Datums serve as a foundation for manufacturing and inspecting your designs. A datum is a theoretically exact reference: plane, line, point or axis from which measurements and part features can be established from.

Datums are crucial for:

• Ensuring consistent measurement and inspection of parts
• Communicating design intent to manufacturing and suppliers
• Enable smooth and easy assembly processes

Datum Features

What is the Difference Between a Datum and a Datum Feature?

• Datum: A theoretical, perfect geometric reference (point, line, plane, or axis) used for measurements and tolerancing
• Datum Feature: The actual physical surface or feature on the part that is used to establish the theoretical datum

For example, a datum plane might be perfectly flat in theory, while the datum feature (the actual surface of the part) may have some inherent surface roughness or imperfections.

How to Show a Datum Feature on a Drawing

Datum features are denoted on a drawing by a capital letter within a square box featuring a leader line to the part anchored with a filled triangle

Datum features are referenced in feature control frames to establish a datum system. The datum feature letter will show in any feature control frame that uses that datum feature as a reference.

A surface or a feature of size?

Datum features can be either a surface or a feature of size, depending on how the symbol is applied.

It is therefore important that datum features used to create our theoretical datums are indicated correctly on the drawing to ensure that the right type of feature is being controlled.