limit tolerance fits by s k mondal

8/10/2019 Limit Tolerance Fits by S K Mondal

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,

By SKMondal

Terminology

Nominal size: Size of a part specified in the drawing

Basic size: Size of a part to which all limits ofvariation (i.e. tolerances) are applied.

Actual size: Actual measured dimension of the part.

The difference between the basic size and the actual

size should not exceed a certain limit, otherwise it will

interfere with the interchangeability of the mating

parts.

Terminology Contd....Limits of sizes: There are two extreme permissible

sizes for a dimension of the part. The largest

permissible size for a dimension is called upper or high

or maximum limit, whereas the smallest size is known

as lower or minimum limit.

Allowance: Minimum clearance or maximum

interference. It is the intentionaldifference between

the basic dimensions of the mating parts. Theallowance may be positive or negative.

Terminology Contd....Tolerance

The difference between the upper limit and lower

limit.

dimension.

The tolerance may be unilateral or bilateral.

Unilateral Limitsoccurs when both maximum limit andminimum limit are eitherabove or below the basic size.

e.g. 25 +0.18+0.10

Basic Size = 25.00 mm

U er Limit = 25.18 mm

Terminology Contd....

.Lower Limit = 25.10 mm

Tolerance = 0.08 mm

e.g. 25 -0.10-0.20

Basic Size = 25.00 mm

Upper Limit = 24.90 mm

Lower Limit = 24.80 mm

Tolerance = 0.10 mm

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For Unilateral Limits, a case may occur when one of thelimits coincideswith the basic size,

e.g.

25

+0.20 ,

25

0

0 0.10


e.g. 250.04

Basic Size = 25.00 mmUpper Limit = 25.04 mmLower Limit = 24.96 mmTolerance = 0.08 mm

BilateralLimits occurwhenthemaximumlimitisaboveandtheminimumlimitisbelowthebasicsize.

Terminology Contd....Zero line: A straight line corresponding to the basic

size. The deviations are measured from this line.

Deviation: Is the algebraic difference between a size

actual max. etc. and thecorres ondin basic size.,

Actual deviation: Is the algebraic difference between

an actual size and the corresponding basic size.

Upper deviation: Is the algebraic difference between

the maximum size and the basic size.


Lower deviation: Is the algebraic difference between

the minimum size and the basic size.

Mean deviation: Is the arithmetical mean of upper

an ower eviations.

Fundamental deviation:This is the deviation, either

the upper or the lower deviation, which is nearest one

to zero line foreither a hole or shaft.

Fits:(assemblyconditionbetweenHole&Shaft)

Hole Afeatureengulfing acomponent

Shaft Afeaturebeingengulfed bya

Fit

component

Hole

Shaft

Min C Max C

Clearance Fits

Tolerancezonesnevermeet

Max. C= UL of hole - LL of shaft

Min. C = LL of hole - UL of shaft

Theclearancefitsmaybeslidefit,easyslidingfit,runningfit,slackrunningfitandlooserunningfit.

ShaftMax I

Max C

Hole

Transition Fits

Tolerancezonesalwaysoverlap

Max. C= UL of hole - LL of shaft

Max. I = LL of hole - UL of shaft

Thetransitionfitsmaybeforcefit,tightfitandpushfit.

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Shaft Min I

Max I

Hole

Interference Fits

Tolerancezonesnevermeetbutcrosseseachother

Max. I = LL of hole - UL of shaft

Min. I = UL of hole - LL of shaft

Theinterferencefitsmaybeshrinkfit,heavydrivefitandlightdrivefit.

It is defined graphically

by the magnitude of the

tolerance and by its

osition in relationto the

Tolerance Zone

m

55

Tolerance Zone

zero line.

Basic Size

5. BasisofFitsHoleBasis

Inthissystem,thebasicdiameteroftheholeisconstant

whiletheshaftsizevariesaccordingtothetypeoffit.

CT I

This system leads to greater

economy of production, as a single

drill or reamer size can be used to

Hole Basis Fits

C - Clearance

T - Transition

I - Interference

Hole

Shaft

Tolerance

Legends:

pro uce a var e y o s y mere y

altering the shaft limits.

The shaft can be accurately

produced to size by turning and

grinding.

Generally it is usual to recommend

hole-base fits, except where

temperature may have a

detrimental effect on large sizes.

BasisofFitsShaftBasis

Aseriesofdrillsandreamersisrequiredforthissystem,thereforeittendstobecostly.

Itmay,however,benecessarytouseitwheredifferentfitsare

C TI

Here the hole size is varied to

produce the required class of fit with a

basic-size shaft.

requiredalongalongshaft.Forexample,inthecaseofdrivingshaftswhereasingleshaftmayhavetoaccommodatetoavarietyofaccessoriessuchascouplings,bearings,collars,etc.,itispreferabletomaintainaconstantdiameterforthepermanentmember,whichistheshaft,andvarytheboreof

theaccessories.

Shaft Basis Fits

Legends:

Hole

Shaft

Tolerance

C - Clearance

T - Transition

I - Interference

IS:LimitsandFits

Limits and fits comprises 18 grades of fundamental

tolerances

There are 25 types of fundamental deviations.A unilateral hole basis system is recommended but if

necessary a unilateral or bilateral shaft basis system

may also be used.

The 18 tolerance grades are designated as IT01, IT0 and

IT1to IT16. These are called standard tolerances.

The standard tolerances for grades IT5 to IT16 aredetermined in terms of standard tolerance unit (i) inmicrons, where

IS:LimitsandFits Contd...

whereDisthesizeorgeometricmeandiameterinmm.

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For hole, H stands for a dimension whose lowerdeviation refers to the basic size. The hole H for whichthe lower deviation is zero is called a basic hole.

Similarly, for shafts, h stands for a dimension whoseupper deviation refers to the basic size. The shaft h for

which the upper deviation is zero is called a basicshaft.

A fit is designated by its basic size followed by symbolsrepresenting the limits of each of its two components,the hole being quoted first.

For example, 100 H6/g5 means basic size is 100 mmand the tolerance grade for the hole is 6 and for theshaft is 5.

IES2006ConventionalFind the limit sizes, tolerances and allowances for a100 mm diameter shaft and hole pair designated byF8h10. Also specify the type of fit that the above pairbelongs to. Given: 100 mm diameter lies in thediameter ste ran e of 80120 mm. The fundamentaldeviation for shaftdesignation f is 5.5 D0.41

The values of standard tolerances for grades of IT 8and IT 10 are 25i and 64i respectively.

Also, indicate the limits and tolerance on a diagram.[15

Marks]

ToleranceSinkA design engineer keeps one section of the part blank

(without tolerance) so that production engineer can

dump all the tolerances on that section which becomes

most inaccurate dimension of the part.

Position of sink can be changing the reference point.

Tolerance for the sink is the cumulative sum of all the

tolerances and only like minded tolerances can be addedi.e. either equally bilateral or equally unilateral.

GATE2003 LimitGaugesPluggauge:usedtochecktheholes.TheGOpluggaugeis

thesizeofthelowlimitoftheholewhiletheNOTGOpluggaugecorrespondstothehighlimitofthehole.

Snap,GaporRinggauge:usedforgaugingtheshaftand

male

components.

The

Go

snap

gauge

is

of

a

size

correspon ngtot e g max mum m to t es a t,whiletheNOTGOgaugecorrespondstothelow(minimumlimit).

Fig.Pluggauge

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Allocationofmanufacturing

tolerancesUnilateralsystem:gaugetolerancezoneliesentirely

withinthe

work

tolerance

zone.

worktolerancezonebecomessmallerbythesumof.

Bilateralsystem:inthissystem,theGOandNOGOgaugetolerancezonesarebisectedbythehighandlowlimitsoftheworktolerancezone.

