Surface profile measurement of a sinusoidal grid using an atomic force microscope on a diamond turning machine

This paper describes the surface profile measurement of a XY-grid workpiece with sinusoidal microstructures using an atomic force microscope (AFM) on a diamond turning machine. The sinusoidal micro-structures, which are fabricated on an aluminum plate by fast tool servo-assisted diamond turning, are a superposition of periodic sine-waves along the X- and Y-directions (wavelength (XY): 150 μm, amplitude (Z): 0.25 μm). A linear encoder with a resolution of 0.5 nm is integrated into the AFM-head for accurate measurement of the Z-directional profile height in the presence of noise associated with the diamond turning machine. The spindle and the X-slide of the machine are employed to spirally scan the AFM-head over the sinusoidal grid workpiece. Experiments fabricating and measuring the sinusoidal grid workpiece are carried out after accurate alignment of the AFM cantilever tip with the spindle centerline.     

Precision nano-fabrication and evaluation of a large area sinusoidal grid surface for a surface encoder

This paper describes the fabrication of a large area sinusoidal grid surface, which is used as the measurement reference of a surface encoder for multi-axis position measurement. The profile of the grid surface is a superposition of sinusoidal waves in the X-direction and the Y-direction with spatial wavelengths of 100 μm and amplitudes of 100 nm. Diamond turning with a fast tool servo (FTS) was chosen as the fabrication method. The constructed FTS, which employs a piezoelectric tube actuator (PZT) to actuate the diamond tool and a capacitance probe as the feedback sensor, was confirmed to have a bandwidth of approximately 2.5 kHz and a tool displacement accuracy of several nanometers in the closed-loop mode. Experiments of fabricating the sinusoidal grid surface were performed on a commercially available precision diamond turning machine. An aluminum alloy workpiece was vacuum chucked on the spindle and the FTS was mounted on the X-slide. Efforts were made to position the tool tip to the center of the spindle (center-alignment) since it was verified that the center-alignment is important for the fabrication accuracy of the sinusoidal grid surface. An evaluation technique based on the two-dimensional (2D) discrete Fourier transform (DFT) of interference microscope images was also developed to evaluate the fabricated grid surface effectively. The fabrication result of a grid surface over an area of 150 mm has indicated the effectiveness of the fabrication system.     

Measurement and compensation of error motions of a diamond turning machine

This paper describes the measurement and compensation of error motions of a diamond turning machine for nanofabrication of large sinusoidal metrology grids. The diamond turning machine has a T-base design, which consists of a spindle with its rotation axis along the Z-direction and a cross-slide with its movement direction along the X-direction. A fast-tool-servo (FTS) unit is mounted on the X-slide to generate sinusoidal microstructures on a flat workpiece surface mounted on the spindle. The error motions of the X-slide and the spindle, which introduce Z-directional profile errors (out-of-flatness) on the grid surface, are measured and compensated. The out-of-straightness of the X-slide is measured to be approximately 60 nm over a travel of 80 mm by using the reversal method. It is also confirmed that the out-of-straightness of the X-slide has a 10-nm periodic component with a period of 11 mm corresponding to the diameter of the needles used in the roller bearing of the X-slide. The angular motion of the spindle is measured to be approximately 0.3″ by using an autocollimator, which can cause a 73-nm out-of-flatness over a workpiece 100 mm in diameter. The axial motion of the spindle is measured to be approximately 5 nm, which is the smallest error motion. The out-of-flatness of the workpiece is reduced from 0.27 to 0.12 μm through compensating for the error motions by utilizing the FTS unit based on the measurement results of error motions.     

Scanning tunneling microscopy-based on-machine measurement for diamond fly cutting of micro-structured surfaces

This paper presents a new on-machine measuring system based on scanning tunneling microscope principle, in order to meet the demand for precision form control of fabricated micro-structures. It is compactly established and mounted on a diamond turning machine, capable of on-machine measuring microstructures with steep slopes and size of tens of micrometers, which remains a challenge task to accomplish undistorted measurement even for general off-line metrological methods. Fly cutting with V-tip fly-cutter was first applied in two directions with interval angle of 60° to form the crossed micro V-grooves, and then efficient on-machine measurement was carried out to characterize the machined surface form without ending the proceeding of the subsequent fabrication. By depth compensation of fly cutting in 120° direction through feedback of on-machine measured results, the surface of rectangular pyramid array with diamond-shaped feature was successfully generated. Experimental results demonstrate that the proposed measuring system is of significance in the fabrication process, owing to its efficient on-machine characterization capability for generating accurate micro-structured surfaces.     

