Unit III Advances in metrology

KIT – KALAIGNAR KARUNANIDHI INSTITUTE OF TECHNOLOGY,
COIMBATORE / ME6504 / V-SEM / 2015 METROLOGY & MEASUREMENTS
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LESSON NOTES
ME6504 METROLOGY AND MEASUREMENTS
UNIT – III

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UNIT III ADVANCES IN METROLOGY

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3.2 BASIC CONCEPTS OF LASER:
A laser is a device that emits light through a process of optical amplification based on the
stimulated emission of electromagnetic radiation. The term "laser" originated as an acronym for
"light amplification by stimulated emission of radiation". The first laser was built in 1960 by
Theodore H. Maiman at Hughes Laboratories, based on theoretical work by Charles Hard Townes
and Arthur Leonard Schawlow.
Principle of LASER :
A laser differs from other sources of light in that it emits light coherently. Spatial coherence allows
a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography.
Spatial coherence also allows a laser beam to stay narrow over great distances (collimation),
enabling applications such as laser pointers. Lasers can also have high temporal coherence, which
allows them to emit light with a very narrow spectrum, i.e., they can emit a single color of light.
Temporal coherence can be used to produce pulses of light as short as a femtosecond.
LASER GENERAL APPLICATIONS:
Electronics : Optical disk drives, laser printers, and barcode scanners, Fiber-optic and free-
space optical communication;
Scientific : Spectroscopy, Heat Treatment, Lunar laser ranging, Photochemistry, Laser
barcode scanners, Laser cooling, Nuclear fusion, Microscopy
Military : Directly as an energy weapon, Defensive countermeasures, Disorientation,
Guidance, Targeting, Target designator, Firearms, Laser sight, Eye-targeted
lasers, Holographic weapon sight, Military and law enforcement devices for
marking targets and measuring range and speed;
Medical : Laser surgery and skin treatments;
Industrial : Surveying and Ranging, cutting and welding materials,
Commercial : Entertainment and recreation Laser lighting displays in entertainment.

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ADVANTAGES (BENEFITS) OF LASER MEASUREMENT SYSTEMS:
Compared to conventional methods, such as dial gauges, and stones, there are lots of good
reasons for investing in a laser system:
 Light and easy-to-use equipment = shorter time for preparations and measurements
 Possible to measure and align at long distances = greater accuracy.
 Possible to measure both X and Y (Z) directions at the same time = saves time.
 The reference (laser beam) is always 100% straight.
 Possible to document the measurement results as PDF and transfer to a PC.
 Ability to compare the measurement results with ISO standards used for machine tool
measurement.
 Possible to read and follow the alignment from where you are standing and making
adjustments.
LASER INSTRUMENTS:
3.2.1 LASER INTERFEROMETER
INTRODUCTION:
Different methods of dimensional measurement using layer.
Laser techniques are used for measurement of dimensions in the following ways.
a. Scanning laser gauges.
b. Photo diode array imaging
c. Diffraction pattern system.
d. Laser triangulation sensors
e. Interferometers.
f. Holography
3.2.2 ADVANTAGES OF LASER (as a light source in interfermetric measurement)
The light emitted in coherent and highly monochromatic enabling interference fringes to be
produced over long distances as opposed to short distances with a conventional discharge lamp.

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The light is of an intensity which enables the fringes produced to be readily detected by suitable
photo-cells, and the signal - to - noise ratio in such that counting speeds up to a million cycles per
second are possible. Further, the light in produced as a narrow parallel beam which eases the
problem of producing the optical components in an interferometer system.
Necessary conditions for interference of light waves:
The following conditions should be satisfied. To observe the phenomenon of sustained or
continuous interference of light waves,
1. Two sources of light should be coherent, ie.
a) The two sources of light should continuously emit waves of same wave length or
frequency.
b) For obtaining interference fringes, the amplitude, of the two interfering wave trains
should be equal or very nearly equal.
c) The two sets of wave trains from the two sources should either have the same phase or
a constant different phase.
2. Two sources should be very narrow.
3. Emitting a set of interfering beans should be very close to each other.
Interferometer measurement and effect:
The line of single for viewing the bands should be nearly perpendicular to the reference
surface of the optical flat. It viewing angle varies by 5 degree, then no error in product. However,
when the viewing angle in bigger, then the actual fringes will be read less and errors of around
15%, 40% and 100% may occur with viewing angles eg 30 degree, 45 degree and 60 degree
respectively.
Monochromatic light in used for interferometer work:
As the white light contains a whole spectrum of wavelengths and since the pitch of the
interference fringes will be different for each, the interference fringes formed will be mixture of all
and it becomes very difficult to distinguish the various dark and light fringes. The whole pattern
looks quite blurred and as the an gap between optical flat and the surface to be tested increases, it
becomes absolutely impossible to distinguish the dark and light fringes at any one point.
In the case of monochromatic light, the spread of wave length is very small and thus fringes
are formed at considerable separations of optical flat and surface. The interference fringe pattern
in much more clearlyed.

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Advantages of light standard of wavelength
Light standard s the length in terms of a standard which is not only constant, but also,
reproducible anywhere in the world. This is the major criterion for any standard. It does not
depend on reference to some particular and possibly whether able piece of metal. It become
possible because at constant pressure and temperature, each pure color of light from a vaporizing
element has a particular and constant wavelength, and with the adventure nuclear physics, it was
possible to obtain pure isotopes of various elements, serving as very pure mono chromatic light
source.
Advantages of using laser beam in interferometer
The laser provides a source of wherence and truly mono chromatic light. Non-laser light is
in coherent and does not exactly follow the sinusoidal wave, but is subject to small random
variations. The property of wherence in laser beam enables it to be projected in a narrow pencil of
beam (without any scatter).
3.2.2 INTERFEROMETER AND TYPES OF INTERFEROMETER.
Interferometer is optical instruments used for measuring flatness and determining the lengths of
slip gauges by direct reference to the wavelength of light.
Types :
1. NPL flatness interferometer
2. Michelson interferometer
3. Laser interferometer
4. Zesis gauge block interferometer
Common source of light used for interferometer.
a) Mercury 198
b) Cad minus
c) Krypton 86
d) Helium
e) Hydrogen
f) Laser mixed radiations
Crust and trough
The light is a form of energy being propagated by electromagnetic waves, which is a sine curve.
The high point of the wave is called crust and the low point is called trough.

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Wavelength
The distance between two crust and two trough is called the wavelength.
Different light sources used for interferometer and their characteristics.
A wide variety of light sources is available for interferometer work but the selection of
proper source for any application depends on the requirements of results to be obtained by
interferometer, cost and convenience. For simple applications like testing of surface geometry,
where the difference between interfering paths is of the order of a few wavelengths only, tungsten
lamp with a filter, transmitting only a narrow band of wavelengths would be adequate.
However, sophisticated applications require the use of light sources such as mercury 198,
cadmium, krypton 86, thallium, sodium, helium, and neon and gas lasers. In these sources, the
discharge lamp is charged with one particular element and contains means to vaporize them. The
atoms of these elements are excited electrically so that they emit radiation at certain discrete
wavelengths.
Characteristics of various light source are summarized below:
i) Mercury. It is les expensive source having high intensity, and green line can be easily
isolated with filters. Since natural mercury contains several isotopes, each isotope emits
light whose wavelength is very slightly different from each other. As a result, natural
mercury light source radiates a mixture of wavelengths which can be treated as
monochromatic only for short path difference.
ii) Mercury 198. It is a pure isotope produced by neutron bombardment of gold. It is
considered to be one of the best sources of very sharply d wavelengths, and fringes are
visible with path difference up to 500 mm. Light is emitted when mercury 198 is excited by
microwave produced electric field. It is the international secondary standard of
wavelength.
iii) Cadmium. This is the only natural material producing a spectral line (red) almost
completely symmetrical, having useful path difference of about 200 mm. Cadmium 114 is
the official secondary international standard of length.
iv) Krypton. It has the advantage of being easily excited, so used in some instruments. It is not
as monochromatic as Krypton 86 because natural krypton is a mixture of isotopes. It can be
used up to path difference of 375 mm.

