Small Polishing Tool Research Articles

Research on High‐Efficiency Curved Surface Polishing with Even Distribution of Slurry Based on Dielectrophoresis

To overcome the problems of low slurry utilization and poor surface uniformity in conventional small tool polishing of curved surfaces, a small tool polishing method based on slurry dielectrophoresis (STP‐DEP) is proposed. The dielectrophoretic effect distributes the slurry evenly and increases its dwell time in the processing area, enabling more abrasives to participate in polishing, thereby improving the material removal efficiency and surface uniformity of the workpiece. The observation experiment indicates that dielectrophoresis can efficiently distribute the slurry evenly. Compared with the traditional small tool polishing, the results show that the polishing efficiency of the STP‐DEP is improved by 34% on the premise of achieving the same surface roughness Ra of the workpiece. The Taguchi experiments are carried out on K9 convex lens. Results show that the optimal surface roughness Ra 3.3 nm (initial Ra 2400 nm, processing time 120 min) is obtained when the tool speed is 400 rpm, the pressing amount is 1 mm, the infeed angle is 15°, and the abrasive size is 3 μm. The research proves that the STP‐DEP successfully solves the problem of slurry uneven distribution, and realizes efficient surface polishing with high surface uniformity.

High-precision turning and ultra-smooth direct polishing of aluminum alloy mirrors.

Due to the high surface roughness requirements of aluminum alloy mirrors used in the visible light band, there are still great challenges in single point diamond turning of high-surface quality aluminum alloy mirrors. In this paper, a processing method for aluminum alloy mirrors is proposed. Based on single point diamond turning technology, the prediction model of aluminum alloy surface roughness was established. The mapping relationship between the surface roughness of the aluminum alloy mirror and each turning parameter was obtained, and the maximum possible surface quality was achieved. On the basis of the turning results, the method of small tool polishing was used to remove the turning texture generated by the copy effect of the tool arc radius, suppress errors of the medium and high-frequency, and reduce the surface roughness. The single abrasive removal efficiency model was established and mechanical removal in the polishing process was analyzed. Combined with the chemical action in the polishing process, two types of polishing liquid-acidic and neutral, were prepared and analyzed. The optimal polishing parameters were obtained through multiple single-factor experiments. On the basis of this, the surface roughness of the aluminum alloy after turning was optimized. The results show that the value was reduced from 4.811 to 1.482 nm, an increase of 69.2%. This method can effectively improve the machining accuracy of aluminum alloy mirrors and provide an important process guarantee for the application of aluminum alloy materials in visible-light systems.

Investigation on Polishing the Concave Surface of Zirconia Ceramics with Magnetic Compound Fluid Enhanced by Hydration Reaction

Zirconia ceramics are prominent engineering materials and are widely used in computers, consumer electronics, and the fifth-generation communication industry. However, zirconia ceramics are a typical hard-to-cut material, and the product structures are more complex as the demanding on the industry increases. In this case, the polishing efficiency should be improved for meeting these requirements. To overcome the problem of polishing concave surfaces of zirconia ceramics, a small polishing tool with a magnetic compound fluid (MCF) was invented. The effect of the polishing parameters on the surface roughness and material removal rate was analyzed by an L9(33) orthogonal experiment. The weight ratio of the parameters was also studied based on the experimental results. With the combination of chemical and mechanical functions, the polishing characteristics were further examined. Based on the soaking experiments, the material removal mechanism is discussed. The results are as follows: (1) the optimal polishing parameters were the revolution speed of the MCF carrier nc of 400 rpm, the working gap h of 0.1 mm, the CIP size D of 5 μm for better surface roughness, the revolution speed of the MCF carrier nc of 400 rpm, the working gap h of 0.1 mm, and the CIP size D of 7 μm for a higher material removal rate. The impact degrees on surface roughness and material removal rate were a revolution speed of the MCF carrier of 54% > working gap of 31%> CIP size of 15% and working gap of 40% > revolution speed of the MCF carrier of 32% > and CIP size of 18%, respectively. (2) Surface roughness was rapidly reduced in the first 20 min and tended to be stable in the last 10 min of polishing. A circular polished area was observed on the flat workpiece for studying the typical material removal curve, and the deepest point was found at the fringe of the material removal curve. The concave workpiece was polished successfully, and the best surface roughness Ra reached 1 nm and 1.2 nm. (3) A pH = 10 with a sodium hydroxide (NaOH) solution has a greater performance in hardness reduction. The chemical and mechanical functions were combined to remove material to enhance the polishing efficiency. All in all, the proposed polishing method with a combination of a small MCF polishing tool and hydration reaction was effective for polishing zirconia ceramics.

