What is the meaning of the diameter symbol in a FCF?

• A. It indicates a spherical tolerance zone.
• B. It indicates a cylindrical tolerance zone.
• C. It indicates a linear dimension.
• D. It indicates a projected tolerance zone.

Answer: (B)

In GD&T, the diameter symbol (Ø) before a tolerance value in a Feature Control Frame indicates a cylindrical tolerance zone. This means the allowable variation is circular in cross-section around the feature’s axis, forming a cylinder whose diameter equals the stated tolerance. The axis of that cylinder is aligned with the feature’s axis, so for a true position or runout tolerance, the feature’s axis must lie within that cylindrical zone. It’s not a spherical zone, not a simple linear dimension, and it doesn’t denote a projected tolerance zone (which uses a different symbol and adds a projection height).