Takingexampleasabove:

Wearallowance:GOgaugeswhichconstantlyrubagainstthesurfaceofthepartsintheinspectionaresubjectedtowearandloosetheirinitialsize.

Thesizeofgopluggaugeisreducedwhilethatofgosnapgaugeincreases.

Toincreaseservicelifeofgaugeswearallowanceisaddedtothegogaugeinthedirectionoppositeto

wear.Wearallowanceisusuallytakenas5%oftheworktolerance.

Wearallowanceisappliedtoanominaldiameter

before

gauge

tolerance

is

applied.

Takingexampleofabove:

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MeasurementofLines&Surfaces

By SKMondal

LinearmeasurementsSome of the instruments used for the linearmeasurements are:

Rules

Vernier

Micrometer

Height gauge

Bore gauge

Dial indicator

Slip gauges or gauge blocks

Bore Gauge: used for measuring bores of differentsizes ranging from smalltolarge sizes.

Provided with various extension arms that can beadded for different sizes.

Dial indicator: Converts a lineardisplacement into a radialmovement to measure over asmall range of movement for theplunger.

The t ical least count that can bobtained with suitable gearingdial indicators is 0.01 mm to 0.001mm.

It is possible to use the dialindicator as a comparator by

mounting it on a stand at anysuitable height. Principleofadialindicator

Applications ofdialindicatorinclude:

centering workpices tomachinetoolspindles

offsettinglathetailstocks

a gnngav ceonam ngmac ne

checkingdimensions

SlipGaugesorGaugeblocks:Used in the manufacturing shops as length standards.

Not to be used for regular and continuousmeasurement.

Rectangular blocks with thickness representing themens on o e oc . e crosssec on o e ocis usually 32 mm x 9 mm.

Are hardened and finished to size. The measuringsurfaces of the gauge blocks are finished to a very highdegree of finish, flatness and accuracy.

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Come in sets with different number of pieces in a given

set to suit the requirements of measurements.

A typical set consisting of 88 pieces for metric units is

.

To build any given dimension, it is necessary to

identify a set of blocks, which are to be put together.

Numberof blocks used should always be the smallest.

Metricslipgaugeset

Example:

ComparatorsComparator is another form of linear measuring

method, which is quick and more convenient forchecking large number of identical dimensions.

During the measurement, a comparator is able to givethe deviation of the dimension from the set dimension.

Cannot measure absolute dimension but can onlycompare two dimensions.

Highly reliable.

To magnify the deviation, a number of principles areused such as mechanical, optical, pneumatic and

electrical.Fig. Principleofacomparator

MechanicalComparatorsThe Mikrokator principle

greatly magnifies anydeviation in size so thateven small deviations

produce large deflections ofthe pointer over thescale.

MechanicalandOptical

Comparators

The EdenRolt Reed system uses a

pointer attached to the end of two

.

plunger, while the other is fixed. As

one reed moves relative to the other,

the pointer that they are commonly

attached to will deflect.

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SineBar

When a reference for a nonsquare angle is required, a

sine bar can be used. Basically a sine bar is a bar of known length. When

gauge blocks are placed under one end, the sine bar

will tilt to a specific angle.

Knowing the height differential of the two rollers in

alignment with the workpiece ,the angle can be

calculated using the sine formula.

ThreadMeasurementsThreadsarenormallyspecifiedbythemajordiameter.

Thoughtherearealargevarietyofthreadsusedinengineering,themostcommonthreadencounteredisthemetricVthreadshowninFig.

Theparametersthatarenormallymeasuredare:

Majordiameter

Micrometer

Pitchdiameter

Screwthreadmicrometer

Wiremethod

Pitch

Screwpitchgauge

Pitchmeasuringmachine

ThreadformOpticalprojector

Surfaces

No surface is perfectly smooth, but the better the

surface quality, the longer a product generally lasts,

and the better is performs.

Surface texture can be difficult to analyse

quantitatively.

Two surfaces may be entirely different, yet still provide

the same CLA (Ra) value.

