Open Access
Add to BibSonomyAbstract
In order to polish microstructured surface without deteriorating its profile, we propose a new vibration-assisted magnetic abrasive polishing (VAMAP) method. In this method, magnetic force guarantees that the magnetic abrasives can well contact the microstructured surface and access the corners of microstructures while vibration produces a relative movement between microstructures and magnetic abrasives. As the vibration direction is parallel to the microstructures, the profile of the microstructures will not be deteriorated. The relation between vibration and magnetic force was analyzed and the feasibility of this method was experimentally verified. The results show that after polishing, the surface finish around microstructures was significantly improved while the profile of microstructures was well maintained.
© 2016 Optical Society of America
1. Introduction
Microstructured surfaces, which are capable of performing many different functions, are increasingly important in the fields such as optics, microfluidics and surface engineering. Examples include light collection and concentration, microfluidic channels and surface characteristics modification (e. g. adhesion, wetting or coefficient of friction) [1–4]. Generally, the technologies used for microelectromechanical systems (MEMS) such as photolithography, etching and electrodeposition are adopted to fabricate microstructures in size less than a few micrometers [5]. For instance, Becker et al. fabricated microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography, galvanoforming, and plastic molding (LIGA process) [6]. Wu et al. proposed a flexible procedure to fabricate three-dimensional (3D) microchannel systems in Polydimethylsiloxane (PDMS) [7]. Recently, a new process was reported to manufacture microstructured surface based on carbon nanotubes growing from a surface [8]. Besides, femtosecond laser micromachining are also used for fabricating microstructured surface for biomedical and optical applications [9, 10]. However, these technologies have the disadvantages that they require specific materials or certain amounts of manual work which usually leads to a long process time and high cost.
For the microstructures in size of tens to hundreds of micrometers, precision machining technologies such as cutting and milling are employed considering productivity and economical cost [11]. Besides, these technologies can fabricate steps on the surface of substrate. Jung et al. used micro end-mills to machine mold inserts for fluidic channel of polymeric biochips [12]. Lee et al. developed a miniaturized diamond tool to fabricate a V-groove on the optical fiber connector [13]. Guo et al. employed precision grinding to fabricate microstructured surfaces on binderless ultrafine tungsten carbide (WC) [14]. However, in some cases, due to the low surface quality attributed to defects such as burrs and tool marks around the microstructures caused by machining processes, post-polishing process is necessary to improve surface finish [15].
In recent years, some methods which show potential and capability of finishing structured and microstructured surfaces have been reported. Brinksmeier et al. proposed conical pin-type and conical wheel-type polishing tools to polish V-grooves and Fresnel lens. By supplying loose abrasives to the polishing area, low surface roughness of the structured surfaces was obtained while the profile of the structures was maintained [16, 17]. The drawback of this method is that due to the sharp tip of the polishing tool, the tool wore rapidly during polishing. This requires frequent replacement of the polishing tools to keep a uniform material removal rate (MRR) on a large area. Additionally, for the application on Fresnel lens which has different cross-section profiles of the V-grooves, several tools with different geometries should be prepared to fit these V-grooves. Vibration-assisted polishing method shows potential to polish the microstructured surfaces as the polishing tool could be miniaturized and arbitrarily shaped [18, 19]. However, as the miniaturized tool wears fast, it is hard to be applied on a relative large area with uniform material removal. Therefore, a flexible tool capable of fitting the geometry of the microstructured surface is preferable.
Magnetic field-assisted polishing, in which magnetic abrasives are employed as the flexible tool, seems to be a good candidate to solve this problem. Kim et al. used magnetorheological fluid mixed with abrasives as a polishing tool to polish three-dimensional silicon channel. The result showed that the roughness of bottom and side surfaces of the silicon channel was reduced by a factor of 5-10, but a slight channel height change was observed due to the different MRR on bottom and top surfaces [20]. Natsume et al. presented the magnetic abrasive machining method by use of workpiece vibration to finish 2D freeform surface [21], and Yin et al. studied polishing characteristics and mechanisms in vibration-assisted magnetic abrasive polishing (VAMAP) and realized the polishing of a 3D micro-curved surface [22]. However, as the magnetic pole was set on top of the workpiece, for microstructured surface finishing, it is difficult for the abrasives to access the corners of the microstructures and generate a uniform pressure distribution. Wang et al. examined the feasibility of magnetic compound fluid slurry on surface finishing of miniature V-grooves. The surface roughness was significantly decreased, but due to the perpendicular motion to the microstructures caused by rotation, the profile of the microstructures was not well maintained [23].
