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Abstract
This paper presents a new localized vibration-assisted magnetic abrasive polishing (VAMAP) method using loose abrasives for V-groove and Fresnel optics finishing. The purpose is to improve the surface quality while maintaining the form of the microfeatures. This method allows abrasives to access the corners of the microfeatures and remove materials locally and uniformly by effectively controlling the magnetic field and vibration which overcomes the limitations of previous research. By using loose abrasives, the method achieved nanometer level surface roughness and damage-free surface while maintaining the form of the microfeatures. The results show that the surface roughness was reduced to about 7 nm Ra from the initial value of over 10 nm Ra while the microfeatures of V-groove and Fresnel optics were well maintained. At the same time, the surface defects including voids, scratches as well as tool marks were clearly removed.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
V-groove and Fresnel optics are key optical components in many optical systems representing imaging, concentration and illumination functions [1, 2]. Applications can be found in optical fiber positioning, solar energy focusing system, grating, and light guiding to enhance the optical performance [3–6]. For instance, Cai et al. demonstrated that the intensity of Circularly Polarized Light (CPL) was greatly enhanced by fabricating V-groove structures on the aperture antennas surface [7]. Benítez et al. presented a high performance Fresnel-based photovoltaic concentrator producing both the desired light concentration with high tolerance and excellent light homogenization by Köhler integration [8]. Rajkumar and McMullin used anisotropically etched V-grooves on silicon wafers for infrared beam-splitting transmission gratings, resulting in more than 70% efficiency possible for equal-intensity beam splitters [9].
With the dramatic increase in activity of V-groove and Fresnel optics, high precision manufacturing processes were developed in recent years to produce V-groove and Fresnel optics with high accuracy and surface quality. Takeuchi et al. achieved the manufacturing of multiple-focus micro Fresnel lenses using a non-rotational cutting tool [10]. Lee et al. used a miniaturized diamond tool to fabricate a V-groove on the optical fiber connector [11]. Wang et al. successfully fabricatd Fresnel microstructure on die steel using a single crystal diamond tool [12]. Suzuki et al. adopted a diamond wheel trued with alloy metal to grind ceramic mold insert for Fresnel optics [13]. Wang et al. proposed a novel method for generating micro V-grooves by diamond cutting and raster milling [14]. Kim and Loh presented ultrasonic elliptical vibration cutting to fabricate micro V-grooves [15]. Fang et al. did some extensive studies to minimize burrs in micro-grooving, and developed step-mirrors with micro grooves for laser-diode beam shaping [16,17]. Zhang et al. presented a high angular accuracy manufacture method for micro V-grooves based on tool alignment [18]. Yan et al. proposed a two-step cutting process for burr-free machining and developed a ductile-mode microgrooving process for fabricating Fresnel lenses on germanium [19, 20]. Although high form accuracy was achieved by these technologies, in some cases, due to the limitation of the achievable surface quality caused by the above-mentioned ultraprecision cutting and grinding technologies, which is attributed to defects such as burrs, tool marks and surface defects, ideal optical performance cannot be obtained. Therefore, a post-polishing process is necessary to improve the surface quality to maximize the optical performance.
However, most of the current existing polishing technologies such as computer controlled optical surfacing (CCOS) and Magnetorheological finishing (MRF) are not suitable for V-groove and Fresnel optics finishing [21, 22]. To date, some methods have been reported for structured and microstructured surfaces finishing. Brinksmeier et al. presented conical pin-type and conical wheel-type polishing tools to polish V-grooves and Fresnel lens [23]. Kim et al. used magnetorheological fluid mixed with abrasives as a polishing tool to polish three-dimensional silicon channels [24]. 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 [25]. Wang et al. examined the feasibility of magnetic compound fluid slurry on surface finishing of miniature V-grooves [26]. Guo et al. did some trials on curved microstructured surface finishing using a dual magnetic roller tool [27]. However, the drawback of conical pin-type and conical wheel-type polishing tools is that due to the sharp tip of the polishing tool, the tool wore rapidly during polishing. The tool by magnetorheological fluid mixed with abrasives caused different MRR on bottom and top surfaces. The VAMAP method and magnetic compound fluid slurry cannot access the corners of the microfeatures and generate a uniform pressure distribution. The dual magnetic roller tool resulted in a round edge of the microfeatures. Therefore, these methods have some limitations when it comes to effectively improving surface finish while maintaining the form of microfeatures.
To solve the problem, in the previous work, a vibration-assisted magnetic abrasive polishing (VAMAP) method has been proposed which shows the capability to finish microfeatured surfaces [28]. However, it is limited to non-magnetic or slightly magnetic materials with straight microfeatures. It is also difficult to prevent surface defects due to the long scratches caused by the large amplitude of linear vibration. As continuous research is undertaken to tackle this challenge, in this paper, a new localized VAMAP method is proposed. It can be used to polish curved microfeatures and the workpiece material could be ferrous. The paper begins with the introduction of principle, followed by the experimental setup and conditions, magnetic flux density distribution analysis, and conclusions obtained from the results regarding the feasibility of the method and achievements in surface quality and form maintenance.
2. Methodology
As shown in Fig. 1, in this method, a magnet is placed under the microstructured surface of workpiece with a small gap to generate a magnetic field. A pole is set on top of the microstructured surface to constrain the magnetic particles in a small area to achieve localized material removal. From the cross-sectional view, through adjusting the designs of magnet and pole including shape, size and material, magnetic flux density distribution can be optimized, and suitable gaps between magnet, microstructured surface and pole can be obtained so that the magnetic particles are attracted to the magnet and therefore contact the microstructured surface closely by the magnetic force. As a result, the magnetic particles are able to conform to the form of the microfeatures and access the corners of microfeatures. As the depth of the microfeatures is just tens to hundreds of micrometers, the difference in contact force between the top and bottom of the microfeatures is negligible.
Fig. 1 Schematic illustration of the VAMAP method. The magnetic particles conform to the form of the microfeatures and access the corners of microfeatures by the magnetic field while a relative movement is generated by vibration between the magnetic particles and microstructured surface, causing materials to be removed.
The source of magnetic field can be a permanent magnet or electromagnet, while the pole can be formed into various shapes to fit the form of microfeatures and manipulate the size and shape of contact area between the magnetic particles and microstructured surface. Then by introducing vibration, a relative movement is generated between the magnetic particles and microstructured surface, causing material to be removed by the abrasives. As the vibration direction traces along the path of the microfeatures, the form of the microfeatures will be maintained.
The localized VAMAP method shows some advantages over the previous work [28]. In this method, both magnetic and non-magnetic materials could be polished as the abrasives can access the corners of microfeatures and remove materials effectively even though the magnetic flux density distributions are different. As the abrasives can be focused in a small area to remove materials locally, curved microfeatures can be polished and long scratches were prevented. Moreover, by using nanometer-sized loose abrasives, nanometric surface finish could be achieved.
3. Experimental
A polishing system was developed to verify the feasibility of the localized VAMAP method. Figures 2(a), 2(b) and 2(c) shows the experimental setups for pole-magnet positioning, V-groove and Fresnel optics polishing, respectively. An iron pole with a length of 24 mm and a diameter of 1.5 mm was mounted on top of the magnet via an aluminium fixture. A round permanent magnet made of Neodymium-Iron-Boron (NdFeB) with a diameter of 16 mm and a thickness of 4 mm was fixed on top of the voice coil motor (VCM), with the polarization being normal to the workpiece. Thus, the magnetic particles formed into clusters which are distributed along the magnetic flux lines and pressed against the microstructured surfaces. The workpiece was mounted on a dynamometer (Type 9119AA2, Kistler Instruments Pte Ltd) through a fixture. The dynamometer was mounted on the Z-axis of a simultaneous 4-axis CNC machine tool which has three linear axes (X, Y & Z) and one rotational axis (A-axis). The A-axis allows the CNC program to trace the path on the workpiece while keeping vibration orientation in tangent or perpendicular to the microfeatures. The VCM was mounted on the A-axis. During polishing, the pole vibrated and scanned a S-curve on the microstructured surfaces. For V-grooves, the vibration direction is parallel to V-grooves to remove waviness and tool marks while for Fresnel optics, the vibration direction is perpendicular to the facets to reduce the tool mark effect caused by diamond turning. The facets near the center of Fresnel optics were polished.
Fig. 2 Experimental setups for (a) pole-magnet positioning, (b) V-groove and (c) Fresnel optics polishing.
The experimental conditions were listed in Table 1. The V-groove and Fresnel optics samples were made of rapidly solidified aluminium RSA905, known for its finer grain size and better mechanical properties compared to conventional aluminium alloys and found its application in optical industry [29]. By performing initial tests, the gap between the pole and sample surface was set to 1 mm, while the gap between the sample surface and magnet was set to 5 mm considering magnetic flux distribution, contact areas and acting force of magnet abrasives on sample surfaces. The vibration frequency was set to 50 Hz with an amplitude of 150 µm. Before polishing process, the V-groove sample was cut by using a single crystal diamond tool. The size of the sample was 25 mm by 40 mm, with a thickness of 3 mm. As shown in the Fig. 3(a), each V-groove had a width of 740 μm and a height of 340 μm. The Fresnel optics sample were fabricated by diamond turning process. The geometry of the sample is shown in Fig. 3(b). It had a pitch of 0.2 mm, and the diameter of the turned area was 30 mm.