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Surfacegeometrycanbequantifiedafewdifferentways.

Realsurfacesarerarelysoflat,orsmooth,butmostcommonlyacombinationofthetwo.

Roughness height: is the parameter with whichgenerally the surface finish is indicated. It is specifiedeither as arithmetic average value or the root meansquare value.

Roughness width: is the distance parallel to thenom na part sur ace w t n w c t e pea s an

valleys, which constitutes the predominant pattern ofthe roughness.

Roughness width cutoff: is the maximum width ofthe surface that is included in the calculation of theroughness height.

Waviness:refers to those surface irregularities that havea greater spacing than that of roughness width.

Determined by the height of the waviness and itswidth.

The greater the width, the smoother is the surface and.

Lay direction: is the direction of the predominantsurface pattern produced on the workpiece by the toolmarks.

Flaw:are surface irregularities that are present which arerandom and thereforewill not be considered.

Diagram Symbol Description

Parallel lay: Lay parallel tothe Surface. Surface isproduced by shaping,planning etc.

Lay

perpendicu lar to theSurface. Surface is producedby shaping and planning

Crossedlay:Layangularinbothdirections.Surface is produced byknurling, honing.

Diagram Symbol Description

Multidirectional lay: Laymultidirectional. Surface isproduced by grinding,lapping, super finishing.

Lay Contd..

Approximately circularrelative to the center.Surface is produced byfacing.

Radiallay:Approximatelyradialrelativetothecenterofthenominalsurface.

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RepresentationofSurfaceRoughness

Roughness RoughnessGradeNumber

RoughnessSymbol

50 N12

25 N11

12.5 N10

6.3 N9

3.2 N8

( )a

R m

1.6 N7

0.8 N6

0.4 N5

0.2 N4

0.1 N3

0.05 N2

0.025 N1

Waviness height the distance from a peak to a valley

Waviness width the distance between peaks orvalleys

Roughness width cutoff a value greater than themaximum roughness width that is the largestsepara on o sur ace rregu ar es nc u e n emeasurements. Typical values are (0.003, 0.010,0.030, 0.100, 0.300)

Lay the direction the roughness pattern shouldfollow

Stylus travel is perpendicular to the lay specified.

EvaluationofSurfaceRoughness

1. Centre line average (CLA) or arithmetic mean

deviation denoted as Ra.

2. Root mean s uare value R : rms value

3. Maximum peak to valley roughness (hmax)

4. The average of the five highest peak and five deepst

valleys in the sample.

5. The average or leveling depth of the profile.

DeterminationofMeanLine

MSystem: After plotting the characteristic of anysurface a horizontal line is drawn by joining two points.This line is shifts up and down in such a way that 50%

area is above the line and 50% area is below the line

DeterminationofMeanLine

ESystem: (Envelop System) A sphere of 25 mmdiameter is rolled over the surface and the locus of itscentre is being traced out called envelope. This envelope

is shifted in downward direction till the area above thene s equa o e area e ow e ne. s s ca emean envelope and the system of datum is called Esystem.

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Arithmetical Average:

Measured for a specified area and the figures are addedtogether and the total is then divided by the number of

measurements taken to obtain the mean orarithmetical average (AA).

CLA value. This in equation form is given by

Theotherparameterthatisusedsometimesistherootmeansquarevalueofthedeviationinplaceofthearithmeticaverage,Thisinexpressionformis

Fig.Surfaceroughnessparameters

MethodsofmeasuringSurfaceRoughness

There are a number of useful techniques for measuring

surface roughness:

Observation and touch the human finger is very

perceptive to sur ace roug ness

stylus based equipment very common

Interferometryuses light wave interference patterns

(discussed later)

ObservationMethods

Human perception is highly relative.

To give the human tester a reference for what they are

touching, commercial sets of standards are available.

Comparison should be made against matched

identical processes.

One method of note is the finger nail assessment of

roughness and touch method.

