What does the term 'datum axis' refer to in GD&T?

What does the term 'datum axis' refer to in GD&T?

• A. A measurement of surface roughness.
• B. The axis of a datum feature used as reference.
• C. The color of the part.
• D. A dimension line.

Answer: (B)

In GD&T, datums establish a reference frame for measurement. The datum axis is the axis of a datum feature used as that reference. For cylindrical features like holes or shafts, this axis is the true geometric line that runs through the center and defines how the part is oriented and located within the datum frame. This axis constrains rotation and translation when applying tolerances, ensuring consistent positioning relative to the datums. It’s not about surface roughness, color, or simply a dimension line.

In GD&T, datums establish a reference frame for measurement. The datum axis is the axis of a datum feature used as that reference. For cylindrical features like holes or shafts, this axis is the true geometric line that runs through the center and defines how the part is oriented and located within the datum frame. This axis constrains rotation and translation when applying tolerances, ensuring consistent positioning relative to the datums. It’s not about surface roughness, color, or simply a dimension line.