40%下降到0。This paper studied the on-line monitor of micro-hole drilling based on electrical current of the principal axis electromotor. Taking 50 high speed steel twist drills with the diameter of 0.5 mm as the samples, the high-point voltage value corresponding to electrical current of the principal axis electromotor was monitored when the micro-drill was broken during drilling H62 brass. Through parameters estimating and tentative proving, the results that the high-point voltage value obeys three parameters Weibull distribution were gained, and the access method of the monitored voltage thresholds corresponding to electrical current of the principal axis electromotor was built up. Finally, the experiments of on-line monitoring were carried out with different reliable levels. The results showed that the drill breaking ratio of decreased from 40% to 0 when reliable level was selected to be 0.99 and the drill utilization ratio was the same.  