Tolerance stack-up

Every manufactured dimension varies within a tolerance. When several parts stack together (a PCB in a housing, a lid on a base, a shaft through bearings) those variations add up. A stack-up analysis follows the chain of dimensions to a critical result, such as a gap, an interference or a gasket compression, and checks whether it stays acceptable.

Worst-case analysis assumes every dimension sits at its limit at the same time: safe, but often expensive. Statistical methods such as root-sum-square (RSS) assume the variations are independent and roughly normally distributed, so they rarely all hit their extremes together. That permits looser, cheaper tolerances in exchange for a small predicted rate of misfit.

Where it bites: stack-ups ignored until the first production parts arrive show up as lids that won't close, sticking buttons, leaking seals and rattles. Tightening every tolerance to compensate drives cost up.