Tolerances

In watchmaking, tolerance is the allowable deviation between the nominal dimension of a component and the one actually produced. The smaller the deviation, the more expensive the component is to manufacture and the more predictably the movement functions over time. You feel it immediately at the bench: a case with tight tolerances sounds full when you close the caseback, one with generous tolerances produces a slight play, a dull sound.

The impact of tolerances is distributed at every level of the movement. In silicon escapements produced with Deep-Reactive Ion Etching (DRIE), we work at the micron — one millionth of a millimeter — without subsequent processing, with absolute repeatability. The MEMS technology applied by Grand Seiko in the caliber 9SA5 brings the escape wheel and lever to tolerances of one millionth of a gram in weight, which translates into components 5% lighter and reduced torque demand on the spring. It's not an aesthetic matter: it's energy engineering.

In ultra-thin watches, tolerance becomes the dominant design constraint. In the Altiplano Ultimate Concept, with 2 mm total thickness, the crystal is at 0.2 mm and some wheels under 0.1 mm. Every material choice, every surface finish must account for the fact that there is no physical margin to compensate for errors. The same principle governs grand feu enameling: the base dial is 1 mm thick, and any geometric imperfection amplifies during firing until it breaks the enamel.

On the opposite front, deliberately more generous tolerances can be a functional choice. In the GCA 3002 caliber, shock resistance was increased by 40% by modifying the tolerances of the screws and internal architecture — no added damping, just redesigned geometry, like how you would change a car's suspension for rough terrain.

Then there is degradation over time. On vintage watches, steel-on-steel contact and cyclic wear produce what in English is called warping: progressive deformation of the case that widens gaps in sealing zones, reducing water resistance even when the movement is intact. It's one of the points I insist on when someone brings me a vintage watch claimed to be waterproof: the original tolerances no longer exist.

Finally, in pieces built by hand like the Greubel Forsey Hand Made 2, tolerance is not the result of a CNC machine but of the craftsman's hand, verified piece by piece. It requires more time and introduces controlled variability: each specimen is technically unique. It's not a defect. It's the price of production without automation.