Abstract: There is disclosed herein an apparatus for automatic monitoring of numerous high speed drill spindles on a numerically controlled drilling machine for purposes of immediate detection of the absence of drilling by a particular spindle because of a broken drill bit. The apparatus detects the absence of drilling, interrupts the automatic cycle of the drilling machine and alerts the operator. Power dissipation in each drill spindle motor is compared just prior to entry of the drill bit into the workpiece and after entry if any. If power dissipation increases above a predetermined threshhold level for all spindles, no interrupt is generated. If power dissipation does not increase in one or more spindle motors, comparator circuits alert a digital processor which interrupts automatic operation of the drilling machine and alerts the operator. Such a system eliminates the expense and error of human visual monitoring and generation of expensive scrap improperly processed before discovery of the broken bit.