Control Charts for Variables with Specified Process Capability Indices

Control charts, also known as Shewhart charts in statistical process control are the statistical tools used to determine whether a manufacturing process is in a state of statistical control or not. If analysis of the control chart indicates that the process is currently under statistical control (i.e. is stable, with variation only co ming fro m sources common to the process) then no corrections or changes to process control parameters are needed or desirable. In addition, data fro m the controlled process can be used to predict the future performance of the process with the help of process capability indices. That is, the suitability and the performance of the manufacturing process are assessed in two stage processes namely control charts and process capability indices. Th is paper deals with the techniques namely, capability based control charts which combines the two stage processes into a single stage process in industrial applicat ions for on line process control. The relative performance of the capability based control charts for variables is assessed with that of existing usual control charts for variables, namely Charts X − and Charts R − . It is observed that the proposed method is simp le to apply and does not warrant any tedious computations both for control charts and for co mputing process capability indices. Fo r the sake o f convenience we have presented the control charts constants for constructing capability based Charts R − and Charts X − . The proposed method is also illustrated with the help of a numerical examp le.