What is a datum axis?

• A. A straight line on a datum feature used to define orientation and position.
• B. A curved line representing the surface contour to guide assembly.
• C. A point used as a reference for coordinates in the part model.
• D. A feature that measures dimensional tolerance.

Answer: (A)

A datum axis is the straight line that runs along the center of a datum feature of size and serves as the reference line for orientation and position of other features. In GD&T, when a cylindrical surface or a hole is designated as a datum, its central axis becomes the datum axis. That axis provides the rotational reference (how parts are oriented around it) and the baseline for locating features relative to it. For example, the axis of a through-hole or the central axis of a shaft is used as the datum axis because it offers a precise, repeatable reference about which tolerances are applied. The other descriptions don’t fit: a curved line describes a surface contour, not a straight reference line; a single point is a datum point, not an axis; and a feature that measures tolerances is a gauge or measurement concept, not the reference axis.