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A comparative study: polishing characteristics and its mechanisms of three vibration modes in vibration-assisted magnetic abrasive polishing
International Journal of Machine Tools & Manufacture 44 (2004) 383–390 www.elsevier.com/locate/ijmatool
A comparative study: polishing characteristics and its mechanisms of three vibration modes in vibration-assisted magnetic abrasive polishing Shaohui Yin a,, Takeo Shinmura b a
Graduate School of Engineering, Utsunomiya University, (now Riken Venture, The Nexsys Corporation), 1-7-13 Kaga, Itabashi-ku, Tokyo321-8585, Japan b Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan Received 19 May 2003; received in revised form 6 October 2003; accepted 15 October 2003
Abstract The automatic precision polishing technique of three-dimensional complicated micro-curved surfaces of components in extremely low surface roughness and high efficiency is greatly demanded by advanced industrial fields. The existing polishing methods have great difficulty in satisfying these demands. Therefore, three modes (horizontal vibration, vertical vibration and compound vibration) of vibration-assisted magnetic abrasive polishing processes have been developed. Previous research focused on each polishing characteristic. The aims of this paper are to characterize effects of vibration of workpiece on magnetic field, polishing pressure, in-process abrasive behavior and polishing performances in three vibration modes and to describe their machining mechanism. Furthermore, a realization of efficient polishing of a 3D micro-curved surface was confirmed to be possible by the process. # 2003 Elsevier Ltd. All rights reserved. Keywords: Magnetic abrasive polishing; Vibration-assisted; Pulsed polishing pressure; Surface roughness; Stock removal
1. Introduction Today’s manufacturing industry is facing challenges from stringent design requirements for high surface quality, high precision and complex shapes. Automatic precision polishing of three-dimensional micro-curved surfaces of various components in extremely low surface roughness and high form accuracy is greatly demanded by advanced industrial fields, especially in ultra precision manufacturing industry. Most product manufacturing procedures have been automated by the introduction of CNC machine tools and CAD/CAM system, however, the complicated micro-curved surface polishing must be completed by highly skilled workers. Many attempts have been made to eliminate the handwork, various types of automatic polishing devices and polishing robots have been developed. However, these devices suffer from some drawbacks, mainly,
Corresponding author. Fax: +81-28-689-6057. E-mail address: [email protected] (S. Yin).
0890-6955/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2003.10.002
most conventional polishing tools are rigid, they need to be manufactured precisely in accordance with the shape of a machined surface, or need to be controlled precisely tool-path by means of CNC systems in polishing curved surfaces. They are rather expensive, may be applicable for restricted part geometry, but not for 3D micro-curved surface. In order to overcome these problems, some researchers proposed that a magnetic abrasive polishing process is one potential polishing technique for 3D microcurved surfaces because its tool (magnetic brush) is flexible in navigation on the surface profile of the workpiece. A series of researches on magnetic abrasive polishing technique have been conducted in recent years. Shinmura [1–4] proposed ‘‘Plane Magnetic Abrasive Polishing Process’’ in which the magnetic pole rotates at a high speed and the workpiece is fed. Anzai [5] also developed a polishing process to keep a magnetic pole standing in the normal direction against the machined surface using a 5-axis machining center. Suzuki proposed a magnetic abrasive polishing method
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S. Yin, T. Shinmura / International Journal of Machine Tools & Manufacture 44 (2004) 383–390
utilizing centrifugal force [6]. Kim [7] developed a twostage magnetic polishing of a curved surface using an abrasive wheel and a magnetic brush. Despite the potential advantages of magnetic abrasive polishing processes, there are several impediments to its more widespread utilization. The key issue is its lower polishing efficiency for polishing complicated 3D curved surfaces. To overcome the problem, one of the authors [8] put forward ‘‘horizontal (x) vibrationassisted magnetic abrasive polishing’’ (hereafter referred to as ‘X mode’) in which the workpiece is vibrated in a horizontal direction. Natsume [9] developed the same method to polish grooves of stainless steel. The authors explored surface polishing characteristics and mechanisms of the polishing mode [10]. However, the X mode is only suitable for polishing 2D curved surfaces from obtained research results. In order to polish 3D micro-curved surfaces with high-efficiency and high precision, we then put forward ‘‘Vertical Vibration-assisted Mode’’ (hereafter referred to as ‘Z mode’) and ‘‘Vertical and horizontal Vibration -assisted Mode’’ (hereafter referred to as ‘ZX mode’) [11]. Three vibration modes (X,Z and ZX mode) have been proposed so far. However, their machining characteristics and mechanisms are not known thoroughly. Depending upon the vibration mode and process parameters, the magnetic brush (abrasive) may exhibit different behavior. In magnetic abrasive polishing processes, the abrasive behavior is undoubtly related to the magnetic field because the abrasive is held magnetically at the polishing area, the magnetic field, or more specifically, the magnetic flux density and the gradient in the polishing area, determines the abrasive configuration and affects the magnetic force acting on the abrasive which, correspondingly, affects the polishing pressure acting on the machined surface. In the vibration-assisted process, polishing characteristics and machining mechanisms should be different from the conventional process because the magnetic field, polishing pressure and relative motion between the abrasive and the workpiece are changed periodically resulting from the vibration of workpiece. However, vibration mode, magnetic field, polishing pressure, machining characteristics, and their relation to this process have not been thoroughly investigated. Therefore, the aim of the present research is to characterize the effects of vibration of workpiece on magnetic field, polishing pressure, in-process abrasives behavior and polishing performances in three vibration modes, compare their differences, understand each machining mechanism, and confirm the possibility of polishing automation for 3D micro-curved surfaces.
2. Processing principle and experimental setup Fig. 1 shows a schematic of the vibration-assisted magnetic abrasive polishing process. The magnetic pole rotates at a speed and the workpiece may be vibrated in vertical (Z), horizontal (X), or ZX directions. After magnetic abrasives are introduced into the machining clearance between the tip of the magnetic pole and the workpiece, the abrasives are magnetically attracted by the magnetic pole and rotated with the magnetic pole. Relative motion is obtained consisting of vibration and rotation between the magnetic brush and workpiece. The polishing apparatus was installed on a table of a vertical milling machine comprising of an electromagnetic coil, core, upper yoke, magnetic pole, workpiece, vibration table, X-Y stage, and lower yoke. The pole with 6 grooves in the experiment was designed to give the magnetic abrasive high magnetic polishing force to achieve efficient surface polishing. Automatic polishing was carried out with this experimental apparatus using mixed magnetic abrasives.
