01
Functional requirement
Identify fit, seal, alignment, motion, load or assembly consequence before tightening a dimension or geometric control.
Evidence route
CTQ rationale and mating/interface reference
Tolerance and metrology planning
CNC tolerances and inspection: control the feature, datum and method together.
A tolerance is not complete control until the requirement, datum system, manufacturing route, measurement method, coverage and release owner agree. PartVerity treats tolerance as a feature-specific decision rather than a universal website number.
Decision premise
Tolerance question: can the selected route repeatedly create the requirement, and can the agreed method credibly verify it?
Engineering control
Six links in a defensible tolerance chain
Tighter is not automatically safer. Unnecessary precision can add setups, grinding, special fixtures, controlled environments and inspection effort without improving function.
02
Datum strategy
Define datums that reflect function and can be established in both machining and measurement without contradictory interpretations.
Evidence route
Datum reference frame and setup alignment
03
Process capability review
Review tolerance against size, geometry, wall condition, material, heat treatment, finishing and supplier route.
Evidence route
Feature-by-feature capability response and exceptions
04
Measurement method
Match instrument, fixture, resolution and access to the characteristic and record limitations that affect the result.
Evidence route
CTQ-to-method table, equipment class and setup reference
05
Coverage and actual results
State FAI, sample, in-process or 100% coverage and keep actual dimensional or geometric results tied to part and lot identity.
Evidence route
Ballooned characteristic list, actual values and sample identity
06
Disposition authority
Define who may accept, hold, rework or seek concession when a measured result falls outside the controlled requirement.
Evidence route
Named release owner, NCR link and disposition status
Decision matrix
Tolerance cost and confidence drivers
The effect comes from the combination of feature, route and verification—not from the nominal tolerance value in isolation.
| Driver | Possible route effect | Evidence to request |
|---|---|---|
| Tight position across setups | Datum transfer, probing, fixture or single-setup route | Setup plan and datum-verification results |
| Thin wall or long slender form | Support strategy, rough/finish sequence and thermal control | Process plan and dimensions after release from clamping |
| Post-machining heat or finish | Stock allowance and final-operation or post-finish inspection | Pre/post-process requirement and final results |
| Form, runout or concentricity | Controlled reference setup and suitable rotary/coordinate method | Datum setup, method and actual geometric result |
| Cosmetic plus dimensional CTQs | Separated visual and metrology acceptance routes | Visual standard, masking and dimensional evidence |
Claim boundaries
What a tolerance table cannot decide
Published platform tolerances do not prove a selected supplier can hold your feature.
A CMM report is not credible without datum, program and part traceability.
A single first article does not prove long-term process stability.
General tolerances do not override explicitly dimensioned or function-critical requirements.
Technical context
Use guidance to frame the review—not to replace project evidence.
The source explains tolerance-cost trade-offs and explicitly frames numeric capability as provider- and geometry-specific. PartVerity publishes no blanket tolerance guarantee.
Protolabs tolerance design guidance