Table 1. Experimental conditions.
The magnetic particles were made of iron powder and the loose abrasive slurry was made of colloidal silica. As shown in Fig. 4(a), the major size of magnetic particles was about 3 µm, with smaller particles adhering around them. From Fig. 4(b), the average particle size of colloidal silica was about 50 nm in diameter. Owing to the small size and round shape of the particles, it can be predicted that nanometric surface roughness will be achieved with low material removal rate. The amount of magnetic media used between the pole and microstructured surfaces was approximately 10 mg and the contact force between the magnetic particle and microstructured surface was 0.1 N as measured by the dynamometer. The force measurement results also indicated that the contact force was 0 without setting the round magnet.
4. Magnetic flux distribution analysis
In order to determine the effect of magnetic field acting on the microstructured surface, the magnetic flux distribution was analyzed using finite element method (FEM). The model was designed in cross-sectional view based on the above-mentioned experimental setups. The magnet, iron pole as well as the gaps were set to the same dimensions with those used in the setups. As colloidal silica was a non-ferrous material, it will not take effect in magnetic flux intensity. The grade of the magnet was N42. The microfeatures of V-groove and Fresnel optics also followed the design geometries. The surrounding medium was set to air, which was bounded in a sphere determining the boundary condition. The abrasives between the iron pole and microfeatures were defined as iron.
The FEM simulated result for V-groove and Fresnel-type structures is shown in Figs. 5(a) and 5(b), respectively. It should be pointed out that the flux density values in this figure may not be quantitatively accurate and are only used for qualitative comparison. The cause of inaccuracy is mainly due to the inconsistency of the real magnetic flux density provided by the magnet and settings in simulation. It is observed that there is a higher magnetic flux density at the valley of the V-grooves and Fresnel-type structures compared to the peak and sides. For Fresnel-type structures, magnetic flux concentrated higher at the edge than the center. By introducing the S-curve scanning, the flux density will become more uniform during the polishing process and result in a uniform material removal.
Fig. 5 Simulation results of magnetic flux density distribution for (a) V-grooves and (b) Fresnel-type structures.
5. Results and discussions
In this section, the feasibility of the localized VAMAP method was verified from the aspects of surface roughness, form accuracy and surface defects. A high-resolution aspheric measurement system (Form Talysurf PGI 2540, Taylor Hobson) was used to precisely evaluate the form and surface roughness. For V-groove structures, the measurement probe scanned along the grooving direction with a small angle of 0–5° so that the complete V-groove profile from peak to valley was covered. For Fresnel-type structures, the probe scanned perpendicular to the tool marks caused by diamond turning. A scanning electron microscope (Ultra Plus FESEM, Carl Zeiss) was employed to examine the surface defects in micrometer level.
5.1 Surface roughness
The surface roughness achieved at different conditions was examined through experiments. The measurement was conducted at 5 different locations on the V-groove structures when the roughness value was saturated. As shown in Fig. 6, after diamond cutting the surface roughness achieved around 13 nm Ra. However, by the magnetic abrasives used in the previous study (KX-600) which were composed of iron powder and alumina powder [26], the surface finish became worse and the roughness value was increased to over 20 nm Ra. By using the abrasives proposed in Section 2 which is the combination of magnetic particles and colloidal silica slurry, the roughness value was reduced to about 10 nm Ra. Then through diluting the colloidal silica slurry using distilled water, when the slurry was diluted to 25%, surface roughness of around 7 nm Ra was obtained. After that, the surface roughness did not improve despite further diluting of the slurry.
Figure 7 shows the change of surface roughness as a function of polishing time for V-groove and Fresnel-type structures, respectively. 25% diluted colloidal silica slurry was used and the surface roughness was measured every 0.5 h. The surface roughness was reduced with the increment of polishing time and became stable after 1.5 h. For Fresnel-type structures, the polishing was not continued after 1.5 h considering not to deteriorate the profile so much because the vibration direction was perpendicular to the microfeatures. Figures 8(a) and 8(b) shows the surface roughness profiles of V-groove structures before and after polishing, respectively. The surface roughness decreased from 14.2 nm Ra to 6.6 nm Ra, a reduction of over 50%. For Fresnel-type structures, the polishing was also conducted using 25% diluted colloidal silica slurry for 1.5 h. As shown in Figs. 9(a) and 9(b), the surface roughness was reduced from the initial value of 10.4 nm Ra to 7.7 nm Ra.
