19Process Control

Process control is concerned with keeping a manufacturing process at a specified stable level. Products that do not meet the manufacturer's specification are unacceptable; they are produced because of changes in the manufacturing process or random variations in the process. To maintain and monitor the quality of the products, samples are taken at regular intervals for inspection. If an unsatisfactorily high number of unacceptable products are observed, some action is taken.

A process that has been running at an acceptable level may suddenly or gradually move off target. The objective of process control is to detect these changes as soon as possible so that appropriate action may be taken.

19.1 Control Charts

The most commonly used technique for detecting changes in a manufacturing process is the control chart. A variable that is characteristic of the quality of the process is plotted against time. For example, a machine may be set to produce items to a certain dimension, called the target value, and usually designated by images. In any process, be it automatic or manual, there will be variations in the dimensions from item to item, even if the target value remains the same, that is, each measurement will have a random variation. The object of control charting is to detect when this variation is “more than random,” when it, in fact, is due to a shift in the target. ...

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