Metrology
The Science of
Measurement
By,
Afaqahmed M J
AIKTC
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Syllabus TH + PR + OR
Module1
1.1 Introduction to Metrology, Fundamental principles and
definitions, measurement standards / primary and tertiary
standards, distinction between precision and accuracy.
1.2 Limits, fits and tolerances, Tolerance grades, Types of fits,
IS919, GO and NO GO gauges- Taylors principle, design of
GO and NO GO gauges, filler gauges, plug gauges and snap
gauges.
Module 2
2.1 Comparators: Constructional features and operation of
mechanical, optical, electrical/electronics and pneumatic
comparators, advantages, limitations and field of applications.
2.2 Principles of interference, concept of flatness, flatness
testing, optical flats, optical interferometer and laser
interferometer.
2.3 Surface texture measurement: importance of surface
conditions, roughness and waviness, surface roughness
standards specifying surface roughness parameters- Ra, Ry, Rz,
RMS value etc., surface roughness measuring instruments
Tomlinson and Taylor Hobson versions, surface roughness
symbols. Page 2
Module 3
3.1 Screw Thread measurement: Two wire and three wire
methods, floating carriage micrometer.
3.2 Gear measurement: Gear tooth comparator, Master gears,
measurement using rollers and Parkinsons Tester.
3.3 Special measuring Equipments: Principles of measurement
using Tool Makers microscope, profile projector & 3D coordinate
measuring machine.
Module 4
Quality Control
Introduction, definition and concept of quality & quality control,
set up policy and objectives of quality control, quality of design
and quality of conformance, compromise between quality & cost,
quality cost and planning for quality.
Module 5
SQC and SQC tools
Importance statistical methods in QC, measurement of statistical
control variables and attributes, pie charts, bar charts/ histograms,
scatter diagrams, pareto chart, GANT charts, control charts, X
chart, X bar charts, R charts, P charts, np charts their preparation,
analysis and applications. Elementary treatment on modern SQC
tools.
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Module 6
Sampling Techniques
Sampling inspection and basic concepts, OC curves,
consumer & producer risk, single & double sampling plans
and use of sampling tables.
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What is metrology?
The science of measurement
(not weather!)
Metrology establishes the
international standards for
measurement used by all
countries in the world in both
science and industry.
Examples: distance, time, mass,
temperature, voltage, values of
physical and chemical constants
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Lets take a trip back in
time
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Prehistoric people
didnt have time to
measure
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But over time.
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People started growing
food
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And developing permanent
settlements
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So, they desired a
system of
measurement
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Or there would be
pandemonium (chaos) !
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History of Measurement
Wall and Leather Painting
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Fast-forward 5000 years
to current measurement
systems
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Career in Metrology
1.Scientific Metrology
Organization and development of
measurement standards and their
maintenance (highest level)
NIST Atomic Clock
Accurate up to 1s / 20
million years
National Institute
Standard &
Technology
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Industrial Metrology
Adequate functioning of measurement
instruments used in industry as well as
production and testing processes
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Who Needs
Accurate Measurements?
Pharmaceutical Industry
Metrology laboratories test weights and
volume standards for pharmaceutical
companies
Products include medicines like aspirin,
antibiotics, vaccines, insulin, & vitamins
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Defense Industry
Metrology laboratories test standards for
many military and defense companies
These companies make the guidance
systems for the Patriot missiles and other
things that are
top secret
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Space Station,
Satellites.
Metrology laboratories test standards for
many companies that provide parts of the
space shuttle
These parts include the metal, heat shield,
electronics, fabrics, o-rings, optics, and
tires
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Standards of Measurement
A standard is defined as something
that is set up and established by an
authority as rule of the measure of
quantity, weight, extent, value or
quality.
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1. Primary standards:
They are material standard preserved under
most careful conditions. These are not used
for directly for measurements but are used
once in 10 or 20 years for calibrating
secondary standards. Ex: International
Prototype meter, Imperial Standard yard.
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1. Secondary standards:
These are close copies of primary standards w.r.t
design, material & length. Any error existing in
these standards is recorded by comparison with
primary standards after long intervals. They are
kept at a number of places under great supervision
and serve as reference for tertiary standards. This
also acts as safeguard against the loss or
destruction of primary standards.
