Reference Chart

Press Brake Tonnage Chart

Air-bending force by material thickness and V-die opening, in metric tonnes per metre and US short tons per foot.

A tonnage chart answers a fast production question: how much force is needed for each unit of bend length at a given thickness and V-die opening? The tables below use the standard air-bending relationship and provide comparable values for mild steel, 304 stainless and 5052 aluminium. They cover common metric and imperial stock sizes, but remain planning values. Verify the actual material tensile strength, machine load chart and tooling rating before bending.

How to read the chart

First choose the material table. Find sheet thickness in the left column, then move across to the column headed by the actual V-opening. The intersection is force per metre for metric tables or US short tons per foot for imperial tables. Multiply that value by the full bend length. A 2 m fold uses twice the metric table value; a 30 in fold uses 2.5 times the tons-per-foot value.

Do not multiply millimetres by tonnes per metre or inches by tons per foot without converting length. For a 650 mm metric bend, multiply by 0.65. For an 18 in imperial bend, multiply by 1.5. If the exact thickness or opening is absent, calculate directly instead of casually interpolating: force changes with thickness squared, so the midpoint between two rows is not the midpoint in force.

Values represent force distributed along a bend. A machine's headline capacity may apply across its full bed and can be lower for concentrated or off-centre loading. Consult its load-distribution diagram. Tool segments also have maximum tonnes per metre or tons per foot; both punch and die must exceed the applied line load.

Metric air-bending charts

All values are metric tonnes-force per metre of bend. Column headings are V-die openings in millimetres.

Mild steel: metric tonnes per metre

304 stainless steel: metric tonnes per metre

5052 aluminium: metric tonnes per metre

Imperial air-bending charts

All values are US short tons per foot of bend. Thickness and V openings are inches.

Mild steel: US tons per foot

304 stainless steel: US tons per foot

5052 aluminium: US tons per foot

The effect of V-die ratio

The die ratio is V divided by material thickness. An 8 mm opening under 1 mm sheet and a 24 mm opening under 3 mm sheet are both 8×T. The familiar 8×T starting point balances force, inside radius, minimum flange and bend control for many mild-steel air bends. It is a starting point, not a universal specification.

Force is inversely proportional to V. Keeping every other input constant, changing from 8×T to 6×T increases force by about one third: 8 ÷ 6 = 1.33. Moving from 8×T to 10×T reduces it by 20 percent: 8 ÷ 10 = 0.80. This is visible across every row—the value halves when the opening doubles.

A wider opening reduces force but creates a larger natural inside radius, approximately 0.16V in typical air bending, and demands a longer minimum flange. A narrow opening produces a tighter radius and supports a shorter flange, but raises tonnage, surface marking and cracking risk. If the punch radius, grain direction, ductility or required inside radius conflicts with the selected ratio, part requirements win over the rule of thumb.

Material factors

Mild steel is the baseline factor 1.00. The stainless values use 700/430, or about 1.63 times the mild-steel force. Aluminium values use 210/430, or about 0.49. Actual grades and tempers vary: annealed aluminium, precipitation-hardened aluminium, S235, S355 and high-strength steels cannot safely inherit a generic factor.

Worked chart examples

For 3 mm mild steel over a 2 m bend in a 24 mm die, find 3 in the mild-steel metric table and V24: 25.3 tonnes per metre. Multiply by 2 to obtain 50.6 tonnes. With 20 percent headroom, plan for at least 60.7 tonnes, while also checking concentrated-load rules.

For the same geometry in the reference stainless, read 41.2 tonnes per metre. The 2 m bend needs about 82.4 tonnes before headroom. This illustrates why substituting stainless on a mild-steel programme without recalculation can overload equipment.

For 0.125 in mild steel with a 1.0 in opening over 30 in, use the imperial table's 0.125 row and V1.0 column, then multiply the displayed tons per foot by 2.5 ft. Keep US short tons distinct from metric tonnes; one metric tonne-force is approximately 1.102 US short tons-force.

Why real results differ

Tensile strength varies between grades and batches. Tool friction, coating, mill scale, bend angle and rolling direction also matter. Chart constants differ between manufacturers, sometimes because they assume another tensile strength or express V ratios differently. A small numerical difference does not necessarily make one chart wrong; it may reflect different assumptions.

Charts are not suitable for bottoming or coining without a validated method-specific calculation. Those processes create much higher force and different contact conditions. Hemming, acute tooling, large-radius forming and bending with urethane pads likewise require specialist guidance. Consult tooling and machine manufacturers when the operation falls outside ordinary air bending.

Using the chart safely

• Confirm material grade, thickness and actual tensile range.
• Use the opening engraved or measured on the installed die.
• Multiply by total bend length in matching units.
• Add justified headroom for variation; never plan at the nameplate limit.
• Check machine capacity at the proposed position and bend length.
• Check punch, die, adapters and clamps for line-load capacity.
• Inspect tooling condition and verify a controlled first bend.

Frequently asked questions