Introduction to Measurement

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302044 Metrology and Quality Control AJB

AISSMS College of Engineering, Pune

Unit 1:

Measurement standards and

Comparators By,

Mr. A J Bhosale

Asst. Professor

Dept. of Mechanical Engineering

AISSMS CoE Pune



Syllabus:

Unit – I Measurement standards and comparators

Principles of Engineering metrology, Measurement

standards, Types and sources of errors, Accuracy and

Precision, introduction to uncertainty in measurement,

linear and angular measuring instruments and their

applications.

Calibration: Concept and procedure, traceability,

Gauge R&R

Comparators: Mechanical, Pneumatic, Optical,

Electrical (LVDT).Checking all geometrical forms.

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Principles of Metrology:

• Definition: Metrology word is derived from two Greekwords such as metro which means measurement and logywhich means science.

• Metrology is the science of precision measurement.

• The engineer can say it is the science of measurement oflengths and angles and all related quantities like width,depth, diameter and straightness with high accuracy.

• Metrology demands pure knowledge of certain basicmathematical and physical principles.

• The development of the industry largely depends on theengineering metrology.

• Metrology is also concerned with the establishment,reproduction and conservation and transfer of units ofmeasurements and their standards



Metrology is mainly concerned with:

(1) Establishing the units of measurements, ensuring the

uniformity of measurements.

(2) Developing methods of measurement.

(3) Errors of measurement(Calibration).

(4) Accuracy of measuring instruments and their care.

(5) Industrial inspection and its various techniques

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Necessity and importance of Metrology:

In design, design engineer should not only check his

design from the point of view of the strength or

economical production, but he should also keep in mind

how the dimensions specified can be checked or

measured.

Higher productivity and accuracy can be achieved by

properly understood, introduced the Metrology.

You can improve the measuring accuracy and

dimensional and geometrical accuracies of the product.



Introduction to Measurement:

• Measurement is defined as the process of numerical

evaluation of a dimension or the process of comparison

with standard measuring instruments.

• The elements of measuring system include the

instrumentation, calibration standards, environmental

influence, human operator limitations and features of the

work-piece.

• The basic aim of measurement in industries is to check

whether a component has been manufactured to the

requirement of a specification or not.

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Types of Metrology:

1) Legal Metrology: 'Legal metrology' is that part of

metrology which treats units of measurements, methods

of measurements and the measuring instruments, in

relation to the technical and legal requirements.

The activities of the service of 'Legal Metrology' are:

(i) Control of measuring instruments;

(ii) Testing of prototypes/models of measuring instruments;

(iii) Examination of a measuring instrument to verify its

conformity to the statutory(legal) requirements etc.

Legal Metrology.pdf



2) Dynamic Metrology. 'Dynamic metrology' is the technique ofmeasuring small variations of a continuous nature. The techniquehas proved very valuable, and a record of continuousmeasurement, over a surface, for instance, has obvious advantagesover individual measurements of an isolated character.

3) Deterministic Metrology.

• Deterministic metrology is a new philosophy in which partmeasurement is replaced by process measurement.

• In this, the system processes are monitored by temperature,pressure, flow, force etc. sensors, these sensors are fast and non-intrusive.

• The new techniques such as 3D error compensation by CNC(Computer Numerical Control) systems and expert systems areapplied, leading to fully adaptive control.

• This technology is used for very high precision manufacturingmachinery and control systems to achieve micro technology andnanotechnology accuracies.

Legal Metrology.pdf

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Institutes & Organisations related to Metrology

• CGPM (General Conference on Weights and Measures)

• CIPM( International Committee for Weights and

Measures)

• BIPM (International Bureau of Weights and Measures)

• NIST(National Institute of Standards & Technology)-

US

• NPL (National Physical Laboratory)-India & UK

• IILM (Indian Institute of Legal Metrology)

• NABL (National Accreditation Board for Testing and

Calibration Laboratories)-India



Measurement Standards:

1) Primary Standards:

• These are fundamental standards like meter that does notchange their value & it is strictly followed & preciselydefined there should be one and one only one materialstandard preserved under most careful conditions.

• This has no direct application to a measuring problemencountered in engineering.

