Abstract
Drill breakage is one of the major problems in micro drilling and in-situ breakage prediction can improve production reliability remarkably. The authors have already reported an in-process detection of small cutting torque by the new torque sensor with a thin-walled elastic cylinder. In this paper, a feed control drilling according to an overload signal from this torque sensor is described. In order to prevent the drill breakage caused by the overrun of a drill feed stage, it is necessary to stop the drill feed quickly enough according to the overload signal. A quick stop mechanism, which consists of z stage for the drill feed motion by regular NC control and z' stage for a quick retreat motion, was developed. The advancing z stage can not be stopped quickly due to electric delays of feed control and the mechanical inertia. The quick retreat motion of z' stage according to overload signal can cancel this time delay of z stage. The automatic canceling motion can prevent drill breakage during the drilling operation. Through a few drilling experiments of multi-layered printed circuit boards, the availability of the developed quick stop mechanism in automatic feed control was verified.
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