Published papers

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.

The fabrication process chain for optically smooth surfaces tends to include several time-consuming grinding and polishing steps. To reduce process time, a hybrid tool is proposed in which a stiff grinding tool and a shape adaptive grinding (SAG) tool are fused together, by taking advantage of the elastic nature of SAG tools. The material removal achieved by the hybrid tool is equivalent to discretely using the stiff grinding and SAG tools in sequence. Under similar processing conditions, a smooth surface of ∼0.02 μm Ra can be obtained on BK7 glass with the proposed tool, instead of ∼0.2 μm Ra with the stiff grinding tool.

We outline requirements for mirror segment edges for extremely large telescopes, and introduce the new CNC machine for process verification. We review a promising approach for edge-control using the Precessions process, and report progress. © 2010 Optical Society of America.

Segment-edges for extremely large telescopes are critical for observations requiring high contrast and SNR, e.g. detecting exo-planets. In parallel, industrial requirements for edge-control are emerging in several applications. This paper reports on a new approach, where edges are controlled throughout polishing of the entire surface of a part, which has been pre-machined to its final external dimensions. The method deploys compliant bonnets delivering influence functions of variable diameter, complemented by small pitch tools sized to accommodate aspheric mis-fit. We describe results on witness hexagons in preparation for full size prototype segments for the European Extremely Large Telescope, and comment on wider applications of the technology. © 2012 Optical Society of America.

© 2020 Author(s). Micro polishing is a process used when a fine surface quality is desired. In optics, it represents the key process to achieve low surface roughness and form error required for optics application. Automated micro polishing with polymer-based tools requires a better understanding of the interaction between tool and workpiece than conventional polishing, and an important phenomenon that has to be taken into account is polymer relaxation. Stress relaxation is due to the non-linear viscoelastic behaviour of polymers. This relaxation is a time-dependent phenomenon that causes a decrease in stress, although the strain is kept constant. In this work, the effect of polymer stress relaxation on polishing is studied. While this effect can be neglected in conventional polishing, it becomes an essential and dominant factor in micro polishing. Characterization tests are conducted on a very common extruded polyurethane (LP-66) used in polishing. Subsequently, bonnet polishing using a spherical tool with a diameter of 1 mm is performed for 8 minutes on a nickel sample. Due to relaxation, the induced stress on the tool decreases during polishing, causing a reduction in the polishing pressure. This implies a reduction in the material removal rate in accordance with Preston's law. Experimental results are in accordance with the physical phenomenon. The polymer relaxation characteristics can be used to compensate the tool path to achieve uniform removal along the polished surface.

The scheduling of feed-rate is an important procedure in CNC finishing of high-precision optical surfaces. Since the polishing process is described as the convolution operation between a tool influence function and a dwell-time map, deconvolution operation gives an optimal dwell-time map. However, physical achievability of the obtained dwell-time map is not considered. This research elucidates the effect of the bulk removal, a constant removal layer across the surface, on the convolution process in spatial domain and time domain. Based on this investigation, we propose a theory that has the potential to analytically adjust dwell-time, feed-rate, and acceleration without compromising the surface accuracy, and to predict the total machining time as a function of the bulk removal.

© 2015 IOP Publishing Ltd. In recent years, rapid prototyping of titanium alloy components for medical and aeronautics application has become viable thanks to advances in technologies such as electron beam melting (EBM) and selective laser sintering (SLS). However, for many applications the high surface roughness generated by additive manufacturing techniques demands a post-finishing operation to improve the surface quality prior to usage. In this paper, the novel shape adaptive grinding process has been applied to finishing titanium alloy (Ti6Al4V) additively manufactured by EBM and SLS. It is shown that the micro-structured surface layer resulting from the melting process can be removed, and the surface can then be smoothed down to less than 10 nmRa (starting from 4-5 μmRa) using only three different diamond grit sizes. This paper also demonstrates application of the technology to freeform shapes, and documents the dimensional accuracy of finished artifacts.

The progressive transition from Excimer to EUV lithography is driving a need for flatter and smoother photomasks. It is proving difficult to meet this next generation specification with the conventional chemical mechanical polishing technology commonly used for finishing photomasks. This paper reports on the application of sub-aperture CNC precessed bonnet polishing technology to the corrective finishing of photomask substrates for EUV lithography. Fullfactorial analysis was used to identify process parameters capable of delivering 0.5 nm rms surface roughness whilst achieving removal rates above 0.1 mm3/min. Experimental results show that masks pre-polished to 300~600 nm P-V flatness by CMP can then be improved down to 50~100 nm P-V flatness using the automated technology described in this paper. A series of edge polishing experiments also hints at the possibility of increasing the quality area beyond the 5 mm defined in the official EUV photomask specification. © 2013 SPIE.

Tungsten carbide is widely used to make replication moulds for small aspheric and freeform optics, like camera lenses. Since aspheric generation by grinding generally fails to reach ultra-precision criteria, a time efficient method for finishing moulds is needed. Using a 7-axis CNC polishing machine, an on-machine tool forming technique and compensation software were developed to correctively polish a plano WC surface of diameter 25 mm from a form error above λ to below λ/20 (29 nm P-V) in under 1 h, while reducing roughness to 1.0 nm Ra. This automatic method and tooling are applicable to aspheric and freeform moulds. © 2011 CIRP.

