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What is the meaning and usage of datum in GD&T?

Hello All,

The concept of Datum is used in GD&T to provide a reference for dimensions and tolerances. It is a Latin word. meaning "given", i.e. an accepted fact. Let’s understand Datum and related terms.

Datum reference frame

Suppose you are given an engineering drawing with dimensions and tolerances to perform some machining activities to produce a part. How would you proceed? There are two primary needs:

1. Restricting movement of the part on some clamp, or so, to perform any operations. In engineering terms, it is a restriction of the degree of freedom. (Recall your studies of six degrees of freedom for a free body in which three are translational and three are rotational movement along X, Y and Z axes,

2. Next, you would need references like point, axis or plane from which dimensions can be measured. These references may be on any feature or we may create any imaginary reference from which dimensions are taken or controlled.

Datum reference frame is created to fulfill the above two requirements. Now let us define few concepts related to datum feature frame.

• Datum: These are “imaginary” reference point, surface, or axis on an object against which measurements are made. In above example, plane A, B, and C were datums, as we started using the term for those planes.

• Datum feature: “Actual” reference point, surface, or axis on an object feature. Example, the face of an object.

• Functional Datum: Datums are chosen based on the connection between parts to fulfil its functionalities. Functional datums are not used for manufacturing.

• Manufacturing Datums: These are datums for the manufacturing process to save cost, improve process speed, and repeatability. Tolerance analysis is used to get manufacturing datums from functional datums. Example,

Functional tolerance + Bonus Tolerance = Manufacturing tolerance.

• Datum Feature Simulator (theoretical): It is theoretically perfect boundary used to establish a datum from a specified datum feature.

• Datum Feature Simulator (Physical): It is the physical boundary used to establish a simulated datum from specified datum features.

Overall, Datum is an important concept used in GD&T which is used for restricting degree of freedom, helping manufacturer to decide the way of operations, reference for quality checks and so on. These concepts have been discussed in detail and with lots of figures in one latest book, Simplified GD&T, which is amazon best seller in India and the USA. It is also becoming popular in Germany, which is one of the best place for mechanical industry. We are sure you would enjoy this book.


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