How is concentricity evaluated?

• A. The feature's axis must lie within a spherical tolerance zone coaxial with a datum axis.
• B. The feature's axis must lie within a cylindrical tolerance zone coaxial with a datum axis.
• C. The feature's axis must be perpendicular to the datum axis within tolerance.
• D. The feature's diameter must match the datum diameter.

Answer: (B)

Concentricity is about keeping the centers of two features on the same axis. In practice, the tolerance is defined by a cylinder whose axis matches the datum axis—the cylindrical tolerance zone is coaxial with that datum axis. The axis of the feature must lie inside this cylinder, which enforces coaxial alignment along the length of the part. This directly controls how well the centers track each other, not their orientation or size.

That’s why this description is the best: it uses a cylindrical tolerance zone coaxial with the datum axis to constrain the feature’s axis, exactly capturing the idea of concentricity. A spherical zone would describe a different type of tolerance and wouldn’t enforce coaxial alignment along the datum axis. Perpendicular or diameter matching issues relate to other tolerancing concepts and don’t describe concentricity.