Figure 13. Axis datum with MMC modifier.

When an (M) modifier is used in the geometric control frame along with a datum letter, the datum axis or the datum center-plane is established by the axis or the center-plane of true gage surface at the WCB of the feature-of-size. In Figure 13, (Case #1) feature-of-size datum A has a virtual condition of 12.6+0.2=12.8. Note that the extra 0.2 is coming due to the straightness tolerance applied to the feature of size. This type of datums with MMC modifier can utilize a fixed gage to establish the datum axis or center-plane. Since the actual size of the FOS can vary, looseness may exist between the part and the gage. This is called datum shift. At the virtual condition of the FOS, there will be no looseness. But as the feature-of-size departs from the virtual condition towards its LMC limit, the datum shift will be the amount of departure, and the maximum possible datum shift is equal to the virtual condition minus LMC. (next)

Figure 14 Datum shift example (next)

Figure 15.Effect of MMC modifier on the same geometry as Figure 12

Figure 15 illustrates the change in gage construction for the same part as shown in figure 12, when MMC modifier is applied to datum. Here, one fixed size pin of diameter 17.6 (virtual size) which is perpendicular to datum plane A, will be used to establish the datum axis B. Similarly, datum center-plane C will be simulated by the center-plane of a true parallel plate of thickness 12.0 and perpendicular to both datum plane A and datum axis B. The position of the two gage elements are given by basic dimension.