• A datum is a theoretical exact point, axis or plane from which the location or geometric characteristic of a part feature are established. It's a starting point or origin.

  • A datum is established by an actual physical part feature called a datum feature. A datum feature typically has an important functional relationship to the part feature being specified. On a drawing, it's identified by a special datum feature symbol.

Example: A flat surface may be used to establish a datum plan. A cylindrical feature, such as a shaft, may be used to establish a datum axis. A slot may be used to establish a datum center plane.

  • By definition,a datum is theoretically exact or perfect. However, the actual part feature used to establish the datum is not perfect. Therefore, the datum is simulated through contact with precision manufacturing or inspection equipment. This provides a more accurate stating point from which to measure.

Example: A datum plane may be simulated through contact with a surface plate.

Simulated datum plane

Datum reference frame
  • As many as three datums may be used to accurately establish the location or geometric characteristics of a part feature.

  • Usually the largest or most important feature of the part is used to establish the primary datum. At least three points of contact are needed between this feature and the machine table or inspection surface.

Once the primary datum is established, the points of contact with the machine table or inspection surface must be maintained. If contact is lost in the process of establishing another datum, then that datum becomes the primary datum.

  • The secondary datum requires at least two points of contact with the machine table or inspection surface, and the tertiary datum requires at least one.

  • Together, the datums form a datum reference frame. It corresponds to the
    X-Y-Z axes on many machines and the three planes used for common inspection procedures.

Datum targets

Datum Targets

  • On irregularly shaped parts, such as castings, forgings or sheet metal, datums are established by datum targets.

  • A datum target is a specified point, line, or area of contact between the part and the machine surface or inspection equipment. It ensures that the part will be located the same way during both production and inspection.

  • On a drawing, a datum target is identified by a datum target symbol.

  • When a datum target is an area on the part, the top of the datum target symbol specifies its diameter.

Example: These drawings show how datum targets are identified on a drawing. In the drawing on the right, the datum target identified as A3 has a diameter of .005.