Chapter 3 Datums 85 Tertiary datum plane 90° 90 ° Example 3-10. The tertiary datum is established to totally confi ne the part in the datum reference frame. Finally, the tertiary datum is established to totally confine the part in the datum reference frame, as shown in Example 3-10. Now, with the part totally confined in the datum reference frame, all measurements made from the simulated datum planes to related features on the part are reliable and have the same origin every time. The surfaces of this inspection equipment are the simulated datums. The datums are the assumed exact planes established by the true geometric counterpart of the datum features. Measurements cannot be made from the datums, because they are only assumed to exist, but do not actually exist. The machine tables, surface plates, or inspection tables are of such high quality that they are used to act as the datums where measurements are taken from and where dimensions are verified. In this way each dimension always originates from the same reliable location. Dimensions are never taken or verified from one surface of the part to another. Dimensions always originate from the datum planes. Refer again to Example 3-7 for review. Notice again the feature control frame associated with the 2X ∅8.0-8.2 dimension. The last three compartments in the feature control frame provide the datum reference. This is known as the datum order of precedence. The primary datum (A) is given first followed by the secondary (B) and tertiary (C). For instructional purposes, this example labels datum feature symbols conveniently as A, B, and C. In industry and later in this book, other letters are used to identify datums, such as D, E, F, or L, M, N, or X, Y, Z. The letters O, Q, and I are avoided because they may resemble numbers. As the part is positioned on the datum reference frame, as illustrated in Example 3-8 through Example 3-10, there must be three points anywhere on the primary datum feature in contact with the first datum plane. At least two points of contact are required to establish the secondary datum feature against its datum plane. At least one point must contact the datum plane on the tertiary datum