Product Engineering
Five tolerances adding to 7.9% stack to 3.74% in reality. Designing to worst case throws capability away.
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Realistic combined tolerance on the finished product
Root sum square requires the contributions to be independent, and textile ones frequently are not — yarn count and crimp both move with tension, so a common cause correlates them and the true stack sits between RSS and worst case. Where a shared cause is known, combine those terms arithmetically first and RSS the groups. The tolerances entered must all be at the same confidence, conventionally three sigma; mixing a three-sigma yarn tolerance with a full-range process tolerance produces a meaningless total. A required tolerance returned as zero means the tightened specification cannot be met by improving that one contributor alone.
Tolerance Stack-Up Calculator for Textile Specifications — free, with the formula and a worked example, at Textile School.