8/15/2014 BIG NOTES: METROLOGY

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Thursday, July 10, 2008
METROLOGY
METROLOGY
Metrology is the science of measurements.
In manufacturing industry the dimensions are measured to control the shape and sizes of an
object
Controlled measurements are necessary in the manufacture of interchangeable parts
Linear Measuring Instruments :
Slip gauges
Dial indicator
Angle Measuring Instruments
Bevel Protractor
Sine bar
Angle gauge
Slip gauges :
Slip gauges are used for precise measurement of parts and for verifying measuring tools such as
micrometers,comparators and various limit gauges
Slip gauges are rectangular blocks made of alloy steel having a cross-section of about 32mm by
9mm
The distance between two opposite faces determines the size of the gauge
During use, the required number of gauge blocks are used together to know the required
dimension.
Dial Indicators
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Dial indicators are used to test and inspect the size and trueness of a finished work
It is measured to an accuracy of 0.01mm
They are also used in conjunction with other measuring instruments like inside calipers,Depth
gauges etc to measure inside and out side dimensions,errors in parallelism,flatness etc
Bevel protractor
A Bevel protractor is used to check angles up to an accuracy of 5min
It consists of a protractor disc carrying degrees,reading from 0 to 90degrees on either side of an
arc
It is free to rotate about fulcrum and can be clamped at a desired angle with a nut
The vernier scale on the disc has 12 equal divisions over a space of 23 degrees on the main scale
One division of vernier scale is equal to 23/12
The difference between two divisions of main scale and one division of vernier scale is 5min
To read the instrument :
Note the whole degrees between zero of main scale and zero on vernier scale
In the same direction,note the division line of vernier scale which coincides with any division line
of main scale
Multiply the number by 5 to give number of minutes to be added to whole degrees
Sine bar
Sine bar used for measuring and setting angles
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It has two hardened steel rollers of equal diameter fastened near the ends
The axis of rollers are mutually parallel and the centers of these cylinders are on a line exactly
parallel with the working face of the bar
It is used on surface plate in conjunction with a set of slip gauges
A sine bar is specified by the distance between the centers of two rollers
From the fig. angle can be obtained by trigonometry
Sin =height of slip gauges(H) / central distance of rollers( L)
Angle gauge
The angle gauges are used for measuring and setting out angles in the work shops
Angle gauges are measured to an accuracy of 3sec
These are wedge shaped steel blocks and their working faces are finished in the same manner as
slip gauges
The angle blocks can be wrung together as in slip gauges
A set of angle gauges consists of 12 separate gauges
These are used in conjunction with precision square block to setup any angle between 0 to 360
These gauges are available in 3 series
Degree series,1,3,9,27and 41
Minute series 1min,3min,9min and 27min
Fraction of minute series 0.1min,0.3min,and 0.5min
Comparator
A comparator works on the relative measurement
It gives only dimensional differences in relation to a basic dimension
A comparator has to compare the unknown dimensions of a part with some standard (Basic Size)
These are commonly used for linear measurement in mass production
Classification
Depending on the amplification system, Comparators are classified as
1. Mechanical Comparators
2. Electrical Comparators
3. Optical Comparators
4. Pneumatic Comparators
Depending on the amplification system, Comparators are classified as
1. Mechanical Comparators
2. Electrical Comparators
3. Optical Comparators
4. Pneumatic Comparators
Mechanical comparators are extremely convenient to use
It does not require any external power for its operation
The measuring head consists of the work contact plunger(Stylus)
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The movement due to size difference between standard and work piece operates a pointer
The pointer moves over a dial
This movement of the plunger is magnified by means of the gear train or levers
The magnification of mechanical comparators is limited to 1000
These are cheap and intended to measure external surfaces
However the mechanical devices are subjected to wear
Types of Mechanical Comparators
1. Dial indicator
2. Reed type mechanical comparator
3. The Sigma comparator
Dial indicator :
The Dial indicators are used to test,inspect the size and trueness of finished work
The measured accuracy is up to 0.01mm
Constructional Details
The dial indicator consists of a base, column,arm,dial
indicator and stylus(work contact plunger)
The arm can be adjusted at required height along
the column
The work table is mounted on the base to provide a
reference surface
The work is placed on the work table during
measurement
Mechanism
Very slight upward pressure moves the stem / plunger upward
The movement is indicated by the dial pointer
The dial is graduated into 100 divisions
The full revolution of the pointer about the scale corresponds to 1mm of the plunger
Thus a turn of the hand by one scale division represents a spindle travel of 0.01mm
The movement of stem is transmitted by means of a toothed rack through a gear train
This movement is obtained around a dial face
The required measuring pressure is provided by small springs incorporated in the mechanism
The indicator is adjusted zero by turning the rim of the dial
The dial indicators are also available for ranges of measurement of 0-3,0-5,0-10mm.
