ToolsTalk ACTA
157
14.3 Capability studies
The purpose of capability studies is to indicate how well a process or machine
performs within the tolerance limits. As for control charts, the method described
below is only applicable to normal distributions.
The most commonly used are the ordinary process capability indexes, CP, and
the modified process capability index, CPK. CP shows the relationship between
the natural process variation - expressed as 6σ − and the tolerance interval:
limitMin -limitMax
CP =
A common requirement is for the CP-value to exceed 1.33. This indicates that
six times the standard deviation covers no more than 75% of the tolerance
interval.
However, this index does not ensure that the distribution is centred in the middle
of the tolerance interval. The process may still perform outside one of the
tolerance limits, even if the scatter is small.
Min Max
Min Max
High CP, badly centred process. Low CP, well-centred process.
To Next Page
Loading...
