- Email: [email protected]
Precision internal grinding with a metal-bonded diamond grinding wheel
Journal of Materials Processing Technology 105 (2000) 80±86
Precision internal grinding with a metal-bonded diamond grinding wheel Jun Qiana,b,*, Wei Lib, Hitoshi Ohmorib a
Materials Fabrication Laboratory, The Institute of Physical and Chemical Research, Hirozawa 2-1, Wako-shi, Saitama-pref., 351-0198, Japan b Mechanical Engineering Institute, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 23 February 1999
Abstract A metal-bonded grinding wheel, compared with conventional grinding wheels, offers the advantage of high hardness, high holding ability and ®ner usable abrasive grit mesh sizes. The truing and dressing of a metal-bonded diamond (MBD) wheel, in practice, are very dif®cult. To grind small-diameter internal cylindrical surface with MBD-wheels, an interval electrolytic in-process dressing (ELID) method was utilized. Experiments were carried out on an ordinary cylindrical grinding machine with an attached internal grinding set-up, and straight type grinding wheels of different grit sizes were used. The grinding wheels were trued, using the electrical discharge method, and the effects of electrode shapes, grinding parameters, and grit sizes were evaluated experimentally. Mirror surface grinding of different materials was carried out with a #4000 CIB-D wheel, incorporated with this interval ELID (ELID II) method. The experimental results are reported. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Cylindrical grinding; Metal-bonded grinding wheel; Dressing; Electrolytic in-process dressing; Precision grinding
1. Introduction Along with the technological advancement of ultra-precision grinding, applications and requirements for precision cylindrical surfaces have increased signi®cantly in the recent years [1]. As a principal processing method for an internal surface, cylindrical grinding has been commonly utilized as a ®nal operation in the production of precision components. Since grinding is usually the most costly of all manufacturing processes, considerable attempts have been focused on the analysis and optimization of the grinding process to minimize machining time [2±6], and on various compensatory control strategies to improve workpiece quality [7±10] in the cylindrical grinding. However, few researches on mirror-surface internal grinding have been reported [5,6,11], probably due to the limitation of abrasive grit size applicable to non-metallic bond grinding wheels [5,7,8,10]. Research on high ef®ciency grinding of advanced materials, by utilizing high-rigidity grinding machines and tough metal-bonded superabrasive wheels, has led to the success*
Corresponding author. Present address: Division of Production Engineering, Machine Design and Automation, Faculty of Engineering, Katholieke Universiteit Leaven, Celestijenlaan 300B, B-3001 Leaven, Belgium. E-mail address: [email protected] (J. Qian).
ful development of cast iron bonded diamond (CIB-D) grinding wheels [12]. These wheels are manufactured by mixing diamond grits, cast iron powder or ®ber, and a small amount of carbonyl iron powder. The wheels are compacted to a desired form under high pressure and then sintered in an atmosphere of ammonia. These wheels are not suitable for continuous grinding for a long period of time for the following reasons: (1) As tougher metal-bonded wheels exhibit poor dressing ability, it is dif®cult to achieve ef®cient and stable dressing simultaneously. (2) Higher rate of material removal in the grinding promotes wear of the abrasive grains, therefore, more frequent redressing of the grinding wheel will be required by stopping the grinding process. (3) While machining metals such as steel, wheel loading (embedding of swarf) occurs, making effective dressing of metallic bond wheel dif®cult in practice. Although a diamond slab incorporated with an abrasive jet sharpening method is able to dress a bronze-bonded wheel to the same topography as an electroplated wheel [12], complex equipment must be added inevitably which cause workingenvironment problem. Dressing by electrical discharge is a good method, but it is dif®cult to conduct on-line dressing, and dressing stripes appear on the wheel periphery when a pair of parallel electrodes is used [13,14]. Electrolytic in-process dressing (ELID) has so far served as the most successful dressing method for metal-bonded wheels. It has been devised and applied successfully in precision surface
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 5 9 6 - 3
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
grinding [15±17]. However, its application to internal grinding has not been well investigated; especially when the internal diameter of the workpiece is just slightly larger than that of the grinding wheel, it is very dif®cult or even impossible to ®x a dressing electrode mounted parallel to the wheel surface as in ordinary ELID grinding [15]. A novel method to carry out ELID grinding of internal cylindrical surfaces on an ordinary grinding tool is presented in this paper. The principle and process of this method, namely interval ELID (ELID II) grinding, is also discussed. Applying ELID II grinding to an ordinary grinding machine, some preliminary experiments have been carried out. Two types of dressing electrodes were used and their dressing effects were investigated. With this technology, four specimens of alumina ceramic, hardened steels SKH51 and SKD11 and bearing steel, were ground to mirror ®nish. The results of this research are presented in the following sections. 2. Principle of interval ELID grinding The interval dressing of an abrasive grinding wheel itself is not a new technology. In fact, using the common mechanical dressing methods, the wheel is usually dressed at intervals. The grinding process is stopped to dress the wheel after grinding one workpiece or several work pieces. The tool life limit can be chatter vibration, surface roughness and burning marks, etc. [2]. However, with the ELID II method, the wheel is dressed at intervals and the abrasives remain protruding, enabling the grinding process to go on without any interruption and consequently realizing high ef®ciency grinding. The interval ELID system is essentially composed of the following elements: (i) a metal-bonded grinding wheel, (ii) an ELID power source, (iii) electrolytic coolant, and (iv) a pipe dressing electrode. The most important feature of this process is that no special machine is required, and in fact the experimental system we used is an auxiliary internal cylindrical grinding attachment on an external cylindrical grinder. The fundamental principle of interval ELID grinding is same as that of ordinary ELID grinding [15]. Fig. 1 shows a schematic diagram of the interval ELID grinding system. The metal-bonded grinding wheel, which is electrically
Fig. 1. Schematic diagram of interval ELID grinding.
