100-Metodos-de-Qualidade-Total

208 100 METHODS FOR TOTAL QUALITY MANAGEMENT
Method 90
Process capability
Purpose
To demonstrate whether a process is capable of meeting a specification and
to calculate an index to show this capability.
When to use
During design, commissioning or post-commissioning to demonstrate the
capability of a product or process.
How to use
The calculation of process capability depends on whether the process is
measured in terms of variables data or attributes data.
For variables data, capability is expressed in terms of an index called Cp o
Cp is dependent upon the actual centre of the process performance being
the same as the centre of the specification. When this is so, the index is
calculated as:
C
_
p -
Total specification width
Total process spread
where the total process spread is estimated using the formula
Total process spread =
6 X R/d2
where R is the mean of the ranges (see Method 69) and d2 is a statistical
constant depending upon the same size as shown below.
2 1.128
3 1.693
4 2.059
5 2.326
Various values of Cp , together with the parts per million (ppm) of items
produced above specification, are as follows: