Abstract
Many methods of measuring the out-of-roundness of nominally circular components or holes have been based, for convenience, upon some form of graphical assessment in which the inspector uses a pair of compasses, a rule and a good measure of subjective judgement. This has changed in recent years with the introduction of methods which can be implemented in an analogue or digital form and which are based normally on a best-fit circle rather than on zonal methods. The author justifies the use of a limacon reference line for roundness measuring instruments and goes on to develop a general expression which enables reference lines to be fitted to components having partial arcs or discontinuous circumferences very often met with in industry. It therefore allows for the first time the meaningful objective assessment of the out-of-roundness or roughness of such parts.