metrology-KSR-1.2

8/10/2019 metrology-KSR-1.2

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METROLOGY & INSTRUMENTAION

K.Srinivasulu ReddyDepartment of Mechanical Engineering

Sreenidhi Institute of Science & Technology

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What is Metrology?

Metrology is the science of measurement ofdimensions, and measurement is the language of

science.

If science is measurement, then without

metrology, there is no science.

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Measurementcan be defined as the determinationof a dimension


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History:

Measurements have been carried out by humans for as

long as civilization has existed. From the primitive populationwho lived in caves to modern man, the need has always

been there to measure and know.

The standard of length evolved from the foot of the

"King", to the Egyptian Royal cubit, to the metallic

metre(1960)and then monochromatic highly stabilized light

source or speed of light in 1983.

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1 royal cubit = 7 palms = 28 fingers


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The Metrebar served as standard until 1960when the metre was

redefined in terms of the wavelength of light emitted by

the krypton-86 isotope.

Historical International Prototype

Metrebar, made of an alloy of

platinum and iridium, that was the

standard from 1889 to 1960.

metre(meterin the US)

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Metre is defined as the length of the path travelled by light in

vacuum in 1/299,97,92458seconds. This can be realized in practice

through the use of an iodine-stabilised helium-neon laser.

The metre was redefined yet again in 1983in terms of the speed of

light.


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Study of metrology is important

Bearings: Shaft in the bush is of

improper dimensions which results

insufficient thin film, and hence friction,

wear, lubrication aspects etc.


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ACCIDENT OF ALASKA AIRLINES FLIGHT 261

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Why metrology is important?

The most serious process error resulted in the loss of Alaska

Airlines on January 31,2000 with 83 passengers.


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Excessive thread wear on the jackscrewassembly resulted in loss

of the horizontal stabilizer.

The mechanic work card stated that thread wear was "withinallowable limits. In fact, the threads on the jackscrew nut were

almost completely worn away.

The process (fixtures) used by the mechanic were not what

Boeing specified and therefore the measurement results were

different and 83 people+crew lost their lives!

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Recovered jackscrew - the spiral 'wire'

wound around the threaded portion is theremains of the acme nut internal screw

thread that has been stripped from the

nut, which, freeing the jackscrew.

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Random procedures produce random

results.


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Subfield Definition

Scientific or

fundamental metrology

concerns the establishment of quantity systems, unit

systems, units of measurement, the development of

new measurement methods.

Applied or

industrial metrology

concerns the application of measurement science to

manufacturing and other processes and their use in

society, ensuring the suitability of measurement

instruments, their calibration and quality control of

measurements.

Legal metrology

concerns regulatory requirements of measurements

and measuring instruments for the protection of

health, public safety, the environment, protection of

consumers and fair trade.

Types of Metrology

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Process of measurement:

1.Measurand 2.Reference 3.Comparator

1.Measurand: Measurand is the physical quantity or property like

length, angle, diameter, thickness etc. to be measured.

2.Reference: It is the physical quantity or property to which

quantitative comparisons are made.

3.Comparator: It is the means of comparing measurand with some

reference

Ex: Fitter has to measure MS flat with steel rule

1.Aligns the zero end of steel rule with one end of MS flat.

2.Compares the length of flat with the graduations on the rule by his eyes.

Here,

length of MS plate is measurand, steel rule is reference and eye is

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6.Sensitivity: The smallest change in a measurement that an

instrument is capable of detecting.

Sensitivity refers to the ability of measuring device to detect smalldifferences in a quantity being measured.

Sensitivity may be defined as the rate of displacement of the

indicating device of an instrument, with respect to the measuredquantity.

Sensitivity= scale spacing/scale division value

Ex: In dial indicator, scale spacing is 1.0 mm and scale division valueis 0.01 mm

Sensitivity= 1/0.01=100= Amplification factor =gearing ratio

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Which is more sensitive?


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7.Calibration: The comparisonof a device with unknown accuracy to a

device with a known, accurate standard to eliminate any variation in

the device being checked.

It is carried out by making adjustments such that the read out device

produces zero output for zero measured input.

Calibration is a premeasurement process, generally carried out bymanufacturers.

The accuracy of an instrument depends on the calibration. Constant

use of instruments affect their accuracy.

If the accuracy is to be maintained, the instruments must be

checked and recalibrated if necessary.

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8.Hysteresis: The delay between the action and reaction of a

measuring instrument.

The phenomenon of hysteresis is due to the presence of dry

frictionas well as the properties of elastic elements.

It results in the loading and unloading curves of the instrument

being separated by a difference called hysteresis error.

It also results in the pointer not returning completely to zero

when the load is removed. Hysteresis is particularly noted in

instruments having elastic elements.

The phenomenon of hysteresis in materials is due mainly to the

presence of internal stresses. It can be reduced considerably by

proper heat treatment.17


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10.Reproducability: is the ability of an entire experimentor study to

be reproduced, either by the researcher or by someone else

working independently.

It is one of the main principles of the scientific method

Reproducibility also refers to the degree of agreement between

measurements or observations conducted on replicate specimens

in different locations by different people, as part of the precisionof

a test method

It may also be expressed quantitatively in terms of the dispersion

of the results.
https://en.wikipedia.org/wiki/Experimenthttps://en.wikipedia.org/wiki/Scientific_methodhttps://en.wikipedia.org/wiki/Replication_(statistics)https://en.wikipedia.org/wiki/Accuracy_and_precisionhttps://en.wikipedia.org/wiki/Test_methodhttps://en.wikipedia.org/wiki/Test_methodhttps://en.wikipedia.org/wiki/Test_methodhttps://en.wikipedia.org/wiki/Test_methodhttps://en.wikipedia.org/wiki/Accuracy_and_precisionhttps://en.wikipedia.org/wiki/Replication_(statistics)https://en.wikipedia.org/wiki/Scientific_methodhttps://en.wikipedia.org/wiki/Scientific_methodhttps://en.wikipedia.org/wiki/Scientific_methodhttps://en.wikipedia.org/wiki/Scientific_methodhttps://en.wikipedia.org/wiki/Experiment


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11.Precision & Accuracy

Precision and accuracy are used in connection with the

performance of the instrument.

Precision is defined as the repeatability of the measuring

process, while the accuracy is the agreement of the result of ameasurement with the true value of the measured quantity.

In most measurements, it is the precision which is of great

importance.

If the instrument is not precise, it will give different results for

the same dimension when measured again and again.


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12.Accuracy:Accuracy is the degree to which the measured value of the

quality characteristic agrees with the true value.

The difference between the true value and the measured value is

know as error of measurement.

It is practically difficult to measure exactly the true value and

therefore a set of observations is made whose mean value is taken as

the true value of the quality measured.

Dimen

sion

Ex: Several measurements are made on a component by different types

of instruments and results are plotted

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Excessive accuracy is a sign of poor breeding - Socrates.


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The most widely known and used of all distributions is the

normal distribution. It fits many human characteristics, suchas height, weight, performance etc.

Many living things in nature, such as trees, animals and

insects have many characteristics that are normallydistributed.

Many variables in business and industry are also normally

distributed.

Normal Distribution(Gaussian Distribution)

very commonly occurring continuous probability

distribution-a function that tells the probability that any real

observation will fall between any two real limits or real

numbers, as the curve approaches zero on either side.


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=

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is Std. deviation


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The formula for Normal distribution


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The set of observations will scatter about the mean. The

scatter of these measurements is designated as sigma(), the

standard deviation,

Standard deviation is used as index of precision. The less the

scattering more preciseis the instrument. Thus ,lower the value

of , the more precise the instrument.

Standard deviation (root mean square deviation) shows how

much variationor dispersion" exists from the average (mean, or

expected value).

A low standard deviation indicates that the data points tend to

be very close to the mean, whereas high standard deviation

indicates that the data points are spread out over a large range

of values. 30

Standard Deviation


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2. Selective or partial interchangeability or selective assembly

Today the consumer not only wants quality, precision and

trouble free products but also he wants them at attractive prices.

This has become possible only by adopting automatic gauging

for selectiveassemblywhereby parts manufactured to rather wide

tolerances fit and function as though they were precisely

manufactured in precision laboratory to very close tolerances.

