Int. J. Nanomanufacturing, Vol. 13, No. 1, 2017 Experimental study on shear thickening polishing method for curved surface Binghai Lyu*, Chenchen Dong, Julong Yuan and Lei Sun Ultra-Precision Machining Center, Zhejiang University of Technology, Hangzhou 310014, China Email: email@example.com Email: firstname.lastname@example.org Email: email@example.com Email: firstname.lastname@example.org *Corresponding author Min Li National Engineering Research Center for High Efficiency Grinding, Hunan University, Changsha 410082, China Email: email@example.com Weitao Dai Ultra-Precision Machining Center, Zhejiang University of Technology, Hangzhou 310014, China Email: firstname.lastname@example.org Abstract: A new polishing method shear thickening polishing (STP) was proposed to improve the polishing efficiency in the process to obtain extremely smooth curved surfaces. A non-Newtonian fluid with shear thickening property was utilised as the base fluid of the polishing slurry, in which the abrasives were dispersed. In this study, the influence of polishing speed, abrasive concentration and abrasive size on the surface roughness and material removal rate were investigated by experiments. The experimental results revealed that the polishing speed has the greatest influence on the polishing effect. With the increase of the polishing speed, the material removal rate increases rapidly and smoother surfaces with better roughness can be obtained. Abrasive concentration affects the polishing results in a manner that is similar to polishing speed. Abrasive size seems to have no effect on the surface roughness, but material removal rate increases as the abrasive size decreases. Finally, surface roughness of bearing steel work-piece (Ø35 mm) was reduced rapidly from Ra = 105.95 nm to Ra = 6.99 nm after one hours’ processing under the appropriate conditions. Keywords: polishing; non-Newtonian fluid; shear thickening; curved surface. Copyright © 2017 Inderscience Enterprises Ltd. 81 82 B. Lyu et al. Reference to this paper should be made as follows: Lyu, B., Dong, C., Yuan, J., Sun, L., Li, M. and Dai, W. (2017) ‘Experimental study on shear thickening polishing method for curved surface’, Int. J. Nanomanufacturing, Vol. 13, No. 1, pp.81–95. Biographical notes: Binghai Lyu received his Master’s degree from Zhejiang University of Technology in 2003 and his PhD from Harbin Institute of Technology in 2007. He worked at Zhejiang University of Technology as an Associate Professor from 2007–2011. He is currently a Supervisor of Master and Professor of Zhejiang University of Technology. His research fields cover ultra-precision manufacturing, precision ceramic ball manufacturing, curved surface polishing and its applications and so on. Chenchen Dong received his Master from Zhejiang University of Technology in 2015. His main field of study is curved surface polishing utilising non-Newtonian fluid. Julong Yuan received his Master’s degree and PhD from Harbin Institute of Technology in 1986 and 1989 respectively. He now works at Zhejiang University of Technology as a Professor and assumes the Director of China Production Engineering Institution and the Chinese Committee for Precision Engineering and Nano Technology, CMES. He has a wide range of research experience and interest in ultra-precision machining technologies. His current research is supported by NSFC and ZJNSF. Lei Sun received his Master from Zhejiang University of Technology in 2014. His main field of study is curved surface polishing and quartz polishing. Min Li is currently a PhD candidate in National Engineering Research Center for High Efficiency Grinding, Hunan University. His current research is curved surface polishing. Weitao Dai is currently a Master student of Ultra-Precision Machining Centre, Zhejiang University of Technology. His main field of study is curved surface polishing utilising non-Newtonian fluid. This paper is a revised and expanded version of a paper entitled ‘Experimental study on shear thickening polishing method for curved surface’ presented at the 4th International Conference on Nanomanufacturing (nanoMan2014), Germany, 8–10 July 2014. 1 Introduction Curved surface plays a key role in a wide range of applications, such as aerospace, astronomy, mould, and automobile. In order to obtain a better performance of imaging optical systems and to reduce the size and weight of lenses, aspheric optical components are used in mirror and lens assemblies. These applications brought an increasing demand for curved surfaces in recent years. Moreover, the shapes of curved surfaces are becoming more complicated, while higher surface quality and form accuracy are required. Experimental study on shear thickening polishing method for curved surface 83 Conventional surface finishing method using a small polishing pad is not suitable for curved surfaces, if high efficiency and low cost is required. Thus, development of new polishing techniques has been a research focus for curved surface. A variety of polishing processes using a special fluid whose