Download CHAPTER: QUALITY CONTROL

Quality Control
The word ‘quality control’ comprises of two words i.e.
‘Quality’ & ‘Control’
Quality is the reflection of your commitment to your customer. It is
the sum of various characteristics of the product such as:
• Shape
• Dimension
• Composition
• Strength and durability
• Finish and colour etc.
MEANING OF QUALITY
CONTROL
Control
It is the process of ensuring that everything happens in
conformity of:
• Plan adopted
• Instructions issued and
• Principles established
The aim is to detect any error or deviation from the set standards,
take corrective actions and prevent recurrence.
It applies on everything• Men
• Things
• Actions
“Quality control means the recognition and removal of identifiable
causes and defects and variables from the set standards.”
- J.A. Shubin
“Quality control includes techniques and systems for the
achievement of the required quality in the articles produced and for
the elimination of the sub-standard goods”
- Tome, Simen, and H.C.Gill
DEFINITIONS
OBJECTS OF QUALITY CONTROL
Establishing Quality Standards
Smooth Production Process
Detecting Deviations in Production Process
Ascertaining Reasons for Low Quality
Segregation of Defective Products
 Clear-cut standards should be fixed.
 Responsibility for quality of product manufactured be fixed.
 Routine checks on uniform application of measures.
 Control should be applied at every stage.
PRINCIPLES OF QUALITY
 Unanimity among workers and management
CONTROL
SYSTEM
 Maintaining proper records of quality.
HOW TO EXERCISE QUALITY
CONTROL
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Fixing Standards of Quality
Control over Raw Material
Control over Production Operations
Inspection of Machinery and Equipment
Promptness in Inspecting Quality
Quality Consciousness
Application of Statistical Methods of Quality Control
FACTORS AFFECTING QUALITY
CONTROL
 Cost of Production
 Requirements of Customers
 Equipment and Tools Used
 Utility of Product
 Attitude of Workers
 Scale of Operations
MEANING
Quality control is a system of inspection ,analysis , and action applied to a
manufacturing process so that, by inspecting a small portion of the product
currently produced, an analysis of its quality can be made to determine what
action is required on the operation in order to achieve and maintain the
desired level of quality.
- JOSEPH MANUEB
According to Manueb , quality control is the system whereby it is determined by
inspection whether the product conform to the quality standards or not.If the
desired quality standard is not achieved then corrective action is also taken in
this system.
FUNCTIONS OF QUALITY CONTROL
DETERMINING STANDARDS: Every firm has its own requirements for quality
standards. Neither high quality nor low quality is good for a firm’s
product. Standards can be set up so that the requirements of the product
can be discussed properly through quality control.
KEEPING RECORD: All the records of various quality standards and
inspection reports should be kept so that proper controls can be
maintained and improved when required.
DETERMINING SAMPLING LIMITS: Because its at times difficult for the firm
to maintain the usual standard, thus, certain quality limits are fixed and
goods complying to those limits are accepted and others are rejected.
DEVELOPMENT OF BETTER QUALITY DEVICES: Through quality control,
management gets to develop better quality control systems so that there
is product efficiency and product economy.
ADVANTAGES OF QUALITY CONTROL
PRODUCERS
QUALITY
CONTROL
HELPS
CONSUMERS
• (i) Reductions in costs: Since only a fraction of output is inspected ,costs of inspection
are greatly reduced. Also, it minimizes the wastage leading to better utilization of
resources.
(ii) Greater efficiency: Not only there is reduction in costs but the efficiency also goes
up because much of the boredom is avoided, the work of inspection being
considerably reduced.
(iii) Easy to apply: An excellent feature of quality control is that it is easy to apply.
Once the system is established, it can be operated by persons who have not had
extensive specialized training or a highly mathematical background. It may appear
difficult only because the statistical principle on which it is based is unrecognized or
unknown, however, as these principles are actually based on common sense, the
quality control method finds wide application.
(iv) Early detection of faults: Quality control ensures an early detection of faults and
hence a minimum waste of reject production. The moment a sample point falls
outside the control limits, it is taken to be a danger signal and necessary corrective
action is taken. On the other hand with 100 per cent inspection unwanted variations
in quality may be detected at a stage when a large amount of faulty products have
already been produced. Thus there would be a big wastage. Control chart, on the
other hand provides a graphic picture of how the production is proceeding and to tell
management where to look for trouble.
(v)
Adherence to specifications: Quality control enables a process to be brought into
and held in a state of statistical control, a state in which variability is the result of
chance causes alone so long as a statistical control continues. Specifications can be
accurately predicted for the future, which even 100 per cent inspection cannot
guarantee. Consequently, it is possible to assess whether the production processes
are capable of turning out precuts which will comply with the given set of
specification.
(vi) In certain cases 100% inspection cannot be carried out without destroying all the
products inspected; for example, while testing breaking strength of chalks, proofing
of ammunition, etc. in such cases if 100% inspection methods are followed then all
the items inspected will be destroyed. Hence sampling must be resorted to .
(vii) To determine the effect of change in process: With the help of control charts one
can easily detect whether or not a change in the production process results in a
significant change in quality.
(viii) Statistical quality control ensures overall coordination. Statistical quality
control provides a basis upon which the difference arising among the various
interests in an organization can be resolved. In some instances. For example,
production engineers may set specifications that are so tight that the operating staff
cannot meet them economically and consequently there is an unnecessary high
scrapping rate. In other instances the specifications may be too loose and product
quality will be sacrificed unnecessarily. In either type of case the control recurs
provide a valuable lad in solving the problem of getting the standing information on
plant capabilities and customer requirements must also be considered in relation to
the quality and the sale value of the product.
QC has a special role to play in a country like India because of the wide variations
encountered in raw materials and in machines. The importance of applying SQC has
become greater in our industries in the context of the need for earning foreign
exchange by supplying quality goods to successfully compete in the world markets.
In fact, one of the most serious problems faced by Indian business community is to
supply quality products and services at competitive rates.
It is heartening to note that the Government of India is conscious of the quality
standard for different products .To the satisfaction of the customers Indian
standards institute (ISI) Bahadur Shah Zafar Marg, New Delhi, set up by the
Government of India is playing a very important role in maintaining quality
standards in the country. After rigorous testing of the quality of the products, the
institute grants ISI label to those products which come up to certain prescribed
standards and a continuous watch is kept to see that these standards are adhered to
and a continuous watch is kept to see that there is another label for standard quality
products. Customers are becoming conscious and often insist on buying products
bearing ISI or Agmark label.
Some other advantages are:
 Consumer satisfaction
 Better industrial relations(cordial relations among employees)
 Better methods of production
 Increased sales
 job satisfaction
 Goodwill
 Facilitates price fixation
 Effective advertising
LIMITATIONS:
Despite the great significance of quality control , ‘it should be remembered
that it is not a panacea for all quality evils.’ They should be matched to the
process being studied. The applications of standard procedures without
adequate study of the process is extremely dangerous, and applied on a
production process are only an information service, and as such used as part of
a general quality awareness, they may only lead to a files with the manager in
charge of the process and not with the statistician. The charts do not reduce the
manger’s responsibility.
METHODS OF QUALITY CONTROL
INSPECTION
STATISTICAL
QUALITY
CONTROL
INSPECTION
Inspection is the most common method of attaining standardization,
uniformity and quality of workmanship. It is the cost art of controlling
the product quality after comparison with the established standards and
specifications. It is the function of quality control.

If the said item does not fall within the zone of acceptability it will
be rejected and corrective measure will be applied to see that the items
in future conform to specified standards.
 Inspection is an indispensable tool of modern manufacturing process. It
helps to :
= control quality,
= reduces manufacturing costs,
= eliminate scrap losses and assignable causes of defective work.
OBJECTIVES OF INSPECTION
= To collect information regarding the performance of the product with
established standards for the use of engineering production, purchasing and
quality control etc.
= To sort out poor quality of manufactured product and thus to maintain
standards.
= Detect source of weakness and failure in the finished products and thus check
the work of designer.
= To establish and increase the reputation by protecting customers from receiving
poor quality products by having quality control techniques.
PURPOSE OF INSPECTION
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To distinguish good lots from bad lots
To distinguish good pieces from bad pieces.
To determine if the process is changing.
To determine if the process is approaching the specification limits.
To rate quality of product.
To rate accuracy of inspectors.
To measure the precision of the measuring instrument.
To secure products – design information.
To measure process capability.
METHODS OF INSPECTION
The inspection may be undertaken in any of the
following ways :
• Remedial and Preventive Inspection
• Centralised and Floor Inspection
Remedial and Preventive Inspection:
Inspection allows the separation of good quality
products from those of low quality. The goods
are compared with the standards set for them.
 If the goods and services are according to the
predecided standards then these are accepted
and if they do not confirm to the standards or
do not fall within the prescribed limits then
they are rejected.
 This method of inspection is known as remedial
inspection.