81
conductive, is connected to the positive terminal of a DCpulse power supply with a smooth brush contact, and a ®xed electrode is made negative. A proper clearance of approximately 0.1 mm is originally set between the positive pole (wheel) and the negative one (dressing electrode). By virtue of electrolysis between two electrodes, the wheel and the dressing electrode, the grinding wheel is dressed and a non-conductive ®lm is formed on the wheel periphery. This occurs upon the supply of current from the power source and the electrically conductive coolant. During the interval ELID grinding process, the protruding grains grind the workpiece and as a result, the grains and the oxide layer wear down. The wheel's electrical conductivity increases, due to the wear of the non-conductive oxide layer. The current in the circuit increases, thus increasing the electrolysis. The abrasive grains therefore become more protruding and an insulating layer is formed. 3. Experimental procedure Usually, interval ELID grinding consists of the following steps. (i) Truing: truing is required to reduce the initial eccentricity of the wheel, especially when a new wheel is used for the ®rst time. It is dif®cult to apply conventional truing methods, such as brake dresser, to metallic bond wheels due to the high bond strength. In this investigation, the cast iron bonded wheel was trued by the electrical discharge (ED) process. (ii) ELID dressing, also known as pre-dressing by electrolysis, presently performed at a much lower wheel rotation speed and higher electric settings. (iii) Grinding: interval ELID grinding. The conditions of electrolysis, during the last two steps, differ due to change in the wheel state and grinding conditions. 3.1. ED-truing It is well known that the truing of a metallic bond grinding wheel is not easy with ordinary mechanical methods, especially in the case of internal cylindrical grinding where the rigidity of the wheel quill is poor because of the large ratio of length to diameter. To true a cast iron bonded diamond wheel at high speed and with high precision, an electrical discharge truing (ED-truing) method was used in this study. Fig. 2 shows the details of this method. A special ED-truing wheel, made of high temperature alloy and insulated from its central shaft, was mounted on the three-jaw chuck of the grinding tool. The ED-truing wheel was connected to the negative pole of an ELID power source originating from ordinary ED power supply, whilst the grinding wheel was linked to the positive pole. Both the ED-truing wheel and the grinding wheel, especially the latter, rotated at a fairly low speed and the ED truing wheel reciprocated along with the machine's saddle. Little and sometimes even no coolant was supplied to the working area to prevent electrolysis to the full and to pursue high truing precision.
82
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86 Table 1 Experimental speci®cation Machine tool Mug27/30-22, Mitsui Seiki Kogyo Co. Ltd. Grinding wheel CIB-D straight wheel, (é)300 mm(L)20 mm(T)3 mm, #325, #1200, #4000 ELID power ED-1560 (peak value: 150 V, 60 A) Coolant 2% dilution of chemical coolant AFM-G Workpiece SKD11, SKH51, (é)36 mm(L)30 mm bearing steel, (é)36 mm (L)15 mm aluminum ceramic, (é)36 mm(L)30 mm Measuring instruments MITUTOYO Surftest 501 Fig. 2. View of the ED-truing set-up.
3.2. Pre-dressing Following the ED-truing, pre-dressing was carried out before starting ELID grinding (see Fig. 1). When the predressing began, the surface of the trued wheel showed a good electrical conductivity. Therefore, the current would be very high and the voltage between the wheel and electrode would be low, varying in accordance with the wheel size and dressing settings. After several minutes, the cast iron ®ber bond material, which is mostly ionized into Fe2, is dissolved by electrolysis. The ionized Fe2 will react with nonconductive ferrous hydroxides and oxides to form a layer on the wheel periphery. This insulating oxide layer would grow on the wheel surface, whereby its electrical conductivity would be reduced. Consequently, the current would decrease and the working voltage would remain quite high (90 V, in case that the originally set open voltage is 100 V) after 20 min. The color of the wheel changed to dark pink, due to the formation of ferrous oxide. 3.3. Interval ELID grinding During the grinding process, the protruding grains grind the workpiece and as a result, the grains and the oxide layer, wear down. The wheel's electrical conductivity increases, due to the wear of the oxide layer. The current in the circuit increases, accelerating the electrolysis, making the abrasive grains more protruding and forming an insulating layer. In the case of internal cylindrical grinding, the metal-bonded grinding wheel is dressed at intervals, i.e. the wheel is dressed when it departs away from the workpiece. When very ®ne grit size abrasive wheel is used and the infeed rate is very low, the insulating layer and the abrasive can ®nish the work surface in a way similar to lapping, achieving a super smooth surface. Preliminary experiments were carried out on an ordinary grinder, and Table 1 lists some of the experimental details. To verify the validity of this method, a coarse grit size grinding wheel of mesh size #170 (80 mm mesh size) was ®rst used to grind alumina ceramic, and two types of
dressing electrodes, a pipe electrode and an arc electrode, were used. The grinding results of these dressing electrodes and those without interval ELID dressing were also evaluated. With the same wheel, the effects of grinding parameters were investigated so as to optimize the parameter combination. Three wheels of different grit sizes, #325, #1200 (12 mm) and #4000 (4 mm), were utilized to grind bearing steel rings. All three bearing rings were ground with a #325 wheel and then two were ground with #1200 and #4000 wheels. Finally, mirror surface grinding of SKD11, SKH11, bearing steel and alumina ceramic was performed through two steps: rough grinding with a #325 wheel and precision grinding with a #4000 wheel. 4. Results and discussions 4.1. ED-truing of CIB-D wheel To start the experiment, a new CIB-D wheel of #325 mesh size was installed on the quill of the internal grinding setup. The run-out of the new grinding wheel, before ED truing, was around 40 mm, so ED-truing was conducted as stated in Section 3. Fig. 3 shows the result of ED-truing, denoting the change of wheel run-out versus truing time. Although both
Fig. 3. Wheel run-out change in ED-truing.
