Cylindricity
Cylindricity is a GD&T form tolerance that controls how closely a cylindrical surface matches a perfect cylinder over its entire length. The tolerance zone is the volume between two coaxial cylinders; every point on the surface must fall within this zone. As a form tolerance, it is datum-independent and is applied to cylindrical features such as shafts, bores, pins, and bushings where rotational accuracy or sealing is critical.
On a CNC lathe or cylindrical grinding cell, cylindricity is used when hitting size is not enough. A part can measure within diameter limits at a few points and still fail cylindricity if it has barrel shape, taper, lobing, or axial bend along the surface. In inspection, cylindricity evaluates the entire surface, not just cross-sections, making it stricter than circularity because it captures roundness and straightness together. This matters for parts that must run true in bearings or seal bores, where localized high spots, tool deflection, or thermal growth can cause intermittent contact, noise, heat, leakage, or premature wear. The practical takeaway for CNC process control is that cylindricity demands stable workholding, low tool pressure, controlled stock removal, and finish strategies that minimize taper, chatter, and residual stress release across the full length of the feature. In precision rotating and sliding features, bearing fit and seal integrity depend on actual surface shape, not just nominal size.
- Passing Diameter, Failing Form: A shaft can measure within diameter at multiple stations yet fail cylindricity due to barreling or waisting between check points; size control does not capture 3D form error.
- Chatter-Induced Waviness: Long pins, deep bores, and extended journal lengths can deflect during machining, leaving a wave pattern or taper that exceeds cylindricity even when roundness at individual sections looks acceptable.
- Post-Machining Distortion: A part may appear perfect off the machine but shift after cooldown or unclamping, introducing axial bow or out-of-round growth that causes cylindricity failure at final inspection.
Is cylindricity a size tolerance?
No. It is a form tolerance; size controls diameter limits, while cylindricity controls the full cylindrical surface form.
Does cylindricity require a datum?
No. It is applied directly to the feature and does not reference a datum.
What geometric errors does cylindricity combine?
It combines the effects of circularity, straightness, and taper across the full cylinder.