We will now look at the three-dimensional (3D) case. Figure 3.1 illustrates a three-plane fixture for determining the position and orientation of a part for measurement. It is analogous to the L-bracket used for the two-dimensional(2D) case.
The 3D part is also shown, its roughness exaggerated for clarity. The -A- datum will be contacted by the three high points of its corresponding surface, as illustrated. Three points determine a plane, so that the highest three points of the surface will position the part so that it can now only slide along the -A- datum. From here, the degrees of freedom of the part are similar to those for the 2D part: one rotational and two translational. As with the 2D part, the next datum will contact the two high points of the B surface. Now the part can only slide along -A- and -B- in a direction perpendicular to -C-. As with the 2D case, one high point of contact with -C- completes the fixturing of the part in space with respect to the datums. Measurements can now be made from the datums with full confidence that whenever or wherever the part is measured, the numbers will be the same.