CNC Polishing Research Articles

Research status and prospect of flexible optimization design methodology of propeller CNC polishing machines.

Among the components of high-tech ships, the structural complexity of the propeller profile requires a high degree of flexibility in the CNC polishing machine. In addressing this requirement, the study formulates the flexible optimization problem pertaining to research on the propeller CNC polishing machine. A comprehensive analysis is undertaken to scrutinize the geometric features of the propeller and the phenomenon of polished contact. The propeller profile-polishing head dynamic contact mechanism is revealed, and the contact force characteristics of propeller polishing are obtained. It is suggested that the propeller configuration-process-polishing machine structure coupling mechanism be explored under the influence of polishing contact force. Subsequently, a dynamic model of the propeller CNC polishing process is formulated. Based on the above model, a simulation of the motion personification and structural flexibility of the propeller CNC polishing machine is proposed to obtain dynamic personification and flexibility rules. Integrating polishing contact force characteristics with dynamic personification and flexibility rules, the dynamic flexible collaborative optimization principle of the propeller CNC polishing machine is revealed. On this basis, multi-objective optimization modeling and solving are carried out, forming a new method for the flexible optimization design of propeller CNC polishing machines.

GRIN pad polishing

AbstractA new concept for CNC polishing pads is given by the gradient index (GRIN) pad polishing tools. Instead of gluing polishing foils as a layer onto metallic tool holders that have the spherical surface to be polished, 3D‐printed GRIN pads are applied directly to the CNC tool axis.

Design of Piezoelectric MEMS Accelerometer Module and its Application in Surface Roughness Prediction of Fused Silica Substrate

This work presents the in-house design, characterization, and development of an integrated PZT-based piezoelectric MEMS vibration sensor module and its usage in surface roughness prediction of fused silica substrate in a CNC polishing machine. The differential electrical configuration of the MEMS device ensures enhanced acceleration sensitivity to out-of-plane motion, two orders larger than in-plane sensitivity. The Thin-film Piezo on Silicon (TPoS) accelerometer with a PZT layer was fabricated using photolithography, frontal etching, and backside release. A fully integrated module comprising MEMS, amplifiers, and a microcontroller was designed and characterized in a shaker system. The designed module provides a sensitivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8.12~mV/{g}$ </tex-math></inline-formula> and a resolution of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5.8~m{g}/\sqrt {Hz}$ </tex-math></inline-formula> . The feed rate, compared to the other polishing parameters, shows the highest correlation with surface roughness. The module’s acceleration-time data were processed by Linear Discriminant Analysis (LDA) and given as input to the neural network together with the feed rate of the polishing spindle. Back Propagation Neural Network (BPNN) algorithm was used to train the machine learning model. The designed module is cost-effective compared to the commercial sensor. The key performance indicators of roughness prediction for the module and the commercial sensor were calculated, showing a Mean Absolute Percentage Error (MAPE) of 6.45% and 5.32%, respectively.

Fundamental study on CNC polishing method with inner channel liquid supply

The various optical components have wide applications in aerospace industries. In the manufacturing process of optical components that are made of optical glass materials, the main challenges still are how to ensure the high requirements in dimensional accuracy and surface quality. In this paper, the CNC polishing method with inner channel liquid supply (ICLS) is proposed to polish glass materials based on the chemical mechanical polishing mechanism. The comparative experiment with both inner and external liquid supply was designed, and it is found that the successive and steady material removal rate (MRR) can be achieved by using the proposed polishing method. Then, the effect of polishing pressure, tool rotation speed, and polishing time on polishing quality was analyzed with single-factor experiments. Finally, the optimal polishing parameters were obtained with the orthogonal experiment. The results show that the average surface roughness is reduced from 238.16 to 1.34 nm, and the nanoscale super-smooth surface is achieved, which proves the feasibility of the CNC polishing method with ICLS.

A New General Paradigm for Understanding and Preventing Li Metal Penetration through Solid Electrolytes

Summary The use of lithium (Li) or sodium (Na) metal anodes together with highly ion-conductive solid electrolytes (SEs) could provide batteries with a step improvement in volumetric and gravimetric energy densities. Unfortunately, these SEs face significant technical challenges, in large part because Li and Na dendrites can penetrate through SEs, leading to short circuits. The ability of such a soft material (Li or Na metal) to penetrate through ceramic is surprising from the point of view of models widely used in the Li-battery field. We introduce a concept, new to the battery field, for preventing penetration of lithium dendrites through SEs by putting the SE surfaces into a state of residual compressive stress. For a sufficiently high compressive stress, cracks have difficulty forming, and cracks that do form are forced to close, inhibiting dendrite penetration. This approach is widely used to solve commercially important stress corrosion cracking problems in metals and static fatigue problems in ceramics and glasses (e.g., Gorilla Glass). However, the technique will not be useful for SEs if the Li-ion transport rate through a SE is substantially reduced when the SE is under compression. Our molecular dynamics calculations for Li-ion transport through a common SE demonstrate that the introduction of even very high residual compressive stresses (∼10 GPa) has only a modest effect on Li-ion transport kinetics, suggesting that this approach is viable and capable of providing a new paradigm for developing high-performance and mechanically stable SEs.

Research on flexible adaptive CNC polishing process and residual stress of blisk blade

Flexible adaptive polishing process can be used to polish blisk. In order to verify the feasibility of polishing blisk and improve the surface integrity of blisk, this paper analyzes residual stress and its effect on deformation. Firstly, the formation mechanism of residual stress and deformation was analyzed. Secondly, the influence of polishing process on surface profile was analyzed and qualitatively analyzed the non-interference polishing of blisk polishing. Thirdly, the effects of polishing process on surface residual stress, residual stress of depth layer, and residual stress uniformity were analyzed. Fourthly, the empirical model of surface residual stress to polishing process parameters was established, and the sensitivity and stable range of the process parameters were analyzed. Finally, the influence of polishing process on deformation was quantitatively analyzed by FEA and experimental testing though contrasting polishing and milling process. The results show that the surface residual stress is reduced by 60 Mpa by flexible polishing process and can be reduced by more than 80 Mpa if appropriate polishing process parameters are used, and the surface residual stress is mostly sensitive to spindle speed, followed by compression depth, granularity, and feed rate. Flexible polishing process can make the surface residual stress become more uniform. Flexible polishing process has no effect on the change rule in the depth direction of residual stress. The second deformation of polished blisk blade is less than the milled blisk blade’s. Flexible polishing process can effectively improve surface integrity and be widely used for polishing blisk.

Flexible CNC polishing process and surface integrity of blades

The surface integrity of aero-engine blades directly affects its mechanical behavior. To improve the surface integrity and address the problem, that is, the blade is mainly polished manually, a flexible polishing process was proposed and surface integrity was investigated. First, flexible polishing technology and the machine tool that the authors researched and developed were introduced. Second, a polishing experiment was conducted on a TC4 titanium alloy blade. Finally, the surface integrity before and after polishing was measured and the results were compared and analyzed. Results showed that the knife marks, pits, unevenness, and other defects of the blade surface were effectively removed through flexible polishing; the surface residual compressive stress was reduced from 483 MPa to 397 MPa; the surface roughness was reduced to Ra 0.4 μm; the surface micro-hardness increased by 18 HV; and the surface metamorphic layer caused by milling was removed. Flexible polishing effectively improved the surface quality without affecting the depth layer quality of the workpiece.

Research on edge-control methods in CNC polishing

BackgroundWe have developed edge-control for the Precessions TM process suitable for fast fabrication of large mirror segments, and other applications sensitive to edge mis-figure. This has been applied to processing of European extremely large telescope (E-ELT) prototype mirror-segments, meeting the specification on maximum edge mis-figure. However we have observed residuals that have proved impossible to correct with this approach, being in part the legacy of asymmetries in the input edge-profiles.MethodsWe have therefore compared different proposed methods experimentally and theoretically and report here on a new edge-rectification step, which operates locally on edges, does not disturb the completed bulk area.ResultsA new toolpath has been developed and experiments have been carried out to demonstrate that local edge rectification can be carried out.ConclusionsWith this method, the residue error on edges can be removed separately and has potential to reduce total process time.

Comparison of tool feed influence in CNC polishing between a novel circular-random path and other pseudo-random paths.

A new category of circular pseudo-random paths is proposed in order to suppress repetitive patterns and improve surface waviness on ultra-precision polished surfaces. Random paths in prior research had many corners, therefore deceleration of the polishing tool affected the surface waviness. The new random path can suppress velocity changes of the polishing tool and thus restrict degradation of the surface waviness, making it suitable for applications with stringent mid-spatial-frequency requirements such as photomask blanks for EUV lithography.

Dimensional optimization of the crossbeam of the spectacle frame CNC polishing machine based on sensitivity analysis

Based on finite element analysis, we obtained the first six natural frequencies and corresponding vibration modes of the crossbeam of the spectacle frame CNC polishing machine, then we proposed some improvement according to the FEM analysis results. In order to determine the reasonable size parameters, we conducted the sensitivity analysis to the mass, total deformation and first frequency of the crossbeam. The results showed that three key parameters had greater impact to the structural characteristics of the crossbeam, while the other parameters had almost no effect. An optimization problem had been established to determine the three parameters (L, H and B), and the mass, total deformation and first frequency of the crossbeam were set as optimization target. With the help of the ANSYS optimization tool, we got three groups of candidate points, and later we used an evaluation function to determine the suitable parameters.

Corrective polishing of freeform optical surfaces in an off-axis three-mirror imaging system

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.

Blade surface uniformity of blisk finished by abrasive flow machining

The blade surface roughness of blisk is of significance to performance of aero-engine on the aspects of thrust weight ratio and service life, etc. However, it is difficult to achieve uniform surface finish because of the strong geometry interferences arising from the complex structures, through the processes of manual finishing, belt grinding, and CNC polishing. In this paper, abrasive flow machining (AFM) process is adopted to polish blade surfaces of blisk with the aim to acquire qualified uniform surface finish, by virtue of AFM’s excellent machining flexibility for parts with structures difficult to machine. Researches on surface finishing are taken for the proposed experimental prototype blisk with straight blades, through the approaches of both experiments and numerical simulations, where abrasive media with different mesh sizes and mass fractions are used. Experimental results show that surface roughness values near regions of leading/trailing edges are higher than those in regions of blades’ center, although surface roughness of the whole blades is improved obviously after AFM process. And, results from numerical simulations indicate that there exist irregular flows of abrasive media and high-pressure gradients near the leading/trailing edges, which provides reasonable explanations why uneven surface finish of blades is not achieved. Based on these analyses, a new fixture with guild blocks is proposed and proper fixture parameters are set to efficiently regulate the abrasive media flows near leading/trailing edges, and the validation experiments show that surface finish uniformity of blades is achieved with this apparatus. The conclusion could be drawn from the studies of this paper that uniform surface finish for blisk is achievable through properly designed AFM fixtures.

Robotic automation in computer controlled polishing

We first present a Case Study – the manufacture of 1.4 m prototype mirror-segments for the European Extremely Large Telescope, undertaken by the National Facility for Ultra Precision Surfaces, at the OpTIC facility operated by Glyndŵr University. Scale-up to serial-manufacture demands delivery of a 1.4 m off-axis aspheric hexagonal segment with surface precision &lt; 10 nm RMS every f̃our days, compared with a typical year or more for an one-off part. This requires a radically-new approach to large optics fabrication, which will inevitably propagate into wider industrial optics. We report on how these ambitious requirements have stimulated an investigation into the synergy between robots and computer numerically controlled (‘CNC’) polishing machines for optical fabrication. The objective was not to assess which is superior. Rather, it was to understand for the first time their complementary properties, leading us to operate them together as a unit, integrated in hardware and software. Three key areas are reported. First is the novel use of robots to automate currently-manual operations on CNC polishing machines, to improve work-throughput, mitigate risk of damage to parts, and reduce dependence on highly-skilled staff. Second is the use of robots to pre-process surfaces prior to CNC polishing, to reduce total process time. The third draws the threads together, describing our vision of the automated manufacturing cell, where the operator interacts at cell rather than machine level. This promises to deliver a step-change in end-to-end manufacturing times and costs, compared with either platform used on its own or, indeed, the state-of-the-art used elsewhere.

Process Automation in Computer Controlled Polishing

We report on an ambitious project in the field of automation, applied to bound and free abrasive processing of precision and ultra-precision surfaces, with potentially far-reaching consequences. This involves two main aspects:- directly processing surfaces using industrial robots, and combining robots with Zeeko CNC polishing machines to automate operations that are currently manual. These form steps towards our ultimate vision of the Integrated Manufacturing Cell for bespoke optics rather than mass-produced, and manufacture of other precision surfaces including prosthetic joint implants. Projects such as the European Extremely Large Telescope provide a relevant case study, where significant numbers of high-value bespoke optics are required.

Process Automation in Computer Controlled Polishing

We report on an ambitious project in the field of automation, applied to bound and free abrasive processing of precision and ultra-precision surfaces, with potentially far-reaching consequences. This involves two main aspects:- directly processing surfaces using industrial robots, and combining robots with Zeeko CNC polishing machines to automate operations that are currently manual. These form steps towards our ultimate vision of the Integrated Manufacturing Cell for bespoke optics rather than mass-produced, and manufacture of other precision surfaces including prosthetic joint implants. Projects such as the European Extremely Large Telescope provide a relevant case study, where significant numbers of high-value bespoke optics are required.