Ultrasonic vibration assisted grinding of hard and brittle linear micro-structured surfaces

In this paper, a novel ultrasonic vibration assisted grinding (UVAG) technique was presented for machining hard and brittle linear micro-structured surfaces. The kinematics of the UVAG for micro-structures was first analyzed by considering both the vibration trace and the topological features on the machined surface. Then, the influences of the ultrasonic vibration parameters and the tilt angle on the ground quality of micro-structured surfaces were investigated. The experimental results indicate that the introduction of ultrasonic vibration is able to improve the surface quality (The roughness SR_a was reduced to 78 nm from 136 nm), especially in guaranteeing the edge sharpness of micro-structures. By increasing the tilt angle, the surface roughness can be further reduced to 56 nm for a 59% improvement in total. By using the preferred UVAG parameters realized by orthogonal experiments, a micro cylinder array with surface roughness of less than 50 nm and edge radius of less than 1 μm was fabricated. The primary and secondary sequence of the grinding parameters obtained by the orthogonal experiments are as follows: feed rate, tilt angle of workpiece, depth of grinding, vibration frequency and amplitude. The spindle speed in the range of 1000 rpm–3000 rpm does not significantly affect the machined micro-structured surface roughness. Finally, more micro-structures including a micro V-groove array and a micro pyramid array were machined on binderless WC as well as SiC ceramic by means of the UVAG technique. The edge radius on the V-grooves and pyramids are both less than 1 μm, indicating the feasibility of UVAG in machining hard and brittle micro-structured surfaces for an improved surface quality.     

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 of elliptically-vibrating ultrasonic actuator for nanocoining

Nanocoining is a method of rapidly creating a cylindrical mold surface covered with features smaller than the wavelength of light. This mold can then be used in a roll-to-roll process to make surfaces whose functionality depends on the wavelength of the illumination. The die replaces the typical diamond tool used to produce overlapping grooves for applications such as reflective signs. The die has a face area approximately 20 μm² that has been patterned in an FIB. It is mounted on a 2D ultrasonic actuator and follows an elliptical path that matches the surface speed of the moving workpiece during the short contact time and creates approximately 6000 features per impact. The spacing of die indents is controlled by the speed of the diamond turning machine axes such that a small overlap exists from previous indents as the die spirals around and along the mold surface. Because the die is small, the indentations must occur rapidly to make nanocoining a feasible process. This work focuses on the design and control of a nominally 40 kHz, 2D resonant actuator that is suitable for this process. A controller to automatically track resonance is described to maintain the elliptical motion during indentation. Methods of tuning the behavior of the actuator and maintaining a constant indent depth are proposed. Finally, 500 nm pitch feature indents were created on a brass workpiece at 40 kHz and scanning electron microscope (SEM) images of the features are provided.     

Evaluation on 3D micro-ground profile accuracy of micro-pyramid-structured Si surface using an adaptive-orientation WLI measurement

