4. Datum precedence rule or 3-2-1 rule
This rule ensures positioning of a part in a repeatable manner for measurement purpose. According to this rule, the minimum number of point of contacts for the primary datum as 3, the secondary datum as 2, and tertiary datum as 1. The first, second and third datum references in the feature control frames are primary, secondary and tartiary datums, respectively. Figure 5 explains the above rule.
Figure 5. Example of 3-2-1 rule.
In the example, the location tolerance control of the two holes are called out with three datum features - D, E, and F. Implicityly these three datum features should be exactly perpendicular to each other, which is unattainable in practice. To gage this part, three simulated datums are established which are accurately perpendicular to each other. Then this 3-2-1 rule states that the primary datum feature D will first be supported on a simulated datum plane by at least 3-point contact. Then the datum feature E will contact the datum plane with at least 2-point contact and lastly F will have at least 1 point of contact. What it all means that, if the datum features are not perfectly flat and square to each other, then the datum plane for the primary datum will be determined by three high points of the surface. Remember that a plane can be defined by at least three points. Once the part is placed on the primary datum plane, then it will be brought to contact with the second datum plane. Depending on the squareness and flatness of the datum feature, the worst case scenario is 2-point contact with the perfectly squared and flat simulated datum plane. And lastly, the third datum feature at the least can have at least on point contact with the third datum plane. Thus the 3-2-1 rule ensures the repeatability of positioning of a part within the datum reference frame.
Note that the only geometric tolerance that specify the datums will be measured from the datum planes. The FOS 36.2-36.0 will not be measured from the datum surfaces. In the example in Figure 5, only the hole locations will be verified from the datums.