Whole Elliptical Surface Polishing Using a Doughnut-Shaped MCF Polishing Tool with Variable Tilt Angle

Abstract

Elliptical elements are essential optical surfaces for modifying optical systems. For polishing the whole elliptical surface using doughnut-shaped MCF polishing tool with variable tilt angle, an experimental investigation was conducted in this work. Firstly, a flat workpiece was polished to determine the polishing feasibility. It was found that the middle portion of the polishing tool had optimal ability to remove materials, and the surface roughness S_a at the material removal peak was changed from 134 nm to 17.5 nm within 50 min of polishing. A smoother surface could be obtained using MCF2 slurry and MCF3 slurry, but the use of MCF1 slurry resulted in a rough surface. Then, the effects of working gap h, revolution speed of MCF polishing tool and polishing time on the polishing results were tested to study the polishing characteristics. S_a 9.6 nm and glossiness 278 Gu were obtained, and form error improved from 2.3 μm to 1.3 μm. Finally, the MCF polishing tool was dried to observe the microstructure of the MCF polishing tool after polishing. Abrasive particles were distributed evenly after polishing. It was seen that the abrasive particles were grabbed by the ferric clusters, and the α-celluloses were interleaved between the clusters.

1. Introduction

As one of the most important aspherical surfaces, elliptical surfaces with high precision and high surface quality compared with spherical surfaces have shown outstanding capacity to improve imaging quality, extend the field of view of devices, simplify the structures of optical systems and reduce the overall cost of optical systems [1,2]. Therefore, elliptical surfaces are considered necessary optical surfaces for replacing spherical elements to improve the performance of optical systems, such as space optical systems, optical inspection systems and smart devices [3].

To manufacture these elliptical surfaces, single-crystal diamond turning (SCDT) and high-precision grinding were usually utilized [4]. Although satisfactory surface roughness can be obtained, there will remain subsurface damage, micro cracks and especially tool marks that mainly affect the reflection ability of the elliptical surfaces [5]. Many effective polishing methods were proposed for polishing aspheric surfaces, including shearing rheological flow [6,7], abrasive jet [8] and magnetic field-assisted polishing [9]. As a soft and flexible polishing method, magnetic field-assisted polishing was realized using a magnetic fluid (MF) or magnetorheological (MR) fluid or magnetic abrasives (MAs) and performed better in polishing optical components. Thus, it was used commercially as a postprocessing technique for delimiting the remaining defects on workpiece surfaces [10]. A diamond-turned PMMA part was polished using magnetorheological finishing (MRF), and RMS 0.5 nm was obtained finally [11]. A ball-end MRF finishing method was used for multiple-dimensional surfaces [12]. The surface roughness of a SLAM55 aspheric element was improved, and the form error was enhanced from 3.7 to 0.2 μm. However, MR was not efficient in polishing due to low magnetic response intensity, and MRF performed worse in the dispersion of abrasive particles. For improving the performance of the traditional magnetic fluid and maintaining the prominent advantages of magnetic field-assisted polishing methods with MRF or MF, a novel MCF (magnetic compound fluid) slurry was invented by Shimada et al. [13,14]. The MCF was prepared by blending micro-sized carbonyl-iron-particles (CIPs), abrasive grains and α-cellulose into a magnetic fluid (MF) which contains nano-sized magnetite particles (MPs). Once a magnetic field is applied, chain-shaped clusters composed of the CIPs and the MPs are instantly formed, leading the MCF slurry to change the state from liquid to solid-like [15,16]. Due to its advantages such as longer and more flexible clusters and better dispersity of abrasives compared with the conventional magnetorheological fluid slurry, the MCF slurry was used to polish planes and V-grooves for Nano precision [17,18]. Guo proposed a rotating magnetic field to maintain the shape of the polishing tool, which significantly improved polishing efficiency and results [19]. Wang used the mountain-shaped MCF polishing tool to polish materials including copper alloy, resin and CVD-SiC, declaring that MCF slurry has a wide application for polishing many materials [20,21].

