interchange ability

7/26/2019 Interchange Ability

1/8

Interchangeability (Metrology)4.3.

An interchangeable part is one which can be

substituted for similar part manufactured tothe same drawing.In earlier times production used to be confined to smallnumber of units and the sameoperator could adjust the mating components to obtaindesired fit. With time the concept ofmanufacturing techniques kept on changing and today

the same operator is no more responsiblefor manufacture and assembly too. With economicoriented approach, mass production techniqueswere inevitable, that led to breaking up of a completeprocess into several smaller activities andthis led to specialisation. As a result various matingcomponents will come from several shops, even

a small component would undergo production onseveral machines. Under such conditions itbecomes absolutely essential to have strict control overthe dimensions of portions which have tomatch with other parts. Any one component selected atrandom should assemble correctly with anyother mating component, that too selected at random.

When a system of this kind is ensured it isknown as interchangeable system. Interchangeabilityensures increased output with reducedproduction cost.In interchangeable system, every operator being


2/8

concerned only with a limited portion ofoverall work, he can easily specialise himself in thatwork and give best results leading to superior

quality. He need not waste his skill in fitting thecomponents by hit and trial and assembly time isreduced considerably. In the case of big assemblies,several units to manufacture individual partscan be located in different parts of country dependingon availability of specialised labour, rawmaterial, power, water and other facilities and final

assembly of all individual componentsmanufactured in several units can be done at oneplace. The replacement of worn out or defectiveparts and repairs is rendered very easy and the cost ofmaintenance is very much reduced and shutdown time also reduced to minimum.Interchangeability is possible only when certain

standards are strictly followed. Universalinterchangeability (i.e. parts drawn from any twoaltogether different manufacturing sources formating purposes) is desirable and for this it is essentialthat common standards be followed by all,and all standards used by various manufacturing unitsshould be traceable to a single source, i.e. international

standards. When all parts to be assembled are madein the same manufacturing unit, local standards maybe followed (condition being known as local interchan-geability) but for reasons of obtaining spares from anyother source it is again desirable that these local stand-


3/8

ards be also traceable to international standards.The required fit in an assembly can be obtainedin two ways, namely (i) universal or full interchan-

geability, and (ii) selective assembly. Full interchan-geability means that any component will mat with any

other mating component without classifyingmanufactured components in subgroup or withoutcarrying out any minor alterations for mating purposes.This type of interchangeability is not mustfor interchangeable production and many times notfeasible also as it requires machines capable of

maintaining high process capability and very highaccuracy, and very close supervision on produc-tion from time to time. (Process capability of a machineis defined as its 3o spread of dimensionsof components produced by it. If a plot is drawn of theactual dimensions of the similar componentsproduced by a machine, it is found to follow natural law

of distribution, i.e. having mean of all thecomponents at central value with a spread of 3ovalue, a being known as standard deviation, and3oas the process capability of machine). For fullinterchangeable assembly it is essential that only
http://lh3.ggpht.com/_X6JnoL0U4BY/S0zB8mvAZzI/AAAAAAAAD4o/rf5DCEq6Jgs/s1600-h/tmp688_thumb1.png


4/8

such machines be selected for manufacturing whoseprocess capability is equal to or less than themanufacturing tolerance allowed for that part. Only then

every component will be within desiredtolerance and capable of mating with any other matingcomponent.4.3.1.

Selective Assembly.

Today the consumer not only wants quality, precisionandtrouble free products but also he wants them atattractive prices. This has become possible only byadopting automatic gauging for selective assemblywhereby parts manufactured to rather widetolerances fit and function as though they were

precisely manufactured in precision laboratory tovery close tolerances. This is a concept which doesaway with old idea of inspection in which partis identified as good or bad; goodpart being used forassembly and bad used to be scraped. Inselective assembly the components produced by amachine are classified into several groups

according to size. This is done both for hole and shaftand then the corresponding groups will matchproperly.If some parts (shaft and holes) to be assembled aremanufactured to normal tolerances of


5/8

0.01 mm (and both are within the curve of normaldistribution), an automatic gauge can segregatethem into ten different groups with a 0.001 mm limit for

selective assembly of the individual parts.Thus parts with tolerances of 0.0001 mm are obtained(due to segregation) and both theconditions of high quality and low cost can be servedby selective assembly technique. However, itis very important that the two component parts to befitted together must be kept within the normal

distribution, i.e. the central or mean value should be atdesired calculated value and the processcapability of two machines producing shafts and holesmust be identical otherwise for somecomponents the mating components will not beavailable. If not so, then other techniques need tobe adopted which are described below :

Fig. 4.7 shows a case in which the process capability ofboth shaft and hole producingmachines is same but tolerances on parts are desiredas one-tenth of process capability of machines.In such a case the parts are segregated by automaticinspection into ten groups and parts in shaftregion Si are matched with parts in hole region Hh S2

with H2 and so on. This results in matching of parts having tolerances l/10th of machine capability.In this case as the process capability of bothmachines is same, equal number of parts are availablein each segregated zone and no wastage will


6/8

be there.Fig. 4.8 shows another case in which the processcapability of hole making machine is much

wider than the tolerance of part but shaft makingmachine can produce component to the desiredtolerance. In such a case the parts with hole aresegregated into adequate number of groupsdepending on the desired tolerance.

This curve when broken into several groups enables to

determine the number of components likelyto be produced in each group. The shafts aretherefore so produced that their setting needs to bechanged as many number of times as there are groupsformed for the holes and at the mean value

Shaft production (Area under curve dictatesnumber of partsto be produced for each setting)Fig. 4.8of each sub-group. The number of shafts to beproduced for each setting is determined by earliercurve for holes.

Another typical case can be when the process
http://lh6.ggpht.com/_X6JnoL0U4BY/S0zCIx8yzuI/AAAAAAAAD44/ZXtUittlFjc/s1600-h/tmp6810_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S0zCHrme92I/AAAAAAAAD4w/oRrAml2i4ww/s1600-h/tmp689_thumb1.pnghttp://lh6.ggpht.com/_X6JnoL0U4BY/S0zCIx8yzuI/AAAAAAAAD44/ZXtUittlFjc/s1600-h/tmp6810_thumb1.pnghttp://lh5.ggpht.com/_X6JnoL0U4BY/S0zCHrme92I/AAAAAAAAD4w/oRrAml2i4ww/s1600-h/tmp689_thumb1.png


7/8

capability of both shaft producing and holemaking machines are different and much wider than thepermissible tolerances. In such a case the

process having worst process capability is broken intoadequate number of sub-groups as describedbefore. The other process is then broken into adequategroups with their set points so adjusted thatthe number of components produced in this processcorresponding to various groups made in earlierprocess will be same and as such no wastage will be

there.It will be noted that selective assembly technique canbe followed for latter two cases alsobut is little complicated and the first case is morepreferred one for selective assembly.It would now be obvious that the selectively assemblyis based on the natural distribution

of components produced by a machine ; and propermeans being available to ensure mean valueand the dispersion of the process and the segregationof parts by automatic gauging. Thus machineproduces parts of wider tolerance (cost being low) butfor assembly, parts of one-tenth of toleranceof machine required for optimum performance are

available. This is the best and cheapest methodof assembling parts and is widely used in industry.Selective assembly is often followed in air craft,automobile industries and in ball and roller bearing unitas the tolerances desired in such industries


8/8

are very narrow and not possible by any sophisticatedmachine at reasonable costs. The selective

assembly, however enables such tolerances to bearranged without their actually being producedas discussed above.

Limits of SizeThere are three considerations in deciding the limitsnecessary for a particular dimension,viz. functional requirement (function of the component,what it is required to do), interchangeability(ease of replacement in the event of failure), and

economics (minimisation of production time andcost). As already pointed earlier, the degree oftolerance thus calls for a compromise. To assist thedesigner in his choice of limits and fits, and toencourage uniformity throughout, a number ofstandards on limit and fit system have been publishedwhich must be strictly followed.

A limit system consists of a series of tolerancesarranged to suit a specific range of sizes andfunctions, so that limits of size may be selected andgiven to mating components to ensure specificclasses of fit.
http://lh6.ggpht.com/_X6JnoL0U4BY/S0zCKOvBExI/AAAAAAAAD5A/8eQph6WDTgM/s1600-h/tmp6811_thumb1.png

interchange ability

Documents