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v) Krypton 86. Krypton 86 lamp produces spectral lines of different wavelengths and,
therefore, a fairy elaborate monochromatic is required to separate them. Further its
excitation takes place at very very low temperatures, therefore, this lamp is used only in
standardizing laboratories. Next to laser, this enables the fringes to be observed with
maximum path difference (about 800 mm).
The orange-red line of krypton 86 isotope, produced under specified conditions, and at a
temperature of 63.3 K temperature of nitrogen triple point, is the new basic international standard
of length-meter being d as exactly 1,650,763.73 wavelengths of this source, measured in vacuum.
vi) Thallium. As 95% of its light is emitted at one green wavelength, it can be used over a
reasonable path difference without the use of my filter.
vii) Sodium. It is used only in applications where interference path difference does not exceed a
few hundred wavelengths. Usually yellow sodium light is used which contains two
separate but closely spaced lines of equal intensity; and because of this the interference
fringes wash out fad because of this the interference fringes wash out for higher path
difference.
viii) Helium. Orange line of helium is used where path difference is not great.
ix) Neon. As conventional neon lamp has too many closely spaced lines (in red part of the
spectrum) and not sharply d, it does not find many applications. Neon in gas laser,
however, has assumed a uniquely important role.
x) Gas lasers. In metrology work gas lasers which produce highly monochromatic and intense
light (1000 times more intense than others) are used to great advantage, enabling
interference fringes to be observed with enormous path differences, up to 100 million
wavelengths. (It may be noted that high-power, intermittently operating ruby laser is not of
interest in metrology). Gas lasers are produced by exciting (by an electric discharge or a
high-frequency field) a mixture of neon and helium

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3.2.3 DC INTERFEROMETER:
Principle and working of Michelson interferometer.
Michelson Interferometer. This is the oldest type of interferometer, which has subsequently
been modified in several respects and lot of sophistication introduced. However, Michelson using
this interferometer, established exact relationship between meter and red wavelengths of
cadmium lamp; so understanding of its working will be of interest to all.
The basic Michelson interferometer consists of
i. Monochromatic light source,
ii. Beam splitter and
iii. Two mirrors.
It relies on the principle of constructive and destructive interference as one mirror remains fixed and
the other is moved.
In schematic form, Michelson interferometer is shown in Fig. 6.16, which utilizes
monochromatic (or single wavelength) light from an extended source. This light falls on a beam
splitter (which is a plain parallel plate having a semi-transparent layer of silver at its back) which
splits the light into two rays of equal intensity at right angles. One ray is transmitted to Mirror M1
and other is reflected through beam splitter to Mirror M2. From both these mirrors, the rays are
reflected back and these reunite at the semi-reflecting surface from where they are transmitted to
the eye as shown in Fig. 6.06.
Mirror M2 is fixed and the reflected ray from M1 serves as reference beam, Mirror M1 is
movable, i.e., it is attached to the object whose dimension is to be measured.

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If both mirrors are at same distance from beam splitter, then light will arrive in phase and
observer will see bright spot due to constructive interference. If movable mirror shifts by quarter
wavelength, then beam will return to observer 180
out of phase and darkness will be observed
due to destructive interference.
Each half wavelength of mirror travel produces a change in the measured optical path of
one wavelength and the reflected beam from the moving mirror shifts through 360
phase change.
When the reference beam reflected from the fixed mirror and the beam reflected from the moving
mirror rejoin at the beam splitter, they alternately reinforce and cancel each other as the mirror
moves. Thus each cycle of intensity at the eye represents /s of mirror travel.
It may be noted that when monochromatic light source is used then fringes can be seen over a
range of path difference that may vary from a few to a million wavelengths, depending on the
source. However, when white light is used, then fringes can be seen only if both ray paths are
exactly equal to a freq. wavelength in total length in glass and air. The lengths themselves are not
important, but only their differences affect fringe formation. So when white light source is used
then a compensator plate is introduced in the path of mirror M1 so that exactly the same amount of
glass is introduced in each of the paths. (In the path of mirror M2, the glass was coming due to
rays passing through beam splitter back surface).
The various sophistications which have undergone to improve the Michelson’s basic apparatus
are:
(i) Use of laser as the light source, which means that the measurements can be made over
longer distances; and also the beam laser compared to other monochromatic sources has
exact and pure wavelength thus enabling highly accurate measurements.
(ii) Mirrors are replaced by cube-corner reflectors (ratio-reflectors) which reflect light
parallel to its angle of incidence regardless of retro reflector alignment accuracy.
(iii) Instead of observing the interference phenomenon by eye, photocells are employed
which convert light-intensity variations in voltage pulses which are processed by
electronic instruments to give the amount and direction of position change.

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Single Frequency DC Interferometer System.
It is much improved system over the Michelson simple interferometer. It uses a single
frequency circular polarized laser beam. On reaching the polarizing beam splitter, the beam splits
into two components. The reflected beam being vertically polarized light and the transmitted
beam being horizontally polarized light. These two beams referred to as reference are and
measurement are respectively travel to their retro reflectors and are then reflected back towards
the beam splitter.
The recombined beam at beam splitter consists of two superimposed beams of different
polarization; one component vertically polarized having traveled around reference arm and other
component horizontally polarized having traveled around the measurement arm. These two
beams being differently polarized do not interface. The recombined beam then passes through a
quarter wave plate which causes the two beams to interfere with one another to produce a beam
of plane polarized light. The angular orientation of the plane of this polarized light depends on the
phase difference between the light in the two returned beams.
The direction of plane of polarization spin is dependent on the direction of movement of the
moving retro reflector. The beam after quarter wave plate is split into three polarization sensitive
detectors. As the plane of polarized light spins, each detector produces a sinusoidal output wave

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form. The polarization sensitivity of the detectors can be set so that their outputs have relative
phases of 0, 90, and 180. The outputs of there detectors can be used to distinguish the direction
of movement and also the distance moved by the moving retro reflector attached to the surface
whose displacement is to be measured.
For linear measurements (positional accuracy of velocity), the retro reflector is attached to the
body moving along the linear axis. For angular measurement. For pitch and yaw), the angular
beam splitter is placed in the path between the laser head and the angular reflector. In this way it
is possible to measure flatness, straightness, rotatory axis calibration. Arrangements also need to
be made for environmental compensation because the refractive index of the air varies with
temperature, pressure and humidity. Heterodyne interferometer, an a.c. device avoids all the
problems encountered in above d.c. device, i.e. effect of intensity level change of source, fringe
contrast changes and d.c. level shifts which can cause fringe miscounting.
Interferometer is now an established and well developed technique for high accuracy and high
resolution measurement.
Twyman – Green Specialization of Michelson Interferometer.
In the Michelson interferometer shown in Fig. 6.18, the rays actually describe a cone, giving rise to
various types of fringe patterns which may be hard to interpret.
Twynman-Green modified Michelson interferometer utilizes a pin-hole source diaphragm and
collimating lenses. In this way, all rays are rendered parallel to the central rays and thus all rays
describe the same path . All modern tow-beam interferometers are based on this arrangement. The
mirrors M1 and M2 are arranged perpendicular to the optical axis. If mirror M1 is kept fixed, and
M2 is moved slowly exactly parallel to itself, the observer will note periodic changes in the
intensity of the field being viewed, from bright to dark for every /2 movement of the mirror.
In fact intensity variation is found to be sinusoidal. It may also be noted that if one of the
mirrors is even slightly inclined to the optical axis then parallel fringes will be seen moving
parallel to themselves by just one fringe for every 2 (half the wavelength of the light source
used) mirror motion. Usually it is quite difficult to count such fringes by eye..

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However, photo detectors connected to high speed counters can do this job very accurately
(accuracy of one part in million being obtainable). It is possible to calibrate the output of counter
directly ion terms of the linear movement of the mirror M2, but several conditions must be met to
achieve these results
FRINGE COUNTING INTERFEROMETER.
A simple arrangement of fringe counting system based on Kosters prism is shown in Fig.6.19.
With the use of Koster s prism, the two interfering paths can be arranged parallel instead of at
right angles. At big advantage is using Koster s prism, is that if slight vibrations exist, then
vibration tends to affect the arms equally and the annoying effect of vibration is nullified. In order
to be able to count the fringes, the following must be taken care of:

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(i) It has been indicated that mirror should travel exactly parallel to itself and no machines have
ways sufficiently straight to maintain uniform fringe fields. The recent trend is to use corner-cube
reflectors which are not all sensitive to their own orientation and return the reflected ray exactly
parallel to the incident beam.
(ii) It is observed that the wavelength of light source is modified by the refractive index of air
which is dependent on pressure, temperature and humidity of air (wavelength is fixed only in
vacuum). The slight changes in wavelength may be immaterial in case of flatness or from
measuring systems, but not in fringe counting and gauge block interferometers. So pressure,
temperature and humidity should be measured and correction factors applied for. If optical paths
are longer then the air currents between optical elements exert more and more influence; and the
system should, therefore, be properly shielded with insulating, and radiation reflecting
enclosures.

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(iii) It has already been indicated that the signal strength becomes poor if the path difference
between the rays corresponding to two mirror systems is high. Thus it limits the range of
movement of movable mirror because its movement means change in path length. It is found that
using cooled mercury 198 lamps, speeds of 12.5 mm/sec. are possible when path lengths are nearly
equal, but the traversing speed has to be reduced to 0.0025 mm/sec., when path difference is about
250 mm due to poor signal to noise ratio.
3.2.4 AC INTERFEROMETER :
Construction and Working:
The measuring capacity in interferometers with lamp as source of light is limited because it is
not possible to maintain the sharpness of interference fringes beyond certain distance due to the
size of the lamp.
Laser interferometer uses A..C. laser as the light source and thus enables the measurements to
be made over longer distance because it is possible to maintain the quality of point interference
fringes over long distances when lamp is replaced by a laser source. It must be understood that
white light emitted by a lamp is combination of waves at different frequencies but laser generates
a continuous train of light waves, resulting into high coherence. Laser represents a source of
intensely monochromatic optical energy, which can be collimated into a directional beam, Also
laser beam wavelength is exact and pure for highly accurate measurements. It utilizes the
principles of both optical techniques and digital electronics; and is a highly accurate and versatile
measuring system that can cope with industrial environments. In case of AC laser interferometer
(ACLI) position information is carried as phase deviation rather than as a signal amplitude
deviation, thus giving a much improved signal to noise ratio over amplitude modulation, because
the noise sources that affect signal amplitude have little effect on phase. In this way, ACLI is much
more tolerant of environmental factors that attenuate the intensity of a laser beam, such as dust,
smoke, air turbulence etc. It requires no warm-up time or standby power.
Thus ACLI has the following advantages: high repeatability and resolution of displacement
measurement (0.1m), high accuracy,, long-range optical path (60m), easy installation, and no
change in performance due to ageing or wear and tear. A single laser source can be used for as
many as six simultaneous measurements in different axes. However, it is very much expensive;
since the basic instrument measures physical displacement in terms of wavelength instead of

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traditional units, conversion instrumentation is required for conventional read out. Highest
possible accuracy is obtainable only by compensating changes in air pressure and temperature
which affect wavelength of the laser beam.
operation of AC Interferometer.
It uses two frequency laser system, thus overcoming the shortcoming of d.c. laser
interferometer. Whereas the d.c. system mixes out of phase light beams of the same frequency, the
a.c. system mixes beams of two different frequencies thus permitting the distance information to
be carried on a.c. waveform. Use is made of the fact that the AC amplifiers are insensitive to d.c.
variation of a.c. inputs.
Two frequency Zee man laser generates light of two slightly different frequencies with
opposite circular polarizations. These beams get split up by beam splitter B1; one part travels
towards B2 and from there to external cube corner where the displacement is to be measured. It
may be noted that mirror is not employed here like Michelson Interferometer, because mirror
alignment is a critical procedures. Thus interferometer, instead, uses cube-corner reflectors (retro
reflectors) which reflect light parallel to its angle of incidence regardless of retro reflector
alignment accuracy. Beam splitter B2 optically separates the frequency f1 which alone is sent to the
movable cube-corner reflector. The second frequency f2 (optically separated) from B2 is sent to a
fixed reflector which then rejoins f1 at the beam splitter B2 to produce alternate light and dark

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interference flicker at about 2 Mega cycles per second. Now if the movable reflector (external cube
corner) moves, then the returning beam frequency will be Doppler-shifted slightly up or down by
∆f1. Thus the light beams moving towards photo-detector P2 have frequencies f2 and (f1 ± ∆f1) and
P2 changes these frequencies into electrical signal. (Photocells convert light-intensity variations
into voltage pulses which can be processed by electronic instruments to give the amount and
direction of position change).
Photo detector P1 receives signal from beam splitter B1 and changes the reference beam
frequencies f1 and f2, into electrical signal. An A..C. amplifier A1 separates frequency difference
signal [(f2- (f1 ± ∆f1). The pulse converter extracts ∆f1, one cycle per half wavelength of motion. The
up-down pulses from the pulse converter are counted electronically and displayed in analog or
digital form on the indicator. It may be noted that output in case of ACLI is in the form of pulses,
whereas in d.c. systems, the output is in the form of a sinusoidal wave, the amplitude (intensity) of
which depends upon laser aging, air turbulence or air pollutant and thus the change of amplitude
leads to improper triggering and counting errors (Refer Fig).
1) Counter operating, if amplitude wave is above counter trigger level.
2) Counter disabled by small amplitude change of sinusoidal wave.
HETERODYNE INTERFEROMETER TECHNIQUE. (A.C. INTERFEROMETER)
Simple d.c. fringe counting techniques suffer from problems of intensity level changes in source
and also on account of motion of source or object. Fringe contrast changes and d.c. level shifts
result in miscounting of the fringes. Heterodyne interferometer is an a.c. device and the problems
of d.c. fringe counting techniques are overcome. In this type of interferometer, a zeeman laser

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source emits two closely spaced orthogonal polarization frequencies separated by around 1 MHz.
A beam splitter placed in front of laser source separates off part of the signal
from both polarizations which are mixed on detector D1 to provide a reference beat f1-f2. The
transmitted component travels up to polarizing beam splitter where it is splitter. Part of it travels
to reference fixed arm and other to measurement arm connected with target movement. The two
signals are recombined at the polarizing beam splitter and detected by detector D2. If target is
stationary, the detected beam is f1-f2. When it moves, then detected beat is f1-f2 ∆f. The reference
and Doppler-shifted beats are counted by two independent counters and subtracted to give ∆f.
Integration of the count over time t measures 2d/.
DUAL-FREQUENCY LASER INTERFEROMETER
This instrument is used to measure displacement, high-precision measurement of lengths,
angles, speeds and refractive indices as well as derived static and dynamic quantities. It operates
on heterodyne principle. The two resonator modes (frequencies f1 and f2) are generated in a laser

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tube such that f1-f2=640MHz. These are controlled so that their maxima are symmetrical to the
atomic transition. This permits a long reliable stability. The frequency stability of He-Ne laser is
responsible for outstanding performance of the interferometer.
An amplitude beam splitter branches off part of the laser output create a reference beam, which
an optical fibre cable relays to a photo detector 1. This detects the beat signal of 640MHz frequency
difference produced by the heterodyning of the two modes. The other portion of the light serves
as measuring beam. Via an interferometer arrangement it is directed to a movable measuring
mirror and a stationary reference mirror, which reflects it on to a photo-detector 2. The two
frequencies in the measuring beam are separated by a polarization-sensitive beam splitter so that
the measuring mirror receives light of frequency f1 only, whereas the light that strikes the
reference consists exclusively of frequency f2. With the measuring mirror at rest, detector 2 also
senses the laser differential frequency of f1-f2 = 640MHz. If the measuring mirror is being displaced
at a speed v, the partial beam of frequency f1 reflected by it is subjected to a Doppler shift df1;
where df1 = (2v)1.

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Accordingly, detector 2 now receives a measuring frequency of f1-f2 ± df1 (+ df1 or df1) depending
on the direction of movement of the measuring mirror. The reference frequency f1-f2 and the
measuring frequency f1-f2 ± df1 are compared with each other by an electronic counting chain. The
result is the frequency shift ± df1 due to the Doppler effect, a measure of the wanted displacement
of the measuring mirror. In a fast, non-hysteric comparator, the
Doppler frequency df1 is digitized and then fed to a counter, which registers the number of zero
passages per unit time.
The forward and return movements of the measuring mirror can be distinguished by out
coupling the measuring signal f1 f2 ± df1 at n phase angles, via a delay line and feeding to n
mixers. The mixers are connected with the reference signal f1 f2 (common feeding point for all
mixers). Thus n Doppler frequencies get shifted in phase by /n at the mixer outputs. They are
symmetrical relative to zero. After comparison they are made available to low-frequency counting
logic as TTL signals. The n phase angles and their tolerances are implemented by the geometry of
the delay line.
This system can be used for both incremental displacement and angle measurements. Due to
large counting range it is possible to attain a resolution of 2.nm in 10 m measuring range. Means
are also provided to compensate for the influence of ambient temperature, material temperature,
atmospheric pressure and atmospheric humidity fluctuations.
Advantages and disadvantages of analog image sensors
Advantages : Resolution, low lighting, contrast, sensitivity, capability to preprocess cost.
Disadvantages : Poor linearity image drift and image burn.