The key problem in microstructured surface finishing is represented as improving surface finish while maintaining the profile of microstructures. An alternative method to solve this is the application of a new VAMAP method which is proposed in this research. The concept of this idea is to make magnetic abrasives conform to the microstructured surface and subsequently remove materials uniformly through effectively controlling the magnetic field. In this method, magnetic force guarantees that the magnetic abrasives can well contact the microstructured surface and access the corners of microstructures while vibration produces a relative movement between microstructures and magnetic abrasives. As the vibration direction is parallel to the microstructures, the profile of the microstructures will not be deteriorated. The paper explains the principle and feasibility study of the VAMAP method which addresses the necessary issues including inter-relationship between process parameters, experimental setup, simulation of magnetic flux density distribution, polishing force measurement and experiment results which will be detailed in the following sections.
2. Principle
Figure 1 shows the schematic illustration of the VAMAP method. A magnet is placed under the microstructured surface of workpiece with a small gap to generate a magnetic field. From the cross-sectional view, the magnetic abrasives are attracted towards to the magnet and therefore contact the microstructured surface closely by magnetic force. As a result, the abrasive particles are able to conform to the profile of the microstructures and access the corners of microstructures. The source of magnetic field can be a permanent magnet or electromagnet, while the magnetic pole can be formed into various shapes to manipulate the size and shape of contact area between the magnetic abrasives and workpiece surface. As the depth of the microstructures is just tens to hundreds of micrometers, the difference of contact force at top and bottom of the microstructures is negligible. When a linear vibration generated by a linear actuator is applied to the magnet, there will be a change of magnetic force and the magnetic abrasives will follow the movement of the magnet. Hence, a relative movement is generated between the workpiece and magnet, causing material to be removed by magnetic abrasives. As the vibration direction is parallel to the microstructures, the profile of the microstructures will not be deteriorated.
Fig. 1 Schematic illustration of the VAMAP method. In this method, due to the magnetic force, the magnetic abrasives were attracted to well contact the microstructured surface and access the corners of microstructures. Then vibration produces a relative movement between microstructures and abrasives so as to remove the material. As the vibration direction is parallel to the microstructures, the profile of the microstructures will not be deteriorated.
As the material is removed due to the relative movement of the magnetic abrasives and workpiece, the inter-relationship between process parameters such as magnetic force, contact force and acceleration of magnetic abrasive during polishing process was analyzed to well understand the material removal process. As shown in Fig. 2, during vibration, the acceleration of the magnetic abrasives which is defined as a can be expressed as
Fig. 2 Illustration of inter-relationship between process parameters. During vibration, the change of θ results in the change of force parameters such as FM and Ff accordingly.
Where FM is the magnetic force, Ff is the friction, µ is the coefficient of friction which is affected by a few parameters such as material, surface roughness and wetness of workpiece surface, and relative speed of workpiece and abrasives. θ is the angle between the force vector FM and its component parallel to the workpiece surface FMx, which changes during vibration. m is mass of the magnetic abrasive. For ease of the explanation, the magnetic abrasives were considered as a whole.
FMx and FMy which is the vector of FM in horizontal and vertical direction respectively are calculated as
According to friction principle, Ff is expressed as
Then according to Eqs. (1)-(4), a is calculated as
According to Preston’s equation [24], MRR is proportional to contact force and velocity of vibration. The velocity of vibration is affected by the acceleration of the magnetic abrasives. So from the change of contact force and acceleration of the magnetic abrasives, the change of MRR can be well predicted.