• Preventive inspection, on the other hand is concerned
mainly to avoid rejections in the future.
• The causes of low quality are studied properly and then steps
are taken so that this is avoided in future.
• Preventive inspection is very useful because the causes of
rejection are found and corrective measures are taken.
CENTRALISED AND FLOOR INSPECTION
• Under centralised inspection system all the inspection work is
centralised. All goods and materials are taken to Inspection department
and then returned to the departments if they conform to the standards.
• Thus, when materials, parts, products, components etc. are sent to the
inspection department for inspection, it is called central inspection. It is
so because inspection is carried out at one central place only.
• Central inspection saves time of inspectors, because they don’t need to
move out in various departments for their work, but they receive in the
inspection department, what they have to inspect. They don’t have to wait
for the jobs to inspect. Moreover, the inspectors can work freely without
any pressure from the people, whose work they inspect. It makes possible
the use of special inspection and testing machines and equipment.
• The records of the rejected items and their scrap are also kept at the
central place. The employing of less inspectors, will be required when the
work is undertaken at a centralised place.
• Floor inspection : Inspection at the job place is also used to
resolve the problems
It is known as floor or patrolling or roving or first piece inspection.
• Here the inspectors move from one place to another place, or from one
department to another department, or from one machine to another
machine or from one process to another process etc. to inspect
materials, parts, components, tools, equipment, products during the
various stages of production and the finished goods.
• They get the machines set right before production starts and also they
catch defective work before a large quantity has been produced.
• Corrective action is immediately taken to set the machines
right.
• This method ensures timely and prompt inspection and
avoid delays.
• Floor inspection keeps the quantity of scrap at the minimum
because the inspector can locate the fault and suggest
rectification.
• It also create direct link between the inspectors and the
work and this helps in proper communication between the
two.
STATISTICAL QUALITY CONTROL (SQC)
• Statistical quality control has assumed tremendous
importance in industrial technology in recent past. Highly
industrialised countries are making maximum use of this
device.
• Walter S. Schewhart and Harold F. Dodge of Bell laboratories
U.S.A introduced SQC shortly after world war I.
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This method is based upon the law of probability and may be described as the
system for controlling the quality of production within specified limits.
The application f these techniques of manufacturing gained momentum
during world war II.
The armed services in U.S.A. successfully applied these techniques in
procurement divisions for establishing quality specification.
This technique makes use of statistical methods and principles which aim to
assess not only the magnitude of “Chance Cause Variation” but also detect
assignable cause variations.
 Since assignable cause variations may be detected, it becomes
easy for an individual undertaking to install statistical control
at various stages in the production or manufacture of a
product.
 Before the introduction of SQC only inspection method was
used to determine the quality of products.
 Because inspection is possible only when production has
already taken place only the quality may be determined.
 When the goods are already produced then poor quality
products will also mean loss.
• In statistical quality control, the quality is checked at every level of
production and necessary corrective measures may be taken at the
earliest.
TOOLS OF STATISTICAL QUALITY CONTROL
•
a)
b)
These are two important tools of statistical quality control:
Quality Control Charts.
Acceptance sampling.
QUALITY CONTROL CHARTS
• This is one of the important methods of SQC . A control chart is the
graphic representation of expected variations in quality. There are two
lines on each graph which indicate the tolerance limits within which the
variations of quality will be permitted. In all production processes, we
need to monitor the extent to which our products meet specifications. In
the most general terms, there are two "enemies" of product quality:
• deviations from target specifications
• excessive variability around target specifications
• During the earlier stages of developing the production process, designed
experiments are often used to optimize these two quality characteristics;
the methods provided in Quality Control are on-line or in-process quality
control procedures to monitor an on-going production process.
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Out line known as average line is drawn on chart. Two lines, one called upper
control limit (UCL) and the other called low control limit, (LCL) are drawn above
and below the average line respectively. A sample is selected from every lot. The
actual measure merits of the sample are plotted on the chart. If analysis of the
control chart indicates that the process is currently under control (i.e., is stable,
with variation only coming from sources common to the process), or the dots
remain within the two lines i.e., UCL and LCL then no corrections or changes to
process control parameters are needed or desired .
In addition, data from the process can be used to predict the future performance
of the process.
If the chart indicates that the monitored process is not in control (i.e., the dots fall
outside the tolerance lines) analysis of the chart can help determine the sources of
variation, as this will result in degraded process performance.[1]
• If the sample is rejected then assignable cause is determined and
immediate action is taken to improve the quality.
• The causes of production process going out of control may be due to poor
materials, negligence of machine operator or defects in machine.
