Calibration is a comparison of instrument
performance to the standards of known accuracy;
calibrations directly link customers
measurement equipment to national and
In other words, calibration means to find out
whether the instrument gives the correct reading
or not. It also includes minor adjustments in the
instrument to minimize error.
The vernier, dial, and digital calipers give a direct
reading of the distance measured to high accuracy.
They are functionally identical, with different ways of
reading the result. These calipers comprise a
calibrated scale with a fixed jaw, and another jaw,
with a pointer, that slides along the scale. The
distance between the jaws is then read in different
ways for the three types.
The simplest method is to read the position of the
pointer directly on the scale. When the pointer is
between two markings, the user can mentally
interpolate to improve the precision of the reading.
This would be a simple calibrated caliper; but the
addition of a vernier scale allows more accurate
interpolation, and is the universal practice; this is the
Vernier, dial, and digital calipers can measure
internal dimensions (using the uppermost jaws),
external dimensions using the lower jaws, and in
many cases depth by the use of a probe that is
attached to the movable head and slides along
the centre of the body.
Parts of a vernier caliper:
1. Outside jaws: used to measure external diameter or width of an object
2. Inside jaws: used to measure internal diameter of an object
3. Depth probe: used to measure depths of an object or a hole
4. Main scale: scale marked every mm
5. Main scale: scale marked in inches and fractions
6. Vernier scale gives interpolated measurements to 0.1 mm or better
7. Vernier scale gives interpolated measurements in fractions of an inch
8. Retainer: used to block movable part to allow the easy transferring of a
VERNIER HEIGHT GAUGE
This is also a sort of vernier caliper equipped with a
special base block & other attachments which makes
the instruments suitable for height measurements.
Along with the sliding jaw assembly, arrangement is
provided to carry a removable clamp.
The vernier height gauge is mainly used in the
inspection of parts and layout work. This can also be
used as scribing instrument.
Micrometer allows a measurement of the size of
It is one of the most accurate mechanical devices
in common use.
PRINCIPLE OF MICROMETER
Micrometer works on the principle of screw and
nut. We know that when a screw is turned
through a nut through one revolution, it
advances by one pitch distance i.e. one revolution
of the screw corresponds to linear movement of a
distance equal to pitch of the thread.
The micrometer screw gauge consists of an accurate screw having
about 10 or 20 threads per cm and revolves in a fixed nut. The
end of the screw forms one measuring tip & the other measuring
tip is constituted by a stationary anvil in the base of the frame.
The screw is threaded for a certain Length & is plain afterwards.
The plane portion is called sleeve & its end is the measuring
surface. The spindle is advanced or retracted by turning a
thimble connected lo the spindle. The spindle is slide fit over the
barrel & barrel is the fixed path attached with the frame. Barrel
is graduated in the unit of 0.005 cm. The thimble has got 25
divisions around its periphery on circular portion. Each division
corresponds to 0,002 cm, A lock nut is provided for locking a
dimension by preventing the motion of the spindle.
MAIN PARTS OF MICROMETER
U-shaped steel frame:--
The outside micrometer has U or C shaped frame. It
holds all the parts of micrometer together .
The frame is generally made of steel, cast steel,
malleable C. I. of light alloy.
It is desirable that the frame of Micrometer be
provided with conveniently placed finger grips.
Anvil and Spindle:
The Micrometer has fixed anvil protruding 3 mm fro
the left hand side of the frame. The diameter of the
anvil is the same as that of the spindle.
Another movable anvil is provided on the front of the
spindle. The anvil are accurately ground and lapped.
The spindle engages with the nut.
A lock nut is provided on the micrometer spindle.
It is used to lock the spindle when the micrometer is at
its correct reading.
Sleeve or Barrel:
The sleeve is accurately divided and clearly marks in
0.5 mm division along its length, which serves as main
It is chrome platted and adjustable for zero setting.
Thimble can be moved over the barrel. It has 50 equal
division around its circumference. Each division
having a value of 0.01mm.
The ratchet is provided at the end of thimble. It is
used for accurate measurement.
This is same as a micrometer only the anvil & spindle
is of different shape. The anvil has an internal V-
shape which fits in the thread. The anvil in this case is
not fixed but is free to rotate- Thus vee of the anvil can
accommodate itself to any rake range of thread. The
spindle on the other hand has the ground conical
shape. When the conical spindle brought into contact
with vee of anvil, micrometer reads zero. Different set
of anvil is provided for different threads sizes.
• Depth gauge micrometers are
used to measure the depth of
blind holes, slots, key ways, etc.
