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Mr.C.Dineshbabu,
AP/MECH,
KNCET
1
UNIT I BASICS OF METROLOGY
Introduction to Metrology
Need
Elements
Work piece, Instruments
their effect
– Persons –
on Precision and
Environment –
Accuracy
Errors
Errors in Measurements
Types
Control
Types of standards.
2
MEASUREMENTS - Introduction
• Measurement is a process of comparing
inputs with pre-defined standard and giving
the output.
• Metrology is a science of measurement.
• Metrology is also concerned with the
inspection and its various
industrial
techniques.
• For every kind of quantity measured, there
must be a unit to measure it.
3
1.Measurand, a physical quantity such as length, weight, and angle to be
measured
2. Reference, to compare the measurand (physical quantity) with a known
Standard for evaluation
3.Standard/Reference, the physical quantity or property to which
quantitative comparisons are to be made, which is internationally
accepted.
STANDARD
(Known Quantity)
MEASURAND
(Unknown Quantity)
COMPARISON
PROCESS
RESULT
(Numerical Value)
MEASUREMENTS - Introduction
4
1. To convert physical parameters into meaningful numbers.
2. To determine the true dimensions of a part.
3. To increase our knowledge and understanding of the world.
4. Needed for ensuring public health and human safety.
5. To test if the elements that constitute the system function as
per the design.
6. For evaluating the performance of a system.
7. To ensure interchangeability with a view to promoting mass
production.
8. To establish the validity of design and for finding new data
and new designs.
NEED FOR MEASUREMENT
5
• Industrial Metrology - Industrial metrology’s purpose is to
ensure that instruments, used in a wide variety of industries,
are functioning properly.
• Scientific Metrology - This form of metrology deals with the
organization and development of measurement standards
and with their maintenance.
• Legal Metrology - Concerned with the measurements that
influence economic transactions, legal metrology is a very
refined type of metrology.
TYPES OF METROLOGY
6
COMPONENTS OF GENERALIZED MEASUREMENT SYSTEM
A generalized measurement system consists of the following
components:
1. Primary Sensing Element
2. Variable Conversion Element
3. Variable Manipulation Element
4. Data Processing Element
5. Data Transmission System
6. Data Presentation Element
7
GENERALISED MEASURING SYSTEM
8
COMPONENTS 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:
Variable manipulation element manipulates and amplifies the output
of the variable conversion element. It also removes noise (if present) in
the signal.
9
COMPONENTS OF GENERALIZED MEASUREMENT SYSTEM
4. Data Processing Element:
It processes the data signal received from the variable manipulation
element and produces suitable 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.
6. Data Presentation Element:
It is used to present the measured physical quantity in a human
readable form to the observer. LED displays are most commonly used
as data presentation elements in many measurement systems.
10
Physical Quantity -
Temperature
Sensing Element - Bulb
Conversion Element
–Pressure
Transmission Element
Manipulation Element
Presentation Element
Element Conversion
COMPONENTS OF GENERALIZED MEASUREMENT SYSTEM
11
STANDARDS
In metrology (the science of measurement), a standard is an object,
or system that bears a defined relationship to a unit of measurement of a
physical quantity.
 Depending on functions and applications, standards of measurement
are classified as follows:
(i) International Standards
(ii) Primary Standards
(iii) Secondary Standards
(iv) Working Standards
12
i. International Standards
 Defined by International agreement
 Periodically evaluated & checked by absolute measurements in terms
of fundamental units of physics
 represent certain units of measurement to the closest possible
accuracy attainable by the science and technology of measurement
 These standards are not available to ordinary uses like measurement
and calibrations.
13
ii. Primary Standards
 Main function is the calibration and verification of secondary
standards
 These are maintained at the National Standards Laboratories in
different countries. For India, it is National Physical Laboratory at
New Delhi.
 The primary standards are not available for the use outside the
National Laboratory.
 These primary standards are absolute standards of high accuracy that
can be used as ultimate reference standards to check, calibrate and
certify the secondary standards.
14
iii. Secondary Standards
 Basic reference standards used by the measurement and calibration
laboratories in industries
 These standards are maintained by the particular industry to which
they belong
 Each industry has its own secondary standard
 Each laboratory periodically sends its secondary standard to the
national standards laboratory for calibration and comparison against
the primary standard
 After comparison and calibration, the National Standards Laboratory
returns the secondary standards to the particular industrial laboratory
with a certification of measuring accuracy in terms of primary
standards
15
iv. Working Standards
 main tools of a measuring laboratory
 used to check and calibrate laboratory instrument for accuracy and
performance.
 For example, manufacturing of mechanical components such as
shafts, bearings, gears etc, use a standard called working standard for
checking the component dimensions. Example: Plug gauge is used for
checking the bore diameter of bearings.
16
UNITS
• Physical quantity is expressed in Units.
• Types:
1. Primary Units – m, Kg, KJ
2. Supplementary Units - rad
3. Derived Units – Kg/KJ
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1. Direct Comparison
2. Indirect Comparison
3. Comparative Method
4. Coincidence Method
5. Fundamental Method
6. Contact Method
7. Transposition Method
8. Complementary Method
9. Deflection Method
10. Contactless method
TYPES OF MEASUREMENTS /
METHODS OF MEASUREMENTS
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1. Direct Method
 Measurements are directly
obtained.
Ex.: Vernier Caliper, Scales.
2. Indirect Method
 Obtained by measuring other quantities.
Ex: Measurement of strain induced in a
bar due to the applied force
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19
3. Comparative Method
It’s compared with other
value.
known
Ex: Comparators.
4. Coincidence Method:
Measurements coincide with certain
lines and signals. Ex: Comparators.
5. Fundamental Method:
Measuring a quantity directly in related with the definition of that
quantity.
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20
6. Transposition Method:
Quantity to be measured is first
balanced by a known value and then
balanced by an other new known value.
Ex: Determination of mass by
balancing methods.
7. Complementary Method:
The value of quantity to be measured is
combined with known value of the same
quantity.
Ex: Determination of the volume of a
solid by liquid displacement Volume.
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8. Deflection Method:
10. Contactless method:
There is no direct contact with the surface to be measured.
E0x6-.Juml-18easurement by optical instruments
The value to be measured is directly
indicated by a deflection of pointer.
Ex: Pressure Measurement.
9. Contact Method:
Sensor/Measuring tip touch the surface area.
Ex: Vernier Caliper.
23
22
1. Deflection and Null type instruments
2. Analog and Digital instruments
3. Active and passive type instruments
4. Automatic and manually operated instruments
5. Absolute and secondary instruments
6. Contacting and non-contacting instruments
7. Intelligent instruments
TYPES OF MEASURING INSTRUMENTS
23
 The weight
indicated by
of the object is
the deflection or
of a a
scale.
pointer on
Ex. Spring
movement
graduated
Balance
1. Deflection and Null type instruments
 The effect caused by the quantity to be
measured is nullified.
 For example, consider the measurement of
weight by an ordinary beam balance as
shown in fig. The unknown weight placed
in one-side causes the beam and the
pointer to deflect. Ex. Beam Balance
24
2. Analog and Digital Instruments
25
 In active instruments, the quantity being
measured just activates the magnitude of
some, external power input source, which
in turn produces the measurement.
 In this type of instruments, another
external energy input source is present
apart from the quantity to be measured.
3. Active and Passive Type Instruments
 In passive type instruments, output is
produced entirely by the quantity being
measured.
26
4. Manual and Automatic Instruments
 Manual instruments require the services of a
human operator.
 When the process of null balance is automated, it
is known termed as automatic instruments.
27
5. Absolute and Secondary Instruments
instruments
 Absolute
give the value of the
are those which
quantity to be
measured, in terms of the constants of the
instrument and their deflection only.
 Secondary Instrument shows deflection
directly in terms of electrical quantity like
voltage, current, power and frequency.
 These instruments are calibrated by
comparison with an absolute instrument.
28
6. Contacting and Non-Contacting Instruments
29
7. Intelligent Instrument
 Microprocessor are incorporated with measuring instrument
30
ACCURACY Vs PRECISION
Measurement is an act of assigning an accurate
and precise value to a physical variable.
What is the difference between
Precision andAccuracy
Accuracy is a measure of rightness.
Precision is a measure of exactness.
31
Accuracy is the ability of the instrument to measure the
accurate value (Conformity).
Precision refers to how closely individual measurements agree
with each other (Repeatability).
ACCURACY AND PRECISION
32
FACTORS AFFECTING ACCURACY AND PRECISION
OF A MEASURING SYSTEM
• A measuring system is made of five basic elements.
These are:
1. Standard
2. Work piece
3. Instrument
4. Person
5. Environment.
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33
FACTORS AFFECTING ACCURACY OF A MEASURING SYSTEM
1. Standard
□ Coefficient of thermal expansion
□ Stability with time
□ Elastic properties
□ Position etc
2. Work piece:
□ Cleanliness surface finish etc.
□ Surface defects
□ Hidden geometry
3. Instrument
□ Inadequate amplification
□ Scale error
□ Deformation while handling heavy w/p
□ Calibration error
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Jul-
e
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peatability & readability 35
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4. Person
□ Training skill
□ Sense of precision appreciation
□ Ability to select measuring instrument & standard
□ Attitude towards personal accuracy achievement
□ Planning for measurement technique to have minimum just with
consistent in precision.
5. Environment
□ Temperature, pressure and humidity
□ Clean surrounding and minimum vibration
□ Adequate illumination
□ Temperature equalization between standard w/p & instrument
Higher accuracy can be achieved if all 5 factors are considered,
analysed & steps are taken to eliminate them
06-Jul-18 36
FACTORS AFFECTING ACCURACY OF A MEASURING SYSTEM
35
What is Error in Measurement?
• Measurement Error (Observational Error) is the
difference between a measured(actual) value and its
true value.
• True size  Theoretical size of a dimension which is
free from errors.
• Actual size  size obtained through measurement
with permissible error.
06-Jul-18 37
ERRORS IN MEASUREMENT
36
Types of Errors in Measurements
Gross / Blunder Errors Measurement Errors
Systematic Errors
Instrumental
Errors
Observational
Errors
Environmental
Errors
Theoretical
Errors
Random Errors
TYPES OF ERRORS IN MEASUREMENT
37
1) Gross or Blunder Errors:
This category of errors includes all the human mistakes while
reading, recording the readings. The best example of these errors is
a person or operator reading pressure gauge 1.01N/m2 as
1.10N/m2.
2) Measurement Error:
 The measurement error is the
measurement of the true value.
result of the variation of a
 Usually, Measurement error consists of a random error and
systematic error.
06-Jul-18 39
TYPES OF ERRORS IN MEASUREMENT
38
a. Systematic Error (Controllable Error)
• A systematic error is a constant error that under the same
operating conditions.
• Systematic error is caused by any factors that systematically affect
measurement .
• Classification of systematic errors:
i. Instrumental Errors – Calibration Error
ii. Environmental Errors – Temp, Pressure, Humidity
iii. Observational Errors - Parallax
iv. Theoretical - Percentage
06-Jul-18 40
TYPES OF ERRORS IN MEASUREMENT
39
Random Errors (Uncontrollable Error)
• Random (or indeterminate) errors are caused by uncontrollable
fluctuations in variables that affect experimental results.
• Random errors are caused by the sudden change in experimental
conditions and noise and tiredness in the working persons. These
errors are either positive or negative.
• These errors may be reduced by taking the average of a large
number of readings.
TYPES OF ERRORS IN MEASUREMENT
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How to Use....
• Required height of
the component: 32.5
mm
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UNIT-III
FORM MEASUREMENT
METROLOGY AND
MEASUREMENTS
149
Thread Measurement: Terminologies, Errors - External
Thread Measurement: Pitch Gauge, Tool Maker's
microscope, Floating Carriage micrometer with One, Two
and Three wires - Internal Thread Measurement: Taper
Parallels and Rollers method.
Gear Measurement: Terminologies, Errors, Gear Tooth
Vernier caliper, Profile Projector, Base pitch measuring
instrument, David Brown Tangent Comparator, Involutes tester,
Parkinson Gear Tester, External and Internal Radius
measurements
Roundness measurement: Circumferential confining gauge,
Assessment using V block and Rotating centres.
150
• Introduction:
• Screw threads are used to transmit power and motion and also used
to fasten two components with the help of nuts, bolts and studs.
• The screw threads are mainly classified into: 1) External Screw
Threads 2) Internal Screw
Threads.
Screw Thread
Measurement
External Screw
Threads
Internal Screw
Threads 151
Screw Thread
terminologies
152
1. Screw Thread: It is a continuous helical groove of specified cross-
section produced on the external or internal surface.
2. Crest: It is the top surface joining two sides of thread.
3. Root: The bottom of the groove between the two flanks of the thread.
4. Flank: It is the surface between crest and root or it is the thread
surface that connects crest with root.
5. Lead: The distance a screw thread advances in one turn. For a single
start threads, lead=pitch, For double start, lead=2xpitch, & so on.
6.Pitch: The distance from a point on a screw thread to a corresponding
point on the next thread measured parallel to the axis.
7. Helix Angle: The angle made by the helix of the thread at the pitch
line with the axis is called
as helix angle.
8. Flank angle: It is half the included angle of the thread or angle made
by the flank of the thread
with the perpendicular to the thread axis. 153
9. Depth of thread: It is the distance between crest and root measured
perpendicular to axis of screw.
10. Angle of thread: It is the angle included between the flanks of a
thread measured in an axial
plane.
11. Major Diameter: This is the diameter of an imaginary cylinder, co-
axial with the screw, which just touches the crests of an external
thread or roots of an internal threads. It is also called as ‘Nominal
diameter’.
12. Minor diameter: This is the diameter of an imaginary cylinder, co-
axial with the screw which just touches the roots of an external
thread or the crest of an internal thread. This is also referred to as
‘root’or ‘core diameter’.
13. Effective diameter or Pitch diameter: It is the diameter of an
imaginary cylinder coaxial with the axis of the thread and intersects
the flanks of the thread such that width of the threads & width of
spaces between threads are equal.
14. Addendum: It is the distance between the crest and the pitch line
154
Tofind out the accuracy of a screw thread it will be necessary to
measure the following:
1. Measurement of Major
diameter:
a. Ordinary micrometer
b. Bench
micrometer.
2. Measurement of Minor diameter:
a. Using taper Parallels b. Using
rollers
3. Measurement of Effective diameter:
a. One wire method b. Two wire method c.
Three wire method
d. Using
Thread MM
4. Measurement of
Pitch:
a. Pitch Measuring
Machine
b. Tool Makers
Microscope
c. Screw Pitch
Gauge
5. Thread angle
and form
Measurement of various elements in
Screw Threads
155
1. Measurement of Major
diameter
Measurement
Processes
a. Ordinary
micrometer
b. Bench micrometer
i) Ordinary Micrometer:
• In this the micrometer is used as a comparator.
