Introduction:
Control charts may be used partly to control variation and partly in the identification and control of the causes which give rise to these variations.
Capability:
Where:
USL=Upper Specification Limit, LSL=Lower Specification Limit, UCL=Upper Control Limit, LCL=Lower Control Limit, n=Sample size
δ=the standard deviation of the individual measurements.
VARIABLES CONTROL CHART:
Steps in constructing M-R Control charts:
Step 1: Plot the calculated means (M) and ranges (R) in two different charts (diagrams) where the abscissa is equal to subgroup number and the ordinate measure are the means and ranges respectively.
Step 2: Calculate the average range and the process average (=the average of the subgroup means):
Step 3: Calculate the control limits UCL (Upper Control Limit) and LCL (Lower Control Limit):
Control limits for the means (=M):
Control Limits for the ranges (=R):
The following control limits can now be calculated: Control limits for the means (=M):
Control limits for the ranges (=R):