•The spindle length can be
changed to set the micrometer
for the desired range of
•To read a depth gauge
micrometer you must visualize
the distance that has been
covered by the thimble.
Depth Gauge Micrometer
Used for measuring cylinder bores, housing bores
Screw pitches same as outside micrometer
Dial gauges are used for
checking flatness of surfaces
and parallelism of bars and
Used for linear
Two pointer arms actuated
by rack and pinion
Rack is cut in spindle and
spindle is made to come in
contact with the work-piece
Linear displacement is converted into rotary
movement of the pointers.
The dial is divided into 100 equal divisions and
each division represents a spindle movement of
For 1mm movement, the bigger arm turns
through one complete revolution.
The smaller arm register the number of full
turns made by bigger arm
Cllinometer is a level mounted on a rotatable member
whose angle of inclination relative to its base can be
measured by a circular scale.
Used for checking angular faces and relief angle on large
For precision work watt clinometers are often employed
(available in four types). The first type contains the front
of a glass circular reading from 0-360 degree ,and used
as similar to optical protractor as already used .The
second type of instrument operates by means of a worm
and quadrant reading direct to n1 seconds and has a
range of 90 degree. The third type of instrument contains
any bubble and their operation depends upon a circle
supported on fine ball bearings and weighted
eccentrically so that, when released, they always take up
the position relative to the true vertical. The angle of the
body is read of 1 second with the aid of vernier.
The fourth type of clinometers illustrated is one of high
precision, reading direct to three seconds. dust tight
body, mounted on an accurately ground base, carries on a
lapped steel bearing, a sensitive spirit level which serves
as a fiducially indicator
An autocollimator or auto collimated telescope is an instrument combining
a telescope and a collimator, such that the graticule and any superimposed
image can be observed through the eyepiece. This instrument is designed to
measure small angular deflections and may be used in conduction with
plane mirror or other reflecting surface .
If a scale is provided on the graticule the tilt of the
reflecting surface may be measured .The angle of the
reflected ray ,relative to the incident ray, is twice the
angle of tilt of the reflecting surface ,so that a direct two
–to –one magnification is obtained .
Most common method for calibrating surface plates.
The instrument is used for measuring the straightness of
the long and relatively narrow surface such as beds of
long lathes and planning machines
Sine bars are always used along with slip gauges
as a device for the measurement of angles very
They are used to
1) Measure angles very accurately.
2) Locate the work piece to a given angle with very
Generally, sine bars are made from high carbon,
high chromium, and corrosion resistant steel.
These materials are highly hardened, ground and
In sine bars, two cylinders of equal diameter are
attached at lie ends with its axes are mutually
parallel to each other.
They are also at equal distance from the upper
surface of the sine bar mostly the distance
between the axes of two cylinders is 100mm,
200mm or 300mm.
The working surfaces of the rollers are finished
to 0.2 μm R value.
The cylindrical holes are provided to reduce the
weight of the sine bar
The working of sine bar is based on trigonometry
To measure the angle of a given specimen, one
roller of the sine bar is placed on the surface
plate and another one roller is placed over the
surface of slip gauges.
Now, ‘h be the height of the slip gauges and ‘L’ be
the distance between roller centers, then the
angle is calculated as
USE OF SINEBAR:
(1,) Locating any’ work to a given angle
Before checking the unknown angle of the
specimen, first the angle (0) of given specimen is
found approximately by bevel protractor.
2)Then the sine bar is set at angle of 0 and
clamped on the angle plate.
3)Now, the work is placed on the sine bar and the
dial indicator set at one end of the work is moved
across the work piece and deviation is noted.
4)Slip gauges are adjusted so that the dial
indicator reads zero throughout the work surface.
Limitations of sine bars
1)Sine bars are fairly reliable for angles than 15°.
2)It is physically difficult to hold in position
3)Slight errors in sine bar cause larger angular
4)A difference of deformation occurs at the point of
roller contact with the surface plate and to the
5)The size of parts to be inspected by sine bar is
Sources of error in sine bars:
The different sources of errors are listed below:
1)Error in distance between roller centers.
2)Error in slip gauge combination.
3)Error in checking of parallelism.
4)Error in equality of size of rollers and cylindricity.
5)Error in parallelism of roller axes with each other.
6)Error in flatness of the upper surface of sine bar.
Bevel protractors are nothing but angular
Types of bevel protractors
The different types of bevel protractors used are:
1)Vernier bevel protractor
A vernier bevel protractor is attached with acute angle
The body is designed its back is flat and no projections
beyond its back. The base plate is attached to the main
body and an adjustable blade is attached to the
circular plate containing vernier scale.