These measurements indicate the movement (left) of the plunger
Reed type Mechanical Comparator
The instrument used to measure linear displacement of spindle with the help of the reed
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mechanism is known as reed type Mechanical comparator.
It is a device magnifying small motions of spindle
Constructional details
The reed mechanism is frictionless device for magnifying small motions of the spindle
It consists of a fixed block `A` which is rigidly fastened to the gauge head case
The floating block `B` carries the gauging spindle
The floating block is also connected horizontally to the fixed block by reeds `C`
A vertical reed is attached to each block with upper ends jointed together
These vertical reeds are shown in fig by letter `D`
A pointer is attached to this joint
Working principle
A linear motion of the spindle moves the floating block vertically upwards
This vertical movement causing the vertical reed on the floating block to slide past the vertical
reed on the fixed block
The movement causes both reeds swing through an arc
As the point is nearly an extension of the vertical reeds `M` it swings through a much wider arc
Sigma Comparator
The Sigma Comparator is a mechanical comparator in which the amplification is obtained by
means of a compound lever
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Advantages: It has got a bold scale and larger indicating
pointer
Dis Advantages: Due to motion of parts ,there is wear in the
moving parts
Constructional details
The vertical beam is mounted on flat steel springs connected to fixed members
The vertical beam which in turn are screwed to a back plate
The assembly provides a frictionless movement with restraint from the springs
The hinged assembly carrying the forked arms incorporates a hardened fulcrums
The metal ribbon attached to the forked arms passes around the spindle
This spindle is to rotate in specially designed miniature ball bearings
The indicating pointer is secured to a boss on the disc
Operating Principle :
The various movements in the Sigma Comparator are shown in figure.
The plunger is mounted on a pair of slit diaphragms in order to have frictionless linear movement
A knife edge is mounted on it and bears upon the face of the moving member of a Cross strip
hinge
The cross strip hinge consists of the moving component and a fixed member
The cross strip hinge then connected by thin flexible strips alternately at right angles to each
other
Thus if an external force is applied to the moving member it will pivot as would a hinge about the
line of intersection of the strips
To the moving member an arm `Y` shape having the effective length `b` is attached
If the distance of the hinge from the knife edge be `a` then the magnification of the first stage
is `b/a
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A phosphor-Bronze strip is attached to the two extremities of the `Y` arm
And is passed around a radius `r` attached to the pointer spindle
If the length of pointer be `R` then R/r is the second stage of magnification
In order to adjust the magnification ,distance `a` must be changed by slackening and tightening
the two screws attaching the knife edge to the plunger
In sigma comparator a damping device must be employed to prevent oscillation of point before
coming to rest
Principle of an Electrical Comparator
The principle of these comparators is to convert the linear
displacement of the measuring stylus into an electric output
Whetstone bridge circuit is used for this comparator
When the circuit is balance no current is detected
Hence the following condition will exit
R1 = R3 R2 R4
Electrical Comparators are extremely sensitive and are
capable of high magnifications up to 30000
If one of these resistors is varied then imbalance in the circuit will result in a current flow through
the galvanometer
By arranging the variable resistor a measuring head mechanism is formed
The galvanometer may be calibrated to read linear displacement
The whetstone bridge circuit is suitable only for D.C supply
Although the operating principle is same certain modification is that the resistors are replaced by
inductors
Two of which are incorporated in the measuring head
When the stylus moves vertically, the circuit will become imbalance
This is due to the movement of iron armature between the inductors L1 and L2
optical comparator
Principle of an optical comparator
