Polishing Experiments Research Articles

MR Polishing을 이용한 비구면 렌즈의 연마 메커니즘 및 연마 특성 분석

The aspherical lens was designed to be able to array a focal point. For this reason, it has very curved surface. The aspherical lens is fabricated by injection molding or diamond turning machine. With the aspherical lens, tool marks and surface roughness affect the optical characteristics, such as transmissivity. However, it is difficult to polish free form surface shapes uniformly with conventional methods. Therefore, in this paper, the ultra-precision polishing method with MR fluid was used to polish an aspherical lens with 4-axis position control systems. A Tool path and polishing mechanism were developed to polish the aspherical lens shape. An MR polishing experiment was performed using a generated tool path with a PMMA aspherical lens after the turning process. As a result, surface roughness was improved from $R_a

Polishing characteristics of optical glass using PMMA-coated carbonyl-iron-based magnetorheological fluid

Soft magnetic carbonyl iron (CI) particles for magnetorheological (MR) polishing encounter corrosion problems as a result of their oxidation, leading to unpredictable polishing results. To overcome this issue, CI particles have been coated with either polymer or inorganic materials for improved MR polishing. In this study, CI particles were coated with poly(methyl methacrylate) to achieve improved MR polishing and anti-corrosion protection. In addition to an analysis of their rheological properties, a series of MR polishing experiments were performed to investigate the material removal rate and surface roughness for BK7 optical glass by changing experimental parameters, such as the wheel rotating speed and magnetic field intensity. A very fine surface roughness (Ra = 0.86 nm for PMMA coated CI/Ra = 0.92 nm for pristine CI) was obtained at a wheel speed of 1256 mm s−1 and a magnetic field intensity of 15.92 kA m−1.

Computational fluid dynamics–based analysis of material removal characteristics in fluid jet polishing

Fluid jet polishing is an enabling ultra-precision machining technology, which has not only been widely used for removing machine tool marks in order to achieve super finished surfaces, but also for controlling the form accuracy in machining freeform surfaces. Due to the complex machining mechanism, it is difficult to model the material removal rate accurately with consideration of a lot of operational parameters in fluid jet polishing. In this article, the optimal operational parameters and the significance of the important parameters are determined by the Taguchi design of experiments. Hence, a computational fluid dynamics–based analysis is built for the determination of the material removal rate in fluid jet polishing. In this model, the impact information of the particles with respect to the workpiece is computed by computational fluid dynamics simulation which is then coupled with a local mechanics erosion model so as to predict the detailed distribution of the material removal rate in fluid jet polishing. To verify the computational fluid dynamics–based erosion model, a series of polishing experiments have been conducted. The experimental results are found to agree well with the predicted form error and the pattern of the material removal rate by the integrated erosion prediction model.

Automatic robotic polishing on titanium alloy parts with compliant force/position control

In this article, an automatic robotic polishing technique and system is developed for the polishing of titanium alloy curved parts. By means of a designed compliant end-effector with a force sensor, the robotic polishing system with a position-based explicit force control architecture is first built to perform the polishing operation. Then, a specially designed multi-axis robotic post-processor based on computer-aided design/computer-aided manufacturing is developed to generate the basic position and posture of the polishing tool without any complicated teaching processes. Subsequently, an adaptive Anti-Saturation Integral Separated Fuzzy PI controller, which is able to imitate the manual polishing operation and prevent undesirable vibrations and mechanical collisions, is designed to control the normal contact force. The basic trajectory is changed each time after the current polishing cycle has been finished by online self-learning, and a new basic trajectory is generated for the next polishing cycle. Finally, the effectiveness of the proposed automatic polishing technique is evaluated by actual polishing experiments on titanium alloy (TC11) parts, and the experimental results show that the proposed automatic robotic polishing technique has a perfect control effect on the contact force and thus can achieve a good and uniform surface quality of the part.

Principle of ultra precision polishing with micro water mist for KDP/DKDP crystals

Potassium dihydrogen phosphate (KDP) or deuterated potassium dihydrogen phosphate (DKDP) crystals are difficult to machine because they are soft and hygroscopic. Based on the deliquescence nature of KDP/DKDP crystals, a novel polishing method with micro water mist for KDP/DKDP crystals was proposed in this paper. The processing principle was investigated and polishing experiments with this method were carried out. The results show that the crystals can be machined by this method and the maximum material removal rate can reach 2.02 µm/min. A polished surface with root-mean-square roughness 47.6 nm was achieved by polishing with micro water mist in 10 min. The material removing principle and the planarisation mechanism were experimentally explained and verified.

