What is the difference between concentricity and symmetry in GD&T?

What is the difference between concentricity and symmetry in GD&T?

• A. Concentricity describes surface finish.
• B. Symmetry controls the alignment of a feature's center with a datum axis.
• C. Concentricity controls the alignment of a feature's center with a datum axis; symmetry controls the alignment of a feature's median plane with a datum plane/axis.
• D. Concentricity and symmetry describe size tolerances.

Answer: (C)

In GD&T, the difference is that concentricity focuses on the alignment of the feature’s central axis with a datum axis, while symmetry focuses on the alignment of the feature’s median plane with a datum plane or axis. Concentricity uses a cylindrical tolerance zone coaxial with the datum axis, so the feature’s axis must lie within that zone to keep the part from wobbling when it rotates. Symmetry looks at the feature’s median plane and requires that plane to be positioned relative to a datum plane or axis so that the material distribution on opposite sides is balanced within the tolerance. A practical way to see it: a pin and its hole would be governed by concentricity to ensure true rotation about a common axis; a disk that must be balanced about its center would use symmetry to ensure equal material distribution about its median plane. The other options misstate the concept—surface finish isn’t what concentricity or symmetry control, symmetry isn’t about the center alignment with a datum axis, and these traits aren’t size tolerances.

In GD&T, the difference is that concentricity focuses on the alignment of the feature’s central axis with a datum axis, while symmetry focuses on the alignment of the feature’s median plane with a datum plane or axis. Concentricity uses a cylindrical tolerance zone coaxial with the datum axis, so the feature’s axis must lie within that zone to keep the part from wobbling when it rotates. Symmetry looks at the feature’s median plane and requires that plane to be positioned relative to a datum plane or axis so that the material distribution on opposite sides is balanced within the tolerance. A practical way to see it: a pin and its hole would be governed by concentricity to ensure true rotation about a common axis; a disk that must be balanced about its center would use symmetry to ensure equal material distribution about its median plane. The other options misstate the concept—surface finish isn’t what concentricity or symmetry control, symmetry isn’t about the center alignment with a datum axis, and these traits aren’t size tolerances.