Control chart is counted one of the tools of Seven QC Tools.
It is a kind of Time Series Analysis.
In this analysis, comparing the image between hope and reality is importat.
In the logic to difine unusual condition, normal distribution or other distributions are used.
But in my experience, real process is controlled by systematic errors not by accidental errors. So this logic is not useful.
I use this logic as one of the refference.
Control limit is one of the logic to difine unusual condition. UCL and LCL is often used.
This value is not the NG limit of the production.
Basically, NG limit is not changed. So we can use it to control the process.
If you use softwares to make the control chart, you may be in trouble because the UCL and LCL change often. This is because some software calculate UCL and LCL every time we use by all data.
So if we use UCL and LCL for daily control, we need to define the period to calculate UCL and LCL.
If we use sensor data without invents, we cannnot get good output.
The invent is sampling of timing to analyze.