• These are used only at rare intervals and solely forcomparison with secondary standards.

• E.g.- BIPM (Bureau International des Poids et Measures)

• The International Bureau of Weights and Measures (BIPM)

• Headquarter- Paris, France, established on 20 May1875.

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2) Secondary Standards:

• These are close copies of primary standards as regards indesign, material and length.

• Any error existing in these bars is recorded by comparisonwith primary standards with long intervals.

• These standards are distributed to number of places for safecustody and used in their turn for occasional comparisonwith tertiary standards.

3) Tertiary Standards:

• Tertiary standards are the first standards to be used forreference purposes in laboratories and workshops.

• These should also be maintained as a reference forcomparison at intervals with working standards.



4) Working Standards:

• These standards are necessary for use in metrology

laboratories and similar institutions

Sometimes standards are classified as:

1) Reference Standards (used for reference purposes)

2) Calibration Standards(used for calibration of inspection

and working standards)

3) Inspection Standards (used by inspectors)

4) Working Standards (used by operators)

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Line Standard:

• The measurement of distance may be made between two

parallel lines or two surfaces.

• When, the length, being measured, is expressed as a

distance between the centres of two engraved lines as in

a steel rule, it is known as line measurement.

• Line standards are used for direct length comparison and

they have no auxiliary devices.

Line Standard

Meter Yard



1) International Standard Prototype- Meter :

The metre or meter is a base unit of length in the metric

system used around the world for general and scientific

purposes.

Historically, the metre was defined by the French

Academy of Sciences as the length between two marks

on a platinum-iridium bar, which was designed to

represent 1⁄10,000,000 of the distance from the equator to

the north pole through Paris.

In 1983, it was redefined by the International Bureau of

Weights and Measures (BIPM) as the distance travelled

by light in free space(vacuum) in 1⁄299,792,458 of a second.

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• It is the distance between the center portions of two

lines etched on a polished surface of a bar of pure

platinum alloy (90%) or iridium alloy (10%).

• It has overall width and depth of 16 mm each and is kept

at 0°C and under normal atmospheric pressure.



• The web section chosen gives maximum rigidity and

economy in the use of costly material.

• The upper surface of web is inoxidizable and needs a

good surface finish for ruling a good quality of engraved

lines.

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2) The Imperial Standard - Yard:

• This standard served its purpose from 1855 to 1960.

• It is made of one inch square cross section bronze bar (82 % copper,13 % tin, 5% zinc) and is 38 inches long.

• This bar has half inch diameter and half inch deep hole, which arefitted with 1/10th inch diameter gold plug.

• The highly polished top surfaces of these plugs three transverselyand two longitudinally engraved lines lying on the neutral axis of thebronze bar.

• The yard is defined as the distance between two central transverselines on the plugs when the temperature of the bar is constant at 620

F and the bar is supported on rollers in specified manner to preventflexure.

• To protect the gold plug from accidental damage, it is kept at theneutral axis, as the neutral axis remains unaffected even if the barbends.

• 1 Yard=0.9144m or 1Meter = 1.09361 Yard



v

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Characteristics of Line Standards:

1. Accurate engraving on the scales can be done but it isdifficult to take full advantage of this accuracy. For example,a steel rule can be read to about ±0.2 mm of true dimension.

2. It is easier and quicker to use a scale over a wide range.

3. The scale markings are not subject to wear althoughsignificant wear on leading end leads to under sizing.

4. There is no 'built in' datum in a scale which would allow easyscale alignment with the axis of measurement, this againleads to under sizing.

5. Scales are subjected to the parallax effect, a source of bothpositive and negative reading errors.

6. For close tolerance length measurement (except inconjunction with microscopes) scales are not convenient tobe used.



2) End Standards:

• Most of the times engineer is concerned with themeasurement between two surfaces.

• When the distance is measured as a separation of twoparallel surfaces, then this is called as End Standards.

• The main difference between line standard and endstandard is that, in case of line standard we take everyvalue of line engraved into consideration and then onlyconclusion is drawn.

• But in the end standard only two faces are taken intoconsideration and no importance is given tointermediate value or reading between that to parallelfaces.