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3.3 STRAIGHTNESS AND ALIGNMENT
INTRODUCTION:
3.3.1 ALIGNMENT TESTS CONDUCTED ON A LATHE.
1. Test for level of installation
(a) In longitudinal direction
(b) In transverse direction
Measuring instruments. Spirit level, gauge block to suit the guide ways of the lathe bed.
Procedure.
The gauge block with the spirit level is placed on the bed ways on the front position, back position
and in the cross wise direction. The position of the bubble in the spirit level is checked and the
readings are taken.
Permissible error
. Front guide ways 0.02 mm/meter convex only. Rear guide ways, 0.01 to 0.02 convexity. Bed level
in cross-wise direction  0.02 meters. Straightness of slide ways (for machines more than 3m
turning length only measurement s taken by measuring taught wire and microscope or long
straight edge). Tailstock guide ways parallel with movement of carriage 0.02mm/m. no twist is
permitted.
The error in level may be corrected by setting wedges at suitable points under the support feel or

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pads of the machine.
3. Straightness of saddle in horizontal plane.
Measuring instruments. Cylindrical test mandrill (600 mm long), dial indicator.
Procedure. The mandrel is held between centres. The dial indicator is mounted on the
saddle. The spindle of the dial indicator is allowed to touch the mandrel. The saddle is then
moved longitudinally along the length of the mandrel. Readings are taken at different places
Permissible error. 0.02 mm over length of mandrel.
1.Alignment of both the centres in the vertical plane.
Measuring instruments. Cylindrical mandrel 600mm long, dial gauge.
Procedure. The test mandrel is held between centres. The dial indicator is mounted on the saddle
in vertical plane as shown in figure. Then the saddle along with the dial gauge is traveled
longitudinally along the bed ways, over the entire length of the mandrel and the readings are
taken at different places.
Permissible error 0.02 mm over 600 mm length of mandrel (tail stock centre is to lie higher only).

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2.True running of taper socket in main spindle.
Instruments required. Test mandrel with taper shank and 300 mm long cylindrical measuring
part, dial gauge.
Procedure. The test mandrel is held with its taper shank in a head stock spindle socket. The dial
gauge is mounted on the saddle. The dial gauge spindle is made touch with the mandrel. The
saddle is then traveled longitudinally along the bed ways and readings are taken at the points A
and B as shown in figure.
Permissible error. Position A, 0.01 mm, position B 0.02 mm.
6. Parallelism of main spindle to saddle movement.
(a) Ina a vertical plane (b) In horizontal plane
Measuring instruments. Test mandrel with taper shank and 300 mm long cylindrical measuring
part, dial gauge.
Procedure. The dial gauge is mounted on the saddle. The dial gauge spindle is made to touch the

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mandrel and the saddle is moved to and fro. It is checked in vertical as well as in horizontal plane.
Permissible errors. (a) 0.02/300 mm mandrel rising towards free and only. (b) 0.02/300 mm
mandrel inclined at fee end towards tool pressure only.
7. Movement of upper slide parallel with main spindle in vertical plane.
Measuring instrument. Test mandrel with taper shank and 300mm long cylindrical measuring
part, dial gauge.
Procedure. The test mandrel is fitted into the spindle and a dial gauge clamped to the upper slide.
The slide is traversed along with the dial gauge plunger on the top of the stationery mandrel.
Permissible error-0.02 mm over the total movement of the slide.
8. True running of locating cylinder of main spindle.
Measuring instrument. Dial gauge.
Procedure. The dial gauge is mounted on the bed, touching at a point on main spindle.
The main spindle is rotated by hand and readings of dial gauge are taken.

25
Permissible error -0.01 mm.
9. True running of head stock centre.
Measuring instrument. Dial indicator.
Procedure : Tailstock sleeve is fed outwards. The dial gauge is mounted on the saddle. Its spindle
is touched to the sleeve at one end and then saddle is moved to and fro, it is checked in H.P. and
V.P. also.
Permissible error. (a) 0.01/100 mm (Tailstock sleeve inclined towards tool pressure only). (b)
0.01/100 mm (Tailstock sleeve rising towards free end only).
10. Parallelism of tail stock sleeve taper socket to saddle movement (a) in V.P (b) in H.P.
Measuring instruments. The mandrel with taper shank and a cylindrical measuring part of 300mm
length, dial gauge.
Procedure. Test mandrel is held with its taper shank in a tail stock sleeve taper socket. The dial
gauge is mounted on spindle. The dial gauge spindle is made touch with the mandrel. The saddle
is the traversed longitudinally along the bed way and readings are taken.
Permissible error.
(a) 0.03/300 mm (mandrel rising towards free and only)
(b) 0.03/300 mm (mandrel inclined towards tool pressure only)

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3.3.2 VARIOUS ALIGNMENT TEST ON A MILLING MACHINE.
Alignment tests on milling machine.
(1) Flatness of work table.
(a) in longitudinal direction
(b) in transverse direction.
Measuring instruments spirit level.
Procedure. A spirit level is placed directly on the table at points about 25 to 30 cm apart, at A B C
for longitudinal tests and D E and F for the transverse test.
The readings are noted.
Permissible error.
Direction A – B – C  0.04 mm
Direction D – E – F  0.04 mm
(2) Parallelism of the work table surface to the main spindle.
Measuring instrument. Dial indicator test mandrel 300 mm long, spirit level.
Procedure. The table is adjusted in the horizontal plane by a spirit level and is then set in its mean
position longitudinally. The mandrel is fixed in the spindle taper. A dial gauge is set on the
machine table, and the feeler adjusted to touch the lower surface of the mandrel. The dial gauge
readings at (A) and (B) are observed, the stand of the dial gauge being moved while the machine
table remains stationery.
Permissible error. 0.02/300 mm.

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(3) Parallelism of the clamping surface of the work table in its longitudinal motion.
Instruments. Dial gauge, straight edge.
Procedure. A dial gauge is fixed to the spindle. The gauge spindle is adjusted to touch the table
surface. The table is then moved in longitudinal direction and readings are noted. If the table
surface is uneven it is necessary to place a straight edge on its surface and the dial gauge feeler is
made to rest on the top surface of the straight edge.
Permissible error. 0.02 up to 50 mm length of traverse, 0.03 up to 1000 mm and 0.04 above 1000
mm length of traverse.

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(4) Parallelism of the cross (transverse) movement of the worktable to the main spindle.
(a) in a vertical plane
(b) in horizontal plane
instruments. Dial gauge, test mandrel with taper shank.
Procedure. The table is set in its mean position. The mandrel is held in the spindle. A dial gauge
field to the table is adjusted so that its spindle touches the surface of the mandrel. The table is
moved cross-wise and the error is measured in the vertical plane and also in the horizontal plane.
Permissible error. 0.02 for the overall traverse movement of the work table.
(5) true running of internal taper of the main spindle.
Instrument 300mm long test mandrel, dial gauge
Procedure. The test mandrel with its taper shank is held in the main spindle. Dial gauge is kept
scanning the periphery of the mandrel. Spindle is rotated and dial gauge readings are noted at
different points say A and B as shown.
Permissible error. A: 0.01 mm, position B: 0.02 mm.

29
(6) Squareness of the centre T-slot of worktable with main spindle.
Instruments. Dial gauge, special bracket.
Procedure. To check the perpendicularity of the locating slot and the axis of the main spindle. The
table should be arranged in the middle position of its longitudinal arranged in the middle position
of its longitudinal movement, and a bracket with a tenon at least 150 mm long inserted in the
locating slot, as shown in figure.
A dial gauge should be fixed in the spindle taper, the feeler being adjusted to touch the

30
vertical face of the bracket. Observe the reading on the dial gauge when the bracket is near one
end of the table, the swing over the dial gauge and move the bracket so that the corresponding
readings can be taken near the other end of the table.
(7) Parallelism of the T-slot with the longitudinal movement of the table.
Instrument. Dial gauge special bracket.
Procedure. The general parallelism of the T-slot with the longitudinal movement of the table is
checked by using 150 mm long braked having a tennon which enters the slot. The dial gauge is
fixed to the spindle taper and adjusted so that its feeder touches the upper surface of the bracket.
The table is then moved longitudinally while the bracket is held stationary by the hand of the
operator and dial gauge deviations from parallelism are noted down.
Permissible error. 0.0125 mm in 300 mm.
(8) Parallelism between the main spindle and guiding surface of the overhanging arm.

31
Instruments. Dial gauge, mandrel
Procedure. The overhanging arm is clamped in its extreme extended position. The dial gauge is
fixed to the arbor support.
The feeler of the dial gauge is adjusted to touch the top or ride of the test mandrel. The
arbor support can then be moved along the overhanging arm and the deviations from parallelism
observed on the dial gauge.
3.3.3 TESTS ON SHAPING MACHINE.
The use of shaping machine is to create flat surfaces accurately. Therefore, the chief
requirements of the shaping machine are that it should cut straight, parallel and face flat.
The important alignment tests on shaping machine are described below:
1. Straightness and flatness of the table.
The straightness and flatness of the table is the fundamental requirement of the shaping
machine to produce accurate work pieces.
Instruments. Spirit level, gauge block.
Procedure. The table is brought in the central position. The spirit level is placed over the gauge
block at several points on the table parallel to and perpendicular to the direction of the table
feed and in all the positions the bubble in the spirit level must be central.
2. Parallelism of top surface of table to its transverse movements.
Instruments. Dial gauge, straight edge.
Procedure. The table is brought to one side end. Dial gauge is then moved in transverse
direction below dial gauge and readings are taken.
3. Parallelism of table top to ram movement (parallelism of the table feed under the tool)
Instrument. Dial gauge, straight edge.

