Time dependent buckling and critical speed in a twisted micro drilling process

Abstract

The buckling and the critical speed of a micro drill in a drilling process were investigated in this study. The trend towards higher density printed circuit boards (PCB), requires smaller holes, down to 0.3 mm in diameter, to be drilled through the board. Due to the micro scale, the drilling fracture is frequently found in a micro drilling process. In all drilling fracture cases, fractures caused by buckling are most frequently found, because of the super high spinning speed and the micro scale. To improve the quality, produce a higher production rate and avoid the drill breakage, the buckling load and critical speed in the micro drilling process must be studied. A stepped pre-twisted beam is used to simulate the micro drill. A moving Winkler-Type elastic foundation is used to approximate the drilling process. A time dependent vibration model for drilling is presented. In an actual engineering application, the drill fracture is frequently found in a micro drilling process. In many cases, it would be caused by the time dependent buckling. The investigation on the buckling and critical speed properties in the micro drilling process to depress the time dependent buckling is presented in this work. The effects of rotating speed, pre-twisted angle and thrust force on the buckling and critical speed of the micro drill are considered. The numerical analysis indicates that the buckling load is reduced suddenly as the micro drill moves into a work piece.