Although MCF slurries exhibit high polishing performance regardless of the work material, the conventional mountain-shaped MCF polishing tool is not suitable for polishing curved surfaces due to the complex ‘W’ shaped material removal profile, which will decrease polishing efficiency because of complex polishing tracks and also induce abrasive wear because all abrasive particles on the tool surface participate in removing material. To develop the appropriate method for aspheric surfaces, a ring-shaped magnet was utilized to replace the traditional disk-shaped magnet in our previous work for polishing convex surface with the doughnut-shaped MCF polishing tool [22]. Polishing with the fringe of doughnut-shaped MCF polishing tool can not only simplify the W- shaped material removal profile to V-shaped material removal profile but also can improve the working life of MCF polishing tool. Although timely investigations on the fundamental spot polishing for aspheric surfaces have also been performed and excellent performance was clarified by previous researchers, whole-surface polishing has not been addressed in detail with MCF slurry. Investigations on whole elliptical surface (a specifical aspheric surfaces) polishing using the novel doughnut-shaped MCF polishing tool with a varied tilt angle were conducted in this work. Firstly, the polishing feasibility and polishing characteristic were investigated in detail, after which the micro constructure of polishing tool was observed. By these optimal conditions, the elliptical surface was polished successfully, and the form error was also improved.

2. Materials and Methods

2.1. Materials

With the development of optical science and technology, higher requirements for optical components have been put forward, which makes the design, manufacturing and installation of optical systems increasingly difficult. Aluminum alloy was an essential material in optical system due to its physical and chemical advantages and the fact that it could be easily machined by the existing processing technology, such as turning and grinding. Thus, AA6061 was employed as the basic material for elliptical elements in this research. Before/after processing experiments, all the samples were rinsed with an ultrasonic cleaner for 30 min in turn with acetone (AR, >99.5%), ethanol (AR, 95%), and distilled water, and then dried by high-pressure dried air-gas.

2.2. Processing Method

Schematic drawings of elliptical surface polishing with doughnut-shaped MCF polishing tool using 6-DOF manipulator and the experimental setup are displayed in the upper side and bottom of Figure 1, respectively. The MCF unit mainly contains a hollow cylinder-shaped neodymium permanent magnet (Ø30 in outer diameter and Ø9 mm in inner diameter, t20 mm, 0.5 T) and a MCF slurry carrier (aluminum plate). In this unit, the magnet was fixed with an appropriate revolution radius r. When the magnet is rotationally driven with the revolution radius r, a rotary magnetic field is generated and then a doughnut-shaped MCF polishing tool is formed after an amount of MCF slurry was put onto the upper surface of MCF carrier. The workpiece is mounted on the end tip of the 6-DOF manipulator. The working gap h (the distance of selected point on the work surface to the upper surface of MCF carrier) and the tilt angle θ (the included angle between revolution axis of workpiece and MCF slurry carrier) were determined by controlling the position of 6-DOF manipulator. Here, n_w, n_c and n_m are the rotation speeds of workpiece, MCF slurry carrier and magnet, respectively. In Figure 1, material removal principle was also shown in the red dashed box in which ferric clusters stirred and refreshed those abrasive particles (APs), and the n_c offered the relative motion between abrasive particles and workpiece surface for removing material. The whole surface is polished by changing continuously tilt angle.

In the experiments, the working gap h, the distance of the points at the generatrix to the upper surface of MCF carrier, was changed from 1 mm to 3 mm with an increment in 0.5 mm. The revolution speed of MCF carrier was increased from 100 rpm to 500 rpm with an increment in 100 rpm. The composition of the MCF slurry was varied; specifically, MCF1, MCF2 and MCF3, where the CIP (mean particle size 4 μm) concentration was varied from 35 to 55 wt.% with an increment of 10 wt.%, while the concentration of water-based magnetic fluid MF was varied from 50 to 30 wt.% with a same increment. The abrasive particles (Al₂O₃, with a mean particle size of 1 μm) and α-cellulose (mean length of 5 μm) were kept constant in 12 wt.%, and 3 wt.%. The experimental conditions were displayed in

Table 1. Experimental condition.

