How are punch radius and die opening related?
In air bending they are locked together by the geometry of the bend. The inner radius the sheet takes is roughly 0.16 times the die opening, and the punch tip radius should be at or below that value, so the sheet forms to the radius the opening allows rather than to the punch.
What radius does each opening produce?
About 0.16 times the opening in mild steel air bending. That single ratio is the most useful number on the shop floor, because it turns a die choice into a radius prediction.
| V opening (mm) | Inner radius ≈ 0.16 × V (mm) |
|---|---|
| 8 | 1.3 |
| 12 | 1.9 |
| 16 | 2.6 |
| 20 | 3.2 |
| 25 | 4.0 |
| 32 | 5.1 |
| 40 | 6.4 |
| 50 | 8.0 |
What happens if the punch radius is too large?
The punch tip supports the sheet over a wider span, so the bend forms around the punch instead of spanning the die. The radius ends up larger than the opening predicts, the tonnage rises and springback grows. The bend looks fine and measures wrong.
- Radius too large: radius follows the punch, force up, springback up.
- Radius too small: the tip concentrates load and can mark or cut the sheet; check the tip load rating.
- Sharp tip in a narrow opening: the classic coining set-up, with the tonnage to match.
How do I match them for a drawing radius?
Work backwards. The drawing fixes the inner radius, the radius fixes the opening at about six times the radius, and the opening fixes the minimum flange. If the resulting flange does not fit the part, the radius has to be negotiated, not the tooling.
Related technical pages
V-die opening chart
Openings against radius, minimum flange and tonnage in one table.
V-die opening calculator
The opening for your thickness, with the radius it will produce.
Radius dies
Bottom dies for deliberate large radii.
Minimum bend radius
The smallest radius each material and thickness accepts.
Next step
Send the drawing radius and material and we will propose the opening and punch profile that delivers it.
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