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Tertiary standards:
The primary or secondary standards exist as the
ultimate controls for reference at rare intervals.
Tertiary standards are the reference standards
employed by National Physical laboratory (N.P.L)
and are the first standards to be used for reference
in laboratories & workshops. They are made as
close copies of secondary standards & are kept as
reference for comparison with working standards.
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4. Working standards
These standards are similar in design to primary,
secondary & tertiary standards. But being less in
cost and are made of low grade materials, they are
used for general applications in metrology
laboratories.
Sometimes, standards are also classified as;
Reference standards (used as reference
purposes)
Calibration standards (used for calibration of
inspection & working standards)
Inspection standards (used by inspectors)
Working standards (used by operators)
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Seven base units:
Length: meter (m)
Mass: kilogram (kg)
Time: second (s)
Electric current: ampere (A)
Thermodynamic temperature: kelvin (K)
Amount of substance: mole (mol)
Luminous intensity: candela (cd)
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Accuracy:
How close you are to the actual
value
Depends on the person
measuring
Calculated by the formula:
% Error = (YV AV) x 100 AV
Where: YV is YOUR measured Value & AV is the
Accepted Value
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Precision:
How finely tuned your
measurements are or how close
they can be to each other
Depends on the measuring tool
Determined by the number of
significant digits
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Accuracy & Precision may be
demonstrated by shooting at a
target.
Accuracy is represented by
hitting the bulls eye (the
accepted value)
Precision is represented by a
tight grouping of shots (they are
finely tuned)
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ACCURACY ACCURACY with PRECISION
PRECISION without ACCURACY No ACCURACY, No PRECISION
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LIMITS, FITS &
TOLERANCES
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TERMINOLOGY
NOMINAL SIZE: It is the size of
a part specified in the drawing.
BASIC SIZE: It is the size of a
part to which all limits of variation
are determined. Or It is the
theoretical size from which limits
of size are derived by the
application of allowances and
tolerances.
ACTUAL SIZE: It is the actual
measured dimension of a part.
Nominal and basic size are often
the same.
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DEVIATION
LOWER UPPER
DEVIATION: It is DEVIATION: It is
the algebraic the algebraic
difference between difference between
the minimum limit of the maximum limit
size and the basic and the basic size.
size. Page 32
LIMIT OF SIZES
There are two
extreme possible
sizes of a
component.
The largest
permissible size for
a component is
called upper limit
and smallest size is
called lower limit.
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BASIS OF LIMIT
SYSTEM
SHAFT BASIS SYSTEM:
In this system, the shaft is kept
as constant member and different
fits are obtained by varying the
hole size.
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BASIS OF LIMIT SYSTEM
HOLE BASIS SYSTEM:
In this system, the hole is kept as
a constant member and
different fits are obtained by
varying the shaft size.
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ZERO LINE
It is the straight line
corresponding to the basic size.
The deviations are measured
from this line.
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Tolerance
Tolerance is the total amount that a
specific dimension is permitted to
vary;
It is the difference between the
maximum and the minimum limits for
the dimension.
For Example a dimension given as
1.625 .002 means that the
manufactured part may be 1.627 or
1.623, or anywhere between these
limit dimensions.
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Tolerances
The Tolerance is 0.001 for the Hole as
well as for the Shaft
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POSITIONAL TOLERANCES
Two types of positional
tolerances are used:
1. Unilateral tolerances
2. Bilateral tolerances
When tolerance is on one side
of basic size, it is called
unilateral and if it is both in
plus and minus then it is known
as bilateral tolerance.
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Specifications of Tolerances
1. Limit Dimensioning
The high limit is placed above the
low limit.
In single-line note form, the low limit precedes the high limit
separated by a dash
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Cumulative Tolerances
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International Tolerance Grade (IT):
`
They are a set of tolerances that varies
according to the basic size and provides a
uniform level of accuracy within the grade.
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IS919 ACT ( REVISED)
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Fits Between Mating Parts
Fit is the general term used to signify the
range of tightness or looseness that may
result from the application of a specific
combination of allowances and
tolerances in mating parts.
There are four types of fits between parts
1. Clearance Fit: an internal member fits in
an external member (as a shaft in a hole)
and always leaves a space or clearance
between the parts.
Minimum air space is 0.002. This is the allowance and is
always positive in a clearance fit Page 46
2. Interference Fit: The internal member is
larger than the external member such
that there is always an actual interference
of material. The smallest shaft is 1.2513
and the largest hole is 1.2506, so that
there is an actual interference of metal
amounting to at least 0.0007. Under
maximum material conditions the
interference would be 0.0019. This
interference is the allowance, and in an
interference fit it is always negative.
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3. TRANSITION FIT:
In this type of fit, the limits for the
mating parts are so selected that either
a clearance or interference may occur
depending upon the actual size of the
mating parts.
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PLAIN GAUGES
Gauges are inspection tools which serve to
check the dimensions of the manufactured
parts. Limit gauges ensure the size of the
component lies within the specified limits.
They are non-recording and do not
determine the size of the part. Plain gauges
are used for checking plain (Unthreaded)
holes and shafts.
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Plain gauges may be classified as follows;
According to their type:
(a) Standard gauges - are made to the nominal
size of the part to be tested and have the
measuring member equal in size to the mean
permissible dimension of the part to be
checked. A standard gauge should mate with
some snugness (Comfort).
(b) Limit Gauges These are also called go and
no go gauges. These are made to the limit
sizes of the work to be measured. One of the
sides or ends of the gauge is made to
correspond to maximum and the other end to
the minimum permissible size. The function of
limit gauges is to determine whether the actual
dimensions of the work are within or outside
the specified limits. Page 50
LIMIT GAUGING
Limit gauging is adopted for checking parts
produced by mass production.
It has the advantage that they can be used
by unskilled persons. Instead of measuring
actual dimensions, the conformance of
product with tolerance specifications can be
checked by a GO and NO GO gauges.
A GO gauge represents the maximum
material condition of the product (i.e.
minimum hole size or maximum shaft size)
and conversely a NO GO represents the
minimum material condition (i.e. maximum
hole size or minimum shaft size)
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1.Plug Gauge
Plug gauges are the limit gauges used
for checking holes and consist of two
cylindrical wear resistant plugs.
The plug made to the lower limit of the
hole is known as GO end and this will
enter any hole which is not smaller than
the lower limit allowed.
The plug made to the upper limit of the
hole is known as NO GO end and this will
not enter any hole which is smaller than
the upper limit allowed.
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1. Plug Gauge
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2. Ring Gauge
Ring gauges are used for gauging
shafts.
They are used in a similar manner to
that of GO & NO GO plug gauges.
A ring gauge consists of a piece of
metal in which a hole of required size
is bored.
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SNAP (or) GAP GAUGES:
A snap gauge usually consists of a plate or
frame with a parallel faced gap of the
required dimension.
Snap gauges can be used for both
cylindrical as well as non cylindrical work as
compared to ring gauges which are
conveniently used only for cylindrical work.
Double ended snap gauges can be used for
sizes ranging from 3 to 100 mm.
For sizes above 100 mm upto 250 mm a
single ended progressive gauge may be
used.
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Taylors Principle of Gauge Design:
GO LIMIT
According to Taylor, Go and No Go
gauges should be designed to check
maximum and minimum material limits which
are checked as below; GO Limit.
This designation is applied to that limit of
the two limits of size which corresponds to
the maximum material limit considerations,
i.e. upper limit of a shaft and lower limit of a
hole.
The GO gauges should be of full form, i.e.
they should check shape as well as size.
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No Go Limit:
This designation is applied to that limit of the
two limits of size which corresponds to the
minimum material condition. i.e. the lower limit
of a shaft and the upper limit of a hole.
No Go gauge should check only one part
or feature of the component at a time, so that
specific discrepancies in shape or size can be
detected.
Thus a separate No Go gauge is required
for each different individual dimension.
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Wear Allowance:
The GO gauges only are subjected to wear
due to rubbing against the parts during
inspection and hence a provision has to be
made for the wear allowance. Wear
allowance is taken as 10% of gauge
tolerance and is allowed between the
tolerance zone of the gauge and the
maximum material condition.
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Filler Gauges L, W
Use for precise spacing inspection by
inserting into the gap between two flat
surfaces
Crucial instrument for measuring gap
between piston and cylinder of automotive
engine
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REFERANCES-
For Video
https://www.youtube.com/watch?v=X5xEE6YTpqI
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