StylusEquipmentuses a stylus that tracks small changes in surface

height, and a skid that follows large changes in surfaceheight.

The relative motion between the skid and the stylus is.

One example of this is the Brown & Sharpe Surfcomunit.

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Profilometer

measuring instrument used to measure a surface'sprofile, in order to quantify its roughness.

Vertical resolution is usually in the nanometre level,though lateral resolution is usually poorer.

ContactprofilometersAdiamondstylusismovedverticallyincontactwitha

sampleandthenmovedlaterallyacrossthesampleforaspecifieddistanceandspecifiedcontactforce.

Aprofilometercanmeasuresmallsurfacevariationsin.

Theradiusofdiamondstylusrangesfrom20nanometresto25m.

NoncontactProfilometers

An optical profilometer is a noncontact method for

providing much of the same information as a stylus

based rofilometer.

There are many different techniques which are

currently being employed, such as laser triangulation

(triangulation sensor), confocal microscopy and digital

holography.

AdvantagesofopticalProfilometers

Because the noncontact profilometer does not touch

the surface the scan speeds are dictated by the light

reflected from the surface and the speed of the

acquisition electronics.

Optical profilometers do not touch the surface and

therefore cannot be damaged by surface wear or

careless operators.

OpticalFlatsOpticalgrade glass structures lapped and polished to

beextremely flaton one or both sides.

Used with a monochromatic light to determine theflatness of other optical surfaces by interference.

en a a sur ace o ano er op c s p ace on eoptical flat, interference fringes are seen due tointerference in the tiny gap between the two surfaces.

The spacing between the fringes is smaller where thegap is changing more rapidly, indicating a departurefrom flatness in one of the two surfaces, in a similar

way to the contour lines on a map.

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Whenthefringesareperfectlystraightandsamefringe

widthfordarkandbrightbandweconcludethatthe

surfaceisperfectlyflat.

orconvexsur ace e r ngescurvearoun epo n

ofcontact.

Forconcavesurfacethefringescurveawayfromthe

pointofcontact.

TalysurfIt is based upon measuring the generated noise due to

dry friction of a metallic blade which travels over thesurface under consideration.

If the frictional force is made small enough to excite, ,

will be proportional to surface roughness, andindependent of the measured specimen size andmaterial.

The specimen surface roughness was measured by awidely used commercial instrument (Talysurf 10), and

the prototype transducer.

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By SKMondal

ClinometerAn optical device for measuring elevation angles above

horizontal.

Compass clinometers are fundamentally just magneticcompasses held with their plane vertical so that a

the sight line.

The clinometer can read easily and accurately angles ofelevation that would be very difficult to measure in anyothersimple and inexpensive way.

A fairly common use of a clinometer is to measure theheight of trees.

Clinometer AutocollimatorAn optical instrument for noncontact measurement

of angles.

Used to align components and measure deflections inoptical or mechanical systems.

An autocollimator works by projecting an image onto atarget mirror, and measuring the deflection of thereturned image against a scale, either visually or bymeans of an electronic detector.

A visual autocollimator can measure angles as small as

0.5 arcsecond, while an electronic autocollimator canbe up to 100 times more accurate.

Visualautocollimatorsareusedforlininguplaserrodendsandcheckingthefaceparallelismofoptical

windowsandwedges.

Electronicanddigitalautocollimatorsareusedasanglemeasurementstandards,formonitoringangular

angularpositionrepeatabilityinmechanicalsystems.

Servoautocollimatorsarespecializedcompactformsofelectronicautocollimatorsthatareusedinhighspeedservofeedbackloopsforstableplatformapplications.

Autocollimator

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LaserScanningMicrometerThe LSM features a high scanning rate which allows

inspection of small workpiece even if they are fragile,

at a high temperature, in motion or vibrating.

Applications :

Measurement of outer dia. And roundness ofcylinder,

Measurement of thickness of film and sheets,

Measurement of spacing if IC chips,

Measurement of forms,

Measurement of gap between rollers.

