Transcript SPC - Lean Enterprise Leadership Information and Resources

Statistical Process Control
Quality Series
© 2009 Factory Strategies Group LLC. All rights reserved.
Outline
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What is Statistical Process Control
Variability
Control Charts
Process Capability Cp
Measuring Instruments
▫ Repeatability & Reproducibility
© 2009 Factory Strategies Group LLC. All rights reserved.
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Statistical Process Control (SPC)
• Invented by Walter Shewhart at Western Electric
• Distinguishes between
▫ common cause variability (random)
▫ special cause variability (assignable)
• Based on repeated samples from a process
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Variability
The world tends to be bell-shaped
Even very rare
outcomes are
possible
(probability > 0)
Fewer
in the
“tails”
(lower)
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Most
outcomes
occur in the
middle
Fewer
in the
“tails”
(upper)
Even very rare
outcomes are
possible
(probability > 0)
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Variability
Even outcomes that are equally likely (like dice),
when you add them up, become bell shaped
Here is why:
Add up the dots on the dice
Probability
0.2
0.15
1 die
0.1
2 dice
0.05
3 dice
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Sum of dots
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“Normal” bell shaped curve
Add up about 30 of most things
and you start to be “normal”
Normal distributions are divide
up into 3 standard deviations
on each side of the mean
Once your that, you know a lot
about what is going on
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Causes of Variability
• Common Causes:
▫
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▫
Random variation (usual)
No pattern
Inherent in process
adjusting the process increases its variation
• Special Causes
▫
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Non-random variation (unusual)
May exhibit a pattern
Assignable, explainable, controllable
adjusting the process decreases its variation
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Control Limits
Sampling
distribution
Process
distribution
Mean
Lower
control
limit
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Upper
control
limit
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Control Chart
Abnormal variation
due to assignable sources
Out of
control
UCL
Mean
Normal variation
due to chance
LCL
Abnormal variation
due to assignable sources
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Sample number
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Process vs. control limits
Distribution of averages
• Variance of
averages < Control limits
variance of
individual items
Specification limits
Distribution of individuals
Process limits
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Shift in Process Average
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Process Capability
• The ratio of process
variability to design
specifications
Text
Text
Text
Text
Text
Text
Natural data
spread
Title
-3σ
-2σ -1σ
µ
+1σ +2σ +3σ
Lower
Upper
Spec
Spec
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The natural spread
of the data is 6σ
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Repeatability and
Reproducibility Studies
• Quantify and evaluate the capability of a measurement
system
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Select m operators and n parts
Calibrate the measuring instrument
Randomly measure each part by each operator for r trials
Compute key statistics to quantify repeatability and
reproducibility
© 2005
Superfactory™.
© 2009 Factory Strategies Group LLC. All rights reserved.
All Rights Reserved.
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