Abstract: An underreamer, hole opener, or expansible rotary drill bit including a body having a bore therethrough, an arm mounted at its upper end to the body, a conical cutter mounted for rotation on the lower end of each arm, a cup-shape thrust bearing between the cutter and the arm, a bolt through the arm to secure the cutter on the arm and means including a ball and groove between the bolt and the cutter to lock the bolt to the cutter. In the underreamer form of the present invention, the arms are pivotally mounted in slots in the body and means is provided for moving their arms to their cutting position with their lower ends positioned radially outward from the body.