Hydraulic bending machine

In a hydraulic bending machine, comprising a bending means to abut against a workpiece to apply a bending force thereto;

• a piston-cylinder hydraulic driving means provided with a reciprocating piston coupled with the bending means and a hydraulic cylinder containing the piston and having two chambers defined by the head of the piston;
• and guide means to hold the workpiece between itself and the bending means;
• the improvement wherein:
• the hydraulic driving means includes biasing means for biasing the piston to a retracted position;
• and wherein the bending machine further comprises:
• an oil hydraulic circuit means including an oil reservoir, a pump, a motor for operating the pump and feeding pressure oil in the oil reservoir into one of the chambers of the cylinder of said piston-cylinder driving means to protrude the piston, a switch for the motor, and a directional control valve coupled to said pump and to said cylinder and having a first port to allow the pressure oil to pass from the pump to one chamber of said cylinder and a second port to allow the pressure oil, forced out of one chamber by the biasing means and returning to the oil reservoir, to pass;
• a rockable change lever coupled to said motor and directional control valve for switching on the motor and shifting the directional control valve so as to let the oil flow through the first port when rocked in one direction as well as for switching off the motor and shifting the directional control valve so as to let the oil flow through the second port when rocked in the opposite direction;
• and
• automatic piston return means including a rod to protrude integrally with the piston according to the protrusion of the piston and a stopper arranged to abut against the change lever to rock the change lever in the opposite direction when the piston attached to the rod has protruded through a prescribed stroke.