How It Works

This skill converts an assembly requirement (gap, alignment, preload, runout, travel)

into a tolerance chain, then evaluates whether the design is robust to manufacturing

variation. It prioritizes functional dimensioning and early discovery of the kinds of

problems that only show up when you put everything together.

The core issue: no tolerance stack-up was performed on critical functional loops, the

wrong datum scheme was used so the chain doesn't match function, or statistical

assumptions (RSS) were applied without verifying process capability. The result is

prototypes that don't assemble without rework, shimming, or selective assembly — even

though every individual part passes inspection.

A correct functional tolerance chain with sign convention for your assembly
Worst-case and RSS calculations (and Monte Carlo when non-linearities or mixed distributions matter)
Identification of the top 20% of contributors causing 80% of the variation
Redesign recommendations: tolerance reallocation, datum changes, adjustability, process changes, or inspection/assembly changes
An inspection plan aligned to the functional chain, not just individual print dimensions
Early prediction of fit risk and expected yield before you cut metal

State your assembly requirement as a measurable functional metric (gap, coaxiality, preload, etc.)
Provide drawing or CAD data with nominal dimensions and tolerances for each contributor
Define the datum references and assembly locating features
Include manufacturing capability data (Cp/Cpk) or historical process variation if available
Run the skill — it builds the chain, runs the analysis, and flags what to fix

If the stack fails, the skill recommends specific fixes: reallocate tolerance to critical contributors, change locating strategy, add adjustability (slots, shims, eccentrics), or change manufacturing process for specific features
Don't forget unmodeled contributors — geometric tolerances, deformation under load, and thermal growth are common blind spots
Validate with a build/inspection plan that measures both the chain contributors and the functional result
Works for any assembly type: bearing stacks, housing fits, optical alignments, preloaded joints, or kinematic linkages
Example: a bearing stack (housing + spacer + cover) meets each part tolerance, but endplay is out of range — build the axial stack, include gasket compression, and redesign the locating faces or add shims

Tolerance Stack-Up Failure in Assemblies