4-quality control

Quality Control

09.09.09

What is Quality Control Denotes all those activities which are directed

to maintaining and improving quality Involves

Setting of Quality Targets

Appraisal of Conformance

Taking corrective action where any deviation is noticed

Planning for improvements in quality

Objectives of Quality Control

To provide products which are dependable, satisfactory and economical

To ensure economical production of products of uniform quality which is acceptable to the customer

1. Minimum scrap, rework and other losses

2. Reduced cost of material and labor

3. Uniform of quality and reliability of products that increase sales turnover

4. Reduced variability and thereby higher quality of products

5. Reduced inspection cost

Benefits of Quality Control


in the long run. First, the improvement in the quality of products and

services. Second, the system is continually evaluated and

modified to meet the changing needs of the customer. Third, a quality control system improves productivity,

which is one of the goals of all organizations. Fourth, such a system reduces costs in the long run.

Fifth, with improved productivity, the lead time on the production of parts and subassemblies is reduced, which may result in an improvement in meeting customer due dates

Keeping the customers satisfied is a fundamental goal.

A company that adopts this philosophy and uses a quality control system to help in meeting this objective is one that will be competitive for a long time.

6. Reduced customer complaints

7. Higher operating efficiency

8. Higher quality consciousness among employees

9. Better utilization of all resources

10. Higher productivity and improved profits


Inspection

Comparing actual quality characteristics of a pdt. With predetermined standards

Purpose to separate good products from bad ones

Rework Dispose

Quality Control

Attempts to find the root cause of defects and to take corrective action to avoid the defects in future production

Reliability

Probability of performing without failure, a

specified function under given conditions for a

specified period of time

Organizing for Quality ControlQuality control dept. – role of watch dog over

production

Quality at Source Aims to put the production worker in the

driver’ seat in controlling quality

Principles:1. Assigns responsibility for product quality to the

production workers

2. Enlarges job of production workers

3. Allows QC personnel do work other than inspection and testing

4. Removes obstacle to cooperation between QC personnel and production workers

5. Gives right to each worker to stop the production line to avoid producing defective parts

6. Uses Statistical control Techniques like SPC to monitor quality- acceptance sampling

7. Organizes workers and managerial personnel into “Quality Circles”

Quality Control StrategyTop management to

1. Establish an organization for quality control

2. Identify the customer’s quality needs and their perception regarding quality

3. Assess the ability of the organization to meet their needs

4. Ensure that the material and services reliably meet the required standards of quality

5. Concentrate on “prevention of defects” rather than “detection of defects”

6. Educate and train all relevant employees for quality improvement

7. Review the QMS for improvement

Company Wide Quality Control (CWQC)Definition:“a system of activities to assure that quality products and services required by customers are economically designed, produced and supplied, while respecting the principle of customer-orientation and the overall public well being”

QC activities: MR, R&D, design, purchasing, production, inspection and sales

Mass Inspection

Inspecting Salvaging Sorting Grading Rectifying Rejecting

Quality Control

Quality control is the process of examining your output

for minimum levels of quality in some dimension. The

quality control team does not actually improve the quality,

they just stop production when the measured quality

drops below a given limit

Quality Manuals Product testing using SQC Basic Quality planning

Quality Assurance

Emphasis on prevention Proactive approach using SPC Advance quality planning

Total Quality Control

All aspects of quality of inputs Testing equipments Control on process

Company wide Quality ControlMeasured in all functions connected with

production such as R&D Design Engineering Purchasing Operations etc

Total Quality Management

Measured in all aspects of business Top management commitment Continuous improvement Involvement & participation of employees

Operations: A Transformation ProcessExhibit

1-1

Inputs

People

Capital

Energy

Materials

Technology

Market andEnvironmental

Forces

TransformationProcesses

Outputs

Goods

Services

1-1

• Heavy use of inspection• Manufacturing & QC are adversaries• Firefighting• Management by crisis• High costs• Lost sales• Loss of competitive position

DetectionMode

• Very little inspection• QC is a resource of Manufacturing - teamwork• Problem elimination• Smooth operations - continual improvement• Decreasing costs• Increased sales• More competitive

PreventionMode

Quality Control Modes

5-7

What does quality control involve? 1. Planning for quality control

i.e. identifying where control is to be exercised And to what extent or frequency

2. Identifying the relevant reference standards E.g. the technical (technological) specifications

applicable

3. Influencing performance by collecting specific information Usually highly technical, standardized data

4. Analyzing and comparing results to standards

5. Reporting results to all parties concerned

As a basis for management decision On appropriate corrective action On-going retro-fixing to maintain end results

Appropriate action as a result of quality control may include

Acceptance Perhaps because the condition is marginal Or of little consequence in the specific location of the

project But still, the customer doesn't get exactly what they want

Rejection Requiring repeat production Or rework to the particular product

Process adjustment I.e. process improvement To avoid the same condition recurring Re-testing If there is doubt about the test results Often occurs where only small samples can be

tested But representing large volumes of product Or because the samples are subject to human error