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Characteristics of End Standards:

1. Highly accurate and well suited to close tolerancemeasurements.

2. Time-consuming in use.

3. Dimensional tolerance as small as 0.0005 mm can beobtained.

4. Subjected to wear on their measuring faces.

5. To provide a given size, the groups of blocks are "wrung"together. Faulty wringing leads to damage.

6. There is a "built-in" datum in end standards, because theirmeasuring faces are flat and parallel and can be positivelylocated on a datum surface.

7.They are not subject to the parallax effect.



1) End Bars:

• These are used for the measurement of larger sizes of

work. These consist of carbon steel round bar about 20

mm in diameter and made in sizes varying from 10 mm

to 1200 mm.

• The faces of End bars lapped and hardened at the ends

are available in sets of various lengths.

• These bars are generally not found in majority of works

but in standardising laboratories.

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2) Slip Gauges:

• Slip gauges are often called as Johannson gauges, asJohannson originated them.

• These are rectangular blocks of high grade cast steel havinga cross section of about 30 by 10 mm.

• These are first hardened to resist wear and carefullystabilised so that they are independent of any subsequentvariation in size or shape.

• Their opposite faces are flat, parallel and accurately thestated distance apart.

• The opposite faces are of such a high degree of surfacefinish, that when the blocks are pressed together with aslight twist by hand, they will “wring” together. They willremain attached to each other.

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• With the set of slip gauges, combination of slip gauge

enables measurements to be made in the range of 0.0025

to 100 mm.

• They are supplied in sets of 103 pieces down to 32

pieces.

• For e.g. 103 pieces

Range Step Pieces

1.01-1.49 0.01 49

0.5-24.50 0.5 49

25,50,75,100 25 4

1.005 0.005 1

Total=103



Set of 45 Pieces-

Set of 87 Pieces-

Range Step Pieces

1.001-1.009 0.001 09

1.01-1.09 0.01 09

1.1-1.9 0.1 09

1-9 1 09

10-90 10 09

Total-45

Range Step Pieces

1.001-1.009 0.001 09

1.01-1.49 0.01 49

0.5-9.5 0.5 19

10-90 10 9

1.005 - 1

Total-87

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Slip gauges are classified according to their guaranteed accuracy:

AA for Master Slip Gauges(Accurate upto- ± 2 micron/meter)

A for Reference purpose,(Accurate upto- ± 4 micron/meter)

B for working slip gauges(Accurate upto- ± 8 micron/meter)

As regards grades or classes of slip gauges, these could also be

designed in five grades as under:

1) Grade 2- Workshop Grade- Used for setting up machine tools,

positioning milling cutters and checking mechanical widths.

2) Grade 1-Used for more precise work such as setting up sine bars,

sine tables, checking gap gauges and setting dial test indicators to

zero.

3) Grade 0-Inspection Grade- Confined to Tool-room or machine

shop inspection.



4) Grade 00-Used in standard room and kept for work ofthe highest precision only.

5) Calibration Grade-Special grade, with the actual sizesof the slips stated or calibrated on a special chartsupplied with the set.

• They are not costly as Grade 00, because these are notmade to specific or set tolerances.

• It must be remembered that a slip gauges, like any otherengineering component, cannot be made to an exactsize.

• As closer the limits the more expensive the slip gaugesbut in case of calibration grade, greater tolerances onlength are permissible.

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Wringing of Slip Gauges:

The phenomenon of Wringing takes place when two flatlapped surfaces are placed in contact with a slidingmovement.

If the surfaces are clean, they will adhere strongly when slidcarefully together.

Various investigators have proved that the force of adhesionof two flat, optically polished, surfaces is much greater thancan be accounted for by atmospheric pressure alone.

It is generally accepted that there is molecular adhesionbetween the surfaces and the liquid film(0.00635µm.)

If two highly finished slip gauges are firmly wrung togetherand left for several days, they may become very stronglyattached.

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Wringing Process:



Wringing Process:

1. Wiping a clean gauge block across an oiled pad.

2. Wiping any extra oil off the gauge block using a dry

pad.

3. The block is then slid perpendicularly across the other

block while applying moderate pressure until they form

a cruciform.

4. Finally, the block is rotated until it is inline with the

other block.

