GD&T Symbol Guide
Straightness
Controls how much a surface line or derived median line may bend away from perfect straight form.
Definition
Straightness has two common readings. Surface straightness controls a line on a surface, while derived median line straightness controls the bend of a feature axis.
The control is independent of datums because it refines form rather than location or orientation.
Application
Use surface straightness on rails, slots, cylinders, and machined edges where a sampled line must remain uniform.
Use axis straightness when a pin, shaft, or bore must remain within a straight envelope, often with MMC for functional gaging.
Surface Line Checked For Straight Form
Surface straightness is applied to a line element. Axis straightness is applied with the feature size dimension.
3D Tolerance Zone
Surface straightness uses two parallel lines; derived median line straightness uses a cylindrical boundary.
Inspection Method
For a surface, sweep along the controlled line and compare the high-to-low variation to the straightness tolerance.
For an axis, evaluate the derived median line from measured cross-sections or use a functional gage when MMC is specified.
Worked Check: Guide Rail Line
A machined guide rail has straightness 0.05 mm along its top edge. The measured line ranges from +0.018 mm to -0.021 mm.
Variation
0.018 - (-0.021) = 0.039 mm
The line element must fit between two parallel lines 0.05 mm apart.
Compare
0.039 <= 0.05
The full sampled line is inside the zone.
Datum
None
No surface or axis is being used as a reference.
The rail passes surface straightness. Orientation to another face would need a separate control.
Comparison Table
| Control | Datum | Zone | Best Read |
|---|---|---|---|
| Form | No | Free-floating | Shape of one feature |
| Orientation | Yes | Locked to datum | Angle relationship |
| Location | Usually | Locked to datum frame | Where a feature belongs |
Notes
Straightness is the 2D cousin of flatness: it controls one line element instead of a full surface.
Axis straightness can gain bonus tolerance as the feature departs from MMC.