Quality Control (QC)

Control – the activity of ensuring conformance to requirements and taking corrective action when necessary to correct problems

Importance Daily management of processes Prerequisite to longer-term improvements

Designing the QC System

Quality Policy and Quality Manual Contract management, design control and purchasing Process control, inspection and testing Corrective action and continual improvement Controlling inspection, measuring and test equipment

(metrology, measurement system analysis and calibration) Records, documentation and audits

Example of QC

1. Hazard analysis2. Critical control points3. Preventive measures with critical limits for

each control point4. Procedures to monitor the critical control

points5. Corrective actions when critical limits are

not met6. Verification procedures7. Effective record keeping and

documentation

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Inspection/Testing Points

Receiving inspection In-process inspection Final inspection

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Receiving Inspection

Spot check procedures 100 percent inspection Acceptance sampling

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Acceptance Sampling

Lot received for inspection

Sample selected and analyzed

Results compared with acceptance criteria

Accept the lot

Send to production or to customer

Reject the lot

Decide on disposition

Pros and Cons of Acceptance Sampling Arguments for:

Provides an assessment of risk

Inexpensive and suited for destructive testing

Requires less time than other approaches

Requires less handling Reduces inspector fatigue

Arguments against: Does not make sense for

stable processes Only detects poor quality;

does not help to prevent it Is non-value-added Does not help suppliers

improve

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In-Process Inspection

What to inspect? Key quality characteristics that are related to cost

or quality (customer requirements) Where to inspect?

Key processes, especially high-cost and value-added

How much to inspect? All, nothing, or a sample

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Economic Model

C1 = cost of inspection and removal of nonconforming itemC2 = cost of repair p = true fraction nonconforming

Breakeven Analysis: p*C2 = C1

If p > C1 / C2 , use 100% inspection

If p < C1 / C2 , do nothing

Human Factors in Inspection

complexitydefect raterepeated inspectionsinspection rate

Inspection should never be a means of assuring quality. The purpose of inspection should be to gather information to understand and improve the processes that produce products and services.

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Gauges and Measuring Instruments Variable gauges Fixed gauges Coordinate measuring machine Vision systems

Examples of Gauges

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Statistical Process Control (SPC)

A methodology for monitoring a process to identify special causes of variation and signal the need to take corrective action when appropriate

SPC relies on

control charts

Quality Assurance

The process whereby quality is at the forefront of every stage of the development, design, marketing, manufacturing and selling process.

‘Quality’ is influenced by the internal philosophy of the business and the external influences -

Quality Assurance

QUALITY MANAGEMENT

is a systematic set of operating procedures which is company wide, documented, implemented and maintained while ensuring the growth of business in a consistent manner

Common Causes

Special Causes

Histograms do not take into account changes over time.

Control charts can tell us when a process changes

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Control Chart Applications Establish state of statistical control

Monitor a process and signal when it goes out of control

Determine process capability

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Commonly Used Control Charts

Variables data x-bar and R-charts x-bar and s-charts Charts for individuals (x-charts)

Attribute data For “defectives” (p-chart, np-chart) For “defects” (c-chart, u-chart)

Developing Control Charts

1. Prepare Choose measurement Determine how to collect data, sample size,

and frequency of sampling Set up an initial control chart

2. Collect Data Record data Calculate appropriate statistics Plot statistics on chart

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Typical Out-of-Control Patterns Point outside control limits Sudden shift in process average Cycles Trends Hugging the center line Hugging the control limits Instability

Shift in Process Average

Identifying Potential Shifts

Cycles

Final Steps

5. Use as a problem-solving tool Continue to collect and plot data Take corrective action when

necessary

6. Compute process capability

Process Capability Capability Indices

mmmmmm

C

LTLUTLC

p

p

0868.0 25.75.10 ision specificatPart :Example

minimum) often the more (1.5 capable as defined is 1 if6

96.00868.06

50.1000.11

pC

Target theis T

12

2 T

CC ppm

10.7171mmat centered is process assumebut above, as same :Example

2 where1

,min3

3

Tolerance

TKKCC

CCC

LTLC

UTLC

ppk

puplpk

pl

pu

Process Capability (2)

086.10868.03

7171.100.11

puC

834.00868.03

5.107171.10

plC

8977.0

868.075.107171.10

1

960.0

2

2

pmC

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Capability Versus Control

Control

Capability

Capable

Not Capable

In Control Out of Control

IDEAL

Process Capability Calculations

Excel Template

Special Variables Control Charts x-bar and s charts x-chart for individuals

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Control Chart Selection

Quality Characteristicvariable attribute

n>1?

n>=10 or computer?

x and MRno

yes

x and s

x and Rno

yes

defective defect

constant sample size?

p-chart withvariable samplesize

no

p ornp

yes constantsampling unit?

c u

yes no

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Control Chart Design Issues Basis for sampling Sample size Frequency of sampling Location of control limits

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Pre-Control

nominal value

Green Zone

Yellow Zones

RedZone

RedZone

LTL UTL

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