Symptom table
| Symptom | What it means | How to check | What to do |
|---|---|---|---|
| Middle angle more open than the ends | Under-crowned | Bend 3 short samples, middle and both ends, measure | Increase the crowning value |
| Middle angle tighter than the ends | Over-crowned | Same test | Decrease the crowning value |
| Short parts consistent, long parts not | Normal — deflection scales with bend length | Compare a 500mm and a 3000mm part | Crown per program, not as a fixed setting |
| Correct for 3mm, wrong for 6mm | Normal — deflection scales with force | Test both thicknesses | Store the crowning value with each program |
| Still random after crowning | It is not deflection — check material and tooling | See the "inconsistent angles" article | Fix material/tooling first, then crown |
The three-sample test
This is the standard method and takes about ten minutes:
- Cut 3 identical samples, 100–200mm wide, from the same lot
- Bend one at the far left, one dead centre, one at the far right — same program, same tooling
- Measure all three with an angle gauge
- Middle more open than the ends → under-crowned. Middle tighter → over-crowned.
- Adjust and repeat until all three are within tolerance
Do not test with a single long sheet. A long sheet brings blank flatness and placement into the result and muddies it.
Mechanical vs. CNC crowning
Mechanical crowning — a wedge system under the bed, set by hand or by one axis. Cheaper, and entirely sufficient for a shop running a few fixed part types.
CNC crowning — the controller computes the value from bend length, thickness and material in the program and sets it automatically. It earns its keep when you change parts several times a day, because the value that is right for one part is wrong for the next.
Both solve the same physical problem. The difference is who enters the number and how many times a day.
The most commonly misjudged point
Crowning is not a set-once setting.
It depends on actual bending force, which depends on thickness, bend length, V opening and material. Change any of those four and the correct crowning value changes with it.
A shop that sets crowning once for its thickest part and leaves it there will over-crown every thinner part — and will see the middle come out tighter than the ends, exactly the reverse of the original symptom.
Store the crowning value with the part program, not in someone's head.