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
the eccentricity and the roundness of the new wheel contribute to the run-out of the wheel, the former, obviously, is the major cause of wheel run-out, which needs to be removed by electrical discharging. At the beginning of ED-truing, the material volume to be removed for reducing the unit of run-out was small and the wheel run-out decreased relatively quickly. As it went on, the removal volume increased and thus the run-out diminution rate slowed down. In practice, when a large electrical discharging speci®cation was chosen, the wheel run-out in some positions even increased to slightly more than that before ED-truing. This is due to the electrical discharging characteristic that the melt material scatters over the wheel surface and solidi®es on it. Thus, the wheel surface becomes rough and run-out increases. However, these protruding points are very easily demolished by electrolysis and have almost no effect on ELID grinding. After ELID grinding of one workpiece, the measured wheel run-out was 4 mm (Fig. 3).
83
4.2. Electrical phenomenon in the grinding process Pre-dressing was conducted immediately after ED-truing. The dressing electrode and the grinding wheel were kept relatively static with a clearance of about 0.1 mm between them. The grinding wheel was rotated slowly and suf®cient coolant was poured into the gap. When the power supply was switched on, the current was around 3.5 A (pipe electrode), and it dropped with time and remained stable after 20 min. After pre-dressing, an insulating layer about 30 mm thickness was generated on the wheel surface, and hence the wheel diameter increased slightly. Fig. 4 shows the wheel surface topography after pre-dressing under a confocal laserscanning microscope (OLS100, OLYMPUS). It can be seen clearly that oxide ®lm has formed on the wheel periphery and the insulating matter. Since the dressing electrode is ®xed on the saddle, which reciprocates relative to the grinding wheel (Fig. 1), the grinding wheel is dressed at intervals in the process of
Fig. 4. Topography of a CIB-D wheel after pre-dressing.
84
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
Fig. 5. Current ¯uctuation in interval ELID grinding.
grinding. Fig. 5 shows the current change in the grinding process. When the wheel moves from one side towards the workpiece, it moves away from the dressing electrode at the same time and the dressing current drops. On the contrary, the dressing current rises when it withdraws from the grinding area from left to right. Thus the average current in the grinding process ¯uctuates, as shown in Fig. 5. 4.3. Comparison of pipe and arc electrodes In internal cylindrical grinding, usually a small diameter grinding wheel is utilized and therefore abrasive wear occurs considerably faster. Two types of dressing electrodes, arc
Fig. 7. In¯uence of depth of cut on Ra.
and pipe electrodes, were used in the experiment. Compared with the arc electrode, the pipe dressing electrode possesses a larger dressing area along the wheel periphery. Hence, its pre-dressing current is higher than that of the arc-dressing electrode (Fig. 6(a)). It also implies that the dressing effect of the pipe electrode is better than the arc electrode and thus a better surface can be expected (Fig. 6(b)). The pipe dressing electrode was therefore applied in subsequent experiments for the sake of providing suf®cient dressing. 4.4. Effect of the grinding parameters To optimize the grinding parameters for interval ELID grinding, several grinding tests were carried out with different grinding settings. Fig. 7 shows that the surface roughness increases when the depth of cut per path increases under each condition. However, with the pipe dressing electrode the best surface roughness Ra can be obtained under the same grinding settings. Fig. 8 shows the in¯uence of traverse speed on the ground surface roughness. It can be seen that the ground surface quality remains almost constant up to a traverse speed of
Fig. 6. Comparison of pipe electrode and arc electrode: (a) current difference in pre-dressing under the same electrical settings; (b) ground surface roughness with the same wheel.
Fig. 8. In¯uence of traverse speed.
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
85
Fig. 10. Bearing steel surface roughness ground by wheels of different grit sizes. Fig. 9. In¯uence of wheel speed.
33 mm/min. Thus the lower the traverse speed, the better will be the surface quality. It also seems that the optimal traverse speed is around 40 mm/min. Due to the limitation of the wheel diameter, the wheel speed could be adjusted only within a small range and the
effect of wheel speed in this work is not signi®cant. Fig. 9 shows the ground surface roughness at different wheel speeds vs in this investigation. The Ra more or less shows the same value in the accessible speed compass. In any case, the higher the wheel speed, the better is the ground surface. Based on these empirical results, it can be concluded that the effects of grinding parameters in interval ELID grinding are
Fig. 11. Samples ®nished with a #4000 CFB-D wheel: (a) surface roughness of different materials; (b) view of ground workpieces.