In optical comparators small displacements of the measuring
plunger are amplified first by a mechanical system consisting of
pivoted levers
The amplified mechanical movement is further amplified by a
simple optical system involving the projection of an image
The usual arrangement employed is such that the mechanical
system causes a plane reflector to tilt about an axis
The image of an index is projected on a scale on the inner
surface of a ground glass screen
The whole system could be explained diagrammatically from fig which gives very simple
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arrangement
In this system mechanical amplification is L2/L1
Optical amplification is ( L2/L1)X2
It is multiplied by 2 because if mirror is tilted by an angle , then the image will be tilted by
2x
Thus overall magnification of this system is
2x ( L2/L1) ( L4/L3)
The important point in optical comparators is that mirror used must be of front reflection type
and not of normal back reflection type
Advantages
1. Optical magnification provides high degree of measuring precision due to reduction of moving
members and better wear resistance qualities
2. Optical magnification is also free from friction,bending,wear etc.
An illuminated scale is provided that enables readings to be taken without regard to the room
lighting conditions
4. These are also used to magnify very small parts such as needles, saw teeth, screw threads etc
Optical Comparator Principal
This is the commercial measuring instrument
This utilizes a plunger tilted mirror, objective lens,prism and observing eye piece to provide a
high degree of magnification
The mirror is mounted on a knife-edge
It can be tilted about the fulcrum by any linear vertical movement of the contact plunger
A beam of light passes through a graticule suitably engraved with a linear scale
The movement of mirror causes this scale to move up (or) down past a translucent screen inside
the observing hood of the instrument
The eye placed near the eye piece views the image of a small scale engraved on scale after
reflection from the plunger actuated mirror
The plan view for the mirror is shown in fig
In the focal plane of the eye piece , a fine reference line ( index 0) is provided
The system of lenses is so arranged that the image of the scale is projected in the same focal
plane
Thus with movement of scale the image can be measured with reference to the fixed line
The division of the scale image opposite the index line indicates the amount of movement of
contact plunger
The image of the scale and the index line could also be viewed through a projection system
The overall magnification of the comparator is given by (2f/d)x eye-piece magnification
Where `f` is the focal length of the lens and `d`is the distance between the knife-edge and the
plunger
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The measurement is made by taking reading using master gauge of known accuracy and
comparing with component reading
It uses a controlled, pressurised jet of air to measure small dimensional variation in the size of
component
The working principle of pneumatic comparator depends on the flow of air between the gauge
head and the component being checked
Important characteristics
Very high magnifications are possible
It can be used to measure diameters, length, squareness, parallelisms, concentricity,taper, and
other geometric conditions.
Internal dimensions can be readily measured , in other words, while measuring a bore it can
reveal complete details of size, taper, straightness,camber and bell mouth etc.
It is independent of the operator skill
High pressure air gauging can be done by cleaning the parts which helps to eliminate errors due to
dirt and foreign matter.
As there is no physical contact made either with the setting gauge (or) the part being measured ,
there is no loss of accuracy because of gear wear.
Gauging pressures can be kept sufficiently low to prevent part deflection
Dimensional variations throughout the length of the shaft (or) cylinder bore can be explored for
out of roundness, taperness, concentricity,regularity and other similar conditions
The total life cost of the gauging heads is much less
It is accurate, flexible, reliable,universal and speedy device for inspecting parts in mass
production
It is best suited for checking multiple dimensions and conditions on a part simultaneously in least
possible time.
