Today, our daily lives and schedules totally depend upon the satisfactory performance and operation of products and services – whether of a city electrical distribution and power system net work, transportation system network, communication system, or even the pen with which we write. This no – alternative situation is new for our society where the customer has become more demanding than before. The customers demand for durability and reliability in products/services has increased tremendously in recent years.
Our markets, which were earlier based only on pricing, now are based on price as well as quality. The systems and procedures adopted to meet this new customer demand i.e., quality have improved and have been updated in recent years as never before.
This pursuit of quality is not new for any manufacturer. It has been a continuous process ever since the inspection of the manufacturing unit. Producer’s policy is to “deliver error – free competitive products and services on time to his customers. Their products take care of their needs of today as well as tomorrow”.
The following are a brief idea of the quality system generally adopted by manufacturers.
Manufacturer must have a well organized Quality Control Department. The department should have skilled supervisory system with necessary tools and work as per the company’s policy and procedures.
QUALITY CONTROL ACTIVITIES
Design and Developments. The drawing at the design stage is reviewed by the representatives from marketing, quality control and manufacturing. After approval from all the representatives, they are released for prototype testing. After finalization and necessary modifications if any, they are released for production.
Raw material and incoming components.
The manufacturer must have established standards for all units’ raw materials.
Tests should be conducted on all incoming materials consignment –wise before unloading it in stores/godown.
They must introduce quality certification by supplier in some of the major areas.
They must develop technically approved source for raw materials and in last two years there should not be any rejection report from the company side.
They may also inspect the incoming materials at the supplier’s end before loading it to delivery.
After receiving drawings from the design department, check lists are prepared for raw materials, and other components and necessary parts as well as for inspection. During this period of time the concerned authorities must follow the components and raw materials testing procedures strictly. And these should be inspected periodically for their accuracy. With the help of design of experiment technique, they must have evaluated and improved testing procedures, reduced testing errors, and time for testing significantly in determining loopholes and faults in raw materials and components/parts which is an original concept.
Improved quality of product by proper checking of each job after each operation is a must. This also saves the money and labor of further operation.
Manufacturers have to introduce inspection cum process cards giving details of process operation, inspection process and details of gauges and instruments to be used. This card should be displayed on each machine enable the concerned person to take immediate action on any faults. Further it is essential that daily conference to be held in each shop, to find out main causes of rejections. Corrective actions should be taken to sort out the problems on the same day. This is a group activity. Pneumatic wrenches, chemical solutions etc have to be used for tightening studs, nuts etc. These controls’ tightening torque and maintains the breakaway torque and prevailing torque and thus minimize the failure rates. Check lists have to be prepared to check sub – assembly and assembly. Ensure that all components or parts using Improved quality of paint, varnishes, grease, lubricants, oil at source and painting procedure for prolonged protection pleasing appearance.
PRINCIPLE OF MEASUREMENT
“When you can measure what you are speaking about and express it in numbers, you know something about it, when you cannot measure and express it in numbers, and your knowledge is of a meager and unsatisfactory kind. It may be the beginning of knowledge, but you have scarcely in your thought advances to the stage of science”
Measurement is the first step leading to scientific analysis. One of the recently developed sciences namely management science began with measurement of work.
In order to measure, we need a unit in terms of which the quantity can be expressed. A unit can be defined as the magnitude of a quality in terms of which magnitudes of other quantities of the same kind are expressed.
Again, in order to measure we need an embodiment of the unit against which the unknown is compared. This physical embodiment of the unit is the standard of measurement.
Any measurement, thus, involves the comparison of the unknown against the standard end finding out how much it varies from the standard.
Precision is the repeat ability of a measuring process or simply, how well identically performed measurements agree with each other.
Accuracy is the agreement of the result of a measurement with the true value of the measured quantity. The true value cannot be known. The best that can be done is to estimate the magnitude of the error.
Precision in measurement is dependent upon the consistency of the set – up employed and datum used, i.e. the process of measurement.
Achieving accuracy is more difficult and more expensive. The standard and the unknown must be as much alike a possible and the operations performed on the standard and on the unknown must be as identical as feasible.
Systematic errors are those due to the measuring procedure. Systematic errors must be identified and rooted out if possible. If not, an allowance must be made in the measurement for a systematic error.
Constant errors are those which affect all measurements in a measurement process by the same amount, or by an amount proportional to the magnitude of the quantity being measured.
It is, therefore, necessary for accuracy in measurement that any standard used be traceable to the defining standard for that quantity. This means that it must have been compared with the defining standard or with some standard in a chain of standards, which leads back to the defining standard. The longer this chain is, the less well known is the value of a standard at the far end of the chain.