86
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
just the same as in ordinary internal grinding and the results coincide with the common grinding principle, while with ELID a better surface can be assured. 4.5. In¯uence of grit size on surface roughness Fig. 10 shows a comparison of ground surface roughness of bearing steel rings using wheels of different grit sizes. The grinding conditions were: wheel speed 10 m/s, workpiece rotation 150 rpm, traverse speed 40 mm/min, in-feed 10 nm per pass. The #325 mesh size wheel was used as the rough grinding wheel. Then a #1200 and a #4000 mesh size grinding wheels were used to achieve ®ne ®nish. It is obvious that the ground surface quality increases along with the wheel mesh size. The advantage of this method is that very ®ne abrasive can be used and suf®cient dressing for continuous grinding is possible. Thus, a very smooth surface can be fabricated. 4.6. Mirror surface grinding of different materials Fig. 11 shows the ground mirror surface results of four materials, bearing steel, SKD11, SKH51 and alumina. These materials were all grounded by a #325 wheel and then ®nished by a #4000 wheel. 5. Conclusions Internal surface grinding with metallic bond grinding wheels, incorporated with electrolytic in-process dressing, was carried out on an ordinary grinding tool. Pipe and arcdressing electrodes were used ®rst to verify their dressing effects and some experiments were conducted to optimize the grinding parameters. Finally, mirror internal surface grinding was accomplished on both metallic materials and alumina ceramic. Based on these experiments, the following conclusion were down: 1. The pipe dressing electrode is superior to electrodes of other shapes in the case of internal grinding with ELID. 2. The grinding conditions have the same effects on grinding qualities in ELID II grinding as in ordinary cylindrical grinding. 3. Mirror internal surface grinding is practicable on ordinary grinding machine using cast iron bonded diamond (CIB-D) wheels with ELID II.
Acknowledgements The authors wish to thank the ELID research group's industry members for their help. Special thanks go to FUJI Die Co. Ltd. for providing the grinding wheels. References [1] B. Komanduri, D.A. Lucca, Y. Tani, Technological advances in ®ne abrasive processes, Ann. CIRP 46 (2) (1997) 545±589. [2] J. Peters, R. Aerens, Optimization procedure of three phase grinding cycles of a series without intermediate dressing, Ann. CIRP 29 (1) (1980) 195±199. [3] S. Malkin, Y. Koren, Optimization infeed control for accelerated spark-out in plunge grinding, ASME J. Eng. Ind. 106 (1984) 70±74. [4] H.K. Toenshoff, M. Zinngrebe, M. Kemmerling, Optimization of internal grinding by microcomputer-based force control, Ann. CIRP 35 (1) (1986) 293±296. [5] G. Xiao, S. Malkin, K. Danai, Autonomous system for multistage cylindrical grinding, ASME J. Dyn. Syst. Meas Control 115 (1993) 667±672. [6] G. Xiao, S. Malkin, On-line optimization for internal grinding, Ann. CIRP 45 (1) (1996) 287±292. [7] I. Inasaki, Monitoring and optimization of internal grinding process, Ann. CIRP 40 (1) (1991) 359±362. [8] H. Kato, Y. Nakano, Transfer of roundness error from center and center hole to workpiece in cylindrical grinding and its control, Ann. CIRP 34 (1) (1985) 287±290. [9] Y.S. Liao, L.C. Shiang, Computer simulation of self-excited and forced vibrations in the external cylindrical plunge grinding process, ASME J. Eng. Ind. 113 (1991) 297±304. [10] K.H. Kim, K.F. Eman, S.M. Wu, Development of a forecasting compensatory control system for cylindrical grinding, ASME J. Eng. Ind. 109 (1987) 385±391. [11] E. Salje, H.-H. Damlos, H. Mohlen, Internal grinding of high strength ceramic workpiece materials with diamond grinding wheels, Ann. CIRP 34 (1) (1985) 263±266. [12] T. Nakagawa, Y. Hagiuda, K. Karimori, Cast iron bonded diamond grinding tool and its application to hard materials, in: Proceedings of the Fifth ICPE, Tokyo, 1984. [13] K. Suzuki, T. Uematsu, T. Nakagawa, On-machine truing/dressing of metal bond grinding wheels by electric-discharge machining, Ann. CIRP 36 (1) (1987) 115±118. [14] J. Tamaki, K. Takahashi, M. Kubota, Electrical dressing of metal bonded grinding wheels, in: Proceedings of the ISEM-X, 1992, pp. 510±515. [15] H. Ohmori, T. Nakagawa, Mirror surface grinding of silicon wafers with electrolytic in-process dressing, Ann. CIRP 39 (1) (1990) 329±332. [16] H. Ohmori, Electrolytic in-process dressing (ELID) gridning technique for ultraprecision mirror surface machining, Int. J. Jpn. Soc. Prec. Eng. 26 (4) (1992) 273±278. [17] H. Ohmori, I. Takahashi, B.P. Bandyopadhyay, Ultra-precision grinding of structural ceramics by electrolytic in-process dressing (ELID) grinding, J. Mater. Process. Technol. 57 (1996) 272±277.
