Press Brake Tonnage Guide

What Press Brake Tonnage Actually Measures

Tonnage is the bending force the press must deliver along the bend line — how hard the ram pushes the punch into the V-die to plastically deform the sheet. It depends on material strength, thickness squared, bend length, and how wide the V-die opening is. It does not tell you how long to make a flat blank — that is bend allowance math. It does not tell you the tightest radius before cracking — that is bend radius guidance.

Fabricators use tonnage estimates to answer: Will this 10-foot 1/4-inch stainless bend exceed my 100-ton brake? Should I stage shorter bend lengths? Do I need a wider V-die to bring force down? The calculator and this guide support those decisions — final numbers still come from the machine's published tonnage chart and test coupons.

Why V-Die Opening Matters as Much as Thickness

The standard air-bending estimate is T ≈ k × 575 × t² × L / V (US tons; t and L in inches, V = die opening in inches, k = material factor). Notice V is in the denominator — a narrower die requires more force for the same thickness. Shops that mount too small a V for thick material often blame the material when the real issue is tooling selection.

Rule of thumb for mild steel air bending: start with V = 8× material thickness. Heavier plate often uses 10× or 12×. The wider die also produces a larger inside radius in air forming (roughly 16% of V), which ties directly to the Bend Radius Calculator and your K-factor in the Bend Allowance Calculator.

Air Bending vs Bottom Bending vs Coining

Air bending (three-point forming) is what the calculator models — the punch does not fully bottom the material into the die. Tolerance is looser but tonnage is lowest. Bottom bending forces the material to contact the die shoulders — tighter angle control, typically 3–5× higher tonnage than air bend for the same part. Coining exceeds bottom bending force further and is used for precision work on dedicated tooling.

Never use air-bend tonnage to set up a bottom-bend job — you will understate force and can overload the machine or snap tooling.

When calculator tonnage looked safe but the brake still overloaded or stalled, see Press Brake Tonnage Troubleshooting.

Material Factors

Mild steel (~60 ksi) uses k = 1.0 in the simplified formula. Stainless (~90 ksi) runs about k = 1.7. Aluminum (~35 ksi) runs about k = 0.5. Actual alloy and temper shift these — 6061-T6 behaves differently from 5052-H32. Convert gauge to decimal with the Sheet Metal Gauge Chart before plugging thickness into any calculator.

Common Mistakes

• Undersized brake — running near 100% rated tonnage damages the ram and bed over time; target 80% or less in production.
• V-die too narrow — spikes tonnage and leaves die marks; step up to the next V width.
• Confusing tonnage with allowance — correct flat length does not mean the brake can form it.
• Ignoring bend length — tonnage scales linearly with length; a 10-foot bend is ten times the force of a 1-foot bend at the same thickness.
• Off-center loading — placing a short bend in the middle of a long ram flexes the machine and changes effective tonnage at the die.

How Tonnage Fits the Sheet Metal Tool Cluster

Typical workflow on TestTalkHQ tools:

1. Gauge chart — thickness in decimal
2. Tonnage calculator — can the brake handle the bend?
3. Radius calculator — minimum and springback
4. Allowance calculator — flat pattern length
5. Sheet Metal Bending Guide — full setup checklist