Free-form Surface Polishing Research Articles

Numerical and experimental investigation on the effect of surface curvature and slope angle on the material removal characteristics in fluid jet polishing

Fluid jet polishing (FJP) has been extensively utilized in the ultra-precision manufacturing of freeform optical components and molds. The form accuracy is mainly improved by controlling the polishing dwell time at different positions, which is determined by the deconvolution of the initial form error and tool influence function (TIF). However, in previous studies, the erosion-induced TIF is considered constant without accounting for the freeform surfaces' curvature and slope angle variations. Due to the deviation of TIF, it is still a challenge to achieve a high form accuracy for freeform surface polishing. In this paper, the TIFs in freeform surface polishing have been modeled by computational fluid dynamics (CFD) analysis. A series of spot polishing experiments subsequently were conducted on the surfaces with different curvature radii and slope angles. The experimentally measured data including cross-section profiles of polishing spots, Peak-to-valley (PV) value, and material removal rate are found to agree well with the model simulation results, which verified the effectiveness of the CFD model. The effect of surface curvature and slope angle on the material erosion distribution was investigated and revealed by the fluid flow field distribution, abrasive impact, and abrasive trajectories. This paper not only provides a deep scientific understanding of the material erosion characteristics in FJP of freeform surfaces, but also offers a simple and cost-effective solution to build the database of TIFs on freeform surfaces under various conditions, which is beneficial for the ultra-precision form control during FJP of freeform surfaces.

Development of CNC machine code and user interface for a 3-axis pneumatically configurable polishing machine

The polishing of freeform surfaces finds applications in a variety of niche areas where the surface characteristics have a defining impact on functional capabilities of the product. Pneumatically Configurable Polishing (PCP) is a recent addition to the available technologies which can generate surface finish of nanometric scales on freeform surfaces. The manipulative ease and determinism in polishing force control offered by the PCP process allow it to finish complex surface profiles in a highly repeatable way. The automation code for a CNC PCP machine-tool is developed to ensure the accuracy in the finished parts being produced. The machine code for CNC interpolation and the parametric process control have been carried out in the Parker Automation Manager software. The user interface has been designed and deployed using the JMobile HMI software. The PAM Visualization allows the user defined part program to be loaded in the controller from an external removable disk. The exchange of tags and process variables between the controller and HMI has been done using the OPC UA platform. A part program created using a CAM software is employed to finish a sample workpiece. The developed CNC machine code is successfully tested and evaluated for its functional performance and process repeatability.

A method for analyzing the texture features of free-form surface polishing paths based on co-occurrence matrix

Surface quality analysis of polished surface has been the subject of many classic studies in surface polishing technology and is a key indicator for evaluating the polishing path. However, as an important part of surface quality, surface texture features have been ignored in many related researches. In this paper, a new method to analyze the texture features of polishing based on co-occurrence matrix is proposed. It provides a new perspective of surface quality analysis focusing on surface texture features. It extends the previous approach termed the residual level co-occurrence matrix (RLCM) focusing on the distribution of surface polishing residues, leading to more targeted and stabilized evaluation results. This method can be used in the path planning stage to estimate the polishing quality of the planned path without physical processing, which can avoid resource waste in the physical world. Furthermore, in this method, the usage of images is avoided, which can ensure that the results are not affected by the light and image quality. Simulation experiments as well as empirical investigations were conducted to verify the feasibility of the method. The results of both consistently reveal that the proposed method is able to accurately describe the texture feature and correctly analyze the texture feature.

Rapid prediction of multi-directionality of polished surface topography based on angular spectrum

The multi-directionality of polished surface topography (PST) is a key index to characterize the polished surface quality and surface integrity, and its prediction before actual machining is of great significance to the toolpath planning, verification, and optimization. However, most related works reported thus far focus mainly on the material removal modeling but few on the prediction of multi-directionality. Hence, this paper proposes a rapid method to precisely predict the multi-directionality of the PST for the pad-polishing process of freeform surfaces. With this method, a pressure distribution model and a material removal profile (MRP) model are first established, in which the MRP is founded subject to the quadratic function distribution. Then, to avoid time-consuming integral operation in the MRP model, an artificial neural network is developed to fit the quadratic function of MRP. With this model, a multi-directionality prediction algorithm is further proposed based on the angular spectrum of the PST. Simulation and experimental studies have shown that the proposed method can predict the multi-directionality of the PST with very high accuracy and efficiency for freeform surface polishing, showing great application potential in promoting the efficiency of toolpath planning and optimization.