Precision internal grinding with a metal-bonded diamond grinding wheel Jun Qiana,b,*, Wei Lib, Hitoshi Ohmorib a
Materials Fabrication Laboratory, The Institute of Physical and Chemical Research, Hirozawa 2-1, Wako-shi, Saitama-pref., 351-0198, Japan b Mechanical Engineering Institute, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China Received 23 February 1999
Abstract A metal-bonded grinding wheel, compared with conventional grinding wheels, offers the advantage of high hardness, high holding ability and ®ner usable abrasive grit mesh sizes. The truing and dressing of a metal-bonded diamond (MBD) wheel, in practice, are very dif®cult. To grind small-diameter internal cylindrical surface with MBD-wheels, an interval electrolytic in-process dressing (ELID) method was utilized. Experiments were carried out on an ordinary cylindrical grinding machine with an attached internal grinding set-up, and straight type grinding wheels of different grit sizes were used. The grinding wheels were trued, using the electrical discharge method, and the effects of electrode shapes, grinding parameters, and grit sizes were evaluated experimentally. Mirror surface grinding of different materials was carried out with a #4000 CIB-D wheel, incorporated with this interval ELID (ELID II) method. The experimental results are reported. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Cylindrical grinding; Metal-bonded grinding wheel; Dressing; Electrolytic in-process dressing; Precision grinding
1. Introduction Along with the technological advancement of ultra-precision grinding, applications and requirements for precision cylindrical surfaces have increased signi®cantly in the recent years [1]. As a principal processing method for an internal surface, cylindrical grinding has been commonly utilized as a ®nal operation in the production of precision components. Since grinding is usually the most costly of all manufacturing processes, considerable attempts have been focused on the analysis and optimization of the grinding process to minimize machining time [2±6], and on various compensatory control strategies to improve workpiece quality [7±10] in the cylindrical grinding. However, few researches on mirror-surface internal grinding have been reported [5,6,11], probably due to the limitation of abrasive grit size applicable to non-metallic bond grinding wheels [5,7,8,10]. Research on high ef®ciency grinding of advanced materials, by utilizing high-rigidity grinding machines and tough metal-bonded superabrasive wheels, has led to the success*
Corresponding author. Present address: Division of Production Engineering, Machine Design and Automation, Faculty of Engineering, Katholieke Universiteit Leaven, Celestijenlaan 300B, B-3001 Leaven, Belgium. E-mail address: [email protected] (J. Qian).
ful development of cast iron bonded diamond (CIB-D) grinding wheels [12]. These wheels are manufactured by mixing diamond grits, cast iron powder or ®ber, and a small amount of carbonyl iron powder. The wheels are compacted to a desired form under high pressure and then sintered in an atmosphere of ammonia. These wheels are not suitable for continuous grinding for a long period of time for the following reasons: (1) As tougher metal-bonded wheels exhibit poor dressing ability, it is dif®cult to achieve ef®cient and stable dressing simultaneously. (2) Higher rate of material removal in the grinding promotes wear of the abrasive grains, therefore, more frequent redressing of the grinding wheel will be required by stopping the grinding process. (3) While machining metals such as steel, wheel loading (embedding of swarf) occurs, making effective dressing of metallic bond wheel dif®cult in practice. Although a diamond slab incorporated with an abrasive jet sharpening method is able to dress a bronze-bonded wheel to the same topography as an electroplated wheel [12], complex equipment must be added inevitably which cause workingenvironment problem. Dressing by electrical discharge is a good method, but it is dif®cult to conduct on-line dressing, and dressing stripes appear on the wheel periphery when a pair of parallel electrodes is used [13,14]. Electrolytic in-process dressing (ELID) has so far served as the most successful dressing method for metal-bonded wheels. It has been devised and applied successfully in precision surface
0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 5 9 6 - 3
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
grinding [15±17]. However, its application to internal grinding has not been well investigated; especially when the internal diameter of the workpiece is just slightly larger than that of the grinding wheel, it is very dif®cult or even impossible to ®x a dressing electrode mounted parallel to the wheel surface as in ordinary ELID grinding [15]. A novel method to carry out ELID grinding of internal cylindrical surfaces on an ordinary grinding tool is presented in this paper. The principle and process of this method, namely interval ELID (ELID II) grinding, is also discussed. Applying ELID II grinding to an ordinary grinding machine, some preliminary experiments have been carried out. Two types of dressing electrodes were used and their dressing effects were investigated. With this technology, four specimens of alumina ceramic, hardened steels SKH51 and SKD11 and bearing steel, were ground to mirror ®nish. The results of this research are presented in the following sections. 2. Principle of interval ELID grinding The interval dressing of an abrasive grinding wheel itself is not a new technology. In fact, using the common mechanical dressing methods, the wheel is usually dressed at intervals. The grinding process is stopped to dress the wheel after grinding one workpiece or several work pieces. The tool life limit can be chatter vibration, surface roughness and burning marks, etc. [2]. However, with the ELID II method, the wheel is dressed at intervals and the abrasives remain protruding, enabling the grinding process to go on without any interruption and consequently realizing high ef®ciency grinding. The interval ELID system is essentially composed of the following elements: (i) a metal-bonded grinding wheel, (ii) an ELID power source, (iii) electrolytic coolant, and (iv) a pipe dressing electrode. The most important feature of this process is that no special machine is required, and in fact the experimental system we used is an auxiliary internal cylindrical grinding attachment on an external cylindrical grinder. The fundamental principle of interval ELID grinding is same as that of ordinary ELID grinding [15]. Fig. 1 shows a schematic diagram of the interval ELID grinding system. The metal-bonded grinding wheel, which is electrically
Fig. 1. Schematic diagram of interval ELID grinding.
