Process Capability And Calculating Process Capability Indices

Calculating Process Capability Indices

There are many process capability indices available. Presented here are several of the most common indices.

1. C _p

This is a process capability index that indicates the process’ potential performance by relating the natural process spread to the specification (tolerance)  spread. It is often used during the product design phase and pilot production phase.

Where USL is the Upper Specification Limit and LSL is the Lower Specification Limit.

2. C_pk (2-Sided Specification Limits)

This is a process capability index that indicates the process actual performance by accounting for a shift in the mean of the process toward either the upper or lower specification limit. It is often used during the pilot production phase and during routine production phase.

C_pku = C_pk (Upper Specification Limit)

C_pkl = C_pk (Lower Specification Limit)

3. C_pk (1-Sided Specification Limits)

Cpk can be calculated even if only one specification limit exists or if a minimum/maximum is specified.

• C_{pk (max)}: C_pk for Upper Specification Limit or maximum specified.

• C_{pk (min)}: C_pk for Lower Specification Limit or minimum specified.

Where F is the number of failures.

Table 2 gives the equivalent C_p  value corresponding to Capability %.