Ultra-precision Machining Process Research Articles

Improving the surface quality and its mechanism in ultraprecision machining of Al–Mg–Si alloy by multiple high energy shot peening pretreatment

Achieving high surface quality is vital for improving the reflectivity of aluminum alloy optical components. However, aluminum alloy materials often have large-sized grain structures, and stress accumulates at the grain boundaries during ultraprecision machining processes, resulting in raised grain boundaries and a decrease in surface quality. In this work, we utilized a multiple high energy shot peening (SP) technique to refine the surface grain structure of Al–Mg–Si alloy. The results revealed that a gradient grain structure was formed within the depth range of 0∼150 μm, accompanied by the occurrence of various substructures such as dislocations, stack faults, and low-angle grain boundaries in the severely deformed layer. This led to a significant 30% increase in hardness. Furthermore, the shot peened sample exhibited remarkable ultraprecision machinability. A comparison with the sample that has not undergone SP treatment revealed a decrease in surface roughness from 2.5 nm to 1.4 nm. The improved surface quality can be attributed to the higher deformation uniformity and reduced grain rotation. Our study provided an innovative and cost-effective method for manufacturing large-sized metal optical components.

Ultra-precision machining process of inner surface considering shear-thickening polishing method

Shear-thickening polishing (STP) is a non-traditional flexible ultra-precision processing method that exploits the polishing fluid’s shear-thickening properties to create a shear rate between the workpiece surface and the potential flow. This increases viscosity and achieves material removal. In the past, the shear-thickening polishing method was mostly employed for the outer surface of rotating bodies. Nevertheless, to apply the STP method to ultra-precision processing of the workpiece’s inner surface, this paper proposes an active formed shear-thickening layer polishing method based on a numerical calculation model. Additionally, the main parameters affecting the machining efficiency were identified, orthogonal experiments were performed on the process parameter levels, and optimal machining conditions were derived. The optimized process was adopted to machine the inner surface of the large-size bearing ring, and the surface roughness in terms of Ra was reduced from the original 131.27 to 27.14 nm in 30 min and further to 19.52 nm in 60 min. The validation experiments highlight that the numerical model can predict the experimental results accurately, demonstrating the proposed method’s feasibility for ultra-precision machining on the inner surface with the STP method.

Comprehensive study of material removal mechanism of polycrystalline copper during ultra-precision cutting using molecular dynamics

Nano-polycrystalline copper as a unique functional and structural material has been drawn great attention. Comprehensive study on its nano-cutting mechanism is of great important to guide the ultra-precision machining process. In this paper, nano-cutting simulation of polycrystalline copper was studied against the cutting depth, tool geometry, grain size and grain orientation. The effect of cutting depth on the shear angle of the material deformation and the proportion of HCP atoms in chips were studied. Furthermore, nano-indentation and nano-cutting of polycrystalline copper with different grain size were simulated. In terms of crystal orientation, the difference of orientation in the bi-crystalline copper could form chips with different shapes and induce different strain distribution within the grain. In addition, the size and distribution of dislocation density are studied in the nano-cutting process of different polycrystalline copper textures. Texture {112}<111>, {236}<385>, and {124}<211> were considered.

In-process monitoring of the ultraprecision machining process with convolution neural networks

ABSTRACT In-process monitoring and quality control are the most critical aspects of the manufacturing industry, especially in ultra-precision machining (UPM) at an industrial scale. However, in-process ensuring product quality has been difficult, as any subtle change in the process influences the UPM process dynamics and the process outcome. In order to meet the increasingly soaring demand for precision components, intelligent monitoring of the machining process is essentially important and much needed. Capturing complex signal patterns through conventional signal processing for the UPM process is often challenging due to the comparably high noise levels in the industrial environment. Signals obtained during UPM are inherent transients and non-stationary, necessitating extensive and accurate features for classification. Accurate detection of anomalies may allow for quick corrective actions, reducing the degree of damage. Earlier research revealed multi-sensor analysis, which yields richer signal feature information, but the unavoidable sensor failure in conjunction with heterogeneous sensing made it challenging. In order to address the challenges, this paper investigates the feasibility of convolution neural network (CNN) for classifying abnormal and normal machining in the UPM process. The vibrational signals obtained from B&J 4533-B accelerometer during diamond turning are transformed into time-frequency-based log-spectrogram images. These images are classified using CNN, and the results show that a proposed convolutional neural network algorithm has demonstrated an accuracy of 85.92% in classifying images and thus the corresponding in-process machining status.

