The Importance of Detail: Tolerances

Abstract

If it were possible to manufacture components to precise dimensions, an engineer’s life would be greatly simplified. It would only be necessary to specify one dimension for each feature of the component, and provided it was correctly specified and manufactured, a perfect fit would be guaranteed. Unfortunately life is not like that. Precise absolute measurements do not exist in the real world. When asked the time at midday you are likely to reply ‘12 o’clock’ even though the actual time will probably be either just before or just after 12 o’clock. What we really mean by our reply is within a minute or two of midday. Our reply is not a precise absolute measurement. We accept that it has an error or tolerance. The same is true of component dimensions. For example, although a diameter may be specified as 55mm, without any further information this would indicate that any value larger then 54.5mm or smaller than 55.5mm would be satisfactory. The dimension is said to have a tolerance of 1mm.