离子束抛光去除函数计算与抛光实验

离子束抛光去除函数计算与抛光实验

Modelling of Grain Motion for Three-body Abrasion

Surface formation for three-body abrasion is essentially more difficult to model than for two-body abrasion. In contrast to grains embedded in the polishing pad and therefore time-invariant tool geometry for two-body abrasion, here a variable tool geometry is prevailing as a result of the rolling abrasive grains. Nevertheless, three-body abrasion is an important abrasive process in the surface finishing of glass and ceramics and the modelling of the surface formation for mechanical abrasive machining with loose grains is required for a better process understanding and enhanced process design.The modelling of the surface formation for three-body abrasion is dealt with by splitting the integral removal volume under the polishing pad into the removal of single grains and distributing the total polishing force according to the interaction of individual grains with the workpiece material. This new methodical approach intends to determine the material removal in discrete time steps for every grain and further on the generated new surface topography. Data from polishing experiments like concentration and form of abrasives, polishing load, surface topography and integral removal are applied to set up this analytic-empiric model. Crucial elements of this model are the geometry of single grains, their rolling motion under load and the principle of material removal. In this paper the simulation of the rolling motion of single abrasive grains using a numerical time step model is introduced.

Deterministic measurement and correction of the pad shape in full-aperture polishing processes

Full-aperture polishing is a significant process in fabricating large optical flats because it restrains Mid-Spatial Frequency errors and removes material quickly on the whole optic surface. Nevertheless, optical flats fabricated by full-aperture polishing generally fail to meet the stringent requirement of surface figure, which has to be corrected by sub-aperture polishing processes. Surface figure of optical flats in full-aperture polishing processes is primarily dependent on the pressure distribution uniformity which correlates intensively with the lap shape. At present, practical and precise means are urgently desired for measuring and correcting the lap shape, especially the polyurethane pad lap. In the study, we present a novel method for deterministic measurement of the pad shape. The method obtains the height of the pad at spirally distributed locations implemented by the revolution of the pad and translation of the laser displacement sensor. The pad shape in terms of matrixes whose elements representing the heights at the corresponding locations is then calculated by interpolation algorithm based on the obtained data. Further, we propose a method for deterministic correction of the pad shape utilizing a small conditioning tool. The dwell time algorithm and implementation strategy for the dwell time are provided for common full-aperture polishers. These solutions for the deterministic measurement and correction of the pad shape have been validated on a full-aperture polisher with polyurethane pad. The polishing experiments revealed that the optic surface figure was obviously improved.

Influence of acid slurries on surface quality of LBO crystal in fixed abrasive CMP

The chemical action, which is generally determined by acid or alkaline regulator in the slurry, plays an important role in fixed abrasive chemical mechanical polishing (CMP). Experiments were conducted to explore the influence of acid slurries on surface quality and material removal in fixed abrasive CMP of LBO (LiB3O5) crystal with soft and brittle, difficult-machining properties. Four organic acid regulators with pH 4.0 (acetic acid, lactic acid, citric acid, and oxalic acid) were screened by the polishing experiments. Further, pH value of the regulator was optimized to the most suitable chemical environment fit for polishing of LBO crystal using three indexes, i.e., material removal rate (MRR), surface topography, and surface roughness. The results indicated that citric acid leads to more robust polishing performance with surface roughness Sa (surface roughness average) 0.52 nm lower than the others. Additionally, MRR decreases with increasing pH value. Surface roughness initially decreases with increasing pH value, then it mutates to the maximum 9.37 nm after reach the minimum. The abrasive-free acid slurry with citric acid at pH 5.0 is effective for fixed abrasive CMP of LBO crystal, obtaining a MRR of 366 nm/min and fine surface quality with surface roughness Sa 0.32 nm with rather slight surface damage.

