Process capability is the range over which the natural variation of a process occurs as determined by the system of common causes, that is, what the process can achieve under stable conditions.
Three types of studies are often conducted to evaluate new processes: peak performance study, process characterization study, component variability study.
The six steps in a process capability study are:
1. Choose a representative segment of the process
2. Define the process conditions
3. Select a representative operator
4. Provide materials that are of standard grade, with sufficient materials for uninterrupted study
5. Specify the gauging or measurement method to be used
6. Provide for a method of recording measurements and conditions, in order, on the units produced.
Process Capability Index is the relationship between natural variation and specifications.
The formula for the Process Capability is: CP = (UTL-LTL) / 6σ
To find the Process Capability Index(CPK), take the lower value of (UTL - μ) / 3σ and (μ-LTL) / 3σ.