81
conductive, is connected to the positive terminal of a DCpulse power supply with a smooth brush contact, and a ®xed electrode is made negative. A proper clearance of approximately 0.1 mm is originally set between the positive pole (wheel) and the negative one (dressing electrode). By virtue of electrolysis between two electrodes, the wheel and the dressing electrode, the grinding wheel is dressed and a non-conductive ®lm is formed on the wheel periphery. This occurs upon the supply of current from the power source and the electrically conductive coolant. During the interval ELID grinding process, the protruding grains grind the workpiece and as a result, the grains and the oxide layer wear down. The wheel's electrical conductivity increases, due to the wear of the non-conductive oxide layer. The current in the circuit increases, thus increasing the electrolysis. The abrasive grains therefore become more protruding and an insulating layer is formed. 3. Experimental procedure Usually, interval ELID grinding consists of the following steps. (i) Truing: truing is required to reduce the initial eccentricity of the wheel, especially when a new wheel is used for the ®rst time. It is dif®cult to apply conventional truing methods, such as brake dresser, to metallic bond wheels due to the high bond strength. In this investigation, the cast iron bonded wheel was trued by the electrical discharge (ED) process. (ii) ELID dressing, also known as pre-dressing by electrolysis, presently performed at a much lower wheel rotation speed and higher electric settings. (iii) Grinding: interval ELID grinding. The conditions of electrolysis, during the last two steps, differ due to change in the wheel state and grinding conditions. 3.1. ED-truing It is well known that the truing of a metallic bond grinding wheel is not easy with ordinary mechanical methods, especially in the case of internal cylindrical grinding where the rigidity of the wheel quill is poor because of the large ratio of length to diameter. To true a cast iron bonded diamond wheel at high speed and with high precision, an electrical discharge truing (ED-truing) method was used in this study. Fig. 2 shows the details of this method. A special ED-truing wheel, made of high temperature alloy and insulated from its central shaft, was mounted on the three-jaw chuck of the grinding tool. The ED-truing wheel was connected to the negative pole of an ELID power source originating from ordinary ED power supply, whilst the grinding wheel was linked to the positive pole. Both the ED-truing wheel and the grinding wheel, especially the latter, rotated at a fairly low speed and the ED truing wheel reciprocated along with the machine's saddle. Little and sometimes even no coolant was supplied to the working area to prevent electrolysis to the full and to pursue high truing precision.
82
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86 Table 1 Experimental speci®cation Machine tool Mug27/30-22, Mitsui Seiki Kogyo Co. Ltd. Grinding wheel CIB-D straight wheel, (é)300 mm(L)20 mm(T)3 mm, #325, #1200, #4000 ELID power ED-1560 (peak value: 150 V, 60 A) Coolant 2% dilution of chemical coolant AFM-G Workpiece SKD11, SKH51, (é)36 mm(L)30 mm bearing steel, (é)36 mm (L)15 mm aluminum ceramic, (é)36 mm(L)30 mm Measuring instruments MITUTOYO Surftest 501 Fig. 2. View of the ED-truing set-up.
3.2. Pre-dressing Following the ED-truing, pre-dressing was carried out before starting ELID grinding (see Fig. 1). When the predressing began, the surface of the trued wheel showed a good electrical conductivity. Therefore, the current would be very high and the voltage between the wheel and electrode would be low, varying in accordance with the wheel size and dressing settings. After several minutes, the cast iron ®ber bond material, which is mostly ionized into Fe2, is dissolved by electrolysis. The ionized Fe2 will react with nonconductive ferrous hydroxides and oxides to form a layer on the wheel periphery. This insulating oxide layer would grow on the wheel surface, whereby its electrical conductivity would be reduced. Consequently, the current would decrease and the working voltage would remain quite high (90 V, in case that the originally set open voltage is 100 V) after 20 min. The color of the wheel changed to dark pink, due to the formation of ferrous oxide. 3.3. Interval ELID grinding During the grinding process, the protruding grains grind the workpiece and as a result, the grains and the oxide layer, wear down. The wheel's electrical conductivity increases, due to the wear of the oxide layer. The current in the circuit increases, accelerating the electrolysis, making the abrasive grains more protruding and forming an insulating layer. In the case of internal cylindrical grinding, the metal-bonded grinding wheel is dressed at intervals, i.e. the wheel is dressed when it departs away from the workpiece. When very ®ne grit size abrasive wheel is used and the infeed rate is very low, the insulating layer and the abrasive can ®nish the work surface in a way similar to lapping, achieving a super smooth surface. Preliminary experiments were carried out on an ordinary grinder, and Table 1 lists some of the experimental details. To verify the validity of this method, a coarse grit size grinding wheel of mesh size #170 (80 mm mesh size) was ®rst used to grind alumina ceramic, and two types of
dressing electrodes, a pipe electrode and an arc electrode, were used. The grinding results of these dressing electrodes and those without interval ELID dressing were also evaluated. With the same wheel, the effects of grinding parameters were investigated so as to optimize the parameter combination. Three wheels of different grit sizes, #325, #1200 (12 mm) and #4000 (4 mm), were utilized to grind bearing steel rings. All three bearing rings were ground with a #325 wheel and then two were ground with #1200 and #4000 wheels. Finally, mirror surface grinding of SKD11, SKH11, bearing steel and alumina ceramic was performed through two steps: rough grinding with a #325 wheel and precision grinding with a #4000 wheel. 4. Results and discussions 4.1. ED-truing of CIB-D wheel To start the experiment, a new CIB-D wheel of #325 mesh size was installed on the quill of the internal grinding setup. The run-out of the new grinding wheel, before ED truing, was around 40 mm, so ED-truing was conducted as stated in Section 3. Fig. 3 shows the result of ED-truing, denoting the change of wheel run-out versus truing time. Although both
Fig. 3. Wheel run-out change in ED-truing.
