The main objective of applying datums on a surface is to limit the movement of the part which will help in repeated measurements.
If a part is free to move in space then it has 6 degrees of freedom are rotation along the x, y or z axis and the movement along the x, y or z axis.
The above image shows the degrees of freedom.
To restrict the 6 degrees of freedom, we tend to use 3 datum planes.
Datum reference frame is formed when three datum planes are used.
A datum reference frame is a set of three mutually perpendicular datum planes.
This datum reference plane helps to identify the origin as well as the direction for dimensional measurements.
There is zero perpendicularity tolerance between the planes of datum reference frame.
Measurements are mostly taken perpendicular to the datum planes.
The 90° angles between planes are mostly basic.
But the 90° angles between the part surface have a tolerance.
The angular tolerance is mostly mentioned in the drawing or in notes section.
The above image shows an example where the part surfaces are not perpendicular to each other.
A measurement on such type of parts would produce different results based on which part surface comes in contact with the datum reference frame first, second and third.
When a location measurement is made on a part feature, the six degree of freedom are restricted with the help of datum reference frame.
The method implemented to bring a part in contact with the datum reference frame has a great impact on the measurement of part dimension.
The feature control frame helps to identify the sequence in which the part surface would come in contact with the datum reference frame.
A feature control frame is read from left to right.
In the above example, datum A is referenced first, datum B is referenced second and datum C is referenced third.
The feature control frame indicates that the part should contact datum A first.
In a dimensional measurement, the first datum plane that the part contacts is known as primary datum.
The primary datum stabilizes the part to datum reference frame.
The part comes in contact with the datum at least at 3 points.
The primary datum restricts 3 degrees of freedom: movement along the z axis, rotation around X axis and rotation around Y axis.
Three degrees of freedom remains unrestricted.
If we check the feature control frame again, the part should come in contact with datum B after the datum A.
Second datum plane is known as the secondary datum.
The secondary datum restricts the movement of part within the datum reference frame.
There must be 2 points of contact on the surface part with the secondary datum.
The secondary datum restricts two additional degrees of freedom: rotation around Z axis and movement along the Y axis.
One degree of freedom still remains.
At last the part surface should come in contact with the datum C.
Tertiary datum is the third datum plane that the part comes in contact with.
The tertiary datum also restricts the part movement within the datum reference frame.
The part may have 1 point of contact with the tertiary datum plane.
Movement along X axis is restricted by datum C.