Chapter 12, Problem 5CS

What is the most glaring error in the figure, and why is it so wrong?

(Hint: It is something one never does with control charts.)

Example: Maintaining accuracy in boring using control charts.

The workpiece shown in Figure CSâ€�12 is made of gray cast iron and is bored to the tolerances indicated (5.5125/5.5115 in.). These parts were bored on a chucking machine. Each of the 18 points plotted on the vertical axis of the control chart represents the average of bore diameter measurements made on four parts (sample size). The horizontal broken lines at +0.0005 and −0.0005 represent upper and lower specified limits, respectively. The solid line $\overline{\stackrel{¯}{X}}=0.00017in$ is the estimate of the process capability based on a study of several samples bored on the machine. The upper and lower control limits are then calculated from $\overline{\stackrel{¯}{X}}$ .

We note that samples 4 through 9 show a definite trend toward undersized bored holes. If the operation had been continued without any changes, the successive bored holes very likely would have been out of tolerance. To avoid this situation (out of control), the boring tools were reset toward the upper control limit before parts in sample 10 and the rest were bored.