The invention includes a stop (33) which prevents movement of the part (23) of the protection device (18) into the non-operating position when the cutting mechanism (2) is located in the work position (A), and which allows movement when the cutting mechanism (2) is located in a different position (B), the activation of the stop (33) being accomplished automatically by means of an activation element (35) when the cutting mechanism (2) is brought from the other position (B) into the work position (A). The invention also includes a limiting element (52) for limiting the amount of pivoting between a support arm (5) and the cutting mechanism (2) in work position (A) and preventing pivoting when the cutting mechanism (2) is in the other position (B). The invention also includes a signalling plate (50) and the activation elements (47)(35) to move the signalling plate (50) to an operative position when the cutting mechanism (2) is moved to position (B) and to an inoperative position when the cutting mechanism is in work position (A).