by wire electrical discharge machining, Journal of the Australasian Ceramic Society 4 (2005) 17–21. B. Bhattacharyya, B.N. Doloi, Experimental investigations into electrochemical discharge machining (ECDM) of non-conductive ceramic materials, Journal of Materials Processing Technology 95 (1999) 145–154. Y.F. Guo, J.C. Bai, H.S. Liu, J.C. Liu, N. Mohri, Y. Fukuzawa, The study of the process of machining insulating ceramics by electrical discharge grilling, Electromachining & Mould 40 (2006) 54–64. Y.T. Tani, N. Mohri, A. Muttamara, Micro hole electrical discharge machining on insulating ceramics with pipe electrode, International Journal of Electrical Machining 8 (2003) 57–61. T. Tani, Y. Fukuzawa, K. Furutani, N. Mohri, Machining process of insulating ceramics by electrical discharge machining, Journal of the Japan Society for Precision Engineering 63 (1997) 1310–1314.
International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035 www.elsevier.com/locate/ijmactool
Effect of machining fluid on the process performance of electric discharge milling of insulating Al2O3 ceramic Y.H. Liu, R.J. Ji, X.P. Li, L.L. Yu, H.F. Zhang, Q.Y. Li School of Mechanical and Electronic Engineering, China University of Petroleum, No. 271 Beier Road, Dongying, Shandong, 257061, China Received 14 August 2007; received in revised form 26 December 2007; accepted 31 December 2007 Available online 11 January 2008
Abstract Wire electric discharge machining (WEDM) and electrical discharge machining (EDM) promise to be effective and economical techniques for the production of tools and parts from conducting ceramic blanks. However, the manufacturing of insulating ceramic blanks with these processes is a long and costly process. This paper presents a new process of machining insulating ceramics using electrical discharge (ED) milling. ED milling uses a thin copper sheet fed to the tool electrode along the surface of the workpiece as the assisting electrode and uses a water-based emulsion as the machining fluid. This process is able to effectively machine a large surface area on insulating ceramics. Machining fluid is a primary factor that affects the material removal rate and surface quality of the ED milling. The effects of emulsion concentration, NaNO3 concentration, polyvinyl alcohol concentration and flow velocity of the machining fluid on the process performance have been investigated. r 2008 Elsevier Ltd. All rights reserved. Keywords: Insulating ceramics; Electric discharge milling; Electric discharge machining; Machining fluid
1. Introduction Engineering ceramics has been widely used in modern industry such as ballistic armor, ceramic composite automotive brakes, diesel particulate filters, a wide variety of prosthetic products, piezo-ceramic sensors and nextgeneration computer-memory products because of their higher hardness and wear resistance, lower thermal expansion coefficient and density, and chemical inertness [1,2]. However, most of the ceramic parts shaped by sintering processes cannot meet the requirements of accuracy and surface quality. Therefore the machining and surface finishing of parts become necessary [3]. Ceramics are known as very difficult-to-machine materials [4]. The main reasons for this are their high hardness, non-electrical conductivity and brittleness. Diamond grinding is one of the most commonly used techniques for insulating ceramics blank shaping but it is costly Corresponding author. Tel.: +86 0546 8932303; fax: +86 0546 8393914. E-mail address: [email protected] (Y.H. Liu).
0890-6955/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2007.12.011
and inefficient. The high hardness of ceramics induces higher grinding force and quick wear of diamond cutting edges [5–7]. Electromachining processes hold the promise of being an effective and economical technique for production of tool and parts from conducting ceramic blanks. Wire electric discharge machining (WEDM) is able to effectively slice conducting ceramics [8,9]. Electrical discharge machining (EDM) and electrical discharge grinding (EDG) shape conducting ceramic blanks at a low cost [10,11]. However, electromachining techniques cannot be directly used to machine insulating ceramics, since these materials are nonconducting. The development of the process for electromachining insulating ceramics is an important research topic in the field of non-traditional machining. Some researchers have used electrolyte as the machining fluid to achieve WEDM, EDM, EDG, arc discharge machining, gas-filled electrochemical discharge machining, and mechanical electrodischarge and electrochemical compound machining of insulating ceramics [12–16]. These processes generate harmful gas during machining. They show low efficiency, and their equipments can be easily
ARTICLE IN PRESS Y.H. Liu et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035
