Ultra-precision Manufacturing Research Articles

Intrinsic multiplex graph model detects incipient process drift in ultraprecision manufacturing

While rare events like process drift and damage in ultraprecision manufacturing are not preventable in most cases, the accurate detection could enable timely corrective actions to substantially reduce the severity of the fallout and the associated treatment cost. Nonetheless, subtle process drift in incipient stage generally nullifies conventional data-driven detection algorithms, which are inadequate to track dynamic behavioral evolution for complex systems. Notably, the inevitable sensor failure in conjunction with heterogeneous sensing characteristics have made it strenuous to synchronize data from multiple sources to monitor process variability. To address those challenges, this paper presents a novel intrinsic multiplex graph approach based on intrinsic time-scale decomposition of the signal collected from one single source for the detection of incipient process drift. A case study in ultraprecision machining indicates that this proposed method detects the formation and sweeping away of built-up edge phenomenon with minimal time delay.

Influence of buried modified layer on crack propagation and diamond turning of silicon

Ductile-regime machining of brittle materials is a critical issue for ultra-precision manufacturing, because the machined surface is prone to be damaged by brittle fracture. Previous studies have showed that surface modification by ion implantation can be used to reduce the brittleness and improve the machinability. However, most researches focus on the formation of a continuous modified layer extending from surface to a specific depth. Although this configuration could provide a remarkable modification, it always requires a large ion dose and is time consuming. For this reason, a buried modified layer model is proposed in this paper. Firstly, the effect of the modified layer on crack propagation is studied by molecular dynamics simulation, which indicates an increase in fracture resistance of the buried modified layer model. Then, ion implantations are conducted on the (100) surface of silicon, and modification on the lattice structure is evaluated by Raman spectrum and transmission electron microscopy. Finally, diamond turning test is conducted and the critical undeformed chip thickness is measured. The results show that the buried modified layer is a potential configuration for improving the diamond turning process on brittle material.

Effect of cupric ion concentration on the etching behavior of copper in Electrogenerated Chemical Polishing (EGCP)

Electrogenerated Chemical Polishing (EGCP) is a stress-free polishing method which has been verified to be an effective method to obtain an ultra-flat and ultra-smooth copper (Cu) surface without defects induced by mechanical force. However, the surface roughness deterioration and removal rate decline emerged after polishing for a long time which prevents the application of EGCP in Cu ultra-precision manufacturing. In this study, apparent activation energy experiments and energy dispersive spectrometer (EDS) analysis were carried out to explore the mechanism of the Cu surface roughness deterioration and removal rate decline. With the increasing of cupric ions (Cu2+) concentration, the apparent activation energy of the Cu/Cu2+ reaction increased which inhibited the material removal rate consequently. Furthermore, it was found that Cu micro bumps were generated on the surface which was speculated to result from the disproportionation reaction of cuprous ion (Cu+) induced by high concentration of Cu2+. Based on this study, lifting the working electrode (WE) with a fixed time interval was verified to be an effective way to avoid the deterioration of surface roughness and the decline of the material removal rate. This study contributes to a further understanding of the chemical etching mechanism of Cu in EGCP.

Edge control in precision robotic polishing based on space-variant deconvolution

In the ultra-precision manufacturing of large optical surfaces, industrial robots with small tools have the potential to become an intelligent and economical choice of surface polishing. But one of the most challenging problems is the severe edge roll-off error caused by the small tools. In this paper, an effective method is proposed to reduce the edge error in the polishing of large mirrors. The convergence rate of the form quality can be improved by adjusting the polishing removal amount. A generic space-variant deconvolution algorithm is developed to precisely calculate the dwell time. Experiments are conducted using an industrial robotic polisher and the edge roll-off error is effectively suppressed. As a consequence the polishing accuracy and efficiency of the robotic polishing technology can be improved significantly.

Region-adaptive path planning for precision optical polishing with industrial robots.

