The abbreviation of Complex Process Capability Index is a metric used by modern enterprises to express process capabilities. Process capability is the ratio of the maximum allowable variation of process performance to the normal deviation of the process. The process capability study is to confirm the degree to which these characteristics meet the specifications, so as to ensure that the defective rate of the finished product is above the required level, as the basis for continuous process improvement.
Once our products have passed the Gager&r test, we can start testing the CPK value.
A higher CPK value indicates better quality.
CPK – Process Capability Index.
cpk= min[ (usl- mu)/3s, (mu - lsl)/3s]
CPK Application Presentation.
1.The Chinese definition of CPK is: process capability index, which is a quantitative response to a certain project or process level, and is also a type of index for engineering evaluation.
2.There are two parameters that are closely related to CPK: CA and CP
ca: Process accuracy. cp: process precision.
3.Relationship between cpk, ca, cp: cpk = cp * 1 - ca|CPK is the neutralization reaction of CA and CP, CA reflects the positional relationship (central trend), and CP reacts the dispersion relationship (discrete trend).
4.When choosing a process station CPK for control, the primary factor that should be considered is the impact of its quality characteristics on the subsequent process.
5.There should be at least 20 25 sets of data for the calculation of sampling data to be representative.
6.In addition to collecting sampling data, it is also necessary to know the upper and lower limits of specifications (USL, LSL) for the quality characteristics to calculate the CPK.
7.First of all, the standard deviation ( ) of the sampled data can be automatically calculated by the "stdev" function of excel, and then the specification tolerance (t) and the specification center value (u) can be calculatedSpecification tolerance t = upper limit of specification - lower limit of specification;The central value of the specification is u=(upper limit of the specification + lower limit of the specification) 2;
8.According to the formula: ca=(x-u) (t 2), the process accuracy: ca value (x is the average of all sampled data).
9.According to the formula: cp = t 6, the process precision: cp value is calculated.
10.According to the formula: cpk=cp(1-|ca|) calculates the process capability index: cpk value.
11.Rating criteria of CPK: (The calculated process capability index can be countered accordingly according to this standard).
Class A++ CPK 20 Premium Cost reductions can be considered.
A+ Level 20 > cpk ≥ 1.67 Superior should be maintained.
Class A 167 > cpk ≥ 1.33 Good ability and stable condition, but should be promoted to A+ grade as best as possible.
Class B 133 > cpk ≥ 1.0 General The state is average, and there is a risk of defects if there is a slight variation in the process factors, so it should be upgraded to A level by using various resources and methods.
Class C 10 > cpk ≥ 0.67 Poor There are many defective processes, and their capabilities must be improved.
D grade 067 > cpk Unacceptable Its capabilities are too poor, and the design process should be reinvented.