The control limits are derived from the data. To be clear, the control limits are not the spec limits set by the engineer on the drawing. Calculate the Upper and Lower Control Limits (UCL, LCL) for each chart.This will be the centerline of the Range chart. Calculate the average of the sample ranges or “R” values.Calculate X-dbar (the average of the averages), which is represented on the X- bar chart by a solid centerline.The range is the difference between the highest and lowest value in each set of 4 sample measurements. Calculate the range of each of the 25 samples of 4 measurements.Calculate the average value for each of the 25 groups of 4 samples.A general rule is to collect 100 measurements in groups of 4 which would result in 25 data points. Start collecting your initial set of samples.Also determine the frequency that the sample measurements will be collected. Remember the sample size should be 8 or less. ![]() Usually 4 or 5 are common sample sizes used in many industries. The following steps are required to build an X-bar and R chart: The range is simply the difference between the highest and lowest value. The Range chart shows the variation within the subgroup. The X-bar chart displays the variation in the sample means or averages. X-bar represents the average or “mean” value of the variable x. One of the most widely used control charts for variable data is the X-bar and R chart.
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