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S.K.P. Engineering College, Tiruvannamalai VI SEM
Mechanical Engineering Department 1 Metrology and Measurements
SKP Engineering College
Tiruvannamalai – 606611
A Course Material
on
Metrology and Measurements
By
Mr.K.Venkatesh, Mr.R.Susenthirar, Mr.R.Manjunathan,
Mr.E.Ravindarsingh
Assistant Professor
Mechanical Engineering Department
S.K.P. Engineering College, Tiruvannamalai VI SEM
Mechanical Engineering Department 2 Metrology and Measurements
Quality Certificate
This is to Certify that the Electronic Study Material
Subject Code: ME6504
Subject Name: Metrology And Measurements
Year/Sem: III / VI
Being prepared by Mr.K.Venkatesh, Mr.R.Susenthirar, Mr.R.Manjunathan,
Mr.E.Ravindarsingh and it meets the knowledge requirement of the University
curriculum.
Signature of the Author
Name: Mr.K.Venkatesh, Mr.R.Susenthirar, Mr.R.Manjunathan, Mr.E.Ravindarsingh
Designation: Assistant Professor
This is to certify that the course material being prepared by R.Susenthirar,
Mr.K.Venkatesh is of the adequate quality. He has referred more than five books and
one among them is from abroad author.
Signature of HD Signature of the Principal
Name: Dr.J. Kuberan Name: Dr.V.Subramania Bharathi
Seal: Seal:
S.K.P. Engineering College, Tiruvannamalai VI SEM
Mechanical Engineering Department 3 Metrology and Measurements
SYLLABUS UNIT I CONCEPT OF MEASUREMENT 9
General concept – Generalized measurement system-Units and standards-measuring instruments: sensitivity, stability, range, accuracy and precision-static and dynamic response-repeatability-systematic and random errors-correction, calibration - Introduction to Dimensional and Geometric Toleranceing - interchangeability UNIT II LINEAR AND ANGULAR MEASUREMENT 9 Definition of metrology-Linear measuring instruments: Vernier, micrometer, Slip gauges and classification, - Tool Makers Microscope – interferometer, optical flats, -Comparators: limit gauges Mechanical, pneumatic and electrical comparators, applications. Angular measurements: -Sine bar, Sine center, bevel protractor and angle Decker. UNIT III FORM MEASUREMENT 9 Measurement of screw threads: Thread gauges, floating carriage micro meter measurement of ear tooth thickness: constant chord and base tangent method-Gleason gear testing machine – radius measurements-surface finish: equipment and parameters, straightness, flatness and roundness measurements UNIT IV LASER AND ADVANCES IN METROLOGY 9
Precision instruments based on laser-Principles- laser interferometer-application in measurements and machine tool metrology- Coordinate measuring machine (CMM): need, construction, types, applications.- computer aided inspection. UNIT V MEASUREMENTOFMECHANICALPARAMETERS 9
Force, torque, power:-mechanical, pneumatic, hydraulic and electrical type-Pressure measurement - Flow: Venturi, orifice, rotameter, pitot tube –Temperature: bimetallic strip, thermocouples, pyrometer, electrical resistance thermistor.
TOTAL: 45 PERIODS CONTENT BEYOND SYLLABUS
Fit and tolerance
Multiple operation
LEARNINGRESOURCES: TEXT BOOKS: 1. Jain R.K., ―Engineering Metrology‖, Khanna Publishers, 2005
S.K.P. Engineering College, Tiruvannamalai VI SEM
Mechanical Engineering Department 4 Metrology and Measurements
2. Alan S. Morris, ―The Essence of Measurement‖, Prentice Hall of India, 1997 REFERENCES
1. Gupta S.C, ―Engineering Metrology‖, Dhanpatrai Publications, 2005 2. Jayal A.K, ―Instrumentation and Mechanical Measurements‖, Galgotia Publications 2000 3. Beckwith, Marangoni, Lienhard, ―Mechanical Measurements‖, Pearson Education, 2006. 4. Donald Deckman, ―Industrial Instrumentation‖, Wiley Eastern, 1985. WEB RESOURCES:
1. www.redoaksys.com (for animations) 2. www.boschrexroth.com 3. www.eaton.in (Vickers) 4. www.compair.com/products/compressor_training_animations.aspx
ADDITIONAL RESOURCES :
1. NPTEL Tutorials (Internal Server) 2. Online Objective Questions 3. YouTube
S.K.P. Engineering College, Tiruvannamalai VI SEM
Mechanical Engineering Department 5 Metrology and Measurements
CONTENTS
S.No Particulars Page
1 Unit – I 6
2 Unit – II 17
3 Unit – III 40
4 Unit – IV 62
5 Unit – V 80
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Mechanical Engineering Department 6 Metrology and Measurements
Unit I
BASICS OF METROLOGY
PART-A
1. Define tolerance and zero line. (ODD 2015)
The basic dimension say 25 mm is the zero line. Any variation to this basic
dimension is the tolerance towards upward or downward limits.
2.Define interchangeable system. (ODD 2015)
Interchangeability means ease of replacement in the event of failure. Any
one component selected at random should assemble correctly with any other mating
component, that too selected at random.
3. Differentiate the terms reproducibility and repeatability. (EVEN 2015)
Reproducibility is the degree of closeness between measurements of the same
quantity where the individual measurements are made under different conditions.
Repeatability is the closeness between successive measurements of the same quantity
with the same instrument by the same operator over a short time span.
4. What is the difference between allowances and tolerance? (EVEN 2015)
Tolerance is the limit of random deviation of a dimension from its nominal value.
Allowance is the amount of designed deviation between two mating dimensions
in a fit, which, in combination with their respective tolerances, results into a
maximum and minimum clearance or interference.
5. What is meant by static response? (ODD 2014)
The static characteristics of an instrument are considered for instruments
which are used to measure an unvarying process conditions
6. Define tolerance and zero line. (ODD 2014)
The basic dimension say 25 mm is the zero line. Any variation to this basic
dimension is the tolerance towards upward or downward limits.
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Mechanical Engineering Department 7 Metrology and Measurements
7.What is measurement? Give its types. (EVEN 2014)
Measurement is a process of comparing the input signal (unknown
magnitude) with a pre-defined standard and giving out the result. Its types are : Direct
comparison method, Indirect comparison method, Primary measurement, Secondary
measurement and Tertiary measurement.
8.What do you mean by sensitivity of a measuring instrument? (EVEN 2014)
Sensitivity may be defined as the following relation:
Sensitivity = Change in the output signal / Change in the input signal.
9. Distinguish between relative error and random error. (EVEN 2013)
a) Relative error: Relative error is defined as the results of the absolute error and
the value of comparison used for calculation of that absolute error. The comparison
may be true value or conventional true value or arithmetic mean for series of
measurement.
b) Random errors: This type of errors occurs randomly and reason for this type of
errors cannot be specified.
10. What is the relationship between sensitivity and range? (EVEN 2013)
Sensitivity: The ratio of the change in output of the instrument to a change of
measured variable is termed as sensitivity.
Range: It is the minimum and maximum values of a quantity for which an
instrument is designed to measure.
PART-B
1. Explain the need of standards of measurements in modern industrial system
and describe the term traceability in connection with standards. (ODD 2015)
Standards The term standard is used to denote universally accepted
specifications for devices. Components or processes which ensure conformity and
interchangeability throughout a particular industry. A standard provides a reference for
assigning a numerical value to a measured quantity. Each basic measurable quantity
has associated with it an ultimate standard. Working standards, those used in
conjunction with the various measurement making instruments.
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The national institute of standards and technology (NIST) formerly called
National Bureau of Standards (NBS), it was established by an act of congress in 1901,
and the need for such body had been noted by the founders of the constitution. In order
to maintain accuracy, standards in a vast industrial complex must be traceable to a
single source, which may be national standards.
The following is the generalization of echelons of standards in the national
measurement system.
1. Calibration standards
2. Metrology standards
3. National standards
1. Calibration standards: Working standards of industrial or governmental
laboratories.
2. Metrology standards: Reference standards of industrial or Governmental
laboratories.
3. National standards: It includes prototype and natural phenomenon of SI (Systems
International), the world wide system of weight and measures standards. Application of
precise measurement has increased so much, that a single national a large country with
high technical development. It has led to the establishment of a considerable number of
standardizing laboratories in industry and in various other areas. A standard provides a
reference or datum for assigning a numerical value to a measured quantity.
Classification of Standards To maintain accuracy and interchangeability it is
necessary that Standards to be traceable to a single source, usually the National
Standards of the country, which are further linked to International Standards. The
accuracy of National Standards is transferred to working standards through a chain of
intermediate standards in a manner given below.
•National Standards
•National Reference Standards
•Working Standards
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Mechanical Engineering Department 9 Metrology and Measurements
•Plant Laboratory Reference Standards
•Plant Laboratory Working Standards
•Shop Floor Standards Evidently, there is degradation of accuracy in passing from the defining standards to the shop floor standards. The accuracy of particular standard
depends on a combination of the number of times it has been compared with a standard
in a higher echelon, the frequency of such comparisons, the care with which it was
done, and the stability of the particular standards itself.
2. Explain the sources of errors in precision measurement with suitable
illustrations. (EVEN 2015)
It is never possible to measure the true value of a dimension there is always
some error. The error in measurement is the difference between the measured value
and the true value of the measured dimension.
Error in measurement = Measured value - True value
The error in measurement may be expressed or evaluated either as an absolute error or
as a relative error.
Absolute Error
True absolute error: It is the algebraic difference between the result of measurement
and the conventional true value of the quantity measured.
Apparent absolute error: If the series of measurement are made then the algebraic
difference between one of the results of measurement and the arithmetical mean is
known as apparent absolute error.
Relative Error: It is the quotient of the absolute error and the value of comparison use
or calculation of that absolute error. This value of comparison may be the true value, the
conventional true value or the arithmetic mean for series of measurement. The accuracy
of measurement, and hence the error depends upon so many factors, such as: -
Calibration
-standard
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-Work piece
-Instrument
-Person
-Environment etc
Types of Errors
1. Systematic Error These errors include calibration errors, error due to variation in
the atmospheric condition Variation in contact pressure etc. If properly analyzed,
these errors can be determined and reduced or even eliminated hence also
called controllable errors. All other systematic errors can be controlled in
magnitude and sense except personal error.
These errors results from irregular procedure that is consistent in action. These errors
are repetitive in nature and are of constant and similar form.
2. 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.
3. Environmental Error These errors are caused due to effect of surrounding
temperature, pressure and humidity on the measuring instrument. External factors like
nuclear radiation, vibrations and magnetic field also leads to error. Temperature plays
an important role where high precision is required. e.g. while using slip gauges, due to
handling the slip gauges may acquire human body temperature, whereas the work is at
20°C. A 300 mm length will go in error by 5 microns which is quite a considerable error.
To avoid errors of this kind, all metrology laboratories and standard rooms worldwide
are maintained at 20°C.
3. What are the various elements of metrology? With example. Explain how these
elements influence the accuracy of measurements? (ODD 2014)
Elements of measuring system:
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Mechanical Engineering Department 11 Metrology and Measurements
1) Measuring Instruments
2) Calibration Standards
3) Work piece
4) Person (who is carrying out the measurement)
5) Environment
The above said five elements composed into the acronym ―SWIPE‖.
Where,
S — Standard
W — Work piece
I — Instrument
P — Person
E — Environment
The factors affecting these five elements:
1. Standard : Affected by Temperature, time, thermal expansion and
elasticity.
2.Workpiece :Surface finish, cleanliness, supporting elements, and elastic
Properties.