5.2 Form accuracy
Besides surface roughness, the changes in form for microfeatures of V-grooves and Fresnel optics before and after polishing were evaluated. As shown in Fig. 10(a), the form difference between initial and polished microfeatures of V-grooves was quite small in the range of a few micrometers. The change on side surface of microfeatures was less than 1 µm. It was relatively large at the peak and valley of microfeatures, indicating that the sharp corners became round after polishing, but the difference was still less than 5 µm. For the microfeatures of Fresnel optics as shown in Fig. 10(b), similar results were obtained that the form change was quite limited on the facets. As the vibration direction was perpendicular to the microstructures, the forms at the peak and valley corners were changed more than those of V-grooves, but still less than 50 µm. As the diamond stylus has a radius of 2 µm at the tip, the form at valley and peak may not be precisely evaluated so it will be further confirmed by SEM observation, which will be shown in section 5.3.
5.3 Surface defects
The surface defects were further examined by SEM. As shown in Fig. 11(a), before polishing voids and scratches can be found on top, side and bottom of V-groove structures. The voids were caused by the loss of higher hardness grains due to the relatively higher stresses during V-grooving process [30]. After polishing, from Fig. 11(b) it can be seen that these defects were clearly removed and a smooth surface was obtained. The form was well maintained although the valley became slightly rounder. It also indicated that the sharpness of valley could be better through reducing the size of magnetic particles. For Fresnel-type structures, the voids were not obvious as the stress induced by diamond turning was lower, but from Fig. 12(a), tool marks can be seen on the surface which will lead to the scattering effect, reducing the optical performance of Fresnel optics. As shown in Fig. 12(b), after polishing the tool marks were shown to be reduced. The sharp edges become slightly rounder as indicated by the increased shininess of the area. However, the affected rounded areas are small and will not affect the light operating functions of the microfeatures.
Fig. 11 Secondary electron images of surface morphologies on top, side and bottom of V-groove structures (a) before and (b) after polishing.
Fig. 12 Secondary electron images of surface morphologies on side and edge of Fresnel-type structures (a) before and (b) after polishing.
5.4 Discussions
In the previous work, although the surface roughness of the samples was improved while the microfeatures was well maintained, it is limited to non-magnetic or slightly magnetic materials and straight microfeatures [28]. Due to the large vibration amplitude, long scratches were easy to be generated which will reduce the surface quality. Besides, according to the theoretical calculation, the vibration amplitude of the magnet must be over than a certain value of tens of millimeters to cause the magnetic abrasives to vibrate. This research overcomes the limitations through realizing localized polishing of curved microstructued surfaces such as V-groove and Fresnel optics. It is applicable to both of non-magnetic and magnetic materials, and straight and curved microstructued surfaces. As the vibration amplitude was just a few micrometers, long scratches were prevented. Moreover, by using loose abrasives, the method achieved nanometer level surface roughness, eliminated surface defects while maintaining the form of microfeatures which is very critical and highly required in optical microstructued surface polishing.
6. Conclusion
In this paper, a localized VAMAP method has been proposed for finishing the microfeatures of V-groove and Fresnel optics. The principle of the method was explained, and a polishing system was developed. Based on the setup, the magnetic flux density distribution was analyzed to evaluate the effect of magnetic field on microstructured surfaces. The experiments were conducted to verify the feasibility of the method and according to the results obtained from this research, some conclusions can be drawn as follows:
- 1. The VAMAP method is capable of improving surface quality and maintaining the form of microfeatures of V-groove and Fresnel optics. It is applicable to both straight and curved microstructured surface finishing.
- 2. The method achieved nanometer level surface roughness and eliminate surface defects by using loose abrasives which will improve the optical performance of V-groove and Fresnel optics.
Therefore, for the first time, the method successfully solved the critical problems of V-groove and Fresnel optics polishing. Future work will consider the polishing of other types of microstructured surfaces, and the optimization of abrasive and process parameters to achieve an even better surface finish.
Funding
Fundamental Research Funds for the Central Universities (Grant No. DUT17RC(3)105); Science Challenge Project (No. JCKY2016212A506-0103); and Science Fund for Creative Research Groups of NSFC (51621064).
Acknowledgments
We would like to express our thanks to Mr. Zhang Jiong (AML, NUS), Dr. Wang Hao (AML, NUS), Mr. Goh Min Hao (MCU, SIMTech), Dr. Zhang Xinquan (MTG, SIMTech), Dr. Huang Rui (AML, NUS) and Ms. Liu Yuchan (MCU, SIMTech) for their kind assistance in the experimental work. This research was supported by the Fundamental Research Funds for the Central Universities (Grant No. DUT17RC(3)105), Science Challenge Project (No. JCKY2016212A506-0103) and Science Fund for Creative Research Groups of NSFC (51621064).
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