Statistical perception of the chaotic fabrication error and the self-adaptive processing decision in ultra-precision optical polishing.

Subaperture polishing is a key technique for fabricating ultra-precision optics. However, the error source complexity in the polishing process creates large fabrication errors with chaotic characteristics that are difficult to predict using physical modelling. In this study, we first proved that the chaotic error is statistically predictable and developed a statistical chaotic-error perception (SCP) model. We confirmed that the coupling between the randomness characteristics of chaotic error (expectation and variance) and the polishing results follows an approximately linear relationship. Accordingly, the convolution fabrication formula based on the Preston equation was improved, and the form error evolution in each polishing cycle for various tools was quantitatively predicted. On this basis, a self-adaptive decision model that considers the chaotic-error influence was developed using the proposed mid- and low-spatial-frequency error criteria, which realises the automatic decision of the tool and processing parameters. An ultra-precision surface with equivalent accuracy can be stably realised via proper tool influence function (TIF) selection and modification, even for low-deterministic level tools. Experimental results indicated that the average prediction error in each convergence cycle was reduced to 6.14%. Without manual participation, the root mean square(RMS) of the surface figure of a ϕ100-mm flat mirror was converged to 1.788 nm with only robotic small-tool polishing, and that of a ϕ300-mm high-gradient ellipsoid mirror was converged to 0.008 λ. Additionally, the polishing efficiency was increased by 30% compared with that of manual polishing. The proposed SCP model offers insights that will help achieve advancement in the subaperture polishing process.

Polishing of Micro Aspheric Mold Based on Viscoelastic Spherical Small Polishing Tool

Polishing of Micro Aspheric Mold Based on Viscoelastic Spherical Small Polishing Tool

Research on constant force polishing method of curved mold based on position adaptive impedance control

The quality and life of the injected product are affected by surface quality of curved mold, and reasonable polishing method is the key to obtain the high-quality surface of curved mold. Small polishing tool is controlled by constant displacement polishing in the conventional method of robot polishing. The polishing force is adjusted to affect the surface quality of mold during mold polishing. The shortcomings of traditional control method of robot are investigated by simulation, and the constant force control approach is proposed to maintain stable force, which is achieved by a position-based impedance control algorithm. The polishing experiments of curved mold are conducted by using two control methods with constant displacement control polishing (CDCP) and constant force control polishing (CFCP), respectively. The experimental results show that the CFCP method can maintain polishing force stability and the reduction of surface roughness in the three groups of experiments using CDCP method is much lower than that of CFCP method, respectively. The feasibility of constant force control polishing method (CFCP) is verified.

Surface Quality Control Strategy of Aspherical Mold Based on Screw Feed Polishing of Small Polishing Tool.

For small aspherical molds, it is difficult for the existing polishing method to take into account the correction of the surface error and the control of the uniformity of the surface roughness (SR) distribution, because the polishing tool is always larger than the small mold. Therefore, we used viscoelastic polyester fiber cloth to wrap the small steel ball as a polishing tool to adapt to the surface shape change of the aspherical mold, and designed a semi-flexible small polishing disc tool with microstructure, which can better adapt to the curvature change of aspherical surface and obtain better SR Ra. At the same time, a combined polishing method of constant speed and variable speed for screw feed was proposed to improve the uniformity of SR distribution in the paper. Then, a series of theoretical analysis and experimental verification were carried out in this paper to predict the tool influence function (TIF) of the two polishing tools and the effectiveness of the combined polishing method. In the experiment, a TIF bandwidth of about 0.46 mm was obtained with a small spherical polishing tool, which favors the surface shape correction of the small aspherical mold. The experiment of uniform removal with a small polishing disc tool was carried out to quickly reduce the Ra. Finally, the surface quality of the aspherical mold was effectively improved, combined with the constant speed and variable speed polishing modes of screw feed of the small spherical polishing tool and the smoothing effect of the small polishing disc tool. The peak valley (PV) of two small aspherical molds with an optical effective diameter less than 13 mm converged from 0.3572 μm and 0.2075 μm to 0.1282 μm and 0.071 μm, respectively. At the same time, the SR dispersion coefficient was reduced from 27.9% and 41.6% to 14.2% and 12.7%, respectively. The study provides a good solution for the surface quality control of small aspherical molds.

