## 1 Introduction  
Manufacturing enterprises have sought process improvement since the dawn of the industrial age. Production lines and division of labour brought early gains in productivity. Automation further increased productivity, especially when paired with computer control systems. Such process improvements are enabled by technological advancement in response to demand. Figure 1 illustrates the forces of technological push and requirements pull as the drivers of improvement. Responding to both push and pull mechanisms, various types of manufacturing systems have been created. Dedicated manufacturing systems led to the rise of mass-produced and more affordable products [2; 3]. Flexible manufacturing systems responded to the need for factories to produce a large variety of products at small scales. These systems represented two extremes: (1) dedicated manufacturing systems with low flexibility and low cost per product, and (2) flexible manufacturing systems with high flexibility and high cost per product. Eventually, the need arose for dedicated manufacturing systems to be more flexible, to be able to respond to fluctuations in market demand and the availability of materials and resources [4]. The response was a reconfigurable manufacturing system, named in reference to the use of configurable tools to achieve small production runs and reasonably quick change-over [5]. Moreover, the manufacturing system is designed around the part family, with the customized flexibility required for producing all parts of this part family [6]. However, the desire for personal and individual products is leading to a rapid increase in the demand for mass customisable products. This is more evident in high-tech products of high value, such as automobiles and aircraft, but the phenomenon is even spreading to traditional products [7]. Increased product customisation and variation places new demands on manufacturing systems. The following effects can be identified [8]:  
* Batch sizes are shrinking to accommodate more product variation, leading to frequent production downtime for system reconfiguration and tool change-over;
* More product variation necessitates more versatile production operations, as a larger variety of material and product transformations are needed;
* A wider range of product specifications and more production possibilities introduce significantly more complexity into the planning and operations management;
* More complexity leads to uncertainty, because the manufacturing processes and resources have a more unpredictable impact on the product. Manufacturing systems must become more flexible to produce more varied products [9]. The extent of the variation is also increasing, with customers exploring the possibilities of truly personalised products [10]. Such variability necessitates ever greater process flexibility and versatile production resources [11; 12; 13]. By positioning manufacturing flexibility as the improvement in Figure 1, we see that demand fluctuation and mass customisation pulls manufacturers towards volume and product flexibility. Meanwhile, new technologies push factories towards technology and equipment flexibility. Market pull and technological push are increasing uncertainty and the demand for manufacturing flexibility. [PERSON] [14] assimilates the two concepts of uncertainty and flexibility into a framework. The argument is made that flexibility is a coping mechanism against the uncertainty inherent to any manufacturing enterprise. Uncertainty is present at every stage of an enterprise and flexibility can help to lessen the effect of uncertainty. More importantly, the overall flexibility of an enterprise is a function of the flexibility at its input, process and output stages. Input flexibility is affected by the flexibility of suppliers and, thus uncertainty can be reduced with surplus inventory or strategic partnerships. Output flexibility is necessary to deal with the uncertainty of demand and customer expectations. Figure 1: Interacting forces towards flexibility (adapted from [1]).