How to size a press brake: tonnage, bending length and tooling

Size a press brake with force (kN) = 650 x S² x L / V for air bending mild steel, using a V-die about 8 times thickness. Multiply by a material factor (about 1.5 for 304 stainless, 0.5 to 0.6 for soft aluminium) and choose a machine rated so that load is at most about 80 percent.

A press brake is sized by three things: how much force your worst bend needs, how long your longest bend is, and what tooling you will run. Get those right and the choice of controller, axes and crowning follows. This guide covers the rule of thumb most fabricators use for air bending and walks through a worked example.

How do you calculate press brake tonnage?

Square the thickness, multiply by the bend length and a constant, and divide by the die opening. Most modern press brake work is air bending: the punch pushes the sheet into a V-die without bottoming out, and the angle is set by how deep the punch goes. For air bending mild steel, the widely used formulas are:

• Metric: force in kN = 650 x S² x L / V, with thickness S and die opening V in millimetres and bend length L in metres. Divide kN by about 9.81 to get metric tonnes.
• Imperial: US tons per foot = 575 x T² / V, with thickness T and die opening V in inches. Multiply by the bend length in feet.

Both assume mild steel with a tensile strength of roughly 410 to 450 MPa (around 60,000 psi) and air bending only. Bottom bending and coining need several times more force and fall outside this rule.

What V-die opening should you use?

Start at about 8 times material thickness. The die opening sets both the force and the inside radius: a narrower die raises tonnage and tightens the radius, a wider die lowers tonnage and opens the radius. Because force is inversely proportional to V, the common choices compare like this:

Die opening	When it is used	Effect compared with 8 x thickness
About 6 x thickness	Thin sheet or a tight inside radius	About a third more tonnage, smaller radius
About 8 x thickness	Standard starting point for most sheet	Baseline
Up to about 12 x thickness	Plate over about 10 mm (3/8 in)	About a third less tonnage, larger radius, lower risk of cracking

In air bending, the inside radius forms as a percentage of the die opening, as long as the punch radius is smaller than that natural radius. So the inside radius on your drawings points you to a die width, and the die width drives the tonnage.

Correction factors for stainless and aluminium

Scale the mild steel result by the strength of the material you actually bend. Common factors and the typical natural inside radius in air bending:

Material	Tonnage factor (mild steel = 1)	Inside radius, percent of V opening	Notes
Mild steel, about 410 to 450 MPa (60,000 psi)	1.0	Roughly 15 to 17 percent	Basis of the formulas above
304 stainless	About 1.4 to 1.6	About 20 percent	Springs back more, so the controller overbends by a few degrees
Soft aluminium such as 5052-H32	About 0.5 to 0.6	About 12 to 15 percent	Less force, slightly tighter radius than mild steel
High-strength and wear-resistant steels	About 1.4 to 2.5 or more	Check with the tooling supplier	Scale by the tensile strength on the mill certificate and check die width and punch radius

Why bending length matters

Required force rises in direct proportion to bend length. A 3 m bend needs three times the force of the same bend 1 m long. That affects two decisions.

• Machine length: choose a bed and ram long enough for your longest part, with some room at the ends. Press brakes range from compact units around 1.5 m (60 in), such as the fully electric SafanDarley E-Brake 40-1600, to 6 m and beyond; many general fabrication shops run something in the 3 m (10 ft) class. Our hydraulic vs electric press brake comparison covers the drive choice.
• Concentrated loads: a machine rated at 100 tonnes over 3 m is not designed to put 100 tonnes onto a 300 mm part. Machine builders and tooling makers specify a maximum load per metre or per foot. Bending short, heavy parts at high tonnage can damage tooling and the bed, so check the per-metre limits of both against your shortest heavy part.

How many tons do you need to bend 3 mm steel 3 m long?

About 75 metric tonnes in mild steel, which points to a 100-tonne class machine once you allow headroom. Here is the working, for a heaviest regular job of a 3 m (about 10 ft) bend in 3 mm mild steel:

1. V-die at about 8 x thickness: 8 x 3 mm = 24 mm.
2. Force: 650 x 3² x 3 / 24 = about 731 kN, or roughly 75 metric tonnes. The imperial formula gives about 84 US tons for the same bend, which is close enough for sizing.
3. Inside radius: roughly 15 to 17 percent of 24 mm, so about 3.6 to 4.1 mm.
4. Headroom: a common recommendation is not to run above about 80 percent of rated capacity. 75 / 0.8 is about 94 tonnes, which points to a 100-tonne class machine at 3 m or longer, such as the 110-ton, 3.1 m Yawei PBH-110/3100 or the 110-ton, 3.2 m Accurl Euro Pro B 32110.

The material mix changes the answer:

Same 3 m bend in 3 mm	Force needed	Machine size at 80 percent load
Mild steel	About 731 kN (75 t)	About 94 t, so a 100 t class machine
304 stainless (x 1.5)	About 1,100 kN (112 t)	About 140 t
5052 aluminium (x 0.5 to 0.6)	About 365 to 440 kN (37 to 45 t)	About 47 to 56 t

A 100-tonne machine that is comfortable for this part in mild steel is undersized for the same part in stainless, where something like the 150-ton, 3.1 m Bystronic Xpert Pro 150/3100 is the better fit.

CNC axes: what you actually need

• Y1 and Y2: the two sides of the ram, controlled independently so the ram stays parallel and the angle stays consistent along the bend.
• X: backgauge depth, front to back. Every CNC press brake has it.
• R: backgauge height, useful when gauging off flanges already bent or when switching between dies of different heights.
• Z1 and Z2: independent left and right movement of the backgauge fingers, which helps with tapered parts and with several bends on one setup across different tool stations.
• Crowning axis: motorised or hydraulic crowning set by the controller, often counted as the "+1".

A common setup for job shops is Y1, Y2, X and R plus CNC crowning, often sold as 4+1 axes. Z1, Z2 and additional backgauge axes earn their keep in high-mix work with staged tooling. More axes cost more and only help if operators use them, so match them to your part mix.

Crowning

Under load, the ram and bed deflect slightly more in the middle than at the ends, so long bends come out with a more open angle in the centre. Crowning compensates by raising the middle of the bed: by hand with manual wedges, or automatically with CNC crowning set from material, thickness and length. If you bend long parts regularly, CNC crowning is worth having.

Tooling

Budget for tooling as part of the machine, not as an afterthought. Decide the tooling style and clamping system, a set of V-dies that covers your thickness range at about 8 x thickness, punches with the right radius and enough clearance for return flanges, and sectioned tools so you can build the lengths you need. Check the load rating on every tool.

Checklist

• List your thickest bend and your longest bend separately, with material grade.
• Note the inside radii and flange lengths on your drawings.
• Calculate tonnage for the worst case in each material, then size the machine so that load is no more than about 80 percent of its rating.
• Check the per-metre load limits of machine and tooling against short, heavy parts.
• Pick axes by part mix: 4+1 suits most job shops; add more for high-mix staged work.
• Have tooling quoted with the machine.

Blanks usually come from a laser or a shear upstream: see our fiber laser power guide, the full range of press brakes and the shears we supply. We quote the machine, tooling, freight, installation and operator training together.