Abrasive machining induced surface layer damage behavior and formation mechanism of monocrystalline β-Ga2O3: A comparative study of nanoindentation and nanogrinding

Beta-phase gallium oxide (β-Ga2O3) has garnered significant attention in recent years as an ultra-wide bandgap semiconductor material. However, its surface layer damage behavior and machining deformation mechanism remain poorly understood. In this work, a systematic comparative study was conducted through nanoindentation and nanogrinding for monocrystalline β-Ga2O3 (−201) crystal plane. Transmission electron microscopy is employed as the primary characterization method. The results show that the microstructural evolution patterns of monocrystalline β-Ga2O3 can be categorized into four stages, i.e. stacking faults, twins, slip bands, and cleavage cracks are observed sequentially. According to the tests results, subsurface damage diagram models based on nanoindentation and nanogrinding are established, both of which exhibit the above evolution patterns as the indentation depth and grit depth of cut increases. However, there are some differences. In the nanogrinding process, nanocrystals and amorphous layers were found near the ground surface, which are related to the high strain rate grinding conditions and the unique material properties of monocrystalline β-Ga2O3 with low stacking fault energy. The results in this work contribute to an enhanced understanding of the deformation characteristics of monocrystalline β-Ga2O3 at macro and micro scales, while offering valuable insights for achieving damage control in ultra-precision machining processes.

Multi-Body Dynamic Analysis of Hydrostatic Bearing with the MMC Material in Micro-Nano Machining.

This study focuses on the analysis of a linear hydrostatic bearing using harmonic frequency response and harmonic response simulations. The aim is to evaluate the feasibility of replacing the existing alloy steel material with a metal matrix composite (MMC) in terms of its performance and dynamic characteristics for both the base and carriage parts. The simulation results indicate that the MMC material exhibits higher resonant frequencies and improved damping capabilities compared to the structural steel material. The higher resonant frequencies observed in the MMC material are attributed to its stiffness and structural properties. These properties contribute to increased natural frequencies and improved vibration damping characteristics. This suggests that incorporating the MMC material in the bearing design could enhance motion control, improving the ability to precisely control and manipulate the movement of components or systems. In the context of ultraprecision machining applications, incorporating the MMC material in the hydrostatic bearing design can also lead to a more accurate and controlled motion, resulting in improved precision and finer machining outcomes. The displacement analysis confirms that both materials meet the specifications provided by the manufacturer, supporting the viability of using MMC as an alternative. However, further experimental validation and considerations of material feasibility, manufacturing factors, and cost-effectiveness are necessary before implementing the MMC material in practical applications. Overall, this research highlights the potential benefits of MMC in the design of linear hydrostatic bearings, paving the way for enhanced performance in ultraprecision machining processes.

Determination of stress waves and their effect on the damage extension induced by surface defects of KDP crystals under intense laser irradiation

The residual crack defects on the surface of potassium dihydrogen phosphate (KDP) crystals are the bottleneck that limits the improvement of laser damage resistance in the application of high-power laser devices. The multiple stress waves introduced by these residual surface lateral cracks on crystals under laser irradiation are the main inducement for damage extension and reduction of laser damage resistance. However, the coupling of these stress waves complicates their propagation in the crystal, and the interaction mechanism between each stress wave and laser damage has not been quantitatively characterized. Herein, a laser damage dynamic model for surface lateral cracks is constructed to reproduce the dynamic behaviors of the evolution of micro-defects to sub-millimeter damage pits under laser irradiation. Combined with the time-resolved pump and probe technique, the distribution of stress waves induced by lateral cracks was detected in situ to determine the type of stress waves. Then, the initiation and extension of laser damage were analyzed quantitatively to establish the correlations between different stress waves and damage extension. It is found that the longitudinal, shear, and Rayleigh waves induced by lateral cracks lead to large crush zones on the surface of KDP crystals, as well as butterfly-like damage sites accompanied by a large number of cracks at the bottom in the longitudinal section. The scale of the damage site can reach up to approximately 150 µm for lateral crack defects with large surface widths. This study ultimately reveals the physical mechanism of damage evolution induced by lateral cracks, providing effective guidance for developing control standards of surface crack defects during optical ultra-precision machining processes. This is of great significance for the improvement of laser damage resistance of KDP crystals in high-power laser systems.