It can be easily used for a line measurement of parts as they are being machined and take
corrective actions.
Working principle of Solex pneumatic comparator
This instrument is produced commercially by Solex air gauges
Ltd.
This is generally designed for internal measurement
But with suitable measuring head it can be used for external
gauging also
The arrangement used in Solex gauge is to pass the high pressure air after filtering through a
flow valve
It consists of a tank in which water is filled up to a certain level and dip tube is immersed into it
up to a depth corresponding to air pressure .
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In the Fig it is represented by `H`
Air is sent at higher pressure than required one
Some air will leak out from the dip tube and bubble out of water
And the air moving towards control orifice will be at desired constant pressure `H`
It is very obvious from Fig that the diameter being measured at any instant is corresponding to
the portion against two jets
To find the concentricity (roundness of any job at any section) The work piece may be revolved
around the measuring gauge
Similarly the diameter can be noted down at several places along the length of bore and thus
tapering of hole is determined
This method is best suited for measuring roundness and taperness of cylinder bores and gun
barrel bores
By having suitable measuring head this can be used for external gauging also
Advantages of Pneumatic Comparators
1. Comparators can be checked with out contact with finished surfaces
2. This method needs little skill to operate
3. No limit to the size of work that can be tested
4. Provide a simple and direct method of high amplifications
5. Dimensional variations can be controlled with close tolerances
Accuracy of a comparator
The absolute accuracy of a comparator is guaranteed by use of standard slip gauges, slip gauges
employed for setting working gauges are themselves checked by micro-comparator for wear effects
For very accurate and for absolute results light wave interference method is used.
The indirect method of testing gauges consists in using two comparators where relative
difference from standard size is obtained
Comparators are available up to the accuracy of 75 millionth of an mm.
Measurement
The surface texture represents the combined effect of several causes,of which the
roughness,waviness and the error of form are more predominant
If only the total height is considered then idea of above three characteristics can be had by
considering different sample lengths
If only surface measurement is considered then the sampling length should be neither too big as
to include the waviness, nor too small as to ignore the occasional deep scratches
Determination of roughness value
To compute the roughness value by taking mean results from the measurement of several
sampling lengths taken consecutively along the profile
So that it gives better indication and is the correct representation of the whole of surface
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roughness
Roughness comparison specimens
The roughness comparison specimens are used for qualitative assessment of the surface
roughness
Qualitative assessment of the roughness of the finished product is evaluated by visual or tactile
comparison with roughness comparison specimen of known roughness value finished by similar
machining process
The roughness comparison specimens are available either in flat form or segmented cylindrical
form
The flat specimens have the size:length-25mm,width-16mm,thickness-1.6 to 10mm
Tracer type profilogram
The surface to be tested is placed on a working table
The table can move to and fro because there is a nut at the bottom of table,which fits into a lead
screw driven by a motor.
The stylus or tracer point which is made to rest on the surface to be tested , is pivoted with
mirror as shown in the fig.
The pivot point is fixed in space with the stand and its position can be changed initially
When the surface to be tested moves, the oscillations of the tracer are transmitted to the mirror
The beam of light strikes on the oscillating mirror and is reflected on to the revolving drum,upon
which sensitized film is arranged
The drum is also rotated from the same lead-screw that traverses the table through bevel gears
The trace obtained on the drum is very much magnified and can be further assessed by various
methods
Light cross-section method
Principle:
The principle of this method is that when a thin film of light strikes the surface to be tested at an
angle of 45, a band of reflected light will appear on the surface
The edge of this band will reproduce the profile of the surface irregularities, to say, it will show
an optical cross-section.
