Papers by Author: Chi Fai Cheung

Abstract: Rolling Imprint Lithography is an Enabling Technique for the Mass Production of Ultra-Precision Micro-Structured Surfaces which are Widely Used in High Value Added Optical Products such as Backlight Guides for Display Devices. the Accuracy of the Produced Micro-Structured Surfaces Relies Heavily on the Master Roller of which Quality is Difficult to be Controlled by Traditional Off-Line Measurement Process due to the Large Dimension and Heavy Weight of the Workpiece. this Paper Presents a CCD Based Non-Contact Measurement System to Perform the on-Machine Measurement of the Structure Pattern on Machined Rollers. Instead of Measuring each Single Structure on the Roller, the System is more Focused on the Characterization of the Conformance of the Structure Pattern with the Designer’s Requirement. Low Angle Monochromatic Light is Used to Extract the Structure Pattern on the Machined Roller Using a High Magnification Objective Lens with a High Resolution CCD Sensor. the Image Processing Techniques are then Employed to Characterize the Captured Image Based on the Designed Pattern Structure. the Developed System has been Mounted on a Single-Point Diamond Turning Machine to Measure the Machined Structured Roller, and the Results Indicate that the Developed Measurement System is Able to Perform the on-Machine Measurement and Characterization of the Structure Pattern of the Machined Roller with Sub-Micrometre Accuracy.

Abstract: This study focuses on the nonlinear dynamic characteristics of tool vibration in ultra precision diamond turning. Considering the first mode vibration of a tool, the tool is modelled as a one degree of freedom spring-mass-damper system with a developed cutting force model. Numerical simulations were carried out to anlyze the nonlinear dynamics of the system, using bifurcation diagrams, Poincaré maps, phase protraits, time histories and power spectral densities. The results indicated that the dynamic response of the tool is dominantly periodic and harmonic at its natural frequency with a little negligible chaos and its natural frequency, more than its fundamental frequency, is slightly shifted by depth of cut and cutting force.

Abstract: Ultra-precision polishing is an emerging technology for producing superfinishing surfaces with sub-micrometer form accuracy and surface finish in nanometer range. It has been applied in superpolishing the freeform bearing surfaces of orthopaedic implants. It is believe that the superfinished surfaces are capable of prolonging the life of the implants. In this paper, an experimental investigation of ultra-precision polishing of orthopaedic implants and the study of the wear characteristics of the superfinished surfaces using a multi-directional pin-on-plate wear test simulator are implemented. Tests were carried out over 3 million cycles using Zeeko IRP200 superfinished cobalt chrome pins articulating against cross-linked UHMWPE plates. The results were compared to that of manually polished pins articulated against the same UHMWPE material. The results show that the Zeeko IRP200 polished pins produced better wear performance that that of the manually polished pins.

Abstract: This paper presents a study of effect of cutting conditions on surface quality in FTS machining of optical microstructures such as micro-lens array. A power spectrum analysis is proposed to characterize the surface quality in FTS machining. It is found that there is a strong relationship between the surface roughness and the power spectrum of the surface profile. This provides an important means for the characterization of surface quality in FTS machining of optical microstructures.

Abstract: Virtual manufacturing (VM) is an emerging technology that provides a digital tool for the optimization of the production efficiency through simulations prior to the start of actual production. A NC program translator is one of the crucial components for virtual machining which allows the simulation and verification of NC tool path before actual machining. In this paper, a framework of a NC program translator for diamond turning of precision optics is presented.

Abstract: In this paper a dynamic non-linear mathematics model is proposed to predict the surface roughness in optical ultra-precision machining, which can be automatically built by evoling computer program of genetic algorithm. The new model can improve the fitting and predicting accuracy, compared with the traditional linear regression model. The numerical simulation test proves the effectiveness and accuracy of new model.

Abstract: One of the remarkable achievements of nanotechnology is the ability to achieve nanometric surface finishes in single-point diamond turning of high-precision components for complex optical surfaces. A better understanding of the surface generation mechanisms is of prime importance for the development for the prediction of the surface roughness. This paper presents a study of mechanisms of nano-surface generation in single-point diamond turning of various types of materials.

Abstract: In this paper, a framework of surface generation model in the fast tool servo (FTS) machining of optical microstructures will be described. The integrated model is totally composed of a tool path generator (TPG), a surface topography model (STM) and an optimization model (OM). To develop the tool path generator, two parts should be involved. The first part is the tool path generated based on cutting conditions such as the feed rate and spindle speed, the geometry of optical microstructures, and diamond tool geometry. Another part is the synchronized motion generated by the tool actuation of the FTS at a bandwidth higher than the rotational frequency of the spindle. The surface topography model will be generated based on the TPG and used to predict the technological aspects of FTS machining. It takes into the account the kinematic and dynamic characteristics of the cutting process. The former includes the tool path generated by the tool path generator. The later includes the relative vibration between the tool and the workpiece caused by the axial error motion of the spindle as well as the synchronized motion of the FTS system. The optimization model will be undertaken by an iterative algorithm, which will be developed based on the TPG and STM. The OM will be expected to output the verified tool path, the suggested optimum cutting conditions, and the diagrams with predicted cutting performance characteristic and process parameters being investigated. Eventually, the successful development of this surface generation model can contribute for the knowledge of ultra-precision machining with FTS and the further development of the performance of the machining system.

Abstract: Ultra-precision raster milling (UPRM) can directly achieve the fabrication of threedimensional non-axisymmetric freeform components with nanometeric surface finish and submicrometric form accuracy. During the past few years, a lot of research on computerized numerical control (CNC) machining technologies has been conducted; however, little of this research makes any contribution to UPRM. This paper studies the mechanism of scallop height generation for UPRM. Through analyzing the cutting edge locus and the scallop height generation mechanism, a model for feed-intervals and for raster-interval scallop height generation, are presented. Parameters such as selection of feed direction (vertical cutting and horizontal cutting), feed rate, spindle speed, tool nose radius, swing distance, feed length and raster distance have been taken into consideration in the scallop generation models.

Abstract: Based on the analysis of characteristics of progressive lens, it can be found that lens uneven change of curvature is an important reason of defect on progressive lens processed by single-point diamond cutting technique. By applying least square fairing method to fair the surface which is fitted and interpolated by Double Cubic B-Spline method, the curvature change of progressive surface becomes even. The processing experiment shows that the method in this paper can eliminate the defect effectively in the ultra-precision processing of progressive lenses.