viscosity can be adjusted by an external field have been developed, such as magnetorheological finishing (MRF) (Kordonski and Jacobs, 1995; Jha and Jain, 2004), electrorheological fluid-assisted polishing (Suzuki et al., 1997), electrophoretic polishing (Kim et al., 2003), magnetic fluid float polishing (Raghunandan et al., 1997; Shimada et al., 2003), magnetic abrasive polishing (Kim and Choi, 1997). MRF is a flexible machining method by using magnetorheological fluid while magnetic field is applied during the processing. MRF has been utilised in actual optical finishing. Electrorheological (ER) fluid-assisted polishing is a useful finishing method of three dimensional micro or meso-scale devices (Suzuki et al., 1997). In ER fluid-assisted polishing process, the ultra-fine abrasive particle is the polishing media. When the electric field is applied, the microstructure and rheological properties of ER polishing fluid will change, with its apparent viscosity and shear yield stress improve significantly. Workpiece surface can be polished by the abrasive particles as the abrasive particles adhering to ER particles. Using external field assisted polishing is a good method to polish curved surfaces, however, an external field and special fluid are needed, which results in very high machining cost. This paper focuses on improving the polishing efficiency and cost reduction in the process of polishing curved surfaces. Shear thickening polishing (STP) method based on the shear-thickening effect of non-Newtonian fluid was proposed. The mechanism of the polishing process and surface finishing were presented. To validate the feasibility of this novel polishing method, a polishing device was developed. Polishing experiments were carried out. The effects of process parameters on the surface roughness and material removal rate were investigated. A summary of experiment results were given in the conclusion section. This study aims at introducing a novel polishing method for curved surface polishing. 2 Principle of STP 2.1 Shear-thickening effect of non-Newtonian fluid Shear thickening fluid (STF) is a typical non-Newtonian fluid, and its viscosity depends on shear rate and shear stress. It is shown in Figure 1 (Galindo-Rosales et al., 2009; Galindo-Rosales et al., 2011), where a slight shear thinning behaviour exists under low shear rate (zone 1). However, when the shear rate exceeds a critical shear rate ηc, the viscosity of STF increases rapidly with shear rate (zone 2) (Galindo-Rosales et al., 2011). STF are now used in bullet-proof and shock-absorption areas. Lee et al. applied STF in the treatment of Kevlar fabrics. Experimental result shows that the STF can make protective equipment with good flexibility and high strength (Lee et al., 2003). Fischer et al. designed sandwich beam which utilised the STF and achieved the goal of controlling vibration response (Fischer et al., 2006). 84 Figure 1 B. Lyu et al. Typical viscosity curve of a shear-thickening fluid (see online version for colours) 2.2 STP slurry STP method utilises STF as the base fluid. Fine abrasives are dispersed evenly in the STF to obtain STP slurry (Min et al., 2015). Figure 2 Viscosity curve of the STP slurry Apparent Viscosity (Pa.s) 1 0.1 0.01 0.1 1 10 100 Shear Rate (1/s) In order to verify the shear-thickening effect of the polishing slurry, rheological property was investigated by dynamic rheometer. Figure 2 shows the relationship between apparent viscosity and shear rate of the polishing slurry. Its apparent viscosity does not change under low shear rate (below 0.2 s–1), show similar properties of the Newtonian fluid. With the increase of shear rate, the phenomenon of shear thinning occurs and this property is shown in Figure 1 zone 1. When the shear rate exceeds 10 s–1, apparent viscosity of the polishing slurry increases drastically, thus, the shear-thickening effect is Experimental study on shear thickening polishing method for curved surface 85 the dominant effect in this region. Unfortunately, shear rate exceeds 100 s–1 was unable to be obtained in this test. Although the abrasives were clustered, shear thickening property of the polishing slurry was still obvious. 2.3 Shear-thickening polishing The principle of STP process is shown in Figure 3. The polishing slurry will be moved relative to the workpiece surface. Then, the shear-thickening phenomenon occurs, and the viscosity of the non-Newtonian fluid in the contact area will increase rapidly. The solid colloidal particle dispersed in the slurry will aggregate to form particle clusters, in which abrasive grit will be surrounded by solid colloidal particles. As a result, the slurry in this area will act like a instant solid, and a flexible ‘fixed abrasive tool’ is formed in the contacting region, and the pullout strength to the abrasives acting on the workpiece are enhanced to accelerate the material removal rate. On the other hand, the fluidity properties of the non-Newtonian fluid guarantee that the ‘fixed abrasive tool’ can fit well with different curved