unit3 inspection

8/3/2019 Unit3 Inspection

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INSPECTION & QUALITY CONTROL

1.0 Definition:Inspection is the process of measuring the quality of a product or service in terms

of established standards.

1.1 Quality:It is fitness for purpose at lowest costIt is the degree of perfection

It is the degree to which it fulfills the requirements of the customer.1.2 Quality control:

It is the control of quality during manufacturing and design.

It is the technique by means of which products of uniform acceptable quality are

manufactured by effectively controlling the application of materials, men,machines, process and working conditions.

QC is concerned with making things right rather than discovering and rejecting

those made wrong.

QC is everybodys business- not only of the inspectors.1.3 QC v/s Inspection:1. Inspection sorts out good and defective pieces2. QC looks into the causes for variations and seeks solutions and remedial action

3. Effective QC reduces the amount of inspection

4. QC verifies the effectiveness of the process and process capability

5. Qc is concerned not only with the quality of the product but also with themethod of manufacture

6. Both inspection and QC try to assure quality1.4 Objectives of inspection:

1. Ensuring the adequate quality of a product

2. Timely detection of defects in raw materials, in-process items and finalproducts3. Preventing further work on defective components

4. Ensures the safe working of products

5. Improving the reputation of the firm6. Helps in reducing the customer complaints

1.5 Kinds of inspection:1. Roving, process, patrolling or floor inspection2. Fixed

3. Key point

4. Final5. Incoming materials

6. In-process

7. Finished goods8. Tools inspection

9. First piece inspection

10. Random inspection

11. Pilot piece inspection


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1.6 Statistical Quality ControlIt is the quality control system using the statistical techniques for controlling the

quality and solving quality problems.The fundamental basis of SQC is the theory of probability and the distribution

pattern.

Evaluates the batch qualityControls the quality of processes and products

The variations in the product parameters may be due to

1. Assignable causes. - Mistakes are assignablePosses greater magnitude

Easily traceable

2. Random or chance causes - Errors are random

No control over themCharacteristic to the process / machine

Effects each component in separate way

They may cancel effects of each other

They will be close to the middle of the extreme values1.7 Probability1. Additional law: for mutually exclusive (only one event can occur at a time)

p = p1 + p2 p1= probability of occurrence of event 1

p2= probability of occurrence of event 2p = probability of occurrence of event 1 or 2

2. Multiplication law: for mutually independent ( not exclusive)

p = p1.p2 p= probability of occurrence of event 1And 2

3. Binomial theorem:

Where p = probability of a component defective

q = probability of a component Non-defective

p + q = 1

The probability of achieving r successes is given by the term p (r) of theabove binomial expansion

1.7.1 Distribution pattern:1. Normal distribution curve

Symmetrical -

Skewed - pulled to one sideLepto kurtic - Peak higher than normal curve

Platy kurtic - Peak lower than normal curve

2. Bimodal - Has 2 peaks ( may be made on 2 m/cs.)

3. Triangular - May be due to defective m/c.


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1.8 Techniques used by SQC:1. Sampling inspection

2. Analysis of the data

3. Control charts1.8.1 100% inspection v/s Sampling inspection:100% Inspection Sampling inspection

Very tiring and expensive Represents the lot in quality

Prone to errors Saving in labor & cost

Can not assure 100% accuracy controls the average outgoing quality

Only way for destructive tests1.8.2 Sampling inspection:

Def: It is a technique to determine whether the lot should be accepted orrejected on the basis of the number of defective parts found inrandomly selected samples from the lot.

Sample: A sample may be defined as the number of items, parts orcomponent drawn from a lot, batch or population for the purposeof inspection to estimate the quality of the lo from which it isdrawn.

Sampling: It is an act of drawing the samples from the batch on random basiswhich represents the lot truly.

Sample size: The number of samples drawn from the lot for accuratelyestimating the quality of the lot and depends on the size of the lot

and the degree of the quality required.Advantages: 1. Less amount of inspection

2. Saving in time and cost

3. Reduced fatigue and boredom of inspectors4. Accuracy can be expected5. Less handling of the lot

6. Only possible way for destructive tests

7. Risk of rejection ensures care and attentionTypes:

Sampling by attributes: Defective non-defective

No quantitative measurement

Compares Go No-Go gauging

Involves % of defects

Sampling by variables: Measuring quality characteristics

Involves average of measurements

Generates more information and trendLot by lot:

Divided into No. of lots

Suitable for batch production


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Continuous: During mass and continuous production samples are drawn at

some regular intervals

1.9 Sampling by attributes:Steps involved in inspection by Attributes:

1. Select a sampling planNo. of samples to be selected (n)

No. of defective parts that can be tolerated to accept the lot (c or a)

(Acceptance number)

Acceptable quality level (AQL):It indicates a small proportion of bad components in a lot such that thelots having less than the proportion of bad components have a high

probability of getting accepted.Lot Tolerance Perfect Defective (LTPD): (Limiting Quality)It indicates a small proportion of bad components some what larger than

AQL in a lot such that the lots having more than this proportion ofdefective components have a small probability of getting accepted.Manufacturers Risk:It is the small probability of a batch being good or even better than AQL

but yielding a bad sample and thus getting rejected.Purchasers Risk:It is the probability of a lot being bad or even worse than LTPD but

yielding a good sample and thus getting accepted.

