Computer-controlled Polishing Research Articles

Toward the fabrication of a 5-μm-resolution Wolter microscope for the National Ignition Facility (invited).

Advancements in computer-controlled polishing, metrology, and replication have led to an x-ray mirror fabrication process that is capable of producing high-resolution Wolter microscopes. We present the fabrication and test of a nickel-cobalt replicated full-shell x-ray mirror that was electroformed from a finely figured and polished mandrel. This mandrel was designed for an 8-m source-to-detector-distance microscope, with 10× magnification, and was optimized to reduce shell distortions that occur within 20mm of the shell ends. This, in combination with an improved replication tooling design and refined bath parameters informed by a detailed COMSOL Multiphysics® model, has led to reductions in replication errors in the mirrors. Mandrel surface fabrication was improved by implementing a computer-controlled polishing process that corrected the low-frequency mandrel figure error and achieved <2.0nm RMS convergence error. X-ray tests performed on a pair of mirror shells replicated from the mandrel have demonstrated <10 μm full-width at half-maximum (FWHM) spatial resolution. Here, we discuss the development process, highlight results from metrology and x-ray testing, and define a path for achieving a program goal of 5 μm FWHM resolution.

A predictable smoothing evolution model for computer-controlled polishing

Quantitative prediction of the smoothing of mid-spatial frequency errors (MSFE) is urgently needed to realize process guidance for computer controlled optical surfacing (CCOS) rather than a qualitative analysis of the processing results. Consequently, a predictable time-dependent model combining process parameters and an error decreasing factor (EDF) were presented in this paper. The basic smoothing theory, solution method and modification of this model were expounded separately and verified by experiments. The experimental results show that the theoretical predicted curve agrees well with the actual smoothing effect. The smoothing evolution model provides certain theoretical support and guidance for the quantitative prediction and parameter selection of the smoothing of MSFE.

Surface removal characteristic of computer-controlled dual-rotor polishing

Computer-controlled dual-rotor polishing, as the final step to correct the optical part surfaces, plays an important role in improving the quality and service life of the surfaces. However, in previous study, the material removal model of dual-rotor polishing was fixed-point polishing model, which could not continuously polish the workpiece. Herein, a novel material removal model of dual-rotor polishing is proposed according to the removal profile orthogonal to the tool path. Firstly, the material removal function per unit time is developed, which is further converted into the material removal function per unit length of the tool path. Secondly, for obtaining the removal profile of dual-rotor polishing, the above function is integrated along the tool path in the polishing area. Then, to verify the effectiveness of the material removal model, the polished profile is studied experimentally and the effects of parameters, such as eccentricity, speed ratio, and main motion feed rate, on material removal of dual-rotor polishing are theoretically analyzed. It is demonstrated that the experimental material removal profiles are in good agreement with the theoretical results; besides, the stable and efficient polishing process can be obtained by optimizing parameters, which have different effects on the material removal depth. This work is of great significance for correcting the optical surface.

Introducing Full-Aperture FJP (faFJP), a Technique for Manufacturing Sub-Millimetre Aspheres

We propose a novel approach, "faFJP", to the computer controlled polishing (CCP) of mini aspheres down to 0:2 mm. The generation of locallytunable, full-aperture footprints is accomplished by applying a Fluid Jet Polishing (FJP) beam larger than the full aperture being polished and varying the incident angle.

Full-shell x-ray optics development at NASA Marshall Space Flight Center.

NASA's Marshall Space Flight Center (MSFC) maintains an active research program toward the development of high-resolution, lightweight, grazing-incidence x-ray optics to serve the needs of future x-ray astronomy missions such as Lynx. MSFC development efforts include both direct fabrication (diamond turning and deterministic computer-controlled polishing) of mirror shells and replication of mirror shells (from figured, polished mandrels). Both techniques produce full-circumference monolithic (primary + secondary) shells that share the advantages of inherent stability, ease of assembly, and low production cost. However, to achieve high-angular resolution, MSFC is exploring significant technology advances needed to control sources of figure error including fabrication- and coating-induced stresses and mounting-induced distortions.

Development and theoretical analysis of novel center-inlet computer-controlled polishing process for high-efficiency polishing of optical surfaces

Although traditional computer-controlled optical surfacing (CCOS) technology has been successfully developed for polishing large aspheric optics, the polishing process continues to pose several challenges, including polishing tool wear and uneven distribution of polishing particles in contact areas. To improve the efficiency and stability of the polishing process, we present a novel center-inlet computer-controlled polishing (CCCP) process and tool for high-efficiency polishing of large-diameter aspheric optics. Compared with traditional CCOS, the most distinctive feature of CCCP is that the polishing slurry is supplied by the central hole of the polishing tool to improve its utilization efficiency. Moreover, a flexible coupling is used to connect the tool and motorized spindle to keep the tool in parallel with the work surface during the polishing process. The experimental apparatus and optimization design of the CCCP tool were first constructed based on CCOS. Then, the material removal mechanisms were explained by a series of theoretical and experimental studies. Results indicated that CCCP can improve the wear resistance of the polishing pad and make fuller and more even abrasive grain supply compared with traditional CCOS. The innovative theoretical model was verified and used for optimizing the epicyclic tool motion in CCCP. The experiments indicated that CCCP can produce a higher material removal rate and a better surface state than can CCOS.

