◎ Coaxiality
Coaxiality · Ubicación · ISO 1101
Example in plan
How to interpret it
The axis of the cylindrical feature must coincide with the axis of datum A within a cylindrical zone of 0.05 mm: both axes must be coaxial along the whole evaluated length.
What controls
Controls how closely the derived axis of a cylindrical feature coincides with the axis of a datum that is also cylindrical. Unlike concentricity (which compares the centers of individual sections), coaxiality compares the full axis of the feature with the datum axis, treating both as derived straight lines.
How to verify
It is measured the same way as concentricity: a dial indicator with the part rotating between centers or in a V-block, or a CMM calculating the actual derived axis and comparing it with the datum axis. On recent ISO drawings it is more common to express it directly as a position tolerance with ⌀ applied to the axis, which avoids the ambiguity of how the "derived axis" is defined in measurement practice.
tolerance zone
Cylinder around the datum axis within which the derived axis of the feature must lie along its entire length.
Datums and notes
The datum is the derived axis of another cylindrical feature, normally the bearing seat or the diameter on which the part runs between centers. If the drawing calls out two datums (|A-B|), the reference axis is the common axis of both diameters, which is exactly what mounting between centers or in two bushings reproduces.
Workshop guideline values
- General
- 0.05–0.15 mm on stepped diameters in general machining.
- Accuracy
- 0.02–0.05 mm on shafts with several close-fit diameters.
- High precision
- 0.005–0.01 mm on rotors or precision shafts.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Assuming coaxiality and concentricity are interchangeable: coaxiality evaluates the full derived axis, not just the centers of individual sections, so it can give a different result for the same part.
- Specifying it without ⌀ when the drawing actually means axis position: check whether the modern frame should be a cylindrical position tolerance instead of the classic ◎ symbol.
How to get it in the workshop / if it comes out of tolerance
On recent ISO drawings it is often expressed as axis position with ⌀. Check whether the drawing uses the classic notation or coaxial position.
Same logic as concentricity: avoid setup changes between the datum diameter and the controlled one. If you need to flip the part, use a floating tailstock center or a repeatable re-seating fixture instead of re-indicating it by eye every time.
Alternative symbol:⌖
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