GD&T for shop floor engineers: the 14 symbols you actually use, with real examples
Most GD&T training is written by engineers, for engineers — heavy on theory, light on the daily reality of running a CNC machine or inspecting a finished part. This guide is the opposite. Here are the 14 GD&T symbols you actually encounter on real machined parts, what each one means in plain language, how to inspect it, and the most common mistakes you'll see on the shop floor.
The 5 categories of GD&T
Every GD&T symbol controls one of five things: form, orientation, location, runout, or profile. You don't need to memorize the taxonomy, but it helps to know which symbol checks what kind of error.
| Category | What it controls | Symbols |
|---|---|---|
| Form | Shape of a surface, by itself | Flatness, Straightness, Circularity, Cylindricity |
| Orientation | Angular relationship to a datum | Perpendicularity, Parallelism, Angularity |
| Location | Position relative to datums | Position, Concentricity, Symmetry |
| Runout | Wobble of a rotating surface | Circular Runout, Total Runout |
| Profile | Shape of a complex surface or line | Profile of a Surface, Profile of a Line |
Form tolerances (control shape, no datum needed)
Flatness
The surface must lie between two parallel planes spaced X apart. No reference to any other feature.
Example: ▱ 0.05 on a casting mating surface = every point on that surface must sit within a 0.05 mm thick zone.
Straightness
An axis or surface line must lie between two parallel lines (or in a cylindrical zone for an axis) spaced X apart.
Example: — 0.02 on a shaft = the axis bow must be under 0.02 mm over the length.
Circularity (Roundness)
Every cross-section of a cylinder or sphere must be circular within X. Each section is checked independently.
Example: ○ 0.01 on a bored hole = at any height, the hole's cross-section is round within 0.01 mm.
Cylindricity
The entire cylindrical surface must lie between two concentric cylinders spaced X apart. Stricter than circularity — combines roundness, straightness, and taper.
Example: ⌭ 0.02 on a precision bushing = the whole O.D. fits in a 0.02 mm thick cylindrical shell.
Orientation tolerances (relative to a datum)
Perpendicularity
A surface or axis must be 90° to the datum, within a tolerance zone of width X.
Example: ⊥ 0.05 | A on a milled face = the face is square to datum A within a 0.05 mm wide zone.
Parallelism
A surface or axis must be parallel to a datum, within a zone of width X.
Example: ∥ 0.02 | B on a guide rail = the rail surface is parallel to datum B within 0.02 mm.
Angularity
A surface must sit at the specified angle (not 0°, not 90°) relative to a datum, within X.
Example: ∠ 0.1 | A with a 30° basic angle = the surface is at 30° to A within 0.1 mm.
Location tolerances (where the feature is)
Position (the most common GD&T symbol on real drawings)
The actual location of a hole's axis (or any feature) must fall within a circular or cylindrical tolerance zone of diameter X around the theoretical perfect location.
Example: ⊕ Ø0.4 | A | B | C on a bolt hole = the hole's axis must lie within a 0.4 mm diameter cylinder around the basic location, measured from datums A, B, C in that priority.
Concentricity (rare, often deprecated)
The median points of a cylindrical feature must lie within a cylindrical zone centered on a datum axis. Mostly replaced by Position or Runout in ASME Y14.5-2018.
Example: ◎ Ø0.03 | A = the centerline of the feature is within 0.03 mm of datum axis A. Hard to measure — most shops use Runout instead.
Symmetry
The median plane of a feature must be centered on a datum plane within X. Also rare in modern drawings — usually replaced by Position.
Example: ⌯ 0.1 | A on a keyway = the keyway's centerline plane is within 0.1 mm of datum A's center plane.
Runout tolerances (for rotating parts)
Circular Runout
As the part rotates around a datum axis, a dial indicator on the surface must not vary more than X at any single cross-section.
Example: ↗ 0.05 | A on a shaft O.D. = dial indicator total runout at any one position is under 0.05 mm.
Total Runout
Same as Circular Runout but the indicator traverses the full length while rotating. Catches taper and bow that Circular Runout misses.
