A quality engineer at an automotive parts plant monitors the lengths of camshafts. Three machines manufacture camshafts for three shifts each day. The engineer measures five camshafts from each machine during each shift.
The quality engineer creates an Xbar-R chart for each machine to monitor the camshaft lengths.
Minitab creates three Xbar-R charts, one chart for each machine. The engineer looks at the R chart first because, if the R chart shows that the process variation is not in control, then the control limits on the Xbar chart are inaccurate.
The R charts for all three machines show that the process variation is in control. No points are out of control, and all the points fall within the control limits in a random pattern.
The Xbar charts show that Machine 2 is in control, but Machines 1 and 3 are not. On the Xbar chart for Machine 2, no points are out of control. But Machine 1 has one out-of-control point, and Machine 3 has two out-of-control points.
TEST 1. One point more than 3.00 standard deviations from center line. |
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Test Failed at points: 8 |
TEST 1. One point more than 3.00 standard deviations from center line. |
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Test Failed at points: 2, 14 |