3-2-1 RULE IN REFERENCE (DATUMMING) STRATEGY

References form the DNA of a part. When the referencing strategy is wrong, tolerances will be faulty, you will not be able to capture the correct values when measuring, and it will take time to find where the error comes from in a faulty part.

There are several rules for proper referencing. One of these is the 3-2-1 rule. This rule basically consists of a plane through 3 points, a line through 2 points, and an origin through 1 point, relative to the XYZ coordinates.

Example:

In this example we see that the 3-2-1 rule is applied as 3Z, 2X and 1Y.

Any referencing strategy that deviates from this rule is wrong, there may be only one exception; a 4th plane point can be used for bulky parts, but this 4th point must be specified as an auxiliary reference.

This article is written for sheet metal part reference. For CNC parts, generally wide surface is selected as plane, narrow surface is selected as straightening, and the narrowest surface is selected as origin and is called ABC.

Example: