红外线聚光非球面透镜的单点金刚石镜面切削方法

根据硬脆性材料的延性域加工机理和面形误差补偿加工方法,研究了圆弧形和平头形刀具的单点金刚石延性域切削方法,在加工中直接获得了镜面切除面;并利用数控技术进行误差补偿,克服了因加工试验、刀具磨损、机械振动、热变形等造成的加工误差导致的非球面的面形精度降低和表面粗糙度恶化.并将该方法用于采用圆弧形刀具对红外线聚光的φ70mm非球面锗透镜进行单点金刚石切削实验中.试验结果表明面形误差补偿加工方法可以进一步消除加工误差,将非球面的面形精度PV值从微米级(1.23μm)提高到亚微米级(0.36μm)的程度,表面粗糙度R_a从亚微米级(0.27μm)改善到超亚微米级(0.04μm)的范围.     

Single-point diamond turning of electroless nickel for flat X-ray mirror

X-ray mirrors require a super-smooth surface to prevent strong X-ray scattering. We examined the fabrication possibility of the X-ray mirror by single-point diamond turning (SPDT) for electroless nickel. The stable and unstable cutting modes for the electroless nickel were obtained by observing the relative position of a diamond tool for machining. A super-smooth surface of 0.95 nm rms was achieved within the stable cutting mode. The surface roughness of the electroless nickel mirror measured with an optical profiler was compared with the X-ray reflectivity measurement. The electroless nickel mirror could be successfully used as a soft X-ray reflector and a low-pass filter for the hard X-rays.     

An accurate interpolator for FTS diamond turning of optical free-form surface

The ultra-precision components with freeform surfaces are used in a wide range of areas such as automotive, aerospace, optical, metrology artifacts, and data storage. Fast tool servo (FTS) diamond turning has been considered as one of the leading techniques to produce such components. We present four points mean value interpolator for FTS diamond turning. Performance is compared between the NURBS interpolations with the proposed interpolation using Matlab, which shows that the proposed interpolation not only improves the surface accuracy of the freeform surface, but also meets the real-time requirement.     

Ultraprecision surface flattening of porous silicon by diamond turning

Porous silicon is receiving increasing interest from a wide range of scientific and technological fields due to its excellent material properties. In this study, we attempted ultraprecision surface flattening of porous silicon by diamond turning and investigated the fundamental material removal mechanism. Scanning electron microscopy and laser Raman spectroscopy of the machined surface showed that the mechanisms of material deformation and phase transformation around the pores were greatly different from those of bulk single-crystal silicon. The mechanism of cutting was strongly dependent on the direction of cutting with respect to pore edge orientation. Crack propagation was dominant near specific pore edges due to the release of hydrostatic pressure that was essential for ductile machining. Wax was used as an infiltrant to coat the workpiece before machining, and it was found that the wax not only prevented chips from entering the pores, but also contributed to suppress brittle fractures around the pores. The machined surface showed a nanometric surface flatness with open pores, demonstrating the possibility of fabricating high-precision porous silicon components by diamond turning.     

Tool force model development for diamond turning

Accurate determination of forces in the three-dimensional turning process is important for the development of a model to describe diamond turning (DT). This paper describes a dynamometer system which measures force magnitude in steady-state cutting. To build a quantitative model, the forces are experimentally separated into components. The response of these components to a variation of cutting parameters is explained in part by a hardness gradient near the part surface. This gradient is due to work hardening by the tool during previous passes. The extent of plastic work, and thus the hardness gradient, is dependent on the tool edge sharpness (≈ 100 nm). Therefore, the turning forces are strongly influenced by the condition of the tool edge. This paper illustrates the feasibility of finding the connection between edge sharpness and tool forces. It also demonstrates the ability to monitor tool forces over extended periods of time. These relationships are important in predicting the diamond tool edge condition from tool forces during a turning operation.     

大型光学零件金刚石车床

介绍美国LODTM(LargeOpticsDiamondTurningMachine)——大型光学零件金刚石车床的基本结构,重点介绍机床的主要关键技术——测量技术、关键元部件技术、数控技术。     

Form error in diamond turning

The main feature of ultraprecision, single point diamond turning (SPDT) is its ability to produce high quality surface finishes on the order of nanometers while meeting tight form tolerances on the order of micrometers. This capability allows for the production of optical devices with minimum post-processing operations. The issue of form error is critical since it may severely compromise the performance of the designed optical system. Tool-workpiece relative vibration is a major cause of this error. In this article, the authors have demonstrated that imbalance of the spindle is a major cause of form error which eventually leads to a structured error called “spindle star” that appears as straight concentric spokes radiating out from the center of the part. The formation of a spindle star pattern can be explained using Campbell's rotordynamics analysis. This analysis explains how assisted air-hammering instability and cross-coupling effect of the air-bearing spindle can contribute to spindle star. This experimental approach used force and accelerometer data with the help of modal analysis to conclude that spindle star is a synchronous error and is a function of rotational frequency of the spindle and its harmonics. The integer harmonic from the Campbell's waterfall diagram predicts the number of spokes in the spindle star. It was also observed that the height of the spindle star undulations increases with higher rotational speed. It was also observed that cutting material and tool geometry has no influence on the spindle star formation; rather this is an inherent characteristic of air-bearing journals. Finally, the analysis was successfully validated by changing the natural frequency of the spindle by adding mass.     

