Machine Designers Reference. J. Marrs
pin. When all three datums are in use to constrain the part, it should not be able to move or rotate. It should be fully constrained.
Figure 3-2: Datum Callouts
It is possible to have more than three datums on a part, and it is also possible to have only one or two datums on a part. The typical case uses three datums. Datums can be patterns of features, such as a pair of dowel holes. In a case where datum A is a planar surface and datum B is a pair of holes, datum C is rarely needed to fully define the part because datum B can be used for location and orientation purposes. Orientation can be related to the imaginary line between the two holes in the pattern defining datum B, and location can be related to the center of either hole.
Symbols
Geometric characteristic symbols are used to specify feature and surface characteristics like orientation, location, shape, symmetry, and runout. These symbols are used as part of a feature control frame represented by a rectangle. This feature control frame contains the geometric characteristic symbol, a total tolerance, any modifiers, and the datums referenced in their order of significance. Figure 3-3 illustrates a typical feature control frame and its contents.
Table 3-13 provides geometric symbols governed by ANSI Y14.5M, which is the unified U.S. standard for both inch and metric units. For a complete coverage of ISO standards governing GD&T, the following standards are required:
•ISO 129: Technical Drawings General Principles
•ISO 2768: General Geometrical Tolerances
•ISO 8015: Fundamental Tolerance Principle
•ISO 406: Linear and Angular Dimensions
•ISO 5459: Datums and Datum Systems
•ISO 2692: Maximum Material Principle
•ISO 2692: Least Material Principle
•ISO 1101: Tolerances of Form, Orientation, Location and Run-Out
•ISO 5458: Positional Tolerancing
•ISO 3040: Cones
•ISO 1660: Profiles
•ISO 10578: Projected Tolerance Zones
•ISO 10579: Non-Rigid Parts
•ISO 7083: Symbols Proportions
When tolerances of position or profile reference datums, the two are related using “basic” dimensions. These dimensions have their value enclosed in a box. Basic dimensions are interpreted as nominal dimensions with no tolerance. The tolerance governing the feature’s position(s) and orientation(s) is applied through the feature control frame attached to the feature that is being positioned using the basic dimension.
Figure 3-3: Feature Control Frame
The following are descriptions of the most commonly used geometric symbols (Table 3-13) and their interpretation:
Straightness is a specification applied mainly to cylindrical features, and less commonly to flat features like rectangular bars. In the case of a cylindrical feature, it defines a tolerance zone within which the longitudinal elements of the feature must lie. Straightness is a tolerance of form, not position, and therefore does not reference any datums. Figure 3-4 demonstrates the straightness callout and its measurement.
Table 3-13: ANSI and ISO Geometric Symbols
Figure 3-4: Straightness Callout and Interpretation
Flatness is a specification applied to flat surfaces. It defines a tolerance zone between two parallel planes. All elements of the actual surface must fall within these two parallel planes. Flatness is a tolerance of form, not position. As a result, it does not reference any datums, and the planes defining the tolerance zone do not need to be parallel to any datums. Figure 3-5 illustrates the flatness callout and its measurement.
Figure 3-5: Flatness Callout and Interpretation
Circularity is a specification applied to cylindrical, conical, or spherical surfaces. On a cylindrical feature, it defines a circular tolerance zone on a plane perpendicular to the axis of the cylinder. An infinite number of planes can be assumed. The feature surface at each planar cross section must fall within the tolerance zone. This tolerance does not define the relationship between the cross sections at different planes. Circularity is a tolerance of form, not position, and therefore does not reference any datums. Figure 3-6 illustrates circularity tolerance for a conical feature.
Figure 3-6: Circularity Callout and Interpretation
Cylindricity is a specification applied to a cylindrical feature. It defines a cylindrical tolerance zone with a straight axis around the cylinder, within which all points on the feature’s surface must lie. Cylindricity is a tolerance of form, not position. As a result, it does not reference any datums. Figure 3-7 illustrates the cylindricity callout and meaning.
Figure 3-7: Cylindricity Callout and Interpretation
Angularity is a specification applied to a flat surface, axis, or midline of a feature. It defines a tolerance zone between two parallel planes at the specified angle from a datum. The angle will be given as a basic dimension. All points on the surface must fall between the tolerance planes. Angularity is a tolerance of orientation relative to a datum. Figure 3-8 illustrates angularity tolerance.
Figure 3-8: Angularity Callout and Interpretation
Perpendicularity is a specification applied to a flat surface, axis, or midline of a feature. It defines a tolerance zone between two parallel planes perpendicular to a datum. All points on the surface must fall between the tolerance planes. Perpendicularity is a tolerance of orientation relative to a datum. Figure 3-9 illustrates perpendicularity tolerance.
Parallelism is a specification applied to a flat surface, axis, or midline of a feature. It defines a tolerance zone between two parallel planes parallel to a datum. All points on the surface must fall between the tolerance planes. Parallelism is a tolerance of orientation to a datum. Figure 3-10 shows the parallelism callout and its meaning.
Figure 3-9: Perpendicularity Callout and Interpretation