Process Capability
Definitions
•
Process Capability – What can it do?
•
Control Chart – Is the process stable / predictable?
•
DOE – (Design of Experiments) What is the full potential?
•
Cp – Potential to meet tolerances w/ inherent variation?
•
Cpk – Ability to meet tolerances accounting for location and inherent variation?
•
Cpm – Relative COPQ for deviations from target with only inherent variation?
Recommendations
1. C – A first step / doesn’t apply for one-sided specification
p
2. C – OK indicator/ include confidence intervals
pk
3. Cpk - Doesn’t guarantee low defect rates
4. Increasing Cpk – Value depends on how
5. Stability - Accomplishing capability is meaningless without it
6. Ppk – Indicator of performance Ok but can be misleading
7. C - Consider its use to maintain focus on true COPQ
pm
8. Priority – Stability / Analysis & reduction of variation / CI
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When to Use What
Application
DOE
Control
Chart
Cp
Cpk
Cpm
Ppk
σ cc
σtotal
Determine Stability
Optimize Process
Determine Potential
Capability
Determine Capability
Indicate Relative
COPQ
Indicate Performance
to Tolerance
σtotal
Total Variation (σ2common cause + σ2special cause)
(territory traversed)
σcc
Formulas
Common Cause Variation (σ2cc original process)
(territory could have traversed)
0
1
2
3
4
5
6
7
μoriginal
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Portland, OR 97210
Info@nwcpe.com
8
9
10
11
12 13
μoverall
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