⌭ Cylindricity
Cylindricity · Forma · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The whole cylindrical surface must fit between two coaxial cylinders 0.03 mm apart radially: it controls roundness in every section, straightness of the generating lines and taper, all at once.
What controls
Combines into a single requirement the circularity of every cross section, the straightness of the surface elements (longitudinal lines) and taper along the entire cylindrical surface. The tolerance zone is two coaxial cylinders separated radially by the stated value, within which the entire actual surface must lie. Because it requires no datum, it is the most complete cylindrical form tolerance and also the most expensive to achieve and inspect.
How to verify
A dedicated cylindricity machine (a precision rotating spindle with a probe that travels axially) gives the most reliable result, evaluating dozens of sections along the part. On a CMM, a point grid is taken (several sections at different heights) and a cylinder is fitted by least squares. With a manual dial indicator: mount the part between centers, rotate it and move the indicator axially to several positions, recording the combined max-min envelope of all sections — slower and less accurate, but valid as a shop check.
tolerance zone
Two coaxial cylinders separated radially by the tolerance value; the entire actual surface must lie between them.
Workshop guideline values
- General
- 0.03–0.08 mm on general-machining cylinders.
- Accuracy
- 0.01–0.02 mm on precision liners or bushings.
- High precision
- 0.002–0.005 mm on high-precision pistons and bearing bores.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Treating it as circularity + straightness checked separately: cylindricity is a single combined requirement, stricter than passing both separately.
- Forgetting that it takes no datum: if the drawing shows a datum next to the ⌭ symbol, the drafter probably meant something else (coaxiality or axis position).
How to get it in the workshop / if it comes out of tolerance
More demanding than circularity + straightness applied separately. Typical on pistons, precision bushings and bearing bores with no explicit datum for form.
Since it combines straightness + circularity, any cause of those two affects it: insufficient support on long parts, eccentric workholding or worn carriage ways. Reduce feed and speed on the finishing pass, use plenty of steady coolant to avoid uneven thermal expansion, and on long parts use a steady rest to minimize the deflection that ruins a straight surface element even when every section comes out round.
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