Parts are graded according to size and only matched grades of

mating parts are assembled

In selective assembly the components produced by a machine

are classified into several groups according to size. This is done

both for hole and shaft and then the corresponding groups will

match properly.


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If some parts (shaft and holes) to be assembled are manufactured

to normal tolerances of 0.01 mm (and both are within the curve of

normal distribution), an automatic gauge can segregate them into ten

different groupswith a 0.001 mm limit for selectiveassemblyof the

individual parts.

Thus parts with tolerances of 0.001 mm are obtained (due tosegregation) and both the conditions of high quality and low cost can

be served by selective assembly technique.

Requirement: Two component parts to be fitted together must be

kept within the normal distribution, the process capability of two

machines producing shafts and holes must be identical.


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Desired mean value of hole Desired mean value of shaft

Process capability ofhole making machine Process capability of shaftmaking machine

Process capability indexUSL=Upper Specification Limit

LSL=Lower Specification Limit

In this parts are graded according to size and only matched grades of mating

parts are sssembled


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Fig. shows a case in which the process capability of both

shaft and hole producing machines is samebut toleranceson parts

are desired as one-tenth of process capability of machines.

In such a case the parts are segregated by automatic inspectioninto ten groupsand parts in shaft region are matched with parts in

hole region.

This results in matching of parts having tolerances l/10th of

machine capability.

In this case as the process capability of both machines is same,

equal number of parts are available in each segregated zone and no

wastage will be there.

Process capability is the ability of a process/machineto produce

output within specification limits
http://en.wikipedia.org/wiki/Process_(engineering)http://en.wikipedia.org/wiki/Specification_(technical_standard)http://en.wikipedia.org/wiki/Specification_(technical_standard)http://en.wikipedia.org/wiki/Process_(engineering)


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No. of groups= process capability/ tolerance desired

LIMITS, FITS &TOLERANCES


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Terms & Definitions

,

What are Limits, Tolerance, Deviation and Allowance ?


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TERMS& DEFINITIONSContd..


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Shaft: The term shaft refers not only to the diameter of a circular

shaft but to any external dimension on a component

Hole: Hole refers not only to the diameter of a circular hole but toany internal dimension on a component

Basic or Nominal size: The size from which the limits of size are

derived by the application of upper and lower deviation.

Basic size is the zero line.

Basic size is same for both the hole and its shaft.

Basic size can be a whole number or a decimal number.Ex: 32,15,8.75 mm etc

Any size more than the basic will be above the zero line and any

size less than basic size will be below the zero line and size equal

to basic size will be at zero line.


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1.Tolerance: The difference between the upper and lower limits is

called the tolerance. (Or) The algebraic difference between upper and

lower deviations, and it is an absolutevalue.

Shaft of dia. 40.00 0.05 = 40.05 mm and 39.95 mm

The dimension 40.05 mm is called the upper limit and the dimension

39.95 mm is called the lower limit.

Tolerance = upper limit lower limit = 40.05 30.95 = 0.10 mm

Tolerance is always a positivequantitative numberFor a shaft:

The maximum metal limit is the upper limit

The minimum metal limit is the lower limit

For a hole:

The maximum metal limit is the lower limit

The minimum metal limit is the upper limit


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Maximum Material Condition (MMC) :The condition when the part

weights the most.

The MMC of a shaft is at the maximum size of the tolerance and the

MMC of the hole is at the minimum size of the hole.

Ex: MMC of the hole of Dia. 4+/- 0.02 mm is Dia. 3.98mm.

MMC of shaft of Dia.10 +0/-0.005 mm is Dia. 10.00 mm.

Least Material Condition (LMC) The condition when the part weights

the least.

The LMC of a shaft is at the minimum size of the tolerance and theLMC of the hole is at the maximum size of the hole.

Ex: LMC of the hole Dia. 4+/-0.02 is Dia. 4.02 mm.

LMC of shaft Dia. 10+0/-0.005 is Dia. 9.995 mm.

TYPES OF TOLERANCE


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TYPES OF TOLERANCE

There are 2 systems of writing tolerances

Unilateral: Dimension of a part is allowed to vary only on one side of basic

size, either above or below it.

