A known bending machine such as press brakes controls the driving amount of a movable die with an NC device according to various kinds of input information to bend a sheet-like workpiece. However, such die driving amount control cannot always avoid errors in bend angle because of variations in the thickness and properties of workpieces to be bent and in processing conditions. One attempt to solve this problem is designed such that the bend angle of the workpiece is measured during the bending process and the driving amount for the die is corrected based on the in-process measurements thereby to ensure high bending accuracy.
When forming a workpiece into particular shapes, bending machines equipped with such a bend angle measuring means are unable to perform bend angle measurement or encounters difficulties in measurement. In these cases, the bending machines fail in eliminating errors in bend angle.
With a view to overcoming the foregoing drawback, the applicant of the present invention has proposed an automatic die driving amount correction method in Japanese Patent Unexamined Publication No. 7-314043 (1995), according to which a correction value for a die driving amount with respect to a first bend is obtained based on measurements of the bend angle thereof and a correction value for a die driving amount with respect to a second bend is calculated from the correction value for the first bend. This correction method makes it possible to ensure high bending accuracy even if the workpiece has bends the angles of which are difficult or impossible to measure and eliminates the need for conducting angle measurement each time when a bend is produced so that significant savings in processing time can be achieved.
The above automatic die driving amount correction method reveals a problem, that is, its inapplicability to some bends if bending order is not appropriately determined when producing a plurality of bends in a workpiece. In addition, since the bending accuracy of the second bend varies depending on the degree of the correlation between the first and second bends in terms of their processing factors, the highest possible accuracy cannot be achieved with some bending orders.