McLeodgaugeUsed to measure vacuum by application of the

principle of Boyle's law.

Works on the principle, "Compression of knownvolume of low pressure gas to higher pressure andmeasurin resultin volume & ressure one cancalculate initial pressure using Boyle's Law equation."

Pressure of gases containing vapours cannot normallymeasured with a McLeod gauge, for the reason thatcompression will cause condensation .

A pressure from 0.01 micron to 50 mm Hg can bemeasured. Generally McLeod gauge is used forcalibration purpose.

PlanimeterAdeviceusedformeasuringtheareaofanyplane

surfacebytracingtheboundaryofthearea.

LVDTAcronym for Linear Variable Differential Transformer,

a common type of electromechanical transducer thatcan convert the rectilinear motion of an object to

which it is coupled mechanically into a corresponding.

LVDT linear position sensors are readily available thatcan measure movements as small as a few millionths ofan inch up to several inches, but are also capable ofmeasuring positions up to 20 inches (0.5 m).

Arotary variable differential transformer (RVDT)is a type of electrical transformer used for measuringangular displacement.

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LVDT ToolMakersMicroscopeAn essential part of engineering inspection,measurement and calibration in metrology labs.

Hence is used to the following:

Examination of form tools, plate and templategauges, punc es an es, annu ar groove anthreaded hobs etc.

Measurement of glass graticules and other surfacemarked parts.

Elements of external thread forms of screw pluggauges, taps, worms and similar components.

Shallow bores and recesses.

TelescopicGaugesUsed to measure a bore's size, by transferring the

internal dimension to a remote measuring tool.

They are a direct equivalent of inside callipers andrequire the operator to develop the correct feel toobtain repeatable results.

CoordinateMeasuringMachine

(CMM)Aninstrumentthatlocatespointcoordinatesonthree

dimensionalstructuresmainlyusedforqualitycontrolapplications.

e g ysens t vemac nemeasuresparts owntothefractionofaninch.

Specifically,aCMMcontainsmanyhighlysensitiveairbearingsonwhichthemeasuringarmfloats.

Advantages,

canautomateinspectionprocess

lesspronetocarelesserrors

allowsdirectfeedbackintocomputersystem

,

Costly

fixturing iscritical

requiresaverygoodtolerancemodel

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TransportEquipmentTransport equipment is used to move material from

one location to another (e.g., between workplaces,

between a loading dock and a storage area, etc.) withina facilityor at a site.

Themajorsubcategoriesoftransportequipmentare:

Conveyors:usedtomovematerialsoverafixedpathbetweenspecificpoints.

Cranes:usedtomovematerialsovervariablepathswithinarestrictedarea.

Industrialtrucks:usedtomovematerialsovervariablepaths,withnorestrictionsontheareacoveredbythemovement(i.e.,unrestrictedarea).

Noequipment:Materialcanalsobetransportedmanuallyusingnoequipment.

ConveyorsystemA common piece of mechanical handling equipment

that moves materials from one location to another.

Useful in applications involving the transportation ofheavy or bulky materials.

Allow quick and efficient transportation for a widevariety of materials, which make them very popular inthe material handling and packaging industries.

Many kinds of conveying systems are available, and areused according to the various needs of different

industries.

TypesofconveyorsGravityrollerconveyor

Gravityskatewheelconveyor

Beltconveyor

Pneumaticconveyor

Beltdrivenliverollerconveyors

Chainconveyor

remes conveyors

Plasticbeltconveyors

Bucketconveyors

Flexibleconveyor

Verticalconveyor

Spiralconveyor

Vibratingconveyor

crewconveyor

Chaindrivenliverollerconveyor

Overheadconveyor

Dustproofconveyors

Pharmaceuticalconveyors

Automotiveconveyors

ChuteConveyorInexpensive

Usedtolinktwohandlingdevices

Usedtoprovideaccumulationinshippingareas

Usedto

convey

items

between

floors

Difficulttocontrolpositionoftheitems

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GravityrollerconveyorAlternativetowheelconveyor

