student slides chapter 10
TRANSCRIPT
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McGraw-Hill/Irwin Copyright 2009 by The McGraw-Hill Companies, Inc. All Rights Reserved.
Chapter 10
Quality Control
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Chapter 10: Learning Objectives
You should be able to:
List and briefly explain the elements in the control proces
Explain how control charts are used to monitor a process
Explain the concepts underlying the use of control charts
Use and interpret control charts
Understand the use of run tests
Assess process capability
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Statistical Process Control (SPC)
Quality control seeks
Quality of Conformance
A product or service conforms to specifications
A tool used to help in this process:
SPC
Statistical evaluation of the output of a process
Helps us to decide if a process is in control or ifcorrective action is needed
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Process Variability
All processes generate output that exhibitssome degree of variability
The fundamental question:
Is the variation within acceptable boundaries?
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Control Chart
UCL
LCL
Mean
Out ofcontrol
Normalvariationdue tochance
Abnormal variation
Abnormal variationdue to assignable causes
Each point on the control chart represents a sample of nobservations
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Control Charts for Variables
Variables generate data that are measured
Mean control charts
Used to monitor the central tendency of a process.
X bar charts
Range control charts
Used to monitor the process dispersion
R charts
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Establishing Control Limits
samplesofnumber
sampleofmean
meanssampleofAverage
where
1
k
ix
x
k
x
x
i
k
i
i
iR
k
R
i
k
i
i
sampleofRange
rangessampleofAverageR
where
R 1
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X-Bar Chart: Control Limits
Used to monitor the central tendency of aprocess
nA
RAxLCL
RAxUCL
x
x
size,sampleonbasedfactorchartcontrolawhere
LimitsControlchartx
2
2
2
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Range Chart: Control Limits
Used to monitor process dispersion
nD
nD
RDLCL
RDUCL
R
R
size,sampleonbasedfactorchartcontrola
size,sampleonbasedfactorchartcontrola
where
LimitsControlChartR
4
3
3
4
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Control Charts for Attributes
Attributes generate data that are counted.
p-Chart
Control chart used to monitor the proportion of
defectives in a process
c-Chart
Control chart used to monitor the number of defects perunit
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Process Capability
Once a process has been determined to be stable, it isnecessary to determine if the process is capable ofproducing output that is within an acceptable range
Tolerances or specifications
Range of acceptable values established by engineeringdesign or customer requirements
Process variability
Natural or inherent variability in a process
Process capability
Process variability relative to specification
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Cp: Process Capability Ratio
limitification)rance(speclower toleLTL
limittion)(specificaranceupper toleUTL
where
6
LTL-UTL
pC
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Cpk: Process Capability Index
Used when a process is not centered at itstarget, or nominal, value
3
LTL,
3
UTLmin
,minxx
CCC plpupk
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Taguchi Loss Function
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Operations Strategy
Quality is a primary consideration for nearly allcustomers
Achieving and maintaining quality standards is ofstrategic importance to all business organizations
Product and service design
Increase capability in order to move from extensive useof control charts and inspection to achieve desiredquality outcomes