In one aspect of the present invention a tool string comprises a jack element substantially coaxial with an axis of rotation. The jack element is housed within a bore of the tool string and has a distal end extending beyond a working face of the tool string. A rotary valve is disposed within the bore of the tool string. The rotary valve has a first disc attached to a driving mechanism and a second disc axially aligned with and contacting the first disc along a flat surface. As the discs rotate relative to one another at least one port formed in the first disc aligns with another port in the second disc. Fluid passed through the ports is adapted to displace an element in mechanical communication with the jack element.