surfaces. It means that complex curved surfaces can be polished conveniently in STP process. The shear-thickening phenomenon is reversible in the sense that when the shearing force is removed, the slurry will revert to the original state. Based on the above arguments, a high efficiency, high quality polishing of the workpiece can be achieved. Figure 3 Material removal mechanism of STP process (see online version for colours) Particle cluster Solid colloidal particle Grit Workpiece Distribution of acting force Shear-thickening Non-Newtonian fluid Shear-thickening effect depends on the relative movement between workpiece and the polishing slurry without any external field, only a relative motion between the workpiece and the slurry is required to make the shear thickening phenomenon occur. The STF can be easily prepared with environment-friendly materials. Thus, STP presented in this study is an environment-friendly, low cost polishing method. 3 Experiments setup and conditions An experimental STP setup was developed, which can provide a relative motion between workpiece and the STP slurry. As shown in Figure 4, the workpiece was clamped under the spindle, which was driven by a stepping motor. The motor and spindle were installed on the Z axis, which can be moved in the vertical direction by a lead screw stage. In addition, the Z axis was installed on the X axis, hence lateral movement can be realised. 86 B. Lyu et al. The polishing plate consists of a circular groove is used to hold STP slurry. The polishing plate was driven by a step motor at the bottom of the device. The experimental setup specifications are listed in Table 1. Above all, lateral, vertical, and rotary motion of the workpiece can be realised with the polishing machining tool. Figure 4 The polishing machining tool (see online version for colours) X axis Z axis Motor STP slurry Spindle Polishing plate Workpiece Table 1 The specifications of polishing machining tool Specification Diameter of workpiece Rotational speed of the motor Diameter of the polishing plate Rotational speed of the polishing plate Value Ø 10~50 mm 0~180 rpm Ø170~320 mm 0~120 rpm The influential factors on the surface roughness and material removal rate include polishing speed, abrasive concentration and abrasive size, were investigated in this study. Single variable method was used in these experiments. As the sphere is a typical curved surface, bearing steel ball was chosen as the workpiece. Experimental parameters are shown in Table 2. Table 2 Experimental parameters Experimental conditions Workpiece Abrasive Abrasive size Parameter Bearing steel ball Ø 35 mm Al2O3 1.000#, 2,000#, 4,000# Abrasive concentration (wt%) 10, 17, 23, 30 Polishing plate (rpm） 10, 20, 40, 60 Rotational speed of the workpiece (rpm) Processing time per trial (h) 100 1 Experimental study on shear thickening polishing method for curved surface 4 87 Results and discussion 4.1 Effect of polishing speed Shear-thickening effect, which depends on the relative movement between the workpiece and polishing slurry, was identified as an important factor in the polishing process. Thus, polishing speed plays a key role in these experiments. Experimental studies were carried out on polishing steel bearing ball in four groups of the polishing plates, each with different speed. The abrasive is Al2O3 with size of 4,000# and abrasive concentration of 23%. Figure 5 Relationship between the polishing speed and material removal rate (see online version for colours) 10rpm 20rpm 40rpm 60rpm 11 Material Removal Rate(mg/h) 10 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 Time (h) Relationship between the polishing speed and the surface roughness (see online version for colours) Surface Roughness (nm) Figure 6 10rpm 20rpm 40rpm 60rpm 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 1 2 3 4 5 6 Time (h) Figure 5 shows the curve of the material removal rate under different polishing plate speed. It was apparent from the figure that material removal rate increases rapidly with 88 B. Lyu et al. the increase of the polishing speed and is tend to rise slowly with the increase of processing time. Material removal rate at 60 rpm was nearly ten times compared that at 10 rpm. Figure 6 shows the result of surface roughness. The surface roughness shows an obvious decrease tendency with increasing polishing speed. The surface roughness decreases rapidly during the first three hours. The surface roughness has very little change during the last three hours of polishing. A smooth surface with Ra = 11 nm was obtained under the condition of 60 rpm after six hours of processing. Figure 7 Images of the machined surface, under different rotational speeds of polishing plate, (a) 10 rpm (b) 20 rpm (c) 40 rpm (d) 60 rpm (see online version for colours) 40 μm 40 μm (a) (b) 40 μm (c) 40 μm (d) Figure 7 shows images of the workpiece surface, which had been processed for six hours under different polishing plate speed. Images were observed under 400 X optical microscope. The workpiece surface showed many scratches with polishing plate speed of 10 rpm. On the contrary, only few scratches were observed and the surface was very smooth under the polishing condition of 60 rpm. Shear-thickening phenomenon is enhanced by the increasing of polishing speed. Abrasives will be surrounded tightly by solid colloidal particles. Cutting ability of the abrasives will be enhanced significantly. Meanwhile, the number of effective abrasives increases dramatically, which leads to rapid increase of material removal rate. The surface roughness also improved. Experimental study on shear thickening polishing method for curved surface 89 4.2 Effect of abrasive concentration Different abrasive concentration had been experimentally studied, and the processing time per trial was one hour with the abrasive type and size being Al2O3 and 4,000# respectively, and the polishing plate speed was at 60 rpm. Figure 8 Relationship between the abrasive concentration and material removal rate (see online version for colours) 10% 17% 23% 30% 16 Material Removal Rate (mg/h) 14 12 10 8 6 4 1 2 3 4 5 6 Time (h) Figure 8 shows the curve of material removal rate under different abrasive concentrations. It can be seen that the material removal rate changed drastically with the increase of abrasive concentration. It reached the maximum value of 15.4 mg/h in the last machining hour with abrasive concentration of 30%. Relationship between abrasive concentration and surface roughness (see online version for colours) Surface Roughness(nm) Figure 9 10% 17% 23% 30% 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 1 2 3 4 5 6 Time (h) Results of the surface roughness were shown in Figure 9. The surface roughness showed an obvious downward tendency with the increase of abrasive concentration. The surface 90 B. Lyu et al. roughness decreases rapidly in the first hour with abrasive concentration of 23% and 30%. The surface roughness reached nearly 10nm. It seems that there is no significant improvement in surface roughness by using abrasive concentration of 10% (from 92 nm to 51 nm after six hours machining). It can be seen from the experimental results that abrasive concentration also had an apparent effect on the material removal rate and surface roughness. When abrasive concentration is too low, the number of abrasive will decrease drastically, resulting in very thin polishing slurry, causing shear-thickening phenomenon to be weakened. Hence, material removal rate will be low and surface roughness improvement insignificant. On the other hand lump phenomena will occur if abrasive concentration is too high in the polishing slurry, which will result in poor experimental accuracy. It is important to choose the appropriate abrasive concentration. 4.3 Effect of abrasive size The abrasives were added as the second dispersed phase in non-Newtonian fluid with shear-thickening phenomenon. Particle size is an important specification of the abrasive. In order to explore the effect of abrasive size, 1,000#, 2,000# and 4,000# Al2O3 abrasive were chosen for the experiment with the polishing plate speed set at 60 rpm and the abrasive concentration of 23%. Material Removal Rate (mg/h) Figure 10 Relationship between the abrasive size and material removal rate (see online version for colours) 1000# 2000# 4000# 11.0 10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 1 2 3 4 5 6 Time (h) As shown in Figure 10, it was a special phenomenon that the removal rate increases as the abrasive size decreases. It is quite different from the principle of traditional polishing method. The average of material removal rate is 5.8 mg/h when abrasive size is 1,000#. However, it was only half of the material removal rate when abrasive size was 4,000#. Experimental study on shear thickening polishing method for curved surface 91 Surface Roughness(nm) Figure 11 Relationship between the abrasive size and the surface roughness (see online version for colours) 140 130 120 110 100 90 80 70 60 50 40 30 20 10 1000# 2000# 4000# 0 1 2 3 4 5 6 Time(h) From Figure 11, it can be seen that three curves of the surface roughness are very similar. In the first hour, the surface roughness of the workpiece observed a sharp fall. During the following five hours, the change of workpiece surface roughness has little change. The surface roughness can reach Ra = 14 nm, Ra = 13 nm and Ra = 11 nm when the abrasive size is 1,000#, 2,000# and 4,000# respectively. These results indicate that the abrasive size has no significant effect on the surface roughness. Material removal is of micro cutting of the workpiced surface by abrasives. Cutting force is provided by STF and the force is basically the same regardless of abrasive size. In other words, the material removal ability of large abrasives is similar to that of smaller abrasives. At the same abrasive concentration, smaller abrasives means large numbers of cutting abrasives in unit area of contact surface, result in increased material removal rate. On the other hand, the cutting process is soft and not rigid in the STP processing. Large abrasives will not cause obvious scratches on workpiece surface and can achieve a smooth surface, just as that achieved with smaller abrasives. This may explain the fact that little differences were observed on surface roughness of workpieced surfaces polished with abrasives with three different sizes. 