3. Instrument : Friction, error, mechanical parts.
4. Person : Ability to measure, training, cost estimation.
5. Environment : Light, Temperature, Humidity.
An example for the elements influence the accuracy of measurements
A generalized measurement system consists of the following components:
1. Primary Sensing Element
2. Variable Conversion Element
3. Variable Manipulation Element
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Mechanical Engineering Department 12 Metrology and Measurements
4. Data Processing Element
5. Data Transmission System
6. Data Presentation Element
In addition to the above components, a measurement system may also have a data
storage element to store measured data for future use. As the above six components
are the most common ones used in many measurement systems, they are discussed in
detail below
Block diagram of generalized measurement system:
1. Primary Sensing Element:
The primary sensing element receives signal of the physical quantity to be
measured as input. It converts the signal to a suitable form (electrical, mechanical or
other form), so that it becomes easier for other elements of the measurement system, to
either convert or manipulate it.
2. Variable Conversion Element:
Variable conversion element converts the output of the primary sensing element
to a more suitable form. It is used only if necessary.
3. Variable Manipulation Element:
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Variable manipulation element manipulates and amplifies the output of the
variable conversion element. It also removes noise (if present) in the signal.
4. Data Processing Element:
Data processing element is an important element used in many measurement
systems. It processes the data signal received from the variable manipulation element
and produces suitable output.
Data processing element may also be used to compare the measured value with
a standard value to produce required output.
5. Data Transmission System:
Data Transmission System is simply used for transmitting data from one element
to another. It acts as a communication link between different elements of the
measurement system. Some of the data transmission elements used to cables, wireless
antennae, transducers, telemetry systems etc.
6. Data Presentation Element:
It is used to present the measured physical quantity in a human readable form to
the observer. It receives processed signal from data processing element and presents
the data in a human readable form. LED displays are most commonly used as data
presentation elements in many measurement systems.
4.With suitable example explain the difference between precision and accuracy.
(EVEN 2014)
Precision
The terms precision and accuracy are used in connection with the performance
of the instrument. Precision is the repeatability of the measuring process. It refers to the
group of measurements for the same characteristics taken under identical conditions. It
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Mechanical Engineering Department 14 Metrology and Measurements
indicates to what extent the identically performed measurements agree with each other.
If the instrument is not precise it will give different (widely varying) results for the same
dimension when measured again and again. The set of observations will scatter about
the mean. The scatter of these measurements is designated as σ, the standard
deviation. It is used as an index of precision. The less the scattering more precise is the
instrument. Thus, lower, the value of σ, the more precise 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.
Accuracy is very often confused with precision though much different. The
distinction between the precision and accuracy will become clear by the following
example. Several measurements are made on a component by different types of
instruments (A, B and C respectively) and the results are plotted. In any set of
measurements, the individual measurements are scattered about the mean, and the
precision signifies how well the various measurements performed by same instrument
on the same quality characteristic agree with each other. The difference between the
mean of set of readings on the same quality characteristic and the true value is called
as error. Less the error more accurate is the instrument. Figure shows that the
instrument A is precise since the results of number of measurements are close to the
average value. However, there is a large difference (error) between the true value and
the average value hence it is not accurate. The readings taken by the instruments are
scattered much from the average value and hence it is not precise but accurate as there
is a small difference between the average value and true value.
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5. Distinguish between (i)Repeatability and reproducibility. (EVEN 2013)
Repeatability: Repeatability is defined as the variation of scale reading and is random in
nature. It is the closeness between successive measurement of the same quantity, with
the same instrument by the same operator over a short time span.
Reproducibility: it is the degree of closeness between measurement of the same
quantity where the individual measurements are made under different conditions like at
different locations, with different measuring instruments, by different operators.
6. Explain detail in static and dynamic response. (ODD 2015)
The static characteristics of measuring instruments are concerned only with the steady-
state reading that the instrument settles down to, such as accuracy of the reading. The
dynamic characteristics of a measuring instrument describe its behavior between the
time a measured quantity changes value and the time when the instrument output
attains a steady value in response. As with static characteristics, any values for dynamic
characteristics quoted in instrument data sheets only apply when the instrument is used
under specified environmental conditions. Outside these calibration conditions, some
variation in the dynamic parameters can be expected. In any linear, time-invariant
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Mechanical Engineering Department 16 Metrology and Measurements
measuring system, the following general relation can be written between input and
output for time (t) > 0:
where qi is the measured quantity, qo is the output reading, and ao...an, bo... bmare
constants. If we limit consideration to that of step changes in the measured quantity
only, then Equation (2) reduces to
UNIT-2
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Mechanical Engineering Department 17 Metrology and Measurements
LINEAR AND ANGULAR MEASUREMENTS
PART-A
1. State any two limitation of sine bar. (ODD 2015)
i.It is physically difficult to hold the position.
ii.slight errors in sine bar cause larger angular solutions.
2. What are the constructional requirements of a good sine bar? (ODD 2015)
i) The sine bar is physically clumsy to hold in position.
ii) The body of the sine bar obstructs the gauge block stack even if relieved.
iii) Slight errors of the sine bar cause large angular errors.
iv) Long gauge stacks are not nearly as accurate as when compared with shorter gauge
blocks.
v) Temperature variation affects the accuracy.
3. What is a comparator? (EVEN 2015)
i) It has less number of moving parts.
ii) Magnification obtained is very high.
iii) Two or more magnifications are provided in the same instrument to use various
ranges.
iv) The pointer is made very light so that it is more sensitive to vibration.
4. Mention the various applications and needs of comparators. (EVEN 2015)
The following are some of the ways in which the comparators used :i) In mass production, where components are to be checked at a very fast rate. ii) As
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laboratory standards from which working or inspection gauges are set and correlated. iii) For inspecting newly purchased measuring gauges and iv) Comparators can be used as working gauges to prevent work spoilage and to maintain required tolerances at all stages of manufacturing by attaching with the machines.
5. Differentiate between sine bar and sine centre. (ODD 2014)
Sine bar is used for locating any work to a given angle and to change unknown angle.The conical work is difficult to mount on sine bars, to overcome this sine centre is used. In this two blocks are mounted on top surface of sine bar at each end, these block have centres and can be clamped at any position.
6. Write any two precaution to be followed when using gauge blocks? (ODD 2014)
The following precautions must be taken.
• Use the minimum number of blocks.
• Wipe the measuring faces clean using soft clean chamois leather.
• Wring the individual blocks together by first pressing at right angles, sliding & then twisting.
7. Why are lasers used in metrology? (EVEN 2014)
The photon emitted during stimulated emission has the same energy, phase and frequency as the incident photon.
This principle states that the photon comes in contact with another atom or molecule in the higher energy level E2 then it will cause the atom to return to ground state energy level E, by releasing another photon.
8. State the working principle of an electronic comparator. (EVEN 2014)
In an electronic comparator, transducer induction or the principle of application of frequency modulation or radio oscillation is followed.
9. What are the advantages of electrical and electronic comparator? (EVEN 2013)
i) It has less number of moving parts.
ii) Magnification obtained is very high.
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iii) Two or more magnifications are provided in the same instrument to use various
ranges.
iv) The pointer is made very light so that it is more sensitive to vibration.
10. Write the constructional requirements of the sine bar for accurate measurement.
(EVEN 2013)
i) The rollers must have equal diameter and equal cylinders.
ii) The rollers must be placed parallel to each other and also to the upper face.
iii) The accurate center to center of rollers must be known.
iv) The top surface of the bar must be flat with high degree of accuracy.
PART-B 1. Explain the working principles of tool maker‟s microscope with neat sketch.
Also list out its applications. (ODD 2015)
Working:
i. Worktable is placed on the base of the instrument
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ii. The optical head is mounted on a vertical column and it can be moved up
and
down.
iii. Work piece is mounted on a glass plate
iv. A light source provides horizontal beam of light which is reflected from a
mirror
by 90O upwards towards the table
v. Image of the outline of contour of the work piece passess through the
objective of
the optical head.
vi. The image is projected by a system of three prisms to a ground glass
screen.
vii. The measurement are made by means of cross line engraved on the
ground glass
screen.
viii. Thescreencan be rotated through 360O
ix. Different types of graduated screens and eyepieces are used.
Application:
-Linear measurement, measurement of pitch of the screw.
-Measurement of pitch diameter.
2. What is comparator? Discuss the different types of comparators and its
applications.
(EVEN 2015)
Comparators are one form of linear measurement device which is quick and more
convenient for checking large number of identical dimensions. Comparators normally
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will not show the actual dimensions of the work piece. They will be shown only the
deviation in size. i.e. During the measurement a comparator is able to give the deviation
of the dimension from the set dimension. This cannot be used as an absolute
measuring device but can only compare two dimensions. Comparators are designed in
several types to meet various conditions. Comparators of every type incorporate some
kind of magnifying device. The magnifying device magnifies how much dimension
deviates, plus or minus, from the standard size. The comparators are classified
according to the principles used for obtaining magnification.
The common types are:
1) Mechanical comparators
2) Electrical comparators
3) Optical comparators
4) Pneumatic comparators
MECHANICAL COMPARATORS Mechanical comparator employs mechanical means
for magnifying small deviations. The method of magnifying small movement of the
indicator in all mechanical comparators are effected by means of levers, gear trains or a
combination of these elements. Mechanical comparators are available having
magnifications from 300 to 5000 to 1. These are mostly used for inspection of small
parts machined to close limits.
1. Dial indicator A dial indicator or dial gauge is used as a mechanical comparator.
The essential parts of the instrument are like a small clock with a plunger projecting at
the bottom as shown in fig. Very slight upward movement on the plunger moves it
upward and the movement is indicated by the dial pointer. The dial is graduated into
100 divisions. A full revolution of the pointer about this scale corresponds to 1mm travel
of the plunger. Thus, a turn of the pointer b one scale division represents a plunger
travel of 0.01mm.
Experimental setup The whole setup consists of worktable, dial indicator and vertical
post. The dial indicator is fitted to vertical post by on adjusting screw as shown in fig.
The vertical post is fitted on the work table; the top surface of the worktable is finely
finished. The dial gauge can be adjusted vertically and locked in position by a screw.
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Procedure Let us assume that the required height of the component is 32.5mm. Initially
this height is built up with slip gauges. The slip gauge blocks are placed under the stem
of the dial gauge. The pointer in the dial gauge is adjusted to zero. The slip gauges are
removed
Now the component to be checked is introduced under the stem of the dial gauge. If
there is any deviation in the height of the component, it will be indicated by the pointer.
Mechanism The stem has rack teeth. A set of gears engage with the rack. The pointer is
connected to a small pinion. The small pinion is independently hinged. I.e. it is not
connected to the stern. The vertical movement of the stem is transmitted to the pointer
through a set of gears. A spring gives a constant downward pressure to the stem.
2. Read type mechanical comparator.
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In this type of comparator, the linear movement of the plunger is specified
by means of read mechanism. The mechanism of this type is illustrated in fig. A spring-loaded pointer is pivoted. Initially, the comparator is set with the help of a known dimension eg. Set of slip gauges as shown in fig. Then the indicator reading is adjusted to zero. When the part to be measured is kept under the pointer, then the comparator displays the deviation of this dimension either in ± or— side of the set dimension.
Advantages
1) It is usually robust, compact and easy to handle.
2) There is no external supply such as electricity, air required.
3) It has very simple mechanism and is cheaper when compared to other types.
4) It is suitable for ordinary workshop and also easily portable.
Disadvantages
1) Accuracy of the comparator mainly depends on the accuracy of the rack and pinion
arrangement. Any slackness will reduce accuracy.
2) It has more moving parts and hence friction is more and accuracy is less.
