V-Die Opening Calculator

Inputs

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Recommend an opening from thickness, or check the die already in the rack.
Sheet or plate thickness.
mm
Rule of 8 for mild steel up to ~3 mm; ~6 for soft aluminum; 10–12 for stainless or thicker plate; 12–15 for high-tensile. Used in recommend mode.
× t
The vee width of the die you want to check. Used in check mode.
mm
Air-formed inside radius as % of the opening — the "20% rule" family: mild steel ≈ 15–17%, stainless 20–22%, soft aluminum less.
% 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)Pass3.84mm

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.

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