Big Notes_ Metrology

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Thursday, July 10, 2008

METROLOGYMETROLOGY

• 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



• 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

• Sinø =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°,27°and 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)



• 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



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



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



• 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



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 δө, then the image will be tilted by

2xδө

• 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



• 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 .



• 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



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 45°with

the tested surface.

• The field of view of the eyepiece contains reading scale called an eyepiece micrometer



• 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



• 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



• 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 setting–up 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 90ºangle stand in three planes

for setting or checking perpendiculars



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 toolmaker’s microscope

• The toolmaker’s 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 toolmaker’s

microscope

• The optical system with projection screen of toolmaker’s

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 toolmaker’s 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



• 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,Posted by BIG NOTES at 4:28 AM


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