eroded by electrolyte. Naotake Mohri et al. [17,18] have developed the techniques of WEDM and EDM nonconducting ceramics using kerosene as machining fluid. In this method, a metal plate or metal mesh is arranged on the surface of the insulating ceramics, as an assisting electrode. With the help of assisting electrode, insulating ceramics can be machined by sinking EDM or by WEDM in work oil. However, these processes of machining a large surface area on insulating ceramics show low efficiency. This paper proposes a new technique of machining insulating ceramics using electric discharge (ED) milling. The effects of emulsion concentration, NaNO3 concentration, polyvinyl alcohol concentration and flow velocity of the machining fluid on the process performance have been investigated. 2. Principle of ED milling The principle of the ED milling is shown in Fig. 1. The tool electrode and the assisting electrode are connected, respectively, to the positive and the negative poles of the pulse generator. The tool is a steel wheel and is mounted onto a rotary spindle, driven by an A.C. motor. The workpiece is an insulating ceramic blank and is mounted onto a numerically controlled (NC) table. The assisting electrode is a thin copper sheet. The stored sheet bobbin is mounted on to a rotary spindle, driven by a D.C. servomotor. The actuating wheel is driven by a D.C. servomotor with gearing. The machining fluid is a waterbased emulsion. During machining, the tool electrode rotates at a high speed; the assisting electrode is fed towards the tool electrode along the surface of the insulating ceramic workpiece driven by the actuating wheel. As short circuits or arcs are generated in the ED milling, the assisting electrode is fed back by the stored sheet bobbin. After the Tool electrode
Workpiece
Nozzle
short circuits or arcs are cleared up, the assisting electrode is fed on again. The machining fluid is flushed to the assisting electrode and discharge gap with the nozzle. The flushing force of the machining fluid presses the assisting electrode close to the workpiece. As the assisting electrode approaches the tool electrode and the distance between them reaches the discharge gap, electrical discharges are produced. Because the assisting copper sheet electrode is very thin and close to workpiece, most of electrical discharge energy acting on the assisting electrode and plasma energy can directly act on the surface of the workpiece. The instantaneous thermal energy of the electrical discharge causes the plasma temperature to research nearly 40,000 K, pressure of the plasma channel rises to 300 MPa. The instantaneous high temperature and pressure cause the insulating ceramics to be removed by ED milling. 3. Experiments and discussion In the following experiments, workpiece material was insulating Al2O3 ceramic, tool electrode material was steel, assisting electrode material was red copper, pulse time was 500 ms, pulse off-time was 400 ms, peak current was 25 A, tool electrode was positive polarity, and machining fluid was a water-based emulsion. 3.1. Effect of emulsion concentration of machining fluid on the process performance The effect of emulsion concentration on material removal rate (MRR) and surface roughness (SR) is illustrated in Figs. 2 and 3; the machining fluid was water+emulsion. As shown in Fig. 2, MRR initially increases fast with increase of emulsion concentration and then decreases Stored sheet bobbin D C servo motor NC table
Machining fluid
NC controller
+ Pulse generator Stepping motor
Stepping motor
Actuating wheel
1031
Assisting electrode
Fig. 1. Schematic illustration of ED milling.
ARTICLE IN PRESS Y.H. Liu et al. / International Journal of Machine Tools & Manufacture 48 (2008) 1030–1035
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MRR (mg/min)
MRR (mg/min)
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200 150 100 50
200 150 100 50
0 0
10 2.5 5 7.5 Emulsion concertration (%)
12.5
Fig. 2. Effect of emulsion concentration on MRR.
0 0
0.25 0.5 0.75 NaNO3 concertration (%)
1
Fig. 4. Effect of NaNO3 concentration on MRR.
12 Voltage wave
Current wave
10
SR (µm)
8 6 4 2 0 0
7.5 2.5 5 10 Emulsion concertration (%)
12.5
Fig. 5. Discharge waves with 5% emulsion+water.
Fig. 3. Effect of emulsion concentration on SR.
Voltage wave
slowly with increase in emulsion concentration. There are many reasons for this. Dielectric strength, washing capability, density and viscosity of machining fluid increase with increase in emulsion concentration. Pinch-effect and energy density of discharge channel are enhanced. Ejection effect of the eroded material increases and, therefore, MRR rises. However, with a very high viscosity of the machining fluid it is difficult to flush away the eroded material, the stability of electrical discharges becomes unsatisfactory, and hence MRR falls. Fig. 3 shows the influence of emulsion concentration on SR. SR increases with increasing emulsion concentration. This is because energy density of the discharge channel increases with increase of emulsion concentration; the crater size generated by a single pulse becomes large and, therefore SR rises with increase in emulsion concentration. 3.2. Effect of NaNO3 concentration of machining fluid on process performance The effects of NaNO3 concentration on MRR and SR are illustrated in Figs. 4 and 7. The machining fluid was 5% emulsion+water+NaNO3. Fig. 4 shows the relationship between MRR and NaNO3 concentrations. The figure indicates that MRR initially
Current wave
Fig. 6. Discharge waves with 5% emulsion+0.5% NaNO3+water.
rises with increase in NaNO3 concentration but decreases with further increase in NaNO3 concentration. The reason for this is that dielectric strength of the machining fluid and breakdown delay time decreases with increasing NaNO3 concentration. It can be seen from Figs. 5 and 6 that discharge time of a single pulse and effective pulse frequency for 0.5% NaNO3 concentration of the machining fluid are higher than that of the machining fluid without NaNO3. Therefore MRR is high. As NaNO3 concentration is higher than a suitable value, electrolysis becomes strong. The gap voltage drop is large and electric discharges become weak. So MRR is low. As shown in Fig. 7, SR initially increases with increase in NaNO3 concentration and then decreases slowly with an
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Fig. 7. Effect of NaNO3 concentration on SR.
