Advantages of GD&T over coordinate tolerancing system

Sometimes designers think that it is faster to dimension a part with coordinate tolerancing than by using geometric tolerancing. This is not true. Let's take the drawing from Figure 1-7 and add geometric tolerances to eliminate the major shortcomings of the coordinate dimensions.

The first major shortcoming of coordinate tolerancing is "square tolerance zones." Let's look at how geometric tolerancing eliminates this shortcoming. In Figure 1-12, the arrow labeled "A" points to a GD&T symbol. This symbol specifies a cylindrical tolerance zone. The square tolerance zone from the coordinate toleranced version (Figure 1-7) is converted into a cylindrical tolerance zone. Notice that the tolerance value is larger than the 0.5 tolerance allowed in Figure 1-7. Figure 1-13 shows how the cylindrical zone provides additional tolerance in comparison' with the square tolerance zone. The additional tolerance gained from using cylindrical tolerance zones can reduce manufacturing costs.

The second major shortcoming of coordinate tolerancing is "fixed-size tolerance zones." Let's look at how geometric tolerancing eliminates this shortcoming. In Figure 1-12 the arrow labeled "B" points to a GD&T symbol. This symbol specifies a tolerance zone that applies when the holes are their smallest diameter. When the holes are larger, this GD&T symbol allows the hole location to have additional tolerance. This additional tolerance allowed by the GD&T symbol can reduce manufacturing costs.

The third major shortcoming of coordinate tolerancing is that it has "ambiguous instructions for inspection." Let's look at how geometric tolerancing eliminates this shortcoming. Geometric tolerancing contains a concept called the "datum system." The datum system allows the designer to communicate the appropriate method of part setup to the inspector. First, symbols are added to the drawing to denote which surfaces touch the gage. See Figure 1-12, arrows labeled "C" and "D." Then, inside the feature control frame (see arrow labeled "E"), the sequence is given for the inspector to address the part to gage surfaces. Using the geometric tolerancing specifications from Figure 1-12, the inspection method would be the one shown in Figure 1-9A.

Now, through the use of geometric tolerancing, the dimensioning shortcomings are eliminated. Let's take a look at what the drawing would look like if we tried to accomplish the same level of drawing completeness with coordinate tolerancing. Figure 1-,,14 shows the vise pad drawing from Figure 1-12. This time the part is dimensioned with coordinate dimensions to the same level of completeness as the GD&T version, but using words instead of symbols. Now, which drawing do you think would be easier to create? When the goal is to dimensionboth drawings to the same degree of completeness, it is faster to use geometric tolerances.

The differences between coordinate tolerancing and geometric tolerancing are summarized in Figure 1-15. When comparing these tolerancing methods, it is easy to understand why geometric tolerancing is replacing coordinate tolerancing.

  • References: