# Can ISO GPS and ASME Torancing Systems Define the Same Functional Requirements?
## 2 From ASME Y14.5 Specifications to the ISO GPS System Drawing
It shall be noted that skipping form deviation of the toleranced axis for perpendicularity tolerance according to ASME Y14.5 will usually give a smaller deviation compare with the perpendicularity deviation assessed according to the default ISO GPS system requirement. However, the pin may have such a shape that deviation assed according to ASME Y14.5 specification is greater (Figure 10b). ASME Y14.5-2018 locates features of size only by tolerances of position. The coaxiality (concentricity) and symmetry symbols that were available in previous editions of this standard have been removed. The coaxiality and symmetry tolerances in ASME Y14.5-2009 controlled the opposing median points of a feature of size. It means that for coaxiality, a set of points has to be examined in each cross-section, which is rarely a functional requirement and is not considered in the ISO GPS system standards. The meaning of the symmetry tolerance in ASME Y14.5-2009 was the same as the meaning of the symmetry tolerance in the ISO GPS system. In ASME Y14.5-2018, the position tolerance is used to control centre distances between features of size (holes, slots, bosses and tabs), location of patterns (groups) of the features of size with respect to the datum system, as well as the coaxial (Figure 11) and symmetrical relationships between features of size. For the location of the derived feature (axis or symmetry plane), ASME Y14.5 applies only position tolerance. According to ASME Y14.5, the position tolerance is applied to the axis (symmetry plane) of an unrelated actual mating envelope, i.e., maximum inscribed feature or minimum circumscribed feature (respectively, cylinder or two parallel planes). The position tolerance (Figure 11a) is specified to control the coaxiality of the two-step shaft flange with respect to the datum A. According to ASME Y14.5, the tolerance feature has the perfect form (Figure 11b). Figure 3:0 (**a**) Specification of perpendicularity tolerance for the pin with respect to the bottom of its flange (ISO GPS) equivalent to the specification given in Figure 9a; (**b**) perpendicularity deviation assessed according to ASME Y14.5 definition may be greater than that assessed according to the default ISO GPS meaning that controls the derived median line of the toleranced feature. In Figure 12a, the coaxiality symbol with the encircled N placed after the tolerance value 0.1 mm is specified to secure in the ISO GPS system the same functional requirement as defined in Figure 11a. Of course, when in the ISO GPS system, the position tolerance symbol is specified instead of the coaxiality symbol; the meaning of specification is the same, but the coaxiality symbol clearly states that the theoretically exact dimension (TED) between the datum axis and the axis of the tolerance zone is equal to 0 mm. Specification of the position tolerance does not convey such information. The part in Figure 12 is simple and there is no doubt that there is no offset between the tolerance zone axis and the datum axis, but if a part will be more complex, application of coaxiality in the ISO GPS system is recommended as it is more informative. According to ASME Y14.5, by default, the _simultaneous requirement_ applies to the position and profile tolerances that are located by basic dimensions related to the set of datum features specified in the same order of precedence without the material modifiers or with the same maximum or least material requirements. In the simultaneous requirement, there is no translation or rotation between the datum reference frames of considered geometrical tolerances, therefore, all features create one pattern. This default approach may be cancelled by indication SEP REQT placed adjacent to each tolerance indicator. According to the ISO GPS system independency principle [23], by default, every GPS specification for a feature or relation between features shall be fulfilled independently of other specifications, except when a modifier that overtules the independency principle is stated. The plate with four holes is given in Figure 13a.