Theoretical and experimental analysis of the effect of error motions on surface generation in fast tool servo machining

The fast tool servo (FTS) machining process provides an indispensable solution for machining optical microstructures with sub-micrometer form accuracy and a nanometric surface finish without the need for any subsequent post processing. The error motions in the FTS machining play an important role in the material removal process and surface generation. However, these issues have received relatively little attention. This paper presents a theoretical and experimental analysis of the effect of error motions on surface generation in FTS machining. This is accomplished by the establishment of a model-based simulation system for FTS machining, which is composed of a surface generation model, a tool path generator, and an error model. The major components of the error model include the stroke error of the FTS, the error motion of the machine slide in the feed direction, and the axial motion error of the main spindle. The form error due to the stroke error can be extracted empirically by regional analysis, the slide motion error and the axial motion error of the spindle are obtained by a kinematic model and the analysis of the profile in the circumferential direction in single point diamond turning (SPDT) of a flat surface, respectively. After incorporating the error model in the surface generation model, the model-based simulation system is capable of predicting the surface generation in FTS machining. A series of cutting tests were conducted. The predicted results were compared with the measured results, and hence the performance of the model-based simulation system was verified. The proposed research is helpful for the analysis and diagnosis of motion errors on the surface generation in the FTS machining process, and throws some light on the corresponding compensation and optimization solutions to improve the machining quality.     

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.     

Modeling and machining evaluation of microstructure fabrication by fast tool servo-based diamond machining

A fast-tool servo-machining process is typically utilized to generate sinusoidal microstructures for optical components only when the clearance angle of the cutting tool is greater than the critical value. This paper focuses on the generation characteristics of microstructures for surface texturing applications when the clearance angle of the cutting tool is smaller than this critical angle. A method for calculating the microstructure profile amplitude and wavelength is introduced for the prediction of microstructure generation. Cutting tests were conducted, and the measured results were quite close to the corresponding calculated results, further verifying the capability of the proposed analytical model.     

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。     

辊筒模具微透镜阵列的慢刀伺服成形加工

目的：为实现辊筒模具表面微透镜阵列高效率、高精度加工,本文对微透镜阵列成形法加工轨迹的拟合方法和机床伺服参数的优化方法进行了理论与实验研究。首先,分析了微透镜阵列的原始轨迹特征,确定了过渡台阶的突变是微透镜表面振纹的主要诱因。其次,为保证加工轨迹二阶导数的连续性,本文提出了三次样条插值与傅里叶级数拟合拼接的方法优化加工轨迹。最后,在优化加工轨迹的基础上,通过调整伺服系统的前馈参数,提高了进给轴响应能力,减小了因驱动质量和阻尼效应而产生的跟踪误差。口径800μm、深度26.7μm的微透镜阵列加工实验表明,采用优化的刀具轨迹和伺服参数,机床加工效率可以达到8Hz,进给轴跟踪误差小于300nm,消除了微透镜阵列的表面振纹。微透镜单元口径的尺寸误差约为设计值的1.075%,随机检测结果表明口径尺寸变化范围为2μm,加工一致性良好。三次样条插值与傅里叶级数拟合优化的加工轨迹可有效抑制进给轴的振动,改善了微透镜阵列表面质量。     

Design,fabrication, and testing of a 3-DOF piezo fast tool servo for microstructure machining

Fast tool servo (FTS) cutting has the superiority of high efficiency and high precision, which has attracted great attention from the field of microstructure machining. 3-degree-of-freedom (DOF) FTS device driven by piezoelectric (PZT) actuator, with high frequency, high precision, and low crosstalk property, are quite appealing for realizing complex microstructure machining. Therefore, a novel 3-DOF piezo-actuated FTS mechanism with high natural frequency and decoupling property is proposed in this paper. First, the static and dynamic models of the mechanism are established by using the compliance matrix and Lagrange equation methods, respectively. Then, the structural parameters of the mechanism are optimized by the genetic algorithm (GA) based Pareto multi-objective optimization algorithm. With the purpose of verifying the property of the above approach, the finite element analysis (FEA) acting on the designed mechanism has been carried out. Moreover, the modeling tasks in terms of cutting principle and trajectory planning are demonstrated in detail. Besides, a series of tests are carried out to verify the performance of the developed 3-DOF FTS. The testing results indicate that the working stroke of the mechanism is up to 40 μm, the natural frequency is above 873 Hz, and the mechanism has excellent motion decoupling performance (within 2%). The error of the trajectory tracking in all three directions are kept within ±0.7 μm. Finally, compared with the desired surface, the error of the machined microstructural surface is kept within ±1.5 μm, which further verifies its satisfactory performance towards ultra-precision FTS machining of microstructure.     

