V-Die Opening Calculator

Air-bending die selection: recommended V opening from the rule-of-8 family (multiplier × thickness), the inside radius the die will float (≈ % of V), the minimum flange it can form (0.77 × V), and the V/t safety check. Metric and imperial. Free, no signup.

Sheet Metal 5 inputs 4 results

Inputs

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Mode

Material thickness (t)

Opening multiplier (m)

× t

Existing die opening (V)

Radius factor

% of V

Results

Default result

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Die opening(V)

24mm

Pass

Round to the nearest die actually in the rack, then re-check here.

Recommended (or echoed) vee width — round to the nearest die in the rack.

Also computed

Air-formed inside radius(IR)3.84mm

Use this radius — not the punch radius — in the bend allowance calculation.

The radius the part floats — the die, not the punch, sets it in air bending.

Minimum flange18.5mm

0.77 × V working rule; the geometric floor is just over half the opening.

V / t ratio8

Inside the classic 6–12 working band.

Below 5 is a tooling-damage risk; the classic band is 6–12.

Method notes 4 notes

Air-bending selection rules (die-maker and trade-press guidance): V = m·t with m ≈ 8 for mild steel to ~3 mm (6 soft aluminum, 10–12 stainless/thicker, 12–15 high-tensile); the part floats an inside radius ≈ the radius factor × V; the leg must reach ≈ 0.77·V so it cannot fall into the vee.
At the classic m = 8 and 16%, the inside radius comes out ≈ 1.3·t — the familiar "radius about one thickness" result for mild steel.
Bottoming and coining follow the punch, not these rules; and the die maker's chart governs where it disagrees.
Feed the inside radius into the bend allowance calculator and the opening into the press-brake tonnage calculator — the three screens share the same V.

In air bending the die does the forming: the sheet bridges the vee and floats a radius set by the opening, so picking V fixes the force, the radius and the smallest formable flange at once. The trade rules run V = m·t (the rule of 8 for mild steel to ~3 mm; ~6 for soft aluminum; 10–12 stainless/thicker; 12–15 high-tensile), inside radius ≈ 15–22% of V by material, and minimum flange ≈ 0.77·V. This calculator runs those rules in both directions — recommend a die from thickness, or check the die already in the rack — and flags V/t below the 5× safety floor.

How to use this calculator

Set the multiplier. Rule of 8 for mild steel; lower for soft aluminum, higher for stainless, thick or high-tensile material.
Round to a real die. Take the recommended V to the nearest opening in the rack, then re-run in check mode.
Carry the radius forward. The air-formed inside radius (≈ radius factor × V) is the IR the bend allowance calculation should use.
Check flange and V/t. Legs shorter than 0.77×V will not form; V/t under 5 is a tooling-damage risk.

How it works

In air bending the die does the forming: the sheet bridges the vee and wraps a radius set by the opening, so die selection fixes three things at once —

V = m·t · IR ≈ r%·V · flange_min ≈ 0.77·V

The multiplier and radius factor are the air-bending trade rules (die-maker guidance), kept user-adjustable rather than baked in. The opening this screen recommends is the same V the press brake tonnage calculator takes, the floated radius feeds the bend allowance calculator, and the K-factor side of that math lives in the K-factor calculator.

Worked example

Verified against the live calculator

3 mm mild steel at the rule of 8, 16% radius factor:

V = 8 × 3 = 24 mm · IR = 0.16 × 24 = 3.84 mm · flange_min = 0.77 × 24 = 18.5 mm

A 24 mm vee — exactly the die the tonnage calculator's example runs — floats a radius of about 1.3 thicknesses and can form legs down to 18.5 mm. Feed 6 mm plate into that same die and the ratio drops to V/t = 4: under the 5× safety floor, where tonnage spikes and die shoulders take the damage. The fix is a wider vee and the bigger radius that honestly comes with it.

Frequently asked questions

How do you choose a V-die opening for air bending?

Multiplier times thickness: the rule of 8 (V = 8×t) for mild steel up to about 3 mm, around 6×t for soft aluminum, 10–12×t for stainless or thicker plate, 12–15×t for high-tensile. Then round to the die actually in the rack — 3 mm mild steel lands on a 24 mm vee.

What inside radius does air bending produce?

The die sets it, not the punch: the part floats a radius of roughly 15–17% of the opening in mild steel (the "20% rule" family — stainless runs 20–22%). A 24 mm die floats about a 3.8 mm radius in 3 mm steel — the familiar "radius ≈ one thickness" result at the rule of 8.

What is the minimum flange a V die can bend?

About 77% of the opening as a working rule — a 24 mm die needs an 18.5 mm leg. The hard geometric floor is just over half the vee (or the leg falls in), but the 0.77 figure is what holds up on the brake with the die shoulder radius accounted.

What happens if the die is too narrow for the material?

Below roughly V = 5×t the bend needs disproportionate tonnage, chews the die shoulders, creases the part and turns inaccurate — 6 mm plate in a 24 mm vee is a 4:1 ratio and firmly in that zone. Go wider and accept the bigger radius, or use a larger-radius punch and bottoming setup designed for it.

Method & assumptions

Air bending only — bottoming and coining take the radius from the punch and run their own tonnage rules.
The multiplier bands (8 mild steel ≤ ~3 mm; 6 aluminum; 10–12 stainless/thick; 12–15 high-tensile) and the 15–22% radius factors follow die-maker and trade-press guidance, verified across two independent sources; the die maker's own chart governs where it disagrees.
0.77·V is the working minimum-flange rule; the geometric floor is just over V/2.
No die chart is embedded — round the recommendation to the openings actually in the rack and re-check.