Since the uncut chip thickness is comparable to the cutting edge radius, in fact, micro cutting is a negative rake angle cutting process. The minimum chip thickness has great effect on the cutting process. This paper presents an experimental investigation of specific cutting energy and surface quality based on the negative effective rake angle in micro turning. A new model is developed to calculate the negative effective rake angle in micro cutting. The effective rake angle is found to be more negative with the decreasing ratio of uncut chip thickness to cutting edge radius. The minimum chip thickness is calculated to be about 0.2-0.3 times of cutting edge radius based on the critical negative rake angle. The turn point of the nonlinear increase of specific cutting energy is observed to be at the critical negative rake angle. The minimum surface roughness also is found to be achieved near the critical negative rake angle.
Aramid fiber-reinforced plastic (AFRP) is widely used in bullet-proof and armor structures, and is difficult to drill because of the high-toughness aramid fibers with ductile fracturing—differently from carbon fiber. Therefore, drilling quality cannot be ensured by the drilling used for carbon fiber-reinforced plastic, and frequently, delamination and burrs occur in the drilling process. This article first established a two-dimensional cutting model for analyzing the fiber deformation and material interface cracking. According to the model, reducing the thrust force and the radial force of the edge on the fibers is an effective way to reduce the fiber deformation, and a three-point step drill is proposed further. Comparative experiments were carried out among twist drilling, candle core drilling and three-point step drilling under three drilling parameters. The results show that the three-point step drill changed the traditional cutting behavior on the drill-exit material into a compound process. Finally, the AFRP was cut effectively with the novel drill with a small thrust force, and the delamination and “burrs area” were reduced through different drilling parameters. In summary, the three-point step drill can drill the AFRP without delamination and burrs with 0.02 mm/rev, which provides a new solution of cost-effective production for AFRP manufacturers.
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