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Title: The adoption of laser micro-cutting and heating in ultra-precision machining process
Authors: Han, Jide
Degree: M.Phil.
Issue Date: 2018
Abstract: Diamond machining is an important method for the manufacturing of high precision components with excellent surface finish and high form accuracy. There are limitations in this method for machining such as single crystal silicon and oxygen-free high conductivity (OFHC) copper which are often used in many cases. In this research, two aspects of using laser to assist machining were investigated. Firstly, a hybrid machining method combining laser grooving and diamond shaping was proposed which aimed at machining silicon lenticular lens mold inserts. An exploratory study on the use of a diamond tool to perform the final shaping of a silicon groove roughly cut by laser was performed. Secondly, the formation of surface defects of OFHC copper in the diamond cutting process was investigated and laser heating was introduced in the cutting process to investigate its influence on the cutting performance of OFHC copper. In machining silicon, cracks are easily formed on the machined workpiece surface when the cutting depth is larger than the critical depth of cut which is far less than 1 µm, so machining microstructures with large feature sizes such as lenticular lens mold inserts is time consuming as well as costly. In this research, a hybrid machining method to fabricate silicon lenticular lens mold inserts by combining laser grooving with diamond shaping was proposed. Firstly, laser micro-machining equipment was developed and used to perform laser grooving experiments on silicon workpiece. The influence of the laser parameters, including laser power, laser scan speed and defocus depth on the laser grooving depth was investigated. Grooves were then generated on the silicon workpiece surface with a designed profile shape. Secondly, diamond cutting of laser generated grooves was conducted to investigate the influence of finish cutting depth on the surface quality of grooves produced by this hybrid method. The experimental results indicated that by reducing the finish cutting depth, the surface quality of the machined grooves could be improved. When the finish cutting depth was 4 µm, the surface roughness was 644 nm, and was reduced to 126 nm when the finish cutting depth was 0.2 µm. This preliminary result shows that it is possible to use this method to fabricate silicon lenticular lens mold inserts, however the surface quality still needs to be further improved to fulfill the requirements of a commercial product.
Due to the high thermal conductivity and high purity of OFHC copper, it is an ideal material to use for making high power laser mirrors, which require high quality defect-free surfaces to make sure a high laser damage threshold could be achieved. While the cutting performance of OFHC copper in ultra-precision machining is sensitive to the machining parameters. The machined surface quality of OFHC copper deteriorates rapidly with increasing cutting depth. In this research, the second part of our work was to investigate the influence of laser heating on the cutting performance of OFHC copper in the diamond cutting process. Firstly, the formation of surface delamination, which caused the machined surface roughness of OFHC copper to increase, was investigated by using taper cutting experiments and the finite element method (FEM). It was suspected that the formation of surface defects in OFHC copper was influenced by the stress and strain conditions of the workpiece material in the cutting region during the diamond cutting process. Diamond cutting experiments assisted with laser heating were undertaken to investigate the influence of laser power on the cutting performance of OFHC copper. It was found that the use of laser heating was beneficial for the suppression of the surface delamination of OFHC copper at large depth of cut in the diamond cutting process, and led to an improvement in the quality of the machined surface. This research is helpful in enhancing our understanding on the formation of surface defects of OFHC copper in the diamond cutting process and is also useful in designing suitable experimental parameters in the manufacturing practice in order to achieve good surface quality in the diamond cutting of OFHC copper.
Subjects: Hong Kong Polytechnic University -- Dissertations
Lasers -- Industrial applications
Pages: xvi, 114 pages : color illustrations
Appears in Collections:Thesis

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