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Fig. 9. Effect of PVA concentration on SR.
400 350 MRR (mg/min)
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Fig. 8. Effect of PVA concentration on MRR.
increase in NaNO3 concentration. This is because under a suitable NaNO3 concentration, discharge energy of a single pulse increases. So the crater generated by a single pulse becomes large and SR rises. As NaNO3 concentration is higher than a suitable value, gap voltage drop is large and electric discharges become weak. The crater generated by a single pulse becomes small and therefore SR decreases. 3.3. Effect of polyvinyl alcohol concentration on process performance The effects of polyvinyl alcohol (PVA) concentration on MRR and SR are illustrated in Figs. 8 and 9. The machining fluid was 5% emulsion+water +PVA. As shown in Fig. 8, MRR initially increases with PVA concentration an increase of emulsion concentration and then decreases with an increase in PVA concentration. The reason for this is that dielectric strength and viscosity of the machining fluid increase with increase in PVA concentration, energy density of the discharge channel and discharge breakdown explosion force rise. Therefore MRR is high. As PVA concentration is higher than a suitable value, the eroded material is difficult to flush away. The stability of electrical discharges becomes unsatisfactory, and hence MRR falls.
Fig. 10. Surface photograph of the workpiece with 5% emulsion+water.
Fig. 9 shows the influence of PVA concentration on SR. SR increases with PVA concentration. This is because energy density of the discharge channel and discharge breakdown explosion force increase with increase in PVA concentration. Crater size generated by a single pulse becomes large. Figs. 10 and 11 show that crater size generated by a single pulse with 0.5% PVA concentration of the machining fluid are higher than that for the machining fluid without PVA. Therefore, SR rises with increase in PVA concentration. 3.4. Effect of flow velocity of machining fluid on process performance The effects of flow velocity on MRR and SR are illustrated in Figs. 12 and 13. The machining fluid was 5% emulsion+water. Fig. 12 shows the relationship between MRR and flow velocity of the machining fluid. MRR increases with flow velocity of the machining fluid. There are many reasons for this. Machining fluids are flushed into the gap between the workpiece and the tool electrode at
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discharge rises. Therefore, MRR increases with flow velocity of the machining fluid. As shown in Fig. 13, SR increases slowly with increase in flow velocity of the machining fluid. The reason for this is that high flow velocity of the machining fluid make the assisting electrode get closer to the surface of the workpiece. Electrical discharge energy acting on the surface of the workpiece increases. Crater size generated by electric discharge becomes large. So SR increases. 4. Conclusions
Fig. 11. Surface photograph of the workpiece with 5% emulsion+0.5% PVA+water.
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300 250 200 150 100 50
(a) Using a thin copper sheet close to the surface of the insulating ceramics as the assisting electrode, insulating ceramics can be easily machined by ED milling. (b) Using a water-based emulsion as the machining fluid, harmful gas is not generated during ED milling, and its equipment is not corroded. (c) With a suitable emulsion concentration, high machining rate and good surface quality of the machining insulating Al2O3 ceramic can be easily obtained by ED milling. (d) Using a suitable chemical additive and its dosage for water-based emulsion, dielectric strength, washing capability and viscosity of the machining fluid can be modified in order to increase MRR of the ED milling. (e) Using high flow velocity of the machining fluid, MRR of ED milling increases and its SR changes less.
0 3
7.5 4.5 6 Flow velocity (m/s)
9
Acknowledgments The work is partially supported by a grant from Chinese National Natural Science Foundation (Grant no. 50675225) and a grant from Ministry of Education of the People’s Republic of China (Grant no. 20040425504).
Fig. 12. Effect of flow velocity on MRR.
16 14
SR (µm)
12
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10 8 6 4 2 0 3
4.5 6 7.5 Flow velocity (m/s)
9
Fig. 13. Effect of flow velocity on SR.
high speed. The eroded material is easily flushed out. Stability of electrical discharges improves. High flow velocity of the machining fluid makes the assisting electrode move close to the surface of the workpiece. Electrical discharge energy acting on the surface of the workpiece increases. The material removed by the electrical
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