Machine learning robot polishing cell

The quality of optical components such as lenses or mirrors can be described by shape errors and surface roughness. With increasing optic sizes, the stability of the polishing process becomes more and more important. Parameters such as chemical stability of the polishing slurry or tool wear are key elements for a deterministic computer controlled polishing (CCP) process. High sophisticated CCP processes such as magnetorheological finishing (MRF) or the zeeko bonnet polishing process rely on the stability of the relevant process parameters for the prediction of the desired material removal. The aim of this work is to monitor many process-relevant parameters by using sensors attached to the polishing head and to the polishing process. Examples are a rpm and a torque sensor mounted close to the polishing pad, a vibration sensor for the oscillation bearings, as well as a tilt sensor and a force sensor for measuring the polishing pressure. By means of a machine learning system, predictions of tool wear and the related surface quality shall be made. The aim is the detection of the critical influence factors during the polishing process and to have a predictive maintenance system for tool path planning and for tool change intervals.

Editorial Note

In this issue, variety of topics is presented as part of the rich world of Robotics and Mechatronics. We present to readers five papers, which are related to image compression technique, product development model, heaving wave energy control, human emotion detection, and computer controlled polishing for modern free-form lenses. The first paper, authored by Falila et al., presents a comparative study on traditional techniques of multi-spectral image compression, such as JPEG, JPEG2000, set partitioning in hierarchical trees (SPIHT), and their proposed modified SPIHT algorithm. The proposed technique is a hybrid compression technique which depends on interband JPEG prediction and the SPIHT algorithm which is extended for multi-spectral case. The experimental results presented indicate that the proposed technique outperformed the other three techniques in term of peak-signal-to noise ratio (PSNR) for multi-spectral image compression and transmission over OFDM communication system. The second paper, authored by Tseng Hsu et al., discusses the implementation of user-oriented model for collaborative product development, illustrated on medical-equipment industry. This type of industry has the characteristics of generating products with short life cycle, high technical complexity and a low chance of success. Therefore, it requires an intelligent system for its collaborative product development. The model contains a knowledge-based operation and maintenance system, a clinical test system, a file management system, an accessibility system, a calendar function and an announcement module. The system proposed by the authors can simplify the originally complex and dispersed process of product development. The third paper, authored by Wahyudie et al. is on optimal control strategy for heaving wave energy converters. The authors proposed a robust hierarchical control strategy which can provide optimal reference signal for various combinations of wave specific heights and frequencies under elevation's constraints. The controller consists of two parts, higher hierarchical controller (HHC) and lower hierarchical controller (LHC). The optimal reference signal is provided by the HHC, and the robust tracking action is performed by the LHC, which is based on a proportional-integral-derivative (PID) controller with sliding mode control (SMC). The proposed controller provides better results in terms of captured mechanical power, converted electrical power and efficiency in comparison to other existing control strategies, such as resistive loading control and conventional hierarchical control system. The fourth paper, authored by Md. Yusoff, et al., proposes human emotion detection system based on thermal imaging. The method uses the Gray Level Co-occurrence Matrix (GLCM) to analyze the heat pattern on the facial skin corresponds to particular affective states, and the k-Nearest Neighbor classifier to classify three human basic emotions (fear, happy and sad). Human-robot interaction has become one of very important topics in Robotics. In order to develop an emotionally intelligent robot with social and interactive skills, it is essential to understand human affective states. The proposed system has successfully detected the three states of emotions with an overall accuracy of approximately 90.3%. Last but not least, the fifth paper, authored by Mosaddegh et al., presents an open-loop fuzzy optimizer for computer controlled polishing (CCP) for optical lens manufacturing. CCP is an alternative approach for manufacturing process of optical lenses. The proposed open-loop fuzzy controller for CCP is designed to optimize the tool feed rate distribution over the toolpath to reduce the surface error in minimum time. Simulation and experimental results indicated that the proposed method has reduced the surface error over 85%. We hope readers find those valuable contributions beneficial and reflect the richness of research and development in the fields of Robotics and Mechatronics. We are looking forward to more high quality contributions in IJRM from all of you!

