Identification and control for micro-drilling productivity enhancement

Micro-hole drilling (holes less than 0.5 mm in diameter with aspect ratios larger than 10) is gaining increased attention in a wide spectrum of precis ion production industries. Alternative methods such as EDM, laser drilling, etc. can sometimes replace mechanical micro-hole drilling, but are not acc eptable in PCB manufacture because they yield inferior hole quality and acc uracy. The major difficulties in microhole drilling are related to the wand ering motions during the inlet stage, high aspect ratios, high temperature, etc. However, of all the difficulties, the most undesirable ones are the i ncreases in drilling force and torque as the drill penetrates deeper into t he hole. This is mainly caused by chip-related effects. Peck-drilling is th us widely used for deep hole drilling despite the fact that it leads to low productivity. Therefore, in this paper, a method for cutting force regulat ion is proposed to achieve continuous drilling. A proportional plus derivat ive (PD) and a sliding mode control algorithm will be implemented and compa red for controlling the spindle rotational frequency. Experimental results will show that sliding mode control reduces the nominal torque and cutting force and their variations better than PD control, resulting in a number of advantages, such as an increase in,drill life, fast stabilization of the w andering motion, and precise positioning of the holes. (C) 1999 Elsevier Sc ience Ltd. All rights reserved.