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3) The range of the instrument is limited since pointer is moving over a fixed scale. 2
ELECTRICAL COMPARATOR: An electrical comparator consists of the following three
major part such as
1) Transducer
2) Display device as meter
3) Amplifier
Transducer An iron armature is provided in between two coils held by a lea spring at
one end. The other end is supported against a plunger. The two coils act as two arms of
an A.C. wheat stone bridge circuit.
Amplifier The amplifier is nothing but a device which amplifies the give input signal
frequency into magnified output Display device or meter The amplified input signal is
displayed on some terminal stage instruments. Here, the terminal instrument is a meter.
Working principle If the armature is centrally located between the coils, the inductance
of both coils will be equal but in opposite direction with the sign change. Due to this, the
bridge circuit of A.C. wheat stone bridge is balanced. Therefore, the meter will read zero
value. But practically, it is not possible. In real cases, the armature may be lifted up or
lowered down by the plunger during the measurement. This would upset the balance of
the wheat stone bridge circuit. Due to this effect, the change in current or potential will
be induced correspondingly. On that time, the meter will indicate some value as
displacement. This indicated value may be either for larger or smaller components. As
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this induced current is too small, it should be suitably amplified before being displayed
in the meter.
Checking of accuracy
To check the accuracy of a given specimen or work, first a standard specimen is placed
under the plunger. After this, the resistance of wheat stone bridge is adjusted so that the
scale reading shows zero. Then the specimen is removed. Now, the work is introduced
under the plunger. If height variation of work presents, it will move the plunger up or
down. The corresponding movement of the plunger is first amplified by the amplifier
then it is transmitted to the meter to show the variations. The least count of this
electrical comparator is 0.001mm (one micron).
ELECTRONIC COMPARATOR
In electronic comparator, transducer induction or the principle of application of
frequency modulation or radio oscillation is followed
Construction details
In the electronic comparator, the following components are set as follows:
i. Transducer
ii. ii. Oscillator
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iii. iii. Amplifier
iv. iv. Demodulator
v. v. Meter
(i) Transducer
It converts the movement of the plunger into an electrical signal. It is connected with
oscillator.
(ii) Oscillator
The oscillator which receives electrical signal from the transducer and raises the
amplitude of frequency wave by adding carrier frequency called as modulation.
(iii) Amplifier
An amplifier is connected in between oscillator and demodulator. The signal coming out
of the oscillator is amplified into a required level.
(iv) Demodulator
Demodulator is nothing but a device which cuts off external carrier wave frequency. i.e.
It converts the modulated wave into original wave as electrical signal.
(v) Meter
This is nothing but a display device from which the output can be obtained as a linear
measurement.
Principle of operation The work to be measured is placed under the plunger of the
electronic comparator. Both work and comparator are made to rest on the surface plate.
The linear movement of the plunger is converted into electrical signal by a suitable
transducer. Then it sent to an oscillator to modulate the electrical signal by adding
carrier frequency of wave. After that the amplified signal is sent to demodulator in which
the carrier waves are cut off. Finally, the demodulated signal is passed to the meter to
convert the probe tip movement into linear measurement as an output signal. A
separate electrical supply of D.C. is already given to actuate the meter.
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Advantages of Electrical and Electronic comparator
1) It has less number of moving parts.
2) Magnification obtained is very high.
3) Two or more magnifications are provided in the same instrument to use various
ranges.
4) The pointer is made very light so that it is more sensitive to vibration.
5) The instrument is very compact.
Disadvantages of Electrical and Electronic comparator
1) External agency is required to meter for actuation.
2) Variation of voltage or frequency may affect the accuracy of output.
3) Due to heating coils, the accuracy decreases.
4) It is more expensive than mechanical comparator.
3. Explain how the measurements are made with optical bevel protactor. (ODD
2014)
1. Vernier bevel protractor is the simplest angle measuring instrument. It consists of
main body, base plate stock, adjustable blade, circular plate containing vernier
scale and acute angle attachment. A universal bevel protractor is used to
measure angles between two planes. The blade can be moved along throughout
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its length and can also be reversed. It is about 150 or 300m long, 13mm wide and
2mm thick. Its ends are beveled at angles of 45˚ and 60˚. The acute angle attachment can be readily fitted into the body and clamped in any position. The
bevel protractors are tested for flatness, squareness, parallelism, straightness,
etc. The main scale is graduated in degrees of arc. The vernier scale has 12
divisions each side of the center zero. These are marked 0-60minutes of arc, so
that each divisions equals 1/12, of 60, that is 5minutes of arc. These 12 divisions
occupy the same space as 23 degrees on the main scale. Therefore, each
divisions of the vernier scale is equal to 1/12 of 23˚ or
12
111
. Since two divisions on
the main scale equals 2 degrees of arc, the difference between two divisions on
the main scale equals 2 degree of arc, the difference between two divisions on
the main scale and one division on the vernier scale is 2˚-
12
111
=
12
1
or 5
minutes of arc.
Procedure
1. First clean the blade of the bevel protractor and fix it over the body of it in the
groove.
2. Then place the specimen to which we have to determine the angle.
3. By adjusting the blade and the stock to slide over that two adjacent side of the
given
specimen, so that we can read the reading from main scale and vernier scale
reading.
4. Hence the angle of the given specimen was determined
4. Write the applications of auto collimeter. (EVEN 2014)
Auto-collimators are used for
1) Measuring the difference in height of length standards.
2) Checking the flatness and straightness of surfaces.
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3) Checking square ness of two surfaces.
4) Precise angular indexing in conjunction with polygons.
5) Checking alignment or parallelism.
6) Comparative measurement using master angles.
7) Measurement of small linear dimensions.
8) For machine tool adjustment testing
5. With a neat diagram explain the working of angle dekkor . (EVEN 2013)
Alignment telescope is used for aligning of bores, surfaces and check
squareness, straightness, flatness, parallelism, vertically and level.
One of the important type of alignment telescope is
Taylor-Hobson alignment telescope.
This instrument can be used to measure angular alignment as well as lateral
displacement and for this purpose the sighting target is mounted in a collimating
unit.
The telescope has an internal-focusing optical system, similar in principle to that of the
surveyor’s level built into a robust unit having a precisely ground external diameter.he focusing knob can be clearly seen in the optical system is shown in fig.
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The collimating unit consists of another steel tube, ground to the same diameter
as the telescope and containing an illuminating system, a graticule G2, a
collimating lens and another graticule G3.
The graticule G2 is graduated with central cross lines, surrounded with scales
and concentric circles and lies exactly at the principal focus of the collimating
lenses.
The graticule G3 contains a central pattern of converging Vee and several
graduated scales lying in two directions at right angles.
The use of telescope with the collimator is given in fig.
If the telescope is aligned with the collimator and sighted on it with its focus
adjusted to infinity target graticuleG2 will appear in the field of view, since rays
from this target will emerge parallel beams from the collimating lens.
Purely lateral displacements of telescope and target will therefore not register,
but any angular misalignment will show as a displacement of the image of the
target.
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If the telescope is now refocused until the target G3 appears in the field, only
lateral displacements of the collimator will be indicated, the parallel beams from
the target G2 being out of focus.
Lateral displacements of the collimator unit will therefore be measured in
thetelescope by means of the scales on graticuleG3.
The ground bores of the telescope and collimator make the instrument
particularly suitable for the alignment of two or more bores, such as bearings of
large engines.
The two units can be located centrally in each bore, using ground bushes where
necessary and both lateral and angular alignment can be measured.
Accurate optical alignment of the telescope with its ground diameter is ensured
by careful centering of the lenses and accuracy of the draw table of the focusing
lens.
The use of the optical micrometer and the accuracy obtainable by rotation of the
telescope are only available for the measurement of lateral displacement of the
target.
The instrument is not equipped for a similar accuracy or angular measurent
without any reason; a micrometer eyepiece would provide the means of doing
this.
6. Calculate the limits for a hole shafts pair designated 25 H8/d9 .Show
graphically the disposition of tolerance zones with reference to the zero line . The
lower deviation for a H, Type hole is zero.25 mm lies in the diameter range 18 mm
to 30 mm . Standard tolerance for IT 8 is 25 I and IT 9 is 40 I,where i is the
standard tolerance unit in microns and given as I (µm)= D+0.001D (Disin mm)
the upper deviation for “d” shaft is -16d ^o.44. (ODD 2015)
Solution:
Using calculations we proceed as under:
D =
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= 23.2379 mm
= 0.45 + 0.001 (23.2379)
= 1.307 microns
IT 8 is 25 i
Limits 25 H 8: 25 + 0.044 mm
- 0.00 mm
Tolerance on Hole = 0.044 mm
Since the tolerance on hole is less than 0.1 mm, no wear allowance will be
provided on the Go gauge.
Taking gauge tolerance to be 10% of work tolerance, tolerance on Go gauge =
0.0044 mm. similarly tolerance on Not-Go gauge is also 0.0044 mm. Disposition of
gauge tolerance with respect to work tolerance is shown in figure.
Fig: Disposition of workshop gauge tolerance
Upper limit of GO gauge = 25.000 mm
Lower limit of GO gauge = 25.0044 mm
Upper limit of Not-GO gauge = 25.044 mm
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Lower limit of Not-GO gauge = 25.025 mm.
IT 9 is 40 i
Limits 25 H 8: 25 + 0.044 mm
- 0.00 mm
Tolerance on Hole = 0.044 mm
Since the tolerance on hole is less than 0.1 mm, no wear allowance will be provided on
the Go gauge.
Taking gauge tolerance to be 10% of work tolerance, tolerance on Go gauge = 0.0044
mm. similarly tolerance on Not-Go gauge is also 0.0044 mm. Disposition of gauge
tolerance with respect to work tolerance is shown in figure.
Fig: Disposition of workshop gauge tolerance
Upper limit of GO gauge = 25.000 mm
Lower limit of GO gauge = 25.0044 mm
Upper limit of Not-GO gauge = 25.044 mm
Lower limit of Not-GO gauge = 25.025 mm.
7. Explain the working principle of AC laser interferometer and explain how the
the straightness is measured? (EVEN 2015)
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Ac Laser Interferometer
It is possible to maintain the quality of interference fringes over longer distance
when lamp is replaced by a laser source:
Laser interferometer uses AClaser as the light source and the measurements to
be madeover longer distance.
Laser is a monochromatic optical energy, which can be collimated into a
directional beamAC.
Laser interferometer (ACLI) has the following advantages.
High repeatability
High accuracy
Long range optical path
Easy installations
Wear and tear
Two-frequency Zeeman laser generates light of two slightly different
frequencies with opposite circularpolarizations. These beams get split up by
beam splitter B1 One part travels towards B2 and from there to external cube
corner here the displacement is to be measured. This interferometer uses cube
corner reflectors which reflect light parallel to its angle of incidence. Beam
splitter B2optically separates the frequency f1 which alone is sent to the
movable cube corner reflector.
The second frequencyf1 from B2is sent to a fixed reflector which then rejoins f1
at thebeam splitter B2to produce alternate light and dark interference flicker at
about 2 Megacycles per second. Now if the movable reflector moves, then the
returning beamfrequency will be Doppler-shifted slightly up or down by (∆f1).
Thus the light beams moving towards photo detector P2have frequencies
f2and(f1± ∆f1)andP2changes these frequencies into electrical signal. (Photocells
convert light-intensity variations into voltage pulses which can be processed by
electronic instruments to give the amount and direction of position change.)
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Photo detector P1 receive signal from beam splitter B1 and changes the
reference beamfrequencies f1and f2into electrical signal.
An AC amplifier A1 separates frequency difference signal (f2 – f1)and A2
separates frequency difference signal [f2- (f1±∆f)]. The pulse converter extracts ∆f, one cycle per half wavelength of motion. The up-down pulses from the pulse
converter are counted electronically and displayed in analog or digital form on
indicator. It may be noted that output in case of ACLI is in the form of pulses,
whereas in D.C systems, the output is in the form of a sinusoidal wave, the
amplitude (intensity) of which depends upon laser aging, air turbulence or air
pollutant and thus the change of amplitude leads to improper triggering and
counting errors.
