Introduction to Metrology

Metrology
The Science of
Measurement
By,
Afaqahmed M J
AIKTC
Slides are subjected to copyright©

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- Taylor’s 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.

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 Parkinson’s Tester.
3.3 Special measuring Equipments: Principles of measurement
using Tool Maker’s 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.

Module