Condition monitoring of three-axis ultra-precision milling machine tool for anomaly detection

Accurately and continuously monitoring ultra-precision machining (UPM) process is the foundation forsubsequent diagnosis and optimization, then facilitating energy-saving, efficient production, and high-quality machining. However, comprehensive monitoring of UPM process has hardly been investigated systematically in previous studies. To cover the gap, this study examined the linkages among these parameters monitored in UPM process using a five-layers network for the first time. Subsequently, we proposed an advanced monitoring platform that integrates G-code command, installation sensors, and controller interface. This proposed platform incorporated with anomalies detection algorithm was finallyemployed and validated onathree-axis ultra-preciisiion miilllliing machiine tooll. Results showed that this proposed platform could successfully achieve anomaly identification using power consumption and X/Y/Z components force signals.

Theoretical and experimental investigation on the radial-fly-cutting process for micro-structured surface fabrication

Radial-fly-cutting (RFC) is a promising technique for the fabrication of micro-structured surfaces. In this paper, the generation of both micro-structured surfaces and hierarchical-structured surfaces under four modes of RFC were theoretically investigated and experimentally validated for the first time. A novel mode of RFC, named servo-assisted helix-feeding radial-fly-cutting (SA-HFRFC) was proposed for fabricating intricate hierarchical-structured surfaces with high efficiency. Considering system configuration and the material removal process during RFC, a novel surface generation algorithm catering for all processing modes was proposed to calculate and predict the intricate micro-structured surfaces. Numerical simulations were conducted to predict the micro-structured surfaces and demonstrate the good effectiveness of the algorithm in the surface calculation. Machining experiments of micro-structured surfaces and hierarchical structured surfaces under four modes of RFC were conducted, which validated the good agreements between the simulations and experiments. Under SA-HFRFC, an intricate primary surface, which is modulated with a 12-cycle sine curve governed by the polar angle, was designed and fabricated, and the secondary microgrooves regularly superimposed on the PS were deterministically fabricated. It was demonstrated that RFC features a strong capacity for the fabrication of both micro-structured surfaces and intricate hierarchical-structured surfaces. This work will contribute to studies on the ultra-precision machining process and deterministic fabrication of both micro-structured surfaces and hierarchical-structured surfaces. • Four modes of RFC, including the newly-proposed SA-HFRFC, were expounded for fabricating intricate hierarchical-structured surfaces. • A novel surface generation algorithm catering for all processing modes was proposed to calculate and predict the surfaces. • Numerical simulations were conducted to predict the micro-structured surfaces and demonstrate the good effectiveness of the algorithm. • Micro-structured surfaces machining validated the good agreements between the simulations and experiments.

Laser damage evolution by defects on diamond fly-cutting KDP surfaces

The unavoidable surface defects on the practical processed potassium dihydrogen phosphate (KDP) crystals, especially the most common protuberance defects, which are prone to damage under intense laser irradiation, seriously limit the application of KDP optics in laser fusion facilities. In this work, a defect-induced laser damage explosion model that couples the electromagnetic field and stress field was constructed to reproduce the dynamic damage process and reveal the laser-induced damage mechanisms innovatively. First, the near-field optical modulation to incident lasers was considered to describe the effect of structural parameters of surface defects on the optical transmission performance. A multi-physics field coupled failure model was then established for the first time to explore the laser damage mechanism through the dynamic behaviors of laser damage that reproduced by this model. It was proved that the protuberance defects on KDP crystal surface would lead to severe laser damage with the characteristics of large transverse spalling and longitudinal cracks due to the Rayleigh and stress waves, which well agrees with the results of laser damage experiments. The established damage model is capable of revealing the hazards of protuberance defects, and providing effective guidance for the suppression of surface defects introduced in the optical ultra-precision machining processes.