Open Loop Fuzzy Optimizer for Computer Controlled Polishing

Computer Control Polishing (CCP) was developed during the last four decades to administrate the operation of almost all polishing methods. To this end, , enhancing the simulation of CCP process not only improves the quality of final products but also trims operation time and, consequently, leads to accelerate the manufacturing of optical lenses. In this study, a new method has been developed and applied for simulation of polishing process based on discretizing continuous toolpath to small distance—100 µm in length—which accordingly heightens the accuracy of simulation process. Smoothing feed rate fluctuation over toolpath is also a side benefit of the represented method that precludes damage to machine and workpiece. To proceed with CCP, we conducted an open-loop fuzzy controller approach to optimize the tool feed rate distribution over the toolpath. The method has the ability to optimize feed rate to reduce surface error for a variety of toolpaths and lens geometries. The simulation and experimental results of series of tests on a BK7 plano-plano lens show about 85% of surface roughness improvement. High percentage of improvement and the resemblance between simulation and experimental results respectively confirmed the accuracy of simulation method and the capability of optimization algorithm.

Modelling of the Polished Profile in Computer-Controlled Polishing by a Sub-Aperture Pad

High-quality part surfaces with high surface finish and form accuracy are increasingly in demand in the mold/die and optics industries. The computer-controlled polishing (CCP) is commonly used as the final procedure to improve the surface quality. This paper presents a theoretical and experimental study on the polished profile of CCP with sub-aperture pad. A material removal model is proposed based on the evaluation of the amount of material removed from the surface along a direction orthogonal to the tool path. The model assumes that the material removal rate follows the Peterson equation. The distribution of the sliding velocity at the contact region is presented. On the basis above, the approaches to calculate the polished profiles are developed for the sub-aperture pad polishing along a straight path and a curved path. The model is validated by a series of designed polishing experiments, which reveals that polishing normal force, angular spindle velocity, feed rate and polishing path all have effects on the polished profile. The result of experiments demonstrates the capability of the model-based simulation in predicting the polished profile.

Investigation of contact pressure and influence function model for soft wheel polishing.

The tool influence function (TIF) is critical for calculating the dwell-time map to improve form accuracy. We present the TIF for the process of computer-controlled polishing with a soft polishing wheel. In this paper, the static TIF was developed based on the Preston equation. The pressure distribution was verified by the real removal spot section profiles. According to the experiment measurements, the pressure distribution simulated by Hertz contact theory was much larger than the real contact pressure. The simulated pressure distribution, which was modeled by the Winkler elastic foundation for a soft polishing wheel, matched the real contact pressure. A series of experiments was conducted to obtain the removal spot statistical properties for validating the relationship between material removal and processing time and contact pressure and relative velocity, along with calculating the fitted parameters to establish the TIF. The developed TIF predicted the removal character for the studied soft wheel polishing.

Investigation into material removal influenced by edge effect in polishing

Edge effect is unavoidable in polishing process when the polishing pad passes by the workpiece edge, which influences the entire form accuracy of free-form surfaces of optical components in computer-controlled polishing, or even reduces their effective aperture. This article focuses on a theoretical and experimental investigation into the material removal influenced by edge effect for polishing along a certain path. The contact pressure distribution models for polishing the workpiece edge are summarized and modified into four representative models: linear model, skin model, linear skin model and divided skin model, which are adopted to calculate the theoretical material removal profiles orthogonal to the straight or curved polishing path, in this article. So, the material removal models are built in the process of polishing along tool path instead of polishing on a single point. And experiments are carried out to choose the most suitable contact pressure distribution model according to the comparison of experimental profiles and the different theoretical profiles. Experimental results reveal the influences of edge effect on material removal and a modified parameter is introduced into the theoretical material removal profile for the curved path to coincide with the experimental profile better. In addition, some qualitative analyses about how to reduce the edge effect are also given in this article.

Modeling and Experimental Study on the Material Removal in the Velocity-dwell-mode Polishing Process

With the demand of improving the form accuracy of free-form surface by computer-controlled polishing, a theoretical investigation is presented on the material removal for the velocity-dwell-mode polishing process. Especially, the material removal profile as the sub-aperture tool polishes at the path corner is modeled. The material removal profile perpendicular to the polishing path can be calculated by integrating the wear index, the polished depth at unit length of polishing path, along the tool path in the contact region. The effects of the straight path and arc path on the polished profile at the bisector of the path arc angle are discussed in different cases. On the basis above, the polished profile at the path corner is modeled and a series of experiments are conducted to verify the proposed model. According to the experimental results, the material removal at the path corner is uneven, and the path radius and arc angle both affect the material removal profile during the velocity-dwell-mode polishing. The proposed model is potentially useful for the planning of tool path and process parameters in the polishing process.