8. Explain the following angular measurement methods using rollers. (ODD 2014)
(i) Measurement of angle by using rollers.
Rollers are precisely manufactured with high accuracy for metrological
applications. It is used to determine both linear and angular dimensions in
conjunction with gauge blocks. These are made of good quality steel and are
hardened and tapered. The length of the roller is equal to the diameter. The use
of precision rollers for determining both linear and angular dimensions is
explained with the help of following examples.
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1. Measurement of angle by using rollers
Angle of the right-tapered piece can be measured by using two rollers of different
sizes, slip gauges and a dial indicator. The two rollers whose diameters are known
and slip gauges are placed on a surface plate as shown in fig. 2.117. The rollers
may be clamped in position against an angle plate by C-clamps. The work is then
placed on top of rollers clamped against the angle plate by C-clamp, if the angle of
the piece all right, then the top edge will be parallel to surface plate. The di indicator
will show no variation when traversed along its surface.
From fig., the triangle 02A 01
Where, 1 = Length of slip gauge pile and
d1 and d2 = Diameters of rollers
From the above equation,
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Thus, initially the length of the slip gauges is calculated by ikabove equation and
the rollers are placed in contact with the slip gauges
(ii) Checking the angle of taper plug guage using rollers
Method of checking the angle of a taper plug gauge using rollers, micrometer and
slip gauges is illustrated by fig. A taper plug is placed on a surface plate. First, two
rollers of equal diameters are placed touching on the opposite sides of the lower
surface of the plug on the slip gauge combination of equal heights.
The distance (M1) between the ends of the roller is measured with a micrometer.
Then the rollers are placed on slip gauge combination of height (112) touching on
the opposite sides of the top portion of the plug. The distance (M2) between the
ends of the rollers in this new position is again measured by means of micrometer.
9. Explain Measuring of included angle of an interval dovetail. (EVEN 2014)
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Dovetail slides are widely used in machine tools as a guide ways. The sloping side
of the dovetail slide acts as guide and prevent the lifting of the female mating part
during sliding operation. This angle can be measured by using two rollers of equal
size, slip gauges and a micrometer similar to the previous case of taper plug gauge.
The two rollers of equal diameters are placed one each at the two corners and
distance 11 is measured across the rollers with a micrometer.
Then the rollers are placed on two sets of equal size slip gauge blocks and the
distance, is measured. It should be noted that the rollers do not extend above the
top surface of dovetail. Let the height of slip gauges be h, then from fig.
Dovetail slides are widely used in machine tools as a guide ways. The sloping side
of the dovetail slide acts as guide and prevent the lifting of the female mating part
during sliding operation. This angle can be measured by using two rollers of equal
size, slip gauges and a micrometer similar to the previous case of taper plug gauge.
The two rollers of equal diameters are placed one each at the two corners and
distance 11 is measured across the rollers with a micrometer.
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10. Explain in detail Measuring interior angle of a profile guage. (EVEN 2013)
Fig. shows the setup for measuring the interior angle of a profile gauge. Two balls
of different diameters and a vernier height gauge are required for measurement.
The small ball of diameter di is first placed in the slot of a profile gauge and the
height hi is measured using vernier height gauge. Then the ball of bigger diameter
d2 is placed after removing smaller one in the slot and the height h2 is measured
using vernier height gauge. The angle of a profile gauge 0 is given by the relation.
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UNIT-3
ADVANCES IN METROLOGY
PART-A
1. Differentiate between straightness and flatness. (ODD 2015)
A line is said to be straight over a given length if the variation of the distance
between the two points on the two planes perpendicular to each other and
parallel to the direction of a line remaining within the specified tolerance limits.
2. Give the advantages of co ordinate measuring system. (ODD 2015)
-Geometric errors in individual components of CMM cause errors. These errors
can be eliminated by compensation.
-Quick and accurate inspection is possible
-Automatic online processing of measured data is possible, even when the
component bring analyzed is still on the work table of CMM
3. State any two applications of laser in machine tool technology. (EVEN 2015)
* Straightness and flatness of guide ways and slide ways of machine tool.
* Flatness of machine tables
* Parallelism, equidistance and alignment of the slide ways.
* True running and alignment of shaft and spindle.
* Lead of lead screw or error in pitch
4. Name the four types of machine vision system. (EVEN 2015)
Image formation
Processing of image
Analysing the image
Interpretation of image.
5. What are touch trigger probes? (ODD 2014)
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A type of contact probe that detects a feature on a part and generates an
electronic signal to record its dimension. Touch trigger probes are the most
accurate and commonly used probes on the CMM.
6. What are diffraction gratings? (ODD 2014)
Diffraction grating are strips of glass or metal ruled with fine equally spaced lines
to from the grating. If the gratings are set at a small angle across the glass and
positioned so that the lines are angled to cross each other, a series of
interference type fringes known as moiré fringes would be produced.
7. Define degree of fullness and degree of emptiness in form factor. (EVEN 2014)
Degree of fullness is defined as the ratio between the area of metal present and
the area of the enveloping portion.
8. What are the advantages of Laser in interferometry? (EVEN 2014)
Laser provides a source of coherence and truly monochromatic light. The
property of clearance enables it to be projected in a narrow pencil of beam
without any scatter.
9. Write the features of CMM. (ODD 2013)
i) In faster machines with higher accuracies, the stiffness to weight ratio has to be
high in order to reduce dynamic forces.
ii) All the moving members, the bridge structure Z-axis carriage and Z-column are
made of hollow box construction.
iii) Errors in machine are built up and fed into the computer system so that error
compensation is built up into the software.
iv) All machines are provided with their own computers and the CMM is able to
measure three-dimensional object from the variable datums.
10. Mention the various geometrical checks made on machine tools. (ODD 2013)
The geometrical checks made on machine tools are :
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* Straightness and flatness of guide ways and slide ways of machine tool.
* Flatness of machine tables
* Parallelism, equidistance and alignment of the slide ways.
PART-B
1. Explain the gear tooth thickness and base tangent length is measured using
vernier gear tooth caliper and flange micrometer. (ODD 2015)
Tooth thickness is generally measured at pitch circle and also in most cases the
chordal thickness measurement is carried out i.e. the chord joining the intersection
of the tooth profile with the pitch circle. The methods which are used for measuring
the gear tooth thickness is
a) Gear tooth vernier caliper method (Chordal thickness method)
b) Base tangent method.
c) Constant chord method.
d) Measurement over pins or balls.
a) Gear tooth vernier method In gear tooth vernier method the thickness is
measured at the pitch line. Gear tooth thickness varies from the tip of the base
circle of the tooth, and the instrument is capable of measuring the thickness at a
specified position on the tooth. The tooth vernier caliper consists of vernier scale
and two perpendicular arms. In the two perpendicular arms one arm is used to
measure the thickness and other arm is used to measure the depth. Horizontal
vernier scale reading gives chordal thickness (W) and vertical vernier scale gives
the chordal addendum. Finally the two values compared.
The theoretical values of W and d can be found out by considering one tooth in the
gear and it can be verified. In fig noted that w is a chord ADB and tooth thickness is
specified by AEB. The distance d is noted and adjusted on instrument and it is
slightly greater than addendum CE.
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Vernier method like the chordal thickness and chordal addendum are dependent
upon the number of teeth. Due to this for measuring large number of gears different
calculations are to be made for each gear. So these difficulties are avoided by this
constant chord method.
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2. Discuss the need, types and constructional features of co ordinate
measuring machine. (ODD 2015)
CO-ORDINATE MEASURING MACHINES
Measuring machines are used for measurement of length over the outer surfaces of
a length bar or any other long member. The member may be either rounded or flat
and parallel. It is more useful and advantageous than vernier calipers, micrometer,
screw gauges etc. the measuring machines are generally universal character and
can be used for works of varied nature. The co-ordinate measuring machine is used
for contact inspection of parts. When used for computer-integrated manufacturing
these machines are controlled by computer numerical control. General software is
provided for reverse engineering complex shaped objects. The component is
digitized using CNC, CMM and it is then converted into a computer model which
gives the two surface of the component. These advances include for automatic
work part alignment on the table. Savings in inspection 5 to 10 percent of the time is
required on a CMM compared to manual inspection methods
Types of Measuring Machines
1. Length bar measuring machine.
2. Newall measuring machine.
3. Universal measuring machine.
4. Co-ordinate measuring machine.
5. Computer controlled co-ordinate measuring machine.
Constructions of CMM
Co-ordinate measuring machines are very useful for three dimensional
measurements. These machines have movements in X-Y-Z co-ordinate, controlled
and measured easily by using touch probes. These measurements can be made by
positioning the probe by hand, or automatically in more expensive machines.
Reasonable accuracies are 5 micro in. or 1 micrometer. The method these
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machines work on is measurement of the position of the probe using linear position
sensors. These are based on moiré fringe patterns (also used in other systems).
Transducer is provided in tilt directions for giving digital display and senses positive
and negative direction.
Types of CMM
(i) Cantilever type The cantilever type is very easy to load and unload, but
mechanical error takes place because of sag or deflection in Y-axis.
(ii) Bridge type Bridge type is more difficult to load but less sensitive to mechanical
errors.
(iii) Horizontal boring Mill typeThis is best suited for large heavy work pieces.
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Working Principle :
CMM is used for measuring the distance between two holes. The work piece is
clamped to the worktable and aligned for three measuring slides x, y and z. The
measuring head provides a taper probe tip which is seated in first datum hole and
the position of probe digital read out is set to zero. The probe is then moved to
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successive holes, the read out represent the co-ordinate part print hole location with
respect to the datum hole. Automatic recording and data processing units are
provided to carry out complex geometric and statistical analysis. Special co-
ordinate measuring machines are provided both linear and rotary axes. This can
measure various features of parts like cone, cylinder and hemisphere. The prime
advantage of co-ordinate measuring machine is the quicker inspection and accurate
measurements.
Causes of Errors in CMM
1) The table and probes are in imperfect alignment. The probes may have a degree
of run out and move up and down in the Z-axis may cause perpendicularity errors.
So CMM should be calibrated with master plates before using the machine.
2) Dimensional errors of a CMM is influenced by
Straightness and perpendicularity of the guide ways.
Scale division and adjustment.
Probe length.
Probe system calibration, repeatability, zero point setting and reversal error.
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Error due to digitization.
Environment
3) Other errors can be controlled by the manufacture and minimized by the
measuring software. The length of the probe should be minimum to reduce
deflection.
4) The weight of the work piece may change the geometry of the guide ways and
therefore, the work piece must not exceed maximum weight.
5) Variation in temperature of CMM, specimen and measuring lab influence the
uncertainly of measurements.
6) Translation errors occur from error in the scale division and error in straightness
perpendicular to the corresponding axis direction.
7) Perpendicularity error occurs if three axes are not orthogonal.
3. What is coordinate measuring machine ? What are the its basic elements ?
(EVEN 2015)
Coordinate measuring machine is one type of measuring machine. For
measurement of large dimensions CMM is used. These machines can take up work
of varied nature.
CMM are used for 3 dimensional measurements. These machines have movement
in 3 coordinates namely x-y-z coordinates. The movement of each coordinate can
be controlled and measured.
CMM has three slides. Each slide is fitted with an accurate and precise linear
measuring transducer to sense both in the positive and negative directions. The
transducer output is obtained digitally.
The work table of the machine is suspended on 3 point suspension
For master guide ways and probe location, principles of kinematic design are used.
Systematic errors in the machine are compensated by the soft wave.
