Tolerance Stack Calculator — Multi-Part Assembly

A tolerance stack is the cumulative effect of individual part tolerances in an assembly. When multiple parts fit together, each part's dimensional variation adds up, potentially causing the assembly to be too tight or too loose.

Worst case assumes every part is at its maximum tolerance simultaneously, which is extremely unlikely. Statistical (RSS) analysis uses root sum of squares to calculate a more realistic expected variation, typically at a 3-sigma confidence level covering 99.7% of assemblies.

A well-calibrated FDM printer typically holds about plus or minus 0.1-0.2mm in XY. Z accuracy is bounded by the layer height and stair-stepping, so it is usually equal to or slightly worse than XY (roughly plus or minus 0.1-0.2mm) — not tighter. SLA printers achieve plus or minus 0.05-0.1mm. Use your own measured printer accuracy for best results.

Design datum surfaces so parts reference from common features. Reduce the number of parts in the tolerance chain. Use locating pins or alignment features. Print critical mating features in the same orientation for consistent shrinkage.