Lord Kelvin’s principle of kinematics states that ‘a body free to move in space has six degrees of freedom’.

There are three degrees of freedom linearly along the x, y and z-axes and three degrees of freedom in a rotational sense around the x, y and z-axes.

To define a datum system for a rectangular part with flat perpendicular surfaces that has its origin in one corner (x= 0, y= 0, z= 0) would require six contact points to restrain the six degrees of freedom and comprises of three elements:

The link between the terms 6 degrees of freedom, datum system and co-ordinate system can be described as the:

1. Primary datum - first datum plane part contacts in a dimensional measurement, defined by a minimum of three-point contact on a surface (xy plane) or defined by the generation of a 3D axis through a minimum of two features.
2. Secondary datum - second datum plane part contacts, defined from a minimum of two points to define a line which lies on the xz plane, relative to the Primary datum.
3. Tertiary datum - third datum plane part contacts, defined by a minimum of one point on the yz plane in relation to the primary and secondary datum’s.

Not all datum systems need to restrain all six degrees of freedom.

For both datum and toleranced features there is a mathematically defined minimum number of points. When measuring these features more than the minimum number is recommended.

Good practice online modules

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• 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