• This micrometer is first set over the cylinder standard having
approx. same dimension.
• This standard is called setting gauge. 156
After taking this reading ‘R1’the micrometer is set on the major
diameter of the thread, and the new reading is ‘R2’and then the
diameter is measured by following equation:.
Then the major
diameter, D=S ±
(R1 - R2) S = Size
of setting gauge
R1 = Micrometer
reading over setting
gauge. R2 =
157
ii) Measurement by Bench
micrometer:
• Bench micrometer is designed by the
NPL to remove
deficiencies inherent in the hand
micrometer.
• In this the fiducial micrometer is used to
ensure that all the readings are taken at the
same pressure.
• The instrument has a micrometer head
having Vernier scale
to read to the accuracy of 0.002mm.
• This instrument is also used as the
comparator to avoid the pitch errors of
micrometer threads, zero error setting etc.
• Then the process is same as of the ordinary
micrometer. Calibrated setting cylinder
having the same diameter as the major
diameter of the thread to be measured is
used as setting standard.
158
2. Measurement of Minor
diameter
Minor diameter is the imaginary diameter of
thread which would touch the roots of the
external and crest of the internal threads.
For measuring minor diameter
of external
following methods are used:
1. Two V pieces method
2. By projecting the thread on
the screen
threa
ds
For measuring minor diameter of internal
thread following methods are used:
1. Using taper parallels
2. By using rollers and slip gauges
159
V pieces
method
• The minor diameter is measured by a comparative method by
using floating carriage diameter measuring machine and small
‘V’ pieces which make contact with the root of the thread.
• These V pieces are made in several sizes, having suitable radii at
the edges. V pieces are made of hardened steel.
• The floating carriage diameter-measuring machine is a bench
micrometer mounted on a
carriage.
160
Measurement Process:
• The threaded work piece is mounted between the centres of the
instrument and the V pieces are placed on each side of the work
piece and then the reading is noted.
• After taking this reading the work piece is
then
replaced cylindrical setting gauge.
• The minor diameter of the thread = D ±
(R2 –R1)
• Where, D = Diameter of cylindrical gauge
by a standard
reference
R2 = Micrometer reading on
threaded workpiece,
R1 = Micrometer reading on
cylindrical gauge.
161
• If the threads are very sharp or have
no radius at the root.
• The measurement of minor
diameter is done by
projecting the thread form on a screen.
• This projected form is compared with
the use of the Tool Makers
Microscope.
By projecting thread
on screen
162
For measuring minor diameter of
internal thread:
a. Using taper Parallels b.
Using rollers
Using taper Parallels
• For the internal thread of the minor
diameter of diameter less than
200mm is measured using the taper
parallels.
• The taper parallels are the pairs of the
wedges having parallel outer edges.
• The taper parallels are inserted inside
the thread and adjusted until firm
contact is not established with the 163
2. Using
Rollers:
• For more than 200mm diameter this
method is used.
• Precision rollers are inserted inside
the thread and
proper slip gauge is inserted between the
rollers.
• The minor diameter is then the length
of slip gauges plus twice the
diameter of roller.
164
• Effective diameter is the imaginary diameter
in between major and minor diameter.
• The effective diameter measurement is carried out
by the following
methods.
1. Wire Methods
2. Thread Micrometer
3. Measurement of Effective
diameter
165
3. Measurement of Effective diameter –
Wire Method
•The effective diameter measurement is carried out by the
following methods.
1. One Wire Method 2. Two Wires Method 3.
Three Wires Method
• This methods are based on the size of the wire.
• The size of the wire whose diameter makes
the contact with the flank of the thread
on the effective diameter this size of wire
is known as Best Size of Wire.
• This size is decided by the following equation:
Where p= pitch and θ=
thread angle
166
• In this method, only one wire is used. The
wire is placed between the two threads at one
side and on the other side the anvil of the
measuring micrometer contacts the crests.
• First, the micrometer reading ‘d1’ is noted on a
standard gauge whose dimension is
approximately same to be obtained by this
method.
• Now, the setting gauge is replaced by thread
and the new reading is taken i.e. ‘d2’ then
effective diameter D = S± (d1-d2).
Where, S = Size of setting gauge.
• Actual measurement over wire on one side
and threads on other side = size of gauge
One Wire
Method
167
The effective diameter can not be
measured directly but can be calculated
from the measurements made.
In this method, wires of exactly known
diameters are chosen such that they
contact the flanks at their straight
portions.
If the size of the wire is such it contacts
the flanks at the pitch line, it is called the
‘best size’ of wire which can be
determined by geometry of screw thread.
The screw thread is mounted between the
centers & wires are placed in the grooves
Two Wire
Method
168
Measuring
Process
Effective diameter E is calculated by E = T + P
Where, T = Dimension under the wires = M - 2d
M = Dimension over the wires
d = Diameter of each wire
P = Compensating factor should be added
to T value and it depends on diameter of wire,
pitch & angle of the screw thread.
Here, The diameter under the wires ‘T’can also be
determined by,
T= S - (R1 – R2)
Where, S = The diameter of the standard.
R1 = Micrometer reading over standard and
wires.
R2 = Micrometer reading over screw thread
andwires
P= 0.866 p - d => For metric thread. 169
 The three-wire method is the accurate method.
 In this method, three wires of equal and precise diameter are
placed in the grooves at opposite sides of the screw.
 In this, one wire on one side and two on the other side are used.
 The wires either may be held in hand or hung from a stand.
This method ensures the alignment of micrometer anvil faces
parallel to the thread axis.
Three Wire
Method
170
Case i) In case of Whitworth Thread:
M = D + 3.1657d – 1.6p
where, D = Outside Diameter
Case ii) In case of
Metric Thread: M
= D + 3d – 1.5155p
We can practically measure the value of M & then compare with the
theoretical values using the formula derived above. After finding
the correct value of M, as d is known, E can be found out.
171
Best Size Wire:
Best size wire is one in which, the wire is having such a diameter
that it makes contact with the flanks of the thread on the
effective diameter or pitch line.
It is recommended that for measuring the effective diameter,
always the best size wire should be used and for this condition the
wire touches the flank at mean diameter line within ±1/5 of flank
length.
172
Effective Diameter
Measurement
 This method is simple and rapid.
 The thread micrometer is same as ordinary
micrometer except that it has special contact points
to suit the end screw threads form that is to be
checked.
 The contact points are selected on the basis of the
types of the thread and the pitch of the thread to be
measured.
 Then the anvils are then made to contact the thread
to be checked and the reading is taken, which will
give the pitch diameter or effective diameter.
 In this the actual reading is the
 measurement of the major diameter on one side and
minor diameter of the other side which gives us the
effective diameter.
Thread
Micrometer
173
i) Pitch Measuring Machine:
 The principle of this method of measurement is
to move the stylus
along the screen parallel to the axis from one
space to the next.
 The pitch-measuring machine provides a
relatively simple and accurate
method of measuring the pitch.
 Initially, the micrometer reading is set near the
zero on the scale.
 Spring loaded head permits the stylus to move
up the flank of the
thread and down into the next space as it is moved
along.
 Accurate positioning of the stylus between the two
flanks is obtained by ensuring that the pointer T
is always opposite to its index mark when
readings are taken.
 When the pointer is accurately placed in position,
the micrometer reading is noted.
4. Pitch
Measurement
174
ii.Tool Makers Microscope:
• Worktable is placed on the base of the
instrument.
• The optical head is mounted on a vertical
column it can be
moved up and down.
• Work piece is mounted on a glass plate.
• A light source provides horizontal beam of
light which is reflected from a mirror by 90
degree upwards towards the table.
• Image of the outline of contour of the
work piece passes
through the objective of the optical head.
• The image is projected by a system of three
prisms to a ground glass screen.
• The measurements are made by means of
cross lines engraved on the ground glass 175
Applications:
1. Linear measurements.
2. Measurement of pitch of the
screw.
3. Measurement of pitch
diameter.
4. Measurement of thread
angle.
5. Comparing thread forms.
6. Centre to center distance
measurement.
176
iii. Screw pitch
gauge:
• It is used to directly compare the pitch by just selecting the
proper pitch value entered in the pitch gauge and comparing it
with the actual screw thread.
177
5. Flank Angle and Thread form
Measurement
Flank angle
•Flank Angle is the angle formed by a flank and
a perpendicular to the thread axis in an axial
plane.
•It is also called the half thread angle.
•For this measurement we have to measure the
thread angle.
•To measure the thread angle the following methods
is used:
1. Optical Projection
178
5. Thread form and flank angle
Measurement
Thread form
The ideal and actual forms are compared for
the measurement.
Types of thread gauges are,
1. Plug Screw Gauge 2.
Ring Screw Gauge
3. Caliper Screw
Gauge
179
The error in screw thread may arise during the manufacturing or storage of
threads. The error either may cause due to the following six main
elements in the thread.2. Minor diameter
error
5. Flank angles
error
3. Effective
diameter error
6. Crest and root
error
1. Major diameter
error
4. Pitch error
1. Major diameter
error
It may cause reduction in the flank contact and interference with the
matching threads.
2. Minor diameter error
It may cause interference, reduction of flank contact.
3. Effective diameter error
If the effective diameter is small the threads will be thin on the
external screw and thick on an
internal screw.
4. Pitch error
Pitch error is defined as the total length of thread engaged either
too high or too small. The various pitch errors may be classified
into
Errors in Screw
Threads
180
This error
occurs
i) Progressive Pitch error:
The pitch of the thread is uniform but is longer or shorter its nominal
value and this is called progressive error. This error occurs whenever the
tool–work velocity ratio is incorrect but constant.
Causes of Progressive error:
1. Incorrect linear and angular velocity ratio.
2. Incorrect gear train and lead screw.
3. Saddle fault.
4. Variation in length due to hardening.
ii) Periodic Pitch error:
In this the pitch error causes the errors to repeat at
certain time of interval. when the tool–work
velocity ratio is not constant.
Causes of Periodic error:
1. Un-uniform tool work velocity ratio.
2. Teeth error in gears.
3. Lead screw error. 181
(iii) Drunken error:
It is error due to the irregular form of helical
groove on a cylindrical surface. In this case pitch
measured parallel to the axis is always same, but
problem is with the thread is not cut to its
true helix. Due to this flank surface will not be as
a straight edge, it will be as curved form.
(iv) Irregular error:
These are the errors randomly take place on threads without
any specific reason.
Causes of Irregular error:
1. Machine fault.
2. Non-uniformity in the material.
3. Cutting action is not correct.
4. Machining disturbances.
Effect of pitch error:
1. It increases the effective diameter of the bolt and decreases
the diameter of nut. 182
• Gears are mechanical drives which transmit power through toothed
wheel.
• In this gear drive, the driving wheel is in direct contact with driven
wheel.
• The accuracy of gearing is very important factor when gears are
manufactured.
• The transmission efficiency is almost 99% for gears.
• So, it is very important to test and measure the gears precisely.
• For proper inspection of gear, it is very important to concentrate on
the raw materials, which are used to manufacture the gears.
• Also very important to check the machining of the blanks, heat
treatment and finishing of
teeth.
Gear
Measurements
183
The most commonly used forms of gear teeth
are
1.Involute 2.Cycloidal
The involute gears also called as straight
tooth or spur gears.
The cycloidal gears are used in heavy and
impact loads.
The involute rack has straight teeth.
The involute pressure angle is either 20̊ or
14.5
Types of Gears
1.Spur gear
Cylindrical gear whose tooth traces is straight
line.
These are used for transmitting power
betwee 184
3. Helical gears
These gears are used to transmit the power between parallel shafts as well
as non-
parallel
and non- intersecting shafts. It is a cylindrical gear whose tooth traces
is straight line.
4. Bevel gears
The tooth traces are straight-line generators of cone. The teeth are cut
onthe
conical
surface. It is used to connect the shafts at right angles.
5. Worm and Worm wheel
It is used to connect the shafts whose axes are non-parallel and non-
intersecting.
6. Rack and Pinion
Rack gears are straight spur gears with infinite radius.
185
Elements of
Spur Gear
186
1. Tooth Profile: It is the shape of any side of gear tooth in its cross section.
2.Base circle: It is the circle of gear from which the involute profile is
derived. Base circle diameter = Pitch circle diameter x Cosine of pressure
angle of gear
3. Pitch circle diameter (PCD): It is the diameter of a circle which will
produce the same motion as the toothed gear wheel.
4.Pitch circle: It is the imaginary circle of gear that rolls without slipping
over the circle of its mating gear.
5. Addendum circle: The circle that coincides with the crests (or) tops of
teeth.
6. Dedendum circle (or) Root circle: This circle that coincides with the
roots (or) bottom of teeth.
7. Pressure angle (α): It is the angle made by the line of action with the
common tangent to the pitch
circles of mating gears.
𝑑
8. Module (m): It is the ratio of pitch circle diameter to the total
number of teeth. m =
Where, d = Pitch circle diameter, n =Number of teeth.
𝑛
Spur Gear
Terminology
187
9. Circular pitch: It is the distance along the pitch circle between
corresponding points of
adjacent teeth.
c 𝑛
P = 𝜋𝑑
=
𝜋m
10. Diametral pitch (Pd): Number of teeth per
inch of the PCD.
d 𝑑
𝑚
P = 𝑛
= 1
Where, m
= Module
11. Addendum: It is the radial distance between tip
circle and pitch circle. Addendum
value = 1 module.
11. Dedendum: It is the radial distance between pitch circle and root
circle.
Dedendum value=1.25 module.
13. Clearance(c): The distance covered by the tip of one gear with the
root of mating gear. Clearance = Difference between Dedendum
and addendum values.
14. Blank diameter: It is the diameter of the blank upto outer
periphery.
Blank diameter = PCD+2m 188
9. Face: It is the part of the tooth in the axial plane lying between tip
circle and pitch circle.
10. Flank: It is the part of the tooth lying between pitch circle and root
circle.
11. Helix angle: It is the angle between the tangents to helix angle.
12. Top land: Top surface of a tooth is called as top land.
13.Lead angle: It is the angle between the tangent to the helix and plane
perpendicular to the axis of cylinder.
14. Backlash: It is the difference between the tooth thickness and the
space into which it meshes. If we assume the tooth thickness as ‘t1 ’
and width ‘t2 ’then
189
1. Profile error: The maximum distance is at any point on the tooth
profile form to the design profile.
2. Pitch error: It is the difference between actual and design pitch.
3. Cyclic error: Error occurs in each revolution of gear.
4.Run out: Total range of a fixed indicator with, the contact points
applied to a surface rotated, without axial movement, about a fixed axis.
5. Eccentricity: It is the half radial run out.
6. Wobble: Run out is measured parallel to the axis of rotation at a
specified distance from the axis.
7. Radial run out: Run out is measured along a perpendicular to the
axis of rotation.