3. Effects of vibration on the magnetic field The magnetic field distribution over the polishing area has a profound impact on the polishing pressure distribution of the magnetic abrasives, which controls the polishing characteristics. Therefore, it is necessary to understand the magnetic field distribution over the polishing area. The magnetic field distribution involves field intensity and gradient at the polishing zone. These parameters are affected by the size, shape, and material of the magnetic poles, the supplied current into the coils, as well as the relative position between the magnetic pole and the workpiece. The distribution defines
Fig. 1.
Schematic of experimental setup.
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the abrasive configuration at the polishing zone, controlling the magnetic force distribution of the abrasive. In conventional plane polishing process (nonvibration mode), the magnetic field distribution may be regarded as static because the workpiece is only fed slowly and the working clearance is invariable, Fig. 2(a) shows the variation of magnetic flux density on the machined surface in the x-direction in the case of the non-vibration mode, measured by a hall sensor. Curves I, II and III express the magnetic flux density in the case of working clearance, 2, 3 and 4 mm, respectively. Magnetic flux density increases toward the center of the pole and remains level within the outer diameter of the pole(x¼ 6 mm), and shows a sharp drop at the pole edge. The peak value in the magnetic flux density Bp increases with a decrease in working clearance S, the peak value of magnetic flux density Bp can be estimated by the following equation: Bp ¼ l0 NI=S
ð1Þ
where l0 is the magnetic permeability in vacuum, N is
385
the number of turns in the coil and I is the magnetizing current. In the Z mode, the working clearance is changed periodically, resulting in a change in the magnetic field distribution. For an example, in the case of working clearance 3 mm and vibration amplitude in the z direction 1 mm, the actual working clearance varies periodically from 2 mm to 4 mm; as a result, the maximum magnetic flux density is changed periodically from 0.8 T to 0.4 T. The value of B varies within curve I (in the case of 2 mm working clearance) and curve III (in the case of 4 mm working clearance) in the non-vibration mode as shown in Fig. 2(a). Fig. 2(b) shows the magnetic field distribution in the x-direction in the X mode (vibration amplitude 3 mm). Because the situation of workpiece is cyclically changed in the x-direction, the magnetic flux density of a given point at the workpiece is cyclically changed. The range of change is mainly affected by vibration amplitude. In the ZX mode, both the working clearance and the situation in the x-direction are changed cyclically, resulting in a change in the magnetic field distribution.
4. Effects of vibration on the polishing pressure and its distribution The polishing pressure, more specifically, the value, distribution and wave shape of dynamic polishing pressure has a profound impact on machining characteristics in polishing processes. In conventional magnetic abrasive processes, the distribution of magnetic flux density B decides the distribution of polishing pressure P; their relation can be expressed by the following equation [2]: B2 1 P¼ 1 ð2Þ l1 2l0
Fig. 2.
Magnetic flux density distribution in the x direction.
where l1 is the relative magnetic permeability of magnetic brush consisting of ferromagnetic material (pure iron), abrasive particle (alumina) and air. As shown in Section 3, value, distribution and wave shape of polishing pressure should be changed because of variation of magnetic field distribution resulting from vibration. On the other hand, because of the effects of mechanical force, polishing pressure must also be changed considerably. To verify this, the polishing pressure was measured by using a pressure gauge (PS-5KB) and a strain amplifier (DPM-713B) (Kyowa Electronic Instrument Co. Ltd). The measurement value was output by a thermal recorder (WR7700) (Graphtec Coda Co). A pressure measuring system is shown is Fig. 3, measuring conditions are shown as Table 1. Fig. 4 shows a variation of polishing pressure in the X-direction in the four polishing modes. In the
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Fig. 3.
case of non-vibration mode as shown in Fig. 5(a). Although the pressure remains level within the pole, as in the X mode and non-vibration mode, the pressure decreased sharply outside of the pole. In the ZX mode, as shown in Fig. 4(d), the polishing pressure is much larger than that of the non-vibration mode resulting from z-direction vibration, and smaller than that of the Z mode resulting from x-direction vibration. It can be seen that the pressure distribution in the x direction is more uniform, compared to the z mode. The pressure distribution is similar than the magnetic field distribution as illustrated in Section 3. Polishing pressure and its distribution have different impacts on the polishing characteristics in the three vibration modes.
Schematic of measuring system.
