metrology and quality

1SME2713 Manufacturing Processes

P repared by Richard Foo Jin [email protected]

SME2713

MANUFACTURING PROCESSES

CHAPTER - 2

MANUFACTURING

ASPECTS-Version 2.0- 10.05.2014



1.0 INTRODUCTION

The main intention of manufacturing is to produce useful products that can be sold. IN order to sell these products, these good has to befunctional, with reasonable pricing and come with high quality that can be acceptable by customer or consumer. In order to produced as permentioned above, the manufacturer has to take a few important aspects related to the manufacturing processes from the stage of design untilthe product is being produced. Those aspects mentioned here involved assembly, specification, standardization, quality control, testing andchecking, limits and fits, and also tolerances.

2.0 MEASURING SYSTEM

The characteristics and quality of a measuring instruments are generally described by certain specific terms as below,

· ACCURACY: The degree of agreement of the measured dimension with its true magnitude.· CALIBRATION: Adjusting or setting an instrument to give readings that are accurate within a reference standard.· LINEARITY: The accuracy of the readings of an instrument over its full working range.· MAGNIFICATION/AMPLIFICATION: The ratio of instrument output to the input dimension.· PRECISION: Degree to which an instrument gives repeated measurement of the same standard.· RESOLUTION: Smallest dimension that can be read on an instrument.· RULE OF 10 (Gage Maker's Rule): An instrument or gage should be 10 times more accurate than the dimensional tolerances of the part

being measured. Similarly, a factor is of 4 is known as the Mil Standard Rule.· SENSITIVITY: Smallest differences in dimension that an instrument can distinguish or detect.· SPEED OF RESPONSE: How rapidly an instrument indicates the measurement, particularly when a number of parts are measured in

rapid succession.· STABILITY/DRIFT: An instrument's capability to maintain its calibration over a period of time.

Selection of an appropriate measuring instrument for a particular application depends on he foregoing consideration. In addition, the size andtype of parts to be measured, the environment (temperature, humidity, dust, and so on), operator skills required, and costs have to beconsidered in the purchase of such equipment.

2.1 MEASUREMENT

Any measurement must be made to an acceptable degree of accuracy, but it must be realized that no measurement is exact. The accuracyof determination is, therefore, as important as the measured dimensions and the inherent errors in any method of measurement should bekept to a minimum. But how can errors be kept to a minimum in the manufacturing environment?

· Selection of appropriate work-piece specifications.· Selection of an appropriate measurement system to meet the chosen specifications.· Provision of a suitable environment for the measurement system.· Applying an predetermined inspection system to check and record production work-piece specifications.· Periodic calibration checking of the adopted measurement system to ensure dimensional reliability of the equipment.· Applying an appropriate process control system as a means of tracking and correcting process errors.

2.2 SURFACE FINISH

Surface finish (or surface technology) can be related to:

· The roughness of surface,· The characteristics of surfaces as they apply to wear and lubrication,· Special surface treatments for enhancing a surfaces performance or appearance,· Fabrication technology is on the measurement of surface roughness.

Surface roughness is varies depending on condition of processing such as the cutting edge shape and condition, cutting speed, feed rate,rigidity, an available power so that, amongst other considerations. All surfaces have their own characteristics and a wide variety ofsurface patterns result from the different production conditions. Each surface texture has its own waviness, roughness, lay and flaws ordefects. These features of a surface have well-defined and measurable quantities which can be specified on a part drawing like shownbelow.

· Lay: The direction of the predominant surface pattern, determined by the production method.



· Flaw: Random irregularities such as scratches, holes, cracks, peaks and ridges.· Waviness: Irregularities on a surface in the form of waves. On smooth machined surfaces such waves measure.· Roughness: Irregularities on a surface more closely spaced than waviness. Roughness may be superimposed on a wavy surface.

2.3 TOLERANCING

Machine can produce high levels of accuracy. However, their ability to continually repeat this accuracy is extremely limited by factorssuch as,

· Temperature changes during processing,· The rate of tool wear,· Deflection due to tool setup and wear in machine parts,· Vibration within the machine tool,· Human error.

When parts are produced singly, it might seems that their size is of little relevance, but more often than not each of these parts must matewith another part in performing a particular function. To achieve the functional requirements of assembled parts, one of the followingthree courses can be taken.

2.3.1 MAKE TO SUIT ASSEMBLY

In this method of assembly, an individual parts and component is produced to be as accurate as possible to the desired size as perdesigned with the use of existing tooling and equipment, so that close fits can be achieved. This method is hard to apply into thehigh quantity production. This is because highly skillful labor is required, high quantity of component can't be produced, and can'tbe selected randomly to be assembled. Other than that, the quality of products is hard to control, time consuming and therefore verycostly. Each time a part is to be replaced, its replacement has to be individually machined.

2.3.2 SELECTIVE ASSEMBLY

Here parts are produced to a given size range that can be easily achieved. Before assembly, they are graded into groups with, say,those whose size is at the high end of the range forming one group, middle of the range size forming another group and those at thelower end of the range form a third group. Thus, selective assembly is a method whereas each and every component is measuredand being graded into different grading according to the range of limits and fits. Say there is 5 grades for a shaft and 5 grades for ahole, so if shaft of grade 5 is assembled with a grade 5 hole, then the assembly will be functioning perfectly. On assembly, themating part can be selected for fit with one from the group giving the best fit. The process is less costly, however some losses areincurred in selecting for assembly. Replacement part must also be selectively chosen, or individually machined. This system is usedunder two condition, that's;

· The component can't be produced economically into the precision required but can be measured and graded.· These assemblies can be changed/replaced in a complete unit/set.



This method is generally used in the assembly of ball bearing, roller, cylinder, or piston on a automobile application.

2.3.3 INTERCHANGEABLE ASSEMBLY

Ideally, an ideal manufacturing method is those which has the following characteristics, they are;

· Using the most economical manufacturing methods. This method normally involved the usage of automated machine, partialautomated machine, in another words, labor involving here generally are those semi skillful labor.

· All of the components can be interchangeable, whereas if there are 1000 shafts and holes, then each and every of them can fix intoeach other and still functioning perfectly.

· All of the assembly produced can be accepted even though there will be a slight variation in the fitting.

Those manufacturing method which can meet the 3 requirement above is said to be a interchangeable manufacturing. The intention ofthis assembly system is that anyone part will fit any of its mating parts as required. With this interchangeability, assemble costs aremuch reduced and maintenance is better facilitated. This type of manufacturing is to be said a result from the usage of standards relatedwith size and fits for mating parts. As mentioned above, this is a system whereas anyone of component produced can be mated with itspair perfectly, in another words, anyone of the component produced can be interchangeable within each other without any modification.Theoretically, even if the same component is being produced in different location, different machine, different worker, still if it isfollowing the same specification and standard, then all of the component produced can be interchange between each another. This willresulted in cost reduction because assembly job is made easy and spare parts can be made in advanced for future usage.

To enable this method, the limits of acceptance of the component must be pre-determined from the standard to suit its application.Manufacturing processes involved must have a high level of accuracy so that the component produced is at its acceptable limits. Furthermore, a good quality control system should be implemented so to assure component produced is within specification. In order forinterchangeability to be possible, close dimensional control is necessary, along with the standardization of different classes of fit.Therefore, when designing for parts that are to fit closely together they should be dimensioned according to the required fit.

2.4 CALIBRATION

Not every company has the facility for product and/or equipment calibration, including the calibration of machine/machine tools.Therefore some companies offer a calibration and re-conditioning service. This is not onlt providing an economical means ofmaintaining internal inspection and quality standards, but also a means of ensuring the maintenance of the link for traceability tonational standards. The calibration is carried out by using standard procedures against recognized primary standards by;

· Repeating a sequence of five point measurement to ensure a direct match with a master standard.· Then, repeating a check on the master standard.· Finally, repeating the whole process if any incomparable measurements are found.

When the calibration is done, a first report and a certificate of calibration is issued as an accepted assurance of the quality of calibrationcarried out on certified equipment. Such calibration fully meet the requirement of major standard around the world such as ISO9000,EN29000, BS5750, and etc.

2.5 SPECIFICATION

Specification is a detail and precise description of certain things. In the context of manufacturing a products, a product can be producedafter we have the technical drawing telling the production on the characteristic of the products we will be making. This drawing did notcontain too much information especially on the quality control specification of the products. Thus, manufacturing engineer has toproduce the working drawing and clear specification so that desired product can be produced. It is very important that every individualin the organization is using the same set of specification and standards so to avoid misunderstanding, confusion and mistakes.

Specifications that normally involved are, materials, finishing requirement, heat-treatment requirement, dimension, tolerances,roundness requirement, straightness requirement, concentricity requirement, and etc. Every requirement normally represented by astandard symbols as shown below (just to list a few);

FlatnessStraightness

Roundness



2.6 STANDARD

Standards can be defined as a defined criteria related to size, shape, quality and etc for a certain things/goods. With the use of standardconcept mentioned above, the production of goods can be made easy, simplified, that's with the use of standard parts of component likescrew, gear and etc, made by standard equipment and machine, and is being inspected using the standard equipment and tools, and so on.Standard methods, standard component, and standard equipment is produced using a certain standard, there is a few functioning bodywhich coordinate the specification such as the International Standard Organization (ISO), British Standard Institution (BSI), AmericanNational Standard Institute (ANSI), and etc. Recent years, due to the global competition, the ISO 9000 Series seems to be a requirementfor every organization to stay competitive. As mentioned above, there's various standards whereas an engineer can refer and use, itincludes dimension, tolerances, limits and fits, specification of material and its characteristics, machine, equipment, inspection methods,assembly and etc. The usage of standards is very important because it gives direct effects towards quality.

2.6.1 THE ISO AND QS STANDARD

With increasing international trade, global manufacturing, and price-sensitive competition have come a wide choice of industrialand consumer products. Customers increasingly are demanding high quality products and services at low prices and are looking forsuppliers that can response to this demand consistently and reliably. In turn, this trend has created the need for internationalconformity and consensus regarding the establishment of methods for quality control, reliability and safety of products. In additionto these considerations, equally important concerns regarding the environment and quality of life are being addressed with newinternational standards.

2.6.2 THE ISO 9000 STANDARD

Published in 1987 and then revised in 1994 and 2000 respectively. The ISO 9000 standard (Quality Management and QualityAssurance Standards) is a deliberately generic series of quality system management standards. The ISO 9000 standard permanentlyhas influenced the manner in which manufacturing companies conduct business in world trade and has become the world standardfor quality. The ISO 9000 series includes the following standards,

· ISO 9001, Quality system: Model for quality assurance in design/development, production, installation, and servicing.

· ISO 9002, Quality system: Model for quality assurance in production and installation.

· ISO 9003, Quality system: Model for quality assurance in final inspection and test.

· ISO9004, Quality management and quality system elements: Guidelines.

Companies voluntarily register for these standards and are issued certificates. For certification, a company's plants are visited andaudited by accredited and independent third-party teams to certify that the standard's 20 key elements are in place and arefunctioning properly. Depending on the extent to which a company fails to meet the requirements of the standard, registration mayor may not be recommended at that time. The audit team does not advise or consult with the company on how to fix discrepanciesbut merely describes the nature of the noncompliance. Periodic audits are required to maintain certification. The certificationprocess can take from six months to a year or more and can cost tens of thousands of dollars. The cost depends on the company'ssize, number of plants, and product line. The ISO 9000 standard is not a product certification, but a quality process certification.Companies establish their own criteria and practices for quality. However, the documented quality system must be in compliancewith the ISO 9000 standards. Thus, a company cannot write into the system any criterion that opposes the intent of the standard.Registration symbolizes a company's commitment to conform to consistent practices, as specified by the company's own qualitysystem (such as quality in design, development, production, installation, and servicing), customers (including government agencies)are assured that the supplier of the product or service (which may or may not be within the same country) is following specifiedpractices. In fact, manufacturing companies are themselves assured of such practice regarding their own suppliers who have ISO9000 registration; thus, suppliers also must be registered.

2.6.3 THE ISO 14000 STANDARDS

Published in 1996 and pertaining to international EMS (Environmental Management System). It concerns the way an organization'sactivities affect the environment throughout the life of its products. A rapidly increasing number of companies in many countrieshave been obtaining certificate for this standard. These activities,



· May be internal or external to the organization,

· Range from production to ultimate disposal of the product after its useful life,

· Include effects on the environment, such as pollution, waste generation and disposal, noise, depletion of natural resources,and energy use.

2.7 LIMITS AND FITS

Engineering drawing do not state the type of fit necessary to allow mating parts to correctly function, but states a tolerance ondimensions to satisfy the needs of each particular class of fit, generally to meet the requirement of interchangeability. Limits can bedefined as the maximum and minimum dimensions of parts. Fits can be defined as the range of looseness or tightness that can resultfrom the application of specific combination of allowance and tolerance in the design of mating part features.

· Shaft: By convention, the term refers to all external features of a part, including features which are not cylindrical.

· Hole: By convention, the term refers to all internal features of a part, including features which are not cylindrical.

· Actual Size: A dimension of a part obtained by measurement.

· Limits of size: The maximum and minimum sizes permitted for a feature. Both limits must always be stipulated in full.

· Basic size: The size by reference to which the limits of size are fixed. The basic size is the same for both members.

· Tolerance: The arithmetic difference between the maximum limit of size and the minimum limit of size for a dimension.

· Allowance: The arithmetic difference between the high limit of size of the shaft and the low limit of size of the hole. (ForClearance Fit: Allowance is always positive; Meanwhile, for Interference Fit: Allowance is always negative)

The interchangeable manufacturing can only be done under these two conditions, that's;

· Types of fits of the mating parts should be of clearance fit, interference fit, or transition fit and etc.

· Tolerance allowance at each dimension.



There is 2 types of standard which is very famous in limits and fits, they are the BS 4500 (ISO limits and fits in metric units) and Newalllimit system (used by Newell Engineering Co. Ltd.) which consist of 6 shaft class and 2 hole class from basic size up until 150 mm andusing the bilateral tolerance. Standard BS 4500 is more widely used and more completed. These standard uses the hole basis system. Itobtained the size range up until 3150 mm. According to the BS4500, some basic size can gives 28 holes ranging from symbol A to ZC.It's the same with shaft which gives 28 shafts ranging from symbol A to ZC too. (IT 01, IT 0, IT 1, …, IT 16). IT is the InternationalTolerance Grade, it is a group of tolerances that vary depending on the basic size, but provide the same relative level of accuracy withina grade. IT 16 will produces a more course finishing product, thus the limits and fits tolerance will be bigger. In reverse, IT 01 willproduce a very fine finishing product, thus the limits and fits tolerance will be very small. In another words, the smaller the IT number is,the finer the tolerances is.

From all the hole and shaft standard in BS4500, the normally used hole standard is H7, H8, H9, and H11. Meanwhile the normally usedshaft standard is c11, d10, e9, f7, g6, h6, k6, n6, p6, and s6.

Example,

Consider one component of size 50 mm diameter is mated together. It was given that the grading of hole is H7 and shaft is n6. You haveto find,

a) Hole tolerance,

b) Shaft tolerance,

c) Hole size range,

d) Shaft size range

e) Minimum looseness,

f) Maximum looseness, and

g) The types of fits.

Solution,

Before doing anything else, you would found out that there is 2 alternatives that can give you the Ф50 mm hole and shaft tolerance asshown below,

Always go for the lower one, that’s alternative –1. So, your answer now is,

Hole, Shaft,

Ф50 mm Ф50 mm

Thus,

a) The hole tolerance will be = (+0.025) - (0) = +0.025 mm

b) The shaft tolerance will be = (+0.033) - (+0.017) = +0.016 mm

c) Hole size range = 50è50.025 mm

+0.025+0.000

+0.033+0.017



d) Shaft size range = 50.017è50.033 mm

e) Minimum clearance = Lower Hole Size - Upper ShaftSize = 50 - 50.033 = -0.033 mm (Interference)

f) Maximum clearance = upper Hole size - lower Shaft Size = 50.025 - 50.017 = +0.008 (Looseness)

g) The type of fits is Transition fits.

ALTERNATIVE - 1

ALTERNATIVE - 2

ALTERNATIVE - 1

ALTERNATIVE - 2



2.8 TYPE OF FITS

In BS 4500, there are 2 fit systems, they are; Shaft-based System: A system of fits in which the different clearance and interference firsare obtained by associating various holes with a single shaft. (More often used, for example, where bar stock is to be used.) Hole-basedSystem: A system of fits in which the different clearance and interference fits are obtained by associating various shafts with a singlehole. (More often used: for example, when standard drills and reamers are to be used. For this reason the Hole-based System is morecommon). Fitting of two mating part is the degree of looseness/tightness of the two mating part. The pair of fitting has to be determinedto ensure the mating part can function properly at a satisfactory level. The degree of fitting between the hole and shaft depends on the;

· Magnitude of tolerance,

· Tolerance and also the basic size,

· Magnitude of basic size.

There are 3 types of fits, they are;

· Clearance fit, when the maximum limit of the shaft is less than the minimum limit of the hole, a clearance fir exists. A gap alwaysexists between the mating parts. This is a fit that allows for rotation or sliding between mating parts.

Minimum clearance = Differences between upper shaft size and lower hole size.

Maximum clearance = Differences between lower shaft size and upper hole size.

ALTERNATIVE - 1

ALTERNATIVE - 2



· Interference fit, when the minimum limit of the shaft is greater than the maximum limit of the hole. The arithmetic differencebetween these two limits is known as the interference. This is a fit that having limits of size so prescribed that interference is alwaysresult when mating part are assembled.

Minimum interference = Differences between upper hole size and lower shaft size before assembled.

Maximum interference = Differences between lower hole size and upper shaft size before assembled.

· Transition fit, when the specified limits for both the hole and the shaft give a range of fits between clearance and interference. (i.e.the tolerance zones for both the hole and the shaft overlap). This is a fit with small clearance or interference that allows for accuratelocating of mating parts. Varies from clearance fits to interference fits.

2.9 TOLERANCES

Whatever effort you put in the manufacturing is with the aim to achieve a production of component/parts size that is pre-determined bythe production drawing. However, as mentioned previously, to manufacture to the exact dimension is not possible to achieve. That's whywe need to have a set of variation to the desired dimension. This variation is called tolerances. It's the design engineer's job to fix thetolerance to all of the dimension, determine how tight or lose one dimension can be so that the component/parts can bemanufacture/produce economically. Because the smaller the tolerances is, then the more expensive the manufacturing cost is.

There are 3 ways in which the tolerances is decided, they are;

· Bilateral tolerance - It's a deviation (plus or minus) from the basic size.

· Unilateral tolerance - It's a deviation in one direction only fromthe nominal dimension.

· Dimension limits - It's useful whenever total tolerance is small or during large manufacturing quantity when gauge is widely used.

The unilateral tolerance is a more preferred method because of its flexibility. The degree of fits between holes and shafts is determinedby the magnitude of tolerance and position from its basic size. The degree of looseness is also depending on the magnitude of basic size.

The value of tolerance given will be depending on the few factors, such as;

· Component/Part's function,

· Available processes, and

· Manufacturing and assembly cost.

The manufacturing and assembly cost will increase if the tolerances is narrowed, because;

· A more advanced/better machine or equipment is required,

· Highly experienced labor required, and

· A more detail inspection and equipment handling required.

These shows the fact that quality and cost is always affecting the manufacturing sector.

3.0 QUALITY ASSURANCE, TESTING, AND INSPECTION

3.1 INTRODUCTION



Manufactured products develop certain external and internal characteristics, which result (in part) from the production processes used.External characteristics most commonly involve dimensions, size, and surface finish and integrity issues such as surface damage fromcutting tools or friction during the processing of the work piece. Internal characteristics include defects, such as porosity, impurities,inclusions, phase transformations, embrittlement, cracks, debonding of laminations, and residual stresses.

Some of these defects may exist in the original stock, and some are introduced or induced during the particular manufacturing operation.Before they are marketed, manufactured parts and products are inspected for several characteristics. This inspection routine is importantin order to;

· Ensure dimensional accuracy so that parts properly fit into other components during assembly,

· Identity products whose failure or malfunction has potentially serious implications, such as bodily injury or fatality. Typicalexamples are the failure of cables, switches, brakes, grinding wheels, railroad wheels, turbine blades, pressure vessels, and weldedjoints.

Product quality always has been one of the most important aspects of manufacturing operations. In view of a global competitive market,continuous improvement in quality is not a major priority, particularly for large corporations in industrialized countries. In Japan, thesingle term term KAIZEN is used to signify never-ending improvement.

The prevention of defects in products and on-line inspection of parts are major goals in all manufacturing activities. We again emphasizethat quality must be built into a product and not merely checked for after the product already has been made. Thus, close cooperationand communication among design and manufacturing engineers and direct involvement and encouragement of the companymanagement are vital.

Major advances in quality engineering and productivity have been made, largely because of the efforts of quality experts such asDeming, Taguchi, and Juran. The importance of the quality, reliability, and safety of products in global economy now is recognizedinternationally by the establishment of various ISO and QSO standards and by the Malcolm Baldrife National Quality Award in theUnited States.

3.2 PRODUCT QUALITY

We all have used term like "poor" quality" or "high quality" to describe a particular product or the products of a particular company;Although we may recognize it when we see or use a product, quality is difficult to define precisely. Simply and generally, quality maybe defined as a product's fitness for use. Several dimensions of quality generally are identified; these include characteristics such asperformance, durability, reliability, robustness, and serviceability, as well as aesthetics and perceived quality. Thus, quality is a broadbased characteristics or property, and its factors consist not only of well-defined technical considerations but also of subjective opinions.

The general perception is that a high quality product is one which performs its functions reliably over a long period of time withoutbreaking down or requiring repairs. We have seen that design and manufacturing engineers have the responsibility of selecting andspecifying materials for the components of the products to be made. However, it is important to note that materials possesses betterproperties generally are more expensive and may be more difficult to process than those with poorer properties. The level of quality thata manufacturer chooses for its products may depend on the market for which the products are intended.

Total quality cost depends on several variables, including the level of automation in the manufacturing plant. Thus, there are many waysfor the engineer to review and modify overall product design and manufacturing processes to minimize a product's cost without affectingthe quality. Contrary to general public perception, high quality products do not necessarily cost more, especially when considering thefact that poor quality product;

· Present difficulties in assembling and maintaining components.

· Result in the need for in-field repairs.

· Have the significant built-in cost of customer dissatisfaction.



Quality standards are essentially a balance between several considerations. This balance is also called Return On Quality (ROQ) andusually includes some limit on the expected life of the product.

3.2 QUALITY ASSURANCE

Quality assurance is the total effort made by a manufacturer to ensure that its products conform to a detailed set of specifications andstandards.. it can be defined as all actions necessary to ensure that quality requirements will be satisfied; whereas, quality control is theset of operational techniques used to fulfill quality requirements. These standards cover several types of parameters, such as dimension,surface finish, tolerances, composition, and color, as well as mechanical, physical, and chemical properties. In addition, standardsusually are written to ensure proper assembly, using interchangeable defect-free components and resulting in a product that performs asoriginally intended by the designers.

Quality assurance is the responsibility of everyone involved with design and manufacturing. The often repeated statement that qualitymust be built into a product reflects this important concept. Although a finished product can be inspected for quality, quality cannot beinspected into a finished product. An important aspect of quality assurance is the capability to,

· Analyze defects as they occur on the production line,

· Promptly eliminate them or reduce them to acceptable levels.

In an even broader sense, quality assurance involves evaluating the product and customer satisfaction. The sum of all these activity isreferred to as the total quality control; and, in a larger sense, total quality management. It is clear that, in order to control quality, wehave to be able to,

· Measure quantitatively the level of quality,

· Identify all of the material and process variable that can be controlled.

The quality level built in during production then can be checked by continuously inspecting the product to determine whether it meetsthe specifications for dimensional tolerances, surface finish, defects, and other characteristics.

3.3 TOTAL QUALITY MANAGEMENT (TQM)

TQM is a system that emphasizes the concept that quality must be designed and built into the product. It is a systems approach in thatboth management and employees make a concerted effort to consistently manufacture high-quality products. Defect prevention (ratherthan defect detection) is the major goal here. Leadership and teamwork in the organization are essential. They ensure that the goal ofcontinuous improvement in manufacturing operation is imperative, because they reduce product variability and they improve customersatisfaction. The TQM concept also requires us to control the processes and not the parts produced, so that process variability is reducedand no defective parts are allowed to continue through the production line.

3.4 STATISTICAL METHODS OF QUALITY CONTROL

Because the numerous variables involved in manufacturing processes and operation, the implementation of statistical techniques isessential. To understand SQC (Statistical Quality Control), let's first review the terms that are used commonly,

· Sample size; The number of parts to be inspected in a sample. The properties of the parts in the sample are studied to gaininformation about the whole population.



· Random sampling; Taking a sample from a population or lot in which each item has an equal chance of being included in thesample. Thus, when taking samples from a large bin, the inspector should not take only those that happen to be within reach.

· Population; The total number of individual parts of the same design from which samples are taken; also called the universe.

· Lot size; A subset of population. A lot or several lots can be considered subsets of the population and may be considered asrepresentative of the population.

The SPC method take certain measures and action to prevent producing further defective parts. This technique consists of variouselements,

· Control charts and control limits

· Capabilities of the particular manufacturing process

· Characteristics of the machinery involved

3.5 SIX SIGMA

A quality standard in increasingly wider practice is the six sigma requirement. Six sigma means that there are only 3.4 defective partsper million parts on each side of the distribution (or a total of 6.8 parts per million). Because of the nature of statistical distribution, thistacitly forces improvements in process capability to reduce variability. The six sigma concept also has been extended to business andoffice processes and operations.

3.6 EQUIPMENT/TOOLING AIDING QUALITY CONTROL

In order to produced and maintain quality products, and inspection is carried out with the help of some equipment or tooling, gauge,destructive test, and un-destructive test. These equipment or tooling is used to measure distance, angle, surface texture, roundness,clearance, and etc on the components/parts. The major aim of inspection is to compare an unknown specimen with a specimen of knownspecification/standard.

Listed below are the equipment/tooling/technique that is usually seen in quality control,

· Equipment/Tooling,

Ruler, combination square, gauge, optical comparator, toolmaker's microscope, protractor, bar sign, plat sign angle gauge, profile,optical flat, profilometer, autocollimator, and etc.

· Un-destructive test

Visual checking, magnetic test, ultra sound, laser, radiography and etc.

3.7 INSPECTION

It's agreeable that there is definitely no 2 identical items. This is also applied in the production of component or parts. As mentionedabove that there is a lot of factors that caused variation in any component even though it is made in the same way, using the samemachine, and by the same worker. Part of the factors is due to the variation of raw material used, machine inaccuracy, incorrect usage oftooling, tool wear, and human error.

The main function of inspection is to assure that those component or part that doesn't meet requirement do not bypass the manufacturingand go straight to the customers. Always remember that there will be a various stage of inspection in as early as incoming materials,work in progress, before critical or expensive process, and final outgoing inspection.

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