|Spoluautoři FOLTA Martin|
Many production processes are subject to tool wear. In this case, the process mean changes systematically and an upward or a downward trend is observed in the control chart. Such processes must be regularly adjusted. When samples are taken from highly capable processes, the within-sample variability is considerably less than the allowable variability given by the specification limits. If the classical Shewhart control chart is used, some points may lie outside control limits giving a signal that an assignable cause exists. Elimination of this special cause is not feasible from an economic point of view and the cutting tool is not replaced as long as the output remains within the specifications. In this case modified control limits or acceptance control charts can be used. A further refinement is to model the trend and control for assignable causes that might change the trend. The approach is applied to the manufacturing process in the automotive sector, namely to machining stoppers used in automotive transmission systems. The modified and regression charts are designed based on the retrospective analysis and a change of the actual process setup allowing to prolong the tool's operational life is recommended.