A 0.1mm tolerance stackup caused 20% scrap

A company designed a shaft-and-housing assembly with a clearance fit. The shaft had a tolerance of +0.0/-0.05mm, the housing bore +0.0/-0.05mm, and a spacer ring between them +0.05/-0.0mm. Each part individually met their limits, but when stacked, the worst-case gap was 0.15mm too small, causing interference and binding.

The problem was that they tolerance each part independently without considering the assembly stackup. The designer assumed the ring thickness would always add clearance, but the tolerance directions actually subtracted clearance. A simple tolerance analysis would have shown the risk.

The fix was to use a tighter tolerance on the ring or change the fit class. This example shows why you should always perform a worst-case or RSS stackup analysis on functional gaps, even for simple assemblies.

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