What is the role of a datum axis in a cylindrical part?

• A. It serves as the reference for color coding.
• B. It is used to locate the feature relative to datums.
• C. It provides a reference axis for alignment of features around the axis.
• D. It defines the surface finish.

Answer: (C)

The datum axis serves as the rotational reference line for a cylindrical part, giving a stable axis around which features are aligned and located. Because a cylinder has circular symmetry, many features are defined in relation to rotation about this axis, so you can control how those features line up around the circumference and along the length. This enables tolerances like concentricity and runout to be specified with respect to the axis, ensuring all features that depend on rotation share a common centerline. It isn’t about color coding or surface finish, which are governed by other kinds of specifications.