7 - Metrication in the Drawing Office. Limits, Fits, and Tolerances

Abstract

This chapter discusses metrication in the drawing office. The United Kingdom is adopting the metric system of S.I. Units primarily for the purpose of improving British export trade. The lack of uniformity of engineering practice and engineering standards between different countries has in the past tended to create barriers to trade. The conversion of dimensions and quantities from one system of units to the other is only a passing phase in the process of metrication. The principal purpose of metrication is to bring British engineering practice into conformity with internationally agreed engineering practices. One of the purposes of machining metal parts is to make them fit for other machined parts. To manufacture parts to fit accurately always proves to be a long and tedious process. If a part is made individually, it can be made to match a mating part by hand fitting. To enable parts to be interchangeable and cheaper to make, a system of manufacturing was introduced into engineering that allowed all parts to be made to a specific size within certain limits.