Engineering · GD&T
Straightness.
A 2D form tolerance that controls how straight a line element of a surface, or the axis of a feature, must be. No datum required — straightness is purely about the feature itself.
01 · Definition
Two interpretations.
Straightness applied to a surface means: every line element of the surface (in the direction indicated by the FCF placement) must lie within a 2D tolerance zone bounded by two parallel lines.
Straightness applied to a feature of size (axis or median plane) means: the derived axis or median plane must lie within a cylindrical or planar tolerance zone of the specified diameter or width.
The two interpretations have very different production consequences. Surface straightness is checked with a straightedge or surface-trace probe. Axis straightness is computed from the actual feature’s derived center axis — usually CMM territory.
02 · Reading the FCF
How to interpret the callout.
The presence of a diameter symbol (Ø) tells you it’s axis straightness — the tolerance zone is a cylinder around the derived axis. Without the Ø, it’s a 2D zone applied to surface line elements.
03 · Worked example
Long shaft with — Ø 0.002 callout.
A 12″-long shaft with a 0.500″ diameter and a — Ø 0.002 axis-straightness callout. Inspection: probe cross-sections along the full 12″ length on a CMM, derive each section’s center, and verify the derived median line lies within the Ø 0.002″ cylindrical tolerance zone.
With Ⓜ (MMC) modifier, bonus tolerance is available as the actual feature departs from MMC. If the OD measures 0.498″ instead of the MMC of 0.500″, you get an additional 0.002″ of straightness tolerance — total allowable departure 0.004″.
For machining, axis straightness on long shafts is typically achieved by Swiss-type turning with guide-bushing support. See Swiss vs. CNC lathe.
04 · Common mistakes
What goes wrong.
- Confusing straightness with size: under Rule #1 (the envelope principle), the size tolerance already limits how far surface line elements can bow. Surface straightness is only meaningful when it is tighter than the size tolerance — equal or looser adds nothing. Axis straightness (“— Ø”, derived median line) is exempt from Rule #1 and may exceed the size tolerance; the virtual condition (MMC size + straightness tolerance) then sets the mating boundary.
- Surface straightness on round features: for cylindrical features, surface straightness applies to longitudinal line elements. Cylindricity covers the entire surface in one tolerance.
- Forgetting the Ø: the difference between “— 0.002” and “— Ø 0.002” is enormous. The first is 2D surface; the second is 3D cylindrical zone around the axis.
- Inspection method matters: hand-gauge straightness with a straightedge ≠ CMM-derived axis straightness. Specify the method when tolerance is tight.
Related
Form tolerances.
- Flatness — 3D version of surface straightness
- Circularity — round-section form
- Cylindricity — combines straightness, circularity, taper
- Full GD&T symbols reference
Keep exploring
Related tools & references
Full feature-control-frame symbol reference.
The 3D surface form control — straightness applied across a whole plane.
Combines straightness, circularity, and taper on a round feature.
Check how form tolerances accumulate across an assembly.
How tight to call out form tolerances — and what it costs.
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