3. Experimental
To verify the feasibility of this method, some experiments were conducted. The setup is shown in Fig. 3. A permanent magnet with a cuboid shape (30 mm × 6 mm × 4 mm) was used as a source of magnetic field. The material of magnet is neodymium iron boron and the polarization direction was normal to the 30 mm x 6 mm face. The magnet was mounted on a pneumatic piston vibrator (F15, OLI Vibrator) which can generate linear vibration at the frequency of 27 Hz with an amplitude of 20 mm at air pressure of 4 bar. The gap between the magnet and workpiece can be adjusted by Z-axis. For the polishing experiments conducted, it was set to 3 mm. The magnetic abrasives were composed of iron powder (20 µm mean diameter) and alumina powder (20 µm mean diameter) with a weight ratio of 9:1. They can well cover and access the corners of the microstructures on workpiece surface. The weight of the magnetic abrasives used on workpiece surface was 0.2 g. From the SEM view of the magnetic abrasives as shown in Fig. 4, it was found that the alumina powder was well bonded with the iron powder. The workpiece was fixed in a fixture which was mounted on a dynamometer and the dynamometer was installed on Z-axis. The thickness of workpiece was 2.5 mm. The whole setup was built in a 3-axis machine tool with a positioning resolution of 1 µm. During polishing, the magnetic abrasives vibrated on workpiece surface while workpiece scans in X-axis direction.
Fig. 3 Experimental setup for feasibility study of the VAMAP method. During polishing process, the magnetic abrasives vibrate on workpiece surface while workpiece scans in X-axis direction.
Fig. 4 A SEM view of the magnetic abrasives used in the experiments. (a) secondary electron image and (b) backscattered electron image.
Two samples were prepared for polishing experiments. One was made of stainless steel 316 which is a slightly magnetic material. It was processed by wire EDM to fabricate some miniature V-grooves on the surface. The geometry of miniature V-grooves is shown in Fig. 5(a). The size of the V-grooves area is 25 mm by 25 mm. V-grooves structures are commonly used for optical applications such as integrated fiber optic alignment and photovoltaic characteristics improvement of solar cell [25, 26]. The other one was made of RSP905, which is a fine grain size aluminium alloy having superior mechanical properties such as hardness and strength than conventional aluminium alloys [27]. Besides, it shows a good machining property by diamond turning and abrasive polishing [28, 29]. Prior to polishing, the sample was fabricated with some rectangular microstructures on its surface as shown in Fig. 5(b) by precision milling process using an endmill tool of 2 mm in diameter. The size of the rectangular microstructures area is 25 mm by 25 mm. Rectangular microstructures can be used to fabricate microchannels which are useful in the applications such as microfluidics [2]. The height to width aspect ratio of the three kinds of rectangular microstructures in width of 200 µm, 100 µm and 50 µm was 1.2, 1.4 and 1.8, respectively.
4. Results and discussions
4.1 Magnetic flux density distribution
The magnetic flux density distribution in cross-sectional view of the setup was analyzed based on the above-mentioned experimental setup. As shown in Fig. 6(a) and 6(b), the simulation results indicated that the magnetic flux well conform to the microstructures on surface for both kinds of workpieces and the density distribution was almost uniform. The density will be more uniform if the length of the magnet is increased. The difference of the magnetic flux density at the top and bottom of microstructures was quite small, which means that the material could be removed uniformly during polishing process.
Fig. 6 Simulation results of magnetic flux density distribution for (a) rectangular microstructures and (b) miniature V-grooves.
4.2 Polishing force condition
As shown in the side view of Fig. 1, the gap which was defined from the workpiece surface to the magnet represents the distance between magnetic abrasives and magnet. As polishing force condition is key to understand material removal process, the contact force between the magnetic abrasives and the workpiece was measured. As shown in Fig. 3, the dynamometer system consists of a multicomponent dynamometer (Type 9119AA2, Kistler Instruments Pte Ltd) and 3 charge meters (Type 5015A, Kistler Instruments Pte Ltd). The force measurement range was set to 50 N with a resolution lower than 0.01 N. The gap was set from 3 mm to 7 mm with an interval of 0.5 mm. The result is shown in Fig. 7. It was found that the contact force dropped dramatically with the increment of the gap. To ensure sufficient polishing force for material removal, a small gap is preferable for the polishing experiment. Additionally, as the force is relatively large compared to the weight of the magnetic abrasives, the effect of the weight of the magnetic abrasives in Eq. (4) can be ignored.