Development and theoretical analysis of novel surface adaptive polishing process for high-efficiency polishing of optical freeform surface

A novel surface adaptive polishing (SAP) process is initially proposed to efficiently improve the surface accuracy of optical freeform surfaces while suppressing their surface and subsurface damage. A multiscale theoretical model is developed to predict the generation of surface microtopography. This model captures considerable fundamental physics of the SAP process, such as the variable curvature characteristics of the workpiece surface, the surface topography of pad, the brittle-ductile removal effect of materials, and the trochoidal motion trajectories of polishing tool. The feasibility and accuracy of the model were quantitatively analyzed through a series of experiments. Moreover, the influence mechanism of process parameters on surface morphology and subsurface damage was studied. The simulated results by the theoretical model agree well with the experimental data, and maximum relative errors of material removal rate (MRR) and surface roughness Sa are 3.91 % and 1.38 %, respectively. Results indicated that the SAP process can produce axisymmetric Gaussian removal functions, significantly remove the damage layer, and improve the surface quality while avoiding the generation of periodic surface textures. The increase in the tool offset changes the material removal mode from ductile to brittle mode, resulting in a higher MRR and Sa. In addition, the Taguchi simulation experiments are conducted to quantitatively evaluate the significance of the process parameters.

Generic three-dimensional model of freeform surface polishing with non-Newtonian fluids

Non-Newtonian fluids are being increasingly considered for application in manufacturing. Their combination with compliant polishing techniques offers a promising approach for ultraprecision finishing of freeform surfaces. However, in-depth understanding of the underlying material removal mechanism and its link with slurry rheology has not been disclosed yet. In this study, a comprehensive three-dimensional dynamic modeling framework is presented, which enables accurate prediction of stress distribution, rheological flow, compliant deformation in 3D, as well as material removal behavior on curved workpieces. Supported by experimental tests and theoretical analysis, it is demonstrated that the viscosity peak gets shifted to a higher frequency by mixing starch and polymer, which makes high speed polishing with improved material removal rate possible. Meanwhile, it is found that the polymer additive prevents large size starch agglomeration and has high affinity with fine abrasives, both of which greatly contribute to the obtained scratch-free and nanoscale surface finish. Finally, the controllability and predictability of the process are demonstrated by polishing a bi-sinusoidal freeform surface onto a planar workpiece coated with nickel.

Laser polishing of metallic freeform surfaces by using a dynamic laser beam preforming system

Laser polishing is an automated polishing solution for polishing metallic freeform surfaces, which is hoped to support or replace the time-consuming and cost-intensive manual polishing in the near future. This technique is based on remelting a thin surface layer by laser radiation. The surface tension leads to a material flow from the peaks to the valleys; thus, the surface resolidifies in a smoother way. Laser polishing of 3D surfaces often leads to a nonperpendicular angle of laser incidence. This results in a projection of the focused circular laser beam on the surface of the workpiece that is primarily approximated with an elliptical shape. The beam geometry in the projection depends on the surface geometry of the workpiece and the actual spatial working position of the laser beam. To ensure a constant polishing quality, the circular symmetry of the laser spot on the workpiece has to be maintained. A method is developed and discussed, which enables the compensation of the elliptical deformation, called elliptical preshaping. This is realized by a combination of an anamorphic zoom lens with a beam rotating device comprising a Pechan prism. It is demonstrated that constant surface qualities up to inclination angles of 60° can be achieved on the tool steel X37CrMoV5-1 using elliptical preshaping.