2. Determine the Operating Characteristic Curve for the chosen plan

Draw the graph of Probability of acceptance (Pa) of he lot v/s actual

Percent Defectives (p) present in the lot.Probability of acceptance can be calculated for various distribution modelslike Binomial, Poissons and Normal etc.

P(a) = d=0 n! . pd (1-p)n-d / d!. (n-d)!- for binomial distribution

P(a) = d=0 d . e- / d! where = n.p- for Poissons

P(a) = d=0 e-1/2(d-/)2 / .2where =n.p; = (n.p(1-p))1/2for NormalDistribution

The OC curve for a particular combination of n and c shows how wellthe sampling plan discriminates the good and bad lots.


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OC curve1.10 Sampling by variables:

The inspection is carried out by measuring the quality characteristics of a product.

Ex.: Dimensions, weight etc.Variables involve the averages of measurement.

Attributes deal with the % of parts rejected

Inspection by variables is more detailed and contains more information,Involves higher inspection and cost per unit.

Attributes A discrete distribution such as binomial or Poissons is used.

Variables Continuous distribution such as Normal is used.

1.10.1 Sampling Plans:

1. Single 2. Double 3. Multiple 4. Sequential

Single sampling Plan:

A lot is accepted or rejected on the basis of a sample drawn from the lot.Method: Draw a single sample of size n parts depending on the size of the

lot and the AQL. The size can be found from the tables available.Inspect the sample and find the No. of defective parts

If the defective parts exceed the acceptance No. C the lot is

rejected.In case of rejection inspect each and every piece of the lot and

replace the defectives or salvage and correct the defects.Characteristics:

1. Easy to design.

2. Lower cost of inspection

3. Accurately estimates the lot quality.

Double Sampling Plan;If it is not possible to decide the fate of the lot on the basis of a single sample, a

second sample is drawn out of the same lot and decision is taken on the basis ofthe combined results of both the samples.

Steps:Let C1 and C2 are the acceptance numbers. (C2>C1)

Inspect a sample of size N1

Count the defectives K1

If K1 >C2 If C1


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Count the defective parts (K1+K2)

If (K1+K2) >C2 If (K1+K2) C2

Reject the lot Accept the lotCharacteristics:1. More expensive 5. Involves more records

2. Involves less inspection than single sampling 6. Not as accurate as of single3. A second chance is given to the lot sampling plan

4. Involves more overheads

Multiple sampling Plan:A lot is accepted or rejected based upon the results obtained from a several

samples drawn from the lot. It involves smaller first sample as compared to single

& double sampling plans.

Sequential sampling Plan:It involves increasing the sample size by one part at a time till the size becomes

sufficiently large and contain sufficient No. of defects to take a decision to acceptor reject.

1.11 Control Charts:

Control chart is a graphical presentation of the measured or otherwise judged

quality characteristics of the items or the samples, being manufactured on day today basis.

The chart show the trend of variations while production is going on, thus helpingto take corrective action at appropriate time.

Purpose and advantages:1. Indicates whether the process is under control or out of control.

2. Determines the process variability and detects unusual variations.

3. Gives warning signal if the process is affected by attributable causes.4. Ensures the process capability and Quality level.

5. Increases the reputation of the firm.

1.1.1 Types:Control charts

Variables control charts Attribute control charts_

X- Chart p- Chart

R- Chart np-chart- Chart c-Chart

U-Chart


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_1.11.2 X & R Charts:

These two chats are used together. The charts are drawn to show the mean

_

X and the range R against the sample numbers on respective graphs.

Sample size : n

No. of samples: S

_Mean of the sample : X

=Mean of the means: X

Range of the values : R = Max. Min. in the sample _Mean of Ranges : R

_

For X chart:

= _UCL: Upper control limit = X +A2.R

= _LCL: Upper control limit = X A2.R

For R- chart:

_UCL: Upper control limit = D4.R

_LCL: Upper control limit = D3.R

A2, D3, D4 are constants based on normal distribution and are available in tables depending

on the sample size.