Mid-Spatial Frequency Error (PSD-2) of optics induced during CCOS and full-aperture polishing

Mid-Spatial Frequency (MSF) Wavefront Error of optics divided into the PSD-1 and PSD-2 ranges plays an important role in the performance of high power laser systems. The present work focuses on the PSD-2 range in terms of short ripples which haven’t been well studied in the literature. Characteristics and origins of these short ripples were detailed, whereafter small tool computer controlled polishing (CCP) and conventional full aperture polishing experiments were conducted on fused silica. It is revealed that PSD2 error is independent of the main process parameters including lap rotating rate and polishing pressure in continuous polishing and tool path pitch and crossfeed velocity in small tool CCP processes. Whereas the type of polishing lap has a decisive effect on PSD2 error of the optics. The pitch lap shows superiority in restraint of short ripples over polyurethane pad. By introducing diamond conditioner for dressing polyurethane pad, the PSD2 error has been greatly decreased.

대형 광학계 연마 장비에 의한 대구경 반사경의 최적 근사 구면 제조 방법에 관한 연구

This paper describes a novel method to surface large optics mirror with an extremely high hardness, which could replace the high cost of the repetitive off-line measurement steps and the large ultraprecision grinding machine with ultra-positioning control of 10 nm resolution. A lot of diamond pellet to be attached on the convex aluminum base consists of a grinding tool for the concave large mirror, and the tool was pressured down on the large mirror blank. The tool motion at an interval on the spiral path was controlled with each feed rate as the dwell time in the conventional computer-controlled polishing. The shape to be surfaced was measured directly by a touch probe on the machine without any separation of the mirror blank. Total 40 iterative steps of the surfacing and measurement could demonstrate the form error of RMS 7.8 μm, surface roughness of Ra 0.2 μm for the mirror blank with diameter of 1 m and spherical radius of curvature of 5400 mm.

Study on Submerged Jet Problem of Concave Optics Surface in Fluid Jet Polishing

Fluid jet polishing (FJP) is a computer controlled polishing (CCP) process. As the water effect of concave optics surface, the slurry won’t spread immediately until component have a basinful, and jet goes through a layer of slurry firstly before impact in FJP process, the submerged jet will influence material removal rate and bring on inaccurate fabrication results if it is not considered. In this paper, submerged jet characteristics are studied, and researches were taken for submerged jet polishing by simulation with FLUENT software and experiments analysis. Based on that, mathematical material removal correction with submerged depth is established for semi-submerged jet polishing. A BK7 concave surface was polished by semi-submerged jet with removal correction. The results show an affirmative proof for correction in submerged jet polishing.

Shaping off-axis aspheric surfaces on automated computer-controlled polishing–lapping machines

Advances in the computer technology of automated shaping of large optical items so that diffraction quality of the image is obtained, developed at Optika Scientific-Manufacturing Organization, make it possible to expand the technological possibilities of shaping aspheric surfaces—in particular, when creating off-axis aspheric mirrors of complex configuration. This paper demonstrates the possibilities of the method of automated shaping of the surface, using as an example the automated shaping of various types of off-axis mirrors about 1000 mm in diameter at the grinding and polishing stage. The technical parameters of fabricated off-axis mirrors are presented, and problems of the automated shaping of off-axis mirrors are indicated, along with technological ways of solving them.

The Computer-Controlled Chemical Polishing Techniques for Precision Optics

The computer-controlled chemical polishing (CCCP) techniques based on the Marangoni effect have been developed to manufacture precision optics on polished fused silica surface. In this study, we present the Marangoni confined chemical-etching process in which the material removal on optical surfaces can be accomplished by etching with buffered HF solution. The process shows stable characteristics and good repeatability while the etching depth can be controlled in the order of ten nanometers. We also present the experimental results of this technology for fabrication of phase corrector and continuous phase plate. Results show that the CCCP’s deterministic sub-aperture-polishing characteristics make it possible to correct the surface error and imprint complex phase structure with spatial scale-length of several millimeters onto optical surface.

Mathematical modelling of influence functions in computer-controlled polishing: Part I

Computer-controlled polishing (CCP) is commonly used to finish high-quality surfaces, such as optical lenses. Based on magnetorheological finishing (MRF), a mathematical model to calculate the polishing tool characteristic (influence function) was developed and verified experimentally. The first part of this paper introduces the model to predict the size and shape of an influence function. The second part of this paper describes the calculation of the distribution of material removal within the size of an influence function. The model supersedes the current cumbersome procedure for determining an influence function and thus results in considerably improved and more economical manufacture. Furthermore, the model enables the quality of the final surface to be enhanced when polishing complex, for example aspherical or free-form, workpiece geometries and provides the first step in the application of time-variant influence functions.

Computer-Controlled Polishing Preparation and Subsequent Mounting of Samples for Low-voltage X-Ray Tomography

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008