The main scale is graduated in degrees from 0° to 90°
in both the directions. The adjustable can be made to
rotate freely about the center of the main scale and it
can be locked at any position.
For measuring acute angle, a special attachment is
provided. The base plate is made fiat for measuring
angles and can be moved throughout its length. The
ends of the blade are beveled at angles of 45° and
The main scale is graduated as one main scale
division is 1° and vernier is graduated into 12
divisions on each side of zero. Therefore the least
count is calculated as
Thus, the bevel protractor can be used to
measure to an accuracy of 5 minutes.
Applications of bevel protractor
The bevel protractor can be used in the following
1.For checking a ‘V’ block:
A comparator works only on relative
measurements i.e. It gives only dimensional
difference in relation to a basic dimension.
so, a comparator has to compare the
unknown dimensions of a part with some
standard or master setting which
represents the basic size and dimensional
variations from the master setting have to
be amplified and measured
CHARACTERISTICS OF COMPARATORS
1. The instrument must be of robust design and construction so
as to withstand the effect of ordinary usage without
impairing its measuring accuracy.
2. The indicating devices must be such that readings are
obtained in least possible time. The system should be free
from errors, wear effects and the inertia should be minimum
3. Provision must be made for maximum compensation for
4. The scale must be linear
5. Indicator should be constant in its return to zero.
1. Instrument must have the maximum versatility, i.e., its
design must be such that it can be used for a wide range of
USES OF COMPARATORSUSES OF COMPARATORS
1. In mass production, where components are to be checked at a very
2. As laboratory standards from which working or inspection gauges
are set and correlated.
3. For inspecting newly purchased gauges.
4. Attached with some machines, comparators can be used as
working gauges to prevent work spoilage and to maintain required
tolerances at all stages of manufacturing.
5. In selective assembly of parts, where parts are graded in three or
more gauges depending upon their tolerance.
The mass production would be impossible if
component parts could not be produced to close
Example: An alluminium piston for a motor car
Very large quantity of this is required to
produce and this means that piston does not only
be mass produced, but also all dimensions must
be checked with some kind of precision and speed
as that used in their manufacture.
If the principle of measurement by
comparison is adopted, say the height of the
piston, then the set-up would appear as
shown in figure
Comparators Are Needed Because
• Determination of accuracy takes only a few seconds
• Little or no skill is required from the operator
• Consistency of measuring operation would be of a high
Elements of a ComparatorElements of a Comparator
1. Sensing device (usually a plunger): which senses the input
signal, may be a change of length or a surface displacement.
2. Magnifying or Amplifying system: to increase the signal to a
suitable magnitude. Mechanical, optical, pneumatic, hydraulic and
electronic methods are used for this purpose.
3. Display system (usually a scale and pointer): which utilizes the
amplified signal to provide a suitable readout?
MECHL MECHANICAL COMPARATORS
‘In these comparators, magnification is obtained by
mechanical linkages and other mechanical devices.’
Systems of Amplifications used in Mechanical Comparators:
1. Rack and Pinion: In it the measuring spindle integral with a
rack, engages a pinion which amplifies the movement of
plunger through a gear train.
2. Cam and gear train: In this case the measuring spindle acts
on a cam which transmits the motion to the amplifying gear
3. Lever with toothed sector: In this a lever with a toothed sector
at its end engages a pinion in the hub of a crown gear sector
which further meshes with a final pinion to produce
Advantages of Mechanical Comparators:
1. These are usually cheaper in comparison to other
devices of amplifying.
2. These do not require any external supply such as
electricity or air and as such the variations in outside
supplies do not affect the accuracy.
3. Usually the mechanical comparators have linear scale
which is easily understood.
4. These are usually robust and compact and easy to
5. For ordinary workshop conditions, these are suitable
and being portable can be issued from a store.
Disadvantages of Mechanical
1. The mechanical comparators have got more moving
parts than other types. Due to more moving parts, the
friction is more and ultimately the accuracy is less.
2. The mechanism has more inertia and this may cause
the instruments to be sensitive to vibration.
3. The range of the instrument is limited as the pointer
moves over a fixed scale.
4. Error due to parallax is possible as the moving pointer
moves over a fixed scale.
There is no pure optical comparator but the instruments classed
as optical comparators obtain large magnification by use of
optical principles though mechanical magnification in these
instruments also contributes for overall magnification.
There are many types of optical
comparators but all of them work on one of
the following two principles
1. The use of the optical lever
2. The use of enlarged image`
PRINCIPLE OF OPTICAL
In principle of optical lever, small displacements of the measuring
plunger are amplified first by a mechanical system consisting of pivoted
levers. The 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 and the image of an index is
projected on a scale on the inner surface of a ground- glass screen.