Modeling and prediction of generated local surface profile for ultrasonic vibration-assisted polishing of optical glass BK7

• Preston coefficient is affected by ultrasonic amplitude. • Contact pressure is affected by ultrasonic amplitude. • Large ultrasonic amplitude helps to improve the polishing ability. • Local surface profile can be well predicted by the proposed calculation model. • Prediction of surface profile is a basis for free-form surface polishing of UVAP. With the rapid progress of science and technology, many applications of hard and brittle materials require advanced and suitable machining-techniques. One technique in particular, ultrasonic vibration-assisted polishing (UVAP), has been studied intensively by researchers worldwide. These studies indicate that UVAP has the capability to improve the surface quality as well as the machining efficiency. However, few studies focus on the deterministic material-removal of UVAP. The difference between generated and theoretical surface profiles determines the surface accuracy. Therefore, this paper focuses on predicting the generated local surface profile and providing a theoretical basis to reduce this error by controlling the polishing parameters. In addition to the axial ultrasonic vibration of polishing tool, ultrasonic atomization was adopted to provide uniformly distributed polishing slurry. To ensure sufficient surface-accuracy, it is necessary to predict the generated surface profile for a fixed spot as a basis research. This means, to enable accurate predictions the following improvements are realized: (i) A novel model for the elastic spherical contact-deformation of the polishing tool is used to calculate both the polishing force and contact radius. It takes into account the nonlinear elasticity of the polishing tool and the lateral extension of the contact area, which makes it more consistent with the actual deformation. (ii) A novel model for the material-removal distribution function for UVAP is proposed, based on the Preston equation. It considers the periodic changes of polishing force and contact radius caused by ultrasonic vibration. (iii) A series of fixed-spot polishing experiments on BK7 glass are carried out to verify the proposed models. The results show that the Preston coefficient is affected by axial ultrasonic amplitude, and large ultrasonic amplitude improves the polishing ability. In addition, the generated local surface profile, the maximum contact radius, as well as the material-removal depth and material-removal rate (MRR) can be predicted well. The new models not only provide a possibility to realize deterministic material removal but also enables in-depth-understanding of UVAP. It also represents a basic theoretical foundation for future, flat or free-form surface-polishing.

Advances in polishing of optical freeform surfaces: A review

Abstract Ultra-precision polishing is one of the main machining processes for components with high surface finish requirements, such as optical molds. Typical optical mold materials include mold steel, silicon carbide and tungsten carbide, which have many excellent properties like strong corrosion resistance, low thermal expansion coefficient, strong oxidation resistance, low density, and high hardness. To ensure that the replicated parts meet high precision form and finish specifications, the molds made in hard and brittle materials must be machined to obtain a high-quality part surface. Therefore, polishing has evolved into ultra-precision polishing, which has a critical impact on the mold service life as well as the precision optical component quality. This paper presents the basic concepts and classification of ultra-precision polishing, introduces typical polishing processes, discusses polishing mechanisms and characteristics, and finally indicates the directions for future development of ultra-precision polishing processes.

Novel hybrid robot and its processes for precision polishing of freeform surfaces

Freeform surfaces have important applications in various industrial fields. However, achieving a high level surface finish on freeform surfaces using polishing is always a challenging task. Hybrid robots are promising alternatives to conventional computer numerical control (CNC) machines and industrial robots for ultra-precision machining. Herein, we present a novel custom-built hybrid robot for freeform polishing. After the laboratory prototype was successfully developed, its automation for a specific freeform surface was a major obstacle preventing its application. This critical issue was addressed by presenting a process to deal with tedious robot programming. Random tool path planning was performed in the task space, and hybrid robot motion planning was conducted in the joint space. By integrating the process flows, a robot programming toolkit was developed to directly output the control program for a specific robot controller. An illustrative example with a freeform surface is provided to verify the functionality of the developed hybrid robot and the proposed control processes, and the corresponding experimental results verify their effectiveness.