non-vibration mode and the x mode, the pressure remains level within the diameter of the magnetic pole and decreases slowly from the pole edge as shown in Fig. 4(a) and (b), respectively. Pressure in the X mode distributes uniformly, and is slightly smaller than that of the case of non-vibration. In the Z mode, with vertical vibration-assisted, as shown in Fig. 4(c), pulsed pressure is produced, the value of the polishing pressure is changed periodically. The polishing pressure can be modeled as symmetrical triangular pulsed pressure shown in Fig. 5(b); its value can be estimated by the following equation in Z mode: 1 1 1 cos nxtc 8Pp X tc 2 Pp þ cosnxt PðtÞ ¼ 2T Ttc n¼1 n2 x2
ð3Þ
where Pp is peak value, T is the vibration period, x is the vibration angular velocity, t is time, tc is net polishing time. Peak value Pp is decided by the magnetic field, working clearance and vibration parameters. The peak and the mean value of the pulsed polishing pressure (Fig. 5(b)) increases remarkably compared with the Table 1 Measuring conditions Workpiece
Vibration of workpiece Magnetic pole
Magnetic abrasives
Lubricant
Stainless steel SUS304, 38 60 mm, thickness: 1.2 mm Feed speed: 0 mm/min Amplitude: 1 mm in Z direction, 3 mm in X direction Frequency: 6 Hz in z and x directions Material: SS400 Size: 12 mm in dia., with six crossing grooves (width: 1mm, depth: 2 mm) Circumferential speed: 1.3 m/min Mixed type magnetic abrasives Iron particles: 4 g supplied, 510 lm in mean dia. WA magnetic abrasive: 1 g supplied, 80 lm in mean dia. Not be added
5. Effects of vibration on abrasive behavior 5.1. X mode In the X mode, x directional vibration offers the following advantages: (a) promotes relative motion of magnetic abrasives against the machined surface needed for the polishing operation by changing the value of relative velocity; (b) abrasives cutting edges may scratch the material in many directions in intermittent cutting mode, the cross-cutting marks are generated and stock removal per working distance is increased by the changing direction of relative velocity [10]; (c) promotes stirring effects. 5.2. Z mode The main feature of the Z mode is the cyclic compression deformation of the magnetic brush, which causes its repetitive movements in the Z direction. Owing to z directional vibration, the abrasives of the magnetic brush are exerted at a higher pulsed pressure against the machined surface, and may closely follow the machined surface. Utilizing this advantage, we may polish the micro-curved surface. i.e. the continued action of decompressing and compressing of the magnetic polishing brush promotes the magnetic abrasives into all machined surfaces, combined inner surface, corner and groove surface. Furthermore, the z directional vibration may also promote stirring effect and cross-cutting effects of the abrasives. Therefore, either flat or extremely complicated micro-curved surfaces can be nicely polished, the following experiments in the next section verify this viewpoint. Fig. 6 shows deformation behavior of the magnetic brush resulting from z directional vibration. When the workpiece is at a low position, the magnetic brush is an easy-to-deformation flexible brush; when the workpiece vibrates from a lower position (see Fig. 6(a)) to a higher position (see
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Fig. 4. Measured polishing pressure distribution.
Fig. 6(b)), working clearance get smaller, the magnetic brush is compressed, the magnetic brush spreads to sides and the abrasives are moved in a lateral direction, the bottom surface and the side surface are possible to be polished. Magnetic flux density on a machined surface and magnetic force acting on the abrasives increase in the compressed state, resulting in an increase of the filling density of the particle group in the brush, furthermore, it increases the number of abrasives participating cutting, obtaining higher polishing efficiency. 5.3. ZX mode
effects of increase in relative motion of abrasives against the machined surface.
6. Effects of vibration on polishing characteristics 6.1. Generation of machined surface In order to compare surface generation processes in these modes, we conducted the following experiment. A brass plate (C2680) was used as the workpiece, the workpiece was not fed. The cross-sectional shape of the polished surface is shown in Fig. 7. With z directional vibration (Fig. 7(b),(d)), the machined depth increases
Resulting from two (z and x) directional vibration, the ZX mode demonstrates characteristics of the above-mentioned two mode, i.e. it offers the following advantages: (a) effects of pulsed vibration cutting; (b) effects of increase in pressure, (c) effects of strengthening stirring behavior; (d) effects of cross-cutting; (e)
Fig. 5. Measured pressures in the center of magnetic brushes and their models.
Fig. 6.
Schematic view of deformation of a magnetic brush.
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Fig. 7.
Cross-sectional shape of a polished surface.
largely resulting from an increase in pressure and effect of pulsed pressure. With x directional vibration, because velocity ‘zero’ point at the center of brush do not exist, material at the center of the magnetic brush will also be removed (Fig. 7(c),(d)). In conventional polishing using a rigid polisher, material removal conforms to Preston’s law, i.e. it is proportional to the relative velocity, polishing pressure and polishing time. In magnetic abrasive polishing using a flexible magnetic brush, although one cannot identify if Preston’s law is conformed to, it is generally agreed from the experimental results that material removal should be closely related to the polishing pressure, the relative velocity, and yet, vibration in the z and x direction have a significant impact on those parameters, and then exert considerably influence to machined surface generation.
polishing pressure increase remarkably, owing to demonstrates effects of pulsed vibration cutting, and enhanced behavior of deformation of the magnetic brushes, it results in higher stock removal than that in the X mode. In the ZX mode, the higher stock removal is attributed to the combined effects of pulsed polishing pressure resulting from z directional vibration and cross cutting effects resulting from x directional vibration, although it demonstrates cross cutting effects, the polishing pressure is lower than that in the Z mode, therefore, there is not a remarkable difference Table 2 Plane polishing experimental conditions Workpiece
1. Brass C2680, 38 60 mm, thickness : 1.2 mm, before polishing: 3~4lmRy (Fig. 7) 2. Stainless steel SUS304, 38 60 mm, thickness: 1.2 mm, before polishing: 3–4 lmRy (Figs. 8 and 9)
Vibration of workpiece
Amplitude: 1 mm in z direction, 3 mm in x direction Frequency: 6 Hz in z and x directions 0 mm/min (Fig. 7), 17 mm/min (Figs. 8 and 9) 25 mm 32 m/min (Figs. 8 and 9)
6.2. Plane polishing characteristics The plane polishing experimental conditions are shown in Table 2. Fig. 8 shows the differences between surface roughness (Ry) and stock removal (M) in three modes in polishing SUS304 stainless steel, in the figure, ‘Non’, ‘ZX’, ‘Z’, and ‘X’ express the non-vibration mode, ZX mode, Z mode, and X mode, respectively. It can be seen that stock removal in the case of the X, Z and ZX modes are much higher than the non-vibration mode, in order, MðzxÞ6MðzÞ >MðxÞ >MðnonÞ. In the X mode, the cross-cutting marks are generated and stock removal per working distance is increased [10], therefore, it results in higher stock removal than that in the non-vibration mode; in the Z mode, pulsed polishing pressure is produced, the mean value and the peak of
Feed speed of workpiece Stroke of workpiece Magnetic pole circumferential speed Magnetic flux density 0.6 T Working clearance 3 mm Magnetic abrasives Mixed type magnetic abrasives Iron particles: 4 g supplied, 330 lm (Figs. 8 and 9) WA magnetic abrasive: 1 g supplied, 80 lm in mean dia. Lubricant Straight oil type grinding fluid: 2 mL
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Fig. 8. steel.