Influence of crystal anisotropy on subsurface damage in ultra-precision cylindrical turning of CaF2

An optical microcavity, which stores light at a certain spot, is an essential component to realize all-optical signal processing. Single-crystal calcium fluoride (CaF₂) theoretically shows a high Q-factor which is a desirable optical property. The CaF₂ microcavity can only be manufactured by ultra-precision cylindrical turning (UPCT). The authors have studied UPCT of CaF₂ and shown the influence of crystal anisotropy and tool geometry on surface roughness and subsurface damage. The study indicated that a smaller nose radius of the cutting tool led to shallower subsurface damage. Thus, it is inferred that a smaller nose radius compared to the previous nose radius (0.05 mm) can further reduce subsurface damage. Nevertheless, the mechanism that causes a difference in subsurface damage due to crystal anisotropy is not sufficiently clear. The influence of subsurface damage on microcavity performance is still unclear. In this study, the UPCT of CaF₂ was conducted using a tool with a nose radius of 0.01 mm. The subsurface damage was investigated by transmission electron microscope (TEM) observation from the viewpoint of the change in crystal lattice arrangement. In our previous study, fast Fourier transfer (FFT) analysis was used for confirmation of change of crystal structure. In this study, FFT analysis was also used to quantitatively evaluate the depth of subsurface damage. In addition, inverse fast Fourier transfer (IFFT) was used to analyze change of crystal lattice arrangement clearly, which enables discussion of the influence of slip systems. Finally, optical microcavities are manufactured without any crack, and the influence of subsurface damage on microcavity performance is experimentally evaluated using a wavelength tunable laser and power meter.     

微透镜阵列单点金刚石车削补偿技术

为了提高微透镜阵列单点金刚石车削的加工精度与一致性,提出了加工误差的理论模型,并针对其补偿方法进行了理论分析和实验研究.将微透镜阵列加工等效为自由曲面加工,通过建立单点金刚石慢刀伺服切削模型,计算了理论曲面在每一个切削点处沿切削方向的曲率半径;结合刀具等效倾斜角模型和机床加工时延模型,进一步得到了慢刀伺服切削微透镜阵列...     

A flexure-based long-stroke fast tool servo for diamond turning

This paper describes the development of the fast tool servo (FTS) in detail and categorizes existing FTSs according to different principles. The characteristics and differences of these FTSs have been analyzed. A flexure-based long-stroke FTS system for diamond turning is presented with displacement range of 1?mm and bandwidths of 10?Hz. The vertical jump is about 0.045?μm, and the full stroke tracking error is less than 0.15%. A voice coil motor and a piezoelectric actuator are used as the driving elements, and two flexure hinges are developed as the guide mechanisms. The FTS utilizes a linear encoder and a capacitive sensor to measure the displacement of the tool for closed-loop control. The electromechanical design of the FTS and its motion analysis are described. Experimental tests have been carried out to verify the performance of the FTS system. This long-stroke FTS has the advantage of easy machining, high resonance frequency, and error compensation in y-axis direction.     

Effects of tool overhang on selection of machining parameters and surface finish during diamond turning

In precision machining leading to nano-metric surface finish, selection of the suitable machining parameters is a critical task. To ensure the desired surface quality, one needs to optimally select the machining parametric matrix. Towards this effort, this paper adds another critical parameter in terms of tool overhang. A well-defined set of machining exercises is carried out with different tool overhangs and machining parameters. In this investigation, an attempt has been made to locate the optimum range of tool overhang with minimum tool vibrations. The interaction between tool overhang with other parameters is also thoroughly investigated. Another important focus of this study is to find out the optimum machining parameters for the situations where it is not possible to select an optimum tool overhang. One such situation occurs when a steep concave parabolic surface needs to be fabricated. In this case a large tool overhang has to be selected. Power spectral density distribution analysis of surface roughness for different tool overhangs is performed to find out significant parameters and their degree of contribution to surface roughness. Analysis of variance is also applied to ascertain statistically significant factors contributing to surface roughness. To model the surface roughness, response surface methodology is being used. The model has been verified by conducting a series of experiments and a steep concave parabolic surface is developed by following the predictions of the developed model.     