8. Undulation: It is the periodical departure of the actual tooth surface
from the design surface.
9. Axial run out: Run out is measured parallel to the axis of rotation at
a speed.
Errors in
Gear
190
The inspection of the gears consists of the following elements in
which manufacturing error
may be present.
1. Runout
2. Pitch
3. Profile
4. Lead
5. Backlash
6. Tooth thickness
Spur Gear
Measurement
191
1. Measurement of
Runout
• the run out is an amount a gear moves in
and out away
from it true centre as it is rotated.
• If runout is excessive, the gear
wobbles as it rotates.
Runout is also the eccentricity in the pitch
circle of gear.
• Gears that are eccentric tend to have
vibration per revolution.
• It may cause an abrupt gear failure.
• The gear is held on a mandrel in the
centers and the dial indicator of the tester
holds a special tip descending upon the
module of gear being tested.
• The tip is inserted between the tooth
spaces and dial indicator reading is
noted.
192
Pitch is the distance between corresponding points on equally spaced and
adjacent teeth. Pitch error is the difference in distance between equally
spaced adjacent teeth and the measured distance between any two
adjacent teeth.
There are two ways for measuring the pitch.
a) Point to point measurement (i.e. One tooth point to next tooth
point)
b) Direct angular measurement
2. Measurement
of Pitch
193
a) Tooth to Tooth measurement:
The instrument has three tips. One is
fixed measuring tip and the second is
sensitive tip, whose position can be
adjusted by a screw and the third tip is
adjustable or guide stop. The distance
between the fixed and sensitive tip is
equivalent to base pitch of the gear. All
the three tips are made in contact with
the tooth by setting the instrument and
the reading on the dial indicator is the
error in the base pitch.
b) DirectAngular Measurement:
It is the simplest method for measuring
the error by using set dial gauge against
a tooth. In this method, the position of a
suitable point on a tooth is measured
after the gear has been indexed by a
2. Measurement of Pitch
194
i. Optical projection method:
The profile of the gear is projected
on the screen by optical lens and
then the projected value is
compared with master profile.
ii. Involute measuring machine:
In this method, the gear is held on
a mandrel and circular disc of
same diameter as the base circle
of gear for the measurement is
fixed on the mandrel. After fixing
the gear on the mandrel, the
straight edge of the instrument is
brought in contact with the base
circle of the disc. Now, the gear
and disc are rotated and the edge
3. Measurement
of Profile
195
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 are
a) Gear tooth Vernier caliper method
b)Constant chord method
c)Base tangent method
d) Measurement over pins or ball
6. Measurement of Tooth
Thickness
196
 It is used to measure the thickness of gear teeth at
the pitch line or chordal thickness of teeth and the
distance from the top of a tooth to the chord.
 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.
 This method is simple and inexpensive.
Disadvantages of Gear Tooth Vernier method:
1.Not closer to 0.05mm.
2.Two Vernier readings are required.
Gear tooth Thickness – Gear
Tooth Vernier
197
• A constant chord is defined as, the chord,
joining those points, on opposite faces of the
tooth, which make contact with the mating
teeth, when the center line of the tooth lies on
the line of the gear centers.
• Constant chord of a gear is measured where
the tooth flanks
touch the flanks of the basic rack.
• The teeth of the rack are straight and inclined
to their centre lines at the pressure angle.
• Also the pitch line of the rack is tangential
to the pitch circle of the gear, the tooth
thickness of the rack along this line is equal
to the arc tooth thickness of the gear round its
pitch circle.
• Now, since the gear tooth and rack space are
in contact in the symmetrical position at the
Gear tooth Thickness – Constant
Chord Method
198
Gear tooth Thickness – Base
Tangent Method
• It is the most commonly used method for checking
the tooth thickness of gear.
• The advantage of this method is that, it depends
only on one vernier reading unlike gear tooth
vernier Caliper where we require two vernier
readings.
• The base tangent length is the distance between
the two parallel planes which are tangential to
the opposing tooth flanks.
• The measurements made across these opposed
involutes by span gauging will be constant and
equal to the arc length of the base circle
between the origins of involutes.
• The value of the distance between two opposed
involutes, or the dimension over parallel faces is
equal to the distance round the base circle
between the points where the corresponding tooth
199
• The master gear is fixed on vertical spindle and
the gear to be tested is fixed on similar spindle
which is mounted on a carriage.
• The carriage which can slide both side and these
gears are maintained in mesh by spring pressure.
• When the gears are rotated, the movement of
sliding carriage is indicated by a dial indicator
and these variations are the measure of any
irregularities in the gear under test.
• The variation's recorded in a recorder which is
fitted in the form
of a waxed circular chart.
• In fig, the gears are fitted on the mandrels and
are free to rotate without clearance.
• Left mandrel moves along the table and the
right mandrel moves
along the spring-loaded carriage.
Parkinson Gear Tester – Gear
profile inspection
200
• If errors occur in the tooth form when gears will be in closer mesh, pitch
or concentricity of pitch line will cause a variation in center distance
from this movement of carriage as indicated to the dial gauge will show
the errors in the gear test.
• The recorder is also fitted in the form of circular or rectangular chart and
the errors are recorded.
Limitations of Parkinson gear tester:
1.Accuracy ±0.001mm
2.Maximum gear diameter is 300mm
3.Errors are not clearly identified.
4.Measurement is dependent upon the master gear.
5.Low friction in the movement of the floating carriage.
201
• By definition, roundness or circularity is the radial uniformity of work
surface measured from the center line of the workpiece.
• Circularity is specified by circularity tolerance. For example, if it is
specified that circularity of a feature is to be 0.1mm, than it means that
the circumference of each cross section of the feature should be
contained between two coplanar concentric circles that are 0.1mm
apart.
• Error in roundness is defined as the radial distance between the
minimum inscribing circle and maximum inscribing circle, that contain
the actual profile of the surface at a section perpendicular to the axis of
rotation.
Roundness
Measurement
202
• Methods for Measuring
Roundness:
a) V-block and dial
indicator method
V-block and dial indicator
method:
b) Roundness Measuring
Machine
• The arrangement consists of a V-block that is placed on the surface
plate. The workpiece to be tested is placed in the V-groove of the V-
block as shown in the figure.
• The feeler of a sensitive dial indicator (held firmly by a stand) is made
to rest on the workpiece.
• Now the workpiece is rotated about the diameter to be checked. The
dial indicator will indicate
variations in the dimensions caused due to out of roundness.
203
• Plotting a Polar Graph: An idea of the actual shape of the workpiece
can be obtained by plotting a polar graph. 12 equispaced markings at an
angle of 30̊ are made in the face of the workpiece to be measured. The
workpiece is placed on the V-block.
• The dial indicator is made to touch the workpiece at its center. Now
when the workpiece is rotated and when the marking comes exactly
under the plunger of the dial indicator, the reading is noted.
• Hence 12 readings will be obtained. The procedure is repeated thrice to
get average values for each marking. Now for plotting the polar graph,
a proper scale is selected.
• A circle of diameter equal to four times the maximum reading of the
dial indicator is drawn. Another concentric circle is drawn in this circle.
The values of the dial indicator are plotted in radial direction by taking
the smallest circle as the reference circle. The individual points are
joined by straight lines to get the actual profile of the workpiece.
•
204
• Error is measured as the radial distance between the maximum and
minimum inscribing circle for the profile obtained.
Roundness
error =
𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑒𝑟𝑟𝑜𝑟 𝑓𝑟𝑜𝑚 𝑝𝑜𝑙
𝑎𝑟 𝑔𝑟𝑎𝑝ℎ 𝐾
Where, K is a constant (This constant depends on the shape of the
workpiece and angle of V-
block)
• The position of the indicating instrument, the number of lobes on the
workpiece and the angle of the V-block have an influence on the
determination of roundness error.
205
Roundness Measuring Machine:
• The machine is also called as Taly-round Instrument or precision
spindle method.
• The main parts of the instrument are a truly running spindle that is
mounted on precision ball bearing and micron indicator.
• The indicating pointer is rotated around the workpiece about an
accurately stable axis. The indicator shows deviations from roundness.
As the output of the indicator is connected to an amplifier unit and pen
recorder, a polar graph of the out line of the workpiece is obtained.
• This is an accurate method. Automatic recording of the exact profile
of the workpiece is
obtained. Waviness also is superimposed on the profile of the
workpiece.
206
Cylindricity
A cylinder is an envelope of a
rectangle rotated about an
axis. It is bound between two
circular and parallel planes.
Runout
Runout is a measure of the
trueness of the
running of a body about its own
axis.
207
UNIT-V
ADVANCES IN METROLOGY
Metrology and Measurements
208
UNIT-IV-MEASUREMENT OF
MECHANICAL PARAMETERS
209
Orifice meter
210
venturimeter
211
212
Flow nozzle
213
Flow nozzle
214
rotameter
215
216
ELECTROMAGNETIC FLOW METER
217
Pitot-tube
It is a device used for measuring the velocity of flow at any point
in a pipe or a channel.
Principle: If the velocity at any point decreases, the pressure at that
point increases due to the conservation of the kinetic energy into
pressure energy.
In simplest form, the pitot tube consists of a glass tube, bent at right
angles.
218
219
PITOT TUBE
220
ULTRASONIC FLOW METER
221
HOT WIRE ANEMOMETER
222
TORQUE MEASUREMENT
223
224
225
PRONY BRAKE ARRANGEMENT
B.P=2πN(WL)/60
226
227
228
229
230
231
232
233
234
TEMPERATURE MEASUREMENT
Bimetallic strip thermometer
Thermocuples
Thermometers
Thermister
Pyrometers
Resistance temperature detectors
235
Bimetallic strip
236
237
thermometer
238
Resistance temperature detector
239
RTD
240
THERMOCOUPLE
241
THERMOCOUPLE
242
243
244
245
246
FILLED THERMAL SYSTEM
247
TOTAL RADIATION PYROMETER
248
INFRARED PYROMETER
249
ELECTRICAL THERMAL RESISTANCE
OR
ELECTRICAL RESISTANCE
THERMOMETER
250
OPTICAL PYROMETER
251
Hydraulic dynamometer
252
Eddy current dynamometer
253
Strain gauge
254
255
Stroboscope method
256
257
258
Capacitive load cell
259
260
261
262
263
264
265
266
267
268
269
270
UNIT-V
ADV
ANCES IN METROLOGY
271
PRECISION INSTRUMENTS BASED
ON LASER
• Laser stands for "Light Amplification by Stimulated Emission of
Radiation". Laser instruments are devices to produce powerful,
monochromatic collimated beam of light in which the waves are
coherent.
• The development of laser gives production of clear coherent light.
The biggest advantage of this coherent light is that whole energy
appears to be coming from a very small point.
• The laser beam can be focused easily into either a parallel beam or
into a very small point by the use of lens.
272
PRINCIPLE OF
LASER
• The principle involved in laser is when the photon emitted during
stimulated emission has the same energy, phase and frequency as
the incident photon.
• The photon comes in contact with another atom or molecule in the
high energy level E2, then it will cause the atom to return to ground
state energy level E1 by releasing another photon.
• The sequence of triggered identical photon from stimulated atom is
known as stimulated emission.
• This multiplication of photon through stimulated emission leads to
coherent, powerful, monochromatic, collimated beam of light
emission. This Light emission is called laser.
273
LASER
METROLOGY
• A laser beam projected directly onto a position detector is a
method of alignment used in a number of commercially available
systems. The laser with its highly controlled frequency modes and
coherent output are used extensively for interferometery
• Laser is suitable for more general applications where a convenient,
collimated and high intensity source is required Precision, accuracy,
no contact and hot moving parts.
274
LASER MEASURING
MACHINES
1. Laser Telemetric System
2. Laser and LED Based Distance Measuring Instruments
3. For Profile Checks
4. Scanning Laser Gauge
275
1. LASER TELEMETRIC
SYSTEM
276
Advantages:
• It is possible to detect changes in dimensions when components
are moving.
• It is possible to detect changes in dimensions when product is in
continuous processes.
• There is no need to wait for taking measurements when the
product is in hot conditions.
• It can be applied on production machines and controlled them
with closed feedback loops.
• 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.
277
2. LASER AND LED BASED DISTANCE
MEASURING
INSTRUMENTS
Advantages:
1. It is very reliable because there
is no moving part.
2. Instrument response time is in
milliseconds.
3. The output is provided as
0 – 20 mA.
278
3. FOR PROFILE
CHECKS
279
4. SCANNING LASER
GAUGE
280
INTERFEROM
ETRY
281
USE OF LASER IN
INTERFEROMETRY
• The laser in interferometery is to find accurate
measurement
length.
• It reduces the most time taken arid skill required like at methods
used for finding the length.
• The accuracy of measurement is the order of 0.1m in 100m.
• In modified laser designs, a single frequency is selected from
the coherent beam and used for interferometric measurement.
282
LASER
INTERFERMETER
• The laser interferometery involves the following components,
1. Two frequency laser source.
2. Optical elements.
3. Laser heads measurement receiver.
4. Measurement display.
283
AC LASER
INTERFEROMETER
284
2. OPTICAL
ELEMENTS
⚫The various optical elements are,
a. Beam splitters.
b. Beam benders.
c. Retro reflectors.
285
a. Beam
splitters
⚫ It is used to divide the laser beam into separate beams along
different axes.
⚫It is possible to adjust the spitted laser's output intensity by having
a choice of beam splitter reflectivities.
286
b. Beam
benders
• It is used to deflect the light
beam around comers on it path
from the laser to each axis.
• The beam benders are just flat
mirrors, but having absolutely flat
and very high reflectivity.
• Normally, the beam deflection is
avoided for not to disturb the
polarizing vectors.
287
c. Retro
Reflectors
• They are plane mirrors, roof prisms or
cube comers.
• The cube comers are three mutually
perpendicular plane mirrors, and the
reflected beam is always parallel to
the incident beam in these devices.
•In case of AC laser interferometer measurements, two retro reflectors are
used.
• When plane mirror is used as retro reflectors in plane mirror
interferometer, it must be flat with in 0.06 micron per cm.
288
3. LASER HEAD'S MEASUREMENT
RECEIVER:
• It is used to detect the part of the returning beam as f1 – f2 and a
Doppler shifted frequency component ∂f .
4. DISPLAY:
• The measurement display has a microcomputer to compute and
display results.
• The signals from reference receiver and measurement receiver
located in the laser head are counted in two separate pulse counters
and subtracted.
• Other input signals for correction are temperature co-efficient of
expansions. Air velocity is also displayed.
289
OTHER TYPES OF INTERFEROMETERS
1. Michelson Interferometer
290
the conditions for improving michelson interferometer are ,
1. Use of laser light source for measuring longer distances
2. Instead of using mirror the cube corner reflector is best suitable
for reflecting the light.
3. Photocells can be employed to convert light intensity variation in
voltage pulses to given direction of pc change.