A novel magnetic field-assisted mass polishing of freeform surfaces

Abstract This paper presents a novel magnetic field-assisted mass polishing (MAMP) technology for high-efficiency finishing of a number of freeform components simultaneously. The MAMP makes use of a rotational magnetic field applied outside an annular chamber which drives the magnetic abrasives to impinge on and remove material from the workpiece mounted inside the chamber. The influence of the magnetic field on the material removal characteristics is analysed by the finite element method. The factors affecting surface generation were studied through polishing experiments. Experimental results show that MAMP is effective for polishing of a number of freeform surfaces with nanometric surface finish.

Experiment Study of Rapid Laser Polishing of Freeform Steel Surface by Dual-Beam

One of the challenges regarding widespread use of parts made from alloy steel is their time-consuming polishing process. A rough freeform surface of part has been often expected to be polished rapidly up to a smooth surface finish. The focus of this study is to develop a fast polishing method of freeform surface by using dual-beam lasers. The dual-beam laser system consists of continuous laser (CW) and pulsed laser based on a five-axis CNC device. In this study, a series of experiments of CW laser polishing present the effects of different spot irradiation on surface topography, then the combination trajectory of zigzag and square waveform of pulsed laser is explored to realize a “melting peak for filling into valley” (MPFV) method. The polishing experiment on a semisphere of S136H steel polished by dual-beam shows that a rough semisphere surface was rapidly polished from initial state value of Sa (=877 nm) to post-polished value of Sa (=142 nm), and the polishing efficiency is as high as 2890 cm2/H.

Generation of material removal map for freeform surface polishing with tilted polishing disk

Polishing is one of the key processes for surface finishing. It can achieve mirror-effect on surface by removing the tool masks caused by milling. Since the process is now automated, tilted polishing disk is applied more and more widely, and the prediction of material removal map is vital for polishing trajectory planning to reduce scrap rate and shorten production cycle. This work proposed an improved material removal model of tilted elastic polishing disk by considering the geometric features of contact region, contact pressure, and velocity distribution while polishing. More specifically, based on the FEM simulation results, the contact region and the pressure distribution are discussed in three diverse types according to the geometric features of surface at contact point. The material removal map is then generated by numerical convolution of this model and trajectory planned, with which the issues of material removal, such as over-polishing, under-polishing, and waviness, can be graphically illustrated. To validate the approach, case studies on both planar and freeform surfaces are conducted. Comparative study reveals that the predicted result by the approach agrees with the actual polishing very well. Moreover, according to the generated material removal map, process optimization can be carried out to adjust process parameters, such as feedrate, interval, and step, and hence, mirror-effect surface quality can be achieved.

Model Predictive Force Control in Grinding based on a Lightweight Robot

The extensive effort in manual labour for mould manufacturing is the main reason for outsourcing manufacturing resources to low-wage countries. Furthermore, the international mould and die sector faces a lack of skilled workers due to the unattractive working conditions, for example the stressing, monotonous work and a partly hazardous working atmosphere (e.g. nickel dust). Especially, the finishing process (e.g. grinding, lapping, polishing) of freeform surfaces is still less automated in metal processing. Therefore, many efforts are initiated in order to fully automate the finishing process, but the today’s approaches are usually limited on prototypal implemented and cost-intensive machine solutions.This work introduces a finishing control approach for freeform surfaces using a lightweight robot. The robot is used as an actuator as well as a sensor for closed-loop force control. The optimal force trajectory is determined by a CAD-model which describes the geometry of the workpiece. The noisy measured force signal is filtered by a Kalman filter which also estimates the system state for the model predictive controller (MPC). Finally, a simple position controller controls the position of the tool center point (TCP) and the MPC controls the force which occur on the tool. The results show that the force can be controlled with a higher accuracy than with a conventional impedance control, which is commercially provided.

Model-based self-optimization method for form correction in the computer controlled bonnet polishing of optical freeform surfaces.

Freeform surfaces have become increasingly widespread in the optical systems for enhanced performance and compact lightweight packaging. The geometrical complexity and high precision requirements of optical freeform surfaces for various functional optical applications, has posed great challenges in the design, precision machining, and measurement of these surfaces. This paper presents a model-based self-optimization approach for precision machining and measurement of optical freeform surfaces in the computer controlled bonnet polishing (CCBP) process. To realize the technical feasibility, the process parameters and motion control are accurately performed through modelling and simulation of machining processes, error compensation, and on-machine metrology.

