Tham khảo tài liệu 'statistical process control 5 episode 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Process capability for variables and its measurement 267 Cpk Promising non-conforming output will occur but there is a very good chance that it will be detected. Cpk 2 High level of confidence in the producer provided that control charts are in regular use Figure . The use of control chart and process capability data The Cpk values so far calculated have been based on estimates of Ơ from R obtained over relatively short periods of data collection and should more properly be known as the Cpk potential . Knowledge of the Cpk potential is available only to those who have direct access to the process and can assess the short-term variations which are typically measured during process capability studies. An estimate of the standard deviation may be obtained from any set of data using a calculator. For example a customer can measure the variation within a delivered batch of material or between batches of material supplied over time and use the data to calculate the corresponding standard deviation. This will provide some knowledge of the process from which the examined product was obtained. The customer may also estimate the process mean values and coupled with the specification calculate a Cpk using the usual formula. This practice is recommended provided that the results are interpreted correctly. An example may help to illustrate the various types of Cpks which may be calculated. A pharmaceutical company carried out a process capability study on the weight of tablets produced and showed that the process was in statistical control with a process mean X of 2504 mg and a mean range R from samples of size n 4 of 91 mg. The specification was USL 2800mg and LSL 2200mg. Hence Ơ R dn 91 and Cpk potential USL - X 3ơ 296 3 X . The mean and range charts used to control the process on a particular day are shown in Figure . In a total of 23 samples there were four warning signals and six action signals from which it is clear that during .
