When a metal plate is bent, its inner side is compressed and its outer side is stretched. The compression of the inner side gradually decreases from the inside to the outside, and the tension of the outer side gradually decreases from the outside to the inside. Near the center of thickness, compression and tension are close to zero, and this plane in the middle of thickness is called neutral layer. In the following, the neutral layer is used as a reference for theoretical calculation of unfolded materials.

When bending, the radius of internal arc R ≥5t (t is the material thickness).

When the radius of the inner arc is greater than or equal to 5 times the thickness of the material, there is no thickness change at the bending part of the material, that is, the neutral layer is on the center line of the material thickness after bending.

B is the distance from the neutral layer to the inner wall of the plate, A is the bending angle T is the plate thickness, and K is the bending coefficient. K=b/T, and k is the bending coefficient of the neutral layer. When the material is bent, it is deformed, the outer material is stretched, the inner material is compressed, and the length of the neutral layer remains unchanged. The material with high hardness has small tensile deformation, the neutral layer is on the outside, the material with low hardness has large tensile deformation, and the neutral layer is on the inside. The neutral layer of ordinary materials tends to be centered. The developed length of the material is the arc length of the neutral layer. It is related to several parameters such as bending radius, bending angle, plate thickness and neutral layer coefficient.

As shown in the figure, the expanded length is: DL=Pi*(R+K*T)*a/ 180.

PROE also uses y factor to calculate the expansion length, y = pi/2 * k.

The formula becomes: DL=(Pi/2*R+Y*T)*a/90.

If there is no special bending table, PROE uses this formula to calculate the unfolded length. Therefore, when we start sheet metal production, we should first

Define the value of k or y, where the value of y is 0.5 and the value of k is 0.3 18, which is equivalent to low carbon steel and copper. If ordinary steel plate is used, it can be set.

Let k be 0.45, that is, y be 0.707.