264 Machine Trades Print Reading Copyright Goodheart-Willcox Co., Inc. As an example of the difference between datums and datum features, a datum plane is a theoretically exact plane established by a surface defined as a datum fea- ture. See Figure 15‑6. The bottom surface of the part shown is identified as the datum feature. A theoreti- cal datum plane is established from the datum feature. In practical application, because datums are theoret- ical, a simulated datum is established from a datum feature simulator to make measurements and verify dimensions. A datum feature simulator is a physical entity, such as a machine table or surface plate, used to establish a simulated datum from a datum feature. In Figure 15-6, the datum feature simulator is in contact with the datum feature. The datum feature simulator establishes a simulated datum plane. A datum axis is a theoretically exact axis established by a cylindrical datum feature. See Figure 15‑7. In Figure 15-8, a datum axis is established by the hole identified as datum feature B. Sometimes, it is impractical to identify an entire sur- face as a datum feature. In these cases, datum targets can be used. Datum targets are specific points, lines, or areas on a part used to establish a datum. A datum target symbol is used to identify a datum target. The datum target symbol is a circle divided by a horizontal line. In the lower half of the circle is the identification of the datum target. This identification includes the datum identifying letter, followed by the number of the target. When the datum target is an area, the upper half of the circle indicates the size and shape of the area. The upper half of the circle is left blank for datum target points and datum target lines. A datum target point is shown on a surface as an X. A leader connects the datum target symbol to the point. See Figure 15‑9. The identifying elements of a datum target symbol are shown in Figure 15‑10. A datum target area is shown as an outline filled with section lines. The outline is a phantom line. See Figure 15‑11. Material Condition and Material Boundary Modifiers Material condition modifiers are applied to tolerances specified for features of size, such as a shaft or hole. A material condition modifier is a modifier applied to a tolerance specification indicating the applicable material condition. The applicable material condition indicates the condition at which the tolerance applies. There are three material condition designations: maxi- mum material condition (MMC), least material condi- tion (LMC), and regardless of feature size (RFS). MMC and LMC are indicated using modifier symbols (an encircled M for MMC and an encircled L for LMC). Regardless of feature size is assumed when no other material condition is specified. There is no modifier symbol for RFS. The formerly used symbol for RFS (an encircled S) may be present on older prints. Theoretical datum plane Datum feature Datum feature contact points Surface of datum feature simulator Actual surface of part Part Datum feature simulator Simulated datum plane Goodheart-Willcox Publisher Figure 15-6. Establishing a datum plane from a datum feature. Datum axis Goodheart-Willcox Publisher Figure 15-7. Datum axis established on a tapered cylinder. Datum axis Goodheart-Willcox Publisher Figure 15-8. A datum axis is established by datum feature B. The datum axis is perpendicular to the datum plane estab- lished by datum feature A.