In the field of ultra-precision manufacturing, industrial robotic polishing has the potential to become a more economical and intelligent method than the conventional polishing machines. But the challenge of the robotic polishing lies in the low control accuracy, which seriously affects the polishing quality. In this paper a new region-adaptive path planning method is proposed, where the path is generated adaptively according to the specific form error. Each time only the regions with form error large enough are processed, thereby improving the polishing stability and efficiency. Smooth paths are generated based on the hexagonal meshing of the processing regions to avoid sharp turning, and then the dwell time is calculated by space-variant deconvolution. The PVr metric of the final form error resulting from the robotic polisher converges down to λ/15. In addition this method can reduce the polishing time by 80%, henceforth the stability and efficiency of robotic polishing can be greatly improved.

A Dirichlet Process Gaussian State Machine Model for Change Detection in Transient Processes

ABSTRACTThe ability to detect incipient and critical changes in real world process—esessential for system integrity assurance—is currently impeded by the mismatch between the key assumption of stationarity underlying most change detection methods and the nonlinear and nonstationary (transient) dynamics of most real-world processes. The current approaches are slow or outright unable to detect qualitative changes in the behaviors that lead to anomalies. We present a Dirichlet process Gaussian state machine (DPGSM) model to represent dynamic intermittency, which is one of the most ubiquitous real-world transient behaviors. The DPGSM model treats a signal as a random walk among a Dirichlet process mixture of Gaussian clusters. Hypothesis tests and a numerical scheme based on this nonparametric representation were developed to detect subtle changes in the transient (intermittent) dynamics. Experimental investigations suggest that the DPGSM approach can consistently detect incipient, critical changes in intermittent signals some 50–2000 ms (20–90%) ahead of competing methods in benchmark test cases as well as a variety of real-world applications, such as in alternation patterns (e.g., ragas) in a music piece, and in the vibration signals capturing the initiation of product defects in an ultraprecision manufacturing process. A supplementary file to this article, available online, includes a Matlab implementation of the presented DPGSM.

Sustainable manufacturing of ultra-precision machining of titanium alloys using a magnetic field and its sustainability assessment

Titanium alloys are widely used in fabrication of certain medical products because of their superior properties. However, titanium alloys are difficult to cut, because of their low thermal conductivity and high sustainability of work hardening at elevated temperature. Therefore, machining of titanium alloys involves lots of machining difficulties especially in ultra-precision machining. On the other hand, an increase in material removal rate (MRR) always reduces surface quality, leading an infeasibility of high MRR machining of titanium alloys. In this study, a novel machining technology, a magnetic field was firstly applied into single point diamond turning (SPDT), which aims to enhance both MRR and the surface quality of machined titanium alloys without requiring complicated equipment and extra natural resources. Under the influence of a magnetic field, the surface quality of machined titanium alloys and diamond tool life in SPDT were improved significantly when machining at relatively high MRR. Furthermore, the sustainability assessment of machined products was demonstrated by product sustainability index (ProdSI), which ProdSI reflects the comprehensive views on the triple bottom line (TBL) of sustainability: economic, environmental and social dimensions. ProdSI of the machined components by SPDT in the presence of magnetic field was 2.39 higher than that by normal SPDT, suggesting an enhancement of product sustainability by the proposed machining technology. The proposed machining technology provides a solid way to increase both the production rate and the precise level of titanium alloy components for promoting sustainable ultra-precision manufacturing.

Yb∶LuScO3晶体的超精密光学加工及其激光性能

Yb：LuScO 3 晶体作为固体激光器的新型增益介质，其面形和表面质量严重影响激光器的光束质量，因此探索Yb：LuScO 3 晶体的超精度光学加工工艺参数具有重要意义。本文系统开展了Yb：LuScO 3 晶体超精密光学加工的工艺参数研究，针对Yb：LuScO 3 晶体在加工过程中容易破裂和表面质量较差的问题，提出了拼接上盘和树脂铜盘抛光垫的关键技术。首先，使用COMSOL Multiphysics有限元软件对拼接工艺中选取的不同保护垫料的应力进行仿真。接着，研磨阶段逐步减小B 4 C磨料的粒径。然后，粗糙阶段使用树脂铜盘作为抛光垫，并对树脂铜盘抛光垫的作用进行了分析。最后，使用激光二极管泵浦加工好的样品进行激光输出实验。实验结果表明：基于该技术加工后的晶体表面粗糙度RMS=0.296 nm，面形精度PV=53 nm。在1 086 nm处获得了8.3 W的连续激光输出，斜效率为58%。该加工方法可以广泛应用于Yb：LuScO 3 晶体的高精度加工。

Experimental and Simulation Study of Nanometric Cutting of Silicon by Diamond Turning

Diamond turning is an ultra-precision manufacturing method which is employed to fabricate components requiring submicron geometric accuracy and atomically smooth surface finish, such as those utilized in the x-ray optics, IR optics and semiconductors industries. This study represents the experimental investigation as well as MD Simulation to elucidate the strong dependence of cutting parameters (tool feed rate, depth of cut and spindle speed) in influencing the ductile-regime machining conditions during cutting of silicon. Precitech Nanoform 200 diamond turning machine is used to perform experiments on 50 mm diameter (111) oriented silicon wafers. To enhance the understanding of actual cutting mechanism, Molecular Dynamic Simulation is being incorporated to mimic the nanometric cutting of silicon. The simulation study encompasses the investigation of phase transformation, cutting force, thrust force and temperature exerted at the cutting zone. The actual cutting mechanism involved during the nanometric cutting of silicon is clearly visualized from MD simulation.

Atomistic scale nanoscratching behavior of monocrystalline Cu influenced by water film in CMP process

The effect of water film on the nanoscratching behavior of monocrystalline Cu was studied by molecular dynamics (MD) simulation. The results indicate that the friction force acting on abrasive particle increases due to the resistance of water film accumulating ahead of particle, but the water film with lubrication decreases friction force acting on Cu surface. The accumulation of water molecules around particle causes the anisotropy of ridge and the surface damage around the groove, and the water molecules remaining in the groove lead to the non-regular groove structure. The dislocation evolution displays the re-organization of the dislocation network in the nanoscratching process. The evaluation of removal efficiency shows the number of removed Cu atoms decreases with water film thickness. It is considered that an appropriate rather than a high removal efficiency should be adopted to evaluate the polishing process in real (chemical mechanical polishing) CMP. These results are helpful to reveal the polishing mechanism under the effect of water film from physical perspective, which benefits the development of ultra-precision manufacture and miniaturized components, as well as the innovation of CMP technology.

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Active Hybrid Control Algorithm with Sky-Hook Damping and Lead-Lag Phase Compensation for Multi-DOFs Ultra-Low Frequency Active Vibration Isolation System

With the improvement of performance in the ultra-precision manufacturing engineering, the requirements for vibration isolation have become more stringent. In order to obtain a wider effective bandwidth and a higher performance of a multi-DOFs active vibration isolation system (AVIS), active hybrid control (AHC) technology is applied in this paper. AHC technology comprises a feedback active control (FBAC) technology and a feedforward active control (FFAC) technology. Absolute velocity feedback is employed to establish a sky-hook damping technology in FBAC technology. Velocity feedforward of base platform is adopted to build a lead-lag phase compensation (LLPC) technology in FFAC technology. Further, a coordinate vector conversion from unit level to system level is mentioned to describe the dynamic characteristic of the six DOFs AVIS applied in the ultra-precision field. And with the assistance of the transformed coordinate vector, the dynamic model of system level is built. Based on the establishment of the dynamic model and the research of AHC, an experimental platform which constitutes three vibration isolators and a real-time active control system is set up. The experimental results indicate that the amplitude of the resonant peak is further reduced significantly, compared to the general feedback control. And simultaneously active effective bandwidth is extended. AHC technology with sky-hook damping algorithm and LLPC control algorithm is verified to be more effective.