Effects of vibration modes in polishing SUS304 stainless
between stock removal in the ZX mode and that in the Z mode (Table 3). On the other hand, in the non-vibration mode, with an initial surface of 3 lmRy pre-polishing gather to 0.35 lmRy. In the Z mode, gathered to 0.53 lmRy, the machined surface was somewhat rougher than that in the non-vibration mode. In the X mode, the machined surface was improved to 0.25 lmRy. In the ZX mode, the machined surface was improved near to 0.25 lmRy in the x direction, and 0.32 lmRy in the y direction, generate a better finish compared with the case of the Z mode and non-vibration mode. Fig. 9 shows microscope photographs of polished surfaces before and after polishing. The results of this experiment show that the ZX mode can produce a better finish than the
Table 3 3d Micro-curved surface polishing experimental conditions Workpiece Vibration of workpiece Magnetic pole circumferential speed Feed speed of workpiece Working clearance Magnetic abrasives
Lubricant
Punch, M50P Cemented carbide, before polishing: 0.8 lmRy Amplitude: 1 mm in Z direction Frequency: 6 Hz 22 m/min
0 mm/min 3 mm Mixed type magnetic abrasives Iron particles: 3 g supplied, 149 lm in mean dia. Diamond powder: 2 g supplied, 2–5 lm in mean dia. Straight oil type grinding fluid: 2 mL
389
Fig. 9. Microscopic photographs of the polished surface of SUS304 stainless steel.
Z mode. In the ZX mode, polishing pressure is improved to distribute uniformly compared with the Z mode as shown in Fig. 4, therefore, obtaining a good finish; on the other hand, different cross-hatching patterns can be generated by varying the rotational speed and vibration amplitude and frequency, abrasive cutting edges scratch the material in many directions in intermittent cutting mode, this also results in a reduction in surface roughness. 6.3. 3D micro-curved surface polishing characteristics The polishing experiment of 3D curved surfaces was conducted using the Z mode. A small punch was used as the workpiece, its material was cemented carbide (M50P, HV1300), the machined surface was a 3D complicated curved surface, the maximum depth in the z direction was approximately 3 mm, initial surface roughness was approximately 0.8 lm Ry. The polishing experimental conditions are shown in Table 3. Fig. 10 shows sample photographs before and after polishing for 15 min, surface roughness at the tip of convex surface (a), bottom of inner surface (b) and bottom of groove surface(c) was measured after cutting workpiece apart. Ry value at (a), (b) and (c) portions were improved to 0.2 lm, 0.25 lm and 0.35 lm respectively. It can be seen that the whole surface was polished, the roughness value of the groove surface was somewhat larger than the inner surface and the concave surface. Without vibration-assistance, polishing the bottom of the inner surface and the groove surface was very difficult, only the plane and a part of the inner surface were polished to some extent.
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Fig. 10. Sample of cemented carbide.
7. Conclusions
References
The results of this research can be summarized as follows.
[1] T. Shinmura, K. Takazawa, E. Hatano, Study on magnetic abrasive process (Application to plane finishing), Bull. of the JSPE 19 (4) (1985) 289–294. [2] T. Shinmura, T. Aizawa, Development of plane magnetic abrasive finishing apparatus and its finishing performance (2nd Report, Finishing apparatus using a stationary type electromagnet), J. Jpn. Soc. Prec. Eng. 54 (5) (1988) 928–933 (in Japanese). [3] T. Shinmura, Study on plane magnetic abrasive finishing (3rd Report on the finishing characteristics of non-ferromagnetic substance), J. Jpn. Soc. Prec. Eng. 55 (7) (1989) 1271–1276, (in Japanese). [4] T. Shinmura, Study on free-form surface finishing by magnetic abrasive finishing process (1st Report, Fundamental experiments), Trans. Jpn. Soc. Mech. Eng. 53 (485C) (1986) 202–208, (in Japanese). [5] M. Anzai, T. Yoshida, T. Nakagawa, Magnetic abrasive automatic polishing of curved surface, RIKEN Review 12 (1996) 15–16. [6] K. Suzuki, T. Uematsu, Development of new surface finishing methods, Journal of the 3rd International Conference on Precision Surface Finishing and Burr Technology (1994) 116–122. [7] J. Kim, M. Choi, Study on magnetic polishing of free-form surface, International Journal of Machine Tools and Manufacture 37 (8) (1997) 1179–1187. [8] T. Shinmura, Y. Takeshi, Study of free form surface finishing by the application of magnetic abrasive machining, Proceedings of JSME Annual Meeting (IV), (in Japanese), (1997) 220–221. [9] M. Natsume, T. Shinmura, K. Sakaguchi, Study of magnetic abrasive machining by use of work vibration system (characteristics of plane finishing and application to the inside finishing of groove), Trans. Jpn. Soc. Mech. Eng. 64 (627C) (1998) 4447–4452, (in Japanese). [10] S. Yin, T. Shinmura, Study of vibration-assisted magnetic abrasive finishing process (Effects of vibration on cylindrical finishing characteristics and its mechanism), Trans. Jpn. Soc. Mech. Eng. 67 (661C) (2001) 3006–3012, (in Japanese). [11] S. Yin, T. Shinmura, Study on vibration-assisted magnetic abrasive finishing process using pulsed finishing pressure, Proceedings of the 10th International Conference on Precision Engineering, (2001) 506–510.
1. In this paper, the effects of three vibration modes on magnetic field, polishing pressure, in-process abrasive behavior and polishing performances were discussed comparatively. Furthermore, their machining mechanisms were described. 2. In the X mode, x directional vibration can help to promote the relative motion of magnetic abrasives against the machined surface and pressure distribution uniformity in the polishing zone, a smoother surface may be obtained. 3. In the Z mode, with z directional vibration-assisted, pulsed polishing pressure may be produced and the value of pressure may be increased hugely compared with the non-vibration mode and the X mode. 4. The polishing experiment of plane demonstrated the Z mode leads to higher polishing efficiency but a little rougher surface than the conventional process; the X mode can obtain a smoother surface; however, the ZX mode can obtain high polishing efficiency and a good finish simultaneously. 5. The polishing experiment demonstrated the realization that efficient polishing of 3D micro-curved surface is possible by this process.
