Ultra-precision Machining Technology Research Articles

A Bionic Type Piezoelectric Actuator Based on Walking Motion and Asymmetrical L-Shaped Flexure Mechanisms

A bionic type piezoelectric actuator based on the walking motion of L-shaped flexure mechanisms has been proposed. By mimicking the walking motion of “human legs,” the proposed actuator is able to not only eliminate the backward motion to improve the motion efficiency, but also achieve the large working stroke with high output load easily. The special L-shaped flexure mechanism is employed as the “leg,” and the bionic walking motion is realized by applying two L-shaped flexure mechanisms alternately. The driving principle of the proposed piezoelectric actuator is discussed in detail, and the simulation is carried out to study the feasibility. An experimental system has been set up to investigate the actual working performance. According to the experimental results, the suitable phase difference between the two L-shaped flexure mechanisms to eliminate the backward motion is around <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">θ</i> = 5°, the maximum speed is <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> = 1887.3 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m/s, the minimum stepping distance is 0.084 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m, and the maximum load is more than <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F_v</i> = 1100 g. The motion speed and the maximum load under the proposed bionic walking motion (two flexure mechanisms work together as “legs”) could be improved greatly for almost 3.7 and 1.7 times, respectively, compared with that of the traditional friction–inertial motion (only one flexure mechanism works). By eliminating the backward motion, the motion efficiency is improved about 52.9% under the condition of 120 V and 1 Hz. This article shows the feasibility of applying the bionic motion to improve the performance of piezoelectric actuators, which may be instructive for the design of piezoelectric actuators with large working stroke. In the future, it may be helpful for the real application of piezoelectric actuators in the fields of ultraprecision machining, optical engineering, and aerospace technology.

A Review of Advances in Fabrication Methods and Assistive Technologies of Micro-Structured Surfaces

Micro-structured surfaces possess excellent properties of friction, lubrication, drag reduction, antibacterial, and self-cleaning, which have been widely applied in optical, medical, national defense, aerospace fields, etc. Therefore, it is requisite to study the fabrication methods of micro-structures to improve the accuracy and enhance the performance of micro-structures. At present, there are plenty of studies focusing on the preparation of micro-structures; therefore, systematic review of the technologies and developing trend on the fabrication of micro-structures are needed. In present review, the fabrication methods of various micro-structures are compared and summarized. Specially, the characteristics and applications of ultra-precision machining (UPM) technology in the fabrication of micro-structures are mainly discussed. Additionally, the assistive technologies applied into UPM, such as fast tool servo (FTS) technology and slow tool servo (STS) technology to fabricate micro-structures with different characteristics are summarized. Finally, the principal characteristics and applications of fly cutting technology in manufacturing special micro-structures are presented. From the review, it is found that by combining different machining methods to prepare the base layer surface first and then fabricate the sublayer surface, the advantages of different machining technologies can be greatly exerted, which is of great significance for the preparation of multi-layer and multi-scale micro-structures. Furthermore, the combination of ultra-precision fly cutting and FTS/STS possess advantages in realizing complex micro-structures with high aspect ratio and high resolution. However, residual tool marks and material recovery are still the key factors affecting the form accuracy of machined micro-structures. This review provides advances in fabrication methods and assistive technologies of micro-structured surfaces, which serves as the guidance for both fabrication and application of multi-layer and multi-scale micro-structures.

Modeling of Rapid Response Characteristics of Piezoelectric Actuators for Ultra-Precision Machining

Piezoelectric actuators are characterized by high positioning accuracy, high stiffness and a fast response and are widely used in ultra-precision machining technologies such as fast tool servo technology and ultrasonic machining. The rapid response characteristics of piezoelectric actuators often determine the overall quality of machining. However, there has been little research on the fast response characteristics of piezoelectric actuators, and this knowledge gap will lead to low precision and poor quality of the final machining. The fast response characteristics of a piezoelectric actuator were studied in this work. Firstly, the piezoelectric actuator was divided into a no-load state and a load state according to the working state. A fast response analysis and output characteristic analysis were carried out, the corresponding dynamic model was established, and then the model was simulated. Finally, an experimental system was established to verify the dynamic model of the piezoelectric actuator's fast response by conducting an experiment in which the piezoelectric actuator bounces a steel ball. The experimental results verify the correctness of the model and show that the greater the cross-sectional area and height of the piezoelectric actuator, the higher the bouncing height of the ball, and the better the dynamic performance of the piezoelectric actuator. It is believed that this study has guiding significance for the application of the dynamic characteristics of piezoelectric actuators in the machining field.

