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