Acknowledgements This research was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society, the authors would like to express their sincere gratitude.
A comparative study: polishing characteristics and its mechanisms of three vibration modes in vibration-assisted magnetic abrasive polishing Shaohui Yin a,, Takeo Shinmura b a
Graduate School of Engineering, Utsunomiya University, (now Riken Venture, The Nexsys Corporation), 1-7-13 Kaga, Itabashi-ku, Tokyo321-8585, Japan b Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan Received 19 May 2003; received in revised form 6 October 2003; accepted 15 October 2003
Abstract The automatic precision polishing technique of three-dimensional complicated micro-curved surfaces of components in extremely low surface roughness and high efficiency is greatly demanded by advanced industrial fields. The existing polishing methods have great difficulty in satisfying these demands. Therefore, three modes (horizontal vibration, vertical vibration and compound vibration) of vibration-assisted magnetic abrasive polishing processes have been developed. Previous research focused on each polishing characteristic. The aims of this paper are to characterize effects of vibration of workpiece on magnetic field, polishing pressure, in-process abrasive behavior and polishing performances in three vibration modes and to describe their machining mechanism. Furthermore, a realization of efficient polishing of a 3D micro-curved surface was confirmed to be possible by the process. # 2003 Elsevier Ltd. All rights reserved. Keywords: Magnetic abrasive polishing; Vibration-assisted; Pulsed polishing pressure; Surface roughness; Stock removal
1. Introduction Today’s manufacturing industry is facing challenges from stringent design requirements for high surface quality, high precision and complex shapes. Automatic precision polishing of three-dimensional micro-curved surfaces of various components in extremely low surface roughness and high form accuracy is greatly demanded by advanced industrial fields, especially in ultra precision manufacturing industry. Most product manufacturing procedures have been automated by the introduction of CNC machine tools and CAD/CAM system, however, the complicated micro-curved surface polishing must be completed by highly skilled workers. Many attempts have been made to eliminate the handwork, various types of automatic polishing devices and polishing robots have been developed. However, these devices suffer from some drawbacks, mainly,
Corresponding author. Fax: +81-28-689-6057. E-mail address: [email protected] (S. Yin).
0890-6955/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2003.10.002
most conventional polishing tools are rigid, they need to be manufactured precisely in accordance with the shape of a machined surface, or need to be controlled precisely tool-path by means of CNC systems in polishing curved surfaces. They are rather expensive, may be applicable for restricted part geometry, but not for 3D micro-curved surface. In order to overcome these problems, some researchers proposed that a magnetic abrasive polishing process is one potential polishing technique for 3D microcurved surfaces because its tool (magnetic brush) is flexible in navigation on the surface profile of the workpiece. A series of researches on magnetic abrasive polishing technique have been conducted in recent years. Shinmura [1–4] proposed ‘‘Plane Magnetic Abrasive Polishing Process’’ in which the magnetic pole rotates at a high speed and the workpiece is fed. Anzai [5] also developed a polishing process to keep a magnetic pole standing in the normal direction against the machined surface using a 5-axis machining center. Suzuki proposed a magnetic abrasive polishing method
384
S. Yin, T. Shinmura / International Journal of Machine Tools & Manufacture 44 (2004) 383–390
utilizing centrifugal force [6]. Kim [7] developed a twostage magnetic polishing of a curved surface using an abrasive wheel and a magnetic brush. Despite the potential advantages of magnetic abrasive polishing processes, there are several impediments to its more widespread utilization. The key issue is its lower polishing efficiency for polishing complicated 3D curved surfaces. To overcome the problem, one of the authors [8] put forward ‘‘horizontal (x) vibrationassisted magnetic abrasive polishing’’ (hereafter referred to as ‘X mode’) in which the workpiece is vibrated in a horizontal direction. Natsume [9] developed the same method to polish grooves of stainless steel. The authors explored surface polishing characteristics and mechanisms of the polishing mode [10]. However, the X mode is only suitable for polishing 2D curved surfaces from obtained research results. In order to polish 3D micro-curved surfaces with high-efficiency and high precision, we then put forward ‘‘Vertical Vibration-assisted Mode’’ (hereafter referred to as ‘Z mode’) and ‘‘Vertical and horizontal Vibration -assisted Mode’’ (hereafter referred to as ‘ZX mode’) [11]. Three vibration modes (X,Z and ZX mode) have been proposed so far. However, their machining characteristics and mechanisms are not known thoroughly. Depending upon the vibration mode and process parameters, the magnetic brush (abrasive) may exhibit different behavior. In magnetic abrasive polishing processes, the abrasive behavior is undoubtly related to the magnetic field because the abrasive is held magnetically at the polishing area, the magnetic field, or more specifically, the magnetic flux density and the gradient in the polishing area, determines the abrasive configuration and affects the magnetic force acting on the abrasive which, correspondingly, affects the polishing pressure acting on the machined surface. In the vibration-assisted process, polishing characteristics and machining mechanisms should be different from the conventional process because the magnetic field, polishing pressure and relative motion between the abrasive and the workpiece are changed periodically resulting from the vibration of workpiece. However, vibration mode, magnetic field, polishing pressure, machining characteristics, and their relation to this process have not been thoroughly investigated. Therefore, the aim of the present research is to characterize the effects of vibration of workpiece on magnetic field, polishing pressure, in-process abrasives behavior and polishing performances in three vibration modes, compare their differences, understand each machining mechanism, and confirm the possibility of polishing automation for 3D micro-curved surfaces.
2. Processing principle and experimental setup Fig. 1 shows a schematic of the vibration-assisted magnetic abrasive polishing process. The magnetic pole rotates at a speed and the workpiece may be vibrated in vertical (Z), horizontal (X), or ZX directions. After magnetic abrasives are introduced into the machining clearance between the tip of the magnetic pole and the workpiece, the abrasives are magnetically attracted by the magnetic pole and rotated with the magnetic pole. Relative motion is obtained consisting of vibration and rotation between the magnetic brush and workpiece. The polishing apparatus was installed on a table of a vertical milling machine comprising of an electromagnetic coil, core, upper yoke, magnetic pole, workpiece, vibration table, X-Y stage, and lower yoke. The pole with 6 grooves in the experiment was designed to give the magnetic abrasive high magnetic polishing force to achieve efficient surface polishing. Automatic polishing was carried out with this experimental apparatus using mixed magnetic abrasives.