• A process that is stable but operating outside of desired limits (e.g., scrap
rates may be in statistical control but above desired limits) needs to be
improved through a deliberate effort to understand the causes of current
performance and fundamentally improve the process.[2]
•
•
This will enables the manufacturer to
take corrective measures.
Control charts are excellent means of
determining quality limits which allows
immediate rectifying measures.
OUTLINE OF A CONTROL CHART
Quality scale
Out of control
UCL
Average
LCL
Out of control
1
2
3
4
5
6
Sample number
7
8
9
10
• As per the information plotted in control chart, one dot is going beyond
the upper control limit and one dot is going beyond the lower control limit
which shows that the quality of the sample is not under control.
• If all the dots are between UCL and LCL then the sample’s quality will be
under control and variations from average line are only due to chance.
ACCEPTANCE SAMPLING
• Acceptance sampling uses statistical sampling to determine whether to
accept or reject a production lot of material. It has been a common
quality control technique used in industry and particularly the military
for contracts and procurement. It is usually done as products leave the
factory, or in some cases even within the factory. Most often a
producer supplies a consumer a number of items and decision to
accept or reject the lot is made by determining the number of
defective items in a sample from the lot. The lot is accepted if the
number of defects falls below where the acceptance number or
otherwise the lot is rejected.[1]
• This is one of the important techniques of statistical quality control. This
method is used to inspect quality just after the product is manufactured or
are in the final stage of production.
• A sample is taken from the manufactured products and quality of these
products is inspected.
• If the quality of sample conforms to the pre-determined standards then it
is presumed that the quality of the whole lot is good.
• This method is used on the presumption that the sample includes
products of varying qualities from the lot and it represents the lot
properly.
• This method is also referred as ‘Sampling Inspection Plan’.
• There is a probability that the sample may not have defective products
but the lot may have or vice-versa .
• The chances are that the sample is accepted but the quality of the lot is
good.
• To avoid this thing we can inspect more than one sample and then take a
decision about the acceptance or rejection of a lot.
There are two limiting levels of quality in an Acceptance
Sampling Plan.
1) Acceptable Quality Level (AQL)
i.e. the lowest percentage of
defectives that the seller
expects to supply and the
buyer expects to receive,
•
AQL involves producer’s risk
or the risk that a lot with an
acceptable quality will be
rejected on the basis of a
sample
2)
Lot percentage Tolerance
Defective (LPTD) i.e. a limit at
which the buyer, or
consumer, wants to be quite
certain that the lot will not be
passed.
a) On the other hand, LPTD
involves consumers’ risk or
the risk that a buyer takes in
accepting a lot of the quality
which does not conform to
the accepted standards.
Role of Acceptance Sampling. Acceptance sampling is widely used due to the
following reasons :
1)
2)
3)
Acceptance sampling is much less expensive than 100 per cent
inspection.
Acceptance sampling also gives good results as compared to 100 per
cent inspection. It is generally agreed that 100% inspection will remove
only between 85% to 95% of defective material while a very good 100%
inspection will remove 99% of defective items. Moreover, 100%
inspection involves fatigue also. Acceptance sampling will be much
better in controlling quality.
Sometimes quality of a product is tested by destroying an item, as in
determining the strength of glass containers. In such a case 100%
inspection is not possible and acceptance sampling will have to be used.
4) In modern manufacturing concerns acceptance sampling is used for
evaluating incoming raw materials, parts at various stages of
manufacturing and final inspection of finished goods.
Advantages of statistical quality control
Some of the important advantages of SQC are discussed as follows:
1)
This method is based on inspecting a sample instead of the whole lot.
This involves lesser cost of inspection and it reduces production costs
too.
2)
SQC results in increasing profits too. This method minimises rejections
which means lesser cost. The rejected products increase cost of other
products. So this method helps in increasing profits by keeping control
on defective products.
3)
It helps in simplifying quality control. The tolerance limits are decided
and the products going beyond the tolerance limits are rejected. This
method is scientific in nature and allows proper and simple quality
control.
4) It helps in creating quality consciousness among employees. The quality
of products is tested immediately after their production, so the
employees remain conscious about quality. This helps in improving
quality of products and rejections are greatly reduced.
5) This helps in improving the goodwill of products among consumers. If the
consumers regularly get good quality products from a particular business
house then the reputation of that concern is enhanced. The consumers
will start patronising the products and will help the company in
increasing sales.
6) It also helps in smooth and unrestricted production. The defects in
products are detected at the earliest and this helps in taking corrective
measures.