Ultra-Precision Machining Technology and Equipment for High-End Optical Elements

Ultra-Precision Machining Technology and Equipment for High-End Optical Elements

Simulation and experimental study on limited cutting and heat effect of silicon carbide

As a typical third-generation semiconductor material, silicon carbide (SiC) presents outstanding properties in terms of the bandgap width, disruptive voltage strength, thermal conductivity and so on. However, there are still technical difficulties in acquiring the nondestructive surface with an optical quality, which seriously constrained the application and development of SiC functional devices. Therefore, in this paper, the material removal mechanism of SiC was firstly investigated according to the MD simulation results. Then, nanocutting experiments were conducted under the vacuum environment of the scanning electron microscope, where the transient behavior of material removal could be observed online with a high resolution, so that the simulation results can be effectively verified. From the simulated and experimental results, it was found that under the research range of parameters, the limited removal thickness (LRT) of SiC decreased with an increase in cutting speed, while the cutting depth had little effect on the LRT. Besides, the polycrystalline SiC presented a smaller LRT compared with the monocrystalline SiC. Therefore, it was advantageous to enhance the machined surface quality and improve the surface integrity with regard to polycrystalline SiC. The cutting temperature increased with an increase in cutting speed during the nanocutting, and a higher cutting speed can prolong the time for the workpiece temperature to stabilize. The findings of this study are able to provide a theoretical basis for the improvement of ultraprecision machining technology especially for hard–brittle and hard-to-process materials. • Material removal mechanism was studied both experimentally and numerically. • Limited removal thickness was studied by labeling and tracing the silicon atoms. • Nanocutting tests were performed with online observation via an SEM. • Experimental results are in good agreement with the MD simulation results.

A Novel Polishing Method for Extending the Service Life of Magnetic Compound Fluid

Magnetic field-assisted magnetic compound fluid (MCF) ultra-precision machining technology is regarded as an effective method to obtain a smooth surface. However, due to the evaporation and splashing of water in the polishing fluid during processing, the service life of the MCF slurry is reduced. This paper presents a material removal model for MCF polishing, and a novel experimental apparatus is proposed to extend the service life by supplying MCF components into the MCF slurry. Firstly, in order to obtain the ideal polishing tool, the appearance morphologies and the formation process of the MCF slurry were observed by an industrial camera. On this basis, the optimum parameters were determined by multi-factor and multi-level orthogonal experiments. Finally, the investigation of the MCF service life was carried out under the optimal processing parameters. The main findings are summarized as follows. (1) Excellent MCF polishing tools are obtained when the eccentric distance r is 4 mm and the MCF slurry supply V is 1 mL. (2) When the eccentric distance increases from 2 mm to 4 mm, the forming time of the MCF tool decreases sharply, but when the eccentricity exceeds 4 mm, the decreasing trend becomes slow. The molding time grows steadily as the supply is increased. (3) When the machining gap Δ, the MCF carrier speed nc, the eccentricity r, and the revolution speed of magnetic nm are 1 mm, 500 rpm, 4 mm, and 600 rpm, respectively, the ideal machining effect can be obtained. (4) It could be proven that the polishing device is feasible to extend the service time of the MCF slurry by adding MCF components.

Design and Fabrication of an Additively Manufactured Aluminum Mirror with Compound Surfaces.

