With SPC, almost anything is possible. If the process average is shifted away from nominal to a point where it becomes equal to one of the specification limits, the result is a Cpk of zero (0), and 50% of the product is being produced outside of the specification limit. Take a look at the Cp value. If the Cp value is within normally acceptable limits (1.00 or larger), then there is hope. The process average can be shifted to the print nominal, and your process will again be producing most of it's product within specification. Further refinement of the process will result in larger values for both Cp and Cpk, increading your capability.
Basic rules for Capability Ratio's Cp and Cpk:
1. Cp tell the process potential (if it were centered). This is the best your process can do at it's current state.
2. Cpk tell you how your process is currently running against it's potential.
3. Cpk can never exceed Cp. If Cpk and Cp are equal, your process is perfectly centered within the specification. Any differences between the two, is an indication of how well your process is centered within specification.
4. A Cpk of 1.33 indicates that ±4 sigma are within the specification limits.
5. A Cpk of 1.00 indicates that ±3 sigma are within the specification limits.
6. A Cpk of zero (0) indicates that nominal is equal to one of the specification limits. (At least 50% nonconforming)
7. A negative Cpk indicates that nominal is outside the specification limits. (>50% nonconforming)
I hope this will help.
