Tolerance stack-up and GD&T guide
Evaluate how part dimensions affect assembly outcome with datum selection and geometric tolerancing examples.
Prepared and technically reviewed by: TORQYX Engineering Team · Methodology and team
This guide is prepared for quick engineering review. Critical design decisions still require the relevant standard, supplier data, and engineering approval.
Problem / Objective
Dimensions that look correct one by one can create a poor assembly result. Tolerance stack-up and datum strategy define how parts come together.
Assumptions
- Assembly function and critical dimension are defined.
- Manufacturing tolerance capability is known.
- Inspection fixture matches the datum strategy.
Step by step method
- Define the critical functional dimension.
- List dimensions in the tolerance chain.
- Separate worst-case and statistical approaches.
- Choose datums based on assembly function.
- Define measurement method in the control plan.
Common mistakes
- Choosing drawing datums randomly.
- Tightening every tolerance equally.
- Thinking about measurement only after production.
Related calculators
Related glossary terms
Quick FAQ
Question: What does tolerance stack-up show?
It shows how multiple dimensional tolerances affect final clearance or interference.
Question: When is GD&T needed?
It is needed when geometric relationship, orientation, or position is critical to function.
Question: How should a datum be selected?
Use surfaces that locate the part in assembly and can be measured repeatedly.
Verification notes
Start from the functional requirement and work backward to the dimensions that control it. Compare worst-case and statistical stack-up only when the production distribution and independence assumptions are justified. The drawing should connect each critical tolerance to a datum scheme, a measurement method, and an acceptance decision.
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