... dimensioning a part. Figure 4-4 shows a part with four holes. The designer selected the back of the part as the primarydatum, datum A, because the back of the part mates with another part, and the ... Part Parts are thought to have six degrees of freedom, three degrees of translationalfreedom, and three degrees of rotational freedom. A part can move back and forth in the X direction, in and ... reference frame—A, B, and C. Datums B and C are thelower and left edges of the part and identified as the secondary and tertiarydatums, respectively. Dimensions are measured from, and are perpendicularto,...