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Procedure. The ram is brought to the end of its edge. The dial gauge is placed on the table top
in the direction of movement of the ram. The ram is then moved backward and forward and
reading are taken.
Permissible error. 0.015 per 300 mm.
4. Trueness and parallelism of vertical ways of column.
Instruments. Dial gauge.
Procedure. The table is brought to its lowest position. The dial gauge is placed on the table so
that its feeler will touch the vertical ways of the column as shown in figure.
The table is then moved up and if the side ways are perfectly parallel and leveled straight, the
dial gauge touching to it will not shows any
5. The accuracy, squareness, and parallelism of T-slots on the label.
Instrument. Dial indicator, angle plate.
Procedure. The angle is inserted in the slot lengthwise and the dial gauge is set in the adjacent
parallel slot as shown in the figure the dial gauge is adjusted so that its feeler just touches the
angle plate. The reading is adjusted to zero and then the dial indicator is moved through the slot
lengthwise and the deflection is noted.
Checking accuracy of T - slots

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3.3.4 VARIOUS ALIGNMENT TESTS ON PILLAR TYPE DRILLING MACHINE.
Before carrying out the alignment tests, the machine is properly leveled in accordance with the
manufacturers instructions.
The various tests performed on pillar drilling machine are:
Instruments. Straight edge, two gauge blocks; feeler gauges.
1. Flatness of clamping surface of base. The test is performed by placing a straight edge on two
gauge block on the base plate in various positions and the error is noted down by inserting feeler
gauges.
Permissible error. The error should not exceed 0.1/1000mm clamping surface and the surface
should be concave only.
2. Flatness of clamping surface of table
The test is performed in the same manner as test (1), but not on the label. The permissible error
is also same.

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3. Perpendicularity of drill guide to the table base plate.
Instruments. Frame level.
The squareness (perpendicularity) of drill head guide to the table is tested.
(a) In a vertical plane passing through the axes of both spindle and column, and
(b) In plane at 90 to the plane at (a).
The test is performed by placing the frame level (with graduations from 0.03 to 0.05 mm) on
guide column and table and the error is noted by noting the difference between the readings of the
two levels.
Permissible error. The error should not exceed 0.25/1000mm guide column for (a) and the guide
column should be inclined at the upper end towards the front, and 0.15/1000mm for (b).
For testing the perpendicularity of drill guide to the base plate the test is similar as above, the only
difference being that the frame level is to be placed on the base instead of a table.
4. Perpendicularity of spindle sleeve with base plate.

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This test is performed in both the place as specified in test (3) and in the similar manner. The
only difference is that the frame levels are to be placed on spindle sleeve and base plate.
Permissible error. The error (i.e. the difference between the readings of the two levels) should not
exceed 0.25/1000mm for plane (a) and the sleeve should be inclined towards column only, and
0.15/100mm for plane (b).
5. True running of spindle taper
Instruments: Test mandrel, dial gauge
Procedure: The test mandrel is placed in the tapered hole of spindle and a dial indicator is fixed on
the table and its feeler made to scan the mandrel. The spindle is rotated slowly and readings on
indicator noted down.
Permissible error. The error should not exceed 0.03/100mm for machines with taper up to Morse
No.2 and 0.04/300mm for machines with taper larger than Morse No.2.
6. Parallelism of the spindle axis with its vertical movements.
Instruments. Test mandrel, dial gauge.
Procedure. This test is performed into two planes (A) and (B) at right angles to each other. The test
mandrel is fitted into the taper hole of the spindle and the dial gauge is fixed on the table with its
feeler touching the mandrel. The spindle is adjusted in the middle position of its travel. The
spindle is moved in upper and lower directions of the middle position of its travel. The spindle is
moved in upper and lower directions of the middle position with slow vertical feed mechanism

36
and the readings of the dial gauge are noted down.
Possible error. For plane (A) and (B) both 0.03/100 mm, 0.05/300mm.
7. Squareness of clamping surface of table to its axis.
Instruments. Dial gauge.
Procedure. The dial indicator is mounted in the tapered hole of the spindle and its feeler is made
to touches the surface of table. The table is then moved slowly and the readings of dial gauge
noted down.
Permissible error. The permissible error should not exceed 0.05/300am diameter.

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8. Squareness of the spindle axis with table.
Instruments. Straight edge, dial gauge.
Procedure. This test is performed by placed the straight edge in position AA’ and BB’. The work
table is arranged in the middle of its vertical travel. The dial gauge is mounted in the tapered hole
of the spindle and its feeler is made to touch the straight edge first at A and readings are taken.
Then the spindle is rotated by 180 so that the feeler touches at point A’ and again the reading is
taken. The difference of these two reading is the error in squareness of spindle axis with table.
Similar readings are taken by placing the straight edge is position BB’.
Permissible error: The permissible errors are 0.08/300mm with lower end of spindle inclined
towards column only for set up AA’ and 0.05/300mm for set up BB’.

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3.4.0 CMM (COORDINATE MEASURING MACHINE)
It is a three dimensional measurements for various components. These machines have precise
movement is x-y-z co-ordinates which can be easily controlled and measured. Each slide in three
directions is equipped with a precision linear measurement transducer which gives digital display
and sense positive and negative direction.
Position accuracy.
It is s as difference between positions read out of machine along an individual axis and value of
a reference length measuring system. Three parameters are needed for position accuracy. Position
accuracy of x axis, y axis and z axis are measured.
Axial length measuring accuracy and volumetric length measuring accuracy.
Axial length measuring accuracy:
It Is d as difference between the reference length of gauges aligned with a machine axis and the
corresponding measurement results from the machine.
Volumetric length measuring accuracy:
It is s as difference between the reference length of gauges, freely in space and the corresponding
measured results from the machine.
3.4.1 Basic Concepts of CMM
Three dimensional measurements are essential for various components. CMMs are useful for this
purpose. These machines have procise movements in a; -y -z coordinates which
can be easily controlled and measured. Each slide in three directions is equipped with a precision
linear measurement transducer which gives digital display and senses + ve/-ve direction. These
are manufactured in both manual and computer-controlled models and come in a wide range of
sizes to accommodate a variety of applications. The measuring head incorporates a probe tip,
which can be of different kinds like taper tip, ball tip etc. Various type of CMMs are shown in Fig.
17.11. All these have very low measuring uncertainty, computer aided measuring runs, vibration
free mechanical structure, and high rigidity. In addition all moving parts must be set verv
accurately, driven by (Measuring head movement in plane perpendicular to paper)

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3.4.2 CONSTRUCTION AND WORKING PRINCIPLE OF VARIOUS TYPES OF CMMS.
Types of co-ordinate measuring machine:
1) Cantilever type : easy to load and unload, but mechanical error may occur due to sag or
deflection.
2) Bridge type : More difficult to load but mechanical errors are less.
3) Horizontal bore mill : It is used for large and heavy work pieces.
4) Vertical bore mill : It is very slow to operate but highly accurate.
Spherical co-ordinate measuring machine : Both linear and rotary axes are incorporated. It can be
used to measure various features of parts like cane, cylinder, hemisphere etc
1. Cantilever Type :
The cantilever type (CMM—refer Fig. 7.11) is easiest to load and unload, but is most susceptible
to mechanical error because of sag or deflection in y-axis beam.
( Or )
1. Cantilever type 2. Bridge Type

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2. Bridge type:
Bridge type is more difficult to load but less sensitive to mechanical errors.
Construction :
3. Column (Horizontal / Vertical boring mill) type:
Horizontal boring mill type is best suited for large heavy workpieces. Vertical bore mill type is
highly accurate but usually slower to operate.
Construction :
3.Column Type 4. Gantry Type
Column Horizontal bore & Vertical Bore Type
4. Gantry type:
A gantry type machine is also available in which the complete bridge can slide in v-direction on
the slides. It has the compromises of both cantilever and bridge type, and is thus fast to operate,
simple in alignment, and rugged construction affords consistent accuracy.
For measuring the distance between two holes, the work piece is clamped to the worktable and
aligned with the machine’s three mutually perpendicular*, y and z measuring slides. The tapered-

