GD&T symbol · Form tolerance

Straightness — Form Tolerance

Controls how straight a line element of a surface or an axis must be.

Definition

Straightness is a Form control: it limits how far a single line element of a surface — or the axis of a feature of size — may depart from a perfectly straight line. Like all Form tolerances it is self-referencing: the zone is established from the feature itself, with no datum reference frame and no basic dimensions. What makes straightness unusual is that it has two genuinely different applications. On a surface it controls line elements in two dimensions; on a feature of size (a pin, shaft, or bore) it controls the derived median line in three. The two cases have different zones, different rules, and — in the axis case — a material-condition option no other Form control gets.

Surface straightness: line elements

Applied to a surface, straightness controls individual line elements, not the surface as a whole. The tolerance zone is the space between two parallel lines a distance t apart, and the requirement is applied one cross-section at a time: each line element of the surface, taken in the direction the callout points, must lie between those lines. Elements are independent — a wavy surface still passes as long as every individual element does. Surface straightness says nothing about how the elements relate to each other, so it is the right tool when only the straightness of a line matters, not the overall form of the surface.

Side view of one surface line element: the zone is two parallel lines 0.1 apart, and the actual element must lie between them. Every element along the surface is checked the same way, one section at a time — surface straightness says nothing about how the elements relate to each other. Schematic, exaggerated.

Axis straightness: a cylindrical zone

Applied to a feature of size — a pin, shaft, or bore — straightness controls the derived median line: the line through the centers of the feature's cross-sections. The tolerance zone is a cylinder of diameter t centered on the feature's nominal axis, and the entire median line must lie within it, evaluated in three dimensions. This is a fundamentally different and generally stricter requirement than the two-dimensional surface case — a shaft can have perfectly round cross-sections everywhere and still fail axis straightness because its centerline bows.

Side view of a shaft: the axis-straightness zone is a cylinder of diameter Ø0.05 around the nominal axis, and the entire derived median line — the line through the centers of the shaft's cross-sections — must lie inside it. The dashed circle at the right end is that cylinder in section, with the median line's actual point offset inside it. Schematic, exaggerated.

MMC and bonus tolerance (axis only)

Axis straightness is the one Form control that may be applied at Maximum Material Condition — a commonly missed nuance, since the other Form controls (flatness, circularity, cylindricity) never take a material modifier. At Ⓜ, the stated tolerance applies when the feature is at MMC, and every departure from MMC adds a bonus equal to that departure. For a shaft, MMC is the largest size, so a shaft produced smaller gains bonus; for a bore, MMC is the smallest size, so a bore produced larger gains it too. The guarantee that matters — a shaft still fits through its mating bore — holds even as the bow increases, because the bonus is bought by clearance.

Schematic example: the feature at MMC (Ø6.0) carries the Ø0.05 zone. Produced at Ø5.6 — 0.4 under MMC — a pin departs from its largest size by getting smaller — the zone grows by exactly that bonus, to Ø0.45. Feature and zone are not drawn to the same scale.

Reading a feature control frame

A straightness frame is simpler than a position frame: no datum references, no basic dimensions. — 0.1 reads as a straightness tolerance of 0.1 width applied to surface line elements. The axis application adds the diameter symbol — the zone is a cylinder, not a width — and may add the MMC modifier: Ø 0.05 Ⓜ reads as a straightness tolerance of 0.05 diameter on the derived median line, evaluated at MMC. The contrast with the position frame ⌖ ⌀0.5 Ⓜ A B C is exactly this: position locates a feature from datums, while straightness references only the feature itself.

The feature control frame, compartment by compartment. Only the axis application adds the diameter symbol and the Ⓜ modifier — and note the absence of any datum reference: straightness, like all Form controls, is self-referencing.

Measurement

Surface straightness — indicator sweep — run a dial indicator along the surface in the controlled direction; the total indicator reading across the traverse must stay within t. Simple and adequate for most rails, ways, and long flat elements.

Axis straightness — CMM or precision spindle — measure cross-sections along the length, derive the median line, and check it against the cylinder around the nominal axis. Rotary-table and V-block setups approximate this but sample only the sections they take; a bow between samples can hide.

When to use it

Long guides and rails where a straight line element carries the function — linear bearing ways, gibs, carriage edges
Shafts and bores that must assemble through a length — a bowed shaft jams even when every cross-section is perfectly round
Features of size where the axis application's MMC modifier buys bonus tolerance while still guaranteeing assembly
Cases where only one direction of form needs control, and a full surface or cylindricity callout would be overkill