– How do vintage watches have such incredible tolerances?

Question

I see vintage watches repaired. When the case is opened the movements are so well-designed and engineered, with gear teeth cut to the 0.1 MM level and thin, perfect jewels. How is such mechanical precision possible from decades ago?

In: Engineering

Answer 1

You’re using the word “tolerance” in a way that implies you don’t know what it really means.

Really old watches didn’t have the parts made to a tolerance, they had each part made to fit that specific watch.

A production tolerance is a fairly modern concept, where you design a product such that all parts within a certain tolerance will fit and function. So the blueprint will call out toleranced dimensions, say 2mm +0 -0.05 would mean anything from 1.95 to 2.00mm is within tolerance. A pin or axle shaft made to that tolerance will fit more or less tightly in a hole toleranced as 2mm +0.05 -0, meaning a hole anywhere from 2.00 to 2.05mm is in tolerance. You design all the parts and their tolerances such that any combination of variations within the tolerance of each part will still make a functional product.

Old timey manufacturing wasn’t based on this same system of tolerances and interchangeable parts. Instead you had more approximate dimensioning, with some parts dimensioned in the blueprint and others made “to fit”. So the watchmaker would have a set of ground jewel bearings, for instance, and those would be whatever dimensions the jewel polisher had made them. Some variation to be expected. He would then make the mating axles to fit the jewels, not to fit a dimension on a blueprint. Simply polish down the axles slowly and test fit along the way, until the fit is perfect. All other mating surfaces are fit similarly- you make parts the right shape and slightly oversize, then carefully polish them down until they fit together just right.

Handmade mechanisms made to fit like that can have even closer fit of parts than most modern production, because you don’t have to allow for slack caused by stacking tolerances (if all parts end up towards the minimum or maximum tolerance you could get either a very loose or tight fit in total). The downside is it’s very labour intensive, and you cannot simply swap in a new part to repair something because parts form one handmade mechanism won’t fit a seemingly identical watch without hand fitting.