41
probe tip is then seated in first datum hole and the probe position digital readout is set to zero.
The probe is then moved to successive holes, at each of which the digital readout represents the
coordinate part-print hole location with respect to the datum hole. Machine is also equipped with
automatic recording and data processing units which are essential when complex geometric and
statistical analysis is to be carried out. In fact, in modern machines, automatic on-line processing
of measurement data is possible when the part is still on the worktable.
5. Spherical co-ordinate measuring machine,
Spherical co-ordinate (R-6) Measuring Machine.
In a special co-ordinate measuring machine, both linear (x and z axes) and rotary axes are
incorporated. The machines can measure various features of parts whose shapes are objects of
revolutions like cones, cylinders and hemispheres. R-Q machines having motions of their
measuring head in R, 0 and $ direction are used for inspecting parts that are basically spherical.
As it is impossible to manufacture a mechanically perfect machine it is important to be able to
analyze the geometry errors associated with each individual CMM and determine their effects on
the machine’s measurement accuracy. The result of such analyses can be used to compensate for
these effects and thus provide a high degree of accuracy that could not otherwise be achieved.
The prime advantage of co-ordinate measuring machine is the quicker inspection coupled with
accurate measurements.
The co-ordinate measuring machine with mechanical gauge makes use of two-axis X and
^positioning tables to bring the work to the probe that engages the holes to the inspected.
Some machines are equipped with an optical comparator as well as travel dial
indicator. Present day co-ordinate measuring machines are three-axis digital read-out type and
work up with an accuracy of 10 microns and resolution of 5 microns. These utilize a measuring

42
element called Inductosyn data element which uses inductive coupling between conductors
separated by a small air gap. As this element is not subjected to wear, it does not develop
inaccuracy. It does not require reference standards or any other external device for its operation.
The workpiece is aligned by a probe and by a switching adjustment on the worktable.
Many machines utilize Moire fringe concept for measurement. Some coordinate measuring
machines are available with accessories like optical viewing screen, (optical comparator),
microscope attachment for the inspection of thin, soft, or delicate workpieces, and automatic print
out. Some machines, in addition to measuring in three axes, are also designed to permit the
checking of angularity, roundness, taper, and concentricity.
Provision of rotary table makes such co-ordinate measuring machine more versatile because
setting of a part need not be changed and all areas can be approached due to positioning of rotary
table. The errors likely to occur in multiple set-ups are thus avoided.
Some co-ordinate measuring machines utilise electronic indicator probe (mounted on the end of
the spindle) which can reach over and under the workpiece to check squareness in a single set up.
Some machines are provided with linear air bearings on the horizontal slide motions to achieve
finer slide position resolution.
3.4.3 (CONSTRUCTIONAL) FEATURES OF CO-ORDINATE MEASURING MACHINES:
 In order to meet the requirement of faster machines with higher accuracies, the stiffness to
weight ratio has to be high in order to reduce dynamic forces.
 To give maximum rigidity to machines without excessive weight, all the moving members,
the bridge structure, Z-axis carriage, and Z-column are made of hollow box construction.
 Principles of kinematic design are used in the three master guideways and probe location.
Even whole machine with its massive granite worktable is supported on a three-point
suspension.
 A map of systematic errors in machine is build up and fed into the computer system so that
error compensation is built up into the software. All machines are provided with their own
computers with interactive dialogue facility and friendly software.
 Thermocouples are incorporated throughout the machine and interfaced with the computer
to be used for compensation of temperature gradients and thus provide increased accuracy

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and repeatability. With the advent of three-axis programming, computers enable CMM to
measure three-dimensional object from variable datums.
 The real benefit of today’s CMM is its total flexibility and programmability, which makes it
capable of handling virtually any measuring requirement within its physical size limit, thus
rendering dedicated or specially designed gauging unnecessary.
 Design improvements allied to a rapid growth in software for 3 and 4 axis movements
enable CMMs to measure straight line relationships between basic features, i.e., hole centre
distances, etc. and also a variety of form measurements, such as turbine blades, cam
profiles etc
Advantages of CMM.
i) The inspection rate is increased
ii) Accuracy is reduced
iii) Operator’s error can be minimized. Skill of the
iv) Operator is reduced.
v) Reduction in calculating, recording and set up time
vi) No need of GO / NOGO gauges
vii) Reduction of scrap and good part rejection.
Disadvantages of CMM.
i. The table and probe may not be perfect alignment
ii. The stylus may have run out
iii. The stylus moving in z-axis may have some perpendicularity errors
iv. Stylus while moving in x and y direction may not be square to each other
v. There may be errors in digital system
3.4.4 APPLICATIONS OF CMM:
i. CMMs find applications in automobile, machine tool, electronics, space, and many other
large companies.
ii. These machines are ideally suited for development of new products and construction of
prototype because of their maximum accuracy, universatility and ease of operation.
iii. Because of high speed of inspection, precision and reproducibility of coordinate measuring
machines, these find application to check the dimensional accuracy of NC produced

44
workpiece in various steps of production.
iv. Regular inspection of work pieces by CMMs provides information on possible trends (due
to tool wear, temperature factors and other influences) obtained through a statistical
evaluation running simultaneously with production, allows efficient process inspection and
control.
v. For safety components as for aircraft and space vehicles, 100% inspection is carried out and
documented using CMM.
vi. CMMs are best suited for the test and inspection of test equipment, gauges and tools.
vii. CMMs can be used for determining dimensional accuracy of the bought in components,
variation on same and thus the quality of the supplier.
viii. These are ideal for determination of shape and position, maximum metal condition, linkage
of results, etc., which other conventional machines can’t do.
ix. Because of verification facility, these can be assessed for their absolute accuracy, running in
characteristics and variation. These make it possible to eliminate human error.
CMMs can also be used for sorting tasks to achieve optimum pairing of components within
tolerance limits.
x. A coordinate measuring machine can replace several single purpose equipment with a low
degree of utilisation like gear tester, gauge tester, length measuring machine, measuring
microscope, etc.
xi. To test a modified component, only a new component program is required whereas
expensive modification of reference gauges is required in conventional machines.
xii. CMMs are also best for ensuring economic viability of NC machines by reducing their
downtime for inspection results. They also help in reducing reject costs, rework costs
through measurement at the appropriate time with a suitable CMM.
3.4.5 PROBES :
Measuring Type Probe System:
It is a small coordinate measuring machine in itself. The buckling mechanism of this system
consists of parallel guideways. (Refer Fig. 17.19). At the moment of probing the spring
parallelograms are deflected from their intitial position. Since the entire system is free from
torsion, play and friction, a defined parallel displacement of probes as compared to their

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Fig. 17.19. Schematic of measuring probe head.
Fig. 17.20. Displacement of parallelogram when probing.
Accuracy Specification for Co-ordinate Measuring Machines.
Two types of accuracies are d in connection with coordinate measuring machines; viz
geometrical accuracy (determined by independent measurement because they make major
contribution to overall accuracy of machine)and ii) total measuring accuracy (determined by
utilising the entire measuring machine system as applied to master gauges ).
Spindle in z-direction.
It establishes the relation between the probing points in the measuring volume and the machine
coordinate system. Trigger Type Probe System
The main features of this system are shown in Fig. 1 .1 a and 1 .1 . The buckling
mechanism is a three point bearing, the contacts of which are arranged at 120° around the

46
circumference. These contacts act as electrical micro switches. When being touched in any
(a) Part section of Probe head.
(b) Out line of Probe head.
Fig. 17.18. Trigger type Probe system.
probing direction one or more contacts is lifted off and the current is broken, thus generating a
pulse. When the circuit is opened, the current co-ordinate positions are read and stored. After
probing, a prestressed spring ensures the perfect zero position of the three point bearing. The
probing force is determined by the prestressed force of the spring. With this probe system data
acquisition is always dynamic and therefore the measuring time is shorter than in static principle.
The following accuracy/test items are carried out for CMM :
1. Measurement Accuracy
(a) Axial length measuring accuracy
(b) Volumetric length measuring accuracy

