MM-UNIT-V.pptx

TOPICS
 Interferometer: NPL Flatness, Laser, Michelson
 Computer Aided Inspection - Digital Devices - Machine
Vision System
 Coordinate Measuring Machine: Basic concept, Types,
Constructional features, Probes, Accessories
 Machine Tool Metrology - Surface Roughness
Measurement - Straightness Measurement - Squareness
Measurement

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.

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.

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.

LASER MEASURING MACHINES
1. Laser Telemetric System
2. Laser and LED Based Distance Measuring Instruments
3. For Profile Checks
4. Scanning Laser Gauge

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.

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.

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.

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.

2. OPTICAL ELEMENTS
⚫The various optical elements are,
a. Beam splitters.
b. Beam benders.
c. Retro reflectors.

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.

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.

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.

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.

OTHER TYPES OF INTERFEROMETERS
1. Michelson Interferometer

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.

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.

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.

LASER INTERFEROMETER APPLICATIONS
1. Linear measurement

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.

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.

• Four elements,
1. Three axis motion structure
2. Probing system
3. m/c controller & computer hardware
4. Application software

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

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

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

1. Cantilever type
• supports probe from
movable vertical support
2. Bridge type
• horizontally suspended
• x-axis carries the bridge

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

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

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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

• 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).

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.

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.

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.

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

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.

60
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.

61
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.

62
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.

63
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)
64

2. PEAK TO VALLEY HEIGHT METHOD
67

68
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
69

70
2. Direct Instrument Measurements
1. Stylus probe instruments
2. Tomlinson surface meter
3. Profilometer
4. Talyor- Bobson - Talysurf

1. STYLUS PROBE INSTRUMENTS
71

4. TALYOR- BOBSON -TALYSURF
74

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.

• 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

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.

79
• Types of Straightness Measurements,
1. Straight edge or Spirit level
2. Auto collimator

1. STRAIGHT EDGE OR SPIRIT LEVEL
80

82
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.
83

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