快速刀具伺服系统加工误差的超前补偿修正

快速刀具伺服系统(fast tool servo,FTS)能够用于复杂面形或微结构工件的精密高效加工,但由于运动惯性和控制信号延迟的影响,将会产生加工误差。根据系统传递函数的幅频、相频特性,采用预期迭代学习控制算法设计了超前补偿器,通过对超前步长和超前增益进行规划,克服了幅值衰减和响应延迟的影响,实现刀具位移的精准输出;对波长70μm的二维正弦面阵进行加工实验,测试结果表明实际面形与设计参数基本一致,偏差小于PV 0.12μm,达到了预期的指标要求。     

Development of a voice coil motor based fast tool servo with a function of self-sensing of cutting forces

This paper presents a fast tool servo (FTS) driven by a voice coil motor (VCM) with a function of self-sensing of cutting forces. Conventionally, cutting force measurement associated with a FTS system is made by integrating an additional force sensor or a dynamometer, which would make the system complicated and influence the dynamic performance of the FTS. Differing from the conventional method, the force measurement in the proposed system is achieved by detecting the current of the VCM and then obtaining the cutting force based on the electromagnetic field distribution of the VCM. Since it is not necessary to integrate additional force sensors, the main body of the FTS could be compact and the dynamics of the FTS would not be influenced by the added function of force measurement. The FTS mainly consists of an air bearing guide driven by a VCM with a stroke of 2.5 mm, an optical encoders feedback system for precision positioning and a hall current sensor for current measurement. To obtain forces from the measured currents, the magnetic field distribution of the VCM is figured out and the nonlinear relationship between the position and the magnetic field distribution is corrected. The basic performances of the FTS for positioning and force measurement were experimentally investigated. It is shown that the system could have a positioning resolution of 20 nm and a force self-sensing resolution of 5 mN based on the proposed method. The proposed method provides a new way for in-process cutting force measurement associated with FTS systems.     

Self-sensing of cutting forces in diamond cutting by utilizing a voice coil motor-driven fast tool servo

Cutting force measurement is important for monitoring the diamond cutting process. In this paper, a new measurement method of thrust cutting force associated with a voice coil motor (VCM) driven fast tool servo (FTS) system has been developed. Instead of integrating additional force sensors to the FTS which would influence the dynamics of the FTS, the force measurement in the proposed system is achieved associated with in-process monitoring the variation of the driving current of the VCM and pre-process determining the system parameters. In this way, the cutting forces are accurately obtained by subtracting the influences of the driving force, the spring force, the damping force and the inertial force associated with the system as well as the cutting process. Based on the proposed method, a microstructure array was machined using the developed VCM-FTS and the cutting force during the machining process was monitored in real time. The measured force signal was in good agreement with the machining result. The surface profile error of the fabricated microstructure could be clearly distinguished by the variation of the measured cutting force signal. This provides a new approach for in-process cutting force measurement associated with FTS based diamond cutting process.     

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.     

A novel robust Gaussian filtering method for the characterization of surface generation in ultra-precision machining

A lot of research work has been focused on the study of the surface generation mechanisms in order to predict the surface topography and provide the optimal machined parameters based on the experiential understanding of relationship of machined conditions and surface features. Although the formation of novel geometrical product specification (GPS) and verification framework system promotes the relevant research work to new characterization methods and draft of international standards, relative little research work was conducted on the application of surface characterization techniques to ultra-precision machining which is very important to evaluate the surface quality. In this paper, a novel robust Gaussian filtering method (RGF) is proposed and used to characterize the surface topography of ultra-precision machined surfaces. Cubic B-spline and M-estimation are used to make the method reliable and robust. Based on the property comparisons of classical weighting functions, a novel auto-developed robust weighting function (ADRF) is defined to improve the robustness of RGF. To verify the characterization feasibility of the proposed method, computer simulation is used and then the real ultra-precision machined surfaces are analyzed. The experimental results indicate that the RGF method cannot only separate the surface components effectively on the whole measured area and but also eliminates the influence of freak outliers.     

Modelling and simulation of structure surface generation using computer controlled ultra-precision polishing

The applications of structured surfaces have been more widespread. However, research on the fabrication of these surfaces is still far from complete. The paper presents a theoretical and experimental investigation of the generation of structured surfaces by using Computer Controlled Ultra-precision Polishing (CCUP). A surface topography simulation model and hence a model-based simulation system for the modelling and simulation of the generation of structure surfaces by using CCUP have been established and verified through a series of simulation and practical polishing experiments. The results of experiments demonstrate the capability of the model-based simulation system in predicting the form error and the pattern of the 3D-texture generated by using CCUP.