Common Datums and Datum Systems

A common datum means a pair of single datums specified to function as one datum. On the other hand, a datum system is a group of datums that is created by combining multiple datums.

Common Datums

A common datum is a single datum consisting of two datum features. Separate datum features are applied to a common practical datum feature.

Indication symbol

Indication symbol

Datum setting

Datum setting
a
Axis of datum A
b
Common datum A-B
c
Axis of datum B
d
Practical datum feature (i.e., contact surface)

For example, indication using a common axis line made by fixing the two smallest circumscribed cylinders—which are coaxial—is described by connecting the two letters of the datums within one frame. This makes a straight line from the axes of datums A and B.

Datum Systems

A datum system is a group of datums created by combining more than one individual datum in order to use it as a reference for toleranced features.

Indication symbol

Indication symbol

Datum setting

Datum setting
  1. Push datum A against target feature
  2. Push datum B against target feature
  3. Push datum C against target feature
    Part (target feature) is completely captured

For example, three datums are used to completely capture the orientation of a part. A datum system comprising three datum planes that are perpendicular to each other is called a three-plane datum system. This enables the optimal surface to be used as a datum.

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