Floating cushion sub

A floating cushion sub for connection to the rotary power head of a drill rig has a mandrel section connectable to a drill string and an outer housing section connectable to the power head. The mandrel and the housing have mating, axially extending splines permitting relative axial movement while also transmitting torque from the power head to the mandrel. Mandrel axial movement is limited by a cap member and by an internal shoulder in the housing. A resilient cushion adjacent the shoulder dampens vibrations when the mandrel and housing are compressed together and a similar cushion adjacent the cap member dampens vibrations when the mandrel and housing are at full extension. The axial movement of the mandrel relative to the housing improves the function of making or breaking the threaded connections with the drill string by permitting proper coordination of the mating thread pitches and reduces the chance of cross-threading.