Table:

Sample size(No. of items in

each sample)

A2

( Limit average )D3

(Range lower limit)D4

( Range upper limit)

2

3

4

56

8

1.88

1.02

0.73

0.580.48

0.37

0

0

0

00

0.14

3.27

2.57

2.28

2.112.00

1.86


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10

12

0.31

0.27

0.22

0.28

1.78

1.72

1.11.3 p- Chart:

This chart is drawn among the fraction defectives or % defectives and the sampleNos. or the dates of production.

Fraction defective p = No. of defectives found

No. of items inspected

Percent defectives %p = 100 x p

_Mean = p = p / No. of samples

_ _ _

UCL = p + 3 [ p-(1-p) / n]

_ _ _LCL = p - 3 [ p-(1-p) / n]

1.1.1 c- chart:

The No. defects in an item, which is big in size, are plotted against No. of items

inspected.

1. The sample size is constant.

2. It controls the No. of defects observed per unit.

3. c- chart is preferred for large and complex parts.

No. of defects found in a part = cMean c = c/No. of parts inspected

_ _UCL = c + 3 (c)

_ _UCL = c - 3 (c)


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p-Chart c- chart2. Quality circles:

Quality circles (QC) are small groups of employees (8 to 10 in a group), whomeet voluntarily for about an hour a week to share the ideas in an attempt to

solve job related quality and productivity problems.

Salient features:

1. Group includes foreman as well as workers who perform similar jobs.

2. Members of QC are taught how to collect data and do the statistical

Analysis to analyze the causes of quality problem.

3. The recommendations of QC are recognized and acted upon by themanagement.

4. QC reflects the philosophy that the responsibility for quality rests with

all the concerned rather than a few inspectors or supervisors.

5. QC are very successful in Japan. More than 80% of workers are

involved in QC.

2.1 Advantages:

1. Improved quality and productivity

2. Reduction defects ( About 2/3 )

3. Job satisfaction and high morale of workmen

4. Reduction in labour turnover

5. Reduction in absenteeism.


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3.0 ISO 900 Quality System:

International Organization for Standardization (ISO) was found after world war- 2The industrialized nations are members of ISO.

ISO does not issue independent standards but recommends some of the standards

prepared by the member countries.

3.1 Need and Evolution of ISO 9000 Quality system:1. Shift from Sellers market to Buyers market

2. Economic liberalization and globalization3. Building customer confidence in suppliers capability to produce Quality

products and services.

4. Exports to Europe - ISO 9000 registration is mandatory.

5. Standard quality system.3.2 Structure:

ISO 9000 Quality management and quality assurance Standards:

Part-1 : ISO 9000-1 Guide lines for selection and use.

Part-2 : ISO 9000-2 Generic guide lines for the application of ISO 9001,

ISO 9002 and ISO 9003.

Part-3 : ISO 9000-2 Guide lines for the application of ISO 9001 to the

Development, supply and maintenance of soft ware.

ISO 9000-x : Quality plan

ISO 9000-y : Configuration control.

ISO 9001-1994: Quality system Model for quality assurance in design,

production, installation and servicing.

ISO 9002-1994: Quality system Model for quality assurance in production,

Installation and servicing.

ISO 9003-1994 : Quality system Model for quality assurance in final inspection

and test.

3.3 Requirements of ISO 9001 Quality system:Salient features:1. Standards on system and not on product. 6.Documented & demonstrable

2. Complementing to product specification. 6. Well defined authority and


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3. 20 elements effecting quality responsibility

4. Establish own procedure and implement 7. Quality manual & procedures

5. Management responsibility 8. Need for training

3.3.1 Elements of ISO 9001 Quality system :

1. Management responsibility 11. Control of inspection, measuring

2. Quality system and test equipment

3. Contract review 12. Inspection & test status4. Design control 13. Control of non- confirming

5. Document & data control product

6. Purchasing 14. Corrective & preventive action

7. Control of customer supplied product 15. Handling, storage, packaging,8. Product identification & traceability preservation & delivery

9. Process control 16. Control of quality records

10. Inspection & testing 17. Internal quality audits

18. Training19. Servicing

20. Statistical techniques

3.3.2 Documentation:

1. Level 1: Quality manual2. Level 2: Executive procedures

3. Level 3: Work instructions

4. Level 4: Records, log sheets and formats

3.4 Advantages:

1. Reduction of quality costs

2. Improvement in work culture & quality of products

3. Involvement & commitment of management & employees4. Good communications through audits & reviews

5. Increased market share & customer confidence

6. Dealing with one certification agency

7. Benefits to the organization & society

3.5 Conclusions:

Approach : Management led

Philosophy: Prevention not detection

Scope : Company wide

Scale : Everyone

Standard : Right first time & every time

Measure : Customer satisfaction

Theme : Continuous improvement


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Vision : Above customer expectation

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