Circularity (Roundness) — Form Tolerance

Controls roundness of a cross-section of a cylinder, cone or sphere.

Definition

Circularity — also called roundness — controls how far a single circular cross-section may depart from a perfect circle. It is a Form control: no datum, evaluated one section at a time. Each cross-section taken perpendicular to the feature's axis must be round within the tolerance, but circularity places no constraint on how the sections relate to each other along the length. A part can be perfectly circular at every section and still be tapered or bowed — that is outside circularity's job. Applied to a cylinder, cone, or sphere, it limits only the shape of each individual section.

The zone: two concentric circles

The tolerance zone is the space between two concentric circles whose radial separation is the tolerance t. The actual profile of the one cross-section under evaluation must fall entirely between the circles. The zone is established from the section itself, so circularity tolerates a uniform change of size from section to section — the circles are free to shrink or grow, and their center is not tied to any datum or to the part axis. That is what makes circularity a pure form control: it measures shape alone.

What circularity does not control

Because each cross-section is evaluated independently, circularity accumulates nothing along the length. Taper, bow, and surface straightness are all outside its reach — those are the job of cylindricity, the composite Form control, or of axis straightness on a feature of size. A hydraulic cylinder bore needs roundness at every section, but it also needs the bore to be straight; calling out circularity alone would let a bowed bore pass. Specify circularity when roundness of sections is the requirement, and pair it with other controls when the length-wise form matters too.

Reading a feature control frame

A circularity frame is minimal: no datum references, no diameter symbol — the tolerance is a radial width, not a diameter — and no material modifier. ○ 0.1 reads as a circularity tolerance of 0.1 radial width on each cross-section. Compare the position frame ⌖ ⌀0.5 Ⓜ A B C: position locates from datums and may take MMC; circularity references only the section itself and never carries a material condition.

Measurement

Roundness machine / precision spindle — the part or probe rotates on a precise spindle while a plot records radial deviation; the only method that reliably catches lobing, because lobed forms (a tri-lobed shape, for example) can read acceptably between two points and hide in two-point measurement.

CMM circular scan — sample a dense set of points around the section and fit the two concentric circles that bound them; the natural choice when the part is already on the CMM.

V-block + dial indicator (shop approximation) — quick screening only: two-point contact can miss odd-numbered lobes entirely, so treat the reading as a check, not a circularity measurement.

When to use it

• Cylindrical surfaces that mate or seal radially — bearing seats, hydraulic cylinder bores, O-ring grooves
• Shaft journals where an out-of-round condition shows up as vibration, uneven wear, or leakage
• Spherical or conical features where only the roundness of each section matters
• Where length-wise form is controlled separately (or not at all), and circularity alone states the real requirement