During World War II, the failure of torpedo guidance mechanisms was found to be due to a lack of control within the mechanical gyroscopes. Up to then, conventional plus / minus tolerancing on location and size dimensions was believed to be all that was needed to control the features on a mechanical workpiece. Stanley Parker, of the Royal Torpedo Factory, is generally credited with correcting the issue in the guidance system by introducing the definition and application of a "true position" control. Parker published Notes on Design and Inspection of Mass Production Engineering Work, recognized as the first text on GD&T, in 1940.
GD&T evolved, and the first documented standard for GD&T was published as British Standard 308 : 1953, by the British Standards Institution. Many nations' standards institutes developed their own national GD&T standards. Recognizing the issues arising from having multiple standards, the standards institutions of Canada, USA, UK consolidated their standards which had each evolved from British Standard 308, into a single standard, ASA Y14.5-1957. This evolved into the USASI Y14.5-1966, then ANSI Y14.5-1973, ANSI Y14.5M-1982, ASME Y14.5-1994, -2009, with the latest revision released as ASME Y14.5-2018.
ASME Geometric Dimensioning
and Tolerancing (GD&T)
The focus of ASME's GD&T system remains the communication of the design intent, or functionality.
Recognizing that the USA had control of the GD&T standard which combined the national standards of Canada, UK USA, Australia and New Zealand, the International Standards Organization (ISO) elected to develop an alternative international GD&T standard. This standard, to be called ISO Geometric Product Specifications, was initially loosely based on the ASME Y14.5 standard, but adopted definitions more consistent with those used in Germany and France at the time. Whereas the ASME GD&T system was a single volume encompassing all geometric controls, the ISO GPS system comprised a number of individual volumes, each of which defines and explains individual controls.
ISO Geometric Product Specifications (GPS)
As the ASME standard has evolved, so too has the ISO collection, though they grew increasingly different despite ongoing efforts to reconcile both to a single standard.
At the root of the differences between the two standards is the focus of each. ASME's Y14.5 standard continues to focus on communicating part functionality with a second standard (Y14.5.1) prescribing the algorithms to be used for data-point based metrology. ISO's GPS defines the controls based on the algorithms used to inspect them, rather than on the part functionality.