Parameters	Value
Slurry	MCF1, MCF2, MCF3
Working gap h	1, 1.5, 2, 2.5, 3 mm
Revolution speed of MCF carrier	100, 200, 300, 400, 500 rpm

.

3. Results and Discussion

3.1. Polishing Feasibility

A flat aluminum alloy workpiece (□50 mm × t5 mm) that can be considered a kind of elliptical elements with infinite long axis and zero short axis was adopted in this experiment using MCF2 slurry. Before polishing, the work surface was blacked. As a typical experimental result, the optical image of polished area and the profiles of cross section A-A after 50 min polishing under the condition of volume of the MCF2 slurry = 2 mL, n_c = 200 rpm in cw, n_m = 500 rpm in ccw, n_w = 450 rpm in cw, the revolution radius r = 4 mm, tilt angle θ (ranging between 0° and 6° with 1°/sec recurrent rate), working gap h = 3.5 mm were provided in the left of Figure 2. It is obvious that a concentric circle polishing area was attained with almost Ø39.3 mm in outer diameter D_o and Ø11.2 mm in inner diameter D_i. In the figure, the peak material removal MRP, namely the maximal removed depth after polishing, occurred at a circle with a radius of about 12.5 mm, almost in the middle of the polished area which corresponded to the middle portion of the polishing tool fringe, meaning that this portion of the polishing tool performed better capacity on removing material. Based on above conditions, the polishing tool can be regarded as a ring-shaped like a regular doughnut and the better capacity on removing material of the polishing tool can be obtained by D = (D_i + D_o)/4, where D_i and D_o are the diameters of inner and outer rings of the MCF tool, namely in the middle of the fringe of MCF polishing tool.

The surface roughness S_a is obtained during polishing, as shown in the right portion of Figure 2. Each plotted point of surface roughness S_a was the mean of five S_a measured at 5 different locations along the middle circle of the polished area with the same interval, which was also adopted in the following experiments. A white light interferometer (Taylor-Hobson CCI) was used for measuring the surface roughness. The surface roughness S_a decreases 75% quickly in the first 20 min, and the workpiece surface continues to be smoothed in following polishing time, finally it decreases from 134 nm to 15 nm after 50 min. Obviously, with the increase in the material removal amount and the increase in polishing time, better surface roughness S_a can be obtained.

Figure 3 shows the pictures of those polished workpieces using different MCF slurries in 50 min. Based on the results, as shown in Figure 3a, the polished area that is marked inside the black circles should be divided into two parts, the major polished area is marked between the two red circles and others are minor polishing areas. It is obvious that the MCF3 slurry leads to not only the maximum polishing area but also the active polishing area which is located between the red circles with dash lines, due to the abundant and longer ferric clusters generated by using MCF3. On the contrary, both minimum polishing area and active polishing area are achieved by using MCF1, which has lowest CIPs concentration. Although material was removed with MCF1 in the minor polishing area, the surface roughness is unable to be modified efficiently after polishing for long time and even to be worse with the increase in the polishing time. The reason is that the slurries and abrasive particles contained within the polishing tool are easy to be squeezed to the fridge, and simultaneously, abrasive particles that are included in those area are hard to be supported just because the number of ferric clusters were inadequate so that they cannot be efficient to support abrasive particles to smooth the surface. The surface was worn due to the inadequate micro cutting of abrasive particles. The 3D microscopic images in Figure 3b displays the final typical values of surface roughness S_a, in which the lowest and the highest value of surface roughness (S_a 7.625 nm and 219.9 nm, respectively) are obtained by using MCF3 and MCF1. Figure 3c shows the typical micro scale view of the polished workpieces captured by a laser microscope (Keyence, VX-1000), it is seen clearly that the work surface is tending to be rough with the increase in time by using the MCF1, many disordered grooves are observed on the surface which makes the surface roughness increase. But by using the MCF2 and MCF3, there are lesser defects in the work surface including scratches and micro cracks, especially, MCF3 can achieve the smoothest surface. The polishing results during 50 min polishing have also been displayed, as shown in the Figure 4, which demonstrated that the value of the surface roughness S_a becomes worse continuously by using MCF1 in 50 min. In the first 20 min, the MCF3 slurry behaves efficiently than MCF2 slurry, then they keep almost the same efficiency to the end. However, due to the less MF within MCF3 slurry, it was found that MCF3 slurry was apt to be dried during polishing. In other words, MCF3 slurry has a short working life, which was a disadvantage compared with MCF2 slurry. Thus, MCF2 slurry was used in the following experiments.

