Non-uniform material removal in robotic compliant grinding for flexible free-form surfaces

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

Chemical mechanical polishing (CMP) is the most important process for global planarization. The micro material removal and planarization of the optical surface is a complicated process, and the surface shape of optics is effected by kinematics, pressure, and chemical conditions. Moreover, it is a remarkable fact that the distribution characterization of polishing particles also has an important effect on material removal uniformity, especially for leather pad and Tin polishing lap. Large optics were always polished to a convex shape for the low density of valid abrasives in optic center. The porosity and grooves distribution of pad plays a major role in slurry delivering. The novel model of contact and material removal is presented in which pad characterization, and polishing particles delivery and distribution effects are included. With the modified pad asperity and optimized grooves, the particles have been inclined towards well-distributed, and experiments validated that the optic figure is significantly promoted.

Development of a fluid line-jet polishing process for rotational axisymmetric surfaces

The application of freeform surface in the off-axis three-mirror imaging system can greatly improve its optical performance. However, it is difficult for fabrication of freeform optical surface to high precision by traditional machining technology. This paper focuses on the corrective polishing of the primary mirror and tertiary mirror, which are fabricated on one monolithic substrate and described by NURBS-based freeform surfaces, in an off-axis three-mirror imaging system. The integrated polishing process system is proposed for polishing the two mirrors on the 4-axis CNC polishing machine by use of spherical polishing tool. The tool influence function (TIF) module, polishing path generation and dwell time calculation module included in the integrated polishing process system are described respectively. The material removal on the measurement line of part surface is predicted to validate the dwell time calculation algorithm. The polishing experiments of the primary mirror and tertiary mirror are performed to verify the proposed integrated polishing process system.

Contact force plan and control of robotic grinding towards ensuring contour accuracy of curved surfaces

This paper describes a methodology for capturing the tacit knowledge of the manual grinding and polishing process. Key Process Variables (KPVs) i.e. contact force, tool path, feed rate, etc. of the manual operator performing the task are captured with a `sensorised' hand-held belt grinder, while the changes to the work-piece geometry is captured using a 3-D laser scanner. These KPVs are fed into an analytical material removal model to generate a material removal profile, which can then be calibrated using the actual material removed determined from the manual surface finishing process. The skill of the surface finishing skill is encapsulated in this material removal model and reduces the need for costly robotic Design of Experiment (DoE) trials with test coupons to develop empirical material removal models. Parts from the production process require different processing variables, but the common objective is to generate various material removal maps in order to manufacture a part of a desired form and dimension. The metal removal rate (MRR) model could then be utilized by the industrial robot to determine suitable polishing parameters to accomplish the polishing task. The characteristics of the skilled worker's captured motions can then be extracted and used for optimization of the industrial robot polishing tool path.

Double side grinding is a process with high processing efficiency in which the wheel and the workpiece are in surface contact. But the phenomenon that the workpiece surface profile is out of tolerance exists because of the material removal non-uniformity in the grinding process. In order to improve the surface integrity of the ground workpiece, the mathematical models of grinding trajectory distribution and material removal were established on the basis of the double side grinding process. The simulation and experimental research were carried out under different grinding parameters. It was shown that when the grinding wheel rotates in the co-rotation to the workpiece, the surface profile of the workpiece was better than that of the opposite direction. The speed ratio of the grinding wheel to the workpiece had a significant influence on the surface trajectory distribution and material removal uniformity. When the speed ratio was irrational, the trajectory distribution was more uniform. At a lower speed ratio, the surface profile of the workpiece was better. Although the simulation results and the experimental results had a good consistency in the trend, the simulated values were obviously smaller than the experimental values, which indicated that the material removal uniformity based on grain trajectories was not the only influencing factor of the phenomenon of convex in the middle of the workpiece during double side grinding.

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.