Analysis and comparison of two different ultra-precision manufacturing methods for off-axis parabolic mirror with single point diamond turning

Off-axis aspheric mirror is an important component widely used in optical system and precision measurement instrument. Off-axis parabolic surface is one typical shape of off-axis aspheric mirrors which can enlarge focal length and widen field-of-view. Two typical ultra-precision turning methods which are turning the cylindrical blank by revolving around the axis of parabolic surface (TPS) and revolving around the axis of cylindrical surface (TCS) are analyzed and compared for off-axis parabolic surface in this article. Three-axis (X, Z, C) turning machine is applied and kinematic analyses of two linear axes (X, Z) are studied during cutting with the single point diamond turning. The tool paths are generated and installation errors are analyzed under two different conditions. Primary experiments are conducted to verify the effectiveness of theoretical analyses. The obtained results demonstrate that the surface machined with TPS method has better machining quality than the one machined with TCS method.

Wear Mechanism of Diamond Tool against Mold Steel in Single Point Diamond by Molecular Dynamics

Owing to the extremely rapid tool wear, ultra-precision manufacturing of mold steel through single point diamond turning (SPDT) operation becomes a challenging task. Molecular dynamics (MD) simulation is an appropriate tool to study nanoscale processes occurring at the femtosecond/picosecond timescale. The wear mechanism of the diamond tool is discussed by means of molecular dynamics in which atomic position variation, temperature fluctuations were involved. According to the analysis results, iron atoms have unpaired d electrons, these iron atoms may connect with diamond carbon atoms through chemical bonds which are the driving force to the diamond removal and making diamond convert into graphite. The simulation results demonstrated that, it is possible to mitigate the wear of diamond tools effectively, if those bindings between unpaired d electrons and diamond carbon atoms of the tool can be under control.

Atomic Step Formation on Sapphire Surface in Ultra-precision Manufacturing.

Surfaces with controlled atomic step structures as substrates are highly relevant to desirable performances of materials grown on them, such as light emitting diode (LED) epitaxial layers, nanotubes and nanoribbons. However, very limited attention has been paid to the step formation in manufacturing process. In the present work, investigations have been conducted into this step formation mechanism on the sapphire c (0001) surface by using both experiments and simulations. The step evolutions at different stages in the polishing process were investigated with atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). The simulation of idealized steps was constructed theoretically on the basis of experimental results. It was found that (1) the subtle atomic structures (e.g., steps with different sawteeth, as well as steps with straight and zigzag edges), (2) the periodicity and (3) the degree of order of the steps were all dependent on surface composition and miscut direction (step edge direction). A comparison between experimental results and idealized step models of different surface compositions has been made. It has been found that the structure on the polished surface was in accordance with some surface compositions (the model of single-atom steps: Al steps or O steps).

Dynamic Behavior of Ultra Precision Feed Axis at Increased Velocities

The processing speed of machine tools for ultra precision manufacturing are low compared to precision manufacturing to satisfy the precision requirements. To increase the productivity and develop machines for ultra precision high performance cutting (UP-HPC) it is essential to examine and compensate dynamic effects. In this work the positioning behavior of an ultra precision feed axis at different setpoint velocities is analyzed and compared to a high speed feed axis. It is observed that the ultra precision feed axis has a large overshoot of several micrometers if the velocities are increased. Further, the decay time of the ultra precision feed axis exceeds the decay time of the high speed feed axis by more than factor two. These two characteristics increase the production time, because of long tool paths and many reversal points, which are often required to keep the kinematic roughness low, especially in ultra precision raster milling.

Machining and Metrology of a Chalcogenide Glass Freeform Lens Pair

Slow tool servo remains a preferred method for ultraprecision manufacture of freeform optics with millimeter-scale departures from an axisymmetric surface. A methodology has been developed for pre-process mapping of, and compensation for, predictable error sources such as tool waviness and machine misalignment, reducing necessity for typical post-process workpiece error correction procedures. This methodology is implemented for manufacture of a chalcogenide glass freeform lens pair for a variable-focus thermal imaging application. Process development, part metrology, and system functional testing are presented.