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3) Wavelength Standards:

• In 1829, Jacqnes Babinet, a French philosopher, suggestedthat wave lengths of monochromatic light might be used asnatural and invariable units of length.

• It was nearly a century later that the Seventh GeneralConference of Weights and Measures in Paris approved thedefinition of a standard of length relative to the meter interms of the wavelength of the red radiation of cadmium.

• In 1892, A. A. Michelson, assisted by J R Benoit at BIPM,made the first determination of the meter in wavelength.

• Material standards are liable to destruction and theirdimensions change slightly with time.

• But with the monochromatic light we have the advantage ofconstant wavelength and since the wavelength is not aphysical one, it need not be preserved.



• This is reproducible standard of length and the error ofreproduction can be of the order of 1 part in 100 millions.

• It is because of this reason that International standardmeasures the meter in terms of wavelength of krypton 86(Kr 86).

• Light wavelength standard, for some time, had to beobjected because of the impossibility of producing puremonochromatic light as wavelength depends upon theamount of isotope impurity in the elements.

• But now with rapid development in atomic energy industry,pure isotopes of natural elements have been produced.

• Krypton 86, Mercury 198 and Cadmium 114 are possiblesources of radiation of wavelength suitable as naturalstandard of length.

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Orange-red line of krypton-86 propagating in a vacuum

1650763.73 x wavelength of the radiation



Advantages of Wavelength Standards:

1. It is not influence by effects of variations of environmental

temperature, pressure, humidity and ageing because it is not a

material standard.

2. There is no need to store it under security and thus there is no

fear of its being destroyed as in the case yard and meter.

3. It is easily available to all standardizing houses, laboratories

and industries.

4. It can be easily transferred to other standards.

5. This standard can be used for making comparative statement

much higher accuracy.

6. It is easily reproducible.

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Accuracy & Precision:

• The purpose of measurement is to determine the true

dimensions of a part. But no measurement can be made

absolutely accurate. There is always some error.

• The amount of error depends upon the following factors:

1. The accuracy and design of the measuring instrument

2. The skill of the operator

3. Method adopted for measurement

4. Temperature variations

5. Elastic deformation of the part or instrument etc.

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Precision:

• Precision is the repeatability of the measuring process.

• It refers to the group of measurements for the same characteristicstaken under identical conditions. It does not have any meaningfor single measurement.

• It indicates to what extent the identically performedmeasurements agree with each other.

• If the instrument is not precise it will give different (widelyvarying) results for the same dimension when measured again andagain.

• The set of observations will scatter about the mean. The scatter ofthese measurements is designated as σ, the standard deviation.

• It is used as an index of precision. The less the scattering moreprecise is the instrument. Thus, lower, the value of σ, the moreprecise is the instrument.



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 known 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.

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Factors affecting the accuracy of the measuring

system:

The basic components of an accuracy evaluation are the

five elements of a measuring system such as:

• Factors affecting the Calibration Standards.

• Factors affecting the Work piece.

• Factors affecting the inherent characteristics of the

Instrument.

• Factors affecting the Person, who carries out the

measurements,

• Factors affecting the Environment.

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1. Factors affecting the Standard: It may be affected by:

- coefficient of thermal expansion,

- calibration interval,

- stability with time,

- elastic properties,

- geometric compatibility

2. Factors affecting the Work piece: These are:

- cleanliness, surface finish, waviness, surface defects etc.,

- hidden geometry,

- elastic properties,

- adequate datum on the work piece,

- arrangement of supporting work piece,

- thermal equalization etc.



3. Factors affecting the inherent characteristics of

Instrument:

- adequate amplification for accuracy objective,

- scale error,

- effect of friction, backlash, hysteresis, zero drift error,

- deformation in handling or use, when heavy work pieces

are measured,

- calibration errors,

- mechanical parts (slides, guide ways or moving elements),

- repeatability and readability,

- contact geometry for both work piece and standard.

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4. Factors affecting Person :

- training, skill,

- sense of precision appreciation,

- ability to select measuring instruments and standards,

- sensible appreciation of measuring cost,

- attitude towards personal accuracy achievements,

- planning measurement techniques for minimum cost, consistent withprecision requirements etc.

