| Summary: | 碩士 === 國立臺灣大學 === 機械工程學研究所 === 102 === Silicon carbide (SiC) is an advanced ceramic material with great mechanical properties, including high hardness, endurance of high temperature and chemical stability. Therefore, SiC often uses in extreme condition. But its great mechanical properties lead to difficulties of machining. The first purpose of this study is to investigate the material removal mechanism of ultrasonic-assisted machining (UAM) of SiC and understand what role is ultrasonic vibration. And second one is to discuss influence of various machining parameters. Last one is to bring up a strategy for better material removal rate (MRR) and workpiece quality.
In this study, the ultrasonic spindle provides longitudinal vibration which is automatically resonating with tools and in the Z-axis direction of machine tool. According to studying results, abrasives path is wavy in drilling process due to the vibration of UAM. Because of the interactive influence between abrasives, the relationship between abrasives and material becomes part separation and part contact in any moment of vibration period. Vibration also generates many cusps and tiny cavities. When abrasives contact with material, the local temperature of cusps increases which decreases material’s local compressive strength; at the same time, vibration continuously generates cusps. This two process revolve repeatedly so the cutting force of UAM decreases significantly. According to studying result, drilling small holes in SiC with UAM could get many good effect, including reducing cutting force, increasing MRR, less defects. High spindle speed and high amplitude get better effect when using ψ1 diamond tool. In ψ1 hole drilling with 0.1 μm/rev feed, the thrust force decreases from 20 to 2 N with UAM, only 10% of original value; chipping in orifice also decreases from 139.5 μm to no chipping. UAM can increase feed from 0.1 to 0.3 μm/rev without tool breakage, so the MRR also increases to 3 times with only few defects. UAM with high feed (0.3 μm/rev) would cause small defects about 10 μm at the center of bottom while UAM with low feed (0.1 μm/rev) would get no defects. If the machining process starts with high feed rate and transfers to low feed rate in the end, there will be good MRR and also workpice quality.
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