What is the difference between cylindricity and roundness in GD&T ?

Question

Accepted Answer

Cylindricity refers to the overall 3D form of a cylinder, while roundness refers to the deviation of a circle from a perfect circle.

Cylindricity is a 3D tolerance that controls the overall form of a cylinder, ensuring that all points on the surface lie within a specified tolerance zone.
Roundness is a 2D tolerance that controls the deviation of a circle from a perfect circle, ensuring that the entire circle lies within a specified tolerance zone.
Cylindricity is used when the feature being measured is a cylinder, while roundness is used when the feature being measured is a circle or circular feature.
Examples: A piston rod would be measured for cylindricity, while a bearing race would be measured for roundness.

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