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CMM has its own computer.
Measurement of the three dimensional objects from different datum is possible
using computers
Advantages of CMM
Geometric errors in individual components of CMM cause errors. These errors can
be eliminated by compensation.
Quick and accurate inspection is possible
Automatic online processing of measured data is possible, even when the
component bring analyzed is still on the work table of CMM
4. List out the various probes used in CMM and explain the working principle of
touch trigger probe. (EVEN 2015)
The various probes used in CMM are
Touch trigger probe (Reinshaw probe)
Touch scanners
Laser probers
Vision probes
Touch trigger probe (ieRenishaw probe)
The buckling mechanism is a three point hearing the contacts which are
arranged at 1200 around the circumference. These contacts act as
electrical micro switches.
When being touched in any probing direction one or f contacts is lifted off
and the current is broken, thus generating a pulse, when the circuit is
opened, the co-ordinate positions are read and stored.
After probing the spring ensures the perfect zero position of the three-
point bearing. The probing force is determined by the pre stressed force of
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the spring with this probe system data acquisition is always dynamic and
therefore the measuring time is shorter than in static principle.
5. Explain the working principle of a DC laser interferometer with a neat diagram ?
(ODD 2014)
Single Frequency DC Interferometer System:
DC laser interferometer is much improved system over the Michelson
simple interferometer. It uses a single frequency circular polarized laser
beam. On reaching the polarizing beam splitter, the beam splits into two
components. The reflected beam is being vertically polarized light. And,
the transmitted beam is being horizontally polarized light. These two
beams referred to as reference are and measurement are respectively
travel to their retro reflectors and are then reflected back towards the
beam splitter.
The recombined beam at beam splitter consists of two superimposed
beams of different polarization; one component vertically polarized having
traveled around reference arm and other component horizontally polarized
having traveled around the measurement arm. These two beams being
differently polarized do not interface. The recombined beam then passes
through a quarter wave plate which causes the two beams to interfere with
one another to produce a beam of plane polarized light. The angular
orientation of the plane of this polarized light depends on the phase
difference between the light in the two returned beams.
The direction of plane of polarization spin is dependent on the direction of
movement of the moving retro reflector. The beam after quarter wave
plate is split into three polarization sensitive detectors. As the plane of
polarized light spins, each detector produces a sinusoidal output wave
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form. The polarization sensitivity of the detectors can be set so that their
outputs have relative phases of 0, 90, and 180.
The output of the detectors can be used to distinguish the direction of movement and
also the distance moved by the moving retro reflector attached to the surface whose
displacement is to be measured.
For linear measurements (positional accuracy of velocity), the retro reflector is attached
to the body moving along the linear axis. For angular measurement, pitch and yaw), the
angular beam splitter is placed in the path between the laser head and the angular
reflector. In this way it is possible to measure flatness, straightness, rotatory axis
calibration. Arrangements also need to be made for environmental compensation
because the refractive index of the air varies with temperature, pressure and humidity.
Heterodyne interferometer, an A.C. device avoids all the problems encountered in
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above D.C. device, i.e. effect of intensity level change of source, fringe contrast
changes and D.C. level shifts which can cause fringe miscounting.
Interferometer is now an established and well developed technique for high accuracy
and high resolution measurement.
6. Write briefly about the various stages involved in machine vision. (ODD 2014)
A Vision system can be defined as a system for automatic acquisition and
analysis of images to obtain desired data for interpreting of controlling an activity. It is a
technique which allows a sensor to view a scene and derive 3 numerical or logical
decisions without further human intervention.
Machine vision can be defined as a means of simulating the image recognition
and analysis capabilities of the human system with electronic and demo mechanical
techniques. Machine vision system are now a day’s used to provide accurate and in
expensive 100% inspection of work pieces. They are used for functions like gauging of
dimensions, identification of shapes, measurement of distances, determining orientation
of parts, quantifying motion-detecting surface shading etc. It is best suited for high
production. These systems function without fatigue. This is suited for inspecting the
masks used in the production of microelectronic devices. Standoff distance up to one
meter is possible. Accuracy of ± 3p is achieved.
Vision System
The schematic diagram of a typical vision system is shown in fig.
This system involves image acquisition; image processing Acquisition requires
appropriate lighting. The camera and store digital image processing involves
manipulating the digital image to simplify and reduce number of data points.
Measurements can be carried out at any angle along the three reference axes x. y and
z without contacting the part the measured values are then compared with the specified
tolerance which stores in the memory of the computer.
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The main advantage of vision system is reduction or tooling and fixture costs,
elimination of need for precise part location for handling robots and integrated
automation of dimensional verification and defect detection.
Principle:
Four types of machine vision system and the schematic arrangement is shown in fig.
(i) Image formation.
(ii) Processing of image in a form suitable for analysis by computer.
(iii) Defining and analyzing the characteristic of image.
Interpretation of image and decision-making
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For formation of image suitable light source is required. It consists of incandescent light.
Fluorescent tube fiber optic bundleandarc lamp. Laser beam is used for triangulation
system for measuring distance. Ultraviolet light is used to reduce glare of increase
contrast. Proper illumination back lighting. Front lighting, structured light is required.
Back lighting is used to obtain maximum image contrast.
The surface of the object is to be inspected by using front lighting. .
For inspecting three-dimensional feature structured lighting is required. An image
sensor vidicon camera. CCD camera is used to generate the electronic signal
representing the image. The image sensor collects light from the scene through a lens,
using photosensitive target converts into electronic signal.
Vidicon camera:
Image is formed by focusing the incoming light through a sense of lenses onto the
photoconductive faceplate of the vidicon tube. The electron beam scans the
photoconductive surface and produces an analog voltage proportional to the variation in
light intensity for each scan line of the original scene.
Solid-stale camera:
The image sensors change coupled device (CCD) contain matrix of small away,
photosensitive elements accurately spaced and fabricated on silicon chips using
integrated circuit technology. Each detector converts in to analog signal corresponding
to light intensity through the camera lens.
Image processor:
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A camera may form an image 30 times per sec at 33 m see intervals. At each time
interval the entire image frozen for processing by an image processor. An analog to
digital converter is used to amen analog voltage of each detector in to digital value.
If voltage level for each pixel is given by either 0 or 1 depending on threshold value. It
is called binary system on the other hand grey scale system assigns upto 256 different
values depending on intensity to each pixel. Thus is addition to black and white many
different shades of grey can be distinguished.
Grey scale system requires higher degree of image refinement, huge storage
processing capability. For analysis 256 x256 pixels image array up to 256 different pixel
values will require 65000-S bit storage locations at a speed of 30 images per second.
Techniques windowing and image restoration are involved.
Windowing:
Processing is the desired area of interest and ignores non-interested part of image.
Image restoration:
Preparation of image during the pre-processing by remove the degrade.
Blurring of lines, poor contrast between images, and presence of noise are the
degrading.
The quality may be improved
(i) By improving the contrast by brightness addition.
(ii) By increasing the relative contrast between high and low intensity
elements.
(iii) By Fourier domain processing.
(iv) Other techniques to reduce edge detection and run length encoding.
Image Analysis:
Digital image of the object formed is analyzed in the central processing unit of the
system. Three important task performed by machine vision system are measuring the
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distance of an object from a vision system camera, determining object orientation and
defining object position.
The distance of an object from a vision system camera can be determined by
triangulation technique. The object orientation can be determined by the methods of
equivalent ellipse.
The image can be interpreted by two-dimensional image. For complex three-
dimensional objects boundary locations are determined and the image is segmented
into distinct region.
Image Interpretation:
This involves identification of an object.
In binary system, the image is segmented on the basis of white and black pixels.
The complex images can be interpreted by grey scale technique and algorithms.
The most common image interpretation is template matching.
FUNCTION OF MACHINE VISION
Block diagram of machine vision is shown.
Lighting and presentation of object to evaluated.
It has great compact on repeatability, reliability and accuracy.
Lighting source and projection should be chosen and give sharp contrast.
Images sensor compressor TV camera may be vidicon camera and state. The vidicon
camera low cost, greater resolution. Solid state cameras have greater accuracy, no
image tat and longer life. Image digitizer is a six to eight bit analog to digital AD
convener which is designed to keep up with the flow of video information from camera
and store the digitized image in memory.
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For simple processing, analog comparator and a computer controller to convert the
video information to a binary image is used Data compactor employs a high speed away
processor to provide high speed processing of the input image data. System control
computer communicates with the operator and make decision about the pan being
inspected. The output and peripheral devices operate the control of the system.
The output enables the vision system to either control a process or provide caution and
orientation information MU a robot. etc. These operate under the control of the system
control of computer.
7.Explain how V-Block and three point probe are used for measurement of
Roundness.Whatare the limitations of V-Block?
The fig. shows three probes with 120° spacing is very, useful for determining
effective size they perform like a 60° V-block.
60° V-block will show no error for 5 or 7 lobes magnify the error for 3-lobed parts
show partial error for randomly spaced lobes.
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7. Explain the following Direct Instrument measurements. (ODD 2013)
STYLUS PROBE TYPE INSTRUMENT:
Principle:
When the stylus be moved over the surface which is to be measured, the
irregularities in the surface texture is measured and it is used to assess the
surface finish of the work piece.
Working:
The stylus type instruments consist of skid, stylus, amplifying device and
recording device.
The skid is slowly moved over the surface by hand or by motor drive. The
skid follows the irregularities of the surface and the stylus moves along with skid.
When the stylus moves vertically up and down and the stylus movements’
are magnified, amplified and recorded to produce a trace.
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Then it is analyzed by automatic device.
Advantage:
Any desired roughness parameter can be recorded.
Disadvantages:
Fragile material cannot be measured.
High Initial cost.
Skilled operators are needed to operate.
8. Explain the Tomlinson surface meter. (ODD 2013)
This instrument uses mechanical-cum-optical means for magnification.
Construction:
In this the diamond stylus on the surface finish recorder is held by spring
pressure against the surface of a lapped cylinder. The lapped cylinder is supported one
side by probe and other side by rollers.The stylus is also attached to the body of the
instrument by a leaf spring and its height is adjustable to enable the diamond to be
positioned and the light spring steel arm is attached to the lapped cylinder and carries at
its tip a diamond scriber which bears against a smoked glass.
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Working:
When measuring surface finish, body is traversed across the surface by a screw rotated
by a synchronous motor. Any vertical movement of the stylus caused by the surface
irregularities causes the horizontal lapped steel cylinder to roll. By its rolling, the light
arm attached to its end provides a magnified movement on a smoked glass plate. This
vertical movement coupled with the horizontal movement produces a trace on the glass
magnified in vertical direction and there being no magnification in horizontal direction.
The smoked glass trace is then, further projected at 50 and 100 magnification for
examination. Finally the movement of scriber together with horizontal movement
produces at race on the smoked glass plate and this trace is magnified by an optical
projector.
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UNIT-IV
FORM MEASUREMENT
PART-A
1. What is floating carriage micrometer? (ODD 2015)
2.List the equipments needed for computer aided inspection. (ODD 2015)
i.micrometer
ii.screw gauge
iii.vernier caliper
iv height gauge
3.What is meant by best size wire in screw thread measurement? (EVEN 2015)
Best size of wire is the diameter of the wire in such a way that it makes
contact with the flanks of the thread on the pitch line.
4.Name the devices used for measurement of roundness. (EVEN 2015)
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The roundness is measured by,
Diametral gauge, Circumferential conferring gauge
Rotating on centre, Three point probe and Accurate spindle
5.Name the various stylus probe instruments used for surface finish
measurement. (ODD 2014)
1. Tomlinson surface meter.
2. Profilometer.
3. Taysurf-surface roughness meter.
6. How is roundness measured in the least squares circle method? (ODD 2014)
A circle is fitted to the measured profile such that the sum squares of the
departure of the profile data from this circle is a minimum. The roundness error is
the difference between the maximum departures of the profile from this circle.
