The properties and variations of sheet metal can significantly impact the press brake bending process. Sheet metal, which is produced on large rolling mills, undergoes either hot or cold rolling to reach its final thickness.
Hot rolling is usually for thicker sheets, while cold rolling is for thinner sheets due to the high heat loss and difficulty in maintaining a constant temperature in thin material. Cold rolling offers better control of thickness tolerances and also hardens the surface layer.
The rolling process stretches the crystal structure, causing the material to have different mechanical properties along its length compared to its width. This results in the material becoming anisotropic, which affects subsequent processing, potentially leading to variations in bend angle during bending.
In addition to this anisotropic nature, unavoidable variations in material properties occur due to small differences in material composition and rolling conditions. This results in variations in stress and strain curves, not only between different batches of sheet metal, but also within a single batch.
Springback” refers to the rebound of a sheet on either side of the bend after the bending tool has been removed. This occurs because there is a zone close to the center of the sheet’s cross-section with low stress, where only elastic deformation occurs even under large bend forces. As a result, this part of the sheet wants to return to its original shape once the bend force is lifted.
The extent of springback depends on the material of the sheet: the stiffer the material, the greater the springback. For example, soft materials exhibit springback limited to 0.5 degrees or less, while steel can have springback of up to 1 degree. However, springback in stainless steel can be as much as 3 degrees.
The bend angle and bend radius also play a role in determining the amount of springback. The smaller the bend angle and bend radius (i.e. the sharper the tool), the smaller the springback.
For example, a 0.8mm thick steel sheet bent with a bend radius of 1S will have a springback of 0.5 to 1 degree. However, if the same sheet is bent with a bend radius of 77S, the springback can be as much as 30 degrees. This deviation of 1 degree in a leg length of 100mm will result in a spatial deviation of 1.7mm at the end of the sheet. This deviation may exceed acceptable tolerance limits for post-processing activities such as robotic welding.
It is possible to correct for springback while bending a sheet if the influential parameters are known. Benson offers a formula for calculating springback in cold-rolled steel: D = R / (2.1 x S), where R is the radius of the angle in mm and S is the sheet thickness in mm. Applying this formula to a 0.8mm thick steel sheet with a bend radius of 20mm and a bend angle of 90 degrees, the springback value is 11.9 degrees. To calculate springback for other materials, Benson uses a correction factor: 0.5 for copper, 0.75 for hot-rolled steel, and 2.0 for stainless steel.
It is important to note that negative springback can occur under certain air-bending conditions, particularly when using dull tools with a large punch angle, as deformations can occur between the punch and die surface.
However, in the case of coining, high pressing pressure and a sharp top tool can press into the sheet past the neutral zone, resulting in plastic deformation everywhere and reducing springback to virtually zero.
Galling of the bend tool, where particles of material or flakes from the part cling to specific spots on the tool during bending, is a particular concern in the bottom tool’s bend radius.
This issue can cause damage to both the tools and the sheet surface.
To minimize this problem, it’s recommended to choose an optimal bend radius for the V-die (refer to Fig. 7) and to harden the corresponding bend radius.
Surfaces that have been hardened are much less susceptible to galling.
When significant force is applied, it is inevitable that the top and bottom tools will experience deflection along their length.
This causes the top and bottom tools to no longer stay parallel during the bending process, leading to variations in the bend angle along the length of the product (refer to Fig. 8).
This can negatively impact subsequent processes, such as robotic welding.
In the past, this issue was often resolved by using shimming on the bottom tool to create a crown that compensated for the deflection.
Nowadays, computer-controlled or centrally adjustable crowning systems can quickly and precisely compensate for deflection across the entire machine length.