Datums & material conditions
The concepts most likely to change how a print is interpreted or inspected.
Machinist reference
Geometric Dimensioning & Tolerancing
An original, shop-focused ASME Y14.5-2018-style reference for interpreting common callouts, tolerance zones, datums, modifiers and inspection intent.
ASME-Style GD&T
Feature control frame — quick anatomy
Read left to right. Not every frame uses every compartment.
⌖
geometric control⌀ 0.20 Ⓜ
zone + tolerance + modifierA
primary datumB Ⓜ
secondary datumC
tertiary datumOther modifiers you may see in or near a frame: Ⓛ least material condition, Ⓟ projected tolerance zone, Ⓕ free state, Ⓣ tangent plane, Ⓤ unequally disposed profile.
Example only. The exact meaning depends on the controlled feature, the drawing's stated standard/edition, and the modifiers actually shown.
Geometric controls
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Shop interpretation
Legacy drawing note
Edition matters
Concentricity ◎ and symmetry ⌯ may appear on older ASME Y14.5 drawings, but their symbols/definitions were removed from Y14.5-2018. On current drawings their functional intent is typically covered by position, profile or runout. Always interpret a drawing according to the standard and edition stated in its title block or governing documentation.
Method & sources
Sources: NIST's 2023 Fundamentals of Geometric Dimensioning and Tolerancing presentations, NIST dimensional-metrology publications and ASME's public Y14.5 overview.
Scope: practical interpretation aid, primarily aligned to ASME Y14.5-2018 concepts. It is not the ASME standard and does not replace the drawing's governing standard, customer requirements, engineering authority or inspection procedure.
NIST: Fundamentals of Geometric Dimensioning and Tolerancing · ASME Y14.5 public overview