7.Define machine vision. (EVEN 2014)
Machine vision is defined as the means simulating the image recognition and
analyse the capabilities of the human system with electronic and
electromechanical techniques.
8. Define straightness of axes. (EVEN 2014)
Is a geometrical shape.
Straightness measuring instrument:
– Collimator
9. Name the four reference circles used in measurement of roundness. (ODD
2013)
i) Least squares circle
ii) Minimum zone or minimum radial separation circles
iii) Maximum inscribed circle and
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iv) Minimum circumscribed circle
10. Derive the expression for „Best size of wire‟ in screw thread measurement. (ODD 2013)
Db=2AP sec x
Where , Db= wire diameter, X= included angle
AP=P/4, Therefore, Db=2(P/4)*sec x, Db=P/2*sec x
PART-B
1. Explain how to use laser interferometer to predict machine tool accuracies.
(ODD 2015)
i. Leveling of the machine
ii. Time running of locating cylinder of main spindle
iii. Axial ship of main spindle and tune running of shoulder face of spindle
nose.
iv. Time running of head stock center
v. Parallelism of main spindle to saddle movement
vi. True running of taper socket in main spindle
vii. Movement of upper slide parallel with main spindle in vertical plane.
viii. Parallelism of tailstock guide ways with the movement of carriage.
ix. Parallelism of tail stock sleeve taper socket to saddle movement.
x. Pitch accuracy of lead screw.
xi. Alignment of lead screw bearings with respect to each other.
2. How major and minimum diameters are measured? (ODD 2015)
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Pitch It is the distance measured parallel to the screw threads axis between the
corresponding points on two adjacent threads in the same axial plane. The basic pitch is
equal to the lead divided by the number of thread starts.
Minor diameter: It is the diameter of an imaginary co-axial cylinder which touches the
roots of external threads.
Major diameter: It is the diameter of an imaginary co-axial cylinder which touches the
crests of an external thread and the root of an internal thread.
Lead: The axial distance advanced by the screw in one revolution is the lead.
Pitch diameter: It is the diameter at which the thread space and width are equal to half
of the screw thread
Helix angle: It is the angle made by the helix of the thread at the pitch line with the axis.
The angle is measured in an axial plane.
Height of thread: It is the distance measured radially between the major and minor
diameters respectively
Addendum: Radial distance between the major and pitch cylinders for external thread.
Radial distance between the minor and pitch cylinder for internal thread.
Dedendum: It is the radial distance between the pitch and minor cylinders for external
thread. Also radial distance between the major and pitch cylinder for internal thread.
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3. How is the straightness of straight edge measured? (EVEN 2015)
A straight edge is a measuring tool which consists of a length of a length of a
steel of narrow and deep section in order to provide resistance to bending in the plane
of measurement without excessive weight. For checking the straightness of any surface,
the straight edge is placed over the surface and two are viewed against the light, which
clearly indicate the straightness. The gap between the straight edge and surface will be
negligibly small for perfect surfaces. Straightness is measured by observing the colour
of light by diffraction while passing through the small gap. If the colour of light be red, it
indicates a gap of 0.0012 to 0.0075mm. A more accurate method of finding the
straightness by straight edges is to place it in equal slip gauges at the correct point for
minimum deflection and to measure the uniformity of space under the straight edge with
slip gauges.
Test for straightness by using spirit level and Autocollimator
The straightness of any surface could be determined by either of these instruments by
measuring the relative angular positions of number of adjacent sections of the surface
to be tested. First straight line is drawn on the surface then it is divided into a number of
sections the length of each section being equal to the length of sprit level base or the
plane reflector’ s base in case of auto collimator. The bases of the spirit level block or
reflector are fitted with two feet so that only feet have line contact with the surface and
the surface of base does not touch the surface to he tested. The angular division
obtained is between the specified two points. Length of each section must be equal to
distance between the centerlines of two feet. The special level can be used only for the
measurement of straightness of horizontal surfaces while auto-collimator can be used
on surfaces are any plane. In case of spirit level, the block is moved along the line equal
to the pitch distance between the centerline of the feet and the angular variation of the
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direction of block. Angular variation can be determined in terms of the difference of
height between two points by knowing the least count of level and length of the base.
In case of autocollimator the instrument is placed at a distance of 0.5 to 0.75m from the
surface to be tested. The parallel beam from the instrument is projected along the
length of the surface to be tested. A block fixed on two feet and fitted with a plane
vertical reflector is placed on the surface and the reflector face is facing the instrument.
The image of the cross wires of the collimator appears nearer the center of the field and
for the complete movement of reflector along the surface straight line the image of cross
wires will appear in the field of eyepiece. The reflector is then moved to the other end of
the surface in steps equal to. The center distance between the feet and the tilt of the
reflector is noted down in second from the eyepiece.
4. Explain the principle and working of an electrical torsion meter with a sketch.
(EVEN 2015)
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Due to the applied torque, there is a relative displacement between the two slotted
discs. Due to this relative displacement of the slotted discs, a phase shift exists between
the pulse generated by the transducers. When these pulses are connected to an
electronic unit, it will show a time lapse between the two pulses. This time lapse
between the two pulses is proportional to the twist of the shaft and the torque of the
shaft.
The teeth produce voltage pulses in the transducers.
When torque is not applied on the shaft, the teeth of the both the discs perfectly align
with each other and hence he voltage pulses produced in the transducers will have zero
time difference.
But when torque is applied on the shaft, there is a relative displacement of the slotted
discs due to twist experienced by the shaft and hence the teeth of both the discs will not
align with each other and hence the voltage pulses produced in the transducer will have
a time difference (that is , time lapse).
This time lapse between the pulses of the two discs is proportional to the twist of the
shaft and hence the torque of the shaft.
A measure of this time lapse becomes of torque when calibrated.
5. How is the tooth thickness of a gear measured in the base tangent method ?
Derive the expression for tooth thickness of a gear in this method. (ODD 2014)
The measurement is done by using micrometer with anvils or by the david brown
tangent computer.
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Two anvils, one is fixed and another immovable.
Fixed anvil is used to set the distance by adjusting locking ring and setting tubes.
6. With a neat diagram explain the working principle of any one stylus type
surface finish measuring instruments. (ODD 2014)
The accuracy of manufactured parts depends on the accuracy of machine tools.
The quality of work piece depends on Rigidity and stiffness of machine tool and its
components. Alignment of various components in relation to one another Quality and
accuracy of driving mechanism and control devices. It can be classified into Static tests
Dynamic tests. Static tests If the alignment of the components of the machine tool are
checked under static conditions then the test are called static test. Dynamic tests If the
alignment tests are carried out under dynamic loading condition. The accuracy of
machine tools which cut metal by removing chips is tested by two types of test namely.
Geometrical tests o Practical tests Geometrical tests In this test, dimensions of
components, position of components and displacement of component relative to one
another is checked. Practical tests In these test, test pieces are machined in the
machines. The test pieces must be appropriate to the fundamental purpose for which
the machine has been designed.
7. How is surface finish indicated in an engineering drawing .What are the various
elements indicated in the symbol? (EVEN 2014)
This instrument uses mechanical-cum-optical means for magnification.
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Construction:
In this the diamond stylus on the surface finish recorder is held by spring
pressure against the surface of a lapped cylinder. The lapped cylinder is supported one
side by probe and other side by rollers. The stylus is also attached to the body of the
instrument by a leaf spring and its height is adjustable to enable the diamond to be
positioned and the light spring steel arm is attached to the lapped cylinder and carries at
its tip a diamond scriber which bears against a smoked glass.
Working:
When measuring surface finish, body is traversed across the surface by a screw rotated
by a synchronous motor. Any vertical movement of the stylus caused by the surface
irregularities causes the horizontal lapped steel cylinder to roll. By its rolling, the light
arm attached to its end provides a magnified movement on a smoked glass plate. This
vertical movement coupled with the horizontal movement produces a trace on the glass
magnified in vertical direction and there being no magnification in horizontal direction.
The smoked glass trace is then, further projected at 50 and 100 magnification for
examination. Finally the movement of scriber together with horizontal movement
produces at race on the smoked glass plate and this trace is magnified by an optical
projector.
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8. With a neat sketch explain the dimensional measurement using
Scanning Laser Guage. (EVEN 2014)
Laser telemetric system is a non-contact gauge that measures with a
collimatedlaser beam. It measures at the rate of 150 scans per second.It
basically consists of three components, a transmitter, and a receiver and
processorelectronics. The transmitter module produces a collimated parallel
scanning laserbeam moving at a high constant, linear speed.
The scanning beam appears a red line. The receiver module collects and photo
electrically senses the laser light transmitted past the object being measured.
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The processor electronics takes the received signals to convert them to
aconvenient form and displays the dimension being gauged.
The transmitter contains a low power helium-neon gas laser and its power
supply, a specially designed collimating lens, a synchronous motor, a multi-
facetedreflector prism, a synchronous pulse photo detector and a protective
replaceable window.
The high speed of scanning permits on line gauging and thus it is possible to
detect changes in dimensions when components are moving on a continuous
product such as in rolling process moving at very high speed.There is no need
of waiting or product to cool for taking measurements. This system can also be
applied on production machines and control then with closed feedback
loops.Since the output of this system is available in digital form, it can run a
process controller limit alarms can be provided and output can be taken on
digital printer. It is possible to write programs for the microprocessor to take care
of smoke, dust and other airborne interference around the work piece being
measured.
(ii)Name the alignment tests performed on milling machine.
(4)
Alignment Tests on Milling Machine:
Machine tools are very sensitive to impact or shock, even heavy cast inn
standards are not always solid and rigid enough to withstand stresses due to
falling during transportation, and deformations may be set up.
Although the machine is always carefully adjusted and aligned when on the test
stand or in the assembly department of the manufacturer, it is well known from
experience that erection in the workshop of the user is not always done with
sufficient care and thus inaccuracies of the work may result from the faulty
erection of the machine.
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So the machine should be carefully levelled up by means of a spirit level before
starting with the actual trial tests. Each trial measurement is based on the correct
erection of the machine. No upright, base etc. can be made so rigid that it will be
thoroughly free from deformation resulting from faulty erection.
Machine tools for the workshop must be able to produce workpieces of given
accuracy within prescribed limits, consistently and without requiring artistic skill
on the part of the operator.
For acceptance test of a machine, its alignment test is performed and to see its
dynamic stability, which may be poor though alignment tests are right, certain
specific jobs are prepared and their accuracy check.
The relative alignment of all parts of machine and the accuracy of the control
devices and driving mechanisms are measured under no load condition. The
result of these measurements must lie within the prescribed limits given by the
manufacturer depending upon the grade of the machine tool
A specification for the alignment tests must comply with the following general
requirements:
The procedure for testing standard machine tools must not require more than 6
to 8 hrs of work provided allthe tooling and measuring equipment are readily
available.
The permissible limits of accuracy of individual measurements must be wide
enough to make economical manufacture possible while on the other hand the
cumulative error of number of superimposed details should not be excessive.
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The various tests performed on the milling machine are shown in (Fig. 16.16) and
described below.
1. Cutter Spindle Axial Slip or Float.
We have to distinguish between axial (or end) play and axial slip of the spindle.
End play means the indispensable freedom of a spindle moving in the axial
direction to prevent it from seizing by heating. This end play is especially
important on high speed machines and it should be within the prescribed limits.
Axial slip is denned as the axial spindle movement which may repeat positively
with each revolution as a consequence of manufacturing errors. It is only this
axial sliding movement which is to be tested, and the specified tolerance applies
only to this movement.
