Engineering · GD&T
Cylindricity.
A 3D form tolerance that controls a cylindrical surface in its entirety. Combines straightness, circularity, and taper into one tolerance — the entire cylindrical surface must lie between two concentric cylinders separated by the tolerance value.
01 · Definition
Three controls in one symbol.
Cylindricity is a single tolerance that simultaneously bounds three errors:
- Out-of-roundness at any cross-section
- Axis straightness over the full length
- Taper or hourglass shape — diameter variation along the length
The tolerance zone is two coaxial cylinders separated by the tolerance value. The actual cylindrical surface must fit somewhere between them.
02 · FCF and example
0.001 cylindricity on a hydraulic spool.
A hydraulic valve spool with a 0.001 cylindricity callout means the entire cylindrical OD must fit within two coaxial cylinders 0.001″ apart in radial separation. Round at every section, straight along the axis, no taper.
Inspection: roundness machine with axial scanning, or CMM scanning probe. Both compute a minimum-zone fit between the surface and two coaxial cylinders.
Production strategy: Swiss-type or precision turning with rigid setups. For long features, in-process probing helps catch drift before tolerance is lost. See Swiss vs. CNC lathe.
03 · When to use cylindricity vs alternatives
Pick the simplest control that does the job.
- Cylindricity: when the full cylindrical surface needs control as a unit. Hydraulic and pneumatic spools, bearing journals, sealing surfaces.
- Circularity only: when you only care about at-section roundness. Cheaper to specify and inspect.
- Total runout: when datum reference matters more than form-by-itself. Total runout is cylindricity referenced to a datum axis.
- Standard size tolerance: often controls cylindricity adequately without explicit callout. Specify cylindricity only when it’s functionally critical and tighter than the size tolerance.
Keep exploring
Related tools & references
Full feature-control-frame symbol reference.
The 2D round-section control cylindricity builds on.
Axis/line form control combined inside cylindricity.
Check accumulated form error on round features.
How tight a cylindricity callout drives cost.
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