— Axis straightness
Axis straightness · Forma · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The derived axis of the cylindrical feature must stay within a cylindrical tolerance zone 0.03 mm in diameter along its whole length, relative to datum A.
What controls
Straightness applied to the derived median line (the theoretical center) of a cylindrical feature, not to its surface. With ⌀ in front of the value, the tolerance zone is an imaginary cylinder of that diameter within which the actual axis must lie along its entire length — not a pair of planes as in line straightness. It is the correct way to control the full axis of a rod or stud; because it applies to a feature of size, it is the only form of straightness that accepts Ⓜ or Ⓛ.
How to verify
On a CMM, several circular sections are taken along the axis, the center of each is calculated by least squares, and the deviation of the resulting axis from a fitted straight line is evaluated. In a shop without a CMM, a practical approximation is to measure runout at several axial positions with the part rotating between centers: if the runout varies systematically (not randomly) along the axis, it indicates real curvature of the axis, not just workholding eccentricity.
tolerance zone
Imaginary cylinder with a diameter equal to the tolerance value; the actual axis must lie inside it along its entire length.
Workshop guideline values
- General
- 0.05–0.15 mm on general-machining shafts and studs.
- Accuracy
- 0.01–0.03 mm on ground shafts or guided bushings.
- High precision
- 0.003–0.008 mm on spindles or precision shafts.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Forgetting the ⌀ in front of the value: without it, the zone is the space between two planes, not a cylinder, and the acceptance criterion changes.
- Measuring only the part diameter and assuming axis straightness is guaranteed: a shaft can meet its diameter dimension along its entire length and still be bent.
How to get it in the workshop / if it comes out of tolerance
Very common on long shafts and studs: the symbol goes in the feature control frame with ⌀ in front of the value. A bent shaft can pass on diameter but fail straightness.
The most common cause on long shafts is deflection from cutting forces: reduce feed and depth of cut, use fixed steady rests or intermediate follow rests, and avoid excessive overhang between chuck and tailstock. If the shaft comes out bent after hardening, it is heat-treat distortion: apply controlled straightening or leave stock to grind after heat treatment, not before.
Alternative symbol:⌀ —
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