4 - Dimensions, Symbols and Tolerances

Abstract

This chapter focuses on some of the fundamental principles of dimensioning and the implications with regard to inspection. Dimensioning is necessary to define the shape and form of an engineering component. Although every aspect of a component has to be dimensioned, some dimensions are naturally more important than others. Some dimensions are critical to the correct functioning of components and these are termed “functional dimensions.” Other dimensions are not critical to correct functioning, which are termed “nonfunctional dimensions.” The standard International Standards Organization (ISO) 129:1985 states that each feature shall be dimensioned only once on a drawing. However, there are instances when there is a need for something to be dimensioned twice for information purposes. There are essentially two methods of dimensioning features: there is the addition of numerical values to dimension lines and there is the use of symbols. The ISO standards recommend that abbreviations and symbols need to be used, wherever possible, to avoid a link to any particular language. Symbols can also be used to dimension holes, and there is an entire ISO standard (ISO 15786:2003) devoted to the simplified representation and dimensioning of holes. Symbology is also used to define welds, the relevant standard of which is ISO 2553:1992.