Will there be an update to the ISO 9000? Is a draft available? It is my understanding that it is reviewed every 5 years and since the last revision was in 2000, it is due.
I am not sure what you mean by "Cpk chart", but I will try to clarify these terms a bit as I understand them. First of all, Cpk is a performance index, all it does is tell you how your process is running long term. It is calculated by taking into account where the process is targeted relative to the specification limits, as well as the variability of the process as estimated by R-bar/d2. For a stable process, approximately normally distributed,and bilaterally toleranced, similar information can be found by obtaining Z-scores from standard normal tables (can be found in statistical texts). From Z-tables, the defect rates can also be obtained. There is a relationship between these methods. A process with a Z-score of 3 is equivalent to a 1.0 Cpk process, and will produce approximately 2700 parts per million defectives.
The task of ongoing control of a production process is seperate from the process performance. The tools of SPC were developed to help identify when a process has changed so the correct action can be taken to correct or improve it. SPC tools are designed to identify the difference between common cause (random) variation and special causes that can be improved or eliminated. When all special causes are eliminated and only common causes remain, the process is said to be in control. At this point it's performance is stable and predictable. Until that point, it is not appropriate to calculate Cpk.
As for what is acceptable, it is your customer that will determine that! If you have a 2 Cpk or +/- 6 sigma process and are producing less than 5 parts per million defectives, your customers may be happier than if you are shipping 20,000 PPM.
At any rate, ideally the control charts on the floor should not display Cpk, or part print limits. The information on the control chart should be used to improve the process and get it stable, the specific information required for offline performance (X-double bar and R-bar)can be obtained by the quality analyst or black belt from the charts.
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mkomarmy 7/2/2003
I am not sure what you mean by "Cpk chart", but I will try to clarify these terms a bit as I understand them. First of all, Cpk is a performance index, all it does is tell you how your process is running long term. It is calculated by taking into account where the process is targeted relative to the specification limits, as well as the variability of the process as estimated by R-bar/d2. For a stable process, approximately normally distributed,and bilaterally toleranced, similar information can be found by obtaining Z-scores from standard normal tables (can be found in statistical texts). From Z-tables, the defect rates can also be obtained. There is a relationship between these methods. A process with a Z-score of 3 is equivalent to a 1.0 Cpk process, and will produce approximately 2700 parts per million defectives.
The task of ongoing control of a production process is seperate from the process performance. The tools of SPC were developed to help identify when a process has changed so the correct action can be taken to correct or improve it. SPC tools are designed to identify the difference between common cause (random) variation and special causes that can be improved or eliminated. When all special causes are eliminated and only common causes remain, the process is said to be in control. At this point it's performance is stable and predictable. Until that point, it is not appropriate to calculate Cpk.
As for what is acceptable, it is your customer that will determine that! If you have a 2 Cpk or +/- 6 sigma process and are producing less than 5 parts per million defectives, your customers may be happier than if you are shipping 20,000 PPM.
At any rate, ideally the control charts on the floor should not display Cpk, or part print limits. The information on the control chart should be used to improve the process and get it stable, the specific information required for offline performance (X-double bar and R-bar)can be obtained by the quality analyst or black belt from the charts.