Microsatellites have a great attraction to researchers due to their high reliability, resource utilization, low cost, and compact size. As the core component of the optical payload, the mirror directly affects the system package size. Therefore, the structural design of mirrors is critical in the compact internal space of microsatellites. This study proposes a closed-back mirror with composite surfaces based on additive manufacturing (AM). Compared with the open-back mirror, it provides excellent optomechanical performance. In addition, AM significantly reduces the intricate mechanical parts’ manufacturing difficulty. Finally, the roughness was better than 2 nm. The surface shape of the AM aluminum mirror reached RMS 1/10λ (λ = 632.8 nm) with the aid of ultra-precision machining technologies such as single-point diamond turning (SPDT), surface modification, and polishing, and the maximum deviation of the surface shape was about RMS 1/42λ (λ = 632.8 nm) after the thermal cycle test, which verified the optical grade application of AM.

Theoretical Modeling Method for Material Removal Characteristics of Abrasive Water Jet Polishing under Rotating Oblique Incidence.

Abrasive water jet polishing (AWJP), as an ultra-precision machining technology, has unique machining advantages. However, the machining application of nozzles in vertical and inclined states is greatly limited because rotational symmetric material removal characteristics and the largest amount of central material removal cannot be obtained. At the same time, considering the many controllable and uncontrollable factors in AWJP, it is difficult to accurately model the removal characteristics obtained by machining. Based on the idea of the Preston equation and the calculation of fluid dynamics, this study first analyzed the material removal characteristics of a single abrasive particle and used FLUENT fluid simulation software to obtain the pressure and velocity distributions at different positions in the processes of nozzle rotation and tilt polishing. By analyzing the influence of the pressure and velocity distributions on material removal and the surface shear stress of the workpiece, a theoretical model of the material removal characteristics of abrasive water jet polishing under rotating oblique incidence was established. Finally, the effectiveness of the theoretical removal model was verified by comparing and analyzing experimental and theoretical results.

Curvature effect-based modeling and experimentation of the material removal in polishing optical surfaces using a flexible ball-end tool.

Optical surfaces with high quality have been widely applied in high-tech industries for their excellent performances. To precision manufacture those surfaces efficiently and effectively, various machining technologies involved become extremely crucial. As one of the promising ultra-precision machining technologies, inflated or solid elastic tool polishing has attracted more attention for its own superiority. However, there is still lack of understanding on material removal mechanisms especially with the consideration of curvature effect, and it is of great importance to determine the surface quality and form control in ultra-precision polishing process. In this paper, originating from the famous macro-scale Preston equation, the curvature effect-based material removal model in polishing using a flexible ball-end tool has been developed successfully on the basis of two key sub-models, one is the generic model of effective relative velocity and the other refers to the semi-experimental contact pressure model. A series of spot polishing experiments subsequently are conducted on concave surfaces with a curvature radius range from 75 mm to 225 mm. The experimentally measured section profiles of polishing spots do match well with the predicted data, which verifies the effectiveness of the proposed material removal model. On the measured polishing spots, it is also observed that there have two nonuniform material removal phenomena, one is analyzed along the central axis and the other is discussed by two regions symmetrical about the central axis. Compared with the effective relative velocity, it is found that, the contact pressure is more sensitive to curvature effect by investigating the variation of maximum removal depth within a broader curvature radius range from 75 mm to 1000 mm. This study can provide a valuable foundation for polishing optical surfaces with deterministic removal.

Measurement of Sub-Surface Microstructures Based on a Developed Ultrasonic Atomic Force Microscopy

Accurate and non-destructive technology for detection of subsurface defect has become a key requirement with the emergence of various ultra-precision machining technologies and the application of ultra-precision components. The combination of acoustic technique for sub-surface detection and atomic force microscopy (AFM) for measurement with high resolution is a potential method for studying the subsurface structure of workpiece. For this purpose, contact-resonance AFM (CR-AFM) is a typical technique. In this paper, a CR-AFM system with a different principle from commercially available instruments is set up and used for the detection of sub-surface Si samples with grating structures and covered by different thickness of highly oriented pyrolytic graphite (HOPG). The influence of subsurface burial depth on the detection capability is studied by simulations and experiments. The thickest HOPG film allowing for sub-surface measurement by the proposed method is verified to be about 30 μm, which is much larger than the feature size of the subsurface microstructure. The manuscript introduces the difference between this subsurface topography measurement principle and the commercially available AFM measurement principle, and analyzes its advantages and disadvantages. The experimental results demonstrates that the technique has the capability to reveal sub-surface microstructures with relatively large buried depth and is potential for engineering application in ultra-precision technologies.

