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Transcript of Quality Control Copyright © 2015 McGraw-Hill Education. All rights reserved. No reproduction or...
Quality Control
Chapter 10
Copyright © 2015 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
10-2
You should be able to:LO 10.1Explain the need for quality controlLO 10.2Discuss the basic issues of inspectionLO 10.3List and briefly explain the elements of the control
processLO 10.4Explain how control charts are used to monitor a
process, and the concepts that underlie their useLO 10.5Use and interpret control chartsLO 10.6Perform run tests to check for nonrandomness in
process outputLO 10.7Assess process capability
Chapter 10: Learning Objectives
10-3
What is Quality Control?Quality Control
A process that evaluates output relative to a standard and takes corrective action when output doesn’t meet standardsIf results are acceptable no further action is
requiredUnacceptable results call for correction action
Inspection alone is not is generally not sufficient to achieve a reasonable level of qualityMost organization rely upon some inspection and a
great deal of process control to achieve an acceptable level of quality.
LO 10.1
10-4
InspectionAn appraisal activity that compares goods or
services to a standardInspection issues:
1. How much to inspect and how often2. At what points in the process to inspect3. Whether to inspect in a centralized or on-site
location4. Whether to inspect attributes or variables
Inspection
LO 10.2
10-5
Sampling and corrective action are only a part of the control process
Steps required for effective control: Define: What is to be controlled? Measure: How will measurement be accomplished? Compare: There must be a standard of comparison Evaluate: Establish a definition of out of control Correct: Uncover the cause of nonrandom variability
and fix it Monitor: Verify that the problem has been eliminated
Control Process
LO 10.3
10-6
Control ChartA time ordered plot of representative sample
statistics obtained from an ongoing process (e.g. sample means), used to distinguish between random and nonrandom variability
Control limitsThe dividing lines between random and nonrandom
deviations from the mean of the distributionUpper and lower control limits define the range of
acceptable variation
Control Charts: The Voice of the Process
LO 10.4
10-7
Control Charts for VariablesVariables generate data that are measuredMean control charts
Used to monitor the central tendency of a process.“x-bar” charts
Range control chartsUsed to monitor the process dispersion
R charts
LO 10.5
10-8
Establishing Control Limits
samples ofnumber
sample ofmean
means sample of Average
where
1
k
ix
x
k
xx
i
k
i
i
iR
k
R
i
k
ii
sample of Range
ranges sample of AverageR
where
R 1
LO 10.5
10-9
Used to monitor the central tendency of a process
X-Bar Chart: Control Limits
nA
RAxLCL
RAxUCL
x
x
size, sampleon basedfactor chart control a
where
Limits Controlchart x
2
2
2
LO 10.5
10-10
Used to monitor process dispersion
Range Chart: Control Limits
nD
nD
RDLCL
RDUCL
R
R
size, sampleon basedfactor chart control a
size, sampleon basedfactor chart control a
where
Limits ControlChart R
4
3
3
4
LO 10.5
10-11
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 per unit
Control Charts for Attributes
LO 10.5
10-12
Run TestsEven if a process appears to be in control,
the data may still not reflect a random process
Analysts often supplement control charts with a run test Run test
A test for patterns in a sequence Run
Sequence of observations with a certain characteristic
LO 10.6
10-13
Nonrandom Patterns
LO 10.6
10-14
Once a process has been determined to be stable, it is necessary to determine if the process is capable of producing output that is within an acceptable range Tolerances or specifications
Range of acceptable values established by engineering design or customer requirements
Process variabilityNatural or inherent variability in a process
Process capabilityThe inherent variability of process output (process width)
relative to the variation allowed by the design specification (specification width)
Process Capability
LO 10.7
10-15
Cp
limit ification)rance(speclower toleLTL
limit tion)(specifica ranceupper tole UTL
where6
LTL - UTL
pC
LO 10.7