An evaluation model for process capability of multiple-process machinery: a case study on the transfer machinery

The major requirements for products machining are speed, cost and quality, in which quality is the most fundamental demand. Process capability index (PCI) is a powerful tool in measuring the process capability of a product. Since the process capability of machinery can’t be evaluated by directly measuring the machinery, we appraise its process capability by the qualities of the product produced by the machine. Since a machined product is normally tolerance asymmetric and nominal-the-best, we developed a generalized measuring index to represent the process capability of a machine, by which its products can be evaluated for both the process yield and quality loss. First, by considering the relationship between process capability index and process yield, we developed a process capability index of a workstation in a machine tool. Secondly, we integrated the process capability indices of all machining workstations to acquire the process capability index for this machinery. Next, an analytic chart for machining with asymmetric tolerances was developed to help to effectively evaluate the process capability of a machine. Finally, a procedure to perform this evaluation model was proposed, and a case study of a transfer machine is given to show the feasibility of this evaluation model.