Curvature-adaptive multi-jet polishing of freeform surfaces

This paper presents a curvature-adaptive multi-jet polishing (CAMJP) method that can achieve high efficiency and cater for adaptation of the variation of the material removal rate (MRR) to the curvature of freeform surfaces. CAMJP makes use of a purposely designed multi-jet nozzle incorporated with a pressure control system. The effect of surface curvature on the MRR is analysed by computational fluid dynamic modelling. The fluid pressure of each jet is controlled independently to vary the MRR according to the curvature of freeform surfaces. Experimental results show that CAMJP is effective in improving the accuracy of polishing freeform surfaces.

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.

Prediction of surface quality considering the influence of the curvature radius for polishing of a free-form surface based on local shapes

When machining a free-form surface automatically and digitally, especially in the case of sophisticated surface shapes, it is very difficult to control the surface quality, and thus sophisticated surfaces are usually polished using manual labor. Over the past few years, there has been little attention to the calculation of material removal depth models and construction of theoretical roughness models considering the influence of the curvature radius. Bonnet polishing can be automatically adapted to polish complex free-form surfaces. This paper explores key problems related to forecasting surface quality with respect to bonnet polishing of free-form surfaces. First, for the convex and planar sub-regions, this paper deduces the relationship expressing the maximum pressure distribution and the curvature radius, and presents a computational expression for material removal depth, taking into consideration the influence of the curvature radius. An expression is also deduced for the dwell time relationship between the adjacent processing points according to the experimental results pertaining to the material removal depth. From this, a theoretical roughness model is constructed that relates the bonnet curvature radius and the workpiece curvature radius. The validity of the experiments is summarized in the conclusion. The research findings provide a basic theory for the prediction of surface quality that can be automatically adapted to a free-form surface shape in bonnet polishing.

Soft computing techniques to model and optimize magnetic field-assisted finishing process and characterization of the finished surface

In magnetic field-assisted finishing process, magnetorheological polishing fluid is used for precision polishing of freeform surfaces in the nanometer range. An efficient model is derived to accurately relate the input and output process parameters for better prediction of finishing performance. In this study, the relationship between the input and output process parameters of magnetic field-assisted finishing process is established using back-propagation neural network technique. Also, a close comparison between the regression analysis and neural network model has been carried out. The simulation results from neural network model better matches with the experimental data. Hence, this particular neural network model can be utilized to predict the response variables. A further optimization study using genetic algorithm and simulated annealing techniques is carried out to optimize the input process parameters for achieving maximum finishing performance. It is found that the results obtained from the genetic algorithm is more accurate and matches with the experimental results than the simulated annealing. Further, a characterization study of the finished workpiece surface is carried out which shows that MFAF process can achieve surface finish in the nanometer range having a minimum surface roughness value of 70 nm.

A practical approach for automated polishing system of free-form surface path generation based on industrial arm robot

In manufacturing environment where a robot arm is programmed to follow a specified path such as in polishing, the geometric coordinate transformation of an automated polishing system frames is needed for polishing workpiece surface. As the presence of kinematic singularities can extremely affect the robot’s performance, singularity regions in the task space for robot are clearly identified using determinant of the robot’s Jacobian matrix. Based on the free-form surface polishing requirements, programing an industrial robot with Teach Pendant manually is skill dependent and time consuming. Therefore, a scheme of an automatic polishing system is proposed, which includes a 6DOF arm robot. An automatic planning and programming system based on data from a CAD system is described to create robot paths. The robot program which contains the polishing path is generated in certain order using RoboGuide software in virtual environment. A Human Machine Interface has been used to control the entire polishing system online. Finally, the generated path is successfully applied in robotic polishing system. The experimental results prove that the proposed method is effective and feasible. The outcomes of this work contribute to enhancing the use of arm robot for polish free-form workpiece surfaces.