GD&T Tutorial : 3-2-1 Principle
Every object in space will have 6 DoF (Degrees of Freedom).
They are 3 linear and 3 rotational degrees of freedom.
At the time of manufacturing and inspection, the part must be in stable condition, hence it becomes necessary to arrest all the degrees of freedom.
3-2-1 Principle describes ways of constraining the 6 degrees of freedom.
When a part is placed on a Primary datum, it requires minimum of 3 points and it constraints 3 degrees of freedom. As the part comes in contact with secondary datum, it requires at least 2 points of contact, and 2 degrees of freedom will be constrained. Contact of the part with tertiary datum requires at least one point to constrain 6th degree of freedom
