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Overview

A tolerance stack-up checks whether the tolerances assigned to individual parts still let the assembly meet its functional requirement — a gap that must stay open, a shaft that must turn freely in a bore, a stack of shims that must land within a target range. Building the chain by hand means tracing every contributing dimension across multiple parts and drawings, then working the arithmetic worst-case or statistical. Nexus builds the dimension chain directly from your CAD models and drawings, runs the stack-up, and tells you whether the requirement holds — and if it doesn’t, which tolerances are driving the overrun.

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What Nexus can help with

  • Worst-case stack-up — Sum contributing tolerances at their extremes to find the guaranteed bound on the resulting dimension.
  • Statistical (RSS) stack-up — Combine contributing tolerances in root-sum-square to estimate the realistic variation, assuming independent, normally distributed contributors.
  • Chain identification — Trace the dimension chain across multiple parts in an assembly, including GD&T call-outs, and confirm nothing in the path was missed.
  • Sensitivity analysis — Rank contributors by how much of the total variation each one accounts for, so you know which tolerance to tighten first.
  • Tolerance allocation — Given a target assembly requirement, recommend a tolerance for each contributor that meets it without over-tightening the whole chain.

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Example prompts

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Tips for tolerance stack-up prompts

  • State the requirement and its direction — say what the resulting dimension is (a gap, a clearance, an offset), its target range, and which direction is critical (minimum, maximum, or both).
  • List every contributor explicitly — name each part, the dimension, and its tolerance. Nexus can trace a chain from the model, but stating it upfront removes ambiguity about which path through the assembly counts.
  • Say worst-case or statistical — the two methods give very different answers. Worst-case guarantees the bound; RSS estimates the realistic spread assuming independent, normally distributed contributors. State which one your requirement calls for.
  • Include GD&T, not just size tolerances — position, concentricity, and MMC/LMC modifiers change the stack-up math. Reference the datum scheme so Nexus applies the right bonus tolerance.
  • Give process capability if you have it — real distributions are rarely centred on nominal. If you have Cpk or historical measurement data for a contributor, share it — the RSS result will be more realistic than assuming ±3σ equals the drawing tolerance.
  • Ask for the driving contributor, not just the answer — when a stack-up fails, “which tolerance should I tighten” is usually more useful than the raw number.