What does the "3-2-1" rule define?

The "3-2-1" rule is fundamental in the context of Geometric Dimensioning and Tolerancing (GD&T) and relates specifically to the constraints and stability of parts during inspection or machining. According to this rule, a part should be supported and located on three points for stability, thereby defining three primary datum points.

This method involves using three contact points to establish a primary datum, which controls three degrees of freedom, hence accommodating translational movement in X, Y, and rotational movement around the Z-axis. For the secondary datum, two more contact points are added which constrain two degrees of freedom, and the tertiary datum requires a single point which constrains the last degree of freedom.

Overall, the "3-2-1" rule ensures that a part is held firmly in place with minimal ambiguity and movement during measurement or assembly, thereby promoting accuracy and repeatability in manufacturing processes. This systematic approach aids in achieving precise alignment and orientation of components, which is crucial for parts requiring tight tolerances.