Research on Mid-Large-Aperture Aspherical Optical Components Rapid Powerful NC Polishing Technology

This paper introduces a new devices and technology of large-aperture quadratic optical components rapid powerful NC polishing, it has greatly improved the efficiency and precision of aspheric polishing. Through self-designed five-axis CNC polishing machine, to achieve the controlling of rotation, revolution integral rapid powerful polishing head. By optimizing the process parameters, determine a reasonable polishing path and polishing mode structure, to obtain good experimental results. Size 340mm × 340mm aspheric optical glass, surface shape accuracy PV λ / 5, surface roughness Rq 1.2nm, processing efficiency has been effectively improved, surface shape convergence stable.

Design and Fabrication of Faceted Mirror Arrays for Light Field Capture

AbstractThe high resolution of digital cameras has made single‐shot, single‐sensor acquisition of light fields feasible, though considerable design effort is still necessary in order to construct the necessary collection of optical elements for particular acquisition scenarios. This paper explores a pipeline for designing, fabricating and utilizing faceted mirror arrays which simplifies this task. The foundation of the pipeline is an interactive tool that automatically optimizes for mirror designs while exposing to the user a set of intuitive parameters for light field quality and manufacturing constraints. We investigate two manufacturing processes for automatic fabrication of the resulting designs: one is based on CNC milling, polishing, and plating of one solid work piece, while the other involves assembly of CNC‐cut mirror facets. We demonstrate results for refocusing in a macro photography scenario. In addition, we observe that traditional photographic parameters take novel roles in the faceted mirror array setup and discuss their influence.

Edge control in CNC polishing, paper 2: simulation and validation of tool influence functions on edges

Edge mis-figure is regarded as one of the most difficult technical issues for manufacturing the segments of extremely large telescopes, which can dominate key aspects of performance. A novel edge-control technique has been developed, based on 'Precessions' polishing technique and for which accurate and stable edge tool influence functions (TIFs) are crucial. In the first paper in this series [D. Walker Opt. Express 20, 19787-19798 (2012)], multiple parameters were experimentally optimized using an extended set of experiments. The first purpose of this new work is to 'short circuit' this procedure through modeling. This also gives the prospect of optimizing local (as distinct from global) polishing for edge mis-figure, now under separate development. This paper presents a model that can predict edge TIFs based on surface-speed profiles and pressure distributions over the polishing spot at the edge of the part, the latter calculated by finite element analysis and verified by direct force measurement. This paper also presents a hybrid-measurement method for edge TIFs to verify the simulation results. Experimental and simulation results show good agreement.

Edges in CNC polishing: from mirror-segments towards semiconductors, paper 1: edges on processing the global surface

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.

Reactive Atom Plasma for Rapid Figure Correction of Optical Surfaces

Nanometre-scale figuring technique at atmospheric pressure for large optical surfaces is a high profile research topic which attracts numerous competing state-of-the-art technologies. In this context, a dry chemical process, called Reactive Atom Plasma (RAP), was developed as a prospectively ideal alternative to CNC polishing or Ion Beam Figuring. The RAP process combines high material removal rates, nanometre level repeatability and absence of subsurface damage. A RAP figuring facility with metre-scale processing capability, Helios 1200, was then established in the Precision Engineering Centre at Cranfield University. The work presented in this paper is carried out using Helios 1200 and demonstrates the rapid figuring capability of the RAP process. First experimental tests of figure correction are performed on fused silica substrates over 100 mm diameter areas. A 500 nm deep spherical hollow shape is etched onto the central region of 200x200 mm polished surfaces. The test is carried out twice for reproducibility purposes. After two iterative steps, a residual figure error of ~16 nm rms is achieved. Subsequently, the process is scaled up to 140 mm diameter areas and two tests are carried out. First, the developed algorithm for 500 nm deep spherical hollow test is confirmed. Residual deviation over processed area is ~18 nm rms after three iterations. Finally, a surface characterised by random topography (79 nm rms initial figure error) is smoothed down to ~ 16 nm rms within three iteration steps. All results presented in this paper are achieved by means of an in-house developed tool-path algorithm. This can be described as a staggered meander-type tool motion path specifically designed to reduce heat transfer and consequently temperature gradient on the surface. Contiguously, classical de-convolution methods are adapted to non-linear etching rates for the derivation of the surface scanning speed maps. The figuring procedure is carried out iteratively. It is noteworthy that iteration steps never exceed ~7 minutes mean processing time.