Effect of material microstructure and tool geometry on surface generation in single point diamond turning

There is a strong desire in industry to improve surface finish when performing ultra-precision, single point diamond turning (SPDT) to reduce the amount of post process polishing required to meet final product specifications. However there are well known factors in SPDT which limit achievable surface finish. This paper focuses on the role of material microstructure, including grain boundary density and the presence of inclusions, as well as tool design on surface roughness using the concept of size effect. Size effect can be described as an interplay between the material microstructure dimension and the relative size of the uncut chip thickness with respect to the cutting edge radius. Since one of the controllable parameters in size effect is grain size and dislocation density, controlled studies were performed on samples whose microstructure was refined by mechanical strain hardening through rolling and a friction stir process (FSP). The use of the ultra-fine grained workpiece prepared using an FSP was observed to reduce side flow as well as grain boundary and inclusion induced roughness. The role of tool geometry on material induced roughness was investigated using a tool with a rounded primary cutting edge and a flat secondary edge. The use of the flat secondary edge was observed to improve surface finish when machining a flat surface. This improvement was primarily attributed to a reduction in side flow and material microstructural effects. By combining these approaches an average surface roughness R_a value of 0.685 nm was achieved when SPDT a flat surface. Furthermore the custom tool has the potential to significantly improve the productivity of SPDT by allowing for a much higher feed rate while still achieving a high quality surface finish.     

Optimisation of surface roughness on turning fibre-reinforced plastics (FRPs) with diamond cutting tools

Fibre-reinforced plastics (FRPs) are used in structural components in various fields of application of mechanical engineering, such as automobile, biomechanics and aerospace industries. Their own properties, particularly the high strength and stiffness and simultaneously low weight, allows the substitution of the metallic materials in many cases. As a result of these properties and potential applications, exist a great necessity to investigate the machining of these composite materials.This paper presents an optimisation study of surface roughness in turning FRPs tubes manufacturing by filament winding and hand lay-up, using polycrystalline diamond cutting tools. A plan of experiments was performed with cutting parameters prefixed in the FRP tubes. The objective was establishing the optimal cutting parameters to obtain a certain surface roughness (R_a and R_t/R_max), corresponding to international dimensional precision (ISO) IT7 and IT8 in the FRP workpieces, using multiple analysis regression (MRA). Additionally, the optimal material removal rates have been obtained.     

A novel surface analytical model for cutting linearization error in fast tool/slow slide servo diamond turning

Fast tool/slow slide servo (FTS/SSS) technology plays an important role in machining freeform surfaces for the modern optics industry. The surface accuracy is a sticking factor that demands the need for a long-standing solution to fabricate ultraprecise freeform surfaces accurately and efficiently. However, the analysis of cutting linearization errors in the cutting direction of surface generation has received little attention. Hence, a novel surface analytical model is developed to evaluate the cutting linearization error of all cutting strategies for surface generation. It also optimizes the number of cutting points to meet accuracy requirements. To validate the theoretical cutting linearization errors, a series of machining experiments on sinusoidal wave grid and micro-lens array surfaces has been conducted. The experimental results demonstrate that these surfaces have successfully achieved the surface accuracy requirement of 1 μm with the implementation of the proposed model. These further credit the capability of the surface analytical model as an effective and accurate tool in improving profile accuracies and meeting accuracy requirements.     

The wear of diamond tools turning mild steel

Previous experiments on the wear of diamond tools turning mild steel have shown that the very high rates of wear of the diamond arise from the degeneration of the diamond to graphite, the details of the wear mechanisms depending on the cutting conditions. The present experiments on a larger number of tools show much greater differences in the wear rates of different diamonds than have been reported previously. It appears probable that these differences in wear rates arise from differences in the chemical constitution of the diamonds.     

Optimal selection of machining parameters for fast tool servo diamond turning

Ultraprecision machined components with micro-structured surfaces in micrometer or nanometer range have gained wide applications especially in optical industry. A technique called fast tool servo (FTS) diamond turning is superior in fabricating precision and complicated micro-structured surfaces with wavelength above tens of microns. However, in order to obtain optimal machined surface quality, the machining parameters need to be selected carefully. In this paper, optimal selection of the machining parameters, including spindle speed, sampling number, feedrate and tool geometry, for fabricating micro-structured surfaces by FTS diamond turning is presented. A simulation system is developed to select feedrate and tool geometry by computing the theoretical surface roughness, spindle speed, and sampling number based on the FTS dynamics and the motion controller capability. Experiments have been carried out to show the effect of the machining parameters. In addition, machining of typical micro-structured surfaces with machining parameters selected by the presented approach proves the effectiveness of the proposed optimal machining parameters selection method and the designed FTS diamond turning machine.     

Design and testing of a fast tool servo for diamond turning

A self-contained and independently servo-operated diamond tool holder was built to increase the resolution and accuracy of a precision lathe. Its static and dynamic repeatability over a range of $\text{± 50 μin (1.27 μm)}$ is better than 0.05 μin (1.3 nm). Its frequency distortion from 0–100 Hz is less than 1.0 μin (25 nm) for a peak displacement of less than 28 μin (0.71 μm).