Tham khảo tài liệu 'process planning episode 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Selection of quality assurance methods 355 Time Figure Example of stratification pattern Time Figure Example of mixture pattern Stratification. This occurs when a number of consecutive plots hug the centre line Evans and Lindsay 1993 . The most common reasons for this are the use of incorrect data SMMT 1994 or the incorrect calculation of the control limits Messina 1987 . A typical pattern of this type is illusttated in Fig. . Mixture. A mixture pattern is when plotted points tend to be close to the control limits as illusttated in Fig. and is also referred to as an erratic pattern Bicheno 1990 . Typical causes of this can be carelessness on the part of the process operators large differences in raw material and overadjustment by the process operators Messina 1987 . Using the above patterns and guidelines as a basis the vast majority of plotted conttol charts can be interpreted in terms of cause and corrective action. Example Consider the forging process studied in Example . Study the control chart produced in Fig. interpret it and identify possible causes of non-conformance. Solution At first glance the control chart would appear to indicate that the process is in control. This is based on the facts that the number of points above and below the centre line is approximately the same the points appear to fall randomly above and below the centre line and finally that the majority of points 75 per cent to be precise fall near to the process mean. However on closer inspection there are three points that actually fall outside the conttol limits these highlighted by being cứcled. It could also be argued that the points that occur close to or outside the control limits indicate a mixture or erratic pattern. Therefore accepting this it is quite likely that the reasons for this are either carelessness or over-adjustment by the operator. Thus the process itself can be considered stable. Process capability In this section the issues of .
