a bending die or matrix mounted around a spindle having an axis y,

a bend arm able to turn around on the same axis y, and

co-operating counteracting means for clamping a section of workpiece to be bent in a start position of the matrix and the bend arm, a workpiece bending operation being performed for a workpiece section corresponding to an angle of rotation of the bend arm and the matrix around the axis y by a driving shaft, the driving shaft having a polygonal portion adapted to engage a corresponding seat, wherein

said bend arm carries on itself a controlled return means able to return said matrix to its start position at the end of said workpiece bending operation,

wherein said controlled return means comprises:

a single-acting cylinder fed with fluid being fixed on said bend arm and having a piston rod with a free end;

a section of flexible transmission having one end being connected to the free end of said piston rod, and the other end being attached to an elastic stretching member comprising a traction resistant helical spring in one end thereof, the other end of the elastic stretching member being fixed to a portion rigidly connected to said bend arm, the elastic stretching member counteracting the piston rod of the single-acting cylinder, said section of flexible transmission being a link chain;

a power transmission element that is a ring gear connected to the matrix and co-operating with said section of flexible transmission, the power transmission element being able to transform a traction movement applied to said section of flexible transmission into a rotation movement of the matrix around said axis y in a direction opposite the rotation that has been caused by said driving shaft to bend the workpiece, in order to return said matrix to a start position,

and wherein return of the bending die or matrix to the start position is performed only after a bent workpiece has been removed from the bending machine.