Abstract: A rotary drive member such as a drill string, has a housing rotatably mounted at its lower end and an axial-throw cam is fixed at the lower end of the drive member in the housing. Chisel blades are axially slidable in the housing and are reciprocated by the cam in such manner that two dimetrically opposed blades are lifted and then forcibly driven downward while other chisel blades are left in contact with the well bottom to intermittently hold the housing against rotation. The chisel blades are sharpened in such a way that they tend to rotate the housing through a small angle at each downward impact.