The sheet metal bending radius is a critical value in sheet metal drawing that can be difficult to determine during the actual processing. This radius is dependent on the material thickness, the pressure of the press brake machine, and the width of the bending die’s lower die groove.

A simple and rough method to determine the bending radius is:

- If the plate thickness is less than 6mm, the bending radius can be equal to the plate thickness.
- If the plate thickness is between 6mm and 12mm, the bending radius is typically 1.25 to 1.5 times the plate thickness.
- If the plate thickness is greater than or equal to 12mm, the bending radius is typically 2 to 3 times the plate thickness.

Experience in actual sheet metal processing shows that when the plate thickness is generally no more than 6mm, the inner radius of sheet metal bending can directly use the plate thickness as the radius. When the bending radius is r = 0.5, the general sheet metal thickness t is equal to 0.5mm.

If a bending radius different from the plate thickness is required, a special die must be used for processing. When the sheet metal drawing calls for a 90-degree bend with a particularly small bending radius, the sheet metal should first be grooved and then bent. Special press brake tooling, such as punches and dies, can also be used.

The relationship between the bending radius of sheet metal and the width of the lower die groove of the bending die has been established through numerous experiments in sheet metal processing.

For example, when a 1.0mm plate is bent with an 8mm groove width, the ideal bending radius is R1. If the groove width is increased to 20mm, the depth of the stretched plate increases, resulting in a larger tensile area and a larger R angle. To avoid damaging the press brake die and to maintain the desired bending radius, it is recommended to bend with a narrow groove, following the standard ratio of 1:8 between plate thickness and groove width. The minimum recommended ratio is 1:6 and bending with a ratio of less than 1:4 is not recommended.

Suggestion: If the strength allows, it is preferable to groove first and then bend in order to achieve a small sheet metal bending radius. The following figure is a table provided by the press brake manufacturer, which shows the corresponding relationship between bending radius, pressure, and minimum bending height.

- V – bend notch width
- R – bend radius
- B – minimum bending height
- S – sheet thickness

Note: The data with gray scale in the table represents the required bending pressure P (KN/m), and the maximum bending force of the press brake machine is 1700KN. There are five available bending knife edges: V = 12, 16, 25, 40, and 50.

Please refer to your available knife edge and bending length to determine the bending radius, which will help you calculate the accurate length of the material to be unfolded.

The above information pertains to the pressure parameters and bending die width of a single press brake. The actual calculations should be based on the pressure and bending die of your own sheet metal processing facility.