A press-brake V-opening is not selected from thickness alone. The familiar thickness ratio provides a starting point, but the final choice also controls air-bend force, achieved inside radius, minimum flange, marking, and the flat pattern.
For air bending steel, a common starting range is V = 6T to 8T.
The outside flange generally must extend a little more than half the V-opening so the workpiece remains supported; confirm the exact minimum with the tooling supplier.
V-opening starting ratios
For conventional 90° air bending of steel, WILA describes 6–8 times thickness as a general rule of thumb. This is a range, not a universal table. Thin material, thick plate, special alloys, acute bends, large radii, and dedicated tooling can require different ratios.
| Thickness | 6T opening | 8T opening |
|---|---|---|
| 1.0 mm | 6 mm | 8 mm |
| 2.0 mm | 12 mm | 16 mm |
| 3.0 mm | 18 mm | 24 mm |
| 0.125 in | 0.750 in | 1.000 in |
Use the closest available die only after checking its angle, shoulder radii, maximum load, and compatibility with the intended punch and part.
How V-opening changes radius and force
In air bending, the die opening largely determines the formed inside radius. A wider opening usually increases inside radius and reduces tonnage; a narrower opening usually reduces radius but raises tonnage and marking risk. Changing V therefore changes bend allowance or deduction as well as force.
Minimum flange support
WILA states that a standard-tool flange should be greater than half the V-opening so the workpiece does not fall into the groove, and separately recommends at least six material thicknesses as a practical regular-tool starting point. Tool shoulder radius, angle, and part geometry affect the exact limit.
For example, a 16 mm opening has a half-width of 8 mm. A nominal 8 mm outside flange sits at the geometric threshold, so “a little more than half” and the specific tool geometry must be respected rather than rounding down.
V-die selection checklist
- Confirm bending method: air bend, bottoming, or coining.
- Use actual thickness and material tensile properties.
- Check the target and achieved inside radius.
- Verify minimum flange, holes near the bend, and collision clearance.
- Calculate force and compare with every component’s load rating.
- Update the flat pattern if the V-opening or radius changes.
- Use simulation or supplier tooling advice for critical bends.
Technical references
Published and formula-checked by MfgWorkbench. Last reviewed September 15, 2026. See our editorial method.
Scope the ratio before extending the chart
WILA’s V-opening article qualifies its 6T–8T steel air-bending guidance for thicknesses up to about 3 mm. The ratio is not a universal rule for thicker plate, other alloys or other bending methods. In particular, 0.125 in equals 3.175 mm: the earlier inch row is ratio arithmetic slightly beyond that stated thickness range, not a supplier-approved selection.
| Steel thickness | 6T | 8T |
|---|---|---|
| 0.8 mm | 4.8 mm | 6.4 mm |
| 1.5 mm | 9 mm | 12 mm |
| 2.5 mm | 15 mm | 20 mm |
Why half the opening is only a geometric warning
For V = 16 mm, V/2 = 8 mm marks a geometric threshold, not a safe minimum flange specification. Actual support depends on bend angle, die shoulders and the tool manufacturer’s flange convention. Use the specific tool drawing or supplier selection tool; see WILA’s short-flange guidance.
Finally compare force and achieved radius for the available openings. A wider die may reduce load while changing the flat pattern and flange support. Recheck the tonnage estimate and part geometry together instead of selecting whichever die gives the lowest force.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.