Spectral analysis of surface roughness features of a lapped ultraprecision single-point diamond machined surface

Single-point diamond turning of soft metals could provide a much shiny surface with an optimal feed rate; however, the machining mark would be left on the machined surface, which caused the roughness cannot be neglected. If the feed rate is too small, the roughness of the surface could be improved but the reflectiveness would be decreased because of damaging the profile. Therefore, it is necessary to develop a lapping method to reduce the roughness by removing the machining mark, while the reflectiveness can be kept at the original level simultaneously. In this study, the novel lapping method, using strands of wool fibers to deliver the abrasive slurry to rub against the lens, was proposed for removing the machining marks on the mold of a lenticular lens by lapping without damaging the profile of the mold. Even though the normal pressure applied by the wool strands onto the mold surface is very low, the coefficient of friction would be increased significantly with the application of the abrasive slurry. The combined effect was to provide a relatively large shear force to lap the surface with a minimal normal force. Therefore, the proposed method could theoretically avoid damaging the lens while effectively removing the irregularities that appeared on the surfaces. In order to evaluate the proposed lapping method, we firstly lapped the machining marks with different lapping parameters (speed, grit size, time, and pressure) to find out the relationship between these parameters and roughness with the same profile of the mold. Secondly, the optimal lapping parameters were designed based on the above lapping results to deduce the best lapping solution for processing the machining marks. Thirdly, the lapped surface profile of the mold was test by optical profiling system, and the features of the surface can be categorized into various spectral distribution groups. Finally, by comparing the variation of the spectral distribution groups, it is verified that based on our proposed methodology, selective removal of surface spectral groups of features becomes possible.     

A theoretical and experimental investigation of orthogonal slow tool servo machining of wavy microstructured patterns on precision rollers

Precision cylinders, or rollers, with patterned microstructures on the surface are the key tooling component in the Roll-to-Roll and Roll-to-Plane fabrication process for precision manufacturing of microstructured plastic films. These films are widely used in optical applications such as the backlight guide and brightness enhancement films in LCD and LED displays. Compared with other fabrication processes, such as lithography, Single-Point Diamond Turning (SPDT), using a Fast Tool Servo (FTS) or Slow Tool Servo (STS) process, is an enabling and efficient machining method to fabricate microstructures. Most studies of the tool servo machining process focus on either machining microstructures in the axial direction for face machining of flat parts or in the radial direction on the surface of a precision roller. There is relatively little research work found on the machining of patterned microstructures on the surface of precision rollers using the tool servo in the axial direction. This paper presents a pilot study on the development of a tool path generator for machining wavy microstructure patterns on precision rollers by using an Orthogonal Slow Tool Servo (OSTS) process. The machining concept of OSTS is first explained, and then the tool path generator is developed in detail for machining wavy microstructure patterns on a roller surface. Modelling and simulation of pattern generation for different microstructures with different wavy patterns and grooves are presented based on the proposed tool path generator. Preliminary experimental work using SPDT on a 4-axis ultra-precision machine tool is presented and clearly shows the generation of unique wavy microstructure patterns on a precision roller. The machined wavy microstructures on the roller surface are measured and analyzed to evaluate the validity of the proposed tool path generator.     

ACHIEVING THRESHOLD BARRIER OF 1 nm ROUGHNESS VALUE OF SILICON SURFACE BY DIAMOND TURNING

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An automated Guilloche machining technique for the fabrication of polygonal Fresnel lens array

Fresnel lenses are gaining wider applications in optoelectronics and photonic devices. They have been evolved into arrays of individual elements with better performance and better capabilities. These arrays may also be realized with individual elements having polygonal shapes such as rectangular, hexagonal, etc to achieve a high fill factor. Due to the fact that the polygonal Fresnel lens arrays are not rotationally symmetrical, they are manufactured by using expensive lithography techniques or time consuming method of having multiple molds assembled together. This paper presents an automated 4-axis ultraprecision machining technique for manufacturing an array of hexagonal Fresnel lenses. In the proposed method, a diamond tool moves as a fixed point on a circle rolling inside a fixed circle which is analogous to a Guilloche machine. The proposed automated machining technique has been experimentally verified through successful manufacturing hexagonal Fresnel lens array in a single process, without having separated sections assembled as a single master mold. From the results, an excellent surface finish and good profile accuracy are also achieved. The developed automated Guilloche machining technique also paves the way for the promising challenges to remove manufacturing barriers of machining freeform surfaces with complex curvatures.     

Efficient microspark erosion-assisted machining of a monocrystalline microdiamond stylus using a heat-avoidance path

Monocrystalline diamond possesses covalent bonding making diamond extremely hard and difficult to machine. In this study, a microdiamond stylus typically used in measuring surface roughness is machined to exemplify the proposed ‘microspark erosion-assisted machining with heat-avoidance path’ technique. Based on the high thermal conductivity and weak electrical conductivity of boron-doped monocrystalline diamond, high-frequency pulsed discharge plasma is employed to efficiently perform microspark erosion machining on an extremely hard monocrystalline diamond blank. It was found that the pulse-on time and servo voltage respectively affect erosion plasma length and the erosion gap during diamond machining. Also, the safety distance and safety height of the erosion path dominate heat transfer to filler metal. These factors all affect the firmness of the brazed diamond blank on the substrate. Three mechanisms for removing carbon atoms from the diamond blank surface were observed. They are vaporization, melting, and graphitization of carbon atoms. This graphitized carbon atoms have weak electrical conductivity, which is conducive to inducing the wire-electrode to generate a greater electric field and secondary discharging, facilitating removal of additional carbon atoms. Experimental results indicate that a microdiamond stylus prototype with a tip of 10 μm can be safely formed using a ‘microspark erosion-assisted machining with heat-avoidance path’ technique, creating 93.7% repeatability of the minimum residual stylus diameter. The tangential micro-grinding facilitates the stylus tip to receive grinding from the grinding wheel's maximum tangential speed and create the precision microdiamond stylus with 1 μm in tip-radius. The applied microspark erosion-assisted machining had a diamond material removal rate that was 54% more efficient than conventional grinding of a commercial microdiamond stylus. The formed microdiamond stylus was inspected by Raman spectroscopy and verified by the surface roughness standard gauge to be up to industry standards.     

Development of machining technology for micropatterns with large surface area

With increasing demand for micropatterns such as V-shaped microgrooves and the trend of large surface areas in developing technologies, precision machining technology for micropatterns with large surface areas is expected to play an increasingly important role in today's manufacturing technology. In large-surface micromachining, machining time is much longer than that in general pattern machining and it is not easy to achieve uniform machining accuracy in the entire machined areas because of various factors. Therefore, systematic machining processes and technical development for achieving precision in each process are essential prerequisites to reduce the errors. In this study, we focused on developing machining technologies, which include a machine vision system for precise tool setting, an on-machine measurement system for large-area measurement, and software for tool path generation and simulation, for the fabrication of large-surface micropatterns in an electroless nickel-plated workpiece with single-crystal diamond tools and a 32-in., 675?×?450-mm mold with tens of V-and pyramid-shaped micropatterns.     

Analysis of profile measurement techniques employed to surfaces planed by an active machining system

The importance of a reliable and robust surface profile measurement system in the inspection of surface finish is beyond any doubt. For years, visual inspection has been employed in industries to determine the quality of surface finish. Since, in most cases, it fails to ensure a consistent minimum standard of finish quality, mechanical stylus based measurement systems have successfully taken over from human inspection. However, in recent years, the trend is to explore other techniques for conducting surface profile measurements. Non-contact optical methods have emerged as one of the leading candidates. In this paper, capabilities of two optical profile measurement methods (namely, light-sectioning and two-image photometric stereo) have been explored for surfaces machined using an active machining system. These profile measurement results have been compared to the ones obtained from a conventional mechanical stylus instrument. An industry-standard Talysurf CLI system has been used to provide the benchmark, traceable to NPL standards, for the measurements. Suitability of different measurement techniques have been discussed based on the results obtained.     

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.     

On-machine measurement of a cylindrical surface with sinusoidal micro-structures by an optical slope sensor

This paper describes the measurement of a cylindrical surface with sinusoidal micro-structures over a large area on a diamond turning machine. The sinusoidal micro-structures, which are fabricated on the periphery surface of a cylinder by the fast tool servo-based diamond turning, are superposition of periodic sine-waves along the cylinder axis and the cylinder circumference with amplitudes of 100 nm and wavelengths of 100 μm, respectively. An optical two-dimensional (2D) slope sensor with a multi-spot light beam is developed for measurement of the 2D local slopes of the sinusoidal micro-structured surface. A cylindrical lens is employed in the sensor for removing the influence of the curvature of the cylinder surface. Experiments of fabrication and measurement of the sinusoidal micro-structured surface on an ultra-precision diamond turning machine are carried out.     

How to diamond turn an elliptic half-shell?

In this research note we are introducing a new slow slide servo (SSS) turning technique which enables fast diamond machining of deep aspheric surfaces which otherwise can only be machined by ball-end milling, if at all. The key idea is to execute the servo motion not only parallel to the axis of rotation, which is the standard mode implemented in commercial SSS software, but in a plane incorporating both directions parallel and perpendicular to the rotational axis. In this way the risk of collisions in non-circular turning between the tool shaft and the machined surface can be reduced significantly. Moreover, when a 180° contour is machined, the acceleration of the servo slide will not increase indefinitely. After a review of the standard SSS turning technique we will outline the generalized SSS concept and demonstrate its applicability by diamond turning of an elliptic half-shell on a commercial diamond turning lathe.     

