SPC Control Charts
|1. Variables Control Charts|
1.1. X bar chart using R chart or X bar chart using s chart
The X bar chart indicates the changes that have occured in the central tendency of a process. These changes might be due to such factors as tool wear, or new and stronger materials.
R chart values or s chart values indicate that a gain or loss in dispersion has occured. Such change might be due to worn bearings, a loose tool, an erratic flow of lubricants to a machine or sloppines on the part of the machine operator.
The two types of charts go hand in hand when monitoring variables, because they measure the two critical parameters: central tendency and dispersion.
1.2. X & MR (moving range) chart
An X & MR chart is used when only one observation per subgroup is taken and process variability needs to be determined. The moving range (MR) is used to estimate variation.
Examples include but are not limited to:
- destructive sampling
- testing of the process characteristics is costly
- long production times
|2. Attributes Control Charts|
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