LIMITS, FITS AND TOLERANCES
LIMITS
The actual size required of an object is called basic size. The allowable variation of a job above and below of basic size is called limit.
TOLERANCE
The difference of high and low limit of job is called tolerance.
ALLOWANCE
The permissible difference in sizes of two parts to be assembled is called allowance. It is intentionally kept to have the required fit during the assembly. In other way, we can say the difference of sizes of hole and shaft is called allowance. It may be positive or negative.
TYPE OF ALLOWANCES
Maximum Allowances: The difference of minimum size of shaft and maximum size of the hole is called maximum allowance.
Minimum Allowance.
The difference of maximum size of shaft and minimum size of hole is called minimum allowance.
DIFFERENCE BETWEEN TOLERANCE AND ALLOWANCE
TOLERANCE
ALLOWANCE
Depends upon basic size.
Depends upon type of fit to be required.
Can be different on different parts of the same job.
Given on two matching parts.
FIT:
The type of relation between the two matching parts is called fit.
TYPE OF FIT
Clearance Fit:
This type of fit occurs when the diameter of the hole is greater than the diameter of the shaft to be assembled. There is distinct clearance between the mating parts.
Example: Running and push fit.
INTERFERENCE FIT:
This type of fit occurs when the diameter of the hole is lesser than the diameter of the shaft to be assembled. There is a distinct interference between mating parts.
Example: Force fit, driving fit, and shrinkage fit.
TRANSITION FIT:
The fit between clearance and interference fit is called transition fit which means, assembly can be obtained by just a slight push.
Example: Sliding fit and push fit.
WHAT IS INSPECTION AND QUALITY CONTROL?
Inspection may be defined as an effort to ensure the production of parts or machine that will be satisfactory to both maker and the user, in other words, inspection as considered as the process of sorting good and bad. It includes the following:
Interpretation of specification.
Measurement of the product.
Comparison of specification with actual measurements.
Quality control may be defined as an effective system for co-coordinating the quality maintenance and quality improvement effort of the various groups in an organization so as to enable production at the most economical levels which satisfy the customer.
WHAT IS QUALITY?
Quality can mean different things to different functional components, within the same organization. Thus it can mean:
Technical specification, to design.
Conformance to specification, to inspection.
Demand for product, to sales.
Usability including visual appeal, to customer.
Rejection and complaints to production.
All the above at lower cost, to management.
Thus, it follows that the most appropriate meaning of quality is achieving ‘FITNESS FOR USE” at economic cost.
WHAT IS STATISTICAL QUALITY CONTROL?
It is a scientific method of analyzing data and using the analysis to solve practical problems.
The word “statistical” means “Having to do with numbers”.
The word “Quality” means much more than the goodness or badness of a product.
The word “Control” means to keep something within boundaries. Taken together, the word Statistical Quality Control means:
STATISTICAL
QUALITY
CONTROL With the help of numbers or data.
We study the characteristics of our process.
In order to make it behave the way we want it to behave.
OBJECTIVE OF S.Q.C.
The major objectives of statistical quality control are:-
To achieve the required quality at economic cost.
Improvement in product quality.
Reduction in operating cost and losses.
Improvement in employee morale and quality consciousness.
Organizing a quality improvement program to solve existing quality problems.
Improving means for maintaining control during production.
Providing for pre-planning of quality.
IMPORTANT TOOLS OF S.Q.C:
Graphical devices.
Frequency distribution.
Control charts.
Process capabilities analysis.
Acceptance sampling.
GRAPHICAL DEVICES
The main important features of graphical devices are:
Collection of data.
Summarization of data.
Scrutiny of data.
Tabulation of data.
Presentation of data.
CONTROL CHARTS
To achieve the objectives of avoiding defects, it is often necessary that the process be uniform. This means that the variability in the product should be due to numerous small chance cause of variation rather than to a few large assignable causes of variation.
In controlling a process, the objectives is to restrict the cause of variation to chance cause only, assignable causes must be eliminated. The key purpose of a control chart is to detect the presence of assignable causes of variation. This is done by taking a series of samples from the process and testing these samples for significant differences from the aggregate of all samples.
CLASSIFICATION OF SAMPLE CHARTS
Even though the objectives remain same, the technique of charting varies with the type of quality characteristics. Table shows the type of control chart and their characteristics.
CHARACTERISTICS AND CONTROL CHART CHARACTERISTICS
Attribute
P-Chart or fraction defective chart.
NP-Chart or number of defective chart.
C-Chart or number of defects per unit chart.
Variable
X-R Chart- Average and range chart.
DIFFERENCE BETWEEN ATTRIBUTE AND VARIABLEATTRIBUTE
VARIABLE
When a record shows the number of items conforming and number of item failing to conform to any specified requirement, it is said to be record by attributes. It provides an estimation that the products either conform or do not conform to the specifications. Attributes charts is usually associated with Go and No Go gauges.
When a record is made of actual measured quality characteristics, a quality is said to be variable.
P-Chart, NP-Chart, C-Chart, and X-R-Chart
Fraction Defective Chart (P-Chart)
It shows the variation in the fraction defectives of output. It is also known as chart for Go and No Go date. Fraction defective of output is defined as the value obtained by dividing the total number of units inspected over number of defectives obtained out of the submitted lot.
NP-Chart
P Chart can readily used where group size is variable and the NP Chart is used, where group size is constant.
No. of defect per unit chart (C-Chart)
In number of cases, it is more convenient to work with number of defects per unit rather than with the fraction defective. There can be number of such situations where C-Chart can be successfully applied.
Examples are: Inspection of Radio, Furniture etc.
Average and Range Chart (X-R Chart)
To secure information to be used in establishing or changing specification in determining whether a given process can meet specifications.
To secure information to be used in establishing or modifying production procedure.
To secure information to be used in establishing or changing inspection procedure or acceptance procedure or both.
PROCESS CAPABILITY ANALYSIS
Process capability refers to the minimum variation that has to be tolerated on any process under the existing situations. Thus process capability is quality – performance capability of the process with given factors under normal, in control conditions. Without an action of fundamental importance like reconditioning the machine or replacement of the machine or improving operator skill etc., the process capability cannot be improved. Two significant elements in this concept of process capability are –
Process factors
Process conditions
The first consideration necessary to the concept of process capability is that a process is made up of a number of distinct factors. These factors include raw material, machine or equipment, the operator skill, measuring devices and the inspector skill. A change in one or more of these factors may change the process capability. Hence a process capability to be meaningful must be stated with respect to a given set of specifically listed process factors.
The second elements implies that for process capability study to be meaningful, the process being analyzed should be one that has measurements normally distributed and in a state of control.
When X-R Chart shows that the process is in control, it means that variations in the dimension subjected to control is within the limits of the natural variation. But it is not possible to say beforehand whether dimensions turned out in a state of statistical control meet the design specifications, or not. The answer to this question depends on the nature of relationship that exists between the processes within control.
It is also necessary that the process average and the process capability should be such as to permit the production of items, almost all of which comply with the specification. Capabilities of the machines must be evaluation periodically or after every maintenance of the machine and compared with specifications of the jobs loaded on the machine. The process capability can be calculated by the expression;
= 6 X R/d2
The process capability is also called natural tolerance.
COMPARISON WITH DESIGN TOLERANCE
A comparison of the process capability ( Natural Tolerance ) with the specified tolerance will bring out the following three types of situations.;
Process capability smaller than the specified tolerance.
Process capability equal to specified tolerance.
Process capability larger than the specified tolerance.
The possible actions/decisions to be taken in the above situations are listed as under:
RELATIONSHIP
ACTIONS/DECISIONS
Case (a)
Process capability (Natural Tolerance) smaller than specified tolerance.
Job can be changed to a less costly machine.
Specified tolerance can be squeezed if it is economically worth – wise.
Inspection interval can be increased and 100% inspection of employed can be dispensed with. The product can be accepted on the basis of control chart as long as it shows the process on control.
Case (b) Process capability equal to specified tolerance.
Greater attention to the centering of the process maintains the process average at the specification should give.
Set the machine at the most economic level if it is advantageous to change the process average at the risk of allowing scrap or re work.
Investigate the possibility of reducing process variability.
Case ( C )Process capability more than the specified tolerance.
Scrap and re work are inevitable.
Report to minimum machine adjustment.
100% inspection must be resorted to.
Examine the possibility of widening the specified tolerance to suit the process capability or affect fundamental changes in the process to reduce its variability.
Maintain X-R Chart and set the machine level at the most economic level.
RESULTS OF S.Q.C
Successful application of S.Q.C. program is expected o yield the following results:
Improvement in quality and Reliability.
Reduction in scrap and rework.
Efficient use of men and machine.
Locating and removing productions bottlenecks.
Scientific evaluation of standards for quality and production.
Decreased inspection costs.
Quality consciousness at all levels.
Improved level of customer satisfaction and quality image in the market.
SAMPLING
In factories, many measures are taken on the basis of data. Since every produced item cannot be inspected individually, these data’s are derived through sampling. It is an over all system containing a range of sampling plan and procedures based on mathematical theory of probability and statistics whereby the result of inspecting one or more samples is used to determine the acceptance or rejection of a lot or to access its quality.
TYPES OF SAMPLING
RANDOM SAMPLING.
TWO STAGE SAMPLING.
STRATIFIED SAMPLING.
CLUSTER SAMPLING.
SELECTED SAMPLING.
PARETO ANALYSIS.
The pare to analysis of failures occurring ensures that when curative action is decided on, efforts are concentrated on those failures that have a major effect on the service under guarantee call rate. This avoids wasting effort on minor problems.
The actions are discussed in committee with quality assurance, laboratory, development, marketing, service and manufacturing.
A Pareto diagram indicates which problem should be solved first in eliminating defects and improving the operation.
The problem contributing to maximum number of defects is tackled first and so on. Such tools are extremely useful in factory quality control. A pareto diagram also reveals whether attempts at improvements produce results.
CAUSE AND EFFECT DIAGRAM
Making cause and effect diagrams (General Steps)
The factors involved in problems with quality are almost uncountable. A cause and effect diagram is useful in sorting out the causes of dispersion and organizing mutual relationships. The following steps are taken in making cause and effect diagram.
STEP-1 Determine the quality characteristics. This is something we would want to improve and control. In this case, most of the factory defectives were due to wobble during rotation and to eliminate the wobble, the causes must be determined.
STEP-2 Write the main factors which may be causing the wobble, directing a branch arrow to the main arrow. It is recommended to group the major possible casual factors of dispersion into such items as raw materials (materials), equipment (machines or tools), method of works (workers), measuring method (inspection), etc. Each individual group will form a branch.
STEP-3 Write in detail the factors which may be regarded as the causes, these will be like twigs. On to each of these factors, write even more details making smaller twigs. Defining and linking the relationships of the possible casual factors should lead to the source of the quality characteristics.
QUALITY ASSURANCE
Quality assurance can be defined as all activities and functions concerned with the attainment of quality. This term has a broader sense as compared to quality control. Quality assurance would broadly cover the following areas. –
Standardization/specifications.
Reliability engineering.
Documentation.
Design reviews.
Vendor appraisal.
Inspection.
Testing.
Market Surveys.
Complaints.
Product rating and audit.
Product rating and audits are gaining increasing importance today. Product ratings involves the following:
Identifying critical parameters of a system.
Selecting a product at random at the final stage.
Check to see if the parameters identified are within specified limits.
Assigning a demerit score if not,
Taking corrective action.
Whereas, a quality audit is a non-executive function, it relates to a quality of a product, process or system. It is carried out on a periodic basis and involves an independent and systematic examination of actions that influence quality. The object is to ascertain compliance with the implementation of the quality system, program, plan and specifications.
RELIABILITY
Reliability was somewhat a vague term, a few years ago and was not given much importance in the design, development, manufacture and use of a system. But today, with the increasing complexity of systems, reliability considerations have entered every aspect of a product cycle. This has resulted in a vast improvement in the form of cost reduction, superior workmanship, a higher probability of satisfactory operation and improved customer-vendor relationship.
Reliability may be defined as the ‘Ability of an item to perform a required function under stated conditions for a stated period of time’. This is linked to the concept of failure. A failure is the termination of the ability of an item to perform a required function. A study of failure would include the following:
Failure cause study.
Failure mode study.
Failure mechanism study.
Failure criteria study.
A reliability study would, there are, consist of the following:
Study of failure based on file surveys, customer complaints and life test.
Documentation of data and analysis.
Implementation in design and/or process of above.
Conduct life tests.
QUALITY COST
Traditionally, the quality control function has been responsible for reporting quality performance to management in terms of rejection and defective material reports. This vital information is often difficult to analyze and interpret in terms of costs. As a result, cost saving opportunities is often overlooked. Successful business requires financial planning and control, it is advisable that quality failures be represented in financial terms.
Quality related costs can be classified as follows-
Prevention cost: The costs of any action taken in to investigate prevent or reduce defects and failures.
Appraisal cost: The costs of assessing the quality achieved.
Internal failure cost: The costs arising within the manufacturing organization, of failure to achieve quality specified (before the transfer of ownership to the customer).
External failure costs: The costs arising outside the manufacturing organization of failure to achieve quality specified (after the transfer of ownership to the customer).
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