Tolerances | Kaelan

A piston that fits perfectly in a cylinder won’t move. Metal expands when hot, contracts when cold. The gap between parts exists by design — clearance, not slop. Tolerance is the acceptable range of variation that still allows a system to function.

Precision isn’t the same as accuracy. A machinist can cut parts to 0.001” precision (repeatability) but be 0.010” off from the target (accuracy). The best work is both: hitting the mark consistently. But in practice, you choose your battles. Not every surface needs to be ground to mirror finish.

Fit describes the relationship between mating parts. A clearance fit has deliberate gap — a shaft spinning freely in a bearing. An interference fit requires force to assemble — a bearing pressed into a housing, held by friction alone. A transition fit could go either way depending on which end of the tolerance band you land on.

Woodworkers think in terms of seasonal movement. A tabletop expands across the grain in humid summers, contracts in dry winters. Attach it rigidly to the base and it will crack. The solution: slots that allow movement, buttons that grip but slide, breadboard ends with elongated holes. The joint accounts for what the material will do.

Accumulation of tolerances is why complex assemblies fail. Each part within spec, but stack ten of them and the errors compound. A door that won’t close. A drawer that binds. The fix isn’t tighter tolerances everywhere — that’s expensive and often impossible. It’s designing so tolerances don’t stack, or providing adjustment at the end.

The craftsman’s trick: make one part, then fit the next to it. Don’t work to drawings alone — work to the actual piece in hand. This is why hand-fit work often exceeds machine work. The human eye and hand close the loop that drawings leave open.