Determining the Minimum Bending Radius

Bending Process Description

The bending process involves changing the shape of a semi-finished product (such as a sheet) around a straight edge. During bending, the metal layers inside the bending angle are compressed and shortened in the longitudinal direction, while they extend in the transverse direction. The outer layers of the bend are stretched and lengthened in the longitudinal direction, while they are compressed in the transverse direction.

Minimum Bending Radius

The minimum bending radius refers to the smallest permissible bending radius inside the part. The size of the minimum radius is determined by the material's resistance to tension on the outer fibers of the semi-finished product. Reducing the minimum radius leads to tension in the material that can cause cracking or even breaking of the material on the outer surface of the part.

Factors Affecting the Minimum Bending Radius:

Physical and Mechanical Properties of the Material

Position of the Bending Line Relative to the Rolling Direction

Bending Angle

Condition of the Edges of the Semi-finished Product (presence or absence of cracks and burrs)

In practical cases, the minimum bending radius for 180° or 90° bends is determined using the relationship derived from experimental data:

rmin​=k⋅t

Where:

• $k$k is a coefficient that depends on the nature and condition of the processed material and the position of the bending lines relative to the rolling fiber direction. It is obtained experimentally.

• $t$t is the material thickness.

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Tables typically present real values of $k$k for various aluminum alloys, depending on:

• Alloy condition

• Sheet thickness

• Tensile strength

• Yield strength

• Brinell hardness

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Improving the Bending Properties of Metals:

1. Applying a Compressing Force: In the sheet plane, to minimize tensile stresses on the outer fibers.

2. Heating the Area to be Bent: This technique helps in improving the material's ductility.

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Factors Reducing the Bending Properties of Metals:

• Increased Edge Roughness: Rough edges act as stress concentrators, increasing the risk of cracking during bending.

• Rolling Direction Relative to Bending Direction: This strongly affects the minimum permissible bending radius. Bending along the rolling direction usually requires a larger radius than bending in the transverse direction.

• Burrs and Their Position Relative to the Bending Line: Burrs can influence material cracking during bending. Therefore, semi-finished products should be positioned with burrs facing inward toward the bending angle, and large burrs are generally not acceptable.

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Example of Determining the Minimum Bending Radius:

For EN 485-2 ALUMINIUM EN AW 1050A, with a bending angle of 90° and H24 condition, and a material thickness of 2 mm:

rmin​=1.0×2=2mm

This material can be bent with a bending device with a minimum radius of 2 mm.

For H14 condition with a material thickness between 0.2 and 0.5 mm:

rmin​=0×2=0

The material has a sharp edge in this case.

For thicknesses greater than 12.5 mm, such as 20 mm, it is not possible to bend the material according to the EN 485-2 standard.

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