What Are Datums?
A datum is a plane, a straight line, or a point that is used as a reference when processing a material or measuring the dimensions of a target.
A datum is defined as follows in the ISO standard:
- ISO 5459：2011
- “One or more situation features of one of more features associated with one or more real integral features selected to define the location or orientation, or both, of a tolerance zone or an ideal feature representing for instance a virtual condition.”
Types of Datums
Datums are established using datum features and simulated datum features. Multiple datums can also be combined to make a datum system that specifies a feature.
- Datum Feature
- A real (non-ideal) integral feature of a target used for establishing a datum (e.g. surface of a part or hole).
- Simulated Datum Feature
- An actual surface with sufficiently precise shape that comes into contact with a datum feature and is used for establishing a datum (e.g. surface plate, bearing, and mandrel).
- Datum System
- A group of datums created by combining more than one individual datum in order to use it as a reference for toleranced features.
The surface of a part indicated as a datum is not an exact shape. Therefore, a surface plate, gauge, or mandrel, which has a more precise surface, must come into contact as a simulated datum.
Drawing Indications of Datum Features
Datums are indicated using the following symbols (datum symbols). Datum symbols use either a white or black triangle.
The letters indicating the datums are always oriented to the direction the reader looks at the drawing.
The position of the datum symbol on the drawing is also relevant to the target area. To express the design intention with strict accuracy, pay attention to the position where you indicate the datum.
Annotating an Axis or Center Plane
A datum feature is indicated by combining the dimension line and datum. The center of the indicated datum feature becomes the datum axis or datum center plane.
Annotating a Generatrix
The dimension line of a datum feature and the datum are indicated apart from each other. The center of the indicated datum feature becomes the datum axis or datum center plane.