Pre-control chart
The pre-control card (also known as traffic light card or process control card) is a type of quality control card for variable characteristics. It serves as an aid for statistical process control (SPC).
Unlike the Shewhart control chart , it does not start from predetermined limit values. The warning and action limits are determined on the basis of the existing process data and thus do not indicate the actual tolerance values. This type of control card is therefore used exclusively to monitor known and capable processes.
The process capability describes the long-term ability of a manufacturing process to remain within the requirements under normal fluctuations. This is indicated using the process capability index c p . If this is greater than / equal to 1.33, the process capability is considered proven. The pre-control card should be used if the c p is greater than two.
application
First, the map is divided into three color-coded areas (green-yellow-red, therefore also called traffic light map). These sections, which indicate whether a value is in an acceptable, critical or negative range, are determined using data from previous processes (not tolerances). In addition to process capability, there is another condition; the first 5 parts of the process must be OK. If this is not the case, the machine must be adjusted and the measurement must be started from the beginning.
As soon as the start conditions are met, two random samples are checked at a conveniently selected time interval. If both parts of the sample are either in the green area, or only one of them is in the yellow area, the machine will continue to operate unchanged. If both parts are in the yellow area, or at least one part is in red, the process must be stopped. The machine is reset or converted and only then continues to be used. After this adjustment, the start condition of the 5 successive parts in the green area must be checked again and fulfilled in order for the process to continue.
advantages
One advantage of this type of control chart is that no prior statistical knowledge is required. It can therefore be used directly on the machine by the operator, which only requires a short teach-in. In comparison, when using other quality control cards, formulas for the warning and action limits must first be calculated. Furthermore, it is a simple and clear way to control the process and to document what can be used to further improve the process.
criticism
A disadvantage of the pre-control chart can be seen in the fact that it is based on a lot of speculation. The warning and action limits are based only on equally large intervals around the average. Furthermore, it is difficult to make retrospective statements about the process, since sources of error are either not recognized or not adequately documented. If you want to show the different types of errors together with their frequency, an error collection card or a Pareto diagram is suitable . It could also be argued that employees are being encouraged to constantly readjust the machine. Any errors that might occur could not be recognized and therefore not remedied.
Individual evidence
- ↑ Ulrich Fischer et al: Metal table book. 42nd edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2002, p. 249
- ↑ Ulrich Fischer et al: Metal table book. 42nd edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2002, p. 249
- ↑ Ulrich Fischer et al: Metal table book. 42nd edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2002, p. 251
- ↑ a b c TQU International GmbH: Archived copy ( memento of the original from June 5, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved June 5, 2016
- ↑ iSixSigma: https://www.isixsigma.com/tools-templates/control-charts/using-control-charts-or-pre-control-charts/ Accessed on June 5, 2016
- ↑ DataNet Quality Systems: http://www.winspc.com/what-is-spc/ask-the-expert/400-pre-control-no-substitute-for-statistical-process-control Retrieved June 5, 2016
