82 Geometric Dimensioning and Tolerancing Geometric Control of Datum Surface The datum feature surface can be controlled by a geometric tolerance such as flatness, straightness, angularity, perpendicularity, or parallelism. Measurements taken from a simulated datum plane do not take into account any variations of the datum feature surface from the simulated datum plane. Any geometric tolerance applied to a datum should only be specified if the design requires the control. Example 3-4 shows the feature control frame and datum feature symbol together. Example 3-5 is a magnified representation that shows the meaning of the drawing in Example 3-4. The geometric tolerance of 0.1 is specified in the feature control frame in Example 3-4. The maximum size that the part can be produced is the upper limit of the dimensional tolerance, or MMC. The MMC is 12.5 + 0.3 = 12.8. The minimum size that the part can be produced is the lower limit of the dimensional tolerance, or LMC. The LMC is 12.5–0.3 = 12.2. Example 3-4. A feature control frame and datum feature symbol. 12.2 minimum 12.8 maximum 0.1 geometric tolerance Datum plane A Example 3-5. The meaning of the drawing in Example 3-4. The Datum Reference Frame Concept The Datum Reference Frame Concept Datum features are selected based on their importance to the design of the part. Generally, three datum features are selected that are perpendicular to each other however, this is not a requirement. These three datum features are used to establish a datum reference frame (DRF). The datum reference frame is used to fully constrain the part in all six degrees of freedom. The datums that make up the datum reference frame are referred to as the primary datum, secondary datum, and tertiary datum. In order to fully constrain all six degrees of freedom, the primary datum is simulated first, followed by the secondary datum, and then the tertiary datum. The order of the datums is designated in the feature control frame. The first datum listed is the primary datum, the second datum listed is the secondary datum, and the third datum listed is the tertiary datum. The datum feature symbols on the drawing relate to the datum features on the part. Notice datum feature symbols A, B, and C as you look at Example 3-7. Also, notice the datum reference order A, B, C in the feature control frame. The datum reference in the feature control frame tells you that Datum A is primary, Datum B is