# Calculating Bending Die Springback: Expert Tips

When designing a bending die with an internal arc, many people either choose to use the same R value as the original product and do not consider the springback, or directly reduce the R value by a certain factor.

For example, if the original product has an R value of 1 and the material is relatively hard, they would choose 0.8 times the R value for the convex mold, which would be 0.8.

If the material is relatively soft, they would choose 0.9 times the R value, which would be 0.9.

If there is any deviation, they modify the mold several times based on experience to achieve tolerance within the range.

However, if this method is used to design a product with a thickness of 0.5 and an internal R value of 200mm, it may be difficult to accurately predict the amount of springback.

Therefore, a universal formula for springback is introduced below, which can be used to calculate the springback value based on numerical input.

In the formula:

r – workpiece fillet radius (mm):
a – the central angle of the arc length of the workpiece fillet;
a1 – the central angle of the arc length of the punch fillet;
t – material thickness;
E – elastic modulus of material;
σs – yield point of the material.

Assuming 3σs/E=A as the simplification coefficient, with values listed in Table 2-27. The calculation formula for the convex die corner radius during the bending of circular section bars is as follows:

The value of A is shown in the table below.

If the necessary materials are not available above, you can also refer to the table below to find the modulus of elasticity and yield strength of the material, and then substitute them into the formula above for calculation.

It is best to establish a commonly used material database and obtain missing physical parameters from suppliers. If the parameters for elastic modulus and yield strength are correct, the bending and rebound of general spring terminals, appearance parts, and profiles are more precise.

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