Previous articles in this series treated a product spec as geometry, then material, then optics. This one is about the number that quietly makes all of them meaningless if you get it wrong: tolerance.
Two materials, two very different meanings of "±1mm"
Ask a factory for a tolerance and you'll get a single number. Ask what it applies to and the conversation changes.
A medium-density fibreboard (MDF) panel and a solid timber panel can both be cut to ±0.5mm. They behave nothing alike afterwards.
| Property | MDF | Solid timber |
|---|---|---|
| Cut accuracy | ±0.5mm, repeatable | ±0.5mm at cut, then it moves |
| Internal stress | Essentially none | Anisotropic — moves across the grain |
| Response to humidity | Thickness swell, recoverable | Warping, cupping, checking |
| Batch consistency | Uniform sheet, uniform result | Every board is a different grain |
| Joining | Screws, dowels, glue hold | Needs allowance for movement |
The tolerance you specified applies at the moment of cutting. Everything after that is governed by the material.
Why a solid wood lid stops closing
A solid wood box lid that fit perfectly in the factory can fail to close two years later — and it isn't a defect in workmanship, it's physics.
Timber expands and contracts across the grain as humidity changes. In a climate that swings from 40% to 90% relative humidity, a 300mm-wide panel can move by a millimetre or more across its width. On a lid-and-base fit with a 0.5mm clearance, that movement is the entire budget.
MDF doesn't do this, because it has no grain direction and no internal stress — it's a pressed fibre mat. It swells slightly with moisture and recovers; it doesn't warp.
So why does anyone still use solid wood?
Three legitimate reasons, and none of them are "it's better":
- Visible end grain. If the design exposes a cut edge, MDF needs edge banding that can read as fake.
- Repairability. Solid timber can be sanded and refinished. Veneered MDF cannot.
- Perceived value at very high price points. Above a certain price, customers inspect the inside of the box, not just the surface.
Everything else — surface grain, colour consistency, flatness, dimensional stability — is easier to control with veneered MDF, not harder. Good veneering is indistinguishable to the eye; you only see the difference by cutting the box open.
What to actually put in the spec
Instead of a bare toleranceMm, specify the condition under which it holds:
type ToleranceSpec = { valueMm: number; measuredAt: 'cut' | 'assembled' | 'in-service'; humidityRange: [number, number]; // %RH temperatureRange: [number, number]; // °C }; A tolerance without a measurement condition is a number, not a promise.
The through-line
Every article in this series has made the same point from a different angle:
- 1 — universal fields vs variable fields
- 2 — geometry needs a
profile - 3 — repeated objects need a list
- 4 — the material owns part of the spec
- 5 — an optical element constrains the interior
- 6 (this one) — a tolerance is only meaningful with the conditions attached
Specs fail not because people forget fields, but because they record values without recording when those values are true.
The case shown is a windowed wooden wine case from TANCYBOX — veneered panel construction, which is what makes the flat, stable face behind the glass possible.
