The Definition and Use of the Process Flexibility Index

This paper introduces an index for assessing the flexibility of candidate engineering designs and manufacturing processes at various stages during product realization. Once the process flexibility index (Cf) is defined, several examples of its use are provided in metal cutting, metal forming, and injection molding. The definition of the process flexibility index: resembles the process capability index, Cp; embraces Taguchi’s loss function; and provides a simple and quantitative basis for practicing the principles of axiomatic design. A high Cp indicates that the product can be consistently manufactured within precisely defined design specifications. A high Cf indicates that the product can be easily manufactured even with inaccurate design specifications. INTRODUCTION Continued global competitiveness has increased standards for product quality and performance while requiring reduced product development time and unit cost. In this new manufacturing paradigm, the efficient development and delivery of robust product and process designs differentiates the market leader from the follower. Consider the following trends in the product development cycle: • reduction of typical product development times by 10% per year (Muccio); • increasing mass customization and proliferation of product variety (Martin); • increasing reliance on electronic design and manufacturing environments (Noaker); • continued global dispersion of the product development team (Blaich); • continued horizontal outsourcing of production. For the design engineer, these trends increase the pressure to rapidly develop functional designs which can be easily manufactured. The design phase is reported to cost only 6% of the total product cost but locks in 85% of the total cost of the product. In theory, improved design and communication tools should enable the designer to more accurately define the product specifications, more quickly synthesize robust design candidates, and more accurately evaluate the design performance and manufacturability. In reality, increased product variety and reduced product development time reduces the allowable engineering design time while the topology of the globally distributed product development team obscures the design interfaces and inhibits communication with manufacturing. This is a common phenomenon in the electronics industry where the product turnover is high – one analyst reports that a major computer manufacturer’s 80% revenue is from products that are less than two years old (Taninecz). For the process engineer, these trends place enormous pressure to deliver functional product at the lowest possible cost. In theory, robust designs will be electronically transmitted through a virtual, agile manufacturing network to be efficiently manufactured and distributed (Deitz). From a fundamental perspective, however, manufacturing processes have not advanced at the same rate as the required manufacturing requirements. While some advances in set-up and control have certainly been made, the processes have not become significantly more capable or flexible. As a result, engineering designs and manufacturing processes must frequently be altered to obtain the desired product performance and/or acceptable quality levels. The lack of robustness in the product development process is sometimes evidenced by long product development cycles, excessive tooling costs, low process yields, and unacceptable product quality. It has been estimated that about 80% of the market share of a new product goes to the first two companies that introduce it, with a six-month overrun in design time causing a 34% loss in revenues over the life time of the product (Brazier). Lack of planning and product evaluation in the design cycle is well illustrated in Figure 1 (Clausing) which compares a “Find and 1 Copyright © #### by ASME Fix” company with one that prevents problems in early design stages. With every change in the cycle, there are associated costs.