32.0 PROCESS CAPABILITY
32.1 Introduction:
Process capability provides quantified prediction of process adequacy .It is the measured inherent variation of the product turned out by a process.
32.2 Process:
This refers to some unique combination of machine ,tools method ,material and people engaged in production
32.3 Capability:
This word is used to in the sense of an ability, based on tested performance to achieve measurable results.
32.4 Uses of process capability information
Predicting the extend of variability that processes will exhibit. Such information when provided to the designers can help in setting realistic specification limits. Most appropriate process for the tolerance to meet can be chosen. It will provide a quantified basis for establishing a schedule and periodic process control check and readjustment It will assist machines to classes of work for which they are best suited. Testing theories of causes of defects during quality improvement program. It will serve as a basis for specifying the quality performance requirement for purchased machines
32.5 Standardized formula
Process capability =± 3 σ (a total of 6 σ)
Where σ =the standard deviation under a state of statistical control
Capability ratio cp= specification range process capability
= USL-LSL
6 σ
*USL=upper specification limit.
*LSL=Lower specification Limit.
| Process capability ratio | Total product |
| 0.5 | 13.36% |
| 0.67 | 4.55% |
| 1.00 | 0.36% |
| 1.33 | 64 ppm |
| 1.63 | 1 ppm |
| 2.00 | 0 |
32.6 The CPK capability index
Process capability ratio as measured by cp refers tom the variation in a process about the average value. In the figure 2 processes have same capabilities. Because 6 σ is same for each distribution. the process aimed at 2 is producing defectives because it is off centre.
Therefore it is useful to have a capability index that reflects both variation and the location of the process average. such on index is CPK
X-LSL , USL-X
Cpk = Min ----------- -------- 6 6
Eg; USL =20, X=16, LSL=8, S=2,
Capability Ratio = USL-LSL/6 = (20-8)/12 =1
Cpk= Min (16-8)/6 , (20-16)/6 =0.67
If the actual average is equal to the midpoint of the specification range then , cpk = cp
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