Essential Manufacturing

27Metrology and Statistical Quality Control

27.1 Introduction

To ensure that quality standards are being met, the actual quality achieved has to be measured. In order to do this, the quality levels must be clearly specified in such a way that measurement is possible. The process of determining whether or not a product, component, material or process conforms to specification is called inspection. In some situations, visual inspection can account for around 10% of the total labour cost of manufactured products, although today, particularly in high volume production, the use of machine vision systems coupled with other automated sensing and handling devices such as industrial robots is widespread. Inspection may be qualitative or quantitative. In qualitative inspection, a product is checked on an accept or reject basis by examining the attributes. For example, a flaw in the glass of a bottle, a crack in a casting or discoloured fruit on a conveyor would be attributes that would cause these items to be rejected. Other aspects of this would be the detection of foreign bodies such as mice or insects on a conveyor in a food processing plant, verification of a completed task in an assembly sequence or checking the degree of swarf build up on a drill bit. Quantitative inspection involves measuring the dimensional features of a product or component and this is the focus of the remainder of this section.

When manufacturing, for example, a shaft and bearing it is not good enough to say ...