291
2. Twyman - Green
Interferometer
⚫Used as a polarizing interferometer with variable amplitude
balancing between sample and reference waves.
⚫ For an exact measurement of the test surface, the
instrument error can be determined by an absolute
measurement.
⚫This error is compensated by storing the same in
microprocessor system and subtracting from the
measurement of the test surface.
292
It has the following advantages,
• It permits testing of surface with wide varying reflectivity.
• It avoids undesirable feed back of light reflected of the tested
surface and the instrument optics.
• It enables utilization of the maximum available energy.
• Polarisation permits phase variation to be effected with the
necessary precision.
293
LASER INTERFEROMETER
APPLICATIONS
1. Linear measurement
294
2. Angular measurement
295
COORDINATE MEASURING
MACHINES
• The term measuring machine generally refers to a single-axis
measuring instrument.
• Such an instrument is capable of measuring one linear dimension
at a time. The term coordinate measuring machine refers to the
instrument/machine that is capable of measuring in all three
orthogonal axes.
• Such a machine is popularly abbreviated as CMM. A CMM
enables the location of point coordinates in a three-dimensional
(3D) space.
• It simultaneously captures both dimensions and orthogonal
relationships. Another remarkable feature of a CMM is its
integration with a computer.
• The computer provides additional power to generate 3D objects as
well as to carry out complex mathematical calculations. Complex
objects can be dimensionally evaluated with precision and speed.
296
CO-ORDINATE MEASURING
MACHINE
Construction of CMM:
• The co-ordinate measuring machine has movements in X-Y-Z which
can be easily controlled and measured.
• Each slide in three in three directions has transducer which gives
digital display and senses +ve or –ve direction.
• The measuring head has a probe tip, which can be different kinds like
taper tip, ball tip etc.
297
298
• Four elements,
1. Three axis motion structure
2. Probing system
3. m/c controller & computer hardware
4. Application software
299
1. Three axis motion
structure
i. The Axis Coordination
- each axis fitted with transducer for positional feedback
- Axis movement through precision guide ways ( air bearing)
- frame material – aluminium alloys, ceramic, SiC
ii. Length Measurement M/C
- measuring scales & scale readers
- stainless steel & glass scale
- having electro optical reader heads for exact position
iii. Base With Table
- attached with base
- Granite material
300
2. Probing System
-for gathering data
- end of probe : hard ball (steel or ruby)
3. M/C Controller & Computer Hardware
- axis controller, probing, programming, control of
measuring m/c, data acquisition and evaluation
-computers can also used to control
4. Application Software
301
TYPES OF
CMM
1. a/c to control system,
a. Manual CM
b. Computer Numerical Control
2. a/c to design of main structure,
a. Cantilever type.
c. Articulated arm
b. Bridge type.
d. Gantry Type
3. a/c to mounting style
a. Bench top
b. Free standing
c. Portable & hand held
302
1. Cantilever type
• supports probe from
movable vertical support
2. Bridge type
• horizontally suspended
• x-axis carries the bridge
303
4. Gantry Type
•frame structure raised
on side supports similar
to bridge style
3. Column Type
• portable or tripod mounted
•probe can be placed in many
different directions
304
5. Horizontal arm CMM
305
TYPES OF PROBES
1. Contact Type,
a) Hard Or Fixed
Type
b) Touch Trigger
c) Displacement Probe
2. Non- Contact Type,
a) Optical Probe
b) Acoustical Probe
c) Laser Probe
d) Vision Probe
306
FEATURES OF
CMM
1. In faster machines with higher accuracies, the stiffness to
weight ratio has to be high in order to reduce dynamic forces.
2. All the moving members, the bridge structure Z- axis carriage
and Z-column are made of hollow box construction.
3. Errors in machine are built up and fed into the computer
system so that error compensation is built up into the
software.
307
4. All machines are provided with their own computers and the
CMM can able to measure three-dimensional object from
variable datums.
5. For compensation of temperature gradient, thermocouples are
connected with the machine and interfaced with the .
computer. This will provide the CMM in high accuracy and
repeatability.
6. Rapid growth in software for three and four axes movements
enable CMM to measure hole center distances and form
measurements such as turbine blades, cam profiles
308
CAUSES OF ERRORS
IN CMM
1. The table of CMM may not have perfect geometric form.
2. The probes may have a degree of run out.
3. Some perpendicularity errors occur when probe is moving up and
down.
4. Dimensional errors of a CMM is influenced by,
a. Straightness and perpendicularity of the guide ways.
b. Scale division and adjustment of scales.
c. Probe length and probe structure.
d. Interpolation error due to digitization.
e. Errors of data feeding by operators into computers.
f. Specimen weigh, clamping, surface finish and hardness.
g. Environment.
5. The other errors can be controlled by the manufacturer and
minimized by the measuring software.
309
6. The length of the probe should be minimum and rigid in order to
reduce deflection.
7. The weight of the work piece may change the geometry of the guide
ways and therefore, the work piece must not exceed maximum weight
8. Variation in temperature of CMM, specimen and measuring lab
influence the uncertainty of measurements.
9. The smoke particle, a finger print, a dust particle and human hair may
introduce uncertainty in measurement.
10.The translational errors result from errors in the scale division and
errors in axis direction.
11. Perpendicularity error occurs if the three axes are not orthogonal.
310
PERFORMANCE OF
CMM
1. Geometrical accuracies such as positioning accuracy, straightness
and squareness
2. Measuring accuracy in terms of axial length measuring accuracy.
3. Volumetric length measuring accuracy and length measuring
repeatability i.e., CMM has to be tested as complete system.
4. Environmental effects have great influence for the accuracy
testing, including parameters, vibrations and relative humidity and
required.
311
APPLICATIONS OF
CMM
1. CMM finds applications In automatic, machine tool, electronics,
space and many other large companies.
2. For development of new products and construction of prototype.
3. It is very much useful in checking NC produced work piece in various
steps of production.
4. For aircraft and space vehicle, hundred percent inspections are
carried out by using CMM.
5. Used for determining the dimensional accuracy of the components.
312
6. Its ideal for determination of shape and position, maximum metal
condition, linkage of results etc.
7. Best suited for the test and inspection of test equipment - gauges &
tools.
8. Sorting tasks to achieve optimum pairing of components with
tolerance limits.
9. Used for low degree of utilization like gear tester, gauge tester, length
measuring machine-measuring microscope etc.
10.For ensuring economic viability of NC machines by reducing their
downtime for inspection results.
11. Helps in reading cost, rework cost at the appropriate time with a
suitable CMM.
313
ADVANTAGES OF
CMM
1. The inspection rate is increased.
2. Improved accuracy of machined parts.
3. Minimisation of operator error.
4. Skill requirements of the operator is reduced.
5. Reduced inspection fixturing and maintenance cost.
6. Uniform inspection quality.
7. Reduction in calculating and recoding time and errors.
314
8. Reduction in setup time.
9.Compensation for misalignment.
10. No need of separate go/no go gauges for each feature.
11. Reduction of scrap and good part rejection.
12. Provision of a permanent record for process Control.
13. Reduction in offline analysis time.
14. Simplification of inspection procedures.
15. Possibility of reduction of total inspection time.
315
DISADVANTAGES OF
CMM
1. The lable and probe may not be in perfect alignment.
2. The probe may have run out.
3. The probe moving in Z-axis may have some perpendicular errors
4. Probe will move in X and Y direction but not be square to each
other.
5. There may be errors in digital system.
316
FEATURES OF CMM
SOFTWARE
1. Measurement of diameter, centre distance, length.
2. Measurement of plane and spatial curves.
3. Minimum CNC programme.
4. Data communications.
5. Digital input and output command.
6. Programme for the measurement of spur, helical, bevel and hypoid
gears.
7. Interface to CAD software.
317
DIGITAL
DEVICES
• Digital indication is better by far when an exact initiative value is
desired.
• The important elements of any electronic signal readout system are
the scale unit or transducer and the Counter of digital readout unit
318
THE ADVANTAGES OF DIGITAL
SYSTEMS
1. Measuring element is free from errors.
2. Learning time is short.
3. More accurate measurement.
4. Excessive reading errors can be eliminated.
5. Clear readability of digital readout is advantageous for persons with
impaired vision.
6. The display can be zero wherever it is desired.
7. BCD output makes the instrument computer compatible.
319
COMPUTER BASED
INSPECTION
MACHINE VISION
•Machine vision is the ability of a computer to see
the object.
•Also called as computer vision or artificial
vision.
•It is technique which allow a sensor to view the
object and derive a mathematical or logical decision
without human intervention.
•Functions of machine vision,
1. image sensing
2. image analysis
3. image interpretation
320
Stages of Machine
Vision
321
• Stages of Machine Vision
1. Image generation and digitization
• The primary task in a vision system is to capture a 2D or 3D image
of the work part. A 2D image captures either the top view or a side
elevation of the work part, which would be adequate to carry out
simple inspection tasks. While the 2D image is captured using a
single camera, the 3D image requires at least two cameras
positioned at different locations. The work part is placed on a flat
surface and illuminated by suitable lighting, which provides good
contrast between the object and the background. The camera is
focused on the work part and a sharp image is obtained. The image
comprises a matrix of discrete picture elements popularly referred
to as pixels. Each pixel has a value that is proportional to the light
intensity of that portion of the scene. The intensity value for each
pixel is converted to its equivalent digital value by an analog-to-
digital converter (ADC). 322
2. Image processing and analysis
• The frame buffer stores the status of each and every pixel. A number
of techniques are available to analyse the image data. However, the
information available in the frame buffer needs to be refined and
processed to facilitate further analysis. The most popular technique
for image processing is called segmentation. Segmentation involves
two stages: thresholding and edge detection. Thresholding converts
each pixel value into either of the two values, white or black,
depending on whether the intensity of light exceeds a given threshold
value. This type of vision system is called a binary vision system. If
necessary, it is possible to store different shades of grey in an image,
popularly called the grey-scale system. If the computer has a higher
main memory and a faster processor, an individual pixel can also
store colour information. Edge detection is performed to distinguish
the image of the object from its surroundings. Computer programs are
used, which identify the contrast in light intensity between pixels
bordering the image of the object and resolve the boundary of the
object. 323
3. Image interpretation
• Once the features have been extracted, the task of identifying the
object becomes simpler, since the computer program has to match the
extracted features with the features of templates already stored in the
memory. This matching task is popularly referred to as template
matching. Whenever a match occurs, an object can be identified and
further analysis can be carried out. This interpretation function that is
used to recognize the object is known as pattern recognition. It is
needless to say that in order to facilitate pattern recognition, we need
to create templates or a database containing features of the known
objects. Many computer algorithms have been developed for template
matching and pattern recognition. In order to eliminate the possibility
of wrong identification when two objects have closely resembling
features, feature weighting is resorted to. In this technique, several
features are combined into a single measure by assigning a weight to
each feature according to its relative importance in identifying the
object. This adds an additional dimension in the process of assigning
scores to features and eliminates wrong identification of an object. 324
4. Generation of actuation signals
• Once the object is identified, the vision system should direct the
inspection station to carry out the necessary action. In a flexible
inspection environment, the work-cell controller should
generate the actuation signals to the transfer machine to transfer
the work part from machining stations to the inspection station
and vice versa. Clamping, declamping, gripping, etc., of the
work parts are done through actuation signals generated by the
work-cell controller.
325
Surface Roughness
Measurement
Factors affecting surface roughness are,
1. Work piece
2. material Vibrations
3. Machining type Tool
4. Fixtures
The geometrical irregularities can be classified as
1. First order
2. Second order
3. Third order
4. Fourth order
326
58
1. First order irregularities
They are caused by lack of straightness of guide ways on which
tool must move.
2. Second order irregularities
They are caused by vibrations.
3. Third order irregularities
They are caused by machining.
4. Fourth order irregularities
They are caused by materials.
327
ELEMENTS OF SURFACE
TEXTURE
328
329
ELEMENTS OF SURFACE
TEXTURE
1. Profile
It is the contour of any section through a surface.
2. Lay
It is the direction of the 'predominate surface grooves that
are produced by machining.
3. Flaws
It is the surface irregularities or imperfection due to cracks,
blow holes, scratches etc.
4. Actual surface
It is the surface of a part which is actually obtained.
330
5. Roughness
It is finely spaced irregularities. It is also called primary texture.
6. Sampling lengths
It is the Length of profile necessary for the evaluation of~
irregularities.
7. Waviness
It is the surface irregularities which are of greater spacing than
roughness.
8. Roughness height
It is rated as the arithmetical average deviation.
331
9. Roughness width
It is the distance parallel to the normal surface between
successive peaks.
10. Mean line of profile
A Line divides the effective profile such that within the
sampling length is called as mean line or profile.
332
Analysis of surface finish
1. The average roughness method.
2. Peak to valley height method
3. From factor
1. Average roughness measurement
The assessment of average roughness is carried out by
a. Centre line average (CLA)
b. Root mean square (RMS)
c. Ten point method
a. CENTRE LINE
AVERAGE (CLA)
333
B. ROOT MEAN SQUARE
(RMS)
334
C. TEN POINT
METHOD
335
2. PEAK TO VALLEY HEIGHT
METHOD
336
337
METHODS OF MEASURING
SURFACE FINISH
The methods used for measuring the surface finish are
classified into,
1. Inspection by comparison
2. Direct Instrument Measurements
1. Inspection by
comparison
a. Touch Inspection.
b. Visual Inspection.
c. Microscopic Inspection.
d. Scratch Inspection.
e. Micro Interferometer.
f. Surface photographs.
g. Reflected Light Intensity
338
339
2. Direct Instrument Measurements
1. Stylus probe instruments
2. Tomlinson surface meter
3. Profilometer
4. Talyor- Bobson - Talysurf
1. STYLUS PROBE
INSTRUMENTS
340
2. TOMLINSON SURFACE
METER
341
3.
PROFILOMET
ER
342
4. TALYOR- BOBSON -
TALYSURF
343
MACHINE TOOL
METROLOGY
• The accurate production of the component parts depends
upon the accuracy of the machine tools.
• The quality of piece depends on,
1. Rigidity and stiffness of machine tool and its components.
2. Alignment of various components in relation to one
another.
3. Quality and accuracy of the control devices and the driving
mechanism.
344
• The alignment accuracy of the machine tools is checked by some
geometric tests. They are,
1. Geometrical Test
• Dimensions of components, position of components and
displacement of component relative to one another are checked.
a. Static tests:
• Checks the alignment accuracy of the varies parts of machine tools
b. Dynamic tests:
• Performed under dynamic loadig conditions
2. Practical Test
345
VARIOUS GEOMETRICAL CHECKS ON
MACHINE TOOL
• Straightness.
• Flatness.
• Parallelism, equidistance and
coincidence.
• Squareness of straight line &
plane.
• Rotations
• Out of round.
