The process capability index (Cpk) is used to measure the ability of a process to product output within the specification limits defined by the customer. In other words, this tool is used to measure the ability to produce a product within a customer's defined tolerance range. It helps answer the question of how close the process is to a given target and how consistent it is around the average performance of that process. In addition to providing the best case scenario for existing process, it also can give future process performance estimates, assuming the performance is consistent over time.
The Cpk is a standard index to state the capability of one process, the higher the Cpk value, the better the process is. For example, if Robot 1 has a Cpk of 1.7 and Robot 2 has a Cpk of 1.1. Using the Cpk value, you can derive that Robot 1 is much better than Robot 2 in terms of capability. Because Cpk using the specification limis and parts variation (sigma), you can also derive the yield processed and losses from the machine.