Failure mode
Camber and Wavy Edge in Sheet & Coil
The sheet curves sideways when laid flat, or the edge ripples instead of lying straight. It feeds badly through a roll former or a CNC table and the operator compensates by hand — until the batch is finished.
What it actually is
The sheet curves sideways when laid flat, or the edge ripples instead of lying straight. It feeds badly through a roll former or a CNC table and the operator compensates by hand — until the batch is finished.
Mechanism
Why it happens
Four causes account for most of it. They are not equally common, and they are not equally within the mill's control.
Uneven roll gap
Roll crown and thermal camber change across the width during rolling. If compensation is not maintained, the two edges come out at different lengths.
Slitting tension
Uneven tension across a slitting line puts more length into one side of the strip than the other.
Levelling setup
Cut-to-length levelling can introduce camber if roller penetration is set for thickness rather than for the actual shape of the coil.
Temper matters
Harder tempers hold residual stress and show shape defects more readily than O-temper, which relaxes.
Verification
How it is measured
Camber is measured as the maximum deviation from a straight edge over a stated length — commonly 2,000 mm — with the sheet free on a flat surface. Wavy edge is measured as wave height against the flat plane. EN 485 and ASTM B209 publish tolerance tables by thickness and width; the limit is not the same at 0.8 mm and at 6 mm.
Specification
How to specify around it
Most of these cost nothing to write into an order and are difficult to enforce afterwards. This is the part worth copying into your next RFQ.
1
Quote a number, not "flat"
Write "camber ≤ 2 mm per 2,000 mm" or whatever your line tolerates. "Flat" is not a measurable requirement.
2
Specify stretch levelling
For cut-to-length sheet feeding automatic equipment, ask for stretch-levelled or tension-levelled material.
3
Give coil weight limits
Coil weight and inner diameter affect how shape defects express themselves on your decoiler. State what your machine accepts.
4
Say which process follows
Deep drawing, roll forming and laser cutting each fail on a different shape defect. The mill can only optimise for one.
Camber and wave height are measured at the mill before loading and recorded on the checklist that ships with the order. For cut-to-length sheet going into automatic equipment, we ask whether stretch levelling was specified and flag it if it was not.
We do not claim this never happens. Metal is rolled, shipped across an ocean and unloaded by someone else. What we do is ask the question before the order runs, put the answer in writing, and be the single party that answers for it afterwards.