CP means or stands for Process capability. That said the CP calculation formula is used to determine the measurable property of a process to the width of the specification. The ultimate solution of the process capability is usually manifested either in the form of calculations or histograms. The formula to calculate CP is as below:-
CP = (USL–LSL)/6×σ
USL = the upper specification limit
LSL = the lower specification limit and
σ = the standard deviation.
What is Process Capability Ratio?
Process capability is used to identify the inconsistency in the process using the upper and lower limits respectively. Elements stated below can be observed from the evaluation and process capability ratio:
If the value of CP is equivalent to 1, the process is perfectly centered i.e. (CP = 1 is a perfect process)
When CP is greater than one, it suggests tolerance> the process. This validates that the process seems to be able to do this. (CP> 1 is process compatible)
If the process length is higher than the tolerance, the process variation is within the range of tolerance, making the process incapable.
CPk makes it feasible to compute the closeness of the performance of the process to the condition set upon by permitting for the natural inconsistency of the process.
If the value of CPk is negative, you are required to ensure that the process you inherited leads to an outcome that is not within the range mentioned by the client and is completely outside. There is a separate CPk calculation formula.
The CPk of 3 suggests that the output is outstanding and the process has 6σ capacities and that is what we require to achieve.
We require a CPk of minimum 1.33, which is 4σs, in order to satisfy most customers.
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Example: Find out CP whose USL, LSL and σ are 12, 7 and 4 respectively?
From the above cp equation, it is given that
USL = 12
LSL = 7
σ = 4
Now using the process capability ratio formula, which is as below;
CP = (USL − LSL) 6σ
CP = (12 − 7)6 × 4
CP = 0.208333