Research on optimization of conformal polishing using continuous precession

Ultraprecision optical elements have found an increasing utilization in various fields for their superior performance so do the manufacturing technologies for optics. With the aim to optimize the conformal polishing (CP) process using continuous precession (Con-P), a theoretical and experimental investigation is presented. Firstly, this study introduced the merits of Con-P and the encountered problem when it is used in CP. By analyzing the removal function and dynamic polishing process, it was shown that the bad performance on CP of Con-P is coursed by the incompatibility of rotation speed of A-axis (RSA) and feeding speed (FS) of bonnet tool. Then, in order to obtain the optimal ranges of RSA and FS to improve the performance on CP, comparative simulations of Con-P were implemented and analyzed. It is shown that the ability for CP is changing with a newly proposed conformal factor f, f = n1 (RSA)/vfx (FS), in addition, only when f reaches a certain threshold (f > 20), Con-P has good performance on CP. On the other hand, it was also indicated that the material removal is only affected by the FS of bonnet tool. Finally, polishing experiments were conducted to validate the simulated optimization. It is revealed that the experimental results are basically consistent with the simulations, which proves the reliability of the optimization process by using conformal factor f.

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.

엔지니어링 세라믹스의 경면연마를 위한 효율적인 슈퍼피니싱 조건의 결정

The Engineering ceramics have some excellent properties as materials for modern mechanical and electrical components. It is, however, not easy to polish them efficiently because they are strong and hard. This study is carried out to obtain a mirror surface on engineering ceramics by surperfinishing with high efficiency. To achieve this, we conducted a series of polishing experiments using representative engineering ceramics, such as <TEX>$Al_2O_3$</TEX>, SiC, <TEX>$Si_3N_4$</TEX> and <TEX>$ZrO_2$</TEX>, using diamond abrasive film from the perspective of oscillations peed, the rotational speed of the workpiece, contact roller hardness, contact pressure and feed rate. Furthermore, the polishing efficiency and characteristics for engineering ceramics are discussed on the basis of optimal polishing time and surface roughness. Our results confirmed that efficient superfinishing conditions and polishing characteristics of engineering ceramics can be determined.

Development and Polishing Process of a Mobile Robot Finishing Large Mold Surface

□ A novel self-determination polishing robot finishing large mold free-form surface is developed, and the finishing process method is researched. Contrary to traditional approaches, our premise is that a large mold surface can be polished by using a small robot. This robot system is mainly composed of a polishing robot part, a computer system and a visual positioning system. A type of robot with four uniform distribution wheels was designed, which has two driving wheels and two driven wheels. Active compliant control of the polishing tool was provided by a pneumatic servo system, and a new special compliant abrasive tool was proposed on the basis of robot characteristics. The process planning steps consisted of subdividing the free-form surface, choosing an abrasive tool, planning the polishing path and optimizing machining parameters. Based on the orthogonal experiment and the grey relational analysis method, the optimal parameter combination was obtained for polishing force, tool speed and feed rate. Aiming to polishing times, the surface roughness method and polishing efficiency method were studied in detail. The polishing experiments were carried out in the robot using process parameters obtained by the efficiency method. These research results provided significant theory foundation and experimental data for a mobile robot planning polishing to realize intelligible process parameter selection.

Study on Polishing Technology for Hard-Brittle Materials by a Micro Abrasive Water Jet

s: Abrasive jet micromachining is considered as a promising precision processing technology for brittle materials such as silicate glass and silicon nitride that are increasingly used in various applications. In this study, some polishing experiments are conducted for hard-brittle materials by a micro slurry jet. The results show that the morphology and the integrity of the material surface are improved greatly after polishing. The average roughness (Ra) value of the silicate glass decrease from 2.32μm to 0.35μm and the average roughness (Ra) value of the Si3N4 decrease from 2.63μm to 0.34μm. The material removal mechanism and the surface formation mechanism are studied. The factors to influence the surface morphology, the surface quality and the surface roughness are analyzed in order to take measures to improve the surface quality and reduce the surface roughness value.

Research on Performance of Fixed Abrasive Tools of Polishing 6H-SiC Wafers

6H-SiC single crystal substrates are considered to be suitable for thin film growth of semi-conductive GaN with wide energy bandgaps, because the lattice mismatch between 6H-SiC and GaN is quite small compared with sapphire substrates. However, the process of SiC wafer prior to epitaxial growth is quite difficult due to its high hardness and chemical stability. The chemical mechanical polishing (CMP) is typically performed in semiconductor device planarization, but the polishing slurry is not environmental friendly and the polishing efficiency is very low. In this paper, polishing experiments were conducted on 6H-SiC wafers using a flexible fixed abrasive film fabricated by sol-gel technique. Other three fixed abrasive tools bought from the different companies were also chosen in these experiments. Abrasive wear resistance on the different tools was observed with the increase of polishing time. Studies were also made on the surface micrographs and roughness of the 6H-SiC wafers polished by the four different tools in order to evaluate the tool performance. The results indicated that poor wear resistance of abrasive were showed in tool C and D, less abrasive worn out in the sol-gel polishing film and tool B. The surface quality of 6H-SiC wafer polished by the sol-gel polishing film was much better than the other three tools, and it attained the lowest surface roughness Ra=0.003 μm. Keywords: Sol-Gel polishing film; Fixed abrasive tools; 6H-SiC wafers