5. Factors affecting Environment:

- temperature, humidity etc.,

- clean surrounding and minimum vibration enhance precision,

- adequate illumination,

- temperature equalization between standard, work piece, and instrument,

- thermal expansion effects due to heat radiation from lights,

- heating elements, sunlight and people,

- manual handling may also introduce thermal expansion.



Repeatability:

• It is the ability of measuring instrument to repeat the

same results for the measurements for the same quantity,

when the measurement is carried out,

- By the same observer,

- With the same instrument,

- Under same conditions,

- Without any change in location,

- Without the change in method of measurement,

- And the measurements are carried out in short interval

of time.

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Reproducibility:

• Reproducibility is the consistency of pattern of variation

in measurement i.e. closeness of the agreement

between the results of measurements of the same

quantity, when individual measurements are carried

out:

by different observers,

by different methods,

using different instruments,

under different conditions, locations, times etc.



Magnification:

• In order to measure small differences in dimensions themovement of the measuring tip in contact with the workmust be magnified.

• For this the output signal from measuring instrument isto be magnified.

• This magnification means increasing the magnitude ofoutput signal of measuring instrument many times tomake it more readable.

Sensitivity:

• The ability of measuring instrument to detect smalldifferences in quantity being measured.

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Types of Error: Error

Static Loading Dynamic

Environmental Reading Systematic RandomCharacteristic

Parallax Interpolation



Systematic Error:

• These error include calibration errors, error due tovariation in the atmospheric condition, variation incontact pressure etc.

• If properly analysed, these errors can be determined andreduced or even eliminated hence also calledcontrollable errors.

• All other systematic errors can be controlled inmagnitude and sense except personal error.

• These errors results from irregular procedure that isconsistent in action. These errors are repetitive in natureand are of constant and similar form

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Random Error:

• These errors are caused due to variation in position of

setting standard and work-piece errors.

• Due to displacement of level joints of instruments, due

to backlash and friction, these error are induced.

• Specific cause, magnitude and sense of these errors

cannot be determined from the knowledge of measuring

system or condition of measurement.

• These errors are non-consistent and hence the name

random errors.



READING ERROR:

Parallax error :

• Occurs when the line of vision is not in line with measuringvalue.

• Error can be reduced by placing a mirror behind the pointer,as this ensures normal reading

Interpolation error:

• This occurs when the pointer shows some value which is between any two graduation marks.

• Depends Thickness of graduation marks, Spacing of the scale divisions Thickness of the pointer.

• Error can be reduced Using a magnifier over the scale or Using a digital read out system

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CHARACTERISTIC ERROR:

• Calibration error and repeatability error are the

examples of this static type.

- Consider the error you may make in a reading a scale as

shown.



Should the gap be wide or narrow for best results?

Measurement 2 is best.

1 and 3 give the wrong readings.

This is called a

parallax error.

It is due to the gap here, between the pointer and

the scale.

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Zero Drift Errors

A particular type of systematic error is the zero Drift error.

Apparatus that does not read zero even when it should.

Over a period of time, the spring may weaken, and so the pointer does not point

to zero.

What affect does this have on ALL the readings?



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Environmental Error:

• Temperature plays an important role. Eg. While using

slip gauges, due to handling the slip gauges may acquire

human body temperature whereas work piece is at 20

degree C.



Elastic Deformation Error:

Contact Error:

Airy’s Formula

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The ± 1 second is called the absolute uncertainty

Every measurement has an uncertainty or error.

e.g. time = 5 seconds ± 1 second

There are three main types of uncertainty.

Random Uncertainties

Systematic Errors

Reading Uncertainties

Uncertainties



Repeated measurements of the same quantity, gives a range of readings.

The random uncertainty is found using:

readings of number

reading minreading maxyuncertaint random

Taking more measurements will help eliminate (or reduce) random uncertainties.

The mean is the best estimate of the true value.

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5

200210209210209length mean

5

1038

mm 208length mean

give the mean to same number of significant figures as measurements

(a)

Example 1Five measurements are taken to determine the length of a card.

209mm, 210mm, 209mm, 210mm, 200mm

(a) Calculate the mean length of card.(b) Find the random uncertainty in the measurements.(c) Express mean length including the absolute uncertainty.