4.4 Polishing results under appropriate experimental condition From the results above, polishing speed is a very important specification. Experimental results will be better with the increase of the polishing speed. Abrasive concentration is another factor. It will get a better result by choosing an appropriate abrasive concentration. Abrasive size had an obvious influence on material removal rate, but, it seems to have no influence on the surface roughness. An experiment had been investigated under appropriate condition that the polishing plate speed was set at 60 rpm, the abrasive concentration was 30% and the abrasive was 4,000# Al2O3. Workpiece surface with Ra = 105.95 nm was rapidly polished down to Ra = 6.99 nm after one hours of processing. 92 B. Lyu et al. Figure 12 3-dimensional texture of the workpiece surface, (a) before processing (b) after processing (see online version for colours) Ra 105.95 nm (a) Ra 6.99 nm (b) Figure 12 shows an image of 3-dimensional texture of the workpiece surface. Pictures of workpiece surface before and after polishing are shown in Figure 13, which indicates that the machined workpiece surface is very smooth and has a reflective effect similar to a mirror. Experimental study on shear thickening polishing method for curved surface 93 Figure 13 Pictures of the workpiece surface, (a) before processing (b) after processing (see online version for colours) (a) (b) 4.5 Comparison experiment In order to verify the effectiveness of the STP method, a comparison experiment was conducted. A fluid with no shear-thickening effect was used as the base fluid of the polishing slurry. In this experiment, abrasive (Al2O3) was dispersed in PFG 200 (polyethylene glycol) after mixing one to two hours to form a new polishing slurry (Al2O3/PEG). Initial apparent viscosity of the new polishing slurry is similar to the STP slurry, but, its viscosity will not increase with shear rate. Polishing plate speed, rotational speed of the workpiece and abrasive concentration were 60 rpm, 100 rpm and 60% respectively. Experimental results are shown in Table 3 after six hours of processing and STP optimised experimental results are given together. Table 3 Comparison parameters Slurry Parameters STP Al2O3/PEG Material removal rate (mg/h) 10.2 1.8 Surface roughness before processing (nm) 105.9 108 6 73 Surface roughness after processing (nm) It can be seen from the experimental results, there was obvious difference between the two experiments. The material removal rate was only 1.8 mg/h and workpiece surface was uneven when using Al2O3/PEG slurry, although its initial apparent viscosity was similar to that of the STP slurry. However, the material removal rate was improved by five times and there were almost no scratches after machining by utilising STP slurry. The results of the comparison experiment confirmed that STP method can achieve high efficiency in polishing curved surfaces and surface roughness can be improved significantly. 94 5 B. Lyu et al. Conclusions A novel polishing method based on the shear-thickening effect of non-Newtonian fluid was proposed to obtain extreme smooth curved surfaces. Experimental studies were carried out to reveal the influence of polishing speed, abrasive concentration and abrasive size on the surface roughness and material removal rate. The main results are summarised as follows: 1 The polishing speed has obvious influence on the material removal rate and surface roughness. With the increase of the polishing speed, the material removal rate increases rapidly and smoother surface with better roughness can be obtained. 2 The abrasive concentration is an important factor that influences polishing effect. The higher the abrasive concentration is, the better the results are. However, the abrasives will be lumped if there are too many them. 3 With the increase of the abrasive size, the material removal rate decreases, and abrasive size has very little effect on surface roughness improvement. 4 A fine workpiece surface with nearly no scratches was obtained, and surface roughness was reduced sharply from Ra = 105.95 nm to Ra = 6.99 nm after one hour of processing. 5 The results from comparison experiment confirmed that STP method can achieve high efficiency machining of curved surfaces. It can also improve surface roughness significantly. Acknowledgements The authors would like to express their thanks for the financial support of the project from National Science Foundation of China (51175166, U1401247). References Fischer, C., Braun, S.A., Bourban, P.E. et al. (2006) ‘Dynamic properties of sandwich structures with integrated shear-thickening fluids’, Smart Materials and Structures, Vol. 15, No. 5, pp.1467–1475. 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