CONTROL CHARTS WITH SLOPING CENTER LINES

Sometimes a process is designed so that drifts or trends from one level to another are planned and expected. For example, cutting tools wear, fluids evaporate, and filters become clogged with contaminants . These conditions require center lines and control limits that show a trend. The control chart describes the rate of changes in the measured characteristic and can be used to predict when adjustments will be necessary. In Chapter 10, we will address this situation in a more detail, as part of our discussion of tool wear.

Control charts with sloping center lines differ from Xbar and R charts because their center lines are not horizontal. Instead, they follow the drift of the measured characteristic. The factors and formulas for calculating the control limits are essentially the same as standard variables control charts.

Sloping center lines may be applied to any variables control chart (Xbar and R, median and range, Xbar and s, X and moving R).