Applications , page 5
Exercise 4 Continue to monitor the hardness data. The process remains in statistical control as shown in the x bar and s charts plotted below.
In practice the data would be plotted as measurements are acquired. The control limits plotted from the first 30 measures remain in effect as long as there has been no change in the process; then the control limits should be recalculated from new samples produced under the changed conditions.

These charts provide a good opportunity to refine chart reading skills.
In figure 19 below, what are the approximate values of the upper and lower control limits and the process center? Write down your answers and then check with the calculated values on page 4.
Figure 19. This x bar chart continues to monitor the process on the previous pages.
The control limits are the same as calculated from samples 1 - 30.
Figure 20. This s chart continues to monitor the process on the previous pages.
The control limits are the same as calculated from samples 1 - 30.
In figure 20.1 below, something has happened to send the process out of the control limits established previously. What has happened, and what could be the cause?
Figure 20.1 An x bar chart of hardness samples 61–90.
The process center has changed1 The new x double bar is up one HRC unit at about 58.5. In this case the blades were not put through the second temper. The cause is under investigation. What are some possible causes, given what you know about the process.
Statistics and SPC: 1 2 3 4 5 6 7
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