• Eccentricity.
• Run out.
• Periodical axial slip.
• Camming.
• Movement of all the working
components.
• Spindle test for
• Concentricity.
• Axial slip.
• Accuracy of axis and position.
346
STRAIGHTNESS
MEASUREMENTS
347
348
• Types of Straightness Measurements,
1. Straight edge or Spirit level
2. Auto collimator
1. STRAIGHT EDGE OR SPIRIT LEVEL
349
2. AUTO
COLLIMATOR
350
351
SQUARENESS MEASUREMENT
•Very often, two related parts of a machine need to meet perfect
squareness with each other.
•In fact, the angle 90° between two lines or surfaces or their
combinations, is one of the most important requirements in engineering
specifications.
• For instance, the cross-slide of a lathe must move at exactly 90° to the
spindle axis in order to produce a flat surface during facing operation.
• Similarly, the spindle axis of a drilling machine and a vertical milling
machine should be perfectly square with the machine table.
•From a measurement perspective, two planes, two straight lines, or a
straight line and a plane are said to be square with each other when
error of parallelism in relation to a standard square does not exceed a
limiting value.
•The standard square is an important accessory for conducting the
squareness test. It has two highly finished surfaces that are
perpendicular to each other to a high degree of accuracy.
SQUARENESS
MEASUREMENT
•Two surfaces need to have a high
degree of squareness. The base of a
dial gauge is mounted on one of
the surfaces, and the plunger is
held against the surface of the
standard square and set to zero.
•Now, the dial gauge base is given a
traversing motion in the direction
shown in the figure, and deviation
of the dial gauge is noted down.
• The
maximum permissible
for a
deviation
specific
traversing distance is the error in
squareness.
352

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Metrology Measurements and All units PPT

  • 2. UNIT I BASICS OF METROLOGY Introduction to Metrology Need Elements Work piece, Instruments their effect – Persons – on Precision and Environment – Accuracy Errors Errors in Measurements Types Control Types of standards. 2
  • 3. MEASUREMENTS - Introduction • Measurement is a process of comparing inputs with pre-defined standard and giving the output. • Metrology is a science of measurement. • Metrology is also concerned with the inspection and its various industrial techniques. • For every kind of quantity measured, there must be a unit to measure it. 3
  • 4. 1.Measurand, a physical quantity such as length, weight, and angle to be measured 2. Reference, to compare the measurand (physical quantity) with a known Standard for evaluation 3.Standard/Reference, the physical quantity or property to which quantitative comparisons are to be made, which is internationally accepted. STANDARD (Known Quantity) MEASURAND (Unknown Quantity) COMPARISON PROCESS RESULT (Numerical Value) MEASUREMENTS - Introduction 4
  • 5. 1. To convert physical parameters into meaningful numbers. 2. To determine the true dimensions of a part. 3. To increase our knowledge and understanding of the world. 4. Needed for ensuring public health and human safety. 5. To test if the elements that constitute the system function as per the design. 6. For evaluating the performance of a system. 7. To ensure interchangeability with a view to promoting mass production. 8. To establish the validity of design and for finding new data and new designs. NEED FOR MEASUREMENT 5
  • 6. • Industrial Metrology - Industrial metrology’s purpose is to ensure that instruments, used in a wide variety of industries, are functioning properly. • Scientific Metrology - This form of metrology deals with the organization and development of measurement standards and with their maintenance. • Legal Metrology - Concerned with the measurements that influence economic transactions, legal metrology is a very refined type of metrology. TYPES OF METROLOGY 6
  • 7. COMPONENTS OF GENERALIZED MEASUREMENT SYSTEM A generalized measurement system consists of the following components: 1. Primary Sensing Element 2. Variable Conversion Element 3. Variable Manipulation Element 4. Data Processing Element 5. Data Transmission System 6. Data Presentation Element 7
  • 9. COMPONENTS 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: Variable manipulation element manipulates and amplifies the output of the variable conversion element. It also removes noise (if present) in the signal. 9
  • 10. COMPONENTS OF GENERALIZED MEASUREMENT SYSTEM 4. Data Processing Element: It processes the data signal received from the variable manipulation element and produces suitable 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. 6. Data Presentation Element: It is used to present the measured physical quantity in a human readable form to the observer. LED displays are most commonly used as data presentation elements in many measurement systems. 10
  • 11. Physical Quantity - Temperature Sensing Element - Bulb Conversion Element –Pressure Transmission Element Manipulation Element Presentation Element Element Conversion COMPONENTS OF GENERALIZED MEASUREMENT SYSTEM 11
  • 12. STANDARDS In metrology (the science of measurement), a standard is an object, or system that bears a defined relationship to a unit of measurement of a physical quantity.  Depending on functions and applications, standards of measurement are classified as follows: (i) International Standards (ii) Primary Standards (iii) Secondary Standards (iv) Working Standards 12
  • 13. i. International Standards  Defined by International agreement  Periodically evaluated & checked by absolute measurements in terms of fundamental units of physics  represent certain units of measurement to the closest possible accuracy attainable by the science and technology of measurement  These standards are not available to ordinary uses like measurement and calibrations. 13
  • 14. ii. Primary Standards  Main function is the calibration and verification of secondary standards  These are maintained at the National Standards Laboratories in different countries. For India, it is National Physical Laboratory at New Delhi.  The primary standards are not available for the use outside the National Laboratory.  These primary standards are absolute standards of high accuracy that can be used as ultimate reference standards to check, calibrate and certify the secondary standards. 14
  • 15. iii. Secondary Standards  Basic reference standards used by the measurement and calibration laboratories in industries  These standards are maintained by the particular industry to which they belong  Each industry has its own secondary standard  Each laboratory periodically sends its secondary standard to the national standards laboratory for calibration and comparison against the primary standard  After comparison and calibration, the National Standards Laboratory returns the secondary standards to the particular industrial laboratory with a certification of measuring accuracy in terms of primary standards 15
  • 16. iv. Working Standards  main tools of a measuring laboratory  used to check and calibrate laboratory instrument for accuracy and performance.  For example, manufacturing of mechanical components such as shafts, bearings, gears etc, use a standard called working standard for checking the component dimensions. Example: Plug gauge is used for checking the bore diameter of bearings. 16
  • 17. UNITS • Physical quantity is expressed in Units. • Types: 1. Primary Units – m, Kg, KJ 2. Supplementary Units - rad 3. Derived Units – Kg/KJ 06-Jul-18 18 17
  • 18. 1. Direct Comparison 2. Indirect Comparison 3. Comparative Method 4. Coincidence Method 5. Fundamental Method 6. Contact Method 7. Transposition Method 8. Complementary Method 9. Deflection Method 10. Contactless method TYPES OF MEASUREMENTS / METHODS OF MEASUREMENTS 06-Jul-18 19 18
  • 19. 1. Direct Method  Measurements are directly obtained. Ex.: Vernier Caliper, Scales. 2. Indirect Method  Obtained by measuring other quantities. Ex: Measurement of strain induced in a bar due to the applied force 06-Jul-18 20 19
  • 20. 3. Comparative Method It’s compared with other value. known Ex: Comparators. 4. Coincidence Method: Measurements coincide with certain lines and signals. Ex: Comparators. 5. Fundamental Method: Measuring a quantity directly in related with the definition of that quantity. 06-Jul-18 21 20
  • 21. 6. Transposition Method: Quantity to be measured is first balanced by a known value and then balanced by an other new known value. Ex: Determination of mass by balancing methods. 7. Complementary Method: The value of quantity to be measured is combined with known value of the same quantity. Ex: Determination of the volume of a solid by liquid displacement Volume. 06-Jul-18 22 21
  • 22. 8. Deflection Method: 10. Contactless method: There is no direct contact with the surface to be measured. E0x6-.Juml-18easurement by optical instruments The value to be measured is directly indicated by a deflection of pointer. Ex: Pressure Measurement. 9. Contact Method: Sensor/Measuring tip touch the surface area. Ex: Vernier Caliper. 23 22
  • 23. 1. Deflection and Null type instruments 2. Analog and Digital instruments 3. Active and passive type instruments 4. Automatic and manually operated instruments 5. Absolute and secondary instruments 6. Contacting and non-contacting instruments 7. Intelligent instruments TYPES OF MEASURING INSTRUMENTS 23
  • 24.  The weight indicated by of the object is the deflection or of a a scale. pointer on Ex. Spring movement graduated Balance 1. Deflection and Null type instruments  The effect caused by the quantity to be measured is nullified.  For example, consider the measurement of weight by an ordinary beam balance as shown in fig. The unknown weight placed in one-side causes the beam and the pointer to deflect. Ex. Beam Balance 24
  • 25. 2. Analog and Digital Instruments 25
  • 26.  In active instruments, the quantity being measured just activates the magnitude of some, external power input source, which in turn produces the measurement.  In this type of instruments, another external energy input source is present apart from the quantity to be measured. 3. Active and Passive Type Instruments  In passive type instruments, output is produced entirely by the quantity being measured. 26
  • 27. 4. Manual and Automatic Instruments  Manual instruments require the services of a human operator.  When the process of null balance is automated, it is known termed as automatic instruments. 27
  • 28. 5. Absolute and Secondary Instruments instruments  Absolute give the value of the are those which quantity to be measured, in terms of the constants of the instrument and their deflection only.  Secondary Instrument shows deflection directly in terms of electrical quantity like voltage, current, power and frequency.  These instruments are calibrated by comparison with an absolute instrument. 28
  • 29. 6. Contacting and Non-Contacting Instruments 29
  • 30. 7. Intelligent Instrument  Microprocessor are incorporated with measuring instrument 30
  • 31. ACCURACY Vs PRECISION Measurement is an act of assigning an accurate and precise value to a physical variable. What is the difference between Precision andAccuracy Accuracy is a measure of rightness. Precision is a measure of exactness. 31
  • 32. Accuracy is the ability of the instrument to measure the accurate value (Conformity). Precision refers to how closely individual measurements agree with each other (Repeatability). ACCURACY AND PRECISION 32
  • 33. FACTORS AFFECTING ACCURACY AND PRECISION OF A MEASURING SYSTEM • A measuring system is made of five basic elements. These are: 1. Standard 2. Work piece 3. Instrument 4. Person 5. Environment. 06-Jul-18 34 33
  • 34. FACTORS AFFECTING ACCURACY OF A MEASURING SYSTEM 1. Standard □ Coefficient of thermal expansion □ Stability with time □ Elastic properties □ Position etc 2. Work piece: □ Cleanliness surface finish etc. □ Surface defects □ Hidden geometry 3. Instrument □ Inadequate amplification □ Scale error □ Deformation while handling heavy w/p □ Calibration error 06-R Jul- e 18 peatability & readability 35 34
  • 35. 4. Person □ Training skill □ Sense of precision appreciation □ Ability to select measuring instrument & standard □ Attitude towards personal accuracy achievement □ Planning for measurement technique to have minimum just with consistent in precision. 5. Environment □ Temperature, pressure and humidity □ Clean surrounding and minimum vibration □ Adequate illumination □ Temperature equalization between standard w/p & instrument Higher accuracy can be achieved if all 5 factors are considered, analysed & steps are taken to eliminate them 06-Jul-18 36 FACTORS AFFECTING ACCURACY OF A MEASURING SYSTEM 35
  • 36. What is Error in Measurement? • Measurement Error (Observational Error) is the difference between a measured(actual) value and its true value. • True size  Theoretical size of a dimension which is free from errors. • Actual size  size obtained through measurement with permissible error. 06-Jul-18 37 ERRORS IN MEASUREMENT 36
  • 37. Types of Errors in Measurements Gross / Blunder Errors Measurement Errors Systematic Errors Instrumental Errors Observational Errors Environmental Errors Theoretical Errors Random Errors TYPES OF ERRORS IN MEASUREMENT 37
  • 38. 1) Gross or Blunder Errors: This category of errors includes all the human mistakes while reading, recording the readings. The best example of these errors is a person or operator reading pressure gauge 1.01N/m2 as 1.10N/m2. 2) Measurement Error:  The measurement error is the measurement of the true value. result of the variation of a  Usually, Measurement error consists of a random error and systematic error. 06-Jul-18 39 TYPES OF ERRORS IN MEASUREMENT 38
  • 39. a. Systematic Error (Controllable Error) • A systematic error is a constant error that under the same operating conditions. • Systematic error is caused by any factors that systematically affect measurement . • Classification of systematic errors: i. Instrumental Errors – Calibration Error ii. Environmental Errors – Temp, Pressure, Humidity iii. Observational Errors - Parallax iv. Theoretical - Percentage 06-Jul-18 40 TYPES OF ERRORS IN MEASUREMENT 39
  • 40. Random Errors (Uncontrollable Error) • Random (or indeterminate) errors are caused by uncontrollable fluctuations in variables that affect experimental results. • Random errors are caused by the sudden change in experimental conditions and noise and tiredness in the working persons. These errors are either positive or negative. • These errors may be reduced by taking the average of a large number of readings. TYPES OF ERRORS IN MEASUREMENT 40
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  • 150. Thread Measurement: Terminologies, Errors - External Thread Measurement: Pitch Gauge, Tool Maker's microscope, Floating Carriage micrometer with One, Two and Three wires - Internal Thread Measurement: Taper Parallels and Rollers method. Gear Measurement: Terminologies, Errors, Gear Tooth Vernier caliper, Profile Projector, Base pitch measuring instrument, David Brown Tangent Comparator, Involutes tester, Parkinson Gear Tester, External and Internal Radius measurements Roundness measurement: Circumferential confining gauge, Assessment using V block and Rotating centres. 150
  • 151. • Introduction: • Screw threads are used to transmit power and motion and also used to fasten two components with the help of nuts, bolts and studs. • The screw threads are mainly classified into: 1) External Screw Threads 2) Internal Screw Threads. Screw Thread Measurement External Screw Threads Internal Screw Threads 151
  • 153. 1. Screw Thread: It is a continuous helical groove of specified cross- section produced on the external or internal surface. 2. Crest: It is the top surface joining two sides of thread. 3. Root: The bottom of the groove between the two flanks of the thread. 4. Flank: It is the surface between crest and root or it is the thread surface that connects crest with root. 5. Lead: The distance a screw thread advances in one turn. For a single start threads, lead=pitch, For double start, lead=2xpitch, & so on. 6.Pitch: The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis. 7. Helix Angle: The angle made by the helix of the thread at the pitch line with the axis is called as helix angle. 8. Flank angle: It is half the included angle of the thread or angle made by the flank of the thread with the perpendicular to the thread axis. 153
  • 154. 9. Depth of thread: It is the distance between crest and root measured perpendicular to axis of screw. 10. Angle of thread: It is the angle included between the flanks of a thread measured in an axial plane. 11. Major Diameter: This is the diameter of an imaginary cylinder, co- axial with the screw, which just touches the crests of an external thread or roots of an internal threads. It is also called as ‘Nominal diameter’. 12. Minor diameter: This is the diameter of an imaginary cylinder, co- axial with the screw which just touches the roots of an external thread or the crest of an internal thread. This is also referred to as ‘root’or ‘core diameter’. 13. Effective diameter or Pitch diameter: It is the diameter of an imaginary cylinder coaxial with the axis of the thread and intersects the flanks of the thread such that width of the threads & width of spaces between threads are equal. 14. Addendum: It is the distance between the crest and the pitch line 154
  • 155. Tofind out the accuracy of a screw thread it will be necessary to measure the following: 1. Measurement of Major diameter: a. Ordinary micrometer b. Bench micrometer. 2. Measurement of Minor diameter: a. Using taper Parallels b. Using rollers 3. Measurement of Effective diameter: a. One wire method b. Two wire method c. Three wire method d. Using Thread MM 4. Measurement of Pitch: a. Pitch Measuring Machine b. Tool Makers Microscope c. Screw Pitch Gauge 5. Thread angle and form Measurement of various elements in Screw Threads 155
  • 156. 1. Measurement of Major diameter Measurement Processes a. Ordinary micrometer b. Bench micrometer i) Ordinary Micrometer: • In this the micrometer is used as a comparator. • This micrometer is first set over the cylinder standard having approx. same dimension. • This standard is called setting gauge. 156
  • 157. After taking this reading ‘R1’the micrometer is set on the major diameter of the thread, and the new reading is ‘R2’and then the diameter is measured by following equation:. Then the major diameter, D=S ± (R1 - R2) S = Size of setting gauge R1 = Micrometer reading over setting gauge. R2 = 157
  • 158. ii) Measurement by Bench micrometer: • Bench micrometer is designed by the NPL to remove deficiencies inherent in the hand micrometer. • In this the fiducial micrometer is used to ensure that all the readings are taken at the same pressure. • The instrument has a micrometer head having Vernier scale to read to the accuracy of 0.002mm. • This instrument is also used as the comparator to avoid the pitch errors of micrometer threads, zero error setting etc. • Then the process is same as of the ordinary micrometer. Calibrated setting cylinder having the same diameter as the major diameter of the thread to be measured is used as setting standard. 158
  • 159. 2. Measurement of Minor diameter Minor diameter is the imaginary diameter of thread which would touch the roots of the external and crest of the internal threads. For measuring minor diameter of external following methods are used: 1. Two V pieces method 2. By projecting the thread on the screen threa ds For measuring minor diameter of internal thread following methods are used: 1. Using taper parallels 2. By using rollers and slip gauges 159
  • 160. V pieces method • The minor diameter is measured by a comparative method by using floating carriage diameter measuring machine and small ‘V’ pieces which make contact with the root of the thread. • These V pieces are made in several sizes, having suitable radii at the edges. V pieces are made of hardened steel. • The floating carriage diameter-measuring machine is a bench micrometer mounted on a carriage. 160
  • 161. Measurement Process: • The threaded work piece is mounted between the centres of the instrument and the V pieces are placed on each side of the work piece and then the reading is noted. • After taking this reading the work piece is then replaced cylindrical setting gauge. • The minor diameter of the thread = D ± (R2 –R1) • Where, D = Diameter of cylindrical gauge by a standard reference R2 = Micrometer reading on threaded workpiece, R1 = Micrometer reading on cylindrical gauge. 161
  • 162. • If the threads are very sharp or have no radius at the root. • The measurement of minor diameter is done by projecting the thread form on a screen. • This projected form is compared with the use of the Tool Makers Microscope. By projecting thread on screen 162
  • 163. For measuring minor diameter of internal thread: a. Using taper Parallels b. Using rollers Using taper Parallels • For the internal thread of the minor diameter of diameter less than 200mm is measured using the taper parallels. • The taper parallels are the pairs of the wedges having parallel outer edges. • The taper parallels are inserted inside the thread and adjusted until firm contact is not established with the 163
  • 164. 2. Using Rollers: • For more than 200mm diameter this method is used. • Precision rollers are inserted inside the thread and proper slip gauge is inserted between the rollers. • The minor diameter is then the length of slip gauges plus twice the diameter of roller. 164
  • 165. • Effective diameter is the imaginary diameter in between major and minor diameter. • The effective diameter measurement is carried out by the following methods. 1. Wire Methods 2. Thread Micrometer 3. Measurement of Effective diameter 165
  • 166. 3. Measurement of Effective diameter – Wire Method •The effective diameter measurement is carried out by the following methods. 1. One Wire Method 2. Two Wires Method 3. Three Wires Method • This methods are based on the size of the wire. • The size of the wire whose diameter makes the contact with the flank of the thread on the effective diameter this size of wire is known as Best Size of Wire. • This size is decided by the following equation: Where p= pitch and θ= thread angle 166
  • 167. • In this method, only one wire is used. The wire is placed between the two threads at one side and on the other side the anvil of the measuring micrometer contacts the crests. • First, the micrometer reading ‘d1’ is noted on a standard gauge whose dimension is approximately same to be obtained by this method. • Now, the setting gauge is replaced by thread and the new reading is taken i.e. ‘d2’ then effective diameter D = S± (d1-d2). Where, S = Size of setting gauge. • Actual measurement over wire on one side and threads on other side = size of gauge One Wire Method 167
  • 168. The effective diameter can not be measured directly but can be calculated from the measurements made. In this method, wires of exactly known diameters are chosen such that they contact the flanks at their straight portions. If the size of the wire is such it contacts the flanks at the pitch line, it is called the ‘best size’ of wire which can be determined by geometry of screw thread. The screw thread is mounted between the centers & wires are placed in the grooves Two Wire Method 168
  • 169. Measuring Process Effective diameter E is calculated by E = T + P Where, T = Dimension under the wires = M - 2d M = Dimension over the wires d = Diameter of each wire P = Compensating factor should be added to T value and it depends on diameter of wire, pitch & angle of the screw thread. Here, The diameter under the wires ‘T’can also be determined by, T= S - (R1 – R2) Where, S = The diameter of the standard. R1 = Micrometer reading over standard and wires. R2 = Micrometer reading over screw thread andwires P= 0.866 p - d => For metric thread. 169
  • 170.  The three-wire method is the accurate method.  In this method, three wires of equal and precise diameter are placed in the grooves at opposite sides of the screw.  In this, one wire on one side and two on the other side are used.  The wires either may be held in hand or hung from a stand. This method ensures the alignment of micrometer anvil faces parallel to the thread axis. Three Wire Method 170
  • 171. Case i) In case of Whitworth Thread: M = D + 3.1657d – 1.6p where, D = Outside Diameter Case ii) In case of Metric Thread: M = D + 3d – 1.5155p We can practically measure the value of M & then compare with the theoretical values using the formula derived above. After finding the correct value of M, as d is known, E can be found out. 171
  • 172. Best Size Wire: Best size wire is one in which, the wire is having such a diameter that it makes contact with the flanks of the thread on the effective diameter or pitch line. It is recommended that for measuring the effective diameter, always the best size wire should be used and for this condition the wire touches the flank at mean diameter line within ±1/5 of flank length. 172
  • 173. Effective Diameter Measurement  This method is simple and rapid.  The thread micrometer is same as ordinary micrometer except that it has special contact points to suit the end screw threads form that is to be checked.  The contact points are selected on the basis of the types of the thread and the pitch of the thread to be measured.  Then the anvils are then made to contact the thread to be checked and the reading is taken, which will give the pitch diameter or effective diameter.  In this the actual reading is the  measurement of the major diameter on one side and minor diameter of the other side which gives us the effective diameter. Thread Micrometer 173
  • 174. i) Pitch Measuring Machine:  The principle of this method of measurement is to move the stylus along the screen parallel to the axis from one space to the next.  The pitch-measuring machine provides a relatively simple and accurate method of measuring the pitch.  Initially, the micrometer reading is set near the zero on the scale.  Spring loaded head permits the stylus to move up the flank of the thread and down into the next space as it is moved along.  Accurate positioning of the stylus between the two flanks is obtained by ensuring that the pointer T is always opposite to its index mark when readings are taken.  When the pointer is accurately placed in position, the micrometer reading is noted. 4. Pitch Measurement 174
  • 175. ii.Tool Makers Microscope: • Worktable is placed on the base of the instrument. • The optical head is mounted on a vertical column it can be moved up and down. • Work piece is mounted on a glass plate. • A light source provides horizontal beam of light which is reflected from a mirror by 90 degree upwards towards the table. • Image of the outline of contour of the work piece passes through the objective of the optical head. • The image is projected by a system of three prisms to a ground glass screen. • The measurements are made by means of cross lines engraved on the ground glass 175
  • 176. Applications: 1. Linear measurements. 2. Measurement of pitch of the screw. 3. Measurement of pitch diameter. 4. Measurement of thread angle. 5. Comparing thread forms. 6. Centre to center distance measurement. 176
  • 177. iii. Screw pitch gauge: • It is used to directly compare the pitch by just selecting the proper pitch value entered in the pitch gauge and comparing it with the actual screw thread. 177
  • 178. 5. Flank Angle and Thread form Measurement Flank angle •Flank Angle is the angle formed by a flank and a perpendicular to the thread axis in an axial plane. •It is also called the half thread angle. •For this measurement we have to measure the thread angle. •To measure the thread angle the following methods is used: 1. Optical Projection 178
  • 179. 5. Thread form and flank angle Measurement Thread form The ideal and actual forms are compared for the measurement. Types of thread gauges are, 1. Plug Screw Gauge 2. Ring Screw Gauge 3. Caliper Screw Gauge 179
  • 180. The error in screw thread may arise during the manufacturing or storage of threads. The error either may cause due to the following six main elements in the thread.2. Minor diameter error 5. Flank angles error 3. Effective diameter error 6. Crest and root error 1. Major diameter error 4. Pitch error 1. Major diameter error It may cause reduction in the flank contact and interference with the matching threads. 2. Minor diameter error It may cause interference, reduction of flank contact. 3. Effective diameter error If the effective diameter is small the threads will be thin on the external screw and thick on an internal screw. 4. Pitch error Pitch error is defined as the total length of thread engaged either too high or too small. The various pitch errors may be classified into Errors in Screw Threads 180
  • 181. This error occurs i) Progressive Pitch error: The pitch of the thread is uniform but is longer or shorter its nominal value and this is called progressive error. This error occurs whenever the tool–work velocity ratio is incorrect but constant. Causes of Progressive error: 1. Incorrect linear and angular velocity ratio. 2. Incorrect gear train and lead screw. 3. Saddle fault. 4. Variation in length due to hardening. ii) Periodic Pitch error: In this the pitch error causes the errors to repeat at certain time of interval. when the tool–work velocity ratio is not constant. Causes of Periodic error: 1. Un-uniform tool work velocity ratio. 2. Teeth error in gears. 3. Lead screw error. 181
  • 182. (iii) Drunken error: It is error due to the irregular form of helical groove on a cylindrical surface. In this case pitch measured parallel to the axis is always same, but problem is with the thread is not cut to its true helix. Due to this flank surface will not be as a straight edge, it will be as curved form. (iv) Irregular error: These are the errors randomly take place on threads without any specific reason. Causes of Irregular error: 1. Machine fault. 2. Non-uniformity in the material. 3. Cutting action is not correct. 4. Machining disturbances. Effect of pitch error: 1. It increases the effective diameter of the bolt and decreases the diameter of nut. 182
  • 183. • Gears are mechanical drives which transmit power through toothed wheel. • In this gear drive, the driving wheel is in direct contact with driven wheel. • The accuracy of gearing is very important factor when gears are manufactured. • The transmission efficiency is almost 99% for gears. • So, it is very important to test and measure the gears precisely. • For proper inspection of gear, it is very important to concentrate on the raw materials, which are used to manufacture the gears. • Also very important to check the machining of the blanks, heat treatment and finishing of teeth. Gear Measurements 183
  • 184. The most commonly used forms of gear teeth are 1.Involute 2.Cycloidal The involute gears also called as straight tooth or spur gears. The cycloidal gears are used in heavy and impact loads. The involute rack has straight teeth. The involute pressure angle is either 20̊ or 14.5 Types of Gears 1.Spur gear Cylindrical gear whose tooth traces is straight line. These are used for transmitting power betwee 184
  • 185. 3. Helical gears These gears are used to transmit the power between parallel shafts as well as non- parallel and non- intersecting shafts. It is a cylindrical gear whose tooth traces is straight line. 4. Bevel gears The tooth traces are straight-line generators of cone. The teeth are cut onthe conical surface. It is used to connect the shafts at right angles. 5. Worm and Worm wheel It is used to connect the shafts whose axes are non-parallel and non- intersecting. 6. Rack and Pinion Rack gears are straight spur gears with infinite radius. 185
  • 187. 1. Tooth Profile: It is the shape of any side of gear tooth in its cross section. 2.Base circle: It is the circle of gear from which the involute profile is derived. Base circle diameter = Pitch circle diameter x Cosine of pressure angle of gear 3. Pitch circle diameter (PCD): It is the diameter of a circle which will produce the same motion as the toothed gear wheel. 4.Pitch circle: It is the imaginary circle of gear that rolls without slipping over the circle of its mating gear. 5. Addendum circle: The circle that coincides with the crests (or) tops of teeth. 6. Dedendum circle (or) Root circle: This circle that coincides with the roots (or) bottom of teeth. 7. Pressure angle (α): It is the angle made by the line of action with the common tangent to the pitch circles of mating gears. 𝑑 8. Module (m): It is the ratio of pitch circle diameter to the total number of teeth. m = Where, d = Pitch circle diameter, n =Number of teeth. 𝑛 Spur Gear Terminology 187
  • 188. 9. Circular pitch: It is the distance along the pitch circle between corresponding points of adjacent teeth. c 𝑛 P = 𝜋𝑑 = 𝜋m 10. Diametral pitch (Pd): Number of teeth per inch of the PCD. d 𝑑 𝑚 P = 𝑛 = 1 Where, m = Module 11. Addendum: It is the radial distance between tip circle and pitch circle. Addendum value = 1 module. 11. Dedendum: It is the radial distance between pitch circle and root circle. Dedendum value=1.25 module. 13. Clearance(c): The distance covered by the tip of one gear with the root of mating gear. Clearance = Difference between Dedendum and addendum values. 14. Blank diameter: It is the diameter of the blank upto outer periphery. Blank diameter = PCD+2m 188
  • 189. 9. Face: It is the part of the tooth in the axial plane lying between tip circle and pitch circle. 10. Flank: It is the part of the tooth lying between pitch circle and root circle. 11. Helix angle: It is the angle between the tangents to helix angle. 12. Top land: Top surface of a tooth is called as top land. 13.Lead angle: It is the angle between the tangent to the helix and plane perpendicular to the axis of cylinder. 14. Backlash: It is the difference between the tooth thickness and the space into which it meshes. If we assume the tooth thickness as ‘t1 ’ and width ‘t2 ’then 189
  • 190. 1. Profile error: The maximum distance is at any point on the tooth profile form to the design profile. 2. Pitch error: It is the difference between actual and design pitch. 3. Cyclic error: Error occurs in each revolution of gear. 4.Run out: Total range of a fixed indicator with, the contact points applied to a surface rotated, without axial movement, about a fixed axis. 5. Eccentricity: It is the half radial run out. 6. Wobble: Run out is measured parallel to the axis of rotation at a specified distance from the axis. 7. Radial run out: Run out is measured along a perpendicular to the axis of rotation. 8. Undulation: It is the periodical departure of the actual tooth surface from the design surface. 9. Axial run out: Run out is measured parallel to the axis of rotation at a speed. Errors in Gear 190