Bilateral: Dimension of the part is allowed to vary on both the sides of the

basic size, the limits of tolerance lie on either side of the basic size, but

may not be necessarily equally disposed about it.


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2.Deviation: The algebraic difference between a size(actual) and

the corresponding basic size.

a.Upper deviation: This is the amount from the basic size or zero

line, to the maximum limit of size for either a hole or a shaft.

Designated by ES for hole , es for shaft

This is +ve when max. limit of size is greater than the basic sizeThis isve when max. limit of size is less than the basic size

b. Lower deviation: This is the amount from the basic size or zero

line to the minimum limit of size.

Designated by EI for hole and ei for shaft.

This is +ve when the min. limit of size is greater than the basic size

This isve when the min limit of size is less than the basic size44

F d l d i i Thi i f h d i i hi h i


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c. Fundamental deviation: This is one of the two deviations which is

conventionally chosen to define the position of the tolerance zone

in relation to the zero line.

This may be upper or lower deviation which is closest to the zero

line.

es: zero line (Basic Size) to superior size of shaft.

ei: zero line (Basic Size) to inferior size of shaft.

ES: zero line (Basic Size) to superior size of hole.

EI: zero line (Basic Size) to inferior size of hole.

French term ecart superieur & ecart inferieur

Basic shaft and Basic hole: The shafts and holes that have zero

fundamental deviation. Basic hole has zero lower deviation where as

basic shaft has zero upper deviation

TOLERANCES ON COMPONENTS


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Tolerance is permissible variation in the dimension of the

component.

Due to inherent inaccuracies in Manufacturing processestolerances have to be provided.

Concepts of basic size, limits, deviations and tolerances - Shaft46



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Concepts of basic size, limits, deviations and tolerances - Hole

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Basic Shaft:

Upper deviation (es) = Basic size Upper limit = 0

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Basic Hole:

Lower deviation (EI) = Basic size Lower limit = 0

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(-ve)

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(+ve)

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Fit: It is the relation between dimensions of two mating parts


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Fit: It is the relation between dimensions of two mating parts

before their assembly.

Theoretically 3 types of fits possible. In actual practice, it is

necessary to define a large variety of fits within the same type toaccount for all possible engineering situations.

Innumerable fits ranging from extreme clearance to those of

extreme interference can be obtained by a suitable combination of

fundamental tolerances and fundamental deviations.

Each of 25 holes has a choice of 18 tolerances.

Holes & Shafts: Based on fundamental deviations, holes and shafts are indicatedby letter symbols (capital letters A to Zcfor holes and small letters a to zc for

shafts.

These are : A,B,C,D,E,F,G,H,JS,J, K, M, N,P,R,S,T,U,V,X,Y,Z,ZA,ZB,ZC


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Standard Tolerance: Various grades of tolerances are defined using

the standard tolerance unit,(i) in m, which is a function of basic

size.

i= 0.004D + 2.1 for D>500 mm

where, D (mm) is the geometric mean of the lower and upperdiameters of a particular diameter step within which the chosen the

diameter D lies.

Diameter steps in I.S.I are: (a-b, where ais above and bis up to)

1-3, 3-6, 6-10, 10-18, 18-30, 30-50, 50-80, 80-120, 120-180, 180-250,

250-315, 315-400 and 400-500 mm

for D


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Grades IT5 IT6 IT7 IT8 IT9 IT10 IT11 IT12 IT13 IT14 IT15 IT16

Values 7i 10i 16i 25i 40i 64i 100i 160i 250i 400i 640i 1000i

For IT01, Tolerance =0.3 + 0.08D

For IT0, Tolerance=0.5+0.12D

For IT1, Tolerance=0.8+0.02D

IT2 to IT4 are regularly scaled approximately, geometrically between

the values of IT1 and IT5

(IT1 is given above and IT5 given in table below)Where D is in millimeters

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INTERNATIONAL TOLERANCE GRADES

