Engineering · GD&T
Circularity (roundness).
A 2D form tolerance for round features. At any single cross-section perpendicular to the axis, the surface must lie between two concentric circles separated by the tolerance value. No datum required.
01 · Definition
Section by section.
Circularity controls roundness at a single cross-section. Slice the cylindrical feature perpendicular to its axis at any point, and the surface profile at that section must fit between two concentric circles separated by the circularity tolerance. The tolerance is checked at every section independently.
Critically, circularity does not check axis straightness or taper. A bowed or tapered shaft can still be perfectly round at every section. For full cylindrical control (round + straight + non-tapered), use cylindricity.
02 · FCF and example
0.0005 circularity on a bearing journal.
A bearing journal with ○ 0.0005 means at any single cross-section, the surface fits between two concentric circles 0.0005″ apart. The journal can taper, bow, or be off-center axially — only the at-section roundness is constrained.
Inspection: rotate the part on a precision spindle (roundness machine) and capture the profile at the section in question. CMM with a circularity calculation also works but is less precise for 0.0001″-class roundness.
03 · Common pitfalls
When circularity isn’t enough.
- Lobed bores: circularity catches in-section out-of-round but doesn’t catch lobes that wander axially. For lobed-bore characterization, supplement with cylindricity or runout.
- Doesn’t control taper: a 0.500″ OD that tapers from 0.5005″ at one end to 0.4995″ at the other can pass any circularity callout. Use cylindricity if taper matters.
- Often redundant with diameter tolerance: a tight diameter tolerance (e.g., ±0.0005″) already constrains in-section variation. Adding circularity only refines the variation pattern within that envelope.
- Easier to achieve on Swiss-type: guide-bushing turning gives best in-section roundness. See Swiss vs. CNC lathe.
Keep exploring
Related tools & references
Full feature-control-frame symbol reference.
Circularity extended along the full length of a cylinder.
Datum-referenced control that bounds circularity plus coaxiality.
See how round-feature form tolerances combine.
Choosing realistic circularity callouts.
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