The thrust exerted on a drill bit (14) is sensed and employed to determine the speed at which the bit is driven. A controller (26) for the drill (10) compares the thrust to a predetermined threshold value and in response to the comparison emits a control signal which shifts a trans­mission (22) in the drill into high or low speed. After each shift, a number of thrust samples are taken in order to compute an average thrust for different strata (A, B, C) of the material (W) being drilled. At the completion of drilling each hole, the average thrusts for the hole are used to recalibrate the shift threshold value. In this manner, the shift threshold is adjusted to compen­sate for drill bit wear. The sensed thrust is also compared to a bit reject threshold value to determine when the bit is too dull for proper drilling.