3. Effects of vibration on the magnetic field The magnetic field distribution over the polishing area has a profound impact on the polishing pressure distribution of the magnetic abrasives, which controls the polishing characteristics. Therefore, it is necessary to understand the magnetic field distribution over the polishing area. The magnetic field distribution involves field intensity and gradient at the polishing zone. These parameters are affected by the size, shape, and material of the magnetic poles, the supplied current into the coils, as well as the relative position between the magnetic pole and the workpiece. The distribution defines
Fig. 1.
Schematic of experimental setup.
S. Yin, T. Shinmura / International Journal of Machine Tools & Manufacture 44 (2004) 383–390
the abrasive configuration at the polishing zone, controlling the magnetic force distribution of the abrasive. In conventional plane polishing process (nonvibration mode), the magnetic field distribution may be regarded as static because the workpiece is only fed slowly and the working clearance is invariable, Fig. 2(a) shows the variation of magnetic flux density on the machined surface in the x-direction in the case of the non-vibration mode, measured by a hall sensor. Curves I, II and III express the magnetic flux density in the case of working clearance, 2, 3 and 4 mm, respectively. Magnetic flux density increases toward the center of the pole and remains level within the outer diameter of the pole(x¼ 6 mm), and shows a sharp drop at the pole edge. The peak value in the magnetic flux density Bp increases with a decrease in working clearance S, the peak value of magnetic flux density Bp can be estimated by the following equation: Bp ¼ l0 NI=S
ð1Þ
where l0 is the magnetic permeability in vacuum, N is
385
the number of turns in the coil and I is the magnetizing current. In the Z mode, the working clearance is changed periodically, resulting in a change in the magnetic field distribution. For an example, in the case of working clearance 3 mm and vibration amplitude in the z direction 1 mm, the actual working clearance varies periodically from 2 mm to 4 mm; as a result, the maximum magnetic flux density is changed periodically from 0.8 T to 0.4 T. The value of B varies within curve I (in the case of 2 mm working clearance) and curve III (in the case of 4 mm working clearance) in the non-vibration mode as shown in Fig. 2(a). Fig. 2(b) shows the magnetic field distribution in the x-direction in the X mode (vibration amplitude 3 mm). Because the situation of workpiece is cyclically changed in the x-direction, the magnetic flux density of a given point at the workpiece is cyclically changed. The range of change is mainly affected by vibration amplitude. In the ZX mode, both the working clearance and the situation in the x-direction are changed cyclically, resulting in a change in the magnetic field distribution.
4. Effects of vibration on the polishing pressure and its distribution The polishing pressure, more specifically, the value, distribution and wave shape of dynamic polishing pressure has a profound impact on machining characteristics in polishing processes. In conventional magnetic abrasive processes, the distribution of magnetic flux density B decides the distribution of polishing pressure P; their relation can be expressed by the following equation [2]: B2 1 P¼ 1 ð2Þ l1 2l0
Fig. 2.
Magnetic flux density distribution in the x direction.
where l1 is the relative magnetic permeability of magnetic brush consisting of ferromagnetic material (pure iron), abrasive particle (alumina) and air. As shown in Section 3, value, distribution and wave shape of polishing pressure should be changed because of variation of magnetic field distribution resulting from vibration. On the other hand, because of the effects of mechanical force, polishing pressure must also be changed considerably. To verify this, the polishing pressure was measured by using a pressure gauge (PS-5KB) and a strain amplifier (DPM-713B) (Kyowa Electronic Instrument Co. Ltd). The measurement value was output by a thermal recorder (WR7700) (Graphtec Coda Co). A pressure measuring system is shown is Fig. 3, measuring conditions are shown as Table 1. Fig. 4 shows a variation of polishing pressure in the X-direction in the four polishing modes. In the
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Fig. 3.
case of non-vibration mode as shown in Fig. 5(a). Although the pressure remains level within the pole, as in the X mode and non-vibration mode, the pressure decreased sharply outside of the pole. In the ZX mode, as shown in Fig. 4(d), the polishing pressure is much larger than that of the non-vibration mode resulting from z-direction vibration, and smaller than that of the Z mode resulting from x-direction vibration. It can be seen that the pressure distribution in the x direction is more uniform, compared to the z mode. The pressure distribution is similar than the magnetic field distribution as illustrated in Section 3. Polishing pressure and its distribution have different impacts on the polishing characteristics in the three vibration modes.
Schematic of measuring system.