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2. Axial Motion Accuracy
(a) Linear displacement accuracy
(b) Straightness
(c) Perpendicularity
(d) Pitch, yaw and roll
The axial length measuring
Accuracy is tested at the lowest position of the z-axis on the opposite side of the main axial guide
of CMM. The length tested are approximately 1/10, 1/5, 2/5, 3/5, and 4/5 of the measuring range of
each axis of CMM. The test is repeated five times for each measuring length and results plotted
and permissible value of measuring accuracy is derived.
Volumetric Length Measuring Accuracy:
The Volumetric length measuring accuracy is tested by measuring artefacts at two points on a
spatial axis at 45° to the x or y axis and about 30° to the x-y plant. Measurments are made as in
case of axial length measuring machine.
Geometrical accuracy concerns the straightness of axes, squareness of axes, and position
accuracy. Total measuring accuracy concern s axial length measuring accuracy, and volumetric
length measuring accuracy.
Performance of CMM.
In evaluating the performance of a coordiante measuring machine, the following major aspects
need consideration.
1.Definition and measurements of "geometrical accuracies", such as positioning accuracy,
straightness and squareness.
2.Master gauge measurement methods to "total measuring accuracy" in terms of "axial
length measuring accuracy, volumetric length measuring accuracy, and length measuring
repeatability, i.e., the coordianted measuring machine has to be tested as complete system.
Measuring systems can be characterised by the combination of "mode of operation" and
probe type. Modes include free floating manual, driven manual, and direct computer
controlled. Probe types are passive, switching, proportional and nulling. The CMM is

48
tested in the mode and with the probe that is commonly used.
3.Since environmental effects have great influence, explicit specification on environmental
conditions for the accuracy testing, including thermal parameters, vibrations and relative
humidity are required.
It is usually difficult to establish a quantitative relationship between any particular
environmental specification and the effect in machine's performance. Thus it is better to what
level of environmental enfluence is acceptable, and maintain those conditions.
The thermal effects dominate the environmental influences affecting a CMM. The sources
of thermally induced errors include deviations of surrounding air temperature from 20oC,
temperature gradients, radiant energy (e.g. Sunlight), utility air temperature, and self-heating in
machines with drive motors. Thermal effects may take the form of differential expansion between
the workpiece and the machine scale system, drift between a workpiece origin and the machine
scale system origin, and distortion of the machine structure leading to significant changes in the
calibration and adjustment of the machine. The dominant effect of vibration is to degrade the
repeatability of a machine. If the indicated relative motion between the machine table and the ram
exceeds 50% of the working tolerance for repeatability, the vibration environment is deemed
unacceptable.
It is important that suitable performance tests capable of testing the machine as a complete
system are performed. It may be mentioned that use of parametric testing (straightness,
squareness, angular motion) does not test the system performance test is carried out by measuring
a mechanical artifact which provides some similarity between the machine testing and actual
measurement of workpieces. Such testing must sample throughout the work zone. For
performance test, linear displacement accuracy is checked by a step bar or a laser interferometer.
These measurements are made along three orthogonal lines through the centre of the work zone to
provide a thorough sampling of many combinations of x, y, and z errors that occur throughout the
work zone of a machine.
Using the socketed ball bar provides a means of sweeping out the surface of a (nearly)
perfect hemispheres with a physical object (ball). The CMM is used to measure the location of the
centre of this ball at many locations on the hemisphere. The actual measurement data is compared

49
to an ideal hemisphere simply by recording the range of the length of the ball bar computed from
the data. The procedure calls for moving the socket defining the centre of the hemisphere to
several locations in the work zone and repeating the measurements. Three different lengths of the
bar also are used. The performance is specified independently for the different lengths.
Computer Controlled Coordinate Measuring Machine (software)
Fig. 17.16 shows the layout of a computer controlled CMM, a flexible measuring centre. The
measurements as well as the inspection of parts for dimension, form, surface characteristics and
position of geometrical elements are done at the same time in a fully coordinated manner
(complete metrological description of a workpiece).
Mechanical systems for the computer controlled CMMs can be subdivided into four basic types as
shown in Fig. 17.17. The selection of a particular type depends on the application.
All these machines use probes (which may be trigger type or measuring type) Probe system is
connected to the
Fig. System components of a computer controlled CMM.

50
(i) Column type
(ii) Bridge type
{iii) Cantilever type
(iv) Gantry type
Fig. Mechanical systems of computer controlled CMMs.
SOFT WARE :
CNC, CMM
A numerical control system can be used with CMM to do calculations while measuring
complex parts. Error can be stored in memory while doing calculations. For automatic calibration
of probe, determination of co-ordinate system, calculation, evaluation and recording etc. special
software are incorporated.
CMM software.
Measurement of diameter, center distance, and length can be measured as follows.
i. Measurement of plane and spatial curves
ii. Minimize CNC programme
iii. Data communications
iv. Digital input and output command
v. Interface to CAD software

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CMM Operation and Programming
• Positioning the probe relative to the part can be accomplished in several ways, ranging
from manual operation to direct computer control.
• Computer-controlled CMMs operate much like CNC machine tools, and these machines
must be programmed.
CMM CONTROLS :
• The methods of operating and controlling a CMM can be classified into four main
categories:
1. Manual drive,
2. Manual drive with computer-assisted data processing,
3. Motor drive with computer-assisted data processing, and
4. Direct Computer Control with computer-assisted data processing.
1. Manual drive:
• In manual drive CMM, the human operator physically move the probe along the
machine’s axes to make contact with the part and record the measurements.
• The measurements are provided by a digital readout, which the operator can record either
manually or with paper print out.
• Any calculations on the data must be made by the operator.
2. Manual drive with computer-assisted data processing,
• A CMM with manual drive and computer-assisted data processing provides some data
processing and computational capability for performing the calculations required to
evaluate a give part feature.
• The types of data processing and computations range from simple conversioons between
units to more complicated geometry calculations, such as determining the angle between
two planes.
3. Motor drive with computer-assisted data processing
• A motor-driven CMM with computer-assisted data processing uses electric motors to drive
the probe along the machine axes under operator control.

52
• A joystick or similar device is used as the means of controlling the motion.
• Motor-driven CMMs are generally equipped with data processing to accomplish the
geometric computations required in feature assessment.
4. Direct Computer Control with computer-assisted data processing.
• A CMM with direct computer control (DCC) operates like a CNC machine tool. It is
motorized and the movements of the coordinate axes are controlled by a dedicated
computer under program control.
• The computer also performs the various data processing and calculation functions.
• As with a CNC machine tool, the DCC CMM requires part programming.
Advantages of computer (software) in processing:
 A particular measurement sequence is strictly adhered to since computer accepts the
information in a sequential manner and also provides necessary guidance to operator in this
regard.
 Inspection time is reduced considerably
 The calculation of final result in available immediately on completion of the last
measurement.
 Rejected readings can be repeated straight away, before the set up is disturbed.
 Scope for copying and calculation errors is virtually eliminated.
 The checking of the result is made much more simple.
 Time for calibration is reduced considerably.
 New Operation can be trained quickly and they need not be highly qualified.
Disadvantages (limitations) of use of computers for this application:
 Computer adheres to a given criteria rigorously and thus all the qualifying requirements and
ability of operator in accepting / rejecting a reading need to be convey (written in software) to
computer clearly without any ambiguity.
 Sometimes a number may be entered incorrectly due to transposing error or key-bounce.
 Strict control is needed over the use, amendment and copying of programme tapes to ensure
that unauthorized modifications are not made.
 Checking procedure to ensure correct loading of program from tape needs to be followed.

53
 Measurement process gets remote from the operator.
 It is difficult to locate the source of problem by normal operator.
 While the effects of drifts, environment influences are hidden or not noticed; but operator
may not get that confidence.
While human eye and memory are extremely good at detecting drifts and averaging high
frequency noise on signals, careful programming has to be undertaken to give a computer a
similar facility.

54
3.5 Basic Concepts of Machine Vision System
Machine vision:
Machine vision can be d as a means of simulating the image recognition and analysis
capabilities of the human system with electronic and electromechanical techniques.
Four basic functions of machine vision system:
i) Image formation
ii) Processing of image
iii) Analyzing the image
iv) Interpretation of image
Gray scale analysis.
In these techniques, discrete areas or windows are formed around only the portions of the
image to be inspected. For determining if brackets are present, high intensity lighting is positions
so that a bracket, when the bracket is missing no shadow will be cash. When the bracket is present,
a large number of darker pixels can be observed in the window due to the cast shadow then when
a bracket is missing. A contrast threshold between the dark and light pixel value area can be set.
This type of discrete area analysis is a powerful tool can be used for inspection of absence, currant
part assembly, orientation, part, integrity etc.
Working principle and the steps involved machine vision system.
The machine vision system involves following four basic steps.
1. Image formation
2. Image Processing (in a form suitable for analysis by computer)
3. Defining and analyzing the characteristics of image
4. Interpretation of image and decision making.
1. Image formation:
For formation of image suitable light source is required. It may consist of incandescent light,
fluorescent tube, fiber-optic bundle, arc lamp, or strobe light. Laser beam is used for triangulation
system for measuring distance. Polarised or ultraviolet light is used to reduce glare or increase

