Tolerance symbols
Tolerance frame (feature control frame)
①Geometric characteristic symbol
②Tolerance value [mm] — preceded by Ø if cylindrical zone
③Material condition modifier: M = MMC, L = LMC (ASME only)
④⑤⑥Primary / secondary / tertiary datum references (if required)
Good to know
ISO 1101 vs ASME Y14.5
- Both use the same 14 symbols — notation differs slightly
- ISO uses ⊙ for position; ASME uses ⊕
- ISO concentricity = axis; ASME concentricity deprecated (use position or runout)
- Datums: ISO uses capital letters in triangle; ASME in square frame
- Unless specified otherwise, assume ISO in European drawings
Datum reference frame (DRF)
- 3-2-1 rule: primary datum constrains 3 DOF, secondary 2, tertiary 1
- Datums are theoretical perfect planes / axes / points
- Form tolerances (straightness, flatness…) never require a datum
- Location and orientation tolerances usually require 1–3 datums
Reading a tolerance frame
Example: | ⊕ | Ø 0.05 | A | B |
- Position tolerance
- Cylindrical tolerance zone Ø 0.05 mm
- Relative to datums A (primary) and B (secondary)
Envelope principle (ISO)
- By default ISO uses independency principle — form and size are independent
- Add Ⓔ to invoke envelope principle (perfect form at MMC)
- ASME Y14.5 applies envelope principle by default
- Always check which standard applies before interpreting a drawing