A datum is:

• established by one or more datum features of a component
• a theoretical geometric characteristic such as an axis, plane, straight line, to which tolerance features are related
• the origin from which the location or geometric characteristics of a component are established
• usually a locating or positioning feature used for measurement

When measuring features of a component it often requires the use of a simulated datum to establish planes or axes. Manufacturing fixtures or inspection equipment, such as a surface table are used to simulate theoretical datum features.

Simulated datum – a real surface of adequately precise form such as a surface plate, bearing, mandrel etc. or a surface with a precise fixture, piece of equipment or machine, which, when place in contact with a datum feature, establishes that datum.

A single point to point measurement could measure the dimension differently and this would affect the function of the component.

Before the dimension can be measured datum needs to be established. To measure this component put the component on a table surface. The datum will be the surface of the table.

3 of 11

Good practice online modules

← Return to the main menu

• Dimensional
• Overview of basic design and interpretation of an engineering drawing
• An introduction to datum
• An introduction to geometric tolerancing
• An introduction to the designer's role when designing a component
• An introduction to the manufacturer's role when designing a component
• An introduction to the inspector's role when designing a component