non-vibration mode and the x mode, the pressure remains level within the diameter of the magnetic pole and decreases slowly from the pole edge as shown in Fig. 4(a) and (b), respectively. Pressure in the X mode distributes uniformly, and is slightly smaller than that of the case of non-vibration. In the Z mode, with vertical vibration-assisted, as shown in Fig. 4(c), pulsed pressure is produced, the value of the polishing pressure is changed periodically. The polishing pressure can be modeled as symmetrical triangular pulsed pressure shown in Fig. 5(b); its value can be estimated by the following equation in Z mode: 1 1 1 cos nxtc 8Pp X tc 2 Pp þ cosnxt PðtÞ ¼ 2T Ttc n¼1 n2 x2
ð3Þ
where Pp is peak value, T is the vibration period, x is the vibration angular velocity, t is time, tc is net polishing time. Peak value Pp is decided by the magnetic field, working clearance and vibration parameters. The peak and the mean value of the pulsed polishing pressure (Fig. 5(b)) increases remarkably compared with the Table 1 Measuring conditions Workpiece
Vibration of workpiece Magnetic pole
Magnetic abrasives
Lubricant
Stainless steel SUS304, 38 60 mm, thickness: 1.2 mm Feed speed: 0 mm/min Amplitude: 1 mm in Z direction, 3 mm in X direction Frequency: 6 Hz in z and x directions Material: SS400 Size: 12 mm in dia., with six crossing grooves (width: 1mm, depth: 2 mm) Circumferential speed: 1.3 m/min Mixed type magnetic abrasives Iron particles: 4 g supplied, 510 lm in mean dia. WA magnetic abrasive: 1 g supplied, 80 lm in mean dia. Not be added
5. Effects of vibration on abrasive behavior 5.1. X mode In the X mode, x directional vibration offers the following advantages: (a) promotes relative motion of magnetic abrasives against the machined surface needed for the polishing operation by changing the value of relative velocity; (b) abrasives cutting edges may scratch the material in many directions in intermittent cutting mode, the cross-cutting marks are generated and stock removal per working distance is increased by the changing direction of relative velocity [10]; (c) promotes stirring effects. 5.2. Z mode The main feature of the Z mode is the cyclic compression deformation of the magnetic brush, which causes its repetitive movements in the Z direction. Owing to z directional vibration, the abrasives of the magnetic brush are exerted at a higher pulsed pressure against the machined surface, and may closely follow the machined surface. Utilizing this advantage, we may polish the micro-curved surface. i.e. the continued action of decompressing and compressing of the magnetic polishing brush promotes the magnetic abrasives into all machined surfaces, combined inner surface, corner and groove surface. Furthermore, the z directional vibration may also promote stirring effect and cross-cutting effects of the abrasives. Therefore, either flat or extremely complicated micro-curved surfaces can be nicely polished, the following experiments in the next section verify this viewpoint. Fig. 6 shows deformation behavior of the magnetic brush resulting from z directional vibration. When the workpiece is at a low position, the magnetic brush is an easy-to-deformation flexible brush; when the workpiece vibrates from a lower position (see Fig. 6(a)) to a higher position (see
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Fig. 4. Measured polishing pressure distribution.
Fig. 6(b)), working clearance get smaller, the magnetic brush is compressed, the magnetic brush spreads to sides and the abrasives are moved in a lateral direction, the bottom surface and the side surface are possible to be polished. Magnetic flux density on a machined surface and magnetic force acting on the abrasives increase in the compressed state, resulting in an increase of the filling density of the particle group in the brush, furthermore, it increases the number of abrasives participating cutting, obtaining higher polishing efficiency. 5.3. ZX mode
effects of increase in relative motion of abrasives against the machined surface.
6. Effects of vibration on polishing characteristics 6.1. Generation of machined surface In order to compare surface generation processes in these modes, we conducted the following experiment. A brass plate (C2680) was used as the workpiece, the workpiece was not fed. The cross-sectional shape of the polished surface is shown in Fig. 7. With z directional vibration (Fig. 7(b),(d)), the machined depth increases
Resulting from two (z and x) directional vibration, the ZX mode demonstrates characteristics of the above-mentioned two mode, i.e. it offers the following advantages: (a) effects of pulsed vibration cutting; (b) effects of increase in pressure, (c) effects of strengthening stirring behavior; (d) effects of cross-cutting; (e)
Fig. 5. Measured pressures in the center of magnetic brushes and their models.
Fig. 6.
Schematic view of deformation of a magnetic brush.
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Fig. 7.
Cross-sectional shape of a polished surface.
largely resulting from an increase in pressure and effect of pulsed pressure. With x directional vibration, because velocity ‘zero’ point at the center of brush do not exist, material at the center of the magnetic brush will also be removed (Fig. 7(c),(d)). In conventional polishing using a rigid polisher, material removal conforms to Preston’s law, i.e. it is proportional to the relative velocity, polishing pressure and polishing time. In magnetic abrasive polishing using a flexible magnetic brush, although one cannot identify if Preston’s law is conformed to, it is generally agreed from the experimental results that material removal should be closely related to the polishing pressure, the relative velocity, and yet, vibration in the z and x direction have a significant impact on those parameters, and then exert considerably influence to machined surface generation.
polishing pressure increase remarkably, owing to demonstrates effects of pulsed vibration cutting, and enhanced behavior of deformation of the magnetic brushes, it results in higher stock removal than that in the X mode. In the ZX mode, the higher stock removal is attributed to the combined effects of pulsed polishing pressure resulting from z directional vibration and cross cutting effects resulting from x directional vibration, although it demonstrates cross cutting effects, the polishing pressure is lower than that in the Z mode, therefore, there is not a remarkable difference Table 2 Plane polishing experimental conditions Workpiece
1. Brass C2680, 38 60 mm, thickness : 1.2 mm, before polishing: 3~4lmRy (Fig. 7) 2. Stainless steel SUS304, 38 60 mm, thickness: 1.2 mm, before polishing: 3–4 lmRy (Figs. 8 and 9)
Vibration of workpiece
Amplitude: 1 mm in z direction, 3 mm in x direction Frequency: 6 Hz in z and x directions 0 mm/min (Fig. 7), 17 mm/min (Figs. 8 and 9) 25 mm 32 m/min (Figs. 8 and 9)
6.2. Plane polishing characteristics The plane polishing experimental conditions are shown in Table 2. Fig. 8 shows the differences between surface roughness (Ry) and stock removal (M) in three modes in polishing SUS304 stainless steel, in the figure, ‘Non’, ‘ZX’, ‘Z’, and ‘X’ express the non-vibration mode, ZX mode, Z mode, and X mode, respectively. It can be seen that stock removal in the case of the X, Z and ZX modes are much higher than the non-vibration mode, in order, MðzxÞ6MðzÞ >MðxÞ >MðnonÞ. In the X mode, the cross-cutting marks are generated and stock removal per working distance is increased [10], therefore, it results in higher stock removal than that in the non-vibration mode; in the Z mode, pulsed polishing pressure is produced, the mean value and the peak of
Feed speed of workpiece Stroke of workpiece Magnetic pole circumferential speed Magnetic flux density 0.6 T Working clearance 3 mm Magnetic abrasives Mixed type magnetic abrasives Iron particles: 4 g supplied, 330 lm (Figs. 8 and 9) WA magnetic abrasive: 1 g supplied, 80 lm in mean dia. Lubricant Straight oil type grinding fluid: 2 mL
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Fig. 8. steel.