Mechanism of Surface Defects in Ultra-Precision Machining of Sesquioxide Laser Crystal Tm: GdScO3.

It is well-known that the surface quality of laser gain crystal elements is very high in order to ensure the stability of laser system and laser output quality. In the ultra-precision machining process of a new sesquioxide laser crystal Tm: GdScO3, it is required to achieve very high surface shape and very low surface defects. In this paper, the molecular dynamics simulation model of single particle grinding was established. It was found that the normal load and tangential friction imposed by abrasive particles on the surface of components cause the spalling of atoms on the substrate surface, which constitutes the removal of materials at the macro-level. At the same time, it causes the displacement of the sub surface atoms, which constitutes the microscopic defects in the structure. Through the structural characterization of macro defects, it was confirmed that the essence of micro defects is the amorphous and distortion of surface structure, and the depth can reach 100 nm. The results of lapping and polishing experiments show that the adjustment of pressure has a limited effect on the improvement of surface defects in the process of machining crystal elements with granular abrasive.

Nanostructure machining and its application in surface information

Surface information is of significance in daily life, such as news displaying and information communication. The conventional methods that generate the surface information are to use chemical inks or dyes, which cause environmental pollution problems. Motived by this, this study proposes the mechanical machining method, named as vibration-assisted ultraprecision machining process, to fabricate orderly nanostructures to generate the surface information. Experimental results showed that the surface information “Beat the COVID-19″ in the form of a quick response code was successfully machined on the workpiece surface. The information also can be hidden by manipulating the facet spacing of the adjacent nanostructures. The information “LOVE”, as an example, was hidden on the workpiece surface. It only can be observed at a certain viewing angle. Besides, a replication material was employed to replicate the generated surface information. Finally, the three-dimensional topographies of these orderly nanostructures were characterized and analyzed. The proposed method for generating, hiding, and replicating surface information extends the application range of the vibration-assisted ultraprecision machining process to information science.

Revealing nanoscale material deformation mechanism and surface/subsurface characteristics in vibration-assisted nano-grinding of single-crystal iron

Ultrasonic vibration-assisted grinding (UVAG) has attracted extensive attention as it can significantly improve the surface integrity of the machined workpiece. However, the atom-scale material deformation mechanism with the presence of ultrasonic vibration is still unclear, which hider the application of the UVAG to the ultra-precision machining process. To fill this gap, we present a molecular dynamics (MD) model for the vibration-assisted nano-grinding process (VANG) of single-crystal iron. The characteristics of the material deformation process induced by ultrasonic vibration are studied, including material accumulation, temperature, dislocation, subsurface damage (SSD), contact area, and grinding force. The results show that vibration changes the dislocation distribution in the plastic zone, forms the phenomena of expansion and contraction and causes the fluctuation of SSD depth. Due to the different distribution of stress and temperature during VANG, the details of plastic deformation, such as dislocation movement, dislocation nucleation and phase transformation, change with the varying vibration frequency. In addition, the reduction of the projected contact area is the main reason for the reduction of grinding force. However, dislocation pileup in the grinding zone and chips further increase the grinding hardness. This research could enrich the understanding of nano-scale deformation mechanisms in vibration-assisted processing of metallic materials.

On-machine measurement and temperature compensation method of NURBS surface interpolation for semicircular narrow neck thickness based on ultra-precision machine tool

Flexible joint is a typical low-rigidity micro part, which has four narrow neck structures evenly distributed around its central axis. In the ultra-precision machining process, it is crucial to control the dimensional accuracy of the narrow necks to improve stiffness consistency. However, the limited space of the neck structures and the high accuracy requirements bring extremely difficulties to the measuring process. In this paper, a novel on-machine measurement method for the semicircular narrow necks was presented and a measuring device was developed based on the combination of high linearity incremental measurement method and on-machine measurement technology. Moreover, the uncertainty of measurement error was analyzed in detail. In view of the measurement error caused by temperature drift, a compensation method based on non-uniform rational B-spline surface interpolation was used to enhance the measurement accuracy. The experimental results showed that the measurement accuracy enhanced over 30% after compensation, which met the measuring requirement of flexible joint.

On-machine measurement and temperature compensation method of NURBS surface interpolation for semicircular narrow neck thickness based on ultra-precision machine tool

Abstract Flexible joint is a typical low-rigidity micro part, which has four narrow neck structures evenly distributed around its central axis. In the ultra-precision machining process, It is crucial to control the dimensional accuracy of the narrow necks to improve stiffness consistency. However, the limited space of the neck structures and the high accuracy requirements bring extremely difficulties to the measuring process. In this paper, a novel on-machine measurement method for the semicircular narrow necks was presented and a measuring device was developed based on the combination of high linearity incremental measurement method and on-machine measurement technology. Moreover, the uncertainty of measurement error was analyzed in detail. In view of the measurement error caused by temperature drift, a compensation method based on non-uniform rational B-spline (NURBS) surface interpolation was used to enhance the measurement accuracy. The experimental results showed that the measurement accuracy enhanced over 30% after compensation, which met the measuring requirement of flexible joint.

Monitoring and Predicting the Surface Generation and Surface Roughness in Ultraprecision Machining: A Critical Review

The aim of manufacturing can be described as achieving the predefined high quality product in a short delivery time and at a competitive cost. However, it is unfortunately quite challenging and often difficult to ensure that certain quality characteristics of the products are met following the contemporary manufacturing paradigm, such as surface roughness, surface texture, and topographical requirements. Ultraprecision machining (UPM) requirements are quite common and essential for products and components with optical finishing, including larger and highly accurate mirrors, infrared optics, laser devices, varifocal lenses, and other freeform optics that can satisfy the technical specifications of precision optical components and devices without further post-polishing. Ultraprecision machining can provide high precision, complex components and devices with a nanometric level of surface finishing. Nevertheless, the process requires an in-depth and comprehensive understanding of the machining system, such as diamond turning with various input parameters, tool features that are able to alter the machining efficiency, the machine working environment and conditions, and even workpiece and tooling materials. The non-linear and complex nature of the UPM process poses a major challenge for the prediction of surface generation and finishing. Recent advances in Industry 4.0 and machine learning are providing an effective means for the optimization of process parameters, particularly through in-process monitoring and prediction while avoiding the conventional trial-and-error approach. This paper attempts to provide a comprehensive and critical review on state-of-the-art in-surfaces monitoring and prediction in UPM processes, as well as a discussion and exploration on the future research in the field through Artificial Intelligence (AI) and digital solutions for harnessing the practical UPM issues in the process, particularly in real-time. In the paper, the implementation and application perspectives are also presented, particularly focusing on future industrial-scale applications with the aid of advanced in-process monitoring and prediction models, algorithms, and digital-enabling technologies.

Optical properties of large-size and damage-free polished Lu2O3 single crystal covering the ultraviolet-visible-and near-infrared (UV–VIS–NIR) spectral region

Lu2O3 has attracted widespread attention in high-power lasers, scintillators, and semiconductors due to its high thermal conductivity, high stability, and large band gap. Due to the limitations of the growth process, high-precision characterization of polished quality and optical properties for large-size Lu2O3 single crystal is extremely scarce, which hinders the design and optimization of optics based on Lu2O3. In this study, we design a systematic machining scheme for large-size Lu2O3 single crystal thick chip to obtain polished crystal with virtually no surface and subsurface damage. The surface/subsurface quality was evaluated utilizing optical profilometer, scanning electron microscope, and transmission electron microscope. Variable-angle spectroscopic ellipsometer was employed to measure the optical constants over wide spectral region (210–1690 nm), removing the lack of optical constants of Lu2O3 single crystal in the ultraviolet region. The band gap is analyzed based on the optical constants results. The consistency of the measured data with the relevant literature verifies the reliability and accuracy of the measurement method along with the data. The surface/subsurface damage inspection method provides an effective approach for ultra-precision machining mechanism research and process optimization of Lu2O3 and other hard and brittle materials. The band gap and the optical constants in wide spectral region provide critical fundamental data for the design and performance optimization of related optics.

