Abstract: A hydraulic drilling machine for cutting into rock formations and the like. The machine has a rotating drill rod with a bit at one end and a rotary hydraulic drive motor at the other end. The drill rod is supported on a carriage adapted for linear travel to advance the rotating bit into the rock formation. The carriage is extended and retracted by means of a hydraulic feed motor. The rotary hydraulic motor and the hydraulic feed motor are connected in series in the hydraulic operating system so that when rotation resistance increases accompanied by a greater pressure drop across the rotary hydraulic motor, there is a consequent decrease in the force exerted by the hydraulic feed motor in order to avoid jamming of the drill bit.