| Summary: | 博士 === 國立臺灣大學 === 機械工程學研究所 === 88 === The study of Wire Saw Machining
Abstract
Wire saw is a machining method that uses free abrasives to cut semiconductor, crystal, various single crystals, oxide semiconductor, magnetic material and other brittle materials. It increases the volume removal rate and reduces defects. Thus, it is a method that can effectively reduce production cost. To understand the behavior of volume removal by single abrasive particle, a theoretical model is first was derived in this paper to explore factors that affect the removal of brittle materials by free abrasives. Next, the Taguchi experimental design to investigate the effect of wire-saw machining parameters on machining characteristics. To improve efficiency, wire-saw machining behaviors with and without vibration are compared.
The mathematical model of brittle material volume removal by single abrasive particle was derived in this paper including brittle fracture and plastic formation. For the brittle fracture, the strain energy density was used as the fracture criterion, and the governing equation of the cutting path of the abrasive particles was formulated. On the other hand, the material removal due to plastic deformation was obtained based on the equation of motion. Actual wire-saw cutting experiments were conducted. Theoretical analysis agrees satisfactorily with experimental results. Based on the derived model, it is found that brittle fracture plays a significant role on the material removal; both wire speed and working load are positively correlated with material removal. When the grain size of the abrasive is too small, there is no material removal irrespective to both wire speed and working load applied to the wire. Under a condition of small coefficient of friction, the lateral crack propagates nearly parallel to the work surface. This in turn leads to more brittle fracture, and as a result material removed is increased.
Taguchi experimental design was used, in conjunction with the theoretical analysis of variance, to understand the effect of various machining parameters on the machining characteristics. The machining parameters discussed in the paper include the following: wire tension, slurry concentration, mixture slurry particles, wire speed and working load. The machining characteristics studied include material removal rate, machined surface roughness of wafer, kerf width, wire wear and flatness. The analysis results indicate that the addition of small abrasive particles slightly improved the machined surface roughness of wafer, while the other machining parameters did not show any significant negative effect. The machining parameters did not have any significant effect on flatness. The addition of small abrasive particles and a smaller slurry concentration resulted in a smaller kerf width. The presence of fine abrasive particles only (e.g. #1000 or even smaller) or that of large particles only (e.g. #300 or even larger) in the slurry led to poor machining efficiency. The slurry concentration of 40%-55% is recommended for the machining model without vibration, which helps to improve the material removal rate.
The comparison between wire saw behaviors with and without vibration shows that the former has a higher machining efficiency than the latter. It is recommended that machining with vibration be used together with 20%-35% of slurry concentration, vibration frequency of 0.9 cyc/s - 1.4 cyc/s, vibration angle of 400 - 600. Such a combination increases the machining efficiency and has a volume removal rate about 37.5% higher than that in the case without vibration. As for the characteristics of machined surface roughness of wafer, planarity, kerf width and wire wear, no significant difference was found between the machining methods with and without vibration.
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