4.3 Results of miniature V-grooves
Figures 8(a) and 8(b) show the pictures and 3D topographies of miniature V-grooves after wire EDM and after polishing, respectively. The 3D topographies were measured by a digital microscope (Keyence VHX-2000, KEYENCE CORPORATION). The total polishing time was about 5 h, and the magnetic abrasives were replaced every 15 minutes considering the life of the magnetic abrasives on the basis of experience. From the view of 3D topographies, it is found that the shape of V-grooves was well maintained while the surface became smoother after polishing. The surface roughness at the bottom of the miniature V-grooves was better than at the side and top.
Fig. 8 Pictures and 3D topographies of miniature V-grooves (a) before polishing (after wire EDM) and (b) after polishing. The size of the miniature V-grooves area is 25 mm by 25 mm.
To precisely evaluate the profile and surface roughness, a high-resolution aspheric measurement system (Form Talysurf PGI 2540, Taylor Hobson Ltd) was adopted. As the wire used for wire EDM has a diameter of 0.2 mm, the corners at the bottom of the V-grooves were round in shape with a radius of 0.1 mm. Therefore, the depth of the V-grooves was in fact less than 0.4 mm. As shown in Fig. 9, the profile of the miniature V-grooves after polishing did not deteriorate but became sharper. The surface roughness was measured from the bottom to the top of the miniature V-grooves. As shown in Fig. 10, after polishing, the surface roughness was reduced from 2.23 µm Ra to 0.32 µm Ra, which had improved by over 80%.
Fig. 9 Results of profiles before and after polishing. The profile of the V-grooves after polishing did not deteriorate, and instead of that, it became sharper.
Fig. 10 Results of surface roughness before and after polishing. After polishing, the surface roughness was reduced from 2.23 µm Ra to 0.32 µm Ra, which had improved by over 80%.
4.4 Results of rectangular microstructures
Figures 11(a) and 11(b) show the pictures of rectangular microstructures before polishing (after precision milling) and after polishing, respectively. The polishing time is around 2h. Figures 12(a), 12(b) and 12(c) show top views and side views of 3D surface topography of rectangular microstructures in width of 200 µm, 100 µm and 50 µm, respectively. The optical 3D surface topographies of the rectangular microstructures were measured by a scanning microscope (Alicona InfiniteFocus, Alicona Imaging GmbH) which has a vertical resolution of up to 10 nm. Top view provides the top and bottom topographies of the rectangular microstructures while side view presents the side topographies of the rectangular microstructures. There was no data in the black areas of top view. To measure the side topographies of the rectangular microstructures, the workpiece was set with an inclination angle of 45 deg. From the results of top view, side view and profiles, it is found that the burrs and tool marks were clearly removed by the polishing process while the width, height and profile of microstructures were well maintained. The surface roughness between the microstructures was measured by Form Talysurf PGI 2540. As shown in Fig. 13, it was found that although some long scratches were generated due to the large amplitude of linear vibration, the surface roughness at tool mark area was reduced from 166.9 nm Ra to 25.4 nm Ra, achieving a mirror finish. The long scratches can be reduced or even eliminated through reducing vibration amplitude, lowering polishing force, and optimizing magnetic abrasive condition.
Fig. 11 Pictures of rectangular microstructures (a) before polishing (after precision milling) and (b) after polishing. The size of the rectangular microstructures area is 25 mm by 25 mm.
Fig. 12 3D topographies of rectangular microstructures before polishing (after precision milling) and after polishing.
Fig. 13 Results of surface roughness (a) before polishing (after precision milling) and (b) after polishing.
4.5 Discussions
The experiment results show that this method has the capability to finish microstructures in size of tens to hundreds of micrometers. To verify the uniformity, the profile and surface roughness at several positions of the two workpiece surfaces after polishing were measured and the results were quite similar. However, this method has two limitations. Firstly, in order to attract magnetic abrasives to the corners of microstructures, workpiece material should be non-magnetic or slightly magnetic. Secondly, as the magnetic flux density decreases dramatically with an increasing gap, the thickness of workpiece needs to be thin. Otherwise, a strong magnetic field is necessary. Currently, straight microstructures are successfully polished by this method, and it shows potential to polish the circular microstructures such as Fresnel structures. For curved microstructures, further improvements on this method are necessary to achieve localized polishing through restricting magnetic abrasives to a small area and vibrating in a few tens of micrometers.
