Cylindricity — Form Tolerance

Simultaneously controls roundness and straightness of a cylindrical surface.

Definition

Cylindricity is the composite Form control: it simultaneously limits roundness, straightness, and taper of a cylindrical surface. The entire surface — not individual cross-sections — must fall between two coaxial cylinders separated by the tolerance t, in one simultaneous evaluation. It is the strictest of the Form controls and the hardest to inspect, precisely because the requirement applies to the whole surface at once rather than slice by slice.

The zone: two coaxial cylinders

The tolerance zone is the space between two coaxial cylinders whose radial separation is the tolerance t. Every point of the actual cylindrical surface must lie between them over the full length. Unlike circularity — which passes or fails each slice on its own — cylindricity evaluates the whole surface as a single shape: a local bulge here and a slight taper there must both fit inside the same pair of envelopes. The zone is established from the surface itself, so like all Form controls it references no datum and tolerates a uniform change in size along the length.

Circularity vs. cylindricity

The two controls answer different questions. Circularity asks: is each cross-section round? Cylindricity asks: is the whole surface cylindrical? A part can pass circularity at every section and still fail cylindricity — a tapered or bowed bore is round everywhere but not contained between two coaxial cylinders. Cylindricity is therefore always at least as strict, and it costs more to verify. When only roundness matters, circularity is the cheaper, honest callout; when roundness and straightness must hold together — a seal running the length of a bore, a journal carrying load along its whole length — cylindricity is the control that states the real requirement.

Reading a feature control frame

A cylindricity frame is minimal: no datum references, no diameter symbol — the tolerance is a radial width between coaxial cylinders — and no material modifier. ⌭ 0.1 reads as a cylindricity tolerance of 0.1 radial width over the entire surface. Compare the position frame ⌖ ⌀0.5 Ⓜ A B C: position locates from datums and may take MMC; cylindricity references only the surface itself and never carries a material condition.

Measurement

Roundness machine with axial traverse — the definitive method: spindle roundness plus a straight traverse produces a three-dimensional map of the surface, checked against the two coaxial cylinders. Expensive and slow; reserved for critical features.

CMM helical scan — a dense helix of points around and along the surface, bounded by the two coaxial cylinders. The usual production compromise between cost and coverage.

Decomposition (shop practice) — because direct cylindricity inspection is costly, production often controls circularity and straightness separately and relies on the process to keep them inside the implied envelope. Only sound when the process is stable enough to guarantee it.

When to use it

• Precision bores where piston or seal function depends on roundness and straightness together — engine cylinders, hydraulic actuators
• Long bearing journals where a taper alone would concentrate load at one end
• Features whose mating surface slides or seals along the entire length, not just at a cross-section
• Use it sparingly elsewhere: when only roundness matters, circularity (plus a straightness or position control if needed) costs far less to verify