With the increasing demands to generate complex parts on ever-more advanced, but difficult-to-cut materials, a group of technologies, generically called High Energy Fluid Jet Machining (HEFJet-Mach) has found niche applications that stimulated further development of science and technology in this field. Here HEFJet-Mach refers to unrestrained fluid jets used to remove and/or deform the workpiece material. The aim of this paper is to systematically present the recent developments of HEFJet-Mach from various perspectives: machine/system designs; modelling of both jet plumes and their interactions with the target surfaces; part quality including material integrity; supervision and control of the process; key aspects of machine maintenance and health and safety. © 2014 CIRP.

In time dependent CNC processes such as polishing, a target material removal profile is obtained by creating and inputting G-codes with varying feed into the controller of a machine tool. The input G-codes are firstly filtered by a CNC interpolator, and the real-time generated signal then goes to a feed drive system of the machine. Through these steps, the original signal is distorted and the ideal feed profile cannot be achieved in some areas. This distortion affects the material removal distribution, and thus the accuracy of the final polished surface profile. While a number of methods have been developed for predicting polished surfaces, most of them fail to consider the influence of controller dynamics. In this paper, a method for accurate prediction of polished surfaces is proposed that considers the error contribution from control signal distortion, and a strategy for compensation of this error. Firstly, the principles of identification of control system behavior in a machine tool are explained, and a process model for Fluid Jet Polishing (FJP) is introduced. Secondly, the transfer functions of the control system are identified, and the process model calibrated against experimental data. Thirdly, a simulator for predicting polished surface profiles is implemented that combines the control system and process model. A micro-groove polishing experiment is carried out that validates the reliability of the proposed method, and shows its limitations. Finally, an algorithm based on particle swarm optimization is proposed for reducing the impact of controller dynamics in micro-groove polishing.

Machining conditions in float polishing method have been determined empirically and there is a need to elucidate the levitation principle of the workpiece and to propose a theoretical method to determine the machining conditions. In this research, the levitation quantity of the workpiece was measured by the displacement sensor and found to be about 4 µm. The order of this value agreed with the prediction in the literature. From the FEM simulation of the thin film fluid based on the Reynolds equation, it was predicted that the pressure distribution under the workpiece was non-uniform and cavitation was generated.

This paper investigates the erosion mechanism and surface quality of microchannel fabrication using airflow-driven suspended abrasives under mask constraints, a process known as submerged pulsating air jet polishing (SPAJP) with a mask. A multiphase Computational Fluid Dynamics (CFD) model is developed to simulate this process and analyze the evolution of jet morphology. An erosion mechanism for microchannel topography at various angles under mask constraints is proposed. It is believed that the erosion topography is primarily influenced by the competitive and synergistic interactions between primary particles entrained in the jet and secondary particles rebounding from the mask sidewalls. Additionally, secondary particles rebounding from the bottom surface play a significant role in edge erosion. An experimental study is conducted to examine the impact of several key factors on microchannel surface quality and the evolution of microstructural topography at different angles. The results show that the integrity of the mask has a significant impact on the surface quality of the fabricated microchannels, while the processing direction affects the uniformity of the removal depth, primarily due to the concentration gradient caused by the precipitation of suspended abrasive particles. The jet angle is found to have a considerable impact on the erosion topography, with the profile evolving from an asymmetric W-type to a biased Gaussian-like shape, and finally to a symmetric Gaussian-like form as the angle increases from 30° to 90°, which aligns with the proposed erosion mechanism. Moreover, a method for predicting surface topography in mask-fabricated microchannels, accounting for edge erosion, is proposed and validated through experimental and simulated comparisons. The results emphasize the critical role of edge erosion in shaping and predicting the topography of mask-fabricated microchannels. This research presents a highly promising, cost-effective, and environmentally friendly processing method and foundation for the high-quality fabrication of microchannels and other microstructures.

Shape adaptive grinding (SAG) is a well-established approach to obtain optical smooth quality on hard ceramic and metallic surfaces. However, the process has been limited to low material removal rate (MRR) when already smooth surfaces are processed using SAG pads. In this work, shape adaptive grinding plus ceria (SAG+) is put forward as an effective approach to increase the MRR, whereby ceria is used as polishing slurry and enables softening of glass/glass-ceramic surfaces through chemical reaction and enhances subsequent material removal through mechanical action of the SAG pad. A macroscopic material removal model is established for SAG+ process on BK7 glass where the effect of ceria on the surface hardness correlates well with the material removal rate. Experiments were performed on BK7 glass with different grades of resin bonded diamond SAG tools to benchmark performance against the standard SAG process. It was observed that the material removal rate increased by 3.5–10 times for SAG+ when all other conditions remain the same. It was also determined that removal rate increases most drastically when the abrasive grit of SAG pad and ceria slurry are of comparable size. Lapped Zerodur™ samples were also processed to check the applicability in pre-polishing of rough glass-ceramic surfaces. A reduction of almost 50 % could be obtained in processing time when using the SAG+ process.