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QUALITY CONTROL
= 7.6 + 1.51 = 9.11 LCL = 7.6 – (0.58 × 2.6) = 6.09 For R chart
UCL = 2.11 × 2.6 = 5.48 LCL = D3 × R = 0 × R = 0
These control limits are marked on the graph paper on either side of the mean value (line). X and R values are plotted on the graph and jointed, thus resulting the control chart. From the X chart, it appears that the process became completely out of control for 4th sample over labels.
(ii) Standard Deviation of the Process, σ, known ILLUSTRATION 3: Twenty-five engine mounts are sampled each day and found to have an average width of 2 inches, with a standard deviation of 0.1 inche. What are the control limits that include 99.73% of the sample means (z = 3)? SOLUTION:
( = 2 − 3 (0.1
UCLX = X + Z σ X = 2 + 3 0.1 LCLX = X − Zσ X
) 25 ) = 2 − 0.06 = 1.94 inches
25 = 2 + 0.06 = 2.06 inches
ILLUSTRATION 4 (Problem on p-Chart): The following are the inspection results of 10 lots, each lot being 300 items. Number defectives in each lot is 25, 30, 35, 40, 45, 35, 40, 30, 20 and 50. Calculate the average fraction defective and three sigma limit for P-chart and state whether the process is in control.