55
contrast. It is important that light source is placed correctly since it influences the contrast of the
image. Selection of proper illumination technique, (viz, back lighting, front lighting-diffused or
directed bright field, or directed dark field, or polarised, structured light) is important. Back
lighting is suited when a simple silhouette image is required to obtain maximum image contrast.
Front lighting is used when certain key features on the surface of the object are to be
inspected. If a three-dimensional feature is being inspected, side lighting or structured lighting may
be required. The proper orientation and fixturing of part also deserve full attention.
An image sensor (like vidicon camera, CCD or CID camera) is used to generate the
electronic signal representing the image. The image sensor collects light from the scene through a
lens and using a photosensitive target, converts it into electronic signal. Most image sensors
generate signals representing two-dimensional arrays (scans of the entire image).
Vidicon Camera: it used in closed – circuit television systems can be used for machine
vision systems. In it, an image is formed by focusing the income light through a series of lenses
onto the photoconductive face plate of the vidicon tube. An electron beam within the tube scans
the ph to conductive surface and produces an analog output voltage proportional to the variations
in light intensity for each scan line of the original scene.
It provides a great deal of information of a scene at very fast speeds. However they tend to
distort the image due to their construction and are subject to image burn-in on the photo
conductive surface. These are also susceptible to damage by shock and vibration.
Solid State Cameras : These are commonly used in machine vision systems. These employ
charge coupled device (CCD) or change injected device (CID) image sensors. They contain matrix
or linear array of small, accurately spaced photo sensitive elements fabricated on silicon chips
using integrated circuit technology, Each detector converts the light falling on it, through the
camera lens, into analog electrical signal corresponding to light intensity. The entire image is thus
broken down into an array of individual picture elements (pixels).
Solid-state cameras are smaller, rugged and their sensors do not wear out with use. They
exhibit less image distortion because of accurate placement of the photodetectors. CCD and CID
differ primarily in how the voltages are extracted from the sensors.
2. Image processing:

56
The series of voltage levels available on detectors representing light intensities over the area of the
image need processing for presentation to the microcomputer in a format suitable for analysis. A
camera may typically from an image 30 times per sec i.e. At 33 m sec intervals. At each time
interval the entire image has to be captured and forzen for processing by an image processor. An
analog to digital converter is used to convert analog voltage of each detector into digital value.
If voltage level for each pixel is given either 0 or 1 value depending on some
threshold value, it is called Binary System. On the other hand gray scale system assigns upto 256
different values depending on intensity to each pixel. Thus in addition to black and white, many
different shades of gray can be distinguished. This thus permits comparison of objects on the
basis of surface characteristics like texture, color, orientation, etc. All of which produce subtle
variations in light intensity distributions. Gray scale systems are used in applications requiring
higher degree of image refinement. For simple inspection tasks, silhoutte images are adequate
and binary system may serve the purpose. It may be appreciated that gray-scale system requires
huge storage processing capability because a 256 x 256 pixel image array with upto 256 different
pixel values will require over 65000-8 bit storage locations for analysis, at a speed of 30 images per
second. The data processing requirements can thus be visualised. It is, therefore, essential that
some means be used to reduce the amount of data to be processed.
Various techniques in this direction are :
a) Windowing: This technique is used to concentrate the processing in the desired area of interest
and ignoring other non-interested part of image. An electronic mask is created around a small
area of an image to be studied.
Thus only the pixels that are not blocked out will be analysed by the computer.
b) Image Restoration. This involves preparation of an image in more suitable form during the
pre-processing stage by removing the degradation suffered. The image may be degraded
(blurring of lines/ boundaries; poor contrast between image regions, presence of background
noise, etc.) due to motion of camera / object during image formation, poor illumination /poor
placement, variation in sensor response, poor contrast on surface, etc.).
The quality may be improved,
( i ) by improving the contrast by constant brightness addition,
( ii ) by increasing the relative contrast between high and low intensity elements by making light
pixels lighter and dark pixels darker (contrast stretching )

57
or
( iii ) by fourier domain processing.
Other techniques to reduce processing are edge detection and run length encoding. In
former technique, the edges are clearly found and d and rather than storing the entire image, only
the edges are stored. In run-length encoding, each line of the image is scanned, and transition
points form black to white or vice versa are noted, along with the number of pixels between
transitions. These data are then stored instead of the original image, and serve as the starting
point for image analysis.
3. Image Analysis.
Digital image of the object formed is analyzed in the central processing unit of the system to draw
conclusions and make decisions. Analysis is done by describing and measuring the properties of
several image features which may belong to either regions of the image or the image as a whole.
Process of image interpretation starts with analysis of simple features and then more complicated
features are added to it completely. Analysis is carried for describing the position of the object, its
geometric configuration, and distribution of light intensity over its visible surface, etc.
Three important tasks performed by machine vision systems are,
i. Measuring the distance of an object from a vision system camera,
ii. Determining object orientation,
iii. Defining object position.
The distance of an object from a vision system camera can be determined by stadimetry
(direct imaging technique, in which distance is judged by the apparent size of an object in the field
of view of camera after accurate focussing), or by triangulation technique, or by stereo vision
(binocular vision technique using the principle of parallax).
The object orientation can be determined by the methods of equivalent ellipse (by
calculating an ellipse of same area as the image of object in two- dimensional plane, and
orientation of object being d by the major axis of the ellipse), the connecting of three points
(defining orientation by measuring the apparent relative position of three points of image), light
intensity distribution (determining orientation based on relative light intensity), structured light
method (in which the workpiece is illuminated by the structured light and the three dimensional
shape and the orientation of the part are determined by the way in which the pattern is distored

58
by the part).
4. Interpretation of image and decision making.
Image can be interpreted by analysis of the fundamental geometric properties of two-dimensional
images. Usually parts tend to have distinct shapes that can be recognized on the basis of
elementary features. For complex three-dimensional objects, additional geometric properties need
to be determined, including descriptions of various image segments (process being known as
feature extraction). In this method the boundary locations are determined and the image is
segment into distinct regions and their geometric properties determined. Then these image
regions are organized in a structure describing their relationship.
An image can also be interpreted on the basis of difference in intensity of light in different regions.
Analysis of subtle changes in shadings over the image can add a great deal of information about
the three-dimensional nature of the object.
Advantages of machine vision system.
i. Reduction of tooling and fixture cash
ii. Elimination of need for precise part location
iii. Integrated automation of dimensional verification
iv. Defect detection
Three important field of machine vision system.
1. Inspection: It is the ability of an automated vision system to recognize well d pattern and if
these pattern match these stored in the system, makes machine vision ideal for inspection of raw
materials, parts, assemblies etc.
2. Part Identification: It is the ability of part recognition provides positive identifications of
an object for decision making purposes.
3. Guidance and control : Machine vision systems are used to provide sensor feedback for
real time guidance and control ranging from visual serving of industrial robots and weld seam
tracking to calculation of geometric off sets for part processing and assembly operations.
Applications of machine vision system.

59
i) This can be used to replace, machine for applications like welding, machining to
maintain relationship between tool and work.
ii) Machine vision systems are used for printed circuit board
iii) These are used for weld seam tracking, robot guidance and control, inspection of
microelectronic devices and tooling, on line inspection in machining operation, online
inspection of a assembling maintaining high speed packaging.
iv) This is for the recognition of object from its image
v) Achieve 100% accuracy.
Advantages of computer in processing:
The advantages of using computer for processing of acquired data and control are as under :
 A particular measurement sequence is strictly adhered to since computer accepts the
information in a sequential manner and also provides necessary guidance to operator in this
regard.
 Inspection time is reduced considerably
 The calculation of final result in available immediately on completion of the last
measurement.
 Rejected readings can be repeated straight away, before the set up is disturbed.
 Scope for copying and calculation errors is virtually eliminated.
 The checking of the result is made much more simple.
 Time for calibration is reduced considerably.
 New Operation can be trained quickly and they need not be highly qualified.
Disadvantages (limitations) of use of computers for this application:
 Computer adheres to a given criteria rigorously and thus all the qualifying requirements and
ability of operator in accepting / rejecting a reading need to be told to computer clearly without
any ambiguity.
 Sometimes a number may be entered incorrectly due to transposing error or key-bounce.
 Strict control is needed over the use, amendment and copying of programme tapes to ensure
that unauthorised modifications are not made.
 Checking procedure to ensure correct loading of program from tape needs to be followed.
 Measurement process gets remote from the operator.

60
 It is difficult to locate the source of problem by normal operator.
 While the effects of drifts, environment influences are hidden or not noticed; but operator
may not get that confidence.
 While human eye and memory are extremely good at detecting drifts and averaging high
frequency noise on signals, careful programming has to be undertaken to give a computer a
similar facility.

Unit III Advances in metrology

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