# Can ISO GPS and ASME Torancing Systems Define the Same Functional Requirements?
## 2 From ASME Y14.5 Specifications to the ISO GPS System Drawing
Of course, the part shown in Figure 7a is simple and it is easy to recognize that the top surfaces are flat, however, for more complex parts, the nominal geometry defined in CAD model specification of the surface profile tolerance may indicate that two surfaces have a free form with low curvature that is excluded when the flatness tolerance is applied. To create coplanarity, two individual tolerance zones should be constrained in location and orientation amongst themselves. The modifier CZ (_combined zone_) indicated after the tolerance value (Figure 8) creates the combined tolerance zone constituted by two parallel planes distance 0.06 mm apart, in which two extracted surfaces shall be contained simultaneously. It is easy to identify in Figure 8 that the tolerance zones are fixed by an implicit theoretically exact dimension TED = 0 mm. Figure 6: (**a**) Roundness tolerance—the default measuring conditions apply (ASME Y14.5); (**b**) roundness tolerance—to indicate the same measuring parameters in the ISO GPS system, measuring conditions should be directly specified. Figure 7: (**a**) Specification of surface profile tolerance for coplanar surfaces (ASME Y14.5); (**b**) each surface shall be within two parallel planes 0.06 mm apart. Two sets of parallel planes are coplanar. The right surface of the actual part is out of the tolerance zone—the requirement is not satisfied. The perpendicularity, parallelism and angularity tolerances have the same symbols in ASME Y14.5 and in the ISO GPS system. The perpendicularity, parallelism and angularity tolerances applied for planar surfaces of a part (integral features) define identical requirements in ASME Y14.5 and in the ISO GPS system. In both systems, the extracted (actual) surface should be contained in a tolerance zone that is limited by two parallel planes separated by a distance that is equal to the tolerance value specified in the second compartment of the tolerance indicator. It means that for the planar surface, each orientation tolerance limits form deviation (flatness) of a toleranced feature. The perpendicularity (Figure 9a), parallelism and angularity tolerances applied for derived features (median line or median plane) create different requirements when specified according to ASME Y14.5 and in the ISO GPS system. According to ASME Y14.5, the orientation tolerance applies to the axis (symmetry plane) of an unrelated actual mating envelope, i.e., the maximum inscribed feature or minimum circumscribed feature (respectively, cylinder or two parallel planes). So, according to ASME Y14.5, the tolerance feature has the perfect form (Figure 9b). According to the ISO GPS system by default the orientation tolerance applies to the derived median line (actual axis) or derived median surface. It means that each orientation tolerance limits form deviation of the tolerance feature (respective median line straightness or median surface flatness). So additional modifier shall be added in specification according to the ISO GPS system to indicate that the specification does not apply to the indicated feature itself, but to a feature associated with it. In this case, the encircled N shall be placed after the tolerance value (Figure 10a) to indicate that the toleranced feature is defined as the derived feature (axis) of the associated minimum circumscribed cylinder - the cylinder that is circumscribed over the actual surface of a pin. Five associated tolerance  
Figure 8: The combination specification element CZ in the ISO GPS system controls top surfaces in the same way as the ASME Y14.5 specification given in Figure 7a. Figure 9: (**a**) Specification of perpendicularity tolerance for the pin with respect to the bottom of its flange (ASME Y14.5); (**b**) minimum circumscribed cylinder axis within the cylindrical tolerance zone. feature specification elements were introduced in 2017 in the fourth edition of ISO 1101. The previous edition from 2012 has no tools to specify requirements identical to ASME Y14.5 specification.