Generalized large optics fabrication multiplexing

High precision astronomical optics are manufactured through deterministic computer controlled optical surfacing processes, such as subaperture small tool polishing, magnetorheological finishing, bonnet tool polishing, and ion beam figuring. Due to the small tool size and the corresponding tool influence function, large optics fabrication is a highly time-consuming process. The framework of multiplexed figuring runs for the simultaneous use of two or more tools is presented. This multiplexing process increases the manufacturing efficiency and reduces the overall cost using parallelized subaperture tools.

Two-dimensional vibration actuated polishing of small surfaces by generating random-like Lissajous trajectories.

Small-surface optical components with complex shapes and high-precision requirements are increasingly needed in the fields of mobile communications, in vivo diagnosis, and other fields. Some scholars have studied and proposed a two-dimensional vibration actuated polishing (2D-VAP) method based on small polishing tools for the preparation of small-surface optical elements. Using the nonresonant 2D-VAP device developed by the author, the frequency and amplitude of 2D-VAP are precisely adjusted to generate a random-like Lissajous polishing trajectory, which can overcome the problem that most of the existing 2D-VAP methods generate a circular or elliptical polishing trajectory at the small polishing tool, resulting in leaving periodic polishing marks on the workpiece surface. The removal function model under the condition of random-like Lissajous polishing motion with a small polishing tool is established. In addition, the removal function verification experiments and surface polishing experiments are carried out. The experimental results show that the measured removal function is in good agreement with that obtained by numerical simulation. Compared with the circular polishing trajectory, the random-like Lissajous polishing trajectory can significantly improve the material removal rate, and there are no obvious periodic polishing marks on the workpiece surface.

Experimental investigation and analytical modelling of the tool influence function of the ultra-precision numerical control polishing method based on the water dissolution principle for KDP crystals

A single crystal of potassium dihydrogen phosphate (KH2PO4, KDP), which possesses unique excellent non-linear electro-optical properties, is currently the only material suitable for electro-optic switches and high power laser-frequency conversion applications in laser-induced inertial confinement fusion. However, KDP crystals are difficult to produce because of their inherent softness, brittleness, and water-solubility, as well as their strong anisotropy and temperature sensitivity. Obtaining high-quality near-lossless KDP elements is an issue that should be solved urgently. In this study, an ultra-precision numerical control polishing method based on the water dissolution principle was introduced to achieve a controllable material removal. A small polishing tool was used to process a large KDP surface, and an accurate tool influence functions is required for deterministic fabrication of KDP surface. In order to quantify the influence of critical parameters (e.g., polishing speed, polishing pressure, water content, and directions of revolution and rotation) on material removal rate distribution, a tool influence function model was established based on the Preston equation. The model was then modified based on experiments, and its accuracy was verified. This modified model lays the foundation for ultra-precision water dissolution polishing of large KDP crystals. A very smoothed surface with high surface accuracy (peak-to-valley value below 0.4 λ) and low surface roughness (Ra 1.598 nm) could indeed be obtained by using the model. This research is also applicable to the polishing of other water-soluble materials.

Mid-spatial frequency removal on aluminum free-form mirror.

Mid-spatial frequency (MSF) errors challenge freeform manufacture, not in the least due to tool-misfit. This can compromise the performance of functional surfaces and is difficult to remove by post-processing. Our previously reported work on an effective process-chain for aluminum polishing demonstrated the ability to remove MSFs by hard-tool grolishing. In this paper, we describe MSF removal on an aluminum mirror, deformed to a saddle-like freeform shape, using power spectral density (PSD) as a diagnostic. CNC Precessions bonnet polishing was optimized to minimize output MSFs, then a non-Newtonian (n-N) tool was used to attenuate the residual MSFs that were present. Our approach was distinct from the approach pioneered by University of Arizona, in that we adopted small-tool polishing on the saddle-like part, with removal rate restored by rotating the n-N tool. In order to define the optimum window of rotation speeds, the dynamic behavior of the n-N material was explored by modelling and experiments. The tool was deployed on an industrial robot, and we describe a novel 'hyper-crossing' tool-path with wide sweeping paths, which is the logical opposite of the unicursal zero-crossing paths we have previously reported. This new path has proved ideally suited to robots, given their high velocity/acceleration capabilities. Detailed results are presented from the PSD viewpoint.

Stabilization of Removal Rate in Small Tool Polishing of Glass Lenses

The demand for improving the image quality of cameras has increased significantly, especially in industrial fields such as broadcasting, on-vehicle, security, factory automation, and medicine. The surface of glass lenses as a key component of cameras is formed and finished by polishing using small tools. The existing polishing technologies, however, exhibit serious problems including an unstable material removal rate over time. In our previous work, the mechanism of time variation in material removal rate was clarified. Based on the findings, a vibration-assisted polishing method using polishing pads containing titanium dioxide particles was developed for improving the stability of the material removal rate with the accumulated polishing time. Our experiments revealed that the proposed polishing method suppressed the time variation significantly in the material removal rate. The developed polishing pads, however, possessed a short life because of their poor wear resistance; as such, they could not be applied to the mass-production process of lenses. In this study, we applied the vibration-assisted polishing method to the polishing process using commercial polishing pads that exhibit sufficient wear resistance for practical use. To investigate the effect of vibration on the stability of the material removal rate, polishing experiments and the observation of slurry flow on the surface of the polishing pads during the vibration-assisted polishing process were conducted. Based on the findings, a new polishing method utilizing a large-amplitude high-frequency vibration applied to the polishing pressure was developed. In addition, a new polishing method utilizing the overhang of a polishing pad, where the polishing pad was moved to hang over the edge of the workpiece for incorporating periodic dressing processes of the polishing pad surface during the polishing process, was also developed. Our polishing experiments revealed that both the proposed polishing methods improved the stability of the material removal rate significantly over the course of the polishing process.

Research on scan polishing flat surfaces with a small diameter tool

High profile accuracy and high surface quality, which are achieved by stable polishing velocity, are needed in the machining of NiP neutron reflection mirror along with suitable machining tools. To respond to the growing demands for high quality reflection mirrors, in this study, a small diameter polishing tool was used on a three-axis CNC ultra-precise polishing machine. Mathematical models of polished profiles based on the Preston equation were built and discussed along with the simulation results of the polished profiles. These models of polished profiles were proved to be effective by numerous experiments under diverse conditions. The polishing characteristics of electro-less plated NiP surface were reviewed, and smooth surfaces could be achieved by scan-type polishing.

Stabilization of removal rate in small tool polishing of glass lenses

Demand for improving image quality of cameras has largely grown, especially in industrial fields such as broadcasting, on-vehicle, security, factory automation and medicine. Surface of glass lenses as a key component of cameras is formed and finished by polishing using small tools. The existing polishing technologies, however, have serious problems including an unstable material removal rate over time. In this study, we investigated mechanism of the variation in the material removal rate, specifically the low material removal rate at the beginning of polishing process. Based on the findings, we clarified that surface of polishing pad with high wettability is effective in stabilizing the removal rate.

Tool paths generation strategy for polishing of freeform surface with physically uniform coverage

The tool path used for polishing applications with characteristic of physically uniform coverage, which is similar to the iso-scallop path in milling operations, is significantly important to facilitate uniform material removal and acquire low surface roughness and consistent surface quality. In this paper, tool path planning method for physically uniform coverage instead of traditional geometrically uniform coverage of polishing path based on scanning mode is further investigated and an efficient iterative approximation algorithm is proposed. Then a complete spiral path generation strategy for small tool polishing of freeform surface is presented, which uses a cyclic iteration correction and driving method to produce spiral path with physically uniform coverage. At the same time, this strategy uses surface expansion and re-parameterization techniques to avoid edge effect in polishing. The effectiveness and robustness of the developed polishing path generation technique are proved by case studies. And the superiority of the planned polishing path over the traditional path in promoting uniformity of material removal is examined through the practical application of polishing.

Optimization technique for rolled edge control process based on the acentric tool influence functions.

In the process of computer controlled optical surfacing (CCOS), the uncontrollable rolled edge restricts further improvements of the machining accuracy and efficiency. Two reasons are responsible for the rolled edge problem during small tool polishing. One is that the edge areas cannot be processed because of the orbit movement. The other is that changing the tool influence function (TIF) is difficult to compensate for in algorithms, since pressure step appears in the local pressure distribution at the surface edge. In this paper, an acentric tool influence function (A-TIF) is designed to remove the rolled edge after CCOS polishing. The model of A-TIF is analyzed theoretically, and a control point translation dwell time algorithm is used to verify that the full aperture of the workpiece can be covered by the peak removal point of the tool influence functions. Thus, surface residual error in the full aperture can be effectively corrected. Finally, the experiments are carried out. Two fused silica glass samples of 100 mm×100 mm are polished by traditional CCOS and the A-TIF method, respectively. The rolled edge was clearly produced in the sample polished by the traditional CCOS, while residual errors do not show this problem the sample polished by the A-TIF method. Therefore, the rolled edge caused by the traditional CCOS process is successfully suppressed during the A-TIF process. The ability to suppress the rolled edge of the designed A-TIF has been confirmed.

