Download - Quality (Part 8)
Quality (Part 8)
Quality Assurance
Chapter 42
Quality Definitions
• Quality is defined as those products and/or service
features that meet or exceed customer expectations
• Quality Assurance (QA) is proactive to quality
planning, instituting system improvements, such
as defects and reliability, while upholding the
after-the-fact QC and audit functions
• Quality Control (QC): a system that identifies and
responds to nonconformities, such as defects and
is reactive in nature
Total Quality Management
(TQM) • A system that ensures that quality is the constant
focus and continuous improvement never stops
• Primary goal is to achieve increasingly higher
levels of customer satisfaction while continuously
improving processes linked to business
measurables such as cost and productivity
• Driven by customer needs and maintains a
constant sense of urgency to improve
Quality Costs
• Poor quality ranges from 20-40% of sales, therefore, exceeding the profit margin
• Four categories:
– internal failure
– external failure
– appraisal
– prevention
Internal Failure Costs
• Costs of defects identified prior to the
customer’s receipt of the product or service
• Examples: rework, unrepairable scrap,
sorting bad lots and decreasing selling price
due to poor quality
External Failure Costs
• The monetary value assigned to defects
found after product shipment
• Examples: warranty charges, complaint
adjustments, and cost to return material
Appraisal Costs
• Expenditures related to determine the degree of conformance to product requirements
• Examples: incoming inspection and testing, final inspection and testing, quality audits of in-process and finished products, calibration of instruments, and measuring devices
Prevention Costs
• Incurred to reduce expenditure on failure
and appraisal costs
• Examples: Quality and process planning,
process control, evaluating suppliers and
training
International Organization
for Standards (ISO) – 9000 • Primary function is the development of international
standards to facilitate international trade
• American National Standards Institute (ANSI) is the U.S.
representative to (ISO)
• Published in U.S. as the ANSI/ASQ Q9000 series
• ISO commonly used as criteria for third party audits
between a supplier of production parts, components, or
equipment and in original equipment manufacturer (OEM)
• ISO 9000 certification implies that a quality system is in
place
Malcom Baldrige National
Quality Award (MBNQA) • Created to improve quality and competitiveness
from American companies
• Public Law 100-107, Passed in 1987
• Instituted by the federal government in reaction to serious foreign encroachment in the domestic market
• Granted annually up to six winners in three categories: large manufacturing, large service companies, and small businesses
MBNQA Criteria
1. Leadership (9.5%)
2. Information and analysis (7.5%)
3. Strategic quality planning (6%)
4. Human resources development and management
(15%)
5. Management of process quality (14%)
6. Quality and operational results (18%)
7. Customer focus and satisfaction (30%)
Statistical Methods of
Quality Control
Chapter 43
Inspection
• Inspection is the process of checking the
conformance of a final product to its
specifications
• Inspection must be a continuous activity
because raw material machines and
operators are all subject to variability
Quality Assurance
• Inspection of variables requires the
quantitative measurement of characteristics
such as dimensions and other physical or
mechanical properties
• Attribute inspection offers the direct
comparison of a dimension to a
specification. Example: Go/No-Go Gage,
presence of a flaw
Contemporary Approach: “Is Preventing
Rather than Identifying Defects”
• Reducing the impact of variation through robust design
• Anticipating problems through design for manufacturability and assembly
• The use of mistake and error proofing and
• Attacking common-cause variation to push process-capability indices high enough to eliminate the need for inspection
Statistical Quality Control (SQC)
Used to Detect Process Variability • Natural Variability: is the inherent,
uncontrolled changes that occur in the
composition of material, the performance of
the operator, and the operation of machines
• Assignable Variability: can be traced to a
specific, controllable cause
Statistical Process Control
• A method to systematically detect
assignable variability in a process
F 43-1
Distribution of Shaft Diameters
F 43-2
Continuous Distribution of Shaft
Diameters
F 43-3
Normal Distribution With
Different Means and Dispersions
Control Charts
• Purpose: to determine whether the quality characteristic is varying within acceptable limits for natural variability or whether the process is “out of control”
• Used to track down assignable causes for changes in process performance as a part of process improvement investigation
Most Used Control Charts
• X Chart shows the quality
characteristic of the average of the
sample
• R Chart shows the quality
characteristic of the range of the
sample
F 43-6
Control charts for Average
Dimension and range
X bar R Chart
n
RRRR
n
xxxx
R
x
n
n
21
21 samples ofnumber n
variance typical therepresents and range average
mean population therepresents and average grand
RAxxLCL
RAxxUCL
xx
xx
2
2
3
3
X bar and R Control Limits
RDRLCL
RDRUCL
RR
RR
3
4
3
3
Control Chart Constants Sample Size A2 D4 D3 d2
2 1.880 3.267 0 1.128
3 1.023 2.575 0 1.693
4 0.729 2.282 0 2.059
5 0.577 2.115 0 2.326
6 0.483 2.004 0 2.534
7 0.419 1.924 0.076 2.704
8 0.373 1.864 0.136 2.847
9 0.337 1.816 0.184 2.970
10 0.308 1.777 0.223 3.078
F 43-9
Various Conditions Monitored on
an X control chart
Process Capability Analysis
• A method to determine or assure that a process can meet specifications
• A process is capable if it is able to consistently produce parts within specification
• Process capability ratios above 1.33 are capable of producing good parts
F 43-10
Relationship of Specified Limits
and Natural Limits
Process Capability
2d
Rˆ
deviation standard process
limit specifiedlower LSL
limit specifiedupper USL 6
LSLUSLC p
Cpk
• Process capability (Cp) does not account for where the
process distribution is relative to the center of the
tolerance.
• A process may have a high capability but produce bad
parts if it is centered towards one of the specifications.
• Cpk indicates where the process is located with respected
to the upper and lower limits of the tolerance.
3
XcNearestSpeCpk
Acceptable Sampling
• Used to determine if parts that have already been produced are acceptable
• Less than 100% of produced parts inspected. Example: destructive testing
• Creates the risk of rejecting good lots and accepting bad parts
• Does not guarantee that all parts in a lot are good
Types of Sampling Plans
• Lot Tolerance Percent Defective (LTPD)
plan specifies a certain quality level with
respect to a percent defect at a given risk of
being accepted by the customer
• Acceptable Quality Level (AQL) plan
specifies a certain quality level with respect
to a percent defective at a given risk of
being accepted by the customer
Dimensional Metrology
Chapter 44
Terminology For Dimensional
Metrology • Accuracy is the closeness to the true answer
• Precision refers to the dispersion of measurements or fineness of the readings
• Reliability is the probability of achieving a desired outcome
• Repeatability of a group of measurements taken with the same instruments taken with the same instrument on the same part is the extent to which they are in agreement
Terminology Continued
• Sensitivity is the minimum input required to produce a noticeable output
• Resolution, in reference to dial indicators and other similar instruments, is the ratio of one scale division to the width of the dial hand
• Discrimination refers to the fineness of an instrument’s scale divisions
Precision and Accuracy
F 44-1 (Wick and Veilleux, 1987)
Types of Measurements
• Absolute or direct measurement
• Comparative measurement
Absolute or Direct Measurement Instruments
Metric (mm) U.S. (in)
Rules and scales .5 .01 or 1/64
Slide calipers .5 .01 or 1/64
Vernier .02 .001
Caliper height gages .01 .001
Dial calipers .02 .001
Digital calipers (LCD) displays
Micrometers: outside, inside, depth
F 44-2
One-Inch Micrometer
•What is the micrometer reading?
•Answer - 0.167 Inches
•What is the micrometer reading?
•Answer - 0.385 Inches
•What is the micrometer reading?
•Answer - 0.778 Inches
•What is the micrometer reading?
•Answer - 0.639 Inches
Comparison Measurement Instruments (Greater Accuracy than Direct Measurement)
• Type A – Spindle parallel to the dial
face (Dial Indicator)
• Type B – Spindle at right angle to the
dial face (Dial Indicator)
• Type C – Measuring contact is a lever
(Test Indicator)
F 44-3 (Wick and Veilleux, 1987)
Dial Indicator
F 44-4
Continuous and Balanced Indicators
F 44-5 (Wick and Veilleux, 1987)
Typical Test Indicator
Optical Comparators or Profile Projector
• A measuring microscope for small
parts
• Magnifications up to 500X
• Only limitation is the size of the screen
F 44-6 (Wick and Veilleux, 1987)
Horizontal Shadow-Lighted Optical
Comparator
Coordinate Measurement
• A two or three-dimensional process to
locate the position of holes, surfaces,
centerlines and slopes
• Six sides of a cube shaped part may be
inspected without repositioning
Elements of a Coordinate
Measuring Machine (CMM)
• X-Y-Z positioning device machine structure
• Probing system to collect data
• Machine control and computer hardware
• Software for geometry analysis
• Configurations: cantilever, column, bridge,
horizontal arm and gantry
F 44-7 (Wick and Veilleux, 1987)
Moving-Bridge Coordinate
Measuring Machine
Advantages of Using CMMs
• Flexibility
• Reduced set up time
• Reduced operator influence
• Improved accuracy
• Improved productivity
Types of Measurement Error
• Instrument – Inherent accuracy limitations due to construction and calibration
• Parallax – Reader to instrument position
• Bias – Operator will unconsciously influence a measurement
• Technique – Manipulate or using the instrument incorrectly
• Condition – Temperature error
F 44-8
Snap and Plug Gage
Qualifying Gages
• Snap Gage: used for external dimensions (shafts)
• Plug gage: used for internal dimensions (holes)
• Gage design principle - It is better to reject a good part than declare a bad part to be within specifications
Gage Tolerances
• Gage Tolerance: the permissible
variation in the manufacture of the
gage. Typically 10% or less of the
working tolerance
• Wear Allowance: compensates for the
wear of the gage surface as a result of
repeated use
F 44-1
Standard Gagemakers’
Tolerances
Gage Block Types
• Class B “Working” Blocks
• Class A “Reference” Blocks
• Class AA “Master” Blocks
Sizes of Gage Blocks in 81-Block Set
One Ten-Thousandth Series – 9 Blocks
.1001” .1002” .1003” .1004” .1005” .1006” .1007” .1008” 1009”
One Thousandth Series – 49 Blocks
.101” .102” .103” .104” .105” .106” .107” .108” .109” .110”
.111” .112” .113” .114” .115” .116” .117” .118” .119” .120”
.121” .122” .123” .124” .125” .126” .127” .128” .129” .130”
.131” .132” .133” .134” .135” .136” .137” .138” .139” .140”
.141” .142” .143” .144” .145” .146” .147” .148” .149” .150”
Fifty Thousandth Series – 19 Blocks
.050” .100” .150” .200” .250” .300” .350” .400” .450” .500”
.550” .600” .650” .700” .750” .800” .850” .900” .950”
Inch Series – 4 Blocks
1.000” 2.000” 3.000” 4.000”
2.6437 2.6437 - .1007 - ,1007
2.5430 2.5430 - .103 - .1430
2.440 2.4000 - .140 - .400
2.300 2.000 - .300 - 2.000
2.000 .000- 2.000 .000
5 BLOCKS 4 BLOCKS