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Title: Development of a serial-parallel machine for precision polishing of freeform surfaces
Authors: Xu, Peng
Degree: Ph.D.
Issue Date: 2019
Abstract: The eternal objective of manufacturing technology is to sufficiently fulfill the demands of industry by improving its quality and efficiency. Nowadays, many polishing processes for freeform surfaces are still carried out manually, resulting in a low efficiency, accuracy and stability. For such reasons, this study focuses on the development of a new tyoe of six degrees-of-freedom (DOF) serial-parallel machine for precision polishing of freeform surfaces. The polishing machine is proposed with a novel design scheme by integrating the technologies of ultra-precision polishing, serial robot, parallel robot, CAD/CAM and CNC. A series of theoretical analyses and investigations about the polishing machine are conducted, including kinematics analysis, error calibration and off-line programming. On these bases, a prototype of the polishing machine is successfully developed. The validity of theoretical results and the effectiveness of the machine are illustrated by calibration experiments and polishing experiments. The main contributions and innovations of this study are summarized as follows: Driven by the practical demands of the freeform surfaces, a novel design scheme of the polishing machine is proposed with a 6-DOF serial-parallel manipulator, which mainly comprises of a 3-DOF parallel manipulator for positioning the workpiece, a 2-DOF serial manipulator for orienting the polishing tool, and a functional extension limb for providing a redundant DOF when polishing axial symmetrical surfaces. An elastic and spherical head, whose center is installed coincidently at the virtual point intersected by the two rotary axes in the serial manipulator, is used as the polishing tool. This mechanical feature ensures that the orientation adjustments of the tool with respect to the workpiece are decoupled from the position adjustments during the synchronous movements. Considering the features reflected in the structural independence and motion decoupling, systematic kinematics analyses of the serial manipulator and parallel manipulator are conducted based on the spatial closed-loop vector method and the exponential product formula, respectively. Related analyses include position, velocity, singularity, workspace and dexterity.
To deal with the geometric errors of the polishing machine, a comprehensive kinematic calibration method is developed by using of the ball-bar distance information. The error models of the parallel manipulator and the serial manipulator are formulated based on the spatial closed-loop vector method and the exponential product formula, respectively. For the two error models, the geometric errors that affect the position error and the orientation error are clearly separated. The relationship between the geometric errors of the whole serial-parallel machine and the length variations of the ball-bar is established by using the two position error models. After eliminating the redundant parameters, an integrated error model that simultaneously satisfies the requirements of completeness, minimality and continuity are obtained. This enables the geometric errors to be quickly identified with the least squares method. To compensate for the geometric errors, a step-by-step decoupling error compensation strategy is built, which simplifies the error compensation process and solves the coupling problem caused by the synchronous motions of the serial manipulator and parallel manipulator. To generate the NC programs for freeform polishing, an off-line programming method is developed for the serial-parallel polishing machine. Due to the problem of mid-spatial frequencies caused by periodic tool paths, a unicursal pseudo-random tool path described in the workpiece frames based on the Hilbert curve is generated in the Bézier space according to the information of freeform surfaces, polishing strategy, polishing tool path, and process parameters. Combining the geometric error compensation strategy with the nominal kinematic model, a special postprocessor for the polishing machine is developed, which can convert the tool path into the machine control data. Taking into account the characteristics of the CNC system and the format of the motion data, corresponding auxiliary information is added to generate the executable NC program. Based on these steps, a programming software is developed to realize the automatic conversion of freeform surface information and process parameters to NC programs. According to the design scheme of the polishing machine, the mechanical system, control system and electrical system of the machine are designed in detail. Hence, a prototype of the polishing machine is developed based on system integration and commissioning. For the developed machine, a calibration experiment is performed by using a ball-bar to identify the geometric errors. With the developed off-line programming software, NC programs are generated and the machine is controlled to polish flat surface, curved surface and Bézier surface. During the experiment, the polishing tool can move according to the planned path and the tool offset is stable. As a result, the effectiveness of the calibration algorithm and the off-line programming method are verified. After polishing, the experimental results demonstrate that both the surface roughness and the maximum profile height of the workpieces with different shapes of surfaces and materials are greatly reduced. As a result, the effectiveness and practicality of the developed polishing machine are also verified.
Subjects: Hong Kong Polytechnic University -- Dissertations
Grinding and polishing
Surfaces (Technology)
Pages: xviii, 235 pages : color illustrations
Appears in Collections:Thesis

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