Values In

MicronsIT01 IT0 IT1

Values For

D In mm0.3+0.008D 0.5+0.012D 0.8+0.020D

INTERNATIONAL TOLERANCE GRADES

Values In

MicronsIT5 IT6 IT7 IT8 IT9 IT10 IT11 IT12 IT13 IT14 IT15 IT16

Values

For D In

mm

7i 10i 16i 25i 40i 64i 100i 160i 250i 400i 640i 1000i

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Table :Formulae for Fundamental Deviations

for Shafts for sizes upto 500 mm


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p

Upper Deviation (es) Lower Deviation (ei)

Shaft

Designation

In microns

(for D in mm)

Shaft

Designation

In microns

(for D in mm)

a

= -(265 + 1.3D)

for D 120

and =3.52Dfor D > 120

J5 to j8 No formula

k4 to k8 = + 0.6 D1/3

b

(140 + 0.85D)

for D 160

=1.82D

for D > 160

k for grade

3 and 4= 0

m = + (IT7-IT6)

c

=52 D 0.2

for D 40

= -(95 + 0.8D)

for D > 40

n = + 5D0.34

p = + IT7 + 0 to 5

r= geometric mean of

values el for p and sd =16D 0.44

s

= IT8 + 1 to 4

for D 50

= + IT7 to + 0.4D

for D > 50

e = -llD0.41

f = -5.5D0.41 t = + IT7 + 0.63D

g = -2.5D0.34 u = + IT7 + D

h = 0

v = + IT7 + 1.25D

x = + IT7 + 1.6D

y = + IT7 + 2D

z = + IT7 + 2.5D

za = IT8 + 3 + 3.15Dzb = + IT9 + 4D

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Symbolic representation for tolerances on shafts and holes

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For shafts a to h the upper deviation is below zero line( ve) and


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For shafts a to h the upper deviation is below zero line(-ve) and

for shafts kto zcit is above the zero line(+ve)

The deviation of the shaft from jto keither +ve orve

For holes Ato H,the lower deviation is above the zero line(+ve)

and for Kto ZC,it is below the zero line(-ve)

The deviation of the hole from Jto Keither +ve orve

Formulas are given to determine the fundamental deviation.

The other deviations(upper & lower) may be derived directly

using the tolerance IT.

Standard tolerances

18 grades: IT01 ,IT0

and IT1-1T16

Fundamental devations

25 types: A- ZC (For holes)

a- zc (For shafts)


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Upper deviation of shafts from a to g are ve and for h it is

zero and lower deviation of the remaining shafts is +ve.

For holes, lower deviation is +ve for holes Ato Gand for Hit iszero and upper deviation of remaining holes isve.

Allowance = Max. metal condition of hole Max. metal condition of shaft

= Low limit of hole High limit of shaft

Allowances: The difference between the hole dimension and shaft

dimension for any type of fit is called allowance.


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D i i f H l Sh f d Fi


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Designation of Holes, Shafts and Fits

A hole or a shaft is completely described if the basic size,

followed by the appropriate letter and the number of tolerancegrade is given.

1. A 50 mm H-hole, with the tolerance grade of IT7, is 50 H7.

2. A 50 mm f-shaft with the tolerance grade IT8 is 50 f8

Afit is designated by the basic size common to both the hole and

the shaft followed by symbols corresponding to each element, the

hole is quoted first.

Thus, if the basic size is 50mm, the hole is H7 and the shaft is f8,

then the fitcan be indicated as 50 H7f8

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APPLICATIONS IT Grade Range

Measuring Instruments andProduction of Gauges

IT01, IT0, IT1, IT2, IT3, IT4,IT5, IT6

General Engineering/Industry and

Precision Fit

IT5, IT6, IT7, IT8, IT9, IT10,

IT11, IT12

Semi Finished Product IT11, IT14, IT15, IT16

Structural Engineering IT16 65


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FITS


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FITS

The relation resulting from the difference between the sizes

before assembly.

Classification of Fits

Clearance Fit Transition Fit Interference fit

Max. size of shaft Min. size of shaft Min. size of shaft

smaller than smallerthan larger than

Min. size of hole Max. size of hole Max. size of hole

orMax. size of shaft

larger than

Max. size of hole

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68Allowance is +ve for clearance fit andve for interference fit.