This profile is magnified and observed by an optical device arranged at an angle of 45
Double microscope
A double microscope is used to evaluate the surface finish by light cross-section method
The general view and optical system of the microscope is shown in the fig.
From a light source a beam of light passes through the condenser and precision slit and is
directed at an angle of 45 to the surface to be tested
The observing microscope having objective and eyepiece is also inclined at an angle of 45with
the tested surface.
The field of view of the eyepiece contains reading scale called an eyepiece micrometer
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Eye piece micromate is used to measure the height of surface irregularities.
Surface roughness
On any finished surface,imperfections are found and these take the form of a succession of hills
and valleys
Which vary both in height and in spacing and result in a kind of texture which in appearance or
feel
Straight edge
Straightedges are commonly used for testing the straightness and flatness of plane surfaces
Flatness is checked by a light test
The narrow edge of the tool is applied to the surface to be tested and kept horizontally at the eye
level
Watching for light between the straightedge and the surface along the straightedge
Light seen at one place or another indicates deviation from straightness
Surface plate
Surface plate is used for testing the flatness of work itself and is also used for marking out work
This is used for small pieces of work while the marking -out table is used for larger jobs
It is used in conjunction with other gauges,the plate becomes a functional gauge in itself
Surface gauge
Surface gauge is used in laying out of parts that have to be
machined
It is used in conjunction with a dial indicator for checking
surface
The tool has a v-shaped groove at the bottom of the base for
resting the block on a round bar
So that dimensions may be set off from the bar to some other part of the work
Visual inspection
The surface roughness can be judged by visual comparison of one surface with the other by
nacked eye
Sense of feel method is most often used as a measure of surface roughness
In this method a finger nail is moved along the surface of the job and a sense of feel indicate the
amount of resistance and depth of irregularities
This can be compared with master roughness scale
Profilometer
Profilometer is a tracer instrument used to measure surface roughness accurately
The two main parts of profilometer are the tracer and an amplifier
The tracer may be moved either manually or mechanically across the surface of the work
Mechanical movements by motor drive give a more consistent and dependable roughness
measurement
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As the tracer is moved across the surface the stylus follows
the contour of the surface
These up and down movements of stylus are converted in to
electrical impulses
These impulses are fed in to the amplifier where it is
amplified to accurate roughness meter
The meter shows roughness heights in micro inches
The instrument is best suited for measuring surface
roughness of deep grooves
Optical flat
Optical flat utilizes interference technique to measure surface roughness
It is made of high quality optical quartz having extremely flat and parallel surfaces of about 25mm
The flat is placed on the surface to be checked and the monochromatic light is directed at the flat
The light is reflected from the back face of the flat and also the surface of the work
The light reflected from the work surface will travel through varying distances causing to the
surface roughness
Some will be in phase and some will be out of phase with light reflected from the optical flat
The result is a series of light and dark bands,which appear to the necked eye
The number of fringes that appear is related to the distance between the part and the bottom
surface of the optical flat
A truly flat work piece surface will not split the light beam and fringes will not appear
When surfaces are not flat,fringes are curved
Autocollimator
The autocollimator is an optical instrument and is used to measure angular displacements with
high accuracy of 0.2 seconds arc
It can also be used to measure the flatness of a large surface
Collimation of light
If a point source of light is placed at the focal point of convex
lens ,parallel rays of light are reflected by the lens. This is
known as collimation of light
A device with a lens suitable for causing light rays from a
point source to become parallel is known as a collimator
Auto collimation of light occurs when reflecting surface is used in conjunction with a collimation
lens system
The collimation rays will be reflected back from the surface
These rays pass through lens ,and unless reflecting surface is perfectly tangential to the lens axis
,will be brought to a focus along side of the light source
Principle of the autocollimator
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From the fig. The light in the form of parallel rays , is reflected from the surface of reflector
After re-entering the objective lens ,the light form an image of target wires in the same plane as
the wire itself
The eyepiece incorporated in micrometer enables the observer to measure any angular
displacement caused by misalignment at the reflector
Autocollimator accessories
Leveling base
Surface plate stand
Steel reflector
Mounted glass corner reflector
Steel cube reflector
Leveling base
It supports the autocollimator and enables it to be leveled to bring its axis parallel to the surface
being measured
It incorporates spring loaded clamps and a circular bubble level
Three pads are included for use under the foot screws
There is no necessity to remove the autocollimator from the base after use as the autocollimator
case is constructed to take both items
Surface plate stand
A multi purpose stand of heavy duty construction for general bench use ,comprises ground cast
iron surface plate, column and bracket
The Autocollimator clamping bracket has independent clamping and rotational adjustments,
enabling the bracket to be turned with out disturbing the height adjustment
Steel reflector
A reflector must be regarded as an integral part of any autocollimator system
Successful auto collimation requires a reflector of adequate flatness reflectivity and diameter
The faces are not perfectly parallel when the reflector is mounted at the back.
