An investigation of factors affecting surface generation in ultra-precision machining with fast tool servo

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An investigation of factors affecting surface generation in ultra-precision machining with fast tool servo

 

Author: Kwok, Tsz-chun
Title: An investigation of factors affecting surface generation in ultra-precision machining with fast tool servo
Degree: M.Phil.
Year: 2011
Subject: Machining.
Diamond turning.
Machine-tools -- Monitoring.
Surfaces (Technology)
Nonotechnology.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Industrial and Systems Engineering
Pages: xix, 207, [15] leaves : ill. ; 30 cm.
InnoPac Record: http://library.polyu.edu.hk/record=b2456176
URI: http://theses.lib.polyu.edu.hk/handle/200/6152
Abstract: Ultra-precision machining with fast tool servo (FTS) is one of the emerging technologies for the fabrication of high-quality optical surfaces. A diamond tool is fixed on the FTS is activated back and forth by a stacked type of piezoelectric actuator. Complex optical surfaces with sub-micrometer form accuracy and nanometric surface finish can be fabricated without the need for any subsequent post processing. Although there has been extensive research work on the design and control of tool actuators for FTS machining, relatively little research work has been reported in the study of nano-surface generation in FTS machining. As a result, this study aims at investigating the factors affecting surface generation in ultra-precision machining with FTS. The factors under investigation include process factors, wear of diamond tool and error motion of FTS actuation. Regarding process factors, a series of experiments were conducted under various cutting conditions such as spindle speed, feed rate, depth of cut, etc. The results indicate that the surface quality of the machined surface can be improved by appropriate selection of cutting conditions. Moreover, a tool compensation method has been developed which is found to be effective in improving the surface quality of machined surfaces. To study the form errors in FTS machining of optical components with complex profiles such as micro-lens arrays, pattern analysis has been employed to determine the surface quality of the machined surface. The experimental results indicate that the influence due to process factors can be minimized with appropriate selection of cutting conditions.
Since a diamond tool with a small radius is usually used in FTS machining, the surface quality of the workpiece is highly susceptible to the tool wear. As a result, the study continues by investigating the effect of the tool wear in FTS machining. A series of cutting experiments were conducted by FTS machining of a tilted flat surface by a predetermined distance. The traditional method for determination of tool wear based on the maximum height and the width of flank wear has been used to characterize the tool wear. In order to investigate the diamond tool wear quantitatively, a digital image processing method has been proposed to quantitatively characterize the tool wear. It is not only found to be effective in exploring the wear phenomena but also quantifying the material loss in various wear stages of the diamond tool. To investigate the effect of tool wear on surface generation in FTS machining, an experimental study has been done by machining tilted flat surface and micro-lense array. It is interesting to note that the form error of the FTS machined surface is adversely affected by tool wear. As FTS machining technology is built based on the single-point diamond turning technology, the error motion of the ultra-precision machine and the stroke error of the FTS play important roles in the surface generation. Based on the experimental results and cutting mechanisms in FTS machining, a theoretical analysis is attempted to address two major error motions, which are due to the systematic characteristics of the ultra-precision machine and the stroke error due to the FTS actuation. Afterwards, a surface generation model was established to predict the surface quality in terms of the form error in FTS machining. The model has been verified through a series of cutting experiments. The result shows that the trend of the theoretical prediction by the simulation system is found to agree reasonably well with the experimental results. On the whole, this study not only contributes significantly to a better understanding of the factors affecting surface generation in FTS machining but also provides an important means for the improvement of the surface quality in ultra-precision machining with Fast Tool Servo. This is vital for the technological achievement of ultra-precision machining technology.

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