Fundamental deviationsStandard tolerances18 grades: IT01 IT0


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25 types: A- ZC (For holes)

a- zc (For shafts)

18 grades: IT01 ,IT0

and IT1-1T16

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FITS


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When two parts are to be assembled, the relation resulting from

the difference between their sizes before assembly is called a fit.

Depending on the actual limits of hole or shaft, the fit may be

clearance fit, transition fit or an interference fit.

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Clearance fit: The largest permitted shaft dia is smaller than thedia of the smallest hole, so that shaft can rotate or slide through

with different degrees of freedom according to the purpose of the

mating members

f fi h i i d di f h h f i l


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Interference fit: The min. permitted dia. of the shaft is larger

than the max. allowable dia. of the hole.

The shaft and the hole members are intended to be attachedpermanently and used as a solid component but according to the

application of this combination, this type of fit can be varied.

Transition fit: The dia. of the largest allowable hole is greater

than that of the smallest shaft, but the smallest hole is smaller

than the largest shaft, so that a small +ve or ve clearance

between the shaft and hole members are employable.

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FITS Contd..


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Clearance Fit

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Maximum shaft dimension < Minimum hole dimension

Clearance Fit Contd


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In a clearance fit, the tolerance zone of the hole is entirely above

the tolerance zone of the shaft.

73Always clearance

Clearance Fit Contd


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Min. clearance=Min. size of hole - Max. size of shaft

Max. clearance=Max.size of hole - Min.size of shaft

In this type of fit, the size limits for mating parts are so selected

that clearance between them always occur.

Clearance fits may be

slide fit, easy sliding fit, running fit, slack running fit and loose

running fit.

Ex: Pully rotates on shaft

Interference Fit (or) Press fit (or) friction fit FITS Contd..


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( ) ( ) FITS Contd..

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Maximum Hole size < Minimum Shaft size

Always interference for all sizes

Interference fit Contd..


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Min. Interference=Max. size of holeMin size of shaft

Max. Interference=Min. size of holeMax. size of shat

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In this type of fit the size limits for the mating parts are so



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In this type of fit, the size limits for the mating parts are so

selected that interferencebetween themalways occur.

In an interference fit, the tolerance zone of the hole is entirelybelow the tolerance zone of the shaft.

The amount of interference determines the degree of force

required to assemble or mate the shaft to the hole.

The quality of surface finish of the mating parts, the size of the

diameters, the metals from which they are made, all affect the

quality of the fit obtained.

Ex: 1.Bearing bushes in their housing

2.Small end of the connecting rod & piston

The small endattaches to the piston pin, gudgeon pinor wrist pin, which is

most often press fit into the connecting rod but can swivel in the piston, a

"floating wrist pin" design.
http://en.wikipedia.org/wiki/Gudgeon_pinhttp://en.wikipedia.org/wiki/Wrist_pinhttp://en.wikipedia.org/wiki/Press_fithttp://en.wikipedia.org/wiki/Press_fithttp://en.wikipedia.org/wiki/Press_fithttp://en.wikipedia.org/wiki/Press_fithttp://en.wikipedia.org/wiki/Wrist_pinhttp://en.wikipedia.org/wiki/Wrist_pinhttp://en.wikipedia.org/wiki/Wrist_pinhttp://en.wikipedia.org/wiki/Gudgeon_pinhttp://en.wikipedia.org/wiki/Gudgeon_pinhttp://en.wikipedia.org/wiki/Gudgeon_pin


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2.Press fit(medium press or light drive fit-H7/s6): Involves heating

or refrigeration of one part powerful forces are brought into play


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or refrigeration of one part, powerful forces are brought into play,

resulting in a permanent joint between the two components.

Ex: Bearing bushes in alloy housings or castings, pump impellershaft

3.Heavy drive fit: Ex: Cylinder liner in a cast iron block, producing apermanent or semi-permanent assembly between liner and block.

large sizes require heating and shrinking to avoid the possibility of

damage ,if we attempt to assemble cold.79


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Wooden wheel of bullock cart with iron rim

FITS Contd..Transition Fit


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Obtained by overlapping of tolerance zones of shaft and hole

Does not guarantee neither clearance nor interference fit

Transition Fit Contd..