(b)

readings of number

reading minreading maxyuncertaint random

5

200210

mm 2

(c) mm 2 mm 208card of length

The “± 2mm” is the absolute uncertainty.

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Reading Uncertainties

A reading uncertainty is how accurately an instruments scale can be read.

Analogue Scales

Where the divisions are fairly large, the uncertainty is taken as:

half the smallest scale division



Where the divisions are small, the uncertainty is taken as:

the smallest scale division

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Digital Scale

For a digital scale, the uncertainty is taken as:

the smallest scale reading

e.g. voltage = 29.7 mV ± 0.1 mV

This means the actual reading could be anywhere from



For a digital meter the uncertainty is taken as the smallest

scale reading.

e.g. Voltage = 29.7 mV ± 1 mV

(actual reading could be from 29.65 to 29.74)???

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Percentage Uncertainty

The percentage uncertainty is calculated as follows:

100reading

yuncertaint absoluteyuncertaint %



Example 1

Calculate the percentage uncertainty of the measurement:

d = 8cm ± 0.5cm

100reading

yuncertaint absoluteyuncertaint %

1008

0.5

100 0.0625

% 6.25

(d = 8cm ± 6.25%)

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COMPARATORS



Comparison Measurement:

• Process consisting of comparing measurement of part toknown standard or master of exact dimension required

• Comparators

– Any instrument used to compare size of workpiece to knownstandard

– Incorporate some means of amplification to compare part sizeto set standard• Standard usually gauge blocks (Slip Gauges).

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Method of Comparison:



Need of Comparators:

I. In mass production, where components are to be checked

at a very fast rate.

II. As laboratory standards from which working or inspection

gauges are set and correlated.

III. For inspecting newly purchased gauges.

IV. Attached with some machines, comparators can be used as

working gauges to prevent work spoilage and to maintain

required tolerances at all stages of manufacturing.

V. In selective assembly of parts, where parts are graded in

three or more groups depending upon their tolerances.

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Characteristics of Good Comparators:

1. It should be compact.

2. It should be easy to handle.

3. It should give quick response or quick result.

4. It should be reliable, while in use.

5. There should be no effects of environment on the comparator.

6. Its weight must be less.

7. It must be cheaper.

8. It must be easily available in the market.

9. It should be sensitive as per the requirement.

10. The design should be robust.

11. It should be linear in scale so that it is easy to read and get uniformresponse.

12. It should have less maintenance.

13. It should have hard contact point, with long life.

14. It should be free from backlash and wear.



Types of Comparator:

COMPARATOR

MECHANICAL ELECTRICAL OPTICAL PNEUMATIC

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1) Mechanical Comparator:

• It works on gears pinions, linkages, levers, springs etc.

Mechanical Comparator

Dial IndicatorsJohansson Mikrokator

Sigma Comparator

Mechanical -Optical

Comparator



Rack and PinionCam and Gear

Tight Taught String

http://www.mechlook.com/wp-content/uploads/2011/10/1230010.jpg

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Dial Indicator:

Work piece Under Measurement



Dial Indicator:

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Applications:

1. Comparing two heights or distances between narrow limits.

2. To determine the errors in geometrical form such as ovality,

roundness and taper.

3. For taking accurate measurement of deformation such as in

tension and compression.

4. To determine positional errors of surfaces such as parallelism,

squareness and alignment, flatness.

5. To check the alignment of lathe centers by using suitable

accurate bar between the centers.

6. To check trueness of milling machine arbours and to check

the parallelism of shaper arm with table surface or vice.

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Johansson “Mikrokator”: - Developed by:

H. Abramson (Swedish Engineer)

and C. E. Johansson (Company)

in 1938.

Also known as Abramson’s

Movement

- Magnification:

(9.11*L) / (W^2*n)

Magnification upto 5000x

possible



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Sigma Comparator:

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If l = Distance from hinge pivot to the knife edgeL = Length of y-armR = Driving drum radiusD Length of the pointerThen the total magnification = (L/l) *(D/R)



Mechanical-Optical Comparator:

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Electrical Comparator:

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Pneumatic Comparator:



Introduction to Measurement

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