Press Brake Tonnage Chart & Air-Bending V-Die Calculator

How Do You Calculate Press Brake Tonnage for Air Bending?

In air bending the punch pushes the sheet into a V-shaped die without bottoming, so the force depends on how much leverage the die opening gives the sheet. The classic shop formula, used by press builders such as Cincinnati Incorporated and in the RMFG tonnage chart, is 575 × t² / V tons per foot for 60,000 psi mild steel. Thickness enters squared — double the plate and the tonnage quadruples — while a wider die lowers the force in proportion.

Other materials scale with tensile strength, the property measured by the standard tensile test (ISO 6892-1 (tensile testing of metals)). This calculator multiplies the mild-steel result by UTS / 413.7 MPa: 304 stainless at 620 MPa needs 26.9 tons/ft for the same 1/4" bend that takes 18.0 tons/ft in A36, and 3 mm 5052-H32 aluminum needs only 4.7 tons/ft. Multiply by the bend length in feet for the total, then keep at least 20% of the machine rating in reserve for crowning and material variation. The US short ton is 2,000 lbf, or 8.896 kN, per the conversion factors in NIST Special Publication 811 (unit conversion factors); metric brakes rated in tonnes-force use 9.807 kN per tonne, and the calculator prints all three units so you can compare against any machine plate. Sheet thickness is the other large source of error: a nominal 10-gauge sheet from a different mill can vary several percent, and because thickness is squared that becomes twice the percentage in tonnage.

Which V-Die Opening Should You Use? (The 8× Thickness Rule)

For sheet up to 1/2" (12.7 mm) the standard choice is a die opening of eight times the thickness; plate from 1/2" to 1" moves to 10t, and above 1" to 12t, because the punch force and the minimum flange both grow quickly. Going narrower is the most common way to overload a brake: 1/4" mild steel in a 1.5" die (6t) needs 24.0 tons/ft instead of 18.0. The calculator warns when V drops below 6t.

The die opening also sets the inside radius in air bending. Following the "20 percent rule" published by The Fabricator, the natural inside radius is about 16% of V for 60 ksi steel, 20–22% for 304 stainless and 13–15% for 5052-H32 aluminum. If the drawing needs a tighter radius than the die produces, change the die or bottom-bend — the punch radius alone will not get you there. Use our bend deduction and K-factor calculator with the radius from this tool to size the flat blank.

The same rules appear in the sheet-metal chapter of Machinery's Handbook (Industrial Press), which also tabulates bend allowances once the inside radius is known.

What Is the Minimum Flange Length for a Given Die?

The flange has to span half the die opening and still rest on the die shoulder while it bends. For a 90° bend that geometry gives b_min ≈ 0.71 × V ((V/2) / cos 45°). With the 2" die above the minimum flange is 36 mm; a shorter flange slips into the die and the bend comes out short and twisted. If the part needs a shorter leg, drop to a smaller die and recheck the tonnage.

How Much Springback Should You Expect?

Springback is the elastic recovery of the bending moment. A purely elastic-plastic model (no hardening) predicts only about 0.4° for 1/4" A36, which is less than any brake operator sees, because real sheet keeps getting stronger as it bends. The calculator therefore uses the plastic-bending moment of a strain-hardening sheet (σ = K·εn, plane strain), the model taught in Hosford and Caddell's Metal Forming: Δθ/θ = 6K′/((n+2)E′) · (ρ/t) · (t/2ρ)n, with K = UTS·(e/n)n, K′ = K·(2/√3)1+n, E′ = E/(1−ν²) and the neutral radius ρ = Ri + t/2. Here e = 2.718 (Euler's number, from the true-strain form of the tensile curve), and the calculator uses typical properties: E = 200 GPa and ν = 0.30 for A36 and Grade 50 (n = 0.20 and 0.15), E = 193 GPa and ν = 0.29 for 304 (n = 0.45), E = 70.3 GPa and ν = 0.33 for 5052-H32 (n = 0.12).

For a 90° bend in 1/4" plate with the 8t die it gives 1.3° for A36, 1.5° for Grade 50, 2.0° for 304 stainless and 1.8° for 5052-H32 — inside the shop ranges reported by The Fabricator (about 0.5–1.5° for mild steel, 2–3° for stainless). Springback grows with a wider die (larger radius) and with stronger material, so use the number as the first overbend, then correct the angle on the first part with the back gauge or the CNC angle compensation.

Common Mistakes on the Brake

• Mixing tons per inch and tons per foot. The 575 constant is per foot; divide by 12 for tons per inch.
• Forgetting the length. A short part concentrates the full load on a small area of the tooling — check the punch and die load rating per foot.
• Bending against the grain without margin. Bends parallel to the rolling direction crack sooner on high-strength and stainless sheet.

Related tools: punch tonnage calculator, bar and plate weight calculator and the hardness conversion chart to estimate tensile strength from a hardness reading.