# Can ISO GPS and ASME Torancing Systems Define the Same Functional Requirements?
## 2 From ASME Y14.5 Specifications to the ISO GPS System Drawing  
Dimensioning and tolerancing should clearly define a designer's intent [20]. All dimensions and tolerances should be selected and specified to secure the functional requirements and mating relationship of a part, and should not allow more than one interpretation. The coupling of the shafts and holes requires the appropriate actual geometry of mating parts. The default Rule #1 in ASME Y14.5 establishes the condition that allows parts to fit every time. According to Rule #1, unless otherwise specified, the limits of the size of an individual feature of size control the form of the feature as well as the size (Figure 1). It means, functionally, that even at maximum shaft and minimum hole diameters, the shaft will be able to freely pass through the mating hole if it is required by a designer. The surface of a shaft shall not extend beyond the boundary (envelope) of the perfect form at maximum material condition. The maximum material size is equal to MMS = 20 mm. The actual local size is defined by a continuously expanding and contracting sphere pulled through the shaft. To meet the local size requirements, the minimum local sphere (LMS = 19.8 mm) shall never exit the material. To find the equivalent specification according to the ISO GPS system notation, it is necessary to start with an explanation (Figure 1b)--two requirements are established: minimum circumscribed size (global size) [21] is equal to 20 mm and spherical size (local size) is equal 19.8 mm. In Figure 2a, the upper limit size is 20 mm due to the modifier minimum circumscribed size (letters GN placed in the elongated circle) and the lower limit size is 19.8 mm due to the modifier local size defined by a sphere (letters LS placed in in the  
Figure 1: (**a**) The size tolerance applied to a pin (ASME Y14.5); (**b**) meaning. elongated circle). Both sizes are defined exactly as ASME Y14.5 default size. It is difficult to verify the local size defined by a sphere, so in most cases, for the lower limit of size 19.8 mm, a two-point size is applied and the specification given in Figure 2b is regarded as equivalent to ASME Y14.5 default size defined according to Rule #1. Dimensional evaluation algorithms for a feature of size according to ASME Y14.5 and the ISO GPS system specifications are discussed in [22]. The term _continuous feature of size_, designated with the \"CF\" symbol, is used in ASME Y14.5 (Figure 3a) to indicate that one or more interrupted features shall be considered as a single feature of size. It means that no portion of the continuous feature crated by two holes shall extend inside one envelope of perfect form at maximum material condition, MMC, (MMS = 18 mm). Almost the same (see Figure 2 for slight difference) functional requirement is controlled in the ISO GPS system by indication CT [21], which stands for _common tolerance feature of size_ (Figure 3b). When ASME Y14.5 feature control frame with the straightness symbol is attached to an extension of the dimension line of a cylindrical feature (Figure 4a), the default Rule #1 is cancelled and the size of the shaft shall be verified as local size defined by continuously expanding and contracting spheres. According to the _independency principle_ formulated in ISO 8015 [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 overrules the independency principle is stated. So, by default, the size in Figure 4b is considered as the two-point size and the straightness tolerance applies to the derived median line. The two-point size is a distance between two opposite points of the shaft's extracted surface, where the connection line between the points includes the associated least square circle centre and the cross-sections are perpendicular to the axis of the associated least square cylinder obtained from the shaft's extracted surface [24].