Technical Note 8

1
Technical Note 8
Process Capability and Statistical Quality Control

• Process Variation
• Process Control Procedures
• Variable data
• Attribute data
• Process Capability

2
Approaches to Quality
3
The Process Focus
4
Types of Statistical Quality Control
Variables
Attributes
5
Basic Forms of Variation

• Assignable variation is caused by factors that
  can be clearly identified and possibly managed

Example A poorly trained employee that creates
variation in finished product output.
Common (natural) variation is inherent in the
production process
Example A molding process that always leaves
burrs or flaws on a molded item.
Statistical Process Control Charts distinguish
between assignable common variation.
6
The Central Limit Theorem
Suppose a population has a mean (?)and a
standard deviation (?)
7
Central Limit Theorem Illustrated
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SPC Control Limits
?
9
X and R Charts
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Process Control Chart Factors
UCL Factorfor Ranges(Range Charts)(D4)
LCL Factorfor Ranges(Range Charts)(D3)
Control LimitFactor forAverages(Mean
Charts)(A2)
Factor forEstimatingSigma( R/d2) (d2)
Sample(Subgroup)Size(n)
2
1.88
1.128
3.27
0
3
1.02
1.693
2.57
0
4
0.73
2.059
2.28
0
5
0.58
2.326
2.11
0
6
0.48
2.534
2.00
0
7
0.42
2.704
1.92
0.08
8
0.37
2.847
1.86
0.14
9
0.34
2.970
1.82
0.18
10
0.31
3.078
1.78
0.22
11
Process Capability Cpk
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Process Capability Cpk Example
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CPK Examples
(a) CPK1.0
(c) CPK2.0
(b) CPK1.33
3s
3s
3s
3s
3s
3s
(e) CPK0.60
(d) CPK1.0
3s
3s
3s
3s
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p - Chart
15
C-Chart Defects in Hotel Suites
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c - Chart for Hotel Suite Inspection
17
End of Technical Note 8