Ch 21 Inspection

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©
2008
Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws a
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hey currently exist.

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Ch
21
Inspection

Principles and Practices

Sections:

1.
Inspection Fundamentals

2.
Sampling vs.
100
% Inspection

3.
Automated Inspection

4.
When and Where to Inspect

5.
Quantitative Analysis of Inspection

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2008
Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws a
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hey currently exist.

No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Inspection

The means by which poor quality is detected and good
quality is assured


Traditionally accomplished using labor
-
intensive methods


Sampling inspection is common


Manual inspection is usually performed after parts are
already made


If defective product is produced, it is too late to correct
during regular processing


Defective parts that are already made must be
scrapped or reworked

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2008
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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

New QC Approaches

to Address these Problems


100
% automated inspection rather than sampling
inspection using manual methods


On
-
line sensor systems for inspection during or
immediately after processing


Feedback control of sensor data to manufacturing
process


Software tools for SPC


Advanced inspection and sensor technologies (e.g.,
CMMs, machine vision)

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2008
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hey currently exist.

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Types of Inspection

1.
Inspection for variables


One or more quality characteristics of the part or
product are measured


Requires appropriate measuring instrumentation

2.
Inspection for attributes


Part or product is inspected to decide whether it
conforms to the accepted quality standard


Sometimes based on judgment of inspector


Sometimes uses a gage


Sometimes involves counting number of defects

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Inspection Procedure

1.
Presentation


item is presented for examination

2.
Examination


consists of measuring or gaging a
quality characteristic, or searching for and counting
defects

3.
Decision


to accept or reject the item?

4.
Action


the item is accepted or rejected


If rejected, can the item be reworked?


Additional action may include adjustments in the
manufacturing process

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Which Features to Inspect?

Key Characteristics


In general, inspecting every feature is unnecessary


Certain features are more important


Key characteristics (KCs) include:


Matching dimensions of assembled components


Surface roughness on bearing surfaces


Straightness and concentricity of high
-
speed rotating
shafts


Finishes of exterior surfaces on consumer products
such as cars


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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Inspection Errors


Errors can occur in the inspection procedure during the
examination and decision steps

1.
Type I error


when a good item is classified as
defective


A “false alarm”

2.
Type II error


when a defective item is classified as
good


A “miss”

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2008
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No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Why Errors Occur

in Manual Inspection


Complexity of the inspection task


Inherent variations in the inspection procedure


Judgment required by inspector


Mental fatigue


Inaccuracies in the measuring or gaging instruments

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2008
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or the exclusive use of adopters of the book

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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Why Errors Occur

in Automated Inspection


Complexity of the inspection task


Resolution of the inspection sensor, as affected by “gain”
or other sensitivity adjustments


Equipment malfunctions


“Bugs” in the computer program controlling the inspection
procedure


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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Inspection Accuracy

The capability of an inspection procedure to avoid type I
and type II errors


Measures of inspection accuracy:


p
1

= probability that a conforming item is classified as
conforming


p
2

= probability that a nonconforming item is classified
as nonconforming


Probability of inspection errors:


Probability of type I error = (
1


p
1
)


Probability of type II error = (
1


p
2
)


Actual fraction defect rate =
q

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Inspection vs. Testing


Inspection is used to assess the quality of a part or
product relative to design specifications


Testing is used to assess the functional aspects of the
product


Does the product operate the way it is supposed to?


Will it operate in environments of extreme
temperature and humidity?


In QC testing, the item is observed during actual
operation or under conditions that might be present
during operation

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No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Sampling Inspection

1.
Variables sampling


the mean value of the quality
characteristic of interest is compared to an allowed
value


The batch is rejected if the mean value compares
unfavorably

2.
Attributes sampling


if the number of defects in the
sample is greater than the acceptance number the
batch is rejected


The allowed value or acceptance number is chosen
so that the probability of rejecting the batch is small
unless the quality level is indeed poor

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or the exclusive use of adopters of the book

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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Acceptance Sampling

In the construction of an acceptance sampling plan, the
supplier and customer must agree on the following
specifications:


Acceptable quality level (AQL)


A quality level that is less than perfect but deemed
acceptable to the customer =
q
0



Lot tolerance percent defective (LTPD)


A lower quality level that is deemed unacceptable =
q
1


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or the exclusive use of adopters of the book

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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Possible Statistical Errors in Sampling

1.
Type I error


rejecting a batch of product that is equal to
or better than the AQL


Actual
q



q
0


Probability of a type I error (

) is called the
producer’s risk

2.
Type II error


accepting a batch of product whose
quality is worse than the LTPD


Actual
q



q
1


Probability of a type II error (

) is called the
consumer’s risk


Supplier and customer must also agree on these risks

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or the exclusive use of adopters of the book

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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Design of an

Acceptance Sampling Plan


Based on the agreed upon values of AQL, LTPD,
producer’s risk (

), and consumer’s risk (

),


The values of the sample size
Q
s

and acceptance
number (or allowed value)
N
a

can be determined


Operating characteristic curve (OC curve) for a given
sampling plan = plot of the probability of accepting
the batch as a function of the fraction defect rate
q

in
the batch


Average outgoing quality curve (AOQ curve) for a
given sampling plan = plot of the mean quality level
passing through the sampling plan

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Operating Characteristic Curve


Typical OC curve for a given
sampling plan shows the
probability of accepting the lot
for different fraction defect
rates of incoming batches


AQL = acceptable quality level
(
q
0
)


LTPD = lot tolerance percent
defective (
q
1
)

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2008
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or the exclusive use of adopters of the book

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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Average Outgoing Quality Curve


AOQ curve for a given
sampling plan shows
the quality level of the
batch as it exits the
inspection plan


AOQL = average
outgoing quality limit
-

the maximum average
defect rate of the batch
and corresponding
incoming defect rate
q

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

100
% Manual Inspection


In principle, the only way to achieve
100
% acceptable
quality is to use
100
% inspection


Two problems with
100
% inspection when performed
manually

1.
Inspection cost per part is applied to every part in the
batch rather than a small portion of the batch (the
sample)

2.
Errors (types I and II) that accompany human
inspection

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or the exclusive use of adopters of the book

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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

OC Curve in
100
% Inspection


Theoretically, the probability of
accepting the batch in
100
%
inspection is
1.0
if the quality level is
better than the AQL and zero if the
quality level is worse than the AQL


Two problems with
100
% inspection
when performed manually:

1.
Expense of inspecting every part

2.
Errors in inspection procedure
(type I and type II errors)

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2008
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hey currently exist.

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Automated Inspection

Automation of one or more steps in the inspection procedure

1.
Automated presentation of parts to human inspector


Human inspector performs examination and decision
steps

2.
Automated examination and decision


Manual loading (presentation) and unloading

3.
Complete automation of entire cycle (presentation,
examination, and decision)

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Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Errors Can Also Occur in

100
% Automated Inspection


Relationship between
sensitivity of an
automated inspection
system and the
probabilities of type I
and type II errors


p
1

= Pr(conforming item
is correctly classified)


p
2

= Pr(nonconforming
item is correctly
classified)

©
2008
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hey currently exist.

No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

More on Automated Inspection

Full potential of automated inspection is best achieved
when


It is integrated into the manufacturing process


100
% inspection is used


Results of the procedure lead to positive action


Feedback process control


To allow compensating adjustments in the process
to reduce variability and improve quality


Parts sortation


Defects are separated from process output

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Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Action Steps in Automated Inspection


Feedback process
control




Sortation into two
or more quality
levels


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Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Off
-
Line and On
-
Line Inspection


Off
-
line inspection


performed away from manufacturing
process, usually after a time delay


On
-
line inspection


performed when the parts are made,
either as an integral step in processing, or immediately
afterward


On
-
line/in
-
process inspection


performed during the
manufacturing process


On
-
line/post
-
process inspection


performed right after
the process

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
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Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Off
-
Line Inspection









Performed away from the manufacturing process


Usually a time delay between processing and inspection

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Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

On
-
Line/In
-
Process Inspection









The inspection procedure is performed during the
manufacturing operation


Allows for corrective action on current work unit

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-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

On
-
Line/Post
-
Process Inspection









Measurement or gaging procedure is accomplished
immediately following the production process


On
-
line because it is integrated with the manufacturing
workstation, and the results can immediately influence the
production process for the next work part

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Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Product Inspection vs.

Process Monitoring


On
-
line/in
-
process inspection is more feasible for process
variables than for product variables


Process monitoring for product quality relies on
assumption of deterministic manufacturing:


Process is in statistical control


Process capability is good


Cause
-
and
-
effect relationships between process
variables and product quality are known, and
mathematical models for these relationships have
been derived

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
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Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Distributed Inspection vs.

Final Inspection


Distributed inspection


when inspection stations are
located along the line of work flow


Most extreme


inspection and sortation are
located after every processing step


More cost effective


inspections are strategically
placed at critical points in the manufacturing
sequence


Final inspection


one comprehensive inspection
immediately before shipment to customer


Quality conscious manufacturers combine the two
approaches

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No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.

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Automation, Production Systems, and Computer
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Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

Quantitative Analysis of Inspection

1.
Effect of defect rate in serial production

2.
Final inspection vs. distributed inspection

3.
Inspection vs. no inspection

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Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

What the Equations Tell Us


Distributed inspection/sortation reduces the number of
parts processed in a sequence of production operations
compared to one final inspection


As the ratio of unit processing cost to unit inspection cost
increases, the advantage of distributed inspection over
one final inspection increases


Partially distributed inspection is less effective than fully
distributed inspection in reducing waste


The “law of diminishing returns” operates in distributed
inspection systems


each additional inspection station
yields less savings

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or the exclusive use of adopters of the book

Automation, Production Systems, and Computer
-
Integrated Manufacturing,
Third Edition,

by Mikell P. Groover.

What the Equations Tell Us


Inspections should be performed immediately following
processes with a high fraction defect rate


Inspection should be performed prior to high cost
processes


Either no inspection or
100
% inspection is more
appropriate than sampling inspection


Whichever is better depends on the relative values of
inspection/sortation cost vs. damage cost (of defects
that pass around the inspection plan)