Principles and Practice Governing Interchangeability and the Specification of Manufacturing Limits of Size,as Influenced by Statistical Considerations

This paper begins with a general summary of the problem of interpreting interchangeability and a limit of size; the general nature of a fit and of the limits of size which determine it are briefly examined. Some statistical problems of assembly and inspection and their effects on interchangeability and on the practical interpretation of a limit of size are investigated. The standard of measurement is considered in some detail as defining gauge tolerances and errors of measurement, and as giving guidance in relation to certain varying degrees of interchangeability that have been shown to exist. The effect of these arguments on the standard of limits and fits is discussed and certain practical pitfalls in the statistical arguments are briefly examined. Low degrees of interchangeability are discussed and also interchangeability in relation to complex forms, position, and angular dimension. It is shown how the various degrees of interchangeability can be indicated by means of suitable provisions in the standard of drawing practice. Finally, the general attitude of mind to design, which prompts solutions of the type suggested, is set out with special emphasis on the matters discussed.