Analysis of the Static Performance of a Cableless Aerostatic Guideway

To develop an innovative aerostatic guideway without cable drag force that can be widely applied to ultra-precision machining and measurement technology, it is necessary to analyze the static performance of the aerostatic guideway. The structural properties of the cableless aerostatic guideway, i.e., the bearing capacity and internal pressure distribution, directly affect the accuracy of processing and measurement. In this work, the relevant flow equations of the cableless aerostatic guideway are established by considering the unbalanced micro-scale air film. Moreover, the finite difference method is used to solve the calculation of the global analysis of air film at different positions. In addition, this work compares and analyzes the pressure and fluid velocity vector distributions in balanced and unbalanced states and investigates the effect of varying the thicknesses of the air film on static characteristics such as bearing capacity and stiffness. By comparing the balanced and unbalanced states under the same conditions, the obtained results show that in the unbalanced situation, the bearing capacity is lower by 11.69 percent and the stiffness is slightly higher (by 16.67 percent). Furthermore, the related experiments verify the predicted dependence of the bearing capacity on the thickness of the air film, and further demonstrate the feasibility of the proposed structure of the cableless aerostatic guideway. This work provides a technical reference for the design of a cableless aerostatic guideway.

A Numerical Analysis of Ductile Deformation during Nanocutting of Silicon Carbide via Molecular Dynamics Simulation.

As a typical third-generation semiconductor material, silicon carbide (SiC) has been increasingly used in recent years. However, the outstanding performance of SiC component can only be obtained when it has a high-quality surface and low-damage subsurface. Due to the hard–brittle property of SiC, it remains a challenge to investigate the ductile machining mechanism, especially at the nano scale. In this study, a three-dimensional molecular dynamics (MD) simulation model of nanometric cutting on monocrystalline 3C-SiC was established based on the ABOP Tersoff potential. Multi-group MD simulations were performed to study the removal mechanism of SiC at the nano scale. The effects of both cutting speed and undeformed cutting thickness on the material removal mechanism were considered. The ductile machining mechanism, cutting force, hydrostatic pressure, and tool wear was analyzed in depth. It was determined that the chip formation was dominated by the extrusion action rather than the shear theory during the nanocutting process. The performance and service life of the diamond tool can be effectively improved by properly increasing the cutting speed and reducing the undeformed cutting thickness. Additionally, the nanometric cutting at a higher cutting speed was able to improve the material removal rate but reduced the quality of machined surface and enlarged the subsurface damage of SiC. It is believed that the results can promote the level of ultraprecision machining technology.

Study on the rheological and polishing properties of electromagnetic two-phase composite particles with abrasive characteristics

Electromagnetic composite rheological polishing (EMRP) is a new ultra-precision machining technology that combines electro-rheological polishing (ERP) and magneto-rheological polishing (MRP). The key technology of the polishing method is the preparation of electromagnetic composite rheological fluid (EMRF) with both ERP and MRP, especially the preparation of composite particles with electro- and magneto-rheological effects. In this study, the EMRF was prepared by using electromagnetic two-phase particles with abrasive characteristics. The electromagnetic two-phase composite particles are synthesized in two steps: the coupling method and sol-gel method. The two-step method successfully prepared the electromagnetic two-phase composite particles with nano-diamond particles embedded randomly on the surface. The electro-rheological shear stress of EMRF can reach 160.7 Pa when the test parameter voltage is 2.5 kV, and magneto-rheological shear stress of EMRF can reach 4076 Pa when the electromagnet excitation current is 3 A. When a fused silica glass is polished under a single magnetic field, the material removal depth reaches a maximum of 2.7 μm at a radius of 13.5 mm. Under the action of the electromagnetic compound field, the removal profile of the work-piece is smoother, and the material removal depth reaches the maximum value of 2.1 μm at a radius of 10.5 mm. This proves that the stiffness distribution of the polishing pad under the electromagnetic composite field is more dispersed than that under a single field. Therefore, the distribution of electromagnetic two-phase composite particles can be controlled by applying an electromagnetic composite field, which provides a good foundation for the abrasive control technology of EMRP.