According to the aperture of the objectives, surfaces with steep topographies greater than approximate 25° are difficult or unable to measure with white light interferometry. Hence, an adaptive-orientation measurement is proposed by adjusting the incidence angle from 51° to 21°. In this study, a micro-grinding with #3000 diamond wheel V-tip was employed to fabricate the micro-pyramid-structured Si surface with 142 μm in depth and 38 nm in surface roughness. The objective is to evaluate the micro-profile accuracy of micro-ground Si surface. First, the four micro-ground surfaces of micro-pyramid-structured surface were measured along the adaptive orientation with an incidence angle, respectively; then iterative closest point (ICP) matching was used to reconstruct the whole micro-ground surface with four adaptive-orientation measured point clouds; finally, 3D reconstruction error and characterized profile error were investigated. It is shown that the ICP matching with denoising and finishing is valid to register four adaptive-orientation measured point clouds for reconstructing an integrated micro-ground surface. Moreover, a decrease in incidence angle to measured surfaces leads to a decrease in 3D reconstruction error, an increase in valid top-topographic point number and a decrease in characterized profile error. It is confirmed that the adaptive-orientation measurement with 21° incidence angle may enhance 3D reconstruction accuracy by about 35%, valid top-topographic point number by about 3 times and characterized profile accuracy by about 38% against the traditional measurement, respectively. The micro-ground form error of 5.5 μm and the characterized profile error of 6.0 μm may be achieved, respectively, thus the micro-grinding is valid for the precision micro-fabrication of micro-structured surface.     

Measurement of lathe Z-slide straightness and parallelism using a flat land

A method is described to measure the straightness of travel of the carriage of a single-point diamond turning machine. The method also measures the slide parallelism to the workhead spindle. A cylindrical artifact was produced on the lathe and used as a straightedge. A flat land was cut in the artifact that was parallel to the workhead axis of rotation. The flat land was measured with a LVDT indicator. The method was demonstrated on a Moore Special Tool Co. M18 Aspheric Generator. The quality of measurements achieved were better than 0.1 μm.     

Vibration-resistant interference microscope with assistant focusing for on-machine measurement of surface topography

The interference microscope is a powerful tool for surface topography measurement, but its high sensitivity to vibration hinders its application to on-machine use. To measure surface roughness on a machine for the ultra-precision machining, a vibration-resistant interference microscope (VRIM) with an assistant focusing function is developed. The basic principle of VRIM is an error-compensated phase-shifting interferometry. An iterative algorithm is presented to calculate the surface phase with the phase shift amounts as unknown variables, where the phase shift amounts are calculated and compensated with least-squares method. A narrow bandwidth illumination is employed to alleviate coherence envelop influence, and a simplified intensity model is established to decouple the variables. Assisting the microscope to find fringe quickly, the focusing is realized by introducing an off-axis thin beam to generate two spots, of which their relative position relates to the defocus. The focusing method is directional and determinant, and has a large range up to 0.3 mm. In the vibration disturbances of 0.2 μm and 0.4 μm amplitudes over 0 Hz to 20 Hz frequency region, the roughness accuracy and repeatability of measuring an ultra-precision machined surface are both up to the sub-nanometer level. The developed instrument is applied to a single-point diamond turning machine and achieves a sub-nanometer accuracy and repeatability.     

Design and testing of a long-range,precision fast tool servo system for diamond turning

A long-range, precision fast tool servo (FTS) system was developed that is capable of accurately translating the cutting tool on a diamond turning machine (DTM) with maximum accelerations of 260 m s^?2 and bandwidths of up to 140 Hz. The maximum displacement range of the cutting tool is 2 mm. The FTS utilizes a flexure mechanism driven by a voice coil actuator, a custom linear current amplifier and a laser interferometer feedback system. This paper describes the design of the electromechanical system, controller configuration and cutting tests to evaluate the system. Initially, low disturbance rejection and poor command following degraded the surface finish of machined test parts. Several techniques to add damping to the dynamic system were investigated to improve the generated surface finishes. Electromotive damping was applied inside the voice coil actuator, and two different viscoelastic damping materials were applied to the flexure mechanism. A control strategy consisting of linear and non-linear feedforward controllers and a proportional, integral and derivative (PID) feedback controller was implemented to accommodate the changed system dynamics. The workpieces were analyzed using form and surface inspection instruments to evaluate the overall system performance. A cylindrical part with five lobes cut across the face had a surface finish value between 20 and 30 nm R_a.     

