Abstract: A casing hammer assembly for a cable tool drilling apparatus and a cable tool drilling apparatus employing the casing hammer assembly. The casing hammer assembly includes a hammer housing which is positioned on a casing pipe and having a central aperture for through passage of a cable tool drilling string. An impact anvil is located in the housing, surrounding the central aperture, and a reciprocal hammer is mounted for striking the anvil. An eccentric arrangement is employed for raising the hammer for repeated striking of the anvil in order to sink the casing pipe. A hydraulic arm is utilized for laterally positioning the hammer housing in relation to the casing pipe.