Optical surfacing via linear ion source

We present a concept of surface decomposition extended from double Fourier series to nonnegative sinusoidal wave surfaces, on the basis of which linear ion sources apply to the ultra-precision fabrication of complex surfaces and diffractive optics. The modified Fourier series, or sinusoidal wave surfaces, build a relationship between the fabrication process of optical surfaces and the surface characterization based on power spectral density (PSD) analysis. Also, we demonstrate that the one-dimensional scanning of linear ion source is applicable to the removal of mid-spatial frequency (MSF) errors caused by small-tool polishing in raster scan mode as well as the fabrication of beam sampling grating of high diffractive uniformity without a post-processing procedure. The simulation results show that optical fabrication with linear ion source is feasible and even of higher output efficiency compared with the conventional approach.

A water dissolution method for removing micro-waviness caused by SPDT process on KDP crystals

Single crystals of potassium dihydrogen phosphate (KDP) are an irreplaceable optical material in laser-induced inertial confinement fusion (ICF), where they are used as electro-optic switches and for laser-frequency conversion. The KDP lenses used in ICF applications must be pristine, high quality, and have a surface roughness of less than 3 nm. Currently, single-point diamond turning (SPDT) is the primary method for performing ultra-precision machining on KDP crystals. However, after the SPDT process, the crystal surface exhibits micro-waviness and subsurface damage, both of which have adverse effects on the optical performance of the KDP elements, especially the laser-induced damage threshold. In this study, a new method of micro-dissolution polishing was developed, which is based on the solubility of KDP crystals in water, to achieve controllable material removal. A material removal function was formulated to describe the material removal in terms of both the path and parameters used to polish the SPDT-machined samples with a small polishing tool. An experimental study showed that this method is able to reduce the surface micro-waviness and surface roughness (from 6.205 to 2.107 nm) significantly. The method is also applicable to the polishing of other water-soluble materials.

Path planning and parameter optimization of uniform removal in active feed polishing

A high-quality ultrasmooth surface is demanded in short-wave optical systems. However, the existing polishing methods have difficulties meeting the requirement on spherical or aspheric surfaces. As a new kind of small tool polishing method, active feed polishing (AFP) could attain a surface roughness of less than 0.3 nm (RMS) on spherical elements, although AFP may magnify the residual figure error or mid-frequency error. The purpose of this work is to propose an effective algorithm to realize uniform removal of the surface in the processing. At first, the principle of the AFP and the mechanism of the polishing machine are introduced. In order to maintain the processed figure error, a variable pitch spiral path planning algorithm and the dwell time-solving model are proposed. For suppressing the possible mid-frequency error, the uniformity of the synthesis tool path, which is generated by an arbitrary point at the polishing tool bottom, is analyzed and evaluated, and the angular velocity ratio of the tool spinning motion to the revolution motion is optimized. Finally, an experiment is conducted on a convex spherical surface and an ultrasmooth surface is finally acquired. In conclusion, a high-quality ultrasmooth surface can be successfully obtained with little degradation of the figure and mid-frequency errors by the algorithm.

Path planning of mechanical polishing process for freeform surface with a small polishing tool

Products with freeform surface are widely applied in industries, and the surface quality plays an important role in order to fulfill the targeted functions. As polishing path of small polishing tool affects the polishing removal function considerably, it is highly necessary to study the polishing path of freeform surface for obtaining good polishing efficiency and well-proportioned surface quality. By combining the Preston polishing removal function, the material removal model of small polishing tool under the control of constant polishing force and pressure is established. Based on this model, the material removal functions of scan line, Archimedean spiral, and Hilbert fractal polishing path are derived. The simulation results show that the Hilbert fractal polishing path has the best comprehensive performance. By using the projection relation of differential geometry, the optimal path generation algorithm of the Bezier surface based on Hilbert fractal polishing path is established. The polishing experiments are conducted on a self-developed polishing machine which is based on a parallel manipulator. The experimental results demonstrate that the surface roughness is improved from level 9 to level 11.