Equipment CPK value definition and standards

The abbreviation for Complex Process Capability Index is an indicator used by modern enterprises to represent process capability. Process capability is the ratio of the maximum allowable range of change in process performance to the normal deviation of the process. Process capability research aims to confirm the degree to which these characteristics meet specifications, in order to ensure that the defect rate of process products that do not meet specifications is above the required level, as a basis for continuous process improvement.

After our product passes the GageR&R test, we can start testing the Cpk value.

The higher the CPK value, the better the quality.

Cpk - Process Capability Index

CPK=Min [(USL Mu)/3s, (Mu LSL)/3s]

Discussion on Cpk Application

The Chinese definition of Cpk is: Process Capability Index, which is a quantitative reflection of a certain project or process level, and is also a type of indicator for engineering evaluation.

2. Two parameters closely related to Cpk: Ca, Cp

Ca: Process accuracy. Cp: Process precision.

3. The relationship between Cpk, Ca, and Cp: Cpk=Cp * (1- | Ca |), Cpk is the neutralization reaction between Ca and Cp, Ca reflects the positional relationship (concentration trend), and Cp reflects the dispersion relationship (dispersion trend)

When choosing the process station Cpk for control, the primary factor to consider is cost, as well as the impact of its quality characteristics on the subsequent process.

5. The sampling data should have at least 20-25 sets of data to be representative.

6. In addition to collecting sampling data, it is also necessary to know the upper and lower limits (USL, LSL) of the specification for the quality characteristic in order to calculate its value smoothly.

7. Firstly, the standard deviation of the sampled data can be automatically calculated using the "STDEV" function in Excel（ σ)， Calculate the specification tolerance (T) and the specification center value (U) again Specification tolerance T=Specification upper limit - Specification lower limit; Specification center value U=(upper specification limit+lower specification limit)/2;

8. According to the formula: Ca=(X-U)/(T/2), calculate the process accuracy: Ca value (X is the average value of all sampled data)

9. According to the formula: Cp=T/6 σ ，  Calculate the process precision: Cp value

10. According to the formula: Cpk=Cp (1- | Ca |), calculate the process capability index: Cpk value

11. Rating criteria for Cpk: (Based on this standard, corresponding countermeasures can be taken for the calculated process capability index)

A++level Cpk ≥ 2.0 with special advantages can consider cost reduction

A+level 2.0>Cpk ≥ 1.67 should be maintained

A-level 1.67>Cpk ≥ 1.33 Good performance, stable condition, but should be improved to A+level as much as possible

B-level 1.33>Cpk ≥ 1.0. In general, there is a risk of adverse effects due to slight variations in process factors. Various resources and methods should be utilized to elevate it to A-level

C-level 1.0>Cpk ≥ 0.67. There are many defects in the differential process, and its ability must be improved

D-level 0.67>Cpk cannot accept its poor ability, and it is necessary to consider redesigning the process.