Design,fabrication and testing of a compact large-field-of-view infrared compound eye imaging system by precision glass molding

Bionic artificial compound eyes inspire a promising field of miniaturized imaging systems. In this research, a novel infrared (IR) three-dimensional (3D) compound eye imaging system, consisting of a double-side molded 3D microlens array and an aperture array, was designed and fabricated by combining modulated slow-tool-servo diamond turning and precision glass molding. To facilitate the complex profiles on the mold inserts, two novel slow-tool-servo strategies were adopted, namely virtual-axis based diamond broaching and adaptive diamond turning. This microlens array consists of 3 × 3 channels for a field of view of 48° × 48° with a thickness of 1.8 mm. The freeform microlens array on a flat surface was employed to steer and focus the incident light from all three dimensions to a two-dimension (2D) infrared imager. Using raytracing, the profiles of the freeform microlenses of each channel were optimized to obtain the best imaging performance. To avoid crosstalk among adjacent channels, a 3D printed three-dimensional micro aperture array was mounted between the microlens array and the IR imager. The imaging tests of the infrared compound-eye imaging system using the molded chalcogenide glass lenses showed that the asymmetrical freeform lenslets were capable of steering and forming images within the designed field of view. Compared to a conventional infrared camera, this novel microlens array can achieve a considerably larger field-of-view while maintaining low manufacturing cost without sacrificing image quality.     

基于FTS的非轴对称微结构表面超精密切削系统研究

介绍了基于快速伺服刀架(FTS)的微结构表面超精密金刚石车削加工系统,并利用该系统成功实现了典型非轴对称结构正弦网格表面的加工。作为FTS的驱动元部件,压电陶瓷微位移驱动器的迟滞、蠕变非线性特性大大影响了系统的动态性能与加工精度。因此,建立了基于拓展输入空间法的FTS神经网络逆模型,并结合PID反馈控制,实现了FTS的闭环控制。实验结果表明,该控制策略可以有效提高FTS的动态性能,其跟踪误差小于150 nm,为微结构表面的加工提供了可靠的保证。     

On-machine measurement of microtool wear and cutting edge chipping by using a diamond edge artifact

This paper presents precision on-machine measurement of microwear and microcutting edge chipping of the diamond tool used in a force sensor integrated fast tool servo (FS-FTS) mounted on a three-axis diamond turning machine. A diamond edge artifact with a nanometric sharpness is mounted on the machine spindle with its axis of rotation along the Z-axis to serve as a reference edge artifact. The diamond tool is placed in the tool holder of the FS-FTS to generate cutting motion along the Z-axis. By moving the X-slide on which the FS-FTS is mounted, the reference edge can be scanned by the diamond tool. During the scanning, the Z-directional position of the tool is closed-loop controlled by the FS-FTS in such a way that the contact force between the tool tip and the reference edge is kept constant based on the force sensor output of the FS-FTS. The tool edge contour can be obtained from the scan trace of the tool tip, whose X- and Z-directional coordinates are provided by the output of the linear encoder of the X-slide and that of the displacement sensor in the FS-FTS, respectively. Since the reference edge artifact has a good hardness and a nanometric sharpness to ensure the lateral resolution of measurement, a microwear on the cutting edge of the diamond tool can be indentified from the measured tool edge contour. Experiments of on-machine measurement of tool edge contour and microtool wear are carried out to demonstrate the feasibility of the proposed system.     

Large-area profile measurement of sinusoidal freeform surfaces using a new prototype scanning tunneling microscopy

This paper presents large-area profile measurement of ultra-precision diamond turned sinusoidal surfaces by using a specially developed scanning tunneling microscopy (STM). The new prototype of STM system employs a long stroke PZT servo actuator as the Z-directional scanner, an integrated capacitance displacement sensor to accurately measure the Z-directional profile height, a motorized stage with long traveling stroke for carrying out large-area scanning. A simple method for self-calibration of the inevitable sample tilt is proposed in order to achieve large-area measurement without tip-crashing or losing of tip-sample interaction. Several types of ultra-precision machined sinusoidal freeform surfaces with different geometrical parameters are measured by the new STM system over large scanning areas at the scale of millimeters. Specially, a sinusoidal surface with peak-valley amplitude of 22 μm and periodical wavelength of 550 μm is successfully measured and imaged by the STM system. The measurement repeatability error, repeatability standard deviation and measured profile deviation are also evaluated. It is confirmed that the new STM system is capable of carrying out large-area as well as large-amplitude measurement of the ultra-precision machined sinusoidal surfaces.     

