The role of measurement and modelling of machine tools in improving product quality

Abstract

Manufacturing of high-quality components and assemblies is clearly recognised by industrialised nations as an important means of wealth generation. A “right first time” paradigm to producing finished components is the desirable goal to maximise economic benefits and reduce environmental impact. Such an ambition is only achievable through an accurate model of the machinery used to shape the finished article. In the first analysis, computer aided design (CAD) and computer aided manufacturing (CAM) can be used to produce an instruction list of three-dimensional coordinates and intervening tool paths to translate the intent of a design engineer into an unambiguous set of commands for a manufacturing machine. However, in order for the resultant manufacturing program to produce the desired output within the specified tolerance, the model of the machine has to be sufficiently accurate. In this paper, the spatial and temporal sources of error and various contemporary means of modelling are discussed. Limitations and assumptions in the models are highlighted and an estimate of their impact is made. Measurement of machine tools plays a vital role in establishing the accuracy of a particular machine and calibrating its unique model, but is an often misunderstood and misapplied discipline. Typically, the individual errors of the machine will be quantified at a given moment in time, but without sufficient consideration either for the uncertainty of individual measurements or a full appreciation of the complex interaction between each independently measured error. This paper draws on the concept of a “conformance zone”, as specified in the ISO 230:1 – 2012, to emphasise the need for a fuller understanding of the complex uncertainty of measurement model for a machine tool. Work towards closing the gap in this understanding is described and limitations are noted.