When testing the axial slip of a spindle the feeler of the dial gauge rests on the
face of the locating spindle shoulder and dial gauge holder is clamped to the
table. The locating spindle shoulder is rotated and change in reading is noted.
This is done at the two spots diametrically opposite to each other. The total error
indicated by the movement of the pointer includes three main sources of errors.
(i)Axial slip due to error in bearing supporting the locating shoulder.
(ii) Face of the locating shoulder not in a plane perpendicular to axis of rotation.
(Hi)
Irregularities of front face. Effects of this will be that in cutting spirals, the pitch
will not be constant and we will get irregular pitch helix. If the feeler touches at
the same spot where the turning tool on the emery wheel has machined the
spindle collar in the assembled machine, then the feeler will not show any
deviation. Therefore axial slip must always be tested at two points 180° apart on
the collar of the spindle.
2. Eccentricity of External Diameter.
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The feeler is placed on the cylindrical surface of the shoulder. The locating
shoulder is rotated and any deviation in reading of dial gauge is noted. It is due to
eccentricity of the spindle in the hole in which it fits.
Due to it, vibrations are produced and the cutter will float sideways and cut over,
or under-size. Face mills may dig in when leading edges cease to cut.
3. True Running of Internal Taper.
The table is set in its main position longitudinally and the mandrel 300 mm long is
fixed in the spindle taper. A dial gauge is set on the machine table and feeler
adjusted to touch the lower surface of the mandrel.
The mandrel is then turned and the dial readings at two points are noted i.e., one
at the place nearest to spindle nose and other at about 300 mm from it. For
shifting the position of dial gauge from A to B cross-slide of the machine is
operated to bring the dial gauge at the bottom of the end of mandrel.
There are can be two errors :
a. Axis of the spindle and the axis of taper may not be parallel.
b. Eccentricity of the taper hole which, if present, should indicate same error
at both the places. The error in first case will give different readings at two
places. Due to this error, cut will not be shared equally between teeth of
cutters, and therefore vibrations and poor finish will result.
4. Table surface parallel with arbor rising towards overarm.
In selecting the permissible errors of horizontal milling machines, care is taken to
the fact that in the direction parallel to the cutter spindle, the work table extends
towards the front face of the knee only, and never slopes down. While working,
the table tends to incline downwards under the influence of the weight of work
and cutting pressure, while the cutter arbor tends to deflect upwards. Great
importance, therefore, has always been attached to the necessity of having the
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direction of table tolerance opposite to the deformation expected under cutting
conditions.
Parallelism between table face and the axis of the main spindle is checked as
follows:
A dial gauge is set on the machine table. A mandrel 300 mm long is fitted in the
spindle taper. The feeler of dial gauge is made to touch the lower surface of the
mandrel. With mandrel in position (mean) the readings at the maximum travel of
the table surface are observed. The stand of the dial gauge is moved and not the
table itself remains stationary. Effect of this error will be that the milled surface
produced will not be square to the base and parallel cross ways.
5. Surface Parallel with Longitudinal Movement.
For this test the dial gauge is fixed to the spindle. Feeler is directed upon the
surface the machine table and latter moved longitudinally. The deviations from
parallelism between the table surface and longitudinal motion are noted down. If
the table is uneven, a straight edge may be placed on the surface.
Due to this error the surface of the table will fluctuate up and down and cutter will
not take equal cuts on the job which is clamped on the table and the milled
surface will not be parallel to the base.
6. Traverse Movement Parallel with Spindle Axis.
(a) In horizontal plane; (b) In vertical plane. The table is set in its mean position and
dial gauge fixed on the table. The table is moved crosswise and any deviation on
reading of dial gauge is noted with feeler on one side of mandrel in horizontal plane and
under the mandrel for error in vertical plane.Due to this error, depth of cut will vary when
cross slide is moved.
7. Central T-Slots Parallel with Longitudinal Movement.
The T-slot, particularly the central one should be well machined on the internal
vertical surface throughout its length because jigs and fixtures are located by T-
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slots. The general parallelism of the central slot with the longitudinal movement
of the table is checked by using a bracket 150 mm long with a tension which
enters the T-slot.
Against the upper surface of the bracket in vertical plane the feeler of the dial
gauge is located. Having fixed the dial gauge to the spindle and adjusting its
feeler to the surface of the bracket the table is moved longitudinal while
thetension block is held stationary by hand and deviations from parallelism are
noted from dial gauge. During the process the tension slides along the slot, thus
eliminating the effects of local error.
Due to this error, the depth of cut will not remain constant as the job will be
inclined according to inclination of T-slots with longitudinal movement and the
axis of job held between tail stock and index head will not be perpendicular to
cutter.
8. Centre T-slot Square with the Arbor.
If the central T-slot is not perpendicular to the arbor, the key way etc. cut on the
machine will not be parallel to the axis of job.
For this test table is adjusted in the middle portion of its longitudinal movement
and a tension block 160 mm long inserted in the central T-slot. A dial gauge is
fixed on the mandrel, the feeler being adjusted to touch the vertical face of the
bracket.
Observe the reading on the dial gauge when the bracket or tension block is near
one end of table. Then swing over the dial gauge and move the tension block so
that the corresponding reading can be taken near the other end of the table.
Generally two tension blocks are used.
9. Tests on Column.
(a) Column ways of knee square with table, inclination front to rear (b) Side
inclination.
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Arrange table is its central position and fix a square with an arm about 300 mm
long (Angle iron type bracket placed on the surface), on the table surface and
attach a dial gauge to the spindle mandrel in such a way that the feeler rests on
the arm (vertical face) of the surface near the bottom edge. Observe the reading
of the dial gauge.
Move the table upwards about 300 mm and again observe the dial gauge. The
difference in readings is a direct indication of the error of perpendicularity of the
table surface (from guiding surface) and knee support or side guiding support.
The above test is conducted for two positions of the square and in first position
the dial gauge touches the square in front and in second position it faces the
side of the square i.e. at 90° to the first position (for testing side inclination).
If column-ways for knee are not square with the table, as the table is fed
upwards in facing operation or end milling, the surface produced will not be
square with the table surface.
10. Overarm parallel with spindle.
(a) In horizontal plane, (b) in vertical plane.
To check the parallelism of the overarm and the spindle, fix the dial gauge on the
table and its feeler under the mandrel. Move the table crosswise and note any
change in the reading.
For error in the horizontal plane, repeat above readings so that feeler is under
overarm and compare the two sets of readings. For vertical plane compare the
readings on mandrel and side of overarm.16.4.10.
Alignment of the main spindle with bore of the bracket of the overarm.
With the mandrel (parallel) in the bore of the overhanging bracket and gauge
holder in the mandrel fitted to the spindle taper, the feeler is adjusted so that it
touches the mandrel in thebore.
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The main spindle is turned slowly and reading of the dial gauge is noted at four
points (opposite ends in horizontal and vertical planes i.e. 90° apart).
The difference between two 180° opposite readings and other two is twice the
eccentricity of the mandrel in the vertical and horizontal directions respectively.
It the axis of the bearing of the supporting bracket is not co-axial with the spindle
axis, the axis of the arbor which is held in spindle and supporting bracket will not
be parallel with table surface and hence the cutter mounted on the arbor will take
more cut on supporting bracket side if the bearing axis is somewhat lower than
spindle axis and less cut if the bearing axis is above.
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UNIT-5
MEASUREMENT OF POWER, FLOW AND TEMPERATURE
PART-A
1. List any two methods employed for measuring torque. (ODD 2015)
Mechanical torsion meter
Electrical torsion meter
2. What are the physical characteristics of temperature measuring sensor? (ODD
2015)
Resistance Temperature Detectors are the sensors used to measure the
temperature by correlating the resistance of the RTD element with temperature.
3. What is the function of load cell? (EVEN 2015)
When the strain gauge – elastic member combination is used for weighing it is
called a load cell. Load cells utilize an elastic member as the primary transducer and
strain gauge as secondary transducer.
4. Name any four instruments used for measuring temperature. (EVEN 2015)
1. Thermocoulping.
2. Electrical thermal resistance.
3. Thermostats.
4. Pyrometers.
5. Define the principle of electrical resistance thermistor. (ODD 2014)
Electrical Resistance thermometers, also called resistance temperature detectors
(RTDs), are sensors used to measure temperature by correlating the resistance of the
RTD element with temperature. Most RTD elements consist of a length of fine coiled
wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it
is often placed inside a sheathed probe to protect it. The RTD element is made from a
pure material, typically platinum, nickel or copper. The material has a predictable
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change in resistance as the temperature changes and it is this predictable change that
is used to determine temperature.
Advantages
• High accuracy
• Low drift
• Wide operating range
• Suitability for precision applications.
6. Name the instrument used for low pressure measurements. (ODD 2014)
Low pressure-Manometer
7. Why are measuring instruments calibrated? (EVEN 2014)
Laser interferometer system can be used to determine linear positioning and
straightness errors in machine axes, to improve performance through targeted
maintenance and correction for linear positioning errors using error compensation
(using sensors for measurement of air and material temperature, air humidity and air
pressure).
8. What is the working principle behind strain gauges? (EVEN 2014)
When force is applied to any metallic wire its length increases due to the strain. A strain
gauge consists of a foil of resistive characteristics, which is safely mounted on a
backing material. When a known amount of stress in subjected on the resistive foil, the
resistance of the foil changes accordingly. Thus, there is a relation between the change
in the resistance and the strain applied. This relation is known by a quantity called
gauge factor. The change in the resistance can be calculated with the help of a
Wheatstone bridge.
9. What is a bimetallic strip? Name its types. (ODD 2013)
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A bimetallic strip is made of two thin strips of metal which have different thermal
co-efficients of expansion. The two metal strips are joined together by brazing,
welding or riveting so that the relative motion between them is arrested.Different
common forms of bimetallic sensors are: * Helix type * Spiral type * Cantilever type
and * Flat type.
10. What is the principle used in thermocouples? (or) What is “Principle of thermo electricity”? (or) What is seebeck effect? (ODD 2013)
The principle states that ― When two conductors of two different metals A
and B are joined together at one end to form a junction, and this junction is heated to a
higher temperature with respect to the free ends, a voltage is developed at the free
ends and if these two conductors of metals at the free ends are connected, then the emf
setup will establish a flow of current‖.
PART-B
1. Explain in briefly any one method of torque measurement. (ODD 2015)
Measurement of applied torques is of fundamental importance in all rotating
bodies to ensure that the design of the rotating element is adequate to prevent failure
under shear stresses.
wer transmitted by
rotating shafts.
The four methods of measuring torque consist of
Measuring the strain produced in a rotating body due to an applied torque
An optical method
Measuring the reaction force in cradled shaft bearings
Using equipment known as the Prony brake.
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Measurement of Induced Strain Measuring the strain induced in a shaft due to an
applied torque has been the most common method used for torque measurement in
recent years. The method involves bonding four strain gauges onto a shaft as shown in
Figure, where the strain gauges are arranged in a d.c. bridge circuit. The output from
the bridge circuit is a function of the strain in the shaft and hence of the torque applied.
It is very important that positioning of the strain gauges on the shaft is precise, and the
difficulty in achieving this makes the instrument relatively expensive. This technique is
ideal for measuring the stalled torque in a shaft before rotation commences. However, a
problem is encountered in the case of rotating shafts because a suitable method then
has to be found for making the electrical connections to the strain gauges. One solution
to this problem found in many commercial instruments is to use a system of slip rings
and brushes for this, although this increases the cost of the instrument still further.
2. How is the pitot tube used for flow measurements? (ODD 2015)
An obstruction type primary element used mainly for fluid velocity measurement is the
Pitot tube.
