2. Proposed method
When sheet metal is blanked by laser cutting, residual tensile stress near the laser cut surface is generated.
The generated compressive stress and residual tensile stresses are well balanced in the sheet metal as shown in Fig. 1(a).
When the balance of the residual stress in the flange surface is disturbed by bending, the “ship-type” warp occurs as shown in Fig. 1(b).
Therefore, it is essential to reduce or control the residual stress occurring near the laser cutting surface for attaining the dimensional accuracy after bending the sheet metal.
Tension leveller and stretch levelling are known as methods of reducing the residual stress. However, as the equipment is extensive, there are problems in the cost and productivity.
Hence, simple methods are demanded for the industrial use.
The proposed method is schematically shown in Fig. 2.
The light press method is concise and its productivity is very high.
In the proposed method, the sheet metal edge, which has been cut by laser, should be pressed by a slight amount in the thickness direction.
The mechanism of reducing the residual stress is explained by the relationship between the shrinkage by laser cutting and the longitudinal expansion by light press method at the edge of the sheet metal.
The cut edge of the sheet metal shrinks at the end of laser cutting process due to rapid cooling.
On the other hand, the rest part does not shrink and constraints the shrinkage of the edge, resulting in the generation of tensile stress at the edge as shown in Fig. 2(a).
The slight thickness reduction elongates the sheet metal in width direction due to volumetric constancy, leading to relaxation of the tensile stress caused by laser cutting as shown in Fig. 2(b).
This method is utilizing plastic deformation in the thickness direction.
Experiments and FEM analysis were carried out to determine the proper conditions of press.