Phase transition and plastic deformation mechanisms induced by self-rotating grinding of GaN single crystals

Despite being the most promising third-generation semiconductor materials, the deformation and removal mechanisms of gallium nitride (GaN) single crystals involved in the ultra-precision machining process are not well revealed and few investigations on the grinding of GaN crystals have been reported, which hinders the development of high-efficiency and ultra-precision manufacturing of GaN components. Self-rotating grinding tests of GaN crystals were performed, and the results indicated that abrasive size had a significant influence on the surface morphology and roughness, in comparison with wheel rotational speed and feed speed. As the abrasive size decreased from 18 μm to 1.6 μm, the brittle fracture-dominated surface gradually changed to a full-plastic surface without brittle fractures and cracks. An ultra-smooth surface with a roughness of 1 nm in Sa was acquired using #8000 grinding wheels and a spark-out time of 10 min, which indicated that the machining technology of “grinding instead of polishing” of GaN crystals was achieved in this work. The plastic deformation mechanism of GaN crystals induced by ultra-precision machining was investigated using a cross-sectional TEM method and MD simulation, and both experimental and simulation results indicated that the plastic deformation involved in the scratching process was caused by the formation of polycrystalline nanocrystals, high-angle lattice misorientations, and close-to-atomic-scale defects, including stacking faults, dislocations and serious lattice distortions, along with a small amount of amorphous and phase transitions. There was an obvious delamination phenomenon in the plastic deformation zone. This research enhances the understanding of the deformation and damage mechanisms of GaN crystals involved in the ultra-precision machining process and is of significance for achieving the high-efficiency and high-accuracy manufacturing of GaN components.

Fabrication and service of all-ceramic ball bearings for extreme conditions applications

Hot isostatic pressing silicon nitride material has properties that most metal materials do not have, such as low specific gravity, high strength, high stiffness, wear resistance, corrosion resistance, extreme temperature resistance, electrical insulation, no magnetic permeability, and excellent dimensional stability Performance, self-lubricating performance, etc. It is one of the ideal alternative materials for the manufacturing of bearings applied to various extreme conditions. This manuscript focuses on the practical needs of bearings for environmental equipment such as extreme temperature and marine corrosion. These bearings are used to improve the ultimate speed and load capacity of the whole steel bearing design system in ultra-high speed applications (such as machine tool spindle and gas turbine engine); to eliminate corrosion and reduce friction under extreme temperature or corrosion conditions, so as to improve system performance. In this paper, from the preparation of ceramic bearing materials to the ultra-precision machining process of ceramic bearing components, the development status of ceramic bearings in recent years is introduced, and the future development and application prospect of ceramic bearings are prospected.

Multi-Physics Fields Based Nonlinear Dynamic Behavior Analysis of Air Bearing Motorized Spindle.

The air bearing motorized spindle (ABMS) is the key component of the ultra-precision machine tool, which plays an important role in the ultra-precision machining process and directly influences machining accuracy. The influence of unbalanced magnetic force (UMF) on the nonlinear dynamic behavior of the ABMS is not understood clearly. To reveal the potential influence of the UMF, a mathematical model of the ABMS considering multiphysics fields is established. The variation trend of the UMF is simulated, and the nonlinear dynamic behavior of the ABMS is analyzed which emphasizes on the stability of the rotating shaft. It is shown that the UMF varies linearly at large rotor eccentricity which meets well with previous research, but it is noteworthy the UMF varies nearly to a quadratic function at small rotor eccentricity. The result of rotor dynamics shows that the UMF can change the converge position of the rotor center and the converge speed. Moreover, when at certain rotor mass and external load, the UMF can enlarge the stability boundary of the rotor. This research provides an example of analyzing the nonlinear dynamic behavior of the ABMS considering multiphysics fields which may help to the further investigation.