Mounted glass corner reflector
This is used in conjunction with a reflector carriage and mounted reflector for calibrating a
surface plate
It enables several calibration lines to be traversed without the autocollimator being moved
Thereby saving settingup time and making the subsequent correlation of reading easier
Steel cube reflector
It can be used as a general purpose reflector and for providing a 90angle stand in three planes
for setting or checking perpendiculars
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Autocollimator applications
To the measurement of straightness and flatness
Precise angular indexing in conjunction with polygons
Comparative measurement using master angles
Assessment of squareness and parallelism of components
The measurement of small linear dimensions
The toolmakers microscope
The toolmakers microscope has a table on which work piece is clamped
The table movements are controlled by micrometer
The light source is contained in the base
The work may be viewed directly as in the case of
conventional microscopes or projected on screen
When looking into the eyepiece of this microscope two thin
straight lines intersecting at the centre of the field may be seen
on the glass at the focal plane
The microscope is focused to show a clear image of a
magnified portion of work piece on a focal plane
To take the measurement ,one cross-line of the eyepiece is aligned with one end of the
dimension and location of table is noted
The table is moved so that the same line in the eyepiece next aligned with the opposite end of
the dimension and location of the table is again noted
The difference in the two readings is the desired measurement
Optical system of toolmakers
microscope
The optical system with projection screen of toolmakers
microscope is shown in fig.
The image of the work profile is focused by the objective lens
system and passed through an inverting prism
To ensure that the projected image is oriented same as the
object on the work stage
The projected light then picks up the image of a graticule
which can be focused by a second lens system before passing
through a right angled prism
Both images are then projected by the final lens ,of the reflecting mirror and on to the screen
Applications of toolmakers microscope
To measure the diameter of holes and distances between centres
Position of keyway with reference to centre of bore
Angles and pitch of threads
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Gear tooth spacing
Interferometer
Interferometer is an optical device used for measuring flatness of a surface and also for
determining the thickness of slip gauges.
It is an extension of the application of optical flat.
Interferometers are based upon the principle of interference and employ wavelength of light
as their measuring units.
The interferometers make use of beam divider that splits an incoming ray into two parts
These two parts of the ray travel along different paths until they are recombined, usually in the
same beam divider.
An interferometer is an optical device which utilizes the effect of interference. Typically, it starts
with some input beam, splits it into two separate beams with some kind of beam splitter (a
partially transmissive mirror),
possibly exposes some of these beams to some external influences (e.g. some length changes or
refractive index changes in a transparent medium), and recombines the beams on another beam
splitter. The power or the spatial shape of the resulting beam can then be used for a measurement
Types of Interferometers
Various types of interferometers used in metrology are as follows.
1) Michelson interferometer
2) N.P.L. Flatness Interferometer
3) Laser Interferometer
Advantage of interferometer
Interferometer is faster and easier than that of optical flats, and is considered as most accurate
measuring instrument,
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