ISO 1101 roundness tolerance and how to measure it

The drawing of a ball bearing seat calls for a roundness tolerance of 0.008 mm. The setter has to check that the actual circular profile in a cross-section lies between two coplanar, concentric circles 0.008 mm apart. A roundness error creates noise and premature wear of the balls.

Definition and inspection methods for a roundness tolerance

Measuring means	Type of assessment	Measuring accuracy	Effect of the V-block
Spindle roundness tester	Actual concentric circles	±0.0005 mm	None (direct measurement)
Dial indicator on a 60° V-block	Measured amplitude variation	±0.002 mm	Suppresses some harmonics
CMM	Reconstruction from 3D points	±0.001 mm	Needs a large number of points

All values are given for the reference environmental conditions of 20 °C.

Measuring roundness on a 60 or 90 degree V-block

Measuring on a V-block means turning the part through 360° under the indicator. The direct reading isn’t pure roundness but runout in the V. To get the exact roundness, a correction factor depending on the number of lobes (3rd or 5th harmonic) has to be applied. To go over the overall tolerances, see our article on the difference between circular runout and total runout.

The roundness tolerance zone under ISO 1101

Roundness is a pure form tolerance that applies section by section. It sets no coaxiality constraint relative to any outside axis.

Example: inspecting a roller seat

A seat with a 0.008 mm roundness tolerance is checked on a roundness tester. 1. Max-min radius difference found: 0.0065 mm. Verdict: the value (0.0065 mm) is within the 0.008 mm limit. The seat conforms.

Common pitfalls when measuring roundness

The first pitfall is measuring roundness with a two-point flat-anvil micrometer, which can’t detect three-lobed shapes. The second is too few points when probing on a CMM. The third comes from confusing roundness with ovality. To record your geometric specifications, explore the CoteCote inspection and report modules.