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.     

Development of a force controlled nanocutting system using a flexible mechanism for adaptive cutting of microstructures on non-planar surfaces

A force controlled nanocutting system based on a flexible mechanism was developed. Instead of utilizing force sensors, the force sensing and control in the developed system is realized by sensing and controlling the deformation of the flexible mechanism. With force feedback control for controlling the cutting force in real time, this system could achieve adaptive cutting along non-planar surfaces without prior knowledge about the surface shape. The finite element method was used to model the flexible mechanism and a Pareto-based multi-objective optimization algorithm with the goals of high force resolution and stability was used to obtain the geometric parameters of the flexible mechanism. During the cutting processing, a capacitive displacement sensor was used to detect the deformation of the flexible mechanism to measure the force in real time, and a piezoelectric ceramic actuator was used to adjust the feed position of the tool to control the cutting force. Nanocutting experiments of microstructures were successfully carried out on inclined and curved surfaces of ductile as well as brittle materials without prior knowledge of their surface forms.     

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.     

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.     

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.     

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.     

Polishing of single point diamond tool based on thermo-chemical reaction with copper

This paper describes a new polishing method for diamond cutting tools. The method is based on the principle of oxidization of copper and deoxidization of copper oxide by carbon. A diamond tool was brought into contact with a copper plate, heated in air to a range of 323–523 K. The depth of the removed layer of diamond increased almost linearly with contact time and reached approximately 7 nm after 6 h. In this erosion process, pre-existing microcracks on the diamond surface were reduced. In comparison with the mechanically polished tool, the thermo-chemically polished tool was highly resistant to chipping and yielded a significant rise in tool life.     

Nanometer edge profile measurement of diamond cutting tools by atomic force microscope with optical alignment sensor

This paper describes an atomic force microscope (AFM) based instrument for nanometer edge profile measurements of diamond cutting tools. The instrument is combined with an AFM unit and an optical sensor for alignment of the AFM probe tip with the top of the diamond cutting tool edge in the submicrometer range. In the optical sensor, a laser beam from a laser diode is focused to generate a small beam spot with a diameter of approximately 10 μm at the beam waist, and then received by a photodiode. The tool edge top and the AFM probe tip are brought to the center of the beam waist, respectively, through monitoring the variation of the photodiode output. To reduce the influence of the electronic noise on the photodiode output so that the positioning resolution can be improved, a modulation technique is employed that modulates the photodiode output to an AC signal by driving the laser diode with a sinusoidal current. Alignment experiments and edge profile measurements are carried out.     

Application of a fast tool servo for diamond turning of nonrotationally symmetric surfaces

The fabrication of nonrotationally symmetric surfaces by diamond turning requires tool actuation at a bandwidth significantly higher than the rotational frequency of the surfaces. This requirement cannot be met by standard slide drives due to their large mass and consequent low natural frequency. This articles describes the development of a laboratory-scale diamond-turning machine with piezoelectric-driven fast tool servo. The capability of this apparatus will be demonstrated for high-speed features such as sine wave, square wave, and ramp-shaped surfaces. Also described is the implementation of this fast tool servo on a commercial diamond-turning machine. Several nonrotationally symmetric surfaces have been machined, and their images are included.     

A flexure-based tool holder for sub-μm positioning of a single point cutting tool on a four-axis lathe

A tool holder was designed to facilitate the machining of precision meso-scale components with complex three-dimensional shapes with sub-μm accuracy on a four-axis lathe. A four-axis lathe incorporates a rotary table that allows the cutting tool to swivel with respect to the workpiece to enable the machining of complex workpiece forms, and accurately machining complex meso-scale parts often requires that the cutting tool be aligned precisely along the axis of rotation of the rotary table. The tool holder designed in this study has greatly simplified the process of setting the tool in the correct location with sub-μm precision. The tool holder adjusts the tool position using flexures that were designed using finite element analyses. Two flexures adjust the lateral position of the tool to align the center of the nose of the tool with the axis of rotation of the B-axis, and another flexure adjusts the height of the tool. The flexures are driven by manual micrometer adjusters, each of which provides a minimum increment of motion of 20 nm. This tool holder has simplified the process of setting a tool with sub-μm accuracy, and it has significantly reduced the time required to set a tool.