GlossaryKind of term: Manufacturing termCircularity (roundness)
What is circularity?
Circularity is a form tolerance that limits how far each cross-section may deviate from a true circle, independent of diameter and with no datum.
Circularity ○ applies to each cross-section perpendicular to the axis of a cylinder or cone, and for a sphere to each section through its centre. In every section the outline has to lie between two concentric circles whose radii differ by the tolerance value. There is no datum: circularity compares the section with itself. A ⌀40 H7 bore can be within size and still be oval or three-lobed; a circularity of 0.005 mm limits that form error to that value.
What does circularity depend on?
Ovality and lobing often come from clamping: a thin-walled bush in a three-jaw chuck distorts under the clamping force, is turned round, and springs back into three lobes when released. Centreless grinding, vibration, out-of-round stock and heat can also cause lobing. The measuring method decides what you find. A two-point measurement with a micrometer misses an odd number of lobes, because every high point faces a low one. A V-block with a dial indicator picks up some of it, depending on the angle. A roundness tester or a coordinate measuring machine with enough points around the circumference gives the form.
What does the standard say about circularity?
The tolerance zone is defined in ISO 1101; terms and default measurement settings, such as filter and reference circle, are in ISO 12181. The result depends on those choices: a least-squares circle or a different filter can give a different value from the minimum zone. Under the independency principle of ISO 8015 a size tolerance does not limit circularity; with the envelope requirement Ⓔ the surface may not violate the envelope of perfect form at maximum material size, so the size tolerance limits form as well.
How does circularity work in practice?
A tight circularity calls for workholding that distorts the part little: soft jaws bored to size, a collet or a fixture that spreads the force around the circumference, and a light finishing cut at reduced clamping pressure. If turning does not hold the value reliably, grinding or honing follows, sometimes subcontracted. Measurement also takes longer than a size check. How much all this adds to route and price depends on the value, wall thickness, material and quantity.
What is circularity often confused with?
Circularity is not cylindricity: circularity limits one cross-section, cylindricity the whole cylindrical surface. Nor is it runout, which is measured while the part turns about a datum axis and also includes any eccentricity. And circularity is not a size tolerance: a bore can be within H7 and still out of round.
Frequently asked questions
Can a bore be within H7 and still not be round?
Yes. Under ISO 8015 a size tolerance, checked by default as a two-point size, does not limit form. An oval or three-lobed bore can pass every size check. If form must stay within size, the drawing carries a circularity or the envelope requirement Ⓔ.
Why not measure circularity with a micrometer?
Because a micrometer touches two opposite points. With an odd number of lobes every high point faces a low one, so the reading stays nearly the same all the way round. Measure circularity with a roundness tester or a coordinate measuring machine with enough points.
Which operation holds a tight circularity?
That depends on the value, the material and the wall thickness. Fine turning with low clamping pressure often goes a long way; grinding and honing go further. The clamping method matters at least as much as the operation.