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In this type of fit, the size limits for the mating parts are so selected

that either a clearance or interference may occur depending upon theactual size of the mating parts. It may be noted that in a transition fit,

the tolerance zones of hole and shaft overlap.

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Transition fit Contd..


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Maximum clearance= Maximum limit size of hole Minimum limit size of shaft

Maximum interference = Minimum limit size of hole Maximum limit size of shaft

The transition fits may be force fit, tight fit and push fit.

Interference is so light that hand pressure is sufficient to cause entry ofthe shaft.

Ex: Hand wheel and indexing dial keyed to shaft (Lathe machine with

lead screw)


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Lower deviation of hole is zero

Upper deviation of shaft is zeroLow limit of hole=basic size

High limit of shaft = basic size

To obtain different types of fits, it is general practice to vary

tolerance zone of one of the mating parts


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HOLE BASED SYSTEM-

Size of hole is kept constant,shaft size is varied

to get different fits.

tolerance zone of one of the mating parts

SHAFT BASED SYSTEM-

Size of shaft is kept constant,

hole size is varied

to get different fits.

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Basic hole is chosen &

Different Fits are obtained

by changing shaft size

Different Fits are obtained

by changing hole size

Hole basis system Shaft basis system


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Hole basis system Shaft basis system

1.Size of hole whose lower deviation is

zero(H-hole) is assumed as the basic size.

Size of shaft whose upper deviation is

zero(h-shaft) is assumed as basic size

2.Limits on the hole are kept constant and

those of shaft are varied to obtain desired

type of fit.

Limits on the shaft are kept constant and

those on the hole are varied to have

necessary fit

3.Hole basis system is prepared in mass

production, because it is convenient and

less costly to make a hole of correct size

due to availability of standard drills and

reamers

This system is not suitable for mass

production because it is inconvenient,

time consuming and costly to make a

shaft of correct size

4.It is much more easy to vary the shaft

according to the fit required

It is rather difficult to vary the hole sizes

according to the fit required

5.Gauging of shafts can be easily and

conveniently done with adjustable gap

gauges.

Being internal measurement, gauging of

holes cannot be easily and conveniently

done.

86

FITS

R d d Fit b d M f t i P d A li ti


87/102

Recommended Fits based on Manufacturing Processes and Application:

87


88/102

88


89/102

89


90/102

90

FITS APPLICATIONS


91/102

91


92/102

92


93/102

Equivalent fits on the Hole-basis and shaft basis system 93


94/102

94


95/102

95


96/102

96

Assume dia. Step of 18 & 24 &

FD of P hole is IT6 + 0 to 5


97/102

97

The fit is interference.

Difference between Tolerance & Allowance


98/102

Tolerance Allowance

It is the permissible variation in thedimension of a part(either a hole or

shaft)

It is the prescribed difference betweenthe dimensions of two mating

parts(hole and shaft)

It is the difference between higher and

lower limits of a dimension of a part

It is the intentional difference between

the lower limit of hole and higher limit

of shaft

The tolerance is provided on the

dimension of a part as it is not possible

to make a part to exact specified

dimension

Allowance is to be provided on the

dimension of mating parts to obtain

desired type of fit

It has absolute value without sign Allowance may be positive(clearance

fit) or negative(interference fit)

98

Geometric Dimensioning and Tolerancing (GD & T)


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99

Geometric tolerancing reading helps to understand to specify and

control the form, location and orientation of the features ofcomponents and manufactured parts.

Geometric Dimensioning and Tolerancing is an efficient method

for describing the tolerancing mandated by the designer of the part.

The Datum axis or Datum planes are to be used for locating other

features.

With GD&T all inspection will result in the same result. It will help

to understand if the dimension is within or out of tolerance.

Geometric Dimensioning and Tolerancing forces the designers to

totally consider functions, manufacturing processes, and inspection

methods.

Tolerance Feature Indication/Feature Control Frame Symbol


100/102

100

Tolerance Feature Indication/Feature Control Frame Symbol.


101/102


102/102

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Documents

metrology-KSR-1.2