From practical experience in sheet metal processing, the following guidelines apply to bending radii:
- When the sheet thickness is no more than 6mm, the inner bending radius of the sheet metal can be directly set to the same value as the sheet thickness during bending.
- When the sheet thickness is greater than 6mm but less than 12mm, the inner bending radius is generally 1.25 to 1.5 times the sheet thickness.
- When the sheet thickness is no less than 12mm, the inner bending radius is typically 2 to 3 times the sheet thickness.
When the bending radius is specified as R=0.5, the thickness (T) of standard sheet metal is usually 0.5mm. If a bending radius larger or smaller than the sheet thickness is required, specialized dies must be used for processing.
When a sheet metal drawing requires a 90° bend with an extremely small bending radius, the sheet should first undergo grooving (or notching) processing before bending. Alternatively, customized upper and lower dies for the bending machine can be used.
The sheet metal bending radius is also related to the groove width of the lower die in the bending machine. Through extensive experiments in sheet metal processing, a correlation between the lower die groove width and the bending radius has been confirmed. For example:
- When bending a 1.0mm-thick sheet using an 8mm-wide die groove, the ideal resulting bending radius is R1.
- If a 20mm-wide die groove is used instead: Since the upper die moves downward during bending to stretch the sheet to the required angle, the 20mm-wide groove has a larger surface area than the 8mm-wide one. This larger stretching area will increase the final bending radius (R-angle).
Therefore, when a specific bending radius is required (while avoiding damage to the bending machine dies), narrower die grooves should be used whenever possible. Under normal circumstances, it is recommended to follow the standard ratio of 1:8 (sheet thickness to die groove width). The minimum allowable ratio is 1:6 (sheet thickness to die groove width); for sheet metal bending, a slightly smaller ratio may be acceptable, but it must not be less than 1:4.
Recommendation: When structural strength permits, prioritize the method of grooving first, then bending to achieve small sheet metal bending radii.
To summarize, the bending radius in sheet metal processing is determined by both the sheet thickness and the width of the bending die’s lower groove. A simple (though approximate) guideline for reference is:
- For sheets ≤ 6mm thick: Inner bending radius = sheet thickness.
- For sheets 6mm < thickness < 12mm: Inner bending radius = 1.25–1.5 × sheet thickness.
- For sheets ≥ 12mm thick: Inner bending radius = 2–3 × sheet thickness.
