Geometric Dimensioning and Tolerancing (GD&T) Practice Exam 2026 – The Comprehensive Guide to Mastering GD&T!

A datum axis is the straight line that comes from a cylindrical datum feature and serves as the orientation reference for measurement. In GD&T, a datum provides a known, repeatable frame of reference; when the datum feature is circular or cylindrical, its axis becomes the datum axis. This axis defines how the part is oriented in the measurement setup and establishes the angular (and, with other datums, positional) relationship used to evaluate features relative to that datum. For example, if a hole is chosen as a primary datum, the axis of that hole is the datum axis and other features are measured with respect to that axis and any additional datums. It’s not related to the maximum material boundary or a generic “datum template”—those describe different aspects (size/clearance states or overall datums), whereas the datum axis specifically refers to the axis of a cylindrical datum feature used for orientation.