In this system, Mechanical amplification
l2/l1 and optical amplification = l4/l3 ×2
It is multiplied by 2, because if
mirror is tilted by an angle δθ, then
image will be tilted by 2 × δθ. Thus
overall magnification of this system
= 2 × (l2/l1) (l4/l3)
TYPES OF OPTICAL COMPARATORS
Zeiss Optotest Comparator
Eden - Rolt Millionth Comparator
ADVANTAGES OF OPTICAL
1. It has small number of moving parts and hence
a higher accuracy.
2. In the optical comparators the scale can be
made to move past a datum line and thus have
high range and no parallax errors.
3. It has very high magnification.
4. Optical lever is weightless.
1. As the instrument has high magnification, heat from
the lamp, transformer. May cause the setting to
2. An electrical supply is necessary.
3. The apparatus is usually large and expensive.
4. When the scale is projected on the screen, then it is
essential to use the instrument to a dark room in
order to take the readings easily.
5. The instruments in which the scale is viewed
through the eye piece of a microscope are not
convenient for continuous use.
Electrical comparators are also known as electro- mechanical
measuring systems as these employ an electro- mechanical device which
converts a mechanical displacement into electrical signal.
Block Diagram of Electro-Mechanical Measuring
Linear Variable Differential Transformer (LVDT)
Phase referenced and absolute value of output voltage
TYPE OF ELECTRICAL
Electro limit gauges
ADVANTAGES OF ELECTRICAL
1. The electrical comparators have got small number of
2. It is possible to have a very high magnification and the
same instrument may have two or more magnifications.
Thus the same instrument can be used for various ranges.
3. The mechanism carrying the pointer is very light and not
sensitive to vibrations.
4. As the instrument is usually operated on A.C. supply, the
cyclic vibration substantially reduces errors due to sliding
5. The measuring unit can be made very small and it is not
necessary that the indicating instrument be close to the
measuring unit, it can be remote also.
It requires an external agency to operate i.e.,
the A.C. electrical supply. Thus the
variations in the voltage or frequency of
electric supply may affect the accuracy.
2. Heating of coils in the measuring unit may
cause zero drift and alter the calibration.
3.This is usually more expensive than
Systems of Pneumatic Gauges:-
Based on physical phenomena on which the
operation of pneumatic gauges is based, these
may be classified as:
1. Flow or Velocity type.
2. Back pressure type.
Air gauging has rapidly increased during some past time
due to the following important characteristics:
Very high amplifications are possible. It can be used to
measure diameters, length, square ness, parallelism,
concentricity, taper, centre distance between holes and
other geometric conditions.
As no physical contact is made either with the setting
gauge or the part being measured, there is no loss of
accuracy because of gauge wear. For this reason, air
spindle and air snap gauges last very long. Also very soft
parts which are easily scratched, can be gauged.
Internal dimensions can be readily measured not only
with respect to tolerance boundaries but also geometric
form. In other words, while measuring a bore it can reveal
complete story of size, taper, straightness, camber and bell
It is independent of operator skill.
High pressure air gauging can be done with cleansing of
the parts which helps to eliminate errors due to dirt and foreign
gauging pressures can be kept sufficiently low to prevent
Dimensional variations throughout the length of shaft or
cylinder bore can be explored for out of roundness, taper ness,
concentricity, regularity and similar conditions.
Not only it measures the actual size, but it can also be used
to salvage oversized pieces for rework or to sort out for selective
assembly, i.e., it is suitable both for variable inspection
(measurement of size) and attribute inspection (GO and NO GO)
gauging and limits.
It is accurate, flexible, reliable, universal and speedy
device for inspecting parts in mass production.
ADVANTAGES OF PNEUMATIC
1. The gauging member does not come into contact with
the part to be measured and hence practically no
wear takes place on the gauging member.
2. It has usually very small number of moving parts
and in some cases none. Thus the accuracy is more
due to less friction and less inertia.
3. Measuring pressure of air helps in cleaning the dust,
if any, from the part to be measured.
4. It is possible to have very high magnification.
5. It is very suitable device for measuring diameter of
holes where the diameter is small compared with the
6. It is probably the best method for determining the
taperness of the circular bores.
1. It requires elaborate auxiliary equipment such as
accurate pressure regulator.
2. The scale is generally not uniform.
3. When indicating device is the glass tube, then high
magnification is necessary in order to avoid the
4. The apparatus is not easily portable and is rather
elaborate for many industrial applications.
5. Different gauging heads are required for different