Cause table
| Symptom | Common cause | How to check | What to do |
|---|---|---|---|
| Angle differs mid-sheet vs. at the ends | Bed and ram deflect under load | Bend 3 identical samples, mid and both ends, measure each | Enable/adjust crowning; see the crowning article |
| Angle varies randomly sheet to sheet | Different lots or suppliers | Micrometer the actual thickness at 5 points on one sheet | Separate lots; run one lot at a time, re-set per lot |
| Angle drifts through a shift | Hydraulic oil warms, viscosity changes | Measure a sample at shift start and after 3 hours | Warm the machine before production; check cooling |
| One end of the sheet always under-bends | Tooling not aligned, or unevenly worn | Remove tooling, check the V with feeler gauges | Re-align; replace the worn segment |
| Correct when bending slowly, wrong when fast | Bend speed too high, material cannot flow | Bend the same part at two speeds | Reduce speed on the final part of the stroke |
The right order to check
- Material first. Micrometer the real thickness at several points on the same sheet. Commercial sheet has thickness tolerance — and thickness is the single strongest variable on the resulting angle.
- Tooling second. Worn V, punch and die not coplanar, segmented tooling not level.
- Crowning third. Only once the first two are clean does a crowning number mean anything.
- Machine parameters last. Adjusting parameters to compensate for any of the three above is the most reliable way to never find the real cause.
The most commonly misjudged point
Many shops reach for crowning first, when the problem is sheet thickness.
Telling them apart is simple: if the deviation follows position on the sheet (middle differs from the ends), it is deflection. If it varies from sheet to sheet but is consistent across any single sheet, it is material. These are opposite problems with opposite fixes.
Crowning to fix a material problem will ruin the sheets that were the correct thickness.
When to call us
When you have worked through all four steps and the angle is still outside drawing tolerance, or when the deviation grows month over month — the second usually points to mechanical wear, not parameters. Send us the part drawing, the steel grade and three measured angles and you get a specific answer, not a guess.