  • 191. The inspection of the gears consists of the following elements in which manufacturing error may be present. 1. Runout 2. Pitch 3. Profile 4. Lead 5. Backlash 6. Tooth thickness Spur Gear Measurement 191
  • 192. 1. Measurement of Runout • the run out is an amount a gear moves in and out away from it true centre as it is rotated. • If runout is excessive, the gear wobbles as it rotates. Runout is also the eccentricity in the pitch circle of gear. • Gears that are eccentric tend to have vibration per revolution. • It may cause an abrupt gear failure. • The gear is held on a mandrel in the centers and the dial indicator of the tester holds a special tip descending upon the module of gear being tested. • The tip is inserted between the tooth spaces and dial indicator reading is noted. 192
  • 193. Pitch is the distance between corresponding points on equally spaced and adjacent teeth. Pitch error is the difference in distance between equally spaced adjacent teeth and the measured distance between any two adjacent teeth. There are two ways for measuring the pitch. a) Point to point measurement (i.e. One tooth point to next tooth point) b) Direct angular measurement 2. Measurement of Pitch 193
  • 194. a) Tooth to Tooth measurement: The instrument has three tips. One is fixed measuring tip and the second is sensitive tip, whose position can be adjusted by a screw and the third tip is adjustable or guide stop. The distance between the fixed and sensitive tip is equivalent to base pitch of the gear. All the three tips are made in contact with the tooth by setting the instrument and the reading on the dial indicator is the error in the base pitch. b) DirectAngular Measurement: It is the simplest method for measuring the error by using set dial gauge against a tooth. In this method, the position of a suitable point on a tooth is measured after the gear has been indexed by a 2. Measurement of Pitch 194
  • 195. i. Optical projection method: The profile of the gear is projected on the screen by optical lens and then the projected value is compared with master profile. ii. Involute measuring machine: In this method, the gear is held on a mandrel and circular disc of same diameter as the base circle of gear for the measurement is fixed on the mandrel. After fixing the gear on the mandrel, the straight edge of the instrument is brought in contact with the base circle of the disc. Now, the gear and disc are rotated and the edge 3. Measurement of Profile 195
  • 196. 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 are a) Gear tooth Vernier caliper method b)Constant chord method c)Base tangent method d) Measurement over pins or ball 6. Measurement of Tooth Thickness 196
  • 197.  It is used to measure the thickness of gear teeth at the pitch line or chordal thickness of teeth and the distance from the top of a tooth to the chord.  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.  This method is simple and inexpensive. Disadvantages of Gear Tooth Vernier method: 1.Not closer to 0.05mm. 2.Two Vernier readings are required. Gear tooth Thickness – Gear Tooth Vernier 197
  • 198. • A constant chord is defined as, the chord, joining those points, on opposite faces of the tooth, which make contact with the mating teeth, when the center line of the tooth lies on the line of the gear centers. • Constant chord of a gear is measured where the tooth flanks touch the flanks of the basic rack. • The teeth of the rack are straight and inclined to their centre lines at the pressure angle. • Also the pitch line of the rack is tangential to the pitch circle of the gear, the tooth thickness of the rack along this line is equal to the arc tooth thickness of the gear round its pitch circle. • Now, since the gear tooth and rack space are in contact in the symmetrical position at the Gear tooth Thickness – Constant Chord Method 198
  • 199. Gear tooth Thickness – Base Tangent Method • It is the most commonly used method for checking the tooth thickness of gear. • The advantage of this method is that, it depends only on one vernier reading unlike gear tooth vernier Caliper where we require two vernier readings. • The base tangent length is the distance between the two parallel planes which are tangential to the opposing tooth flanks. • The measurements made across these opposed involutes by span gauging will be constant and equal to the arc length of the base circle between the origins of involutes. • The value of the distance between two opposed involutes, or the dimension over parallel faces is equal to the distance round the base circle between the points where the corresponding tooth 199
  • 200. • The master gear is fixed on vertical spindle and the gear to be tested is fixed on similar spindle which is mounted on a carriage. • The carriage which can slide both side and these gears are maintained in mesh by spring pressure. • When the gears are rotated, the movement of sliding carriage is indicated by a dial indicator and these variations are the measure of any irregularities in the gear under test. • The variation's recorded in a recorder which is fitted in the form of a waxed circular chart. • In fig, the gears are fitted on the mandrels and are free to rotate without clearance. • Left mandrel moves along the table and the right mandrel moves along the spring-loaded carriage. Parkinson Gear Tester – Gear profile inspection 200
  • 201. • If errors occur in the tooth form when gears will be in closer mesh, pitch or concentricity of pitch line will cause a variation in center distance from this movement of carriage as indicated to the dial gauge will show the errors in the gear test. • The recorder is also fitted in the form of circular or rectangular chart and the errors are recorded. Limitations of Parkinson gear tester: 1.Accuracy ±0.001mm 2.Maximum gear diameter is 300mm 3.Errors are not clearly identified. 4.Measurement is dependent upon the master gear. 5.Low friction in the movement of the floating carriage. 201
  • 202. • By definition, roundness or circularity is the radial uniformity of work surface measured from the center line of the workpiece. • Circularity is specified by circularity tolerance. For example, if it is specified that circularity of a feature is to be 0.1mm, than it means that the circumference of each cross section of the feature should be contained between two coplanar concentric circles that are 0.1mm apart. • Error in roundness is defined as the radial distance between the minimum inscribing circle and maximum inscribing circle, that contain the actual profile of the surface at a section perpendicular to the axis of rotation. Roundness Measurement 202
  • 203. • Methods for Measuring Roundness: a) V-block and dial indicator method V-block and dial indicator method: b) Roundness Measuring Machine • The arrangement consists of a V-block that is placed on the surface plate. The workpiece to be tested is placed in the V-groove of the V- block as shown in the figure. • The feeler of a sensitive dial indicator (held firmly by a stand) is made to rest on the workpiece. • Now the workpiece is rotated about the diameter to be checked. The dial indicator will indicate variations in the dimensions caused due to out of roundness. 203
  • 204. • Plotting a Polar Graph: An idea of the actual shape of the workpiece can be obtained by plotting a polar graph. 12 equispaced markings at an angle of 30̊ are made in the face of the workpiece to be measured. The workpiece is placed on the V-block. • The dial indicator is made to touch the workpiece at its center. Now when the workpiece is rotated and when the marking comes exactly under the plunger of the dial indicator, the reading is noted. • Hence 12 readings will be obtained. The procedure is repeated thrice to get average values for each marking. Now for plotting the polar graph, a proper scale is selected. • A circle of diameter equal to four times the maximum reading of the dial indicator is drawn. Another concentric circle is drawn in this circle. The values of the dial indicator are plotted in radial direction by taking the smallest circle as the reference circle. The individual points are joined by straight lines to get the actual profile of the workpiece. • 204
  • 205. • Error is measured as the radial distance between the maximum and minimum inscribing circle for the profile obtained. Roundness error = 𝑀𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑒𝑟𝑟𝑜𝑟 𝑓𝑟𝑜𝑚 𝑝𝑜𝑙 𝑎𝑟 𝑔𝑟𝑎𝑝ℎ 𝐾 Where, K is a constant (This constant depends on the shape of the workpiece and angle of V- block) • The position of the indicating instrument, the number of lobes on the workpiece and the angle of the V-block have an influence on the determination of roundness error. 205
  • 206. Roundness Measuring Machine: • The machine is also called as Taly-round Instrument or precision spindle method. • The main parts of the instrument are a truly running spindle that is mounted on precision ball bearing and micron indicator. • The indicating pointer is rotated around the workpiece about an accurately stable axis. The indicator shows deviations from roundness. As the output of the indicator is connected to an amplifier unit and pen recorder, a polar graph of the out line of the workpiece is obtained. • This is an accurate method. Automatic recording of the exact profile of the workpiece is obtained. Waviness also is superimposed on the profile of the workpiece. 206
  • 207. Cylindricity A cylinder is an envelope of a rectangle rotated about an axis. It is bound between two circular and parallel planes. Runout Runout is a measure of the trueness of the running of a body about its own axis. 207
  • 208. UNIT-V ADVANCES IN METROLOGY Metrology and Measurements 208
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  • 218. Pitot-tube It is a device used for measuring the velocity of flow at any point in a pipe or a channel. Principle: If the velocity at any point decreases, the pressure at that point increases due to the conservation of the kinetic energy into pressure energy. In simplest form, the pitot tube consists of a glass tube, bent at right angles. 218
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  • 235. TEMPERATURE MEASUREMENT Bimetallic strip thermometer Thermocuples Thermometers Thermister Pyrometers Resistance temperature detectors 235
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  • 250. ELECTRICAL THERMAL RESISTANCE OR ELECTRICAL RESISTANCE THERMOMETER 250
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  • 272. PRECISION INSTRUMENTS BASED ON LASER • Laser stands for "Light Amplification by Stimulated Emission of Radiation". Laser instruments are devices to produce powerful, monochromatic collimated beam of light in which the waves are coherent. • The development of laser gives production of clear coherent light. The biggest advantage of this coherent light is that whole energy appears to be coming from a very small point. • The laser beam can be focused easily into either a parallel beam or into a very small point by the use of lens. 272
  • 273. PRINCIPLE OF LASER • The principle involved in laser is when the photon emitted during stimulated emission has the same energy, phase and frequency as the incident photon. • The photon comes in contact with another atom or molecule in the high energy level E2, then it will cause the atom to return to ground state energy level E1 by releasing another photon. • The sequence of triggered identical photon from stimulated atom is known as stimulated emission. • This multiplication of photon through stimulated emission leads to coherent, powerful, monochromatic, collimated beam of light emission. This Light emission is called laser. 273
  • 274. LASER METROLOGY • A laser beam projected directly onto a position detector is a method of alignment used in a number of commercially available systems. The laser with its highly controlled frequency modes and coherent output are used extensively for interferometery • Laser is suitable for more general applications where a convenient, collimated and high intensity source is required Precision, accuracy, no contact and hot moving parts. 274
  • 275. LASER MEASURING MACHINES 1. Laser Telemetric System 2. Laser and LED Based Distance Measuring Instruments 3. For Profile Checks 4. Scanning Laser Gauge 275
  • 277. Advantages: • It is possible to detect changes in dimensions when components are moving. • It is possible to detect changes in dimensions when product is in continuous processes. • There is no need to wait for taking measurements when the product is in hot conditions. • It can be applied on production machines and controlled them with closed feedback loops. • 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. 277
  • 278. 2. LASER AND LED BASED DISTANCE MEASURING INSTRUMENTS Advantages: 1. It is very reliable because there is no moving part. 2. Instrument response time is in milliseconds. 3. The output is provided as 0 – 20 mA. 278
  • 282. USE OF LASER IN INTERFEROMETRY • The laser in interferometery is to find accurate measurement length. • It reduces the most time taken arid skill required like at methods used for finding the length. • The accuracy of measurement is the order of 0.1m in 100m. • In modified laser designs, a single frequency is selected from the coherent beam and used for interferometric measurement. 282
  • 283. LASER INTERFERMETER • The laser interferometery involves the following components, 1. Two frequency laser source. 2. Optical elements. 3. Laser heads measurement receiver. 4. Measurement display. 283
  • 285. 2. OPTICAL ELEMENTS ⚫The various optical elements are, a. Beam splitters. b. Beam benders. c. Retro reflectors. 285
  • 286. a. Beam splitters ⚫ It is used to divide the laser beam into separate beams along different axes. ⚫It is possible to adjust the spitted laser's output intensity by having a choice of beam splitter reflectivities. 286
  • 287. b. Beam benders • It is used to deflect the light beam around comers on it path from the laser to each axis. • The beam benders are just flat mirrors, but having absolutely flat and very high reflectivity. • Normally, the beam deflection is avoided for not to disturb the polarizing vectors. 287
  • 288. c. Retro Reflectors • They are plane mirrors, roof prisms or cube comers. • The cube comers are three mutually perpendicular plane mirrors, and the reflected beam is always parallel to the incident beam in these devices. •In case of AC laser interferometer measurements, two retro reflectors are used. • When plane mirror is used as retro reflectors in plane mirror interferometer, it must be flat with in 0.06 micron per cm. 288
  • 289. 3. LASER HEAD'S MEASUREMENT RECEIVER: • It is used to detect the part of the returning beam as f1 – f2 and a Doppler shifted frequency component ∂f . 4. DISPLAY: • The measurement display has a microcomputer to compute and display results. • The signals from reference receiver and measurement receiver located in the laser head are counted in two separate pulse counters and subtracted. • Other input signals for correction are temperature co-efficient of expansions. Air velocity is also displayed. 289
  • 290. OTHER TYPES OF INTERFEROMETERS 1. Michelson Interferometer 290
  • 291. the conditions for improving michelson interferometer are , 1. Use of laser light source for measuring longer distances 2. Instead of using mirror the cube corner reflector is best suitable for reflecting the light. 3. Photocells can be employed to convert light intensity variation in voltage pulses to given direction of pc change. 291
  • 292. 2. Twyman - Green Interferometer ⚫Used as a polarizing interferometer with variable amplitude balancing between sample and reference waves. ⚫ For an exact measurement of the test surface, the instrument error can be determined by an absolute measurement. ⚫This error is compensated by storing the same in microprocessor system and subtracting from the measurement of the test surface. 292
  • 293. It has the following advantages, • It permits testing of surface with wide varying reflectivity. • It avoids undesirable feed back of light reflected of the tested surface and the instrument optics. • It enables utilization of the maximum available energy. • Polarisation permits phase variation to be effected with the necessary precision. 293
  • 296. COORDINATE MEASURING MACHINES • The term measuring machine generally refers to a single-axis measuring instrument. • Such an instrument is capable of measuring one linear dimension at a time. The term coordinate measuring machine refers to the instrument/machine that is capable of measuring in all three orthogonal axes. • Such a machine is popularly abbreviated as CMM. A CMM enables the location of point coordinates in a three-dimensional (3D) space. • It simultaneously captures both dimensions and orthogonal relationships. Another remarkable feature of a CMM is its integration with a computer. • The computer provides additional power to generate 3D objects as well as to carry out complex mathematical calculations. Complex objects can be dimensionally evaluated with precision and speed. 296
