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.