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True Precision Machining
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Engineering quick reference

GD&T Symbol Reference

The 14 geometric tolerance characteristics — symbol, category, datum requirement, and plain-English definition (ASME Y14.5).

True Precision Machining

How to read a feature control frame — true position Ø0.010 at MMC, datums A | B | C

Anatomy of a feature control frame: a section view of a plate shows the frame's leader pointing at a hole, with datum feature A on the seating face. The frame itself reads, left to right, position symbol, diameter symbol, 0.010, circled M, then datums A, B, C — each compartment labeled characteristic, zone shape, tolerance, modifier, and primary, secondary, tertiary datum. A top view shows the actual hole axis as a dot inside the cylindrical 0.010 tolerance zone centered at true position, with datum features B and C on the plate edges. A ON THE DRAWING Section view — the frame’s leader points at the hole. A is the seating-face datum. Ø 0.010 M A B C ZONE SHAPE Ø = CYLINDRICAL MODIFIER Ⓜ = AT MMC SECONDARY DATUM ALIGNS · 2 DOF CHARACTERISTIC WHAT IS CONTROLLED TOLERANCE TOTAL ZONE — NOT ± PRIMARY DATUM SEATS · 3 DOF TERTIARY DATUM STOPS · 1 DOF FEATURE CONTROL FRAME Read left to right — what is controlled, the zone, the modifiers, then the datums that anchor it. Ø0.010 ZONE B C THE TOLERANCE ZONE Basic dims from B and C set true position; axis (dot) must stay in the Ø0.010 zone (exaggerated).

Key: Req = datum reference required · — = none · Ⓜ MMC · Ⓛ LMC · no modifier = RFS · FIM = full indicator movement

Sym Characteristic Datum Definition
Form No datum — feature judged against itself
— Straightness — All points on a feature lie within a tolerance zone bounded by two parallel lines (axis or surface).
▱ Flatness — All points on a planar surface lie within a tolerance zone bounded by two parallel planes.
○ Circularity (roundness) — At any cross-section, the surface must lie between two concentric circles separated by the tolerance value.
⌭ Cylindricity — Combines straightness, circularity, and taper into one tolerance — the whole cylindrical surface lies between two coaxial cylinders.
Orientation Datum required
∥ Parallelism Req A surface or axis must be parallel to a referenced datum within the tolerance zone.
⟂ Perpendicularity Req A surface or axis must be perpendicular to a referenced datum within the tolerance zone.
∠ Angularity Req Surface or axis at a specific basic angle to a datum, within the tolerance zone.
Location Datum required
⌖ Position (true position) Req An axis or feature center must lie within a cylindrical or other-shape tolerance zone at the basic position relative to datums.
◎ Concentricity Req Median points of a feature lie within a cylindrical zone coaxial with a datum axis. Withdrawn in ASME Y14.5-2018 (still seen on legacy Y14.5-2009 drawings) — replace with position-of-axis or runout.
⌯ Symmetry Req Median points of feature lie within a planar zone symmetric to a datum plane. Withdrawn in ASME Y14.5-2018 (still seen on legacy Y14.5-2009 drawings) — replace with position.
Runout Datum axis required — checked while rotating
↗ Circular runout Req FIM (full indicator movement) at any single cross-section as the part rotates about a datum axis.
↗↗ Total runout Req FIM across the entire surface as the part rotates and the indicator traverses axially.
Profile Datum optional
⌒ Profile of a line — A line element of a surface lies within a 2D tolerance zone defined by basic dimensions. Used for 2D profiles like a cross-section.
⌓ Profile of a surface — A 3D surface lies within a 3D tolerance zone defined by basic dimensions. Most commonly applied to complex curved or contoured surfaces.

ASME Y14.5-2018 defines 12 current geometric characteristics. Concentricity and symmetry were withdrawn in the 2018 revision (still found on legacy Y14.5-2009 and earlier drawings) — replace concentricity with position-of-axis or runout, and symmetry with position. Feature control frame reads left to right: symbol · tolerance (with Ø for a cylindrical zone) · material modifier (Ⓜ MMC / Ⓛ LMC / none = RFS) · datum references A B C. For worked examples and datum guidance, see the full GD&T symbols reference.

Free engineering reference from True Precision Machining
trueprecisionmachining.com · 805-964-4545 · Buellton, CA · AS9100 / ISO 9001 / ITAR
Per ASME Y14.5-2018 — 12 current characteristics plus concentricity & symmetry (withdrawn in 2018, still seen on legacy drawings). · September 2026