Example: ⌰ 0.08 | A | B on a long shaft = total error across the entire length, including taper, is under 0.08 mm.
Profile tolerances (for complex shapes)
Profile of a Surface
A 3D surface (free-form, casting, airfoil) must lie within a 3D tolerance zone of thickness X, centered on the theoretical surface.
Example: ⌓ 0.5 | A | B on a casting blend = the actual surface stays within a 0.5 mm thick zone wrapped around the CAD model.
Profile of a Line
Like Profile of a Surface, but only along a single cross-section line. Used for 2D profiles (extruded shapes, sheet metal edges).
Example: ⌒ 0.2 | A on a stamped edge = each cross-section of the edge fits in a 0.2 mm thick line zone.
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Open the Balloon Tool — Free →Datums explained (the part everyone gets wrong)
GD&T tolerances are useless without something to measure FROM. That something is a datum. On a drawing, datums are marked with a square containing a capital letter — A, B, C in order of priority.
Three rules:
- Order matters. A datum reference like
⊕ Ø0.4 | A | B | Cmeans: align to A first, then to B (constrained by A), then to C (constrained by A and B). Reverse the order and you'll get a different inspection result. - Datums are theoretical, not physical. The datum is a perfect plane or axis derived from the actual surface. A real surface is wavy; the datum is the best-fit perfect feature.
- 3-2-1 rule. Primary datum (A) constrains 3 degrees of freedom, secondary (B) constrains 2 more, tertiary (C) constrains the last 1. Six degrees of freedom locked → the part is fully located.
Material condition modifiers
You'll often see a small Ⓜ or Ⓛ next to a tolerance value inside a feature control frame:
- Ⓜ (Maximum Material Condition) — the tolerance applies when the feature is at its maximum material size (smallest hole, largest pin). As the feature departs from MMC, you get bonus tolerance.
- Ⓛ (Least Material Condition) — opposite. Tolerance applies at the LMC size; bonus tolerance available toward MMC.
- No modifier (RFS — Regardless of Feature Size) — tolerance applies regardless of the feature's actual size. The default if nothing's shown.
Practical impact: Ⓜ gives you bonus tolerance, which is why it's preferred for clearance holes on assembly drawings. RFS is stricter and used where fit is critical (bearings, gears).
Reading a complete feature control frame
A feature control frame (FCF) is the boxed thing on a drawing that ties it all together:
│ ⊕ │ Ø 0.4 Ⓜ │ A │ B Ⓜ │ C │
Read left to right:
- ⊕ — control type (position)
- Ø 0.4 — tolerance value and zone shape (diameter 0.4 mm cylindrical zone)
- Ⓜ — material condition (MMC)
- A — primary datum
- B Ⓜ — secondary datum at MMC
- C — tertiary datum at RFS
Three GD&T mistakes that wreck inspection reports
- Treating GD&T tolerances as a "± value". They're not bilateral. A position tolerance of 0.4 means a 0.4 mm diameter zone around the basic location — not ±0.4 mm. Confusing the two leads to false rejects (or worse, accepting bad parts).
- Ignoring datum order. Measuring position with datum priority B-A-C instead of A-B-C is a different inspection. The numbers won't match and you'll spend hours arguing with the customer.
- Forgetting the basic dimensions. Basic dimensions (boxed numbers on the drawing) are theoretical — they don't have tolerances. The tolerance lives entirely in the GD&T frame. Measuring against a non-existent ± on a basic dim is a beginner's mistake.
How CadNexa handles GD&T
When you auto-balloon a drawing with GD&T frames, CadNexa detects each frame as a separate inspection feature, parses the symbol type (position, perpendicularity, etc.), extracts the tolerance value and material condition, and links the datum references. The FAI report then groups GD&T frames into their own section so the inspector knows which surface to fixture against.
For drawings where the GD&T symbols are partially garbled by OCR — common on scanned or low-resolution PDFs — CadNexa shows the original cropped image of the frame in the report instead of the parsed text, so the inspector always has the source of truth.
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