A tolerance without a measuring method is a future argument. These are the five that decide whether the blank works, the numbers we hold, and the six lines that remove the argument.
A tolerance is only meaningful when it names what is being measured and over what distance. “±0.5 mm” on a drawing tells a supplier almost nothing; “thickness ±0.10 mm, flatness 3 mm per metre, burr ≤0.1 mm, diagonal difference ≤2 mm on a 1200 × 800 panel” tells them exactly what to build — and what to charge for.
| Characteristic | What it controls | Typical hold | What goes wrong if missed |
|---|---|---|---|
| Thickness | Film adhesion, wind load, channel fit | ±0.10 mm under 2 mm; ±0.15 mm above | Film bubbles over high spots; blank rocks in the frame |
| Flatness | Whether the face reads flat at 100 m | 3 mm per running metre | Visible ripple under grazing sunlight; film lifts at the crown |
| Burr / edge | Handling safety, film edge seal | ≤0.1 mm, no rolled edge | Film cannot seal, water enters, edge lifts in one season |
| Squareness | Frame fit, printing registration | Diagonal difference ≤2 mm on 1200 × 800 | Channel mitres open; printed border runs out at the corner |
| Hole position | Post fit on site | ±0.5 mm from datum, not from each other | Signs that fit in the workshop and not on the road |
“Typical hold” is what our line achieves on a standard run. Tighter is possible on a sorted lot, at a price; looser is possible too, and sometimes correct.
Thickness is the tolerance buyers argue about and the one that is least understood. A coil is rolled to a nominal thickness with a tolerance band; a blank cut from the middle of the coil and a blank from the edge can differ within that band and still both be “2 mm”. If your film applicator assumes a flat surface, a 0.15 mm step across a panel is enough to trap air.
The practical answer is not to demand a tighter coil — it is to state the tolerance you need and let the supplier choose the coil. On a laminated sign we hold ±0.10 mm up to 2 mm because that is what our applicator needs; on a bare blank for a buyer who will powder-coat it themselves, ±0.15 mm is fine and cheaper.
Flatness is measured with the panel on a table and a straight edge across the face. Three millimetres per metre is the usual commercial figure, and it is enough for a sign viewed from a car. It is not enough for a large gantry face, where sunlight at a shallow angle turns a 3 mm bow into a visible wave, and it is not enough for a panel that will be laminated with high-grade microprismatic film, which shows every deviation.
If the sign will be seen against the sky at dawn or dusk — which is most overhead signage — write 1.5 mm per metre, and expect to pay for sorted sheets.
Six lines in a purchase order remove most of the arguments that otherwise arrive with the container.
Sometimes, and we say so up front. Thickness and flatness inside our standard band come free; sorted sheets for 1.5 mm per metre flatness add a sorting cost, and we quote it as a separate line rather than hiding it in the unit price.
State it. We quote bare-metal thickness as standard, because that is what the mill certificate certifies. With 0.1 mm film applied, a micrometer reading will be about 0.1 mm higher, and that difference has failed more incoming inspections than any real defect.
0.1 mm maximum, with the burr on the face that will not carry film. Below that you are paying for a deburring operation that adds nothing; above it, the film edge will not seal.
Specification questions are answered by the same person who quotes the job.