An Ultra-Low Frequency Two DOFs’ Vibration Isolator Using Positive and Negative Stiffness in Parallel

With the improvement of performance in the ultra-precision manufacturing engineering, the requirements for vibration isolation have become increasingly stringent. In order to get wider effective bandwidth and higher performance of vibration isolation in multiple DOFs system, an ultra-low frequency two DOFs’ vibration isolator with positive and negative stiffness in parallel (PNSP) is proposed. The two DOFs’ isolator which combines a positive stiffness (PS) air spring with a negative stiffness (NS) magnetic spring in parallel and combines a PS flat spring with an NS inverted pendulum in parallel is designed to reduce the natural frequency and broaden the effective bandwidth in horizontal and vertical direction. Based on this structure, stiffness models of different components in different directions are established. Compared with a PS isolator, it possesses the characteristic of high-static-low-dynamic stiffness. The simulation curves also provide strong evidence. Last, a real-time active control system and a spectrum testing and analysis system are used for the contrast experiment between the mentioned PNSP structure and PS only. The experimental results demonstrate that the isolator with PNSP can obviously reduce the natural frequency to 1 Hz and simultaneously maintain the stability of the system and consequently verify the validity and superiority of the mentioned structure.

The ultra-precision Ud-lap grinding of flat advanced ceramics

The present study focuses on the Ud-lap grinding process and its machine-tool design aiming at ultra-precision (UP) manufacturing of advanced ceramics. Impacts of three different overlapping factors on a dressing (Ud) and three abrasive grit sizes of conventional SiC grinding wheels were analyzed on flat nanometric surface finishing of dense discs of 3Y-TZP in a ductile regime of material removal. Microhardness, contact and optical profilometry, SEM-FEG, Raman spectroscopy, XRD, and confocal epi-fluorescence were applied to characterize the ceramic material. Mechanical and electric-electronic designs of the machine-tool were developed toward the UP ceramic grinding. The design methodology was successful for supporting the achievement of all design requirements of the CNC Fiocchi Lap Grinder. The machine-tool and the Ud-lap grinding process were capable of manufacturing flat 3Y-TZP surfaces with nanometric finishing without introducing critical defects. The best finishing, Ra=60.63nm, came from the #300 grinding wheel dressed with Ud=5. A flatness deviation of 0.308μm was obtained through the #800 grinding wheel and Ud=3. Differences between theoretical and real macro and micro effects over the grinding wheels after single-point diamond dressing, epoxy bond strength, abrasive protrusion, abrasive grit size and abrasive friability play a key role in the Ud-lap grinding. There is no report of an abrasive process capable of achieving similar nanometric finishing without introducing critical defects with the same micrometric grit size and type of abrasive. The Ud-lap grinding can replace the engagement of processes such as grinding, lapping, and polishing of advanced ceramics.

Reduction of vibrations of passively-isolated ultra-precision manufacturing machines using mode coupling

Ultra-precision manufacturing (UPM) machines are used to fabricate and measure complex parts having micrometer-level features and nanometer-level tolerances/surface finishes. Therefore, random vibration of the machine due to ground excitations and residual vibration stemming from onboard disturbances must be mitigated using vibration isolation systems. A long-standing rule of thumb in vibration isolation system design is to locate the isolators in such a way that the vibration modes of the isolated machine are decoupled. However, prior work by the authors has demonstrated that coupling vibration modes of passively-isolated UPM machines could provide conditions for drastic reduction of residual vibrations compared to decoupling. The authors’ analysis was based on the restrictive assumption that the isolated machine was modally damped. The key contribution of this paper is in investigating the effect of mode coupling on the residual vibrations of UPM machines with non-proportional (NP) damping—which is more realistic than modal damping. It also analyzes the effects of mode coupling on the reduction of ground vibrations (i.e., transmissibility). The analyses reveal that, even though NP damping changes the vibration behavior of the machine compared to modal damping, mode coupling still provides ample opportunities to reduce residual vibration and transmissibility. Guidelines for properly designing a UPM machine to best exploit mode coupling for vibration reduction are provided and validated through simulations and experiments. Up to 40% reduction in residual vibration and 50% reduction in transmissibility are demonstrated.