Calculate bending force from sheet thickness, V opening and material strength.
Choosing the right tonnage when bending on a press brake determines both part quality and machine life. Insufficient tonnage leads to angle errors, while excessive tonnage damages tooling and the machine. Here is the logic of tonnage calculation.
The basic formula
In air bending, the approximate force is found with this formula:
F = 1.42 × Rm × s² × L / V
- F: Bending force (N)
- Rm: Tensile strength of the material (N/mm²)
- s: Sheet thickness (mm)
- L: Bend length (mm)
- V: Lower die V opening (mm)
A worked example
Let us bend 3 mm St37 steel sheet with Rm = 450 N/mm² over 1 metre in a V = 24 mm die (8 × s rule):
F = 1.42 × 450 × 3² × 1,000 / 24 = 239,625 N ≈ 24.4 tonnes
So a 3-metre bend needs about 73 tonnes. For safety margin and tooling life, it is recommended to work at least 20% below the machine's nominal tonnage; a 100-tonne press brake suits this job.
Factors that affect force
- V opening: Force increases as V gets smaller. The general rule is V = 8 × s, 6 × s for thin sheet and 10-12 × s for thick sheet.
- Material: Stainless steel (Rm ≈ 700) needs about 55% more force than carbon steel; aluminum about 40% less.
- In bottoming, the force increases 3-5 times.
Minimum flange and inside radius
Minimum flange length is roughly 0.7 × V. The inside radius formed in air bending is about 16% of the V opening. If the radius on the drawing is not close to this value, the tooling selection must change.
Common mistakes
- Calculating force for the part length only and loading it onto a short die segment
- Choosing the V opening based on the dies at hand rather than sheet thickness
- Ignoring the grain direction of the sheet; bending perpendicular to the grain lowers the risk of cracking
- Confusing the die tonnage limit (tonnes per metre) with the machine tonnage
Crowning and angle measurement
On long parts, a crowning system compensates for deflection in the middle; CNC crowning and an angle-measurement system help hit the right angle on the first part. These are offered as options on our AB-100/3100 and AB-220/4100 models.