红外菲涅尔透镜的注射超声辅助成型优化

为了提升聚合物红外菲涅尔透镜的光学性能,以其表面微沟槽的成型质量为目标,提出了一种高效的注射超声辅助成型方法,并对工艺参数进行了综合质量优化。首先分析了超声振动对聚合物的加热和加压效应,设计了一套一模两腔的对比试验模具;接着以红外菲涅尔透镜的调制传递函数MTF和齿形平均高度h为优化质量目标,设计了四步骤的多目标优化流程,通过试验设计、基于BP神经网络的质量目标与注射工艺参数关系建模、基于NSGA-Ⅱ的多目标优化和试验验证进行工艺参数的综合优化。实验结果表明:该多目标优化流程具有很高的精度,MTF和h的平均预测误差MPE分别为4.16%和3.32%;注射超声辅助成型的菲涅尔透镜微沟槽具有更高的复制质量,其齿沟槽平均高度h增加了15.6%,且h值的波动量随着h值的增大而增大,MTF值受齿高h均匀性的影响大于齿高h对其的影响。     

光学自由曲面快速刀具伺服车削误差的补偿

受各种误差因素以及周期性变化的切削力的影响,快速刀具伺服金刚石车削技术往往难以用一次车削获得满足光学性能要求的自由曲面。本文提出了一种利用线性差动传感器(LVDT)实现高精度接触式自由曲面在位测量的方法。该方法结合两自由度快速刀具伺服系统,实现了基于快速刀具伺服(FTS)的自由曲面车削加工的误差补偿。试验结果表明,该技术将自由曲面的加工精度提高了20%,表面粗糙度降低18.1%,解决了FTS系统与机床运动的同步问题,可补偿机床xyz三向运动误差,可用于自由曲面加工误差的修正。该方法还可用于不对称幅度较大的曲面或硬脆性材料的加工等,故促进了高精度光学自由曲面的推广应用。     

超精密车削表面粗糙度的控制与优化

金刚石车削是利用高精度机床与锋利的单晶金刚石刀具加工出尺寸精度高、表面完整性好的零件的一种金属加工技术。用回归分析的方法，根据金刚石车削铝合金的实验结果可以建立表面粗糙度预测模型，这种方法能够以较少的实验次数获得大量的加工信息。在一定条件下，利用优化设计软件可以实现切削参数的优选，用优选得到的最优切削参数组合进行超精密加工，能够获得超光滑加工表面。     

椭圆振动辅助车削7075铝合金表面微织构及其特性

为研究椭圆振动车削技术辅助微织构制备的工艺及其加工后微表面特性，采用椭圆振动原理和微织构的形成原理分析了不同转速和进给速度下的微织构变化规律，进行椭圆振动轨迹测量试验和椭圆振动车削试验，分析了装置振动轨迹、表面形貌、单个微织构几何尺寸及表面粗糙度变化情况。试验结果表明，在切削试验过程中，相比传统车削，由于车刀的空间椭圆运动轨迹，超声椭圆振动车削后表面呈现类椭圆状的微织构凹槽。由理论分析和试验结果得出，随着转速和进给速度的改变，微织构凹槽的几何尺寸、轮廓高度及表面粗糙度均会发生规律性变化。     

An instantaneous cutting force model for disc mill cutter based on the machining blisk-tunnel of aero-engine

The fast tool servo (FTS) system is widely used for micro-structure manufacturing by diamond turning. Conventional FTS had been generally researched regarding the front/back-axial swing direction. However, development of more complex machining technology is demanded for the FTS. In this paper, we developed a right/left-horizontal swing FTS (HFTS) and analyzed its characteristics with various hinge structures. Flexure hinge structures were designed four types, considering the influence of mass and the number of hinges. Also, to compare the difference in machining performance with hinge structure, we compared the results of a machined microwave pattern on a Ni-coated steel roll mold. Comparing the machining results, the influence of mass was minor; however, machining performance varied with the number of fixed hinges. Having two fixed hinges was not suitable for precision machining due to strong over-constraint conditions. Thus, a single-hinge HFTS provides the appropriate hinge structure for precision machining of a roll mold.     

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.     

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.     

Fractal roughness structures of precision-machined WC-Co- and Inconel 625-coated steel rods

Thermally sprayed WC-Co coatings on steel rods were machined by grinding and turning using diamond tools, and thermally sprayed Inconel 625 coatings on steel rods were machined by turning using various WC tools. Four of these samples were selected for surface characterization using a stylus roughness tester. The results show that precision-machined WC-Co and Inconel-625 surfaces can be identified as self-affine fractals in the stochastic sense within the correlation length. The root-mean-square roughness (R _q ) depends on the scale of cut-off length (=sampling length in this paper) as a power law. The R _q of the machined WC-Co surface was found to be dependent on the scale of cut-off length rather than the scale of evaluation length. The roughness exponent is a very useful parameter and can be used to predict the roughness value of the surfaces at any scale length, if the scale is within the correlation length and provided that one such value of a scale is known. It may be suitable to compare surfaces using roughness exponents, even if different cut-off lengths or scanning scales are used in the measurements to obtain roughness exponents.     

超精密加工的三维表面形貌预测

描述并建立了仿真超精密加工的三维表面形貌模型。三维表面形貌模型由切削参数,刀具几何形状及刀具与工件的相对运动来表征,通过将预测的表面粗糙度轮廓线性映射到网格 面元上来生成已加工表面的形貌,实际的加工和测量实验表明,仿真的三维表面形貌和由激光干涉形貌仪测量得到的三维形貌具有很好的相似性。该模型可用来确定如刀具切削运动的迹线,表面波度等表面特征。