Straightness is a GD&T form control that limits how much a single line can bow or wave. On a surface it applies to each line element on its own; on a pin or hole it can apply to the axis instead. It never uses a datum.
Straightness has two quite different uses, and the feature control frame tells you which one you have. When the frame points at a surface (the leader arrow touches the surface or an extension line of it), every individual line element along that surface must be straight. Think of laying a straight edge along a shaft at one spot, then rotating it and laying it again: each of those lines is checked separately.
When the frame sits under the size dimension of a pin or hole, it controls the derived median line: the line through the centres of all the cross-sections. That is the version you use when a long pin must still slide into a bore even if its surface is slightly wavy, and it is the only version that can take a material condition modifier.
Surface straightness says nothing about how the line elements relate to each other. A part can pass straightness on every line and still be barrel-shaped or tapered, which is why long precision diameters often get cylindricity instead.
For a surface line element, the zone is two parallel lines, the tolerance value apart, in the plane that contains the line being checked. The line may sit anywhere and tilt freely between them; only its own waviness counts.
For a derived median line, the zone is a cylinder whose diameter is the tolerance value (the frame shows a diameter symbol). The centre points of every cross-section must fall inside that cylinder.
Straightness is a pure form control, so it never references a datum. The feature is compared only with a perfect line of its own; where that line sits on the part does not matter.
| Modifier | With straightness |
|---|---|
| MMC or LMC | Only when straightness is applied to a feature of size (the derived median line). The tolerance then grows as the pin or hole departs from that material condition. Surface straightness cannot take a modifier. |
| Diameter symbol | Used for derived median line straightness, because the zone is a cylinder. Never used for a surface line element. |
| Free state | Can be added for thin or flexible parts measured unrestrained. |
Example: a Ø12 mm guide pin, 80 mm long, with this frame pointing at its surface
Result: Every line element is straight within 0.05 mm, so the pin passes straightness. Its size must still be checked separately against the Ø12 limits.
Every GD&T symbol page: GD&T symbols chart
No. Straightness is a form control and compares the feature only with a perfect line. Adding a datum to a straightness frame is a drafting error.
Surface straightness applies to each line element on the surface and uses two parallel lines as the zone. Axis (derived median line) straightness applies to the centre line of a feature of size, uses a cylindrical zone and can take MMC or LMC.
Surface straightness cannot usefully exceed the size tolerance when Rule #1 applies, because the envelope already limits it. Derived median line straightness can exceed it, because it is allowed to break the perfect-form envelope.
Usually by running an indicator along each line on a surface plate, or by probing points on a CMM and fitting the minimum zone. For axis straightness at MMC, a full-length ring gauge is a fast functional check.
No. Straightness controls one line at a time; flatness controls the whole surface. A surface can have straight lines in one direction and still be twisted.
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