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Consider a nominally 1.00-inch-thick block that has three progressively more stringent dimensioning schemes applied to it (Figure 1). Beginning with the block dimensioned at 1.00 plus or minus 0.100, it can be assumed that the designer was communicating a relative lack of importance on the overall height by assigning such a liberal tolerance, be it for economical manufacturing or other reasons.
In any event, the ASME Y14.5M-1994 national dimensioning and tolerancing standard, says, "In the absence of a geometric tolerance, the conventional tolerance prescribes the limits of variation permitted on a given feature."
Therefore, in the example in Figure 1, any irregular surface confined between the limits of the conventional X-Y coordinate dimension (in this case, 0.90 and 1.10) is technically acceptable. Yet the designer may need to further control this feature by applying additional measures in the feature control frames shown in Figure 1 (c) and (d). The frames contain geometric dimensioning and tolerancing (GD&T) symbols, as well as the tolerance and datums pointing out the part's most critical features in terms of form, fit, function, processing, tooling and inspection.
Under (c) in Figure 1, a flatness symbol controls the amount of variation permitted the top surface of the block to 0.010 inch. In (d), a second geometric symbol—parallelism—added to the top surface further constrains the feature and ties it to the datum. Through both the conventional and geometric dimensioning systems, the designer precisely communicates three interrelated conditions that must be simultaneously met.
This example illustrates the importance of geometric dimensioning and tolerancing, a system with a bedrock of 14 control characteristic symbols (Figure 2). When sparingly applied in combination with X-Y coordinate dimensions, GD&T affords designers and engineers a precise, cost-effective method of completely describing the dimensional configuration of any part.
The 14 Geometric Characteristics
1. The straightness tolerance can be applied to either a cylindrical or flat feature. For a cylindrical feature, there are two possible intentions. When applied to the cylinder's surface, the symbol's intent is to control the straightness of each individual line element making up that surface. When attached to the diameter dimension line and the diameter symbol is contained within the feature control frame, the cylinder's centerline is being controlled within a cylindrical tolerance band of a specific size. When the straightness symbol is attached to an otherwise flat surface, the line elements lying on the surface of the part and running parallel to the straightness symbol are being controlled. Note that no datum is called out, since straightness is defined as a line drawn through two points in space and is measured only in reference to itself.
2. Flatness may be applied to any flat surface in addition to customary size controls whenever the acceptable limits of variation prescribed (e.g., flatness) need to be held tighter than the size limits alone. Like straightness, flatness is referenced to itself and so is applied without need for or reference to a datum.
3. Circularity, or roundness, controls any surface feature created by a set of points intended to be equidistant from an axis. Parts that are round in cross section, such as cylinders, cones, O-rings and spheres, give examples of simple geometric features that, in addition to a conventional size dimension and tolerance, may be further controlled with the circularity tolerance.
4. Cylindricity typically applies to rigid cylindrical shapes when the straightness and roundness characteristics need to be simultaneously controlled.
5. Profile-of-a-line controls the allowable variation of one specific non-straight line element. Conceptually, the profile of a line and the straightness tolerances are the same; one controls curved lines while the other controls straight lines.