Micro-structuring of glass with features less than 100 μm by electrochemical discharge machining

Micro-electrochemical discharge machining (ECDM) was studied in order to improve the machining of 3D micro-structures of glass. To minimize structures and obtain good surface microstructures, the effects of the electrolyte, the pulse on/off-time ratio, the voltage, the feedrate, the rotational speed, and the electrolyte concentration in the drilling and milling processes were studied.In ECDM, voltage is applied to generate a gas film and sparks on a tool electrode; however, high voltage produces poor machining resolution. To obtain a stable gas film over the whole surface of the tool at a low voltage, a new mechanical contact detector, based on a loadcell, was used; the immersion depth of the tool electrode in the electrolyte was reduced as much as possible. In this study, various micro-structures less than 100 μm in size, such as Ø 60 μm micro-holes, a 10 μm-thin wall, and a 3D micro-structure were fabricated to demonstrate the potential for micro-machining of glass by ECDM.     

UG NX4在光学超精密车削加工中应用

针对光学超精密加工中数控程序的编写问题,采用UG NX4对实际的锗单晶零件进行建模,在其加工环境中,创建加工几何体,依据Tylor Ultra Form机床实际加工参数合理选择刀具,对车削的加工过程进行了三维动态仿真模拟,并生成刀具系统源文件,转换成机床可识别的数控加工程序,应用于机床车削出Φ38mm实际锗单晶零件,利用高精度轮廓仪进行加工后的面形测量,其表面粗糙度为0.1507μm,远远达到了实际生产要求。     

Ultra-precision grinding of optical glasses using mono-layer nickel electroplated coarse-grained diamond wheels. Part 2: Investigation of profile and surface grinding

After finishing the precision conditioning of mono-layer nickel electroplated coarse-grained diamond wheels with 151 μm (D151), 91 μm (D91) and 46 μm (D46) grain size, resp., profile and surface grinding experiments were carried out on a five-axis ultra-precision grinding machine with BK7, SF6 optical glasses and Zerodur glass ceramic. A piezoelectric dynamometer was used to measure the grinding forces, while an atomic force microscopy (AFM), white-light interferometer (WLI)) and scanning electron microscope (SEM) were used to characterize the ground surface quality in terms of micro-topography and subsurface damage. Moreover, the wear mechanics of the coarse-grained diamond wheels were analyzed and the grinding ratio was determined as well, in aiming to evaluate the grinding performance with the conditioned coarse-grained diamond wheels. Finally, the grinding results were compared with that of the fine-grained diamond wheels with regard to the ground specimen surface quality, process forces and wheel wear as a function of stock removal. The experimental results show that the precision conditioned coarse-grained diamond wheels can be applied in ductile mode grinding of optical glasses with high material removal rates, low wheel wear rates and no dressing requirement yielding excellent surface finishes with surface roughness in the nanometer range and subsurface damage in the micrometer range, demonstrating the feasibility and applicability of the newly developed diamond grinding technique for optical glasses.     

Design and experimental validation of an ultra-precision Abbe-compliant linear encoder-based position measurement system

The design and development of an Abbe-compliant linear encoder-based measurement system for position measurement with a targeted 20 nm uncertainty (k = 2) in machine tools and CMMs is presented. It consists of a linear scale and a capacitive sensor, mounted in line on an interface which is guided in the scale's measurement direction and driven by a linear motor based on the output signal of the capacitive sensor. The capacitive sensor measures the displacement of a target surface on the workpiece table. The functional point, which is the center of a tool or touch probe, is always aligned with the scale and capacitive sensor such that this configuration is compliant with the Abbe principle. Thermal stability is achieved by the application of a thermal center between the scale and capacitive sensor at the tip of the latter, which prevents both components to drift apart. Based on this concept, a prototype of a one-DOF measurement system was developed for a measurement range of 120 mm, together with an experimental setup aimed at verifying the reproducibility of the system for changing ambient conditions of ±0.5 °C and ±5%rh and the repeatability during tracking of a target surface over a short period of time. These experiments have shown that the measurement uncertainty of the one-DOF system is below 29 nm with a 95% confidence level.