Review of field-assisted ultraprecision machining difficult-to-machine materials

<p indent="0mm">Field-assisted ultraprecision machining is an advanced manufacturing technology innovated from the traditional ultraprecision machining technologies by combining laser, ultrasonic vibration, and magnetism, among others. It achieves a nano- or subnanometer machining accuracy when manufacturing difficult-to-machine materials. This paper reviews the development of field-assisted techniques in ultraprecision machining, including turning, grinding, and milling. Considering the surface quality, machining efficiency, forces, tool wear, and other experimental results, field-assisted ultraprecision machining has obvious advantages compared to traditional ultraprecision machining. This review summarizes the present shortcomings and prospective challenges in field-assisted ultraprecision machining, which is beneficial to the development of the advanced hybrid multienergy field-assisted machining technology.

Investigation on the surface roughness modeling and analysis for ultra-precision diamond turning processes constrained by the complex multisource factors

Ultra-precision machining is widely used in the manufacturing of national defense and sophisticated civil products because of its high precision, nearly no surface damage and other advantages. The surface roughness of a workpiece is the key indicator of ultraprecision machining technology, which is affected by many factors in the process of machining. However, the existing surface roughness prediction model only considers a single error factor between the tool and workpiece, lacking comprehensive consideration of multisource error factors. Therefore, based on the research of the single error factor, multisource errors that affect the surface roughness of a workpiece in machining was investigated in this paper. A prediction model of workpiece surface roughness under multisource error affecting was established to improve the accuracy of prediction models. According to the vibration model, the sensitive direction of roughness can be determined. In the axial direction, the error of each point on the surface is superimposed with the theoretical surface shape to obtain the final surface profile. The three-dimensional morphology was established and the two-dimensional topography can be obtained by intercepting the three-dimensional topography. The roughness simulation value is obtained by extracting the peak value of each point of the two-dimensional topography. A single-point diamond turning experiment is performed to verify the simulation models. The result concluded from the experiment is compared with the simulation result, showing that the error between the simulation and the experiment is less than 4%, which is of great significance to optimizing machining process parameters and improving machining precision of workpiece surfaces for ultraprecision single point diamond turning.

A high-fidelity digital twin approach for the optimisation of fluid jet polishing process

Fluid Jet Polishing (FJP) is an Ultra-Precision Machining (UPM) technology for super-fine finishing of small and complex components. FJP has distinctive advantages compared to other polishing methods, including high polishing accuracy, no heat generation, no tool wear, applicability for various types of materials, and suitability for various freeform surfaces. Nevertheless, previous research work on FJP focuses mainly on theoretical modelling and simulation of the polishing mechanisms with experimental validations, a large amount of process uncertainties happened during the polishing process have been overlooked. These uncertainties could cause variations of the surface quality of workpieces in terms of material removal rate and surface roughness. Recent advancements of Digital Twin (DT) technology have shown great potential in addressing this issue. However, high-fidelity DT for FJP has not been investigated to date. In this paper, we propose a novel high-fidelity DT approach for the optimisation of FJP process. First, related research on FJP and DT is reviewed to identify the limitations of the existing approaches. Second, we propose a conceptual framework of the high-fidelity DT for FJP process. Third, the key enabling technologies and major challenges for the development of the high-fidelity DT are identified and discussed. Finally, a conceptual application scenario of the in-process control optimisation for FJP of freeform surfaces is presented. This work attempts to integrate smart manufacturing technologies into FJP process and will contribute to the theoretical development of high-fidelity DT for various UPM technologies.

Gradient nanotwinned CrCoNi medium-entropy alloy with strength-ductility synergy

In this study, a high-strain rate ultra-precision machining technology named single point cubic boron nitride turning is developed to fabricate a gradient nanotwinned CrCoNi medium entropy alloy layer. The grain size of the ~ 150 µm-thick gradient layer is gradually refined from the original ~ 17 µm to ~ 25 nm in the topmost surface, exhibiting a significantly enhanced yield strength (from ~ 450 MPa to ~ 1100 MPa) and well-retained ductility of ~ 27%. High-resolution transmission electron microscope and atomistic simulations were mainly performed to unveil the size-dependent twinning mechanisms governing the gradient refinement process from the core to the topmost surface, i.e. transiting from the parallel twins segmenting ultrafine grains, twin-twin intersections refining rhombic blocks and rotating the intersected nanograins, and finally to the zero-macrostrain deformation nanotwinning in the refined nanograins. The machining process provides sufficient equivalent stress to activate the twinning partials for forming the gradient nanotwinned structure.

Study of the tool path generation method for an ultra-precision spherical complex surface based on a five-axis machine tool

The improvement of ultra-precision machining technology has significantly boosted the demand for the surface quality and surface accuracy of the workpieces to be machined. However, the geometric shapes of workpiece surfaces cannot be adequately manufactured with simple plane, cylindrical, or spherical surfaces because of their different applications in various fields. In this research, a method was proposed to generate tool paths for the machining of complex spherical surfaces based on an ultra-precise five-axis turning and milling machine with a C-Y-Z-X-B structure. Through the proposed tool path generation method, ultra-precise complex spherical surface machining was achieved. First, the complex spherical surface model was modeled and calculated, and then it was combined with the designed model to generate the tool path. Then the tool paths were generated with a numerically controlled (NC) program. Based on an ultra-precision three-coordinate measuring instrument and a white light interferometer, the machining accuracy of a workpiece surface was characterized, and the effectiveness of the tool path generation method was verified. The surface roughness of the machined workpiece was less than 90 nm. Furthermore, the surface roughness within the spherical region appeared to be less than 30 nm. The presented tool path generation method in this research produced ultra-precision spherical complex surfaces. The method could be applied to complex spherical surfaces with other characteristics.

Ultra-precision direct diamond shaping of functional micro features

• Direct Diamond Shaping is presented as a novel machining method, producing unique features using the UPM. • Rotary and linear features can now be machined continuously with a turning tool. • 2D and 3D complex functional micro features can now be realized using the Virtual Rotating Tool Shaping algorithm. • Successfully machined a Bi-focal segmented Fresnel lens by Direct Diamond Shaping without the need for assembly. • Successfully machined a serpentine microfluidic channel with linear and rotary features by Direct Diamond Shaping. • Comprehensive geometrical analysis on tool limitations during shaping of channels is presented. As technological components continue to utilize space effectively within devices without compromise to their operational efficiency, the relevance of accurate and precise parts with good tolerances and surface finishing has shot to prominence. Ultra-Precision Machining (UPM) technology has constantly risen to the challenge by realizing components with superior geometrical accuracies and mirror-like surface finishes, much of which are required in the optical industry. However, conventional diamond shaping processes are usually limited to rotary symmetric components or linear gratings. Thus, the developed Virtual Rotating Tool Shaping (VRTS) addresses these issues by enabling the UPM to fabricate non-rotary symmetric, intricate and freeform elements by Direct Diamond Shaping (DDS). VRTS synchronizes the linear and rotational motions of the ultra-precision machine to constantly align the tool with the cutting direction, while compensating in the linear axes for the run-off. As such, DDS can now achieve a combination of linear and rotary features by planing. In this paper, the developed DDS technique is presented with experimental verifications of the algorithm on a segmented bi-focal Fresnel lens and a serpentine micro-channel, followed by the corresponding analyses.

Ultra-precision machining technology of off-axis paraboloid surface based on PMAC time-based control

Ultra-precision machining technology of off-axis paraboloid surface based on PMAC time-based control