RECOMMENDED RESOURCES
•L. Foster, Geo-Metrics III, Addison-Wesley Publishing Company, New York, NY, 1994
•Oberg, Jones, Horton, Ryffel, Machinery’s Handbook, 28th Ed., Industrial Press, New York, NY, 2008
•ANSI Y14.5M: “Dimensioning and Tolerancing”
•ANSI B4.1: “Preferred Limits and Fits for Cylindrical Parts”
IMPLIED TOLERANCES
To simplify drawings, most drawing title blocks contain a list of implied tolerance values. These tolerance values are applied to all dimensions unless otherwise specified. Tables 3-11 and 3-12 give some commonly used implied tolerances, but any tolerances may be applied to a drawing as implied tolerances. Wider tolerances generally reduce manufacturing cost, so generous implied tolerances are preferred if possible.
Table 3-11: Common Implied Tolerances (Inch)
Table 3-12: Common Implied Tolerances (Metric)
GEOMETRIC DIMENSIONING AND TOLERANCING (GD&T)
The use of Geometric Dimensioning and Tolerancing (GD&T) is the subject of many excellent books. GD&T allows the user to specify tolerances and relationships based on physical features and can yield drawings that are truer to the design intent that traditional Cartesian tolerancing. This section will serve as a basic introduction to the subject. Refer to the recommended resources for a full treatment of the proper use of GD&T and its symbols.
Datums
GD&T uses datums and a system of symbols to communicate the relationships between and tolerances of part features and surfaces. Datums are theoretically exact points, axes, or planes that are used as references to define the location and orientation of features on a part. Their typical appearance is illustrated in Figure 3-2. Although datums are usually associated with physical features, they are theoretical and have no tolerance or deviation from ideal even if the actual feature deviates. Datums should be selected to represent the function and mating relationship of a part. When selecting datums, it is helpful to use the following procedure:
1.Select datum A to be the primary constraining surface to contact the mating part when this part is placed into an assembly. This is the primary functional datum. For convenience, this surface is often a flat plane. Often this will be the bottom surface of a part, or a side surface if the part attaches on its side.
2.Select datum B to be the second constraining feature to locate the part to another part when the part is placed into an assembly. This datum is often a dowel hole or other locating feature. This is the secondary functional datum.
3.Select datum C to be the third constraining feature (if there is one) to contact the mating part when this part is placed into an assembly. This is the tertiary functional datum. This is often a slotted hole for a dowel 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