  • 297. CO-ORDINATE MEASURING MACHINE Construction of CMM: • The co-ordinate measuring machine has movements in X-Y-Z which can be easily controlled and measured. • Each slide in three in three directions has transducer which gives digital display and senses +ve or –ve direction. • The measuring head has a probe tip, which can be different kinds like taper tip, ball tip etc. 297
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  • 299. • Four elements, 1. Three axis motion structure 2. Probing system 3. m/c controller & computer hardware 4. Application software 299
  • 300. 1. Three axis motion structure i. The Axis Coordination - each axis fitted with transducer for positional feedback - Axis movement through precision guide ways ( air bearing) - frame material – aluminium alloys, ceramic, SiC ii. Length Measurement M/C - measuring scales & scale readers - stainless steel & glass scale - having electro optical reader heads for exact position iii. Base With Table - attached with base - Granite material 300
  • 301. 2. Probing System -for gathering data - end of probe : hard ball (steel or ruby) 3. M/C Controller & Computer Hardware - axis controller, probing, programming, control of measuring m/c, data acquisition and evaluation -computers can also used to control 4. Application Software 301
  • 302. TYPES OF CMM 1. a/c to control system, a. Manual CM b. Computer Numerical Control 2. a/c to design of main structure, a. Cantilever type. c. Articulated arm b. Bridge type. d. Gantry Type 3. a/c to mounting style a. Bench top b. Free standing c. Portable & hand held 302
  • 303. 1. Cantilever type • supports probe from movable vertical support 2. Bridge type • horizontally suspended • x-axis carries the bridge 303
  • 304. 4. Gantry Type •frame structure raised on side supports similar to bridge style 3. Column Type • portable or tripod mounted •probe can be placed in many different directions 304
  • 305. 5. Horizontal arm CMM 305
  • 306. TYPES OF PROBES 1. Contact Type, a) Hard Or Fixed Type b) Touch Trigger c) Displacement Probe 2. Non- Contact Type, a) Optical Probe b) Acoustical Probe c) Laser Probe d) Vision Probe 306
  • 307. FEATURES OF CMM 1. In faster machines with higher accuracies, the stiffness to weight ratio has to be high in order to reduce dynamic forces. 2. All the moving members, the bridge structure Z- axis carriage and Z-column are made of hollow box construction. 3. Errors in machine are built up and fed into the computer system so that error compensation is built up into the software. 307
  • 308. 4. All machines are provided with their own computers and the CMM can able to measure three-dimensional object from variable datums. 5. For compensation of temperature gradient, thermocouples are connected with the machine and interfaced with the . computer. This will provide the CMM in high accuracy and repeatability. 6. Rapid growth in software for three and four axes movements enable CMM to measure hole center distances and form measurements such as turbine blades, cam profiles 308
  • 309. CAUSES OF ERRORS IN CMM 1. The table of CMM may not have perfect geometric form. 2. The probes may have a degree of run out. 3. Some perpendicularity errors occur when probe is moving up and down. 4. Dimensional errors of a CMM is influenced by, a. Straightness and perpendicularity of the guide ways. b. Scale division and adjustment of scales. c. Probe length and probe structure. d. Interpolation error due to digitization. e. Errors of data feeding by operators into computers. f. Specimen weigh, clamping, surface finish and hardness. g. Environment. 5. The other errors can be controlled by the manufacturer and minimized by the measuring software. 309
  • 310. 6. The length of the probe should be minimum and rigid in order to reduce deflection. 7. The weight of the work piece may change the geometry of the guide ways and therefore, the work piece must not exceed maximum weight 8. Variation in temperature of CMM, specimen and measuring lab influence the uncertainty of measurements. 9. The smoke particle, a finger print, a dust particle and human hair may introduce uncertainty in measurement. 10.The translational errors result from errors in the scale division and errors in axis direction. 11. Perpendicularity error occurs if the three axes are not orthogonal. 310
  • 311. PERFORMANCE OF CMM 1. Geometrical accuracies such as positioning accuracy, straightness and squareness 2. Measuring accuracy in terms of axial length measuring accuracy. 3. Volumetric length measuring accuracy and length measuring repeatability i.e., CMM has to be tested as complete system. 4. Environmental effects have great influence for the accuracy testing, including parameters, vibrations and relative humidity and required. 311
  • 312. APPLICATIONS OF CMM 1. CMM finds applications In automatic, machine tool, electronics, space and many other large companies. 2. For development of new products and construction of prototype. 3. It is very much useful in checking NC produced work piece in various steps of production. 4. For aircraft and space vehicle, hundred percent inspections are carried out by using CMM. 5. Used for determining the dimensional accuracy of the components. 312
  • 313. 6. Its ideal for determination of shape and position, maximum metal condition, linkage of results etc. 7. Best suited for the test and inspection of test equipment - gauges & tools. 8. Sorting tasks to achieve optimum pairing of components with tolerance limits. 9. Used for low degree of utilization like gear tester, gauge tester, length measuring machine-measuring microscope etc. 10.For ensuring economic viability of NC machines by reducing their downtime for inspection results. 11. Helps in reading cost, rework cost at the appropriate time with a suitable CMM. 313
  • 314. ADVANTAGES OF CMM 1. The inspection rate is increased. 2. Improved accuracy of machined parts. 3. Minimisation of operator error. 4. Skill requirements of the operator is reduced. 5. Reduced inspection fixturing and maintenance cost. 6. Uniform inspection quality. 7. Reduction in calculating and recoding time and errors. 314
  • 315. 8. Reduction in setup time. 9.Compensation for misalignment. 10. No need of separate go/no go gauges for each feature. 11. Reduction of scrap and good part rejection. 12. Provision of a permanent record for process Control. 13. Reduction in offline analysis time. 14. Simplification of inspection procedures. 15. Possibility of reduction of total inspection time. 315
  • 316. DISADVANTAGES OF CMM 1. The lable and probe may not be in perfect alignment. 2. The probe may have run out. 3. The probe moving in Z-axis may have some perpendicular errors 4. Probe will move in X and Y direction but not be square to each other. 5. There may be errors in digital system. 316
  • 317. FEATURES OF CMM SOFTWARE 1. Measurement of diameter, centre distance, length. 2. Measurement of plane and spatial curves. 3. Minimum CNC programme. 4. Data communications. 5. Digital input and output command. 6. Programme for the measurement of spur, helical, bevel and hypoid gears. 7. Interface to CAD software. 317
  • 318. DIGITAL DEVICES • Digital indication is better by far when an exact initiative value is desired. • The important elements of any electronic signal readout system are the scale unit or transducer and the Counter of digital readout unit 318
  • 319. THE ADVANTAGES OF DIGITAL SYSTEMS 1. Measuring element is free from errors. 2. Learning time is short. 3. More accurate measurement. 4. Excessive reading errors can be eliminated. 5. Clear readability of digital readout is advantageous for persons with impaired vision. 6. The display can be zero wherever it is desired. 7. BCD output makes the instrument computer compatible. 319
  • 320. COMPUTER BASED INSPECTION MACHINE VISION •Machine vision is the ability of a computer to see the object. •Also called as computer vision or artificial vision. •It is technique which allow a sensor to view the object and derive a mathematical or logical decision without human intervention. •Functions of machine vision, 1. image sensing 2. image analysis 3. image interpretation 320
  • 322. • Stages of Machine Vision 1. Image generation and digitization • The primary task in a vision system is to capture a 2D or 3D image of the work part. A 2D image captures either the top view or a side elevation of the work part, which would be adequate to carry out simple inspection tasks. While the 2D image is captured using a single camera, the 3D image requires at least two cameras positioned at different locations. The work part is placed on a flat surface and illuminated by suitable lighting, which provides good contrast between the object and the background. The camera is focused on the work part and a sharp image is obtained. The image comprises a matrix of discrete picture elements popularly referred to as pixels. Each pixel has a value that is proportional to the light intensity of that portion of the scene. The intensity value for each pixel is converted to its equivalent digital value by an analog-to- digital converter (ADC). 322
  • 323. 2. Image processing and analysis • The frame buffer stores the status of each and every pixel. A number of techniques are available to analyse the image data. However, the information available in the frame buffer needs to be refined and processed to facilitate further analysis. The most popular technique for image processing is called segmentation. Segmentation involves two stages: thresholding and edge detection. Thresholding converts each pixel value into either of the two values, white or black, depending on whether the intensity of light exceeds a given threshold value. This type of vision system is called a binary vision system. If necessary, it is possible to store different shades of grey in an image, popularly called the grey-scale system. If the computer has a higher main memory and a faster processor, an individual pixel can also store colour information. Edge detection is performed to distinguish the image of the object from its surroundings. Computer programs are used, which identify the contrast in light intensity between pixels bordering the image of the object and resolve the boundary of the object. 323
  • 324. 3. Image interpretation • Once the features have been extracted, the task of identifying the object becomes simpler, since the computer program has to match the extracted features with the features of templates already stored in the memory. This matching task is popularly referred to as template matching. Whenever a match occurs, an object can be identified and further analysis can be carried out. This interpretation function that is used to recognize the object is known as pattern recognition. It is needless to say that in order to facilitate pattern recognition, we need to create templates or a database containing features of the known objects. Many computer algorithms have been developed for template matching and pattern recognition. In order to eliminate the possibility of wrong identification when two objects have closely resembling features, feature weighting is resorted to. In this technique, several features are combined into a single measure by assigning a weight to each feature according to its relative importance in identifying the object. This adds an additional dimension in the process of assigning scores to features and eliminates wrong identification of an object. 324
  • 325. 4. Generation of actuation signals • Once the object is identified, the vision system should direct the inspection station to carry out the necessary action. In a flexible inspection environment, the work-cell controller should generate the actuation signals to the transfer machine to transfer the work part from machining stations to the inspection station and vice versa. Clamping, declamping, gripping, etc., of the work parts are done through actuation signals generated by the work-cell controller. 325
  • 326. Surface Roughness Measurement Factors affecting surface roughness are, 1. Work piece 2. material Vibrations 3. Machining type Tool 4. Fixtures The geometrical irregularities can be classified as 1. First order 2. Second order 3. Third order 4. Fourth order 326
  • 327. 58 1. First order irregularities They are caused by lack of straightness of guide ways on which tool must move. 2. Second order irregularities They are caused by vibrations. 3. Third order irregularities They are caused by machining. 4. Fourth order irregularities They are caused by materials. 327
  • 329. 329 ELEMENTS OF SURFACE TEXTURE 1. Profile It is the contour of any section through a surface. 2. Lay It is the direction of the 'predominate surface grooves that are produced by machining. 3. Flaws It is the surface irregularities or imperfection due to cracks, blow holes, scratches etc. 4. Actual surface It is the surface of a part which is actually obtained.
  • 330. 330 5. Roughness It is finely spaced irregularities. It is also called primary texture. 6. Sampling lengths It is the Length of profile necessary for the evaluation of~ irregularities. 7. Waviness It is the surface irregularities which are of greater spacing than roughness. 8. Roughness height It is rated as the arithmetical average deviation.
  • 331. 331 9. Roughness width It is the distance parallel to the normal surface between successive peaks. 10. Mean line of profile A Line divides the effective profile such that within the sampling length is called as mean line or profile.
  • 332. 332 Analysis of surface finish 1. The average roughness method. 2. Peak to valley height method 3. From factor 1. Average roughness measurement The assessment of average roughness is carried out by a. Centre line average (CLA) b. Root mean square (RMS) c. Ten point method
  • 334. B. ROOT MEAN SQUARE (RMS) 334
  • 336. 2. PEAK TO VALLEY HEIGHT METHOD 336
  • 337. 337 METHODS OF MEASURING SURFACE FINISH The methods used for measuring the surface finish are classified into, 1. Inspection by comparison 2. Direct Instrument Measurements
  • 338. 1. Inspection by comparison a. Touch Inspection. b. Visual Inspection. c. Microscopic Inspection. d. Scratch Inspection. e. Micro Interferometer. f. Surface photographs. g. Reflected Light Intensity 338
  • 339. 339 2. Direct Instrument Measurements 1. Stylus probe instruments 2. Tomlinson surface meter 3. Profilometer 4. Talyor- Bobson - Talysurf
  • 343. 4. TALYOR- BOBSON - TALYSURF 343
  • 344. MACHINE TOOL METROLOGY • The accurate production of the component parts depends upon the accuracy of the machine tools. • The quality of piece depends on, 1. Rigidity and stiffness of machine tool and its components. 2. Alignment of various components in relation to one another. 3. Quality and accuracy of the control devices and the driving mechanism. 344
  • 345. • The alignment accuracy of the machine tools is checked by some geometric tests. They are, 1. Geometrical Test • Dimensions of components, position of components and displacement of component relative to one another are checked. a. Static tests: • Checks the alignment accuracy of the varies parts of machine tools b. Dynamic tests: • Performed under dynamic loadig conditions 2. Practical Test 345
  • 346. VARIOUS GEOMETRICAL CHECKS ON MACHINE TOOL • Straightness. • Flatness. • Parallelism, equidistance and coincidence. • Squareness of straight line & plane. • Rotations • Out of round. • Eccentricity. • Run out. • Periodical axial slip. • Camming. • Movement of all the working components. • Spindle test for • Concentricity. • Axial slip. • Accuracy of axis and position. 346
  • 348. 348 • Types of Straightness Measurements, 1. Straight edge or Spirit level 2. Auto collimator
  • 349. 1. STRAIGHT EDGE OR SPIRIT LEVEL 349
  • 351. 351 SQUARENESS MEASUREMENT •Very often, two related parts of a machine need to meet perfect squareness with each other. •In fact, the angle 90° between two lines or surfaces or their combinations, is one of the most important requirements in engineering specifications. • For instance, the cross-slide of a lathe must move at exactly 90° to the spindle axis in order to produce a flat surface during facing operation. • Similarly, the spindle axis of a drilling machine and a vertical milling machine should be perfectly square with the machine table. •From a measurement perspective, two planes, two straight lines, or a straight line and a plane are said to be square with each other when error of parallelism in relation to a standard square does not exceed a limiting value. •The standard square is an important accessory for conducting the squareness test. It has two highly finished surfaces that are perpendicular to each other to a high degree of accuracy.
  • 352. SQUARENESS MEASUREMENT •Two surfaces need to have a high degree of squareness. The base of a dial gauge is mounted on one of the surfaces, and the plunger is held against the surface of the standard square and set to zero. •Now, the dial gauge base is given a traversing motion in the direction shown in the figure, and deviation of the dial gauge is noted down. • The maximum permissible for a deviation specific traversing distance is the error in squareness. 352