|Title:||Novel vibration-assisted ultraprecision machining system for generating micro/nanostructured surfaces|
|Advisors:||To, Suet (ISE)|
Vibration -- Industrial application
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xxiv, 179 pages : color illustrations|
|Abstract:||Micro/nanostructured surfaces with regular patterns are drawing ever-increasing attention and are widely used in a range of modern industries due to their tribological, optical, and antibacterial properties, etc. For instance, the micro/nanostructures on the lotus leaf exhibit higher superhydrophobicity and self-cleaning properties. The micro-dimple surfaces can decrease the friction force by 80%. How to machine these functional micro/nanostructured surfaces has become the current research focus. The lithography-based or laser-based machining process is the widely used machining method, but they are limited by the high cost or low surface finish. Vibration-assisted machining is a promising process to solve these problems. However, existing vibration-assisted machining systems still suffer from low bandwidth or high coupling ratio, which affects the machining efficiency and machining flexibility.|
In this thesis, a novel two-degree-of-freedom vibration-assisted ultraprecision machining (2DOF-VAUM) system with high machining efficiency and high machining flexibility is designed to machine these micro/nanostructures. In addition to the design of the 2DOF-VAUM system, the modeling of the cutting force, the functional applications of the 2DOF-VAUM system are also studied. The thesis includes four main research parts as follows:
(1) The first part designs the novel 2DOF-VAUM system with a bandwidth of 3000Hz and a low coupling ratio of less than 5%. In the mechanical design, a quasi-ellipse amplification mechanism is proposed to amplify the displacement of the piezoelectric actuator. In comparison with the existing amplification mechanisms, the proposed quasi-ellipse amplification mechanism possesses a compact structure, which can efficiently enhance the bandwidth of the machining. Besides, the orthogonal layout of the amplification mechanism can decrease the coupling ratio. A detailed multi-physics finite element method is proposed to precisely simulate the working performance of this designed 2DOF-VAUM system. Considering the three-dimensional shape of the cutting tool, machining parameters, and elastic recovery of the workpiece material, a numerical simulation algorithm of the micro/nanostructure generation is then developed to predict the surface topography and provide the guidance in the machining parameter selection.
(2) The second part focuses on the cutting force modeling during micro/nanostructure machining. A practical cutting force prediction model with high accuracy is established with consideration of the periodical vibration motion, tool geometry, and workpiece material, which fills the gap in the cutting force prediction in micro/nanostructure machining. In this model, the instantaneous shear angle is calculated based on the time-varying vibration trajectory. The shear strain and shear strain rate inside the primary shear zone is evaluated by the analysis of the non-equidistant shear zone. The influence of the cutting tool geometry on the cutting force is also investigated considering the relative size between the tool radius and the maximal depth-of-cutting. The shuttle-shaped and bamboo-shaped microstructures are machined on the surface of the difficult-to-machining material, pure titanium TA2, which validates the effectiveness of the proposed cutting force prediction model at different machining parameters.
(3) The third part investigates the application of the developed 2DOF-VAUM system on the magnesium alloy surfaces. Micro/nanostructure generation on the magnesium alloy surface remains a challenge due to its flammability and ignition. The developed 2DOF-VAUM system is applied to effectively solve this problem. Various shape microstructures and sawtooth-shaped nanostructures are successfully machined on the magnesium alloy surface. In this micro/nanostructure machining, no burn marks are found, which demonstrates the safety of the developed 2DOF-VAUM system. Besides, the sawtooth-shaped nanostructures induce the optical effect to generate the colorful letters and colorful flower image.
(4) The fourth part investigates another application of the developed 2DOF-VAUM system. The developed 2DOF-VAUM system is used to generate and hide optical information on the workpiece surface. This study extends the function of the nanostructures to the field of information science. The quick response (QR) code containing the optical information "Let's Beat COVID-19 Together" is machined on the workpiece surface. The content of the QR code can be easily read via the smartphone. By adjusting the facet spacing of the nanostructures, the optical information "LOVE" can be hidden. The hidden information is read when the view angle reaches the certain angle. For other view angles, only disturbance information "8888" is viewed.
Overall, this research develops a novel 2DOF-VAUM system to generate the various micro/nanostructures and carries out the systemic study on the machining system design, the cutting force modeling, and the machining system application. The main contributions and significance of this thesis can be concluded as follows: (1) a novel two-degree-of-freedom vibration-assisted ultraprecision machining system with superior working performances was developed, which can generate various micro/nanostructures on workpiece surfaces. (2) A practicable cutting force prediction model was established, which fills the gap in cutting force prediction in the field of micro/nanostructure and also helps to better understand the material removal mechanism during micro/nanostructure machining. (3) The micro/nanostructure generation on the magnesium alloy surfaces was systematically investigated, which expands the function of the magnesium alloy to the field of optics. (4) The generation and hiding method of optical information was proposed based on the periodic nanostructures, which makes the function of nanostructures extend to information storage, information communication, and information hiding.
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