What does the diameter symbol indicate in a GD&T frame?

• A. The tolerance applies to a cylindrical zone around the axis; the dimension is a diameter.
• B. The tolerance applies to a spherical zone around the axis; the dimension is a diameter.
• C. The tolerance applies to a rectangular zone around the axis; the dimension is a diameter.
• D. The tolerance applies to a cylindrical zone around the axis; the dimension is a radius.

Answer: (A)

The diameter symbol signals a cylindrical tolerance zone around the feature’s axis, and the value shown is the diameter of that cylinder. In GD&T, this means every point on the controlled surface must lie within a tube coaxial with the feature’s axis, with the tube’s diameter equal to the specified value, giving equal radial allowance in all directions around the axis. Since the dimension is a diameter, you’re not dealing with a radius or a spherical/rectangular zone. This approach is appropriate for round features like holes or shafts, where you want consistent tolerance around the axis.