5. Conclusion
In this paper, the VAMAP method has been proposed and developed to polish microstructured surfaces. The principle of the method was explained, and the inter-relationship between process parameters was analyzed. Based on the setup, the magnetic flux density distribution was simulated and the results indicate that the material around microstructures could be removed uniformly during polishing process. The polishing force was measured and it was found that the force dropped dramatically with the increment of gap. Some polishing experiments were conducted to verify the feasibility of the VAMAP method. For the miniature V-grooves, the surface roughness was improved from 2.23 µm Ra to 0.32 µm Ra after polishing while the profile did not deteriorate but became sharper. For the rectangular microstructures in width of 200 µm, 100 µm and 50 µm, it was found that burrs and tool marks were clearly removed and a mirror surface finish of 25.4 nm Ra was achieved after polishing while the width, height and profile of microstructures were well maintained. These results demonstrated that this method is feasible for finishing microstructured surface without deteriorating the profile of microstructures. Further study will be focused on the issues such as modelling of the material removal, optimization of process parameters and evaluation of lifetime of the magnetic abrasives so as to improve machining efficiency and surface finish.
Acknowledgments
We would like to express our grateful thanks to Ms. Liu Yuchan (Measurements and Characterization Unit, SIMTech) for her kind assistance during profile and roughness measurement of microstructures, and Mr. Goh Min Hao and Ms. Ng Fern Lan (Measurements and Characterization Unit, SIMTech) for their help on SEM observation of magnetic abrasives. We would also like to express our gratitude Dr. Liu Shiyu (Surface Technology Group, SIMTech) and Dr. Wang Pan (Forming Technology Group, SIMTech) for their valuable technical comments. The acknowledgment is extended to Mr. Ng Seow Tong (MTG, SIMTech) for his kind support.
References and links
1. C. J. Evans and J. B. Bryan, “‘Structured’, ‘textured’, or ‘engineered surfaces,’,” CIRP Annals-Manufacturing Technology 48(2), 541–556 (1999). [CrossRef]
2. S. K. Mitra and S. Chakraborty, eds., Microfluidics and Nanofluidics Handbook: Fabrication, Implementation, and Applications, Vol. 2 (CRC Press, 2011).
3. P. C. H. Li, Fundamentals of Microfluidics and Lab on a Chip for Biological Analysis and Discovery (CRC Press, 2010).
4. R. Seemann, M. Brinkmann, E. J. Kramer, F. F. Lange, and R. Lipowsky, “Wetting morphologies at microstructured surfaces,” Proc. Natl. Acad. Sci. U.S.A. 102(6), 1848–1852 (2005). [CrossRef] [PubMed]
5. G. Bissacco, H. N. Hansen, P. T. Tang, and J. Fugl, “Precision manufacturing methods of inserts for injection molding of microfluidic systems,” in Proceedings of the ASPE Spring topical meeting (2005), 57–63.
6. E. Becker, W. Ehrfeld, P. Hagmann, A. Maner, and D. Münchmeyer, “Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography, galvanoforming, and plastic moulding (LIGA process),” Microelectron. Eng. 4(1), 35–56 (1986). [CrossRef]
7. H. Wu, T. W. Odom, D. T. Chiu, and G. M. Whitesides, “Fabrication of complex three-dimensional microchannel systems in PDMS,” J. Am. Chem. Soc. 125(2), 554–559 (2003). [CrossRef] [PubMed]
8. M. De Volder, S. Park, S. Tawfick, and A. J. Hart, “Strain-engineered manufacturing of freeform carbon nanotube microstructures,” Nat. Commun. 5, 4512 (2014). [CrossRef] [PubMed]
9. K. Sugioka, J. Xu, D. Wu, Y. Hanada, Z. Wang, Y. Cheng, and K. Midorikawa, “Femtosecond laser 3D micromachining: a powerful tool for the fabrication of microfluidic, optofluidic, and electrofluidic devices based on glass,” Lab Chip 14(18), 3447–3458 (2014). [CrossRef] [PubMed]
10. M. Elbandrawy and M. C. Gupta, “Optical characteristics of femtosecond laser micromachined periodic structures in Si <100>,” Appl. Opt. 45(27), 7137–7143 (2006). [CrossRef] [PubMed]
11. E. Brinksmeier, O. Riemer, and R. Stern, “Machining of precision parts and microstructures,” in Initiatives of Precision Engineering at the Beginning of a Millennium (Springer, 2002), pp. 3–11.