Characterization of the tool influence function in a dual-axis wheel polishing process to achieve high material removal rates

Today, CVD SiC mirrors are readily available in the market. However, it is well known to the community that the key surface fabrication processes and, in particular, the material removal characteristics of the CVD SiC mirror surface varies sensitively depending on the shop floor polishing and figuring variables. We investigated the material removal characteristics of CVD SiC mirror surfaces using a new and patented polishing tool called orthogonal velocity tool (OVT) that employs two orthogonal velocity fields generated simultaneously during polishing and figuring machine runs. We built an in-house OVT machine and its operating principle allows for generation of pseudo Gaussian shapes of material removal from the target surface. The shapes are very similar to the tool influence functions (TIFs) of other polishing machine such as IRP series polishing machines from Zeeko. Using two CVD SiC mirrors of 150 mm in diameter and flat surface, we ran trial material removal experiments over the machine run parameter ranges from 12.901 to 25.867 psi in pressure, 0.086 m/sec to 0.147 m/sec in tool linear velocity, and 5 to 15 sec in dwell time. An in-house developed data analysis program was used to obtain a number of Gaussian shaped TIFs and the resulting material removal coefficient varies from 3.35 to 9.46 um/psi hour m/sec with the mean value to 5.90 ± 1.26(standard deviation). We report the technical details of the new OVT machine, of the data analysis program, of the experiments and the results together with the implications to the future development of the OVT machine and process for large CVD SiC mirror surfaces.

Computer controlled optical surfacing (CCOS) requires accurate knowledge of the tool influence function (TIF) for the polishing tool. The linear Preston's model for material removal has been used to determine the TIF for most cases. As the tool runs over the edge of the workpiece, however, nonlinear removal behavior needs to be considered to model the edge TIF. We reported a new parametric edge TIF model in a previous paper.** This model fits 5 parameters to measured data to accurately predict the edge TIF. We present material from the previous paper, and provide a library of the parametric edge TIFs for various tool shape and motion cases. The edge TIF library is a useful reference to design an edge figuring process using a CCOS technique.

Computer controlled polishing requires accurate knowledge of the tool influence function (TIF) for the polishing tool (i.e. lap). While a linear Preston's model for material removal allows the TIF to be determined for most cases, nonlinear removal behavior as the tool runs over the edge of the part introduces a difficulty in modeling the edge TIF. We provide a new parametric model that fits 5 parameters to measured data to accurately predict the edge TIF for cases of a polishing tool that is either spinning or orbiting over the edge of the workpiece.

Ultrasonic-Magnetorheological Compound finishing (UMC finishing) is a new technique for the ultraprecision machining of aspheric surfaces, especially for the ultraprecision machining of small-radiused concave surfaces and freeform surfaces. The principle of UMC finishing is introduced. Material removal in UMC finishing is studied when the flat workpiece is hold still. A model for material removal in UMC finishing is developed. Experiments are carried out to testify the proposed model. The results show that the shape of the material removal depth is close to the simulation results. Furthermore, the maximum material removal depth of UMC finishing is approximately proportional to the polishing time.

Aiming at the even removal of hard and brittle materials such as optical components, glass ceramics, and silicon wafers, a novel processing technology with functionally graded and composite-structured lapping and polishing plate (FG/CS-LPP) was proposed. The FG/CS-LPP was mainly made of the composites materials including room temperature-vulcanized (RTV) silicone rubber as polymer matrix and SiC abrasive particles as particle reinforcement. In addition, it has the characteristics of required quasi-continuous distribution of Young’s modulus in the radial direction and composite structure in the longitudinal direction. Firstly, the processing principle and composites preparation were described. Secondly, the simulation of the contact stress distribution and calculation of the product of stress and velocity (PSV) were performed. Lastly, the experimental validation and result analysis of contact stress distribution and material removal characteristic were discussed. According to the experimental result, it is proofed that this novel technology has the following benefits: the smooth inverse proportional stress distribution in the radial direction, processing parameters decoupling, quasi-uniform effective action rate of abrasive particles, quasi-uniform wear characteristic, and better attenuation uniformity of material removal rate (MRR) are obtained. Furthermore, the difference between the maximum and minimum MRR and the wear resistance of the FG/CS-LPP are decreased by 47.75% and increased by 38% than that of the functionally graded lapping and polishing plate (FG-LPP) respectively. Therefore, this novel processing technology effectively improves the uniformity of material removal.

Theoretical investigation and implementation of nonlinear material removal depth strategy for robot automatic grinding aviation blade