An improved power drill having means for sensing when the drill bit has penetrated through the workpiece and for then retracting the drill bit from the workpiece. An electrical signal responsive to the axial force between the drill and the drill bit is used to sense when the drill bit pierces the workpiece. A sudden decrease in the axial force triggers a mechanism for retracting the drill bit from the workpiece. Means are included to detect broken bits, missing bits and previously drilled holes.