A Global Highway for Mechanical Engineering
Communications: ASME Y14.5, Dimensioning
and Tolerancing
by Lowell W. Foster
[excerpted from page 8 & 9 of ASME Codes and Standards]
ASME Y14.5M-1994, Dimensioning and Tolerancing, which specifies engineering drawing requirements, originated in the 1950s. Over the years it has incorporated technical innovations such as the new electronic compatible systems. Its original goal was to delineate and define mechanical part hardware and to create a common technical drawing language for standardized drawing practices. It was also recognized that representing a perfect part on the drawing must include a permitted “tolerance” as a deviation from the perfect part (because perfection cannot be achieved in real production). This explains the dimensions and tolerances emphasis of Y14.5M. This standard is designated as an “Internationally Recognized Standard,” the standard of choice throughout much of the world.
ASME Y14.5M predominantly illustrates the Geometric Dimensioning and Tolerancing (GDT) language to capture the “function and relationships” of part features. This shows that the design requirements and dynamics of part features (e.g., holes, pins, slots, surfaces), as they relate in part function, fit, or assembly with mating parts, can be captured and specified with “geometric characteristics,” “datum references,” and the other tools of the system. For example, in a mating part situation where four pins on one part are to assemble with four mating part holes, would it not stand to reason that if a hole (any one of them and each individually) was produced away from its worst case low limit size as an individual feature (i.e., be larger within its size tolerance), that any permitted error (tolerance) of location of that hole could be increased an equal amount? In other words, size of features can have an effect on, compensate for, and assist in meeting the location tolerance. This is known as the “maximum material condition” principle and increases possible location tolerances, brings down costs of production, and makes using functional gaging principles possible, which ensures assembly and other advantages. The product designer can select this option or decide on a more stringent method (called “regardless of feature size”) when this dynamic is prohibited. Other controls delineate part form, orientation, profile, and location of features relative to design requirements. This is all done through the system’s symbolic language and the rules and guidelines provided in the standard. This useful, powerful, and widely recognized GDT language is now established as the best method for communicating design requirements. Through use of the GDT system, production, quality, inspection, tooling, programming, and all other supporting operations can meet the requirements using a greater range of methods.