3.2. Polishing Characteristic

Glossiness is a physical quality that evaluates the ability of material surface to reflect light under a set of geometric conditions. The greater the glossiness of the polished surface, the better reflection performance of the workpiece. Three angle (20°, 60°, 85°) glossiness meter (HG268 by 3NH Co., Ltd.) was employed for measuring the glossiness based on ISO 2813 standard. In these experiments, the initial surface roughness S_a was around 75 nm and the initial glossiness was around 130 Gu. The workpiece was an elliptical element with semellipse surface that had a long axis in 15 mm and short axis in 5 mm. The tilt angle was varying between 0°–90° with 10°/s recurrent rate. The normal line of the workpiece generatrix was vertical to the rotation axis of the polishing tool and 5 points were seletced evenly along the workpiece generatrix for measuing the experimental results. The revolution speeds of the magnet and workpiece were kept at 500 rpm in ccw and 450 rpm in cw, respectively. The volume of supplied MCF2 slurry was 2 mL in each test.

The experimental results for the effects of parameters including working gap h, revolution speed of MCF carrier n_c and polishing time on the surface roughness S_a and glossiness were shown in Figure 5. As the increase in working gap h, the surface roughness and glossiness became worse gradually. When the working gap h is 1 mm, a perfect surface roughness and glossiness were achieved simultaneously. It was concluded that smaller working gap h impacted both surface roughness and glossiness because as the workpiece gradually moved to the magnet, namely, working gap h decreased, the polishing force increased, indicating that the material removal rate was enhanced and the defects were eliminated with higher efficiency.

According to the results of the influence of the revolution speed of MCF carrier n_c on the surface roughness and glossiness, it was found that the value of surface roughness and glossiness increased when the revolution speed of MCF carrier n_c increased from 100 rpm to 300 rpm, then surface roughness and glossiness started to be worse when revolution speed of MCF carrier n_c increased from 300 rpm to 500 rpm, because an amount of MF contained in MCF slurry was spilt out by the overlarge centrifugal force, leading that the abrasive particles started to run away and the polishing force of the MCF polishing tool provided by magnetic field was decayed. The best surface quality can be achieved when revolution speed of MCF carrier n_c is 300 rpm.

Based on the results mentioned above, the effect of polishing time on the polishing results is also shown in Figure 5 under the condition of working gap h = 1 mm and revolution speed of MCF carrier n_c = 300 rpm in cw. The results shown that the surface roughness was continuously decreased and glossiness was continuously increased. The decrease rate of surface roughness was decreased with the increase in polishing time and simultaneously the increase rate of glossiness was also decreased. Finally, S_a 9.6 nm and glossiness 278 Gu are obtained after 30 min polishing. Furthermore, based on the tendency between surface roughness and glossiness in the three results, it was concluded that smoother workpiece surface obtained better glossiness.

To investigate the comprehensive reason why the surface roughness and glossiness were improved during polishing. The surface morphologies in the top tip of workpiece and their profiles were displayed in Figure 6 with its initial, 15 min polishing and 30 min polishing. Tool marks remained by SCDT process were the typical surface morphologies on the elliptical surface. According to the cross-section profile, the initial distance between the two peaks or valleys was around 110 μm and initial height between peak and valley was around 160 nm. After 15 min polishing, the distance was still around 110 μm and height was shortened to 65 nm. Then, the distance and height were decreased to 60 nm and 20 nm after final 15 min polishing. In other words, it was seen that the tool marks which caused the intermediate frequency errors were observed obviously in the initial and were gradually diminished by the MCF polishing tool within 30 min polishing, thus, the refection ability of the surface was enhanced and the surface roughness S_a was decreased, leading that optical performance was improved.

The 3D whole views of elliptical surface (captured by KEYENCE VR-6000) before/after polishing were shown in Figure 7, it was clear that the surface was smoothed obviously with the elimination of remarkable defects on the surface and the form error was improved from 2.3 μm to 1.3 μm. Therefore, the whole surface was polished successfully, demonstrating that this method can well improve not only the surface roughness and surface glossiness, but also the form error.

3.3. Polishing Tool

In order to comprehensivly analyse the polishing characteristic of the MCF polishing tool for the whole surface polishing with varied tilt angle, the tool sample was made for obsering the microstructure of the polishing tool. After polishing 30 seconds under the condition of volume of the MCF2 slurry = 2 mL, n_c = 300 rpm in cw, n_m = 500 rpm in ccw, n_w = 450 rpm in cw, the revolution radius r = 4 mm, working gap h = 1 mm and especially the tilt angle varying from 0°–90° with 10°/sec. the sample was dried naturally for 36 h. The obtained MCF polishing tool sample was shown in Figure 8.

The typical used portion of the MCF polishing tool was highlighted with the red solid line in area 1 and area 2 where abundant ferric clusters can be observed directly. The ferric clusters within the marked working area of the MCF polishing tool were further studied using the scanning electron microscopy (SEM, ZEISS Merlin) and energy-dispersive X-ray spectroscopy (EDX). Ferric clusters in area 1 (facial side (a)) and area 2 (cross section (b)) were observed, respectively. In area 2, the orientations of ferric clusters were different at different positions corresponding to those of the magnetic lines of force. Starting from out fringe of MCF polishing tool, the clusters gradually changed their orientations clockwise, the clusters at the middle location between inner fringe and outer fringe were almost vertical to the upper surface of the MCF carrier. Then, the clusters continued to change the orientations clockwise from being vertical to being in inclined with the upper surface of the MCF carrier. The Al elements within abrasive particles which are marked by green color in Figure 1a,b were found at the surface and inside of the MCF polishing tool, meaning that the abrasive particles can reach to the surface of MCF polishing tool and remove materials during polishing, which is important for polishing performance.

The distribution of the main compositions including α-celluloses, CIPs and abrasive particles were studied in Figure 8c–e. These α-celluloses were interleaved between the clusters, resulting the improvement in the shear strength of the MCF polishing tool. The abrasive particles were grabbed by the ferric clusters. In a word, Once the iron clusters changed their orientation due to the rotating magnetic field, the cutting edge of the abrasive particles was altered by the stir motion of clusters, resulting in a reduction in abrasive wear.

4. Conculsions

This paper proposed the whole surface polishing method for elliptical surface using MCF (Magnetic Compound Fluid) for the nano-precision surface finishing. The experimental setup was constructed to realize the proposed method. Experimental investigations on the fundamental polishing feasibility, polishing characteristics and polishing tool were conducted in this work by considering surface roughness and glossiness, microstructure of MCF polishing tool. The whole surface of the employed elliptical element was polished by these optimal polishing conditions. The conclusions are as followings:

The middle portion of the polishing tool had optimal ability in removing materials. The concentration of CIPs affects deeply to the performance of MCF slurry. The working gap h, revolution speed of MCF polishing tool and polishing time can determine the surface quality of the polished workpiece. The smaller working gap h and higher revolution speed of MCF polishing tool, the better surface roughness and glossiness. The smoother surface, the better reflection ability of polished elliptical surface. Further, once the iron clusters changed their orientation due to the rotating magnetic field, the cutting edge of the abrasive particles can be altered by the stir motion of clusters, resulting in a reduction in abrasive wear.