A single datum refers to a datum identifier like A or AC that refers to only one datum feature. A single datum may refer to one of the following types of features:
Plane
Line
Point
Cylinder
Circle
Sphere
Cone
Width
Most datums in the geometric tolerance command are like the datum features on an inspection gage:
They have a fixed distance and orientation with respect to each other.
They engage the part in a fixed order of precedence.
Comparison to past practice:
Under XactMeasure, PC-DMIS treated datum features like alignment features in most cases. They defined the level, rotation, and origin.
The geometric tolerance command is more accurate, because it simulates how the datum reference frame contacts the mating part.
A datum feature can represent a planar surface with a plane feature, a line feature measured on a surface, or a point feature measured on a surface.
While you can measure a surface line or surface point feature on a non-planar feature, PC-DMIS always treats them as coming from planar surfaces when referenced as datums in the geometric tolerance command. For more details, see "How PC-DMIS Solves Datums".
A datum feature can represent a cylindrical surface with a cylinder feature or a circle feature. While you can measure a circle on a non-cylindrical feature, PC-DMIS always treats circles as coming from cylindrical surfaces when referenced as datums in the geometric tolerance command. For more details, see "How PC-DMIS Solves Datums".