Failure mode
Thickness Out of Tolerance
A sheet specified at 1.0 mm measures 0.92 mm, or the centre is on gauge while the edges are thin. The part still assembles, but the section modulus is not what the calculation assumed.
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.
Minus-tolerance rolling
Rolling to the lower half of the tolerance band saves metal on every tonne. It is normal practice, and it is why the standard and not the nominal number has to be named in the order.
AGC not engaged
Automatic gauge control holds thickness across the length. Without it the coil drifts as the mill warms up.
Edge drop
Thickness falls away near the strip edge as a natural consequence of the roll gap profile. It matters when the edge is a functional surface.
Post-processing movement
Levelling, heat treatment and milling all remove or redistribute material after the gauge was set.
Verification
How it is measured
Measure with a micrometer at a stated distance from the edge — the distance matters, because measuring on the edge drop gives a falsely low reading. ASTM B209 and EN 485 publish tolerance tables by thickness and width. Cross-width variation needs multiple points across the sheet, not one reading in the middle.
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
Name the standard
"Thickness tolerance to ASTM B209" is enforceable. "1.0 mm" alone is not.
2
Say whether minus tolerance is acceptable
If your design has no margin, state that the tolerance is bilateral.
3
Confirm the mill can hold a tighter band
A tighter tolerance than the standard default is available but it is a mill capability question, and it changes price.
4
Ask for the measurement record
A mill measuring across the width has the record. That is the difference between a claim and an argument.
Thickness is checked at nine points across the width before loading rather than at one point in the centre, and the readings go on the checklist. Where an order has a tolerance tighter than the standard default, we confirm the mill can hold it before we quote the price.
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.