Principle
Consider Figure which shows flow around a solid body. When a solid body is held
centrally and stationary in a pipeline with a fluid streaming down, due to the presence of
the body, the fluid while approaching the object starts losing its velocity till directly in
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front of the body, where the velocity is zero. This point is known as the stagnation point.
As the kinetic head is lost by the fluid, it gains a static head. By measuring the
difference of pressure between that at normal flow line and that at the stagnation point,
the velocity is found out. This principle is used in pitot tube sensors
Pitot tube
A common industrial type of pitot tube consists of a cylindrical probe inserted into the air
stream, as shown in Figure. Fluid flow velocity at the upstream face of the probe is
reduced substantially to zero. Velocity head is converted to impact pressure, which is
sensed through a small hole in the upstream face of the probe. A corresponding small
hole in the side of the probe senses static pressure. A pressure instrument measures
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the differential pressure, which is proportional to the square of the stream velocity in the
vicinity of the impact pressure sensing hole. The velocity equation for the pitot tube is
given by,
Advantages
1. No pressure loss.
2. It is relatively simple.
3. It is readily adapted for flow measurements made in very large pipes or ducts
Disadvantages
1. Poor accuracy.
2. Not suitable for dirty or sticky fluids and fluids containing solid particles.
3. Sensitive to upstream disturbances.
3. Explain the working of bimetallic strip type temperature measuring system.
(EVEN 2015)
Two dissimilar metals are bonded together into what is called a bimetallic strip, as
sketched to the right.
As temperature increases, metal B expands more than does metal A, causing the
bimetallic strip to curl upwards as sketched.
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bimetallic strip is used as the arm of a switch between electrical contacts. As the room
temperature changes, the
bimetallic strip bends as discussed above. When the bimetallic strip bends far enough, it
makes contact with electrical leads that turn the heat or air conditioning on or off.
perature indicates over-current,
which shuts off the circuit.
thermometers. These thermometers consist of a bimetallic strip wound up in a spiral,
attached to a dial that is calibrated into a temperature scale.
4. What are the thermocouples?State its applications. (EVEN 2015)
Thomas Johan Seeback discovered in 1821 that thermal energy can produce
electric current. When two conductors made from dissimilar metals are connected
forming two common junctions and the two junctions are exposed to two different
temperatures, a net thermal emf is produced, the actual value being dependent on the
materials used and the temperature difference between hot and cold junctions. The
thermoelectric emf generated, in fact is due to the combination of two effects: Peltier
effect and Thomson effect. A typical thermocouple junction is shown in fig. 5. The emf
generated can be approximately expressed by the relationship
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Where, T1 and T2 are hot and cold junction temperatures in K. C1 and C2 are
constants depending upon the materials. For Copper/ Constantan thermocouple,
C1=62.1 and C2=0.045.
Thermocouples are extensively used for measurement of temperature in industrial
situations. The major reasons behind their popularity are:
(i) They are rugged and readings are consistent
(ii) They can measure over a wide range of temperature
(iii) Their characteristics are almost linear with an accuracy of about 0.05%.
However, the major shortcoming of thermocouples is low sensitivity compared
to other temperature measuring devices (e.g. RTD, Thermistor).
5.With a sketch explain the torque measurement using Strain Guages. (ODD 2014)
Measuring the strain induced in a shaft due to an applied torque has been the
most common method used for torque measurement in recent years. Torque
transducers based on strain measurement are normally made by applying strain gauges
to a shaft to measure the shear strain caused by torsion. The shear stress causes
strains to appear at 45° to the longitudinal axis of the shaft. So, the strain gauges must
be placed precisely at 45° to the shaft axis as shown in fig.
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(ii) Otherwise, the arrangement is sensitive to bending and axial stress in
addition to those caused by torsion. The output is increased by using four
gauges so that the adjacent arms have strains of opposite nature. Also this
arrangement provides complete thermal compensation. For taking signals in
and out of the rotating shaft, slip rings and brushes are used. The
arrangement of slip rings and brushes are shown on fig .
(iii) It is easier to measure bending strains rather than strains due to torque at 45°
and so an arrangement using beams may be employed, in which the
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transmitted torque results in bending the beams. This arrangement is shown
in fig.
(iv) A slip ring arrangement results in noise due to change in contact resistance
also slip rings and brushes wear out and hence it needs to be renewed. A
non-contacting type of arrangement as shown in fig 5.24.is preferred. This
bridge supply and output signals are transmitted between the rotating and
stationary member through transformers. Through AC supply of the bridge, an
amplitude modulated AC voltage proportional to the torque is obtained as the
output of the bridge. The AC voltage necessary for supply, the strain gauge
bridge and the measurement signal can be transmitted via rotating
transformer.
6. Explain the different types of torque measurements techniques with example.
(ODD 2014)
Measurement of applied torques is of fundamental importance in all rotating bodies to
ensure that the design of the rotating element is adequate to prevent failure under shear
stresses.
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Torque measurement is also a necessary part of measuring the power transmitted by
rotating shafts.
The four methods of measuring torque consist of
1.Measuring the strain produced in a rotating body due to an applied torque
2.An optical method
3.Measuring the reaction force in cradled shaft bearings
4.Using equipment known as the Prony brake.
1.Measuring the strain induced in a shaft due to an applied torque has been the
most common method used for torque measurement in recent years.
The method involves bonding four strain gauges onto a shaft as shown in Figure, where
the strain gauges are arranged in a d.c. bridge circuit. The output from the bridge circuit
is a function of the strain in the shaft and hence of the torque applied. It is very
important that positioning of the strain gauges on the shaft is precise, and the difficulty
in achieving this makes the instrument relatively expensive. This technique is ideal for
measuring the stalled torque in a shaft before rotation commences. However, a problem
is encountered in the case of rotating shafts because a suitable method then has to be
found for making the electrical connections to the strain gauges. One solution to this
problem found in many commercial instruments is to use a system of slip rings and
brushes for this, although this increases the cost of the instrument still further.
2. Optical Torque Measurement
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Optical techniques for torque measurement have become available recently with the
development of laser diodes and fiber-optic light transmission systems. One such
system is shown in Figure. Two black-and-white striped wheels are mounted at either
end of the rotating shaft and are in alignment when no torque is applied to the shaft.
Light from a laser diode light source is directed by a pair of fiber-optic cables onto the
wheels. The rotation of the wheels causes pulses of reflected light, which are
transmitted back to a receiver by a second pair of fiber-optic cables. Under zero torque
conditions, the two pulse trains of reflected light are in phase with each other. If torque
is now applied to the shaft, the reflected light is modulated. Measurement by the
receiver of the phase difference between the reflected pulse trains therefore allows the
magnitude of torque in the shaft to be calculated. The cost of such instruments is
relatively low, and an additional advantage in many applications is their small physical
size.
3. Reaction Forces in Shaft Bearings
Any system involving torque transmission through a shaft contains both a power source
and a power absorber where the power is dissipated. The magnitude of the transmitted
torque can be measured by cradling either the power source or the power absorber end
of the shaft in bearings, and then measuring the reaction force, F, and the arm length, L,
as shown in Figure. The torque is then calculated as the simple product, FL. Pendulum
scales are used very commonly for measuring the reaction force. Inherent errors in the
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method are bearing friction and windage torques. This technique is no longer in
common use.
4. Prony Brake
The Prony brake is another torque-measuring system that is now uncommon. It is used
to measure the torque in a rotating shaft and consists of a rope wound round the shaft,
as illustrated in Figure. One end of the rope is attached to a spring balance and the
other end carries a load in the form of a standard mass, m. If the measured force in the
spring balance is Fs, then the effective force, Fe, exerted by the rope on the shaft is
given by
Fe = mg - Fs If the radius of the shaft is Rs and that of the rope is Rr, then the effective radius, Re,
ofthe rope and drum with respect to the axis of rotation of the shaft is given by
Re = Rs+ Rr The torque in the shaft, T, can then be calculated as
T= FeRe
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While this is a well-known method of measuring shaft torque, a lot of heat is generated
because of friction between the rope and shaft, and water cooling is usually necessary.
Prony brake
7. Explain any one type of pyrometer for measuring temperature.
Pyrometers(Thermocouple
8. Describes with neat sketches thermocouples.
INTRODUCTION:
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The most common method of temperature used the thermocouples. It is based on setback effect. The thermocouple temperature measurement is based on creation of an electromotive force. BASIC PRINCIPLE: The basic principle of thermocouple ―when two dissimilar metals are joined together an e.m.f will exist between the two points A and B which is primarily a functions of the junction temperature. The above said to be principle is see-back effect.
CONSTRUCTION: The thermocouple consists of one hot junction and the cold junction. The hot junction will be introduced into the place where temperature it be measured. The other cold junction is maintaining at constant reference temperature. Also one voltage-measuring instrument is connected to the free ends of the thermocouple. OPERATION: In thermocouple, the known temperature is called reference temperature. The temperature which is to be measured is introduced in the thermocouples hot junction. A common arrangement for establishing the reference temperature is lath. The reference temperature is controlled to be at a constant temperature of 00c The temperature difference between the ice bath and the hot junction temperature will an e.m.f. this e.m.f production is measured in the millivoltmeter. So, this
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change in e.m.f rate will give the measure of temperature. The output voltage is, Where T is the temperature in 0c and A, B and c are constant depending upon the thermocouple material. THERMOCOUPLE LAWS:
o Law of intermediate metals.
o Law of intermediate temperature. LAW OF INTERMEDIATE METALS If a third metal is introduced into a thermocouple circuit, as shown in fig. the net e.m.f of the circuit is no affected as long as the new connections are at the same temperature.
9. With a neat diagrams explain the working of bimetallic strip and thermometer.
(EVEN 2014)
The bimetallic strip temperature measurement is based on change in dimension of the metal. A very widely used method of temperature measurement is the bimetallic strip. Already discussed the basic principle in bimetallic temperature measurement is based on the change in dimension of metals i.e., expand or contract when there is a change in temperature.
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The expansion or contraction is based on the thermal expansion coefficient. It changes from metal to metal contact.
WORKING: Two pieces of metal with different coefficient of thermal expansion are bonded together to form the bimetallic strip as shown in fig. it is in the form of a cantilever beam. When the strip is subjected to a temperature higher than the bonding temperature. It will be bent in one direction. Suppose, it is subjected to a temperature lower than the bounding temperature, it will be bent in the other direction. The radius of curvature `r` may be calculated as [ ]
Where,
T=combined thickness of the bonded strip mm.
1=Lower coefficient of expansion peroc
2=Higher coefficient of expansion per0c.
m=Ratios of thickness of low to high expansion material.
n=Ratio of modulus of elasticity of low to high expansion material
T=Temperature in 0c
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T0=Initial bounding temperature in 0c
While selecting the bimetallic trip materials, it should have high coefficient of expansion, high ductility, high modules of elasticity, high electrical conductivity. There properties are very important. The various material used in bimetallic strip and their mechanical properties are given below. MATERIAL THERMAL COEFFICIENT OF
EXPANSION PER0C MODULUS OF ELASTICITY(GN/m2)
YELLOW BRASS 2.02 x 10-5 96.5
INVAR 1.7 x 10-5 147
STAINLESS STELL 1.6 x 10-5 193
MONEL 400 1.35 x 10-5 179
INCONEL 702 1.25 x 10-5 217
APPLICATIONS:
Bimetallic strips are frequently used in simple ON-OFF switches. The bimetallic strips are also used in control witches.
ADVANTAGES:
The cost is low when compared to other temperature measuring instruments. It has neglible maintenance expense. Stable operation over extended periods of time Accuracy of this type of instruments is between 2 to 5% of the scale.
DISADVANTAGES:
It is not suitable for more than above 4000c The permanent deformation of metallic strip may occur.