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Guide

How to calculate press brake tonnage

Before you choose a press brake, estimate the force your bends need. The bending force depends on the material strength, the sheet thickness, the bend length and the width of the die opening.

What affects the force

The force grows with the tensile strength of the material and with the bend length. It grows with the square of the thickness, so doubling the thickness needs about four times the force. A wider die opening reduces the force.

The formula for air bending

For air bending, the bending force in kilonewtons can be estimated from the tensile strength in N/mm², the thickness and the die opening in millimetres and the bend length in millimetres.

F (kN) = 1.42 × Rm × t² × L / (1000 × V)

Worked example

Mild steel with 400 N/mm², 3 mm thick, a 1000 mm bend and a 24 mm die opening needs about 213 kN, which is roughly 22 tonnes.

Choosing the die opening

A common rule is a V opening of about six to eight times the sheet thickness. A narrower opening gives a tighter inside radius but needs more force.

Add a margin

Choose a press brake with a margin above the result and check the tonnage limit of your tooling. Bottoming and coining need much more force than air bending.

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