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
the eccentricity and the roundness of the new wheel contribute to the run-out of the wheel, the former, obviously, is the major cause of wheel run-out, which needs to be removed by electrical discharging. At the beginning of ED-truing, the material volume to be removed for reducing the unit of run-out was small and the wheel run-out decreased relatively quickly. As it went on, the removal volume increased and thus the run-out diminution rate slowed down. In practice, when a large electrical discharging speci®cation was chosen, the wheel run-out in some positions even increased to slightly more than that before ED-truing. This is due to the electrical discharging characteristic that the melt material scatters over the wheel surface and solidi®es on it. Thus, the wheel surface becomes rough and run-out increases. However, these protruding points are very easily demolished by electrolysis and have almost no effect on ELID grinding. After ELID grinding of one workpiece, the measured wheel run-out was 4 mm (Fig. 3).
83
4.2. Electrical phenomenon in the grinding process Pre-dressing was conducted immediately after ED-truing. The dressing electrode and the grinding wheel were kept relatively static with a clearance of about 0.1 mm between them. The grinding wheel was rotated slowly and suf®cient coolant was poured into the gap. When the power supply was switched on, the current was around 3.5 A (pipe electrode), and it dropped with time and remained stable after 20 min. After pre-dressing, an insulating layer about 30 mm thickness was generated on the wheel surface, and hence the wheel diameter increased slightly. Fig. 4 shows the wheel surface topography after pre-dressing under a confocal laserscanning microscope (OLS100, OLYMPUS). It can be seen clearly that oxide ®lm has formed on the wheel periphery and the insulating matter. Since the dressing electrode is ®xed on the saddle, which reciprocates relative to the grinding wheel (Fig. 1), the grinding wheel is dressed at intervals in the process of
Fig. 4. Topography of a CIB-D wheel after pre-dressing.
84
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
Fig. 5. Current ¯uctuation in interval ELID grinding.
grinding. Fig. 5 shows the current change in the grinding process. When the wheel moves from one side towards the workpiece, it moves away from the dressing electrode at the same time and the dressing current drops. On the contrary, the dressing current rises when it withdraws from the grinding area from left to right. Thus the average current in the grinding process ¯uctuates, as shown in Fig. 5. 4.3. Comparison of pipe and arc electrodes In internal cylindrical grinding, usually a small diameter grinding wheel is utilized and therefore abrasive wear occurs considerably faster. Two types of dressing electrodes, arc
Fig. 7. In¯uence of depth of cut on Ra.
and pipe electrodes, were used in the experiment. Compared with the arc electrode, the pipe dressing electrode possesses a larger dressing area along the wheel periphery. Hence, its pre-dressing current is higher than that of the arc-dressing electrode (Fig. 6(a)). It also implies that the dressing effect of the pipe electrode is better than the arc electrode and thus a better surface can be expected (Fig. 6(b)). The pipe dressing electrode was therefore applied in subsequent experiments for the sake of providing suf®cient dressing. 4.4. Effect of the grinding parameters To optimize the grinding parameters for interval ELID grinding, several grinding tests were carried out with different grinding settings. Fig. 7 shows that the surface roughness increases when the depth of cut per path increases under each condition. However, with the pipe dressing electrode the best surface roughness Ra can be obtained under the same grinding settings. Fig. 8 shows the in¯uence of traverse speed on the ground surface roughness. It can be seen that the ground surface quality remains almost constant up to a traverse speed of
Fig. 6. Comparison of pipe electrode and arc electrode: (a) current difference in pre-dressing under the same electrical settings; (b) ground surface roughness with the same wheel.
Fig. 8. In¯uence of traverse speed.
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
85
Fig. 10. Bearing steel surface roughness ground by wheels of different grit sizes. Fig. 9. In¯uence of wheel speed.
33 mm/min. Thus the lower the traverse speed, the better will be the surface quality. It also seems that the optimal traverse speed is around 40 mm/min. Due to the limitation of the wheel diameter, the wheel speed could be adjusted only within a small range and the
effect of wheel speed in this work is not signi®cant. Fig. 9 shows the ground surface roughness at different wheel speeds vs in this investigation. The Ra more or less shows the same value in the accessible speed compass. In any case, the higher the wheel speed, the better is the ground surface. Based on these empirical results, it can be concluded that the effects of grinding parameters in interval ELID grinding are
Fig. 11. Samples ®nished with a #4000 CFB-D wheel: (a) surface roughness of different materials; (b) view of ground workpieces.
