○ Circularity / Roundness
Circularity / Roundness · Forma · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
In the indicated cross-section, the circular outline may not deviate more than 0.02 mm from a perfect circle. No datum is required: it only evaluates the form of that section.
What controls
Limits the radial variation of a circular cross section from a perfect circle, evaluated in a plane perpendicular to the axis at a given location — not along the whole part (that is what cylindricity is for). The tolerance zone is two concentric circles separated radially by the stated value; the actual section must lie between them. It needs no datum: it is the circularity of that section relative to itself, not relative to an external reference axis.
How to verify
The reference method is a roundness tester (a precision rotary table with a probe): the part turns on an extremely accurate axis of rotation and the probe records the true radial variation, independent of how the part is centered. On a CMM, a full circular section is taken and fitted by least squares (or minimum zone, depending on the standard cited) to calculate the error. A dial indicator with the part turned by hand in a V-block gives a quick shop approximation, but it mixes circularity with workholding eccentricity if not done carefully.
tolerance zone
Two concentric circles separated radially by the tolerance value; the actual section must lie between them.
Workshop guideline values
- General
- 0.02–0.05 mm on turned or milled diameters for general use.
- Accuracy
- 0.005–0.015 mm on sliding fits or bearing seats.
- High precision
- 0.001–0.003 mm on precision bearing races.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Confusing it with circular runout: circularity does NOT need a datum and evaluates the section against itself; if you measure with the part rotating about an axis other than its actual one, you are measuring runout, not circularity.
- Checking only two crossed diameters with a micrometer: that detects 2-lobe ovality but not 3- or 5-lobe (polygonal) shapes, typical of 3-jaw chucking.
How to get it in the workshop / if it comes out of tolerance
Different from out-of-roundness found through runout: circularity requires no datum. An oval hole can meet position but fail circularity.
Before starting, check the TIR (total indicator reading) of the collet or chuck: eccentric workholding transfers out-of-roundness directly to the part. On a lathe, use constant surface speed (G96) on parts with varying diameters so roundness is not lost when the Ø changes. A faceted or chipped edge leaves polygons instead of a circle: change the insert before the finishing pass, not after a bad measurement.
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