x-bar and R Charts are time oriented line graphs measuring the performance of a process. Having them allows the process to be monitored in real time in a problem prevention mode. The the specification limits reflect if the process is within tolerance. control limits reflect the amount of variation that is inherent in the process data. The control limits do not relate to the specification limits.
The x-bar chart monitors changes in the location (central tendency) of the histogram. The desired direction of the x -bar chart is toward the middle of the chart.
The desired direction of an R Chart is down ward. The R chart monitors changes in the width of the histogram.
Steps to Create an x-bar & R Chart
- Measurement – Accuracy and repeatability are of paramount concern. The 10% discrimination rule and good gaging techniques should be adhered to.
- Relevant Information – Information such as machine, material, and method should also be collected to help identify sources of assignable cause variation.
- Sampling Techniques – Select sample size and frequency.
- Data Collection – Collect a minimum of 100 measurements, for example, or 20 subgroups of 5 each.
- Compute x-bar & R for each subgroup.
- Compute the center lines & control limits.
- Plot the values from steps 6 and 7.
- Analyze the charts Look for assignable cause variation.
- Revise the center lines and control limits when the process is no longer reflected by the current center lines and control limits.
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