What size press brake do I need?

The 575 rule: where press brake tonnage numbers come from

Every tonnage chart taped to a toolbox comes from one formula: tons per foot = 575 × thickness² ÷ V-die opening (inches). The 575 is the constant for mild steel at roughly 60ksi tensile. Square the thickness, divide by the V opening. Example: 1/4" plate in a 2" die → 575 × 0.0625 ÷ 2 ≈ 18 tons per foot.

Two things about that formula matter more than the number itself. First, thickness is squared — double the thickness and you need four times the force. That's why 1/2" plate surprises people who are used to 1/4". Second, halving the die opening doubles the tonnage. Tight die, thick material, and suddenly you're asking a 100-ton brake for 200 tons of work.

Charts round things off and bake in assumptions; the formula answers any combination. Real material varies ±10% with actual tensile strength, which is exactly what the safety margin is for. Run your numbers in the calculator.

3-axis vs 6-axis backgauge: which do you actually need?

Short version: count the independent fingers. X sets flange depth, R sets finger height, Z slides fingers along the bed — and numbered pairs (X1/X2, R1/R2, Z1/Z2) mean each finger moves on its own.

If you bend simple brackets all day, a 2-axis (X, R) backgauge is everything you'll ever program — the rest is money on the table for the dealer. Tapered or asymmetric work pushes you to 4-axis. High-mix shops running complex parts with multi-station setups are where 6-axis pays for itself in fewer setups.

And read the fine print on the brochure: "9-axis" usually counts the ram (Y1/Y2), crowning, and sheet followers too. Ask which axes are backgauge axes before comparing. See the full explainer with diagrams.

The stainless trap: why 304 needs 60% more tonnage

The classic sizing mistake: quoting and sizing stainless like it's mild steel. 304 stainless needs roughly 1.6× the force at the same thickness and die opening. A job that's 100 tons in A36 is 160 tons in 304 — which is exactly how a 135-ton brake ends up stalled halfway through a bend with a part stuck in the die.

It gets worse: stainless work-hardens and wants a larger inside radius, which means a wider V-die, which changes the tonnage math again. Size the machine for the stainless and the mild steel takes care of itself. Aluminum goes the other way at roughly half the force of mild — but keep it out of tight dies or it'll crack on you.

Check both materials in the calculator before you commit to a machine — or a quote.

What bend radius will I actually get? The 20% rule

Your print says 1/32" inside radius. Your brake has other plans. In air bending, the inside radius floats — it's set mostly by the V-die opening, not by what you wish for. The rule: inside radius ≈ 16–20% of the V opening for mild steel, about 20–22% for 304 stainless. A 1" V gives you roughly a 3/16" radius whether the print likes it or not.

Want it tighter? Smaller V — and the tonnage that comes with it — or switch to bottoming. Want it bigger and gentler? Open up the V. The punch nose radius matters less than most people think in air bending; the die opening runs the show.

Design rule that keeps you out of trouble: inside radius ≥ material thickness for mild steel, ≥ 1.5–2× thickness for aluminum. Call for anything sharper and you're asking for cracks — or a bottoming die and a much bigger tonnage number. See what your die choice does to tonnage.

Minimum flange length: the 0.77×V rule

The flange has to sit on both shoulders of the die. Too short and it tips into the V mid-bend — slipped, distorted, scrap. The rule: minimum flange ≈ 0.7–0.77 × the V-die opening. On a 2" die, don't try to form a flange much under 1-1/2".

This is why small flanges force small dies, and small dies force big tonnage — it's all one equation. If you're designing parts, check your smallest flange against the die you'll actually run before you quote the job. Finding out on the brake is the expensive way to learn it.

Why your bends are off in the middle: crowning

Classic symptom: the ends hit 90° and the middle comes out at 88°. That's not you — under full tonnage, the bed and ram bow apart in the center. On a 10-footer at full load, that deflection is real and measurable.

Crowning pushes the middle back up to compensate: hydraulic crowning (CNC-controlled, built into the bed) on newer machines, mechanical wedge systems on older ones. No crowning on your brake? Then you're shimming the die or living with it.

Shopping used? Bend a full-length test piece and check the angle in five places across the bed. That tells you more about the machine's condition than the spec sheet ever will.

Springback: how much, and how to beat it

Metal wants to spring back when the punch lifts: roughly 1–3° per bend for mild steel, more for stainless and aluminum (high yield strength relative to stiffness). The fix is overbending — program 88° to land on 90°.

Air bending has the most springback of the three methods; bottoming and coining kill it with tonnage. Grain direction matters too: bending across the grain is stronger but springier, bending with the grain risks cracking on tight radii.

One thing estimators get wrong: springback changes the angle, not the flat pattern length. Don't adjust your blank size for it — adjust your bend angle.

K-factor, bend allowance, bend deduction — in plain English

Bend a sheet and the outside stretches while the inside compresses. Somewhere in the middle sits the neutral axis — the line that doesn't change length. The K-factor is just where that axis lives, expressed as a fraction of thickness. For air bending mild steel, start at 0.44.

Bend allowance is the arc length the bend consumes: BA = (π/180) × angle × (radius + K × thickness). Bend deduction is the flip side — what you subtract from your flange dimensions to get the flat. They are not interchangeable; mixing them up flips your flat pattern by roughly twice the allowance.

The honest truth: K-factor moves with your die opening and your material batch. Your CAD default is a starting guess, not gospel — confirm with test bends and keep a per-material table.

Buying a used press brake: tonnage is only the first question

Everyone shops tons × length. That's step one — run your actual work through the calculator above instead of guessing. Step two is the backgauge: check the axes tab, and don't pay for 6-axis if you bend brackets all day.

Step three: bend a full-length test piece and check the angle in five places. Inconsistent angles mean crowning problems — the most expensive thing to discover after the check clears. Step four: hydraulics. Leaks, ram drift, strange noises under load. Step five: the control. Can you — or your guy — actually program it? A cheap brake nobody can run is the most expensive brake.

And price the tooling that comes with it. A brake with a full die library is worth real money over a bare machine.