Effects of vibration modes in polishing SUS304 stainless
between stock removal in the ZX mode and that in the Z mode (Table 3). On the other hand, in the non-vibration mode, with an initial surface of 3 lmRy pre-polishing gather to 0.35 lmRy. In the Z mode, gathered to 0.53 lmRy, the machined surface was somewhat rougher than that in the non-vibration mode. In the X mode, the machined surface was improved to 0.25 lmRy. In the ZX mode, the machined surface was improved near to 0.25 lmRy in the x direction, and 0.32 lmRy in the y direction, generate a better finish compared with the case of the Z mode and non-vibration mode. Fig. 9 shows microscope photographs of polished surfaces before and after polishing. The results of this experiment show that the ZX mode can produce a better finish than the
Table 3 3d Micro-curved surface polishing experimental conditions Workpiece Vibration of workpiece Magnetic pole circumferential speed Feed speed of workpiece Working clearance Magnetic abrasives
Lubricant
Punch, M50P Cemented carbide, before polishing: 0.8 lmRy Amplitude: 1 mm in Z direction Frequency: 6 Hz 22 m/min
0 mm/min 3 mm Mixed type magnetic abrasives Iron particles: 3 g supplied, 149 lm in mean dia. Diamond powder: 2 g supplied, 2–5 lm in mean dia. Straight oil type grinding fluid: 2 mL
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Fig. 9. Microscopic photographs of the polished surface of SUS304 stainless steel.
Z mode. In the ZX mode, polishing pressure is improved to distribute uniformly compared with the Z mode as shown in Fig. 4, therefore, obtaining a good finish; on the other hand, different cross-hatching patterns can be generated by varying the rotational speed and vibration amplitude and frequency, abrasive cutting edges scratch the material in many directions in intermittent cutting mode, this also results in a reduction in surface roughness. 6.3. 3D micro-curved surface polishing characteristics The polishing experiment of 3D curved surfaces was conducted using the Z mode. A small punch was used as the workpiece, its material was cemented carbide (M50P, HV1300), the machined surface was a 3D complicated curved surface, the maximum depth in the z direction was approximately 3 mm, initial surface roughness was approximately 0.8 lm Ry. The polishing experimental conditions are shown in Table 3. Fig. 10 shows sample photographs before and after polishing for 15 min, surface roughness at the tip of convex surface (a), bottom of inner surface (b) and bottom of groove surface(c) was measured after cutting workpiece apart. Ry value at (a), (b) and (c) portions were improved to 0.2 lm, 0.25 lm and 0.35 lm respectively. It can be seen that the whole surface was polished, the roughness value of the groove surface was somewhat larger than the inner surface and the concave surface. Without vibration-assistance, polishing the bottom of the inner surface and the groove surface was very difficult, only the plane and a part of the inner surface were polished to some extent.
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Fig. 10. Sample of cemented carbide.
7. Conclusions
References
The results of this research can be summarized as follows.
[1] T. Shinmura, K. Takazawa, E. Hatano, Study on magnetic abrasive process (Application to plane finishing), Bull. of the JSPE 19 (4) (1985) 289–294. [2] T. Shinmura, T. Aizawa, Development of plane magnetic abrasive finishing apparatus and its finishing performance (2nd Report, Finishing apparatus using a stationary type electromagnet), J. Jpn. Soc. Prec. Eng. 54 (5) (1988) 928–933 (in Japanese). [3] T. Shinmura, Study on plane magnetic abrasive finishing (3rd Report on the finishing characteristics of non-ferromagnetic substance), J. Jpn. Soc. Prec. Eng. 55 (7) (1989) 1271–1276, (in Japanese). [4] T. Shinmura, Study on free-form surface finishing by magnetic abrasive finishing process (1st Report, Fundamental experiments), Trans. Jpn. Soc. Mech. Eng. 53 (485C) (1986) 202–208, (in Japanese). [5] M. Anzai, T. Yoshida, T. Nakagawa, Magnetic abrasive automatic polishing of curved surface, RIKEN Review 12 (1996) 15–16. [6] K. Suzuki, T. Uematsu, Development of new surface finishing methods, Journal of the 3rd International Conference on Precision Surface Finishing and Burr Technology (1994) 116–122. [7] J. Kim, M. Choi, Study on magnetic polishing of free-form surface, International Journal of Machine Tools and Manufacture 37 (8) (1997) 1179–1187. [8] T. Shinmura, Y. Takeshi, Study of free form surface finishing by the application of magnetic abrasive machining, Proceedings of JSME Annual Meeting (IV), (in Japanese), (1997) 220–221. [9] M. Natsume, T. Shinmura, K. Sakaguchi, Study of magnetic abrasive machining by use of work vibration system (characteristics of plane finishing and application to the inside finishing of groove), Trans. Jpn. Soc. Mech. Eng. 64 (627C) (1998) 4447–4452, (in Japanese). [10] S. Yin, T. Shinmura, Study of vibration-assisted magnetic abrasive finishing process (Effects of vibration on cylindrical finishing characteristics and its mechanism), Trans. Jpn. Soc. Mech. Eng. 67 (661C) (2001) 3006–3012, (in Japanese). [11] S. Yin, T. Shinmura, Study on vibration-assisted magnetic abrasive finishing process using pulsed finishing pressure, Proceedings of the 10th International Conference on Precision Engineering, (2001) 506–510.
1. In this paper, the effects of three vibration modes on magnetic field, polishing pressure, in-process abrasive behavior and polishing performances were discussed comparatively. Furthermore, their machining mechanisms were described. 2. In the X mode, x directional vibration can help to promote the relative motion of magnetic abrasives against the machined surface and pressure distribution uniformity in the polishing zone, a smoother surface may be obtained. 3. In the Z mode, with z directional vibration-assisted, pulsed polishing pressure may be produced and the value of pressure may be increased hugely compared with the non-vibration mode and the X mode. 4. The polishing experiment of plane demonstrated the Z mode leads to higher polishing efficiency but a little rougher surface than the conventional process; the X mode can obtain a smoother surface; however, the ZX mode can obtain high polishing efficiency and a good finish simultaneously. 5. The polishing experiment demonstrated the realization that efficient polishing of 3D micro-curved surface is possible by this process.
Acknowledgements This research was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society, the authors would like to express their sincere gratitude.