Forheavydutyapplications

Slopeforgravitymovementdependsonloadweight

Foraccumulatingloads

Live(Powered)RollerConveyorBeltorchaindriven

Forcesensitivetransmissioncanbeusedtodisengagerollersforaccumulation

Foraccumulatingloadsandmerging/sortingoperations

Provideslimitedinclinemovementcapabilities

MonorailconveyorUnit+Overhead+Accumulate

Overheadsingletrack(i.e.,monorail)ortracknetworkonwhichoneormorecarriersride

Carriercanrangefromasimplehooktoahoisttoanintelligentvehiclelikedevice

Singlecarrier,singletrackmonorailsimilartobridgeorgantrycrane

Monorailconveyor

BucketconveyorBulk+OnFloor

Usedtomovebulkmaterialsinaverticalorinclinedpath

Bucketsareattachedtoacable,chain,orbelt

conveyorrun

BeltconveyorUnit+OnFloor+NoAccumulate

Fortransportinglightandmediumweightloadsbetweenoperations,departments,levels,and

buildingsWhenaninclineordeclineisrequired

Providesconsiderablecontrolovertheorientationandplacementoftheload.

Nosmoothaccumulation,merging,andsortingonthebelt

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Beltconveyor KilledsteelDefinition: Thetermindicatesthatthesteelhasbeencompletelydeoxidisedbytheadditionofanagent

suchas

silicon

or

aluminium,

before

casting,

so

that

thereispracticallynoevolutionofgasduring.

Killedsteelsarecharacterisedbyahighdegreeofchemicalhomogeneityandfreedomfromporosity.

DyePenetrant InspectionDefinition:AqualitycontrolandNDEtechnique

usedforcrackdetectionandotherdiscontinuitiesonthesurfaceinnonmagneticalloys.

Aftersoakinginmetalpenetrant,thesuspectedpartofma er a o e es e sc eane , r e an r e yimmersedindeveloper.

Thepenetrantwillbleedtothesurfaceindicatingthepresenceofanimperfection

TransferMachinesAtransfermachineisanautomatedflowline.

Workpiecesareautomaticallytransferredfromstationtostation,fromonemachinetoanother.

Operationsareperformedsequentially.

Ideally,workstationsperformtheoperation(s)simultaneouslyonseparateworkpieces,withthenumberofpartsequaltothenumberofstations.Eachtimethemachinecycles,apartiscompleted.

ScrewThreadStandardizationThe size listings of metric threads start with " M and

continue with the outside diameter in millimeters.

Most ISO metric thread sizes come in coarse, medium,and fine pitches.

When a coarse thread is desi nated it is not necessarto spell out the pitch.

Example: A coarse 10mmOD thread is called out as"M 12." This thread has a pitch of 1.75 mm, but thepitch may be omitted from the callout.

A fine 12mmOD thread is available. It has a 1.25mmpitch and must be designated "M 12 x 1.25." An extrafine 12mmOD thread having a 0.75mm pitch wouldreceive the designation "M 12 x 0.75.

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Fig.Majorelementsofjigandfixtures

321LocatingPrincipleA workpiece, just like any free solid body, has six

degrees of freedom (some researchers have referred

this to the twelve degrees of freedom by consideringthe +/movements in each category)

For locating it is necessary to arrest all these six degreesof freedom to ensure the mechanical stability.

A single locator in Plane 1 would arrest the linearmotion along the Xaxis.

A second locator in the same plane would arrest therotary motion about the Zaxis.

Another locator placed in the same plane would arrestthe rotary motion about the Yaxis.

Adding one more locator in Plane 1 would not serve anypurpose.

So fourth locator is placed in Plane 2 which isperpendicular to Plane 1. This would restrict the linearmotion along the Yaxis.

The fifth locator is placed in the Plane 2 which canarrest the rotational motion about the Xaxis.

