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14.4.1 Using control charts
If the mean value and standard deviation of the variable that is to be controlled are
not known, they need to be estimated. The variable is generally measured over a
trial period.
As many as 20-25 groups, each group consisting of five separate samples,
should be measured. One of the reasons for this is that at least 20 sample groups
are necessary to determine whether the process is stable or not.
Calculations to establish the control limits in X-bar chart and R chart respectively
are shown below. Most software that is used for statistical process control
automatically calculates these values. For further information, please contact your
local Atlas Copco representative.
Once the control limits have been established, the control chart can be put into
use. The variable is measured at regular intervals and the readings are plotted on
the chart. It is usually recommended that sample groups of five separate
observations be measured at each interval.
There are different types of control charts. Two of the most common are the X-
bar chart and the R chart. Through the X-bar chart the mean, i.e. the level of the
process, is controlled. The R chart, where R signifies range, is a means of
controlling the process variation.
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