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

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.

The air-bending rule of thumb

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:

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.
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.

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.

Choosing the V-die: about 8 times thickness

The die opening sets both the force and the inside radius. The usual starting point is a V opening of about 8 times material thickness. A narrower die raises tonnage and tightens the radius; a wider die lowers tonnage and opens the radius. For thick plate, many shops move to 10 to 12 times thickness to keep tonnage manageable and reduce the 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. For cold-rolled mild steel it is roughly 15 to 17 percent of the V opening; for 304 stainless roughly 20 to 22 percent; for soft aluminium slightly less than mild steel. 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:

304 stainless: about 1.4 to 1.6 times mild steel. It also springs back more, so the controller has to overbend by a few degrees.
Soft aluminium such as 5052-H32: about 0.5 to 0.6 times mild steel.
High-strength and wear-resistant steels: much higher. Scale by the tensile strength on the mill certificate, and check die width and punch radius with the tooling supplier.

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 come in lengths from compact units of about 1.2 m to 6 m and beyond; many general fabrication shops run something in the 3 m (10 ft) class.
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.

A worked example

Say your heaviest regular job is a 3 m (about 10 ft) bend in 3 mm mild steel.

V-die at about 8 x thickness: 8 x 3 mm = 24 mm.
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.
Inside radius: roughly 15 to 17 percent of 24 mm, so about 3.6 to 4.1 mm.
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.
Material mix: the same part in 304 stainless, at about 1.5 times, needs roughly 112 tonnes, or about 140 tonnes of machine with headroom. In 5052 aluminium, at 0.5 to 0.6 times, it needs roughly 37 to 45 tonnes.

A 100-tonne machine that is comfortable for this part in mild steel is undersized for the same part in stainless.

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.

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 and add about 20 percent headroom.
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.

See the press brakes and shears in our catalog, and request a quote with your part list; we will quote the machine, tooling, freight, installation and operator training together.