Straightness

Controls

Straightness limits how far a line element or feature axis may deviate from a perfect straight line. It has two distinct applications: surface straightness — applied per line element running along a flat or cylindrical surface; and axis straightness — applied to the derived median line of a feature of size such as a shaft or pin.

The tolerance zone is two parallel straight lines (surface) or a cylinder of diameter t (axis) within which the element must lie.

Example

A turned shaft Ø40h6 requires axis straightness Ø0.05. Even though each circular cross-section is perfectly round and within size, the axis could be bowed. The straightness callout ensures the shaft assembles into aligned bearing housings on a gearbox without binding or inducing bending stress.

Inspection

• Straight edge + feeler gauge — for surface line elements on flat faces
• Height gauge on V-blocks — rotate and traverse to find deviation
• CMM — probe along a line or fit a cylinder and extract the axis deviation
• Laser alignment — preferred for long shafts where sag is a concern

Datum & Modifiers

MMC and LMC are not permitted on surface line element straightness — only on features of size.