PURPOSE: To maintain bending accuracy of a metal sheet at a high level by making the opening angle of the groove of a die smaller than the value obtained when the maximum angle of the spring back quantity estimated to the metal sheet is subtracted from the bending angle given to the metal sheet.
CONSTITUTION: A metallic mold composed of a die 2 provided with a V-shaped groove 20 and punch 3 which bends a metal sheet by indenting the plate 1 into the groove 20 is used. The opening angle θ1 of the groove 20 of the die 2 is set in such a way that, when the sheet 1 is bent at a bending angle θ2, the opening angle θ1 is made smaller than the value obtained by subtracting the maximum value of the spring back quantity Δθ estimated to the sheet 1 from the bending angle θ2. When, for example, the bending angle θ2 and the maximum value of the spring back quantity ΔV are 90° and 3°, respectively, the opening angle 1 of the groove 20 is set to 84° which is smaller then the 87°. Therefore, even when the sheet 1 is indented into the groove until the bending angle becomes 87°, no coining-bend state takes place and bending can be performed to the sheet 1 in an air bending state regardless of the spring back quantity. Thus the bending accuracy can be maintained at a high level.
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TOKUNAGA YOSHINORI
YOSHIDA TAKASHI
