Press Brake Bending Force — Air Bending

Air bending is the process most press brake work uses, and it is worth being precise about what it is. The sheet rests on the two shoulders of a V die and the punch pushes down in the middle. The material touches the tooling along three lines only, and the punch stops short of the bottom of the die — the angle of the bend is set by how deep the ram goes, so one set of tooling makes any angle. The force it takes is $F = K_b \cdot UTS \cdot L t^2 / V$.

Read the two levers in that expression, because between them they are most of the practical knowledge of a press brake.

The thickness is squared. Going from 2 mm to 3 mm in the same tooling multiplies the tonnage by 2.25. The step from a gauge a press handles comfortably to one it cannot manage at all is much smaller than it feels — and run the other way, doubling the available force buys only 41% more thickness, because the inverse of a square is a square root.

The die opening is in the denominator. This is the cheapest tonnage in the shop: going from a 12 mm die to a 16 mm die takes a quarter off the force, and it costs nothing but changing a tool you probably already own. It is not free, though. In air bending the die opening is what sets the inside bend radius — a wider die gives a larger radius with the same punch in the same sheet — and a changed radius means a changed K-factor, a changed bend allowance and a changed bend deduction. Widen the die to make a bend and every flat pattern developed for the old setup is now wrong. Redevelop them; do not just swap the tool. Two other constraints bound the die opening: the flange has to stay long enough to sit on both die shoulders, which puts a hard ceiling on how wide a die a short flange can use, and a very narrow die drives the tonnage up steeply while making a radius tight enough to crack the outside of the bend in a material with limited ductility.

This is the air-bending equation and nothing else. The other two bending processes are different in kind and different in force. Bottoming drives the material down until it conforms to the die, so the angle is set by the tooling rather than by ram position, and it takes several times this force. Coining goes further still, squeezing the material at the bend hard enough to yield it through the full thickness, and runs several times higher again — an order of magnitude above air bending is normal. Both buy accuracy and repeatability, and coining largely eliminates springback because the material at the bend has been plastically crushed rather than merely folded. Sizing a press from the air-bending number and then bottoming on it is how machines get hurt.

$K_b$ here is not the K-factor. They share a letter and nothing else, and the collision causes real confusion. This $K_b$ is an empirical bending factor, commonly quoted near 1.33, that lumps together die geometry and friction. The K-factor on the bend-allowance page describes where the neutral axis sits inside the thickness and runs about 0.3 to 0.5. One is a force multiplier a bit above 1; the other is a position fraction well below 1. Like the K-factor, $K_b$ is best fitted from your own measured bend rather than taken from a chart, and like the K-factor it does not travel between setups.

Two practical notes on capacity. A press's rating is usually quoted for a bend spread over the full bed, and a short bend concentrated near one end of the ram can overload the frame at a total force well inside the nameplate — machines carry a distance-from-centre derating for exactly this. And when a part is too long for the press, the answer is a wider die rather than a bigger machine; stepping along a bend line in sections leaves witness marks at every stop and an angle that wanders down the length.

Finally, and as everywhere in this shard: springback is not in this equation. The force is what the punch has to push while the material is loaded. What angle the part settles at when the ram lifts is a separate question, governed by the material's ratio of strength to elastic modulus and by the bend radius, and it is answered by overbending, by bottoming or coining, or by measuring the recovery on your own material — never by this arithmetic.