12. W. C. Jung, Y. M. Heo, G. S. Yoon, K. H. Shin, S. H. Chang, G. H. Kim, and M. W. Cho, “Micro machining of injection mold inserts for fluidic channel of polymeric biochips,” Sensors (Basel Switzerland) 7(8), 1643–1654 (2007). [CrossRef]
13. J. H. Lee, S. R. Park, S. H. Yang, and Y. S. Kim, “Fabrication of a V-groove on the optical fiber connector using a miniaturized machine tool,” J. Mater. Process. Technol. 155, 1716–1722 (2004). [CrossRef]
14. B. Guo, Q. Zhao, and M. J. Jackson, “Precision grinding of binderless ultrafine tungsten carbide (WC) microstructured surfaces,” Int. J. Adv. Manuf. Technol. 64(5–8), 727–735 (2013). [CrossRef]
15. E. Brinksmeier and O. Riemer, “Deterministic production of complex optical elements,” Int. J. Prod. Eng.Comput. 4(5), 63–72 (2002).
16. E. Brinksmeier, O. Riemer, and A. Gessenharter, “Finishing of structured surfaces by abrasive polishing,” Precis. Eng. 30(3), 325–336 (2006). [CrossRef]
17. E. Brinksmeier, O. Riemer, A. Gessenharter, and L. Autschbach, “Polishing of structured molds,” Ann. CIRP 53(1), 247–250 (2004). [CrossRef]
18. J. Guo, S. Morita, M. Hara, Y. Yamagata, and T. Higuchi, “Ultra-precision finishing of micro-aspheric mold using a magnetostrictive vibrating polisher,” Annals of CIRP 61(1), 371–374 (2012). [CrossRef]
19. J. Guo, H. Suzuki, and T. Higuchi, “Development of micro polishing system using magnetostrictive vibrating polisher,” Precis. Eng. 37(1), 81–87 (2013). [CrossRef]
20. W. B. Kim, S. H. Lee, and B. K. Min, “Surface finishing and evaluation of three-dimensional silicon microchannel using magnetorheological fluid,” J. Manuf. Sci. Eng. 126(4), 772–778 (2004). [CrossRef]
21. M. Natsume, T. Shinmura, and 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(627), 4447–4452 (1998). [CrossRef]
22. S. Yin and T. Shinmura, “A comparative study: polishing characteristics and its mechanisms of three vibration modes in vibration-assisted magnetic abrasive polishing,” Int. J. Mach. Tools Manuf. 44(4), 383–390 (2004). [CrossRef]
23. Y. Wang, Y. Wu, and M. Nomura, “Feasibility study on surface finishing of miniature V-grooves with magnetic compound fluid slurry,” Precis. Eng. 45, 67–78 (2016). [CrossRef]
24. F. W. Preston, “The theory and design of plate glass polishing machines,” J. Soc. Glass Technol. 11, 214–256 (1927).
25. OZ Optics Ltd, http://www.ozoptics.com/products/hermetic_feedthru.html
26. P. Li, J. Xie, J. Cheng, and Y. N. Jiang, “Study on weak-light photovoltaic characteristics of solar cell with a microgroove lens array on glass substrate,” Opt. Express 23(7), A192–A203 (2015). [CrossRef] [PubMed]
27. R. S. P. Technology, http://www.rsp-technology.com/
28. K. Abou-El-Hossein, O. Olufayo, and Z. Mkoko, “Diamond tool wear during ultra-high precision machining of rapidly solidified aluminium RSA 905,” Wear 302(1), 1105–1112 (2013). [CrossRef]
29. R. T. Horst, M. D. Haan, G. Gubbels, R. Senden, B. V. Venrooy, and A. Hoogstrate, “Diamond turning and polishing tests on new RSP aluminium alloys,” Proc. SPIE 8450, 84502M (2012). [CrossRef]
