— Straightness

How to verify

Rest the part on two V-blocks (or on a granite surface plate if it is a flat edge) and sweep a dial indicator along the specified line, recording the maximum and minimum readings: the difference between them is the straightness deviation, not each reading on its own. An optical instrument (autocollimator) or a CMM with a linear scan gives better accuracy on long or demanding parts. Avoid measuring over tool marks, burrs or stuck chips: they throw off the reading.

Workshop guideline values

General

0.1–0.3 mm on edges or surface elements of conventionally machined parts.

Accuracy

0.02–0.05 mm on ground guideways or close-fit parts.

High precision

0.005–0.01 mm on gauges, master straightedges or precision grinding.

Guidance only: the drawing's value always wins.

Common mistakes when interpreting it

• Confusing it with axis straightness: without ⌀ in front of the value, it does NOT control the full axis of a cylinder, only the specific line indicated on the drawing (an edge or surface element).
• Applying it as if it were parallelism: without a datum, straightness does not compare anything to another face; it only evaluates the line against itself.