Abstract: The device employs a number of detectors (1) mounted at the level of the upper surface of the die (2) and a numerical control system (5). The purpose of the detectors is to signal to the numerical control the moment that the part b of the sheet begins to rise. The device allows the automatic determination of the thickness of the sheet in the area where the bend is to be located, during the course of the forming operation. The thickness of the sheet is then used by a numerical control system to correct the travel of the tool, without interrupting the forming cycle, in such a way as to obtain with high precision the required bend angle.