A non-planar tool path for spring back compensation in incremental sheet forming

Spring back (SB) in incremental sheet forming (ISF) occurs in three stages. SB1 (spring back 1) occurs in stage 1 when the tool is moving along its path during forming. SB2 occurs in stage 2 due to removing the clamps from the formed part. If the formed part is trimmed in stage 3, SB3 also occurs. SB2 and SB3 are not important when the formed part is to be mounted on a stiff structure, and in this paper, it is focused on SB1 only. Compensation for SB1 is carried out by applying a correction to the tool path so that the formed part after SB1 is close to the designed target geometry. Conventional “planar” tool path is found from intersections of closely spaced horizontal planes with the target geometry, and considering the tool nose radius. The tool moves in horizontal loops keeping the same forming depth (Z) in each loop. In this paper a new approach to tool path design and compensation for SB1 is presented, in which the tool path projection on the plane of the unformed sheet (XY plane) is created. Z position of the tool is found such that the tool is made to move tangent to the target. After forming and spring back, a new surface referred to as “forming surface”, is constructed based on the deviation of the formed geometry from the target. Tool path projection on the XY plane is not changed, and the compensated tool Z position is found such that the tool is made to move tangent to this new forming surface. The new non-planar tool path has a variable forming depth (Z) when it moves in each loop. An approach to accelerate conversion of the tool path correction is also presented. The effectiveness and efficiency of this new approach to tool path creation, compensation for spring back and conversion acceleration have been demonstrated through finite element analyses.