Don’t get bog down by these words.. like Cp and Cpk…
You may get lot of material related to Cp and Cpk but below diagram I made is self-explanatory and you can remember it for life long.
If you are given a rifle to shoot on the white board with a target at the centre. If four persons try these rifle shots named A, B, C & D. Then above results of these four shooters can be depicted in below way.
- In case of A the shots are the centre(good Cpk) and are very near to each other (good Cp).
- In case of B the shots are the centre(good Cpk) and are not near to each other (bad Cp).
- In case of C the shots are away from centre(bad Cpk) but are very near to each other (good Cp).
- In case of D the shots are away from the centre(bad Cpk) and are also not near to each other (bad Cp).
Points to ponder
- Cp is Process capability and Cpk indicates Process capability index.
- Cp is showing how repeated is the process
- Cpk indicates how much near the readings to centre or target.
- So obviously Cpk can be corrected by giving some offsets.
- But it is very difficult to correct Cp. For Cp correction you may have to correct many parameters in the process and machine also. Like axis repeatability, if grinding process then grinding wheel, dressing, diamond used for dressing etc.
Spend few minutes on understanding the picture I’ve depicted above and you will be always clear about Cp, Cpk.