The sixth locator placed in Plane 2 would not serve anypurpose.

So, sixth locator is placed in Plane 3 which is

perpendicular both the planes 1 and 2. This wouldarrest the linear motion along the Zaxis.

Fig.Acomponen w sixlocators

Considering12DOFYou must fix all the 12 degrees of freedom except the three

transitional degrees of freedom (X,Y andZ) in order tolocate the work piece in the fixture. So, 9 degrees offreedomof the work piece need to be fixed.

Rest the work piece on three non

collinear points of the,the +Z, CROTX, ACROTX, CROTY and ACROT

Ydegrees of freedom.

Now, rest the work piece attwopoints of side surface (XZ),and you will be able to fix the+YandACROTZdegrees offreedom.

Now, rest the work piece at one point of the adjacentsurface (YZ), and you will be able to fix the +XandCROTZ degrees of freedom.

Pointstoponder

When more than one locator is placed on a surface

(plane), they should be distributed as far apart as

possible on the surface.

While selecting the surface for the largest locators,

consideration should be given to the largest area of the

workpiece.

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DifferentmethodsusedforLocation

Flat Locator : Used for location of flat machinedsurfaces of the component.

Cylindrical Locators: Used for locating componentshaving drilled holes.

The cylindrical component to be located is gripped by a

cylindrical locator fitted to the jigs body and insertedin the drilled hole of the component.

Conical Locator : Used for locating the workpieceshaving cylindrical hole.

It is superior as it has a capacity to accommodate aslight variation in the hole diameter of the component

without affecting the accuracy of location.

Jack Pin Locator : Used for supporting rough

workpieces.

A suitable method to accommodate the

.

Drill Bush Locator : Used for holding and locatingcylindrical workpieces.

The bush has conical opening for locating purposeand it is sometimes screwed on the jigs body for

the adjustment of height of the work.

Vee Locators: Quick and effective method of locating

the workpiece.

Used for locating the circular and semicircular type of

.

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ScrewClampsA much faster way of applying clamping is to make use

of either a swing washer or a ceewasher if the

workpiece has a bore for clamping.A swing washer can be used to clamp a part having a

.

This helps in loading and unloading the part quickly.

The only condition is that the hole used for clappingshould be larger than the nut used for clamping.

A ceewasher is similar to a swing washer, whichremains loose unlike a swing washer. Otherwise,application is very similar.

Fig.

A

swing

washer

used

for

clampingapartwithhole

CamClampsProvide clamping force because of the contour of the cam

surface that comes into contact with the plate used for theclamping.

Plate is pushed down by the cam against the springpressure to hold the part in place.

Cam clamps are quick in operation. Cam c amps are o t ree types, eccentr c cam, at sp ra

cam and cylindrical cam. The design shown in Fig. is flat spiral and is the most

commonly used clamp.

Fig.Acamclampusedforquickandeasy

clampingapart

The design shown is indirect pressure clamping wherethe pressure is transmit to the part through the plate.This is more stable and the vibrations duringmachining do not affectthe a part clamping.

Fig.Anexampleofafixtureheldbyacamclamp

ToggleClampsA toggle clamp is a quick acting mechanical linkage where

twoof the elements make up a toggle action.

Toggle clamps are mainly used because of their fast actionfor clamping and unclamping, their ability to completely

clear the work piece and the force Fixture amplification.

Fig.Apushpulltypetoggleclamp

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EqualizersWhen the clamping force is to be applied at more than

one location then an equalizing clamp is useful. In this

type of clamp the link arm system is being used toapply an equally divided clamping force to a pair ofclam s actin on the sam com onent. It is alsopossible to use this system of clamping to clamp twoparts.

This is particularly useful in a condition where theoperator may be denied easy access to one or other ofthe clamps.

Fig.Anequalizingclamp

JigsandFixtures

Q. No Option Q. No Option

1 C 6 C

3 B 8 D

4 B 9 C

5 C

limit tolerance fits by s k mondal

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