Study on Pre-Mixed Micro Abrasive Water Jet Machining System

This paper conducts a study on a pre-mixed micro abrasive water jet machining system. A new kind of abrasive mixing tank and the jet system are designed and tapped for the pre-mixed micro abrasive water jet machining system. The performances of the pre-mixed micro AWJ machining system are tested, and some polishing experiments are conducted for hard-brittle materials such as silicon nitride. The results show the feasibility and the advantage of the pre-mixed micro abrasive water jet machining system for polishing hard-brittle materials.

The Role of Pad Topography in Chemical-Mechanical Polishing

In this paper, the role of pad topography on material removal rate (MRR) in chemical-mechanical polishing (CMP) is investigated. First, based on the mechanics of pad/particle and particle/wafer sliding contacts at an asperity of the polishing pad a new MRR model is developed. The model is then extended to multi-asperity contacts, taking into account the statistics of the asperity heights. The single-asperity model reveals that the removal rate at relatively low pressure strongly depends on the pressure and the area at a sliding asperity contact. However, removal rate per asperity becomes independent of the contact pressure once it exceeds a critical value, which is determined by the asperity hardness and the particle concentration. Material removal by multi-asperity sliding contacts increases due to the increase in real contact area, provided a large number of asperity contacts at pressures greater than the critical. The plasticity index is identified as a key parameter that determines the contact area ratio and proportion of asperities in contact at pressures greater than the critical, thus the overall MRR. The model suggests that MRR in CMP can be greatly increased by controlling the surface topography of the pads. Results of polishing experiments on Cu thin films validate the model.

Method to calculate the error correction ability of tool influence function in certain polishing conditions

Tool influence function (TIF) is quite important for computer-controlled optical surfacing (CCOS) technology. Quantitatively investigating the error correction ability of TIF in frequency domain is essential for CCOS to restrain different spatial frequency errors. The smoothing spectral function (SSF) is a newly proposed parameter to evaluate the error correction ability of CCOS process. Based on the SSF, a new method to calculate the error correction ability of TIF in certain polishing conditions will be proposed. The basic mathematical model for this method will be established in theory. A set of polishing experiments with rigid conformal (RC) tool are performed to calculate the error correction ability of TIF. The calculated results can quantitatively indicate the error correction ability of TIF for different spatial frequency errors in certain polishing conditions.

Study on Burr Removal Technology of Micro-Channelsby a Micro Slurry Jet

Micro-channels are widely used in many fields. There are a lot of burrs in the process of micro-channel machining, which affect the performance of micro-channels. In this study, some polishing experiments are conducted for the burr removal of micro-channels by the micro slurry jet on the home made micro slurry jet machining system. The morphology and the roughness of the micro-channels are measured. The factors to influence the burr removal and the micro-channel surface quality are analyzed in order to take measures to improve the surface quality. The results show that the micro slurry jet polishing not only removes the micro channel burrs, but also improves the micro channel surface quality. It is helpful to increase the flow of medium inside micro channel.

Surface roughness prediction and parameters optimization in grinding and polishing process for IBR of aero-engine

In order to improve surface quality and reduce surface roughness of integrally bladed rotors (IBRs), the optimizations of grinding and polishing process parameters, such as abrasive size, contact force, belt linear velocity, and feed rate, are carried out in this study. Firstly, the optimal range of each factor is obtained by single-factor experiment, and a surface roughness prediction model is generated according to the central composite design experiments and quadratic regression design technology. Secondly, the optimal level of each factor is confirmed through the surface roughness prediction model, and the response surface of the optimal level of each factor is drawn using Matlab. Then, the optimum process parameters are determined by analyzing the response surface. Finally, the optimum process parameters are selected in the grinding and polishing experiments for IBR. The experimental results show that grinding and polishing with the optimum parameters obtained in this study enhances the machining quality significantly. The surface roughness of IBR improved greatly (nearly 25 %) compared with grinding using normal parameters.