High speed boring of deep holes with conventional boring bars is difficult due to the low specific stiffness (E/ρg) and low damping of conventional boring bar materials. Since the stability of metal cutting of a boring bar or a reaming bar is proportional to the static stiffness and damping, the structure of the boring bar or reaming bar should have high static stiffness and high damping. In addition, the high specific stiffness is beneficial because the natural frequency of bars is proportional to the square root of the specific stiffness of bar material. Recently tungsten carbide alloy has been used for a boring bar or a reaming bar substituting conventional steel for the machining of deep holes because the static stiffness of tungsten carbide alloy is 2.2 times higher than that of steel. However, the maximum rotating speed of the tungsten carbide boring bar cannot be improved much because the specific stiffness of tungsten carbide alloy is only 1.5 times higher than that of steel.
In this study, a very high modulus carbon fiber/epoxy composite whose specific stiffness and damping are 10 times higher than those of tungsten carbide alloy was used for boring bar material in order to improve dynamic characteristics such as natural frequency and damping and to reduce chatter problem. The effects of the core shape and material, steel cover thickness and the adhesive thickness between the composite material and the steel cover on the dynamic performance of the composite boring bar were investigated experimentally. From the experimental results, design parameters of the composite boring bar were determined. Then the dynamic characteristics and metal cutting ability of the developed composite boring bar were investigated from vibration test and metal cutting test of aluminum specimens. From the experiments, it was found that the chatter was not initiated up to the ratio of length to diameter of 10.7 at the rotating speed of 2,500 rpm with the feed rate of 0.04 mm/rev and the depth of cut of 0.25 mm.