86
J. Qian et al. / Journal of Materials Processing Technology 105 (2000) 80±86
just the same as in ordinary internal grinding and the results coincide with the common grinding principle, while with ELID a better surface can be assured. 4.5. In¯uence of grit size on surface roughness Fig. 10 shows a comparison of ground surface roughness of bearing steel rings using wheels of different grit sizes. The grinding conditions were: wheel speed 10 m/s, workpiece rotation 150 rpm, traverse speed 40 mm/min, in-feed 10 nm per pass. The #325 mesh size wheel was used as the rough grinding wheel. Then a #1200 and a #4000 mesh size grinding wheels were used to achieve ®ne ®nish. It is obvious that the ground surface quality increases along with the wheel mesh size. The advantage of this method is that very ®ne abrasive can be used and suf®cient dressing for continuous grinding is possible. Thus, a very smooth surface can be fabricated. 4.6. Mirror surface grinding of different materials Fig. 11 shows the ground mirror surface results of four materials, bearing steel, SKD11, SKH51 and alumina. These materials were all grounded by a #325 wheel and then ®nished by a #4000 wheel. 5. Conclusions Internal surface grinding with metallic bond grinding wheels, incorporated with electrolytic in-process dressing, was carried out on an ordinary grinding tool. Pipe and arcdressing electrodes were used ®rst to verify their dressing effects and some experiments were conducted to optimize the grinding parameters. Finally, mirror internal surface grinding was accomplished on both metallic materials and alumina ceramic. Based on these experiments, the following conclusion were down: 1. The pipe dressing electrode is superior to electrodes of other shapes in the case of internal grinding with ELID. 2. The grinding conditions have the same effects on grinding qualities in ELID II grinding as in ordinary cylindrical grinding. 3. Mirror internal surface grinding is practicable on ordinary grinding machine using cast iron bonded diamond (CIB-D) wheels with ELID II.
Acknowledgements The authors wish to thank the ELID research group's industry members for their help. Special thanks go to FUJI Die Co. Ltd. for providing the grinding wheels. References [1] B. Komanduri, D.A. Lucca, Y. Tani, Technological advances in ®ne abrasive processes, Ann. CIRP 46 (2) (1997) 545±589. [2] J. Peters, R. Aerens, Optimization procedure of three phase grinding cycles of a series without intermediate dressing, Ann. CIRP 29 (1) (1980) 195±199. [3] S. Malkin, Y. Koren, Optimization infeed control for accelerated spark-out in plunge grinding, ASME J. Eng. Ind. 106 (1984) 70±74. [4] H.K. Toenshoff, M. Zinngrebe, M. Kemmerling, Optimization of internal grinding by microcomputer-based force control, Ann. CIRP 35 (1) (1986) 293±296. [5] G. Xiao, S. Malkin, K. Danai, Autonomous system for multistage cylindrical grinding, ASME J. Dyn. Syst. Meas Control 115 (1993) 667±672. [6] G. Xiao, S. Malkin, On-line optimization for internal grinding, Ann. CIRP 45 (1) (1996) 287±292. [7] I. Inasaki, Monitoring and optimization of internal grinding process, Ann. CIRP 40 (1) (1991) 359±362. [8] H. Kato, Y. Nakano, Transfer of roundness error from center and center hole to workpiece in cylindrical grinding and its control, Ann. CIRP 34 (1) (1985) 287±290. [9] Y.S. Liao, L.C. Shiang, Computer simulation of self-excited and forced vibrations in the external cylindrical plunge grinding process, ASME J. Eng. Ind. 113 (1991) 297±304. [10] K.H. Kim, K.F. Eman, S.M. Wu, Development of a forecasting compensatory control system for cylindrical grinding, ASME J. Eng. Ind. 109 (1987) 385±391. [11] E. Salje, H.-H. Damlos, H. Mohlen, Internal grinding of high strength ceramic workpiece materials with diamond grinding wheels, Ann. CIRP 34 (1) (1985) 263±266. [12] T. Nakagawa, Y. Hagiuda, K. Karimori, Cast iron bonded diamond grinding tool and its application to hard materials, in: Proceedings of the Fifth ICPE, Tokyo, 1984. [13] K. Suzuki, T. Uematsu, T. Nakagawa, On-machine truing/dressing of metal bond grinding wheels by electric-discharge machining, Ann. CIRP 36 (1) (1987) 115±118. [14] J. Tamaki, K. Takahashi, M. Kubota, Electrical dressing of metal bonded grinding wheels, in: Proceedings of the ISEM-X, 1992, pp. 510±515. [15] H. Ohmori, T. Nakagawa, Mirror surface grinding of silicon wafers with electrolytic in-process dressing, Ann. CIRP 39 (1) (1990) 329±332. [16] H. Ohmori, Electrolytic in-process dressing (ELID) gridning technique for ultraprecision mirror surface machining, Int. J. Jpn. Soc. Prec. Eng. 26 (4) (1992) 273±278. [17] H. Ohmori, I. Takahashi, B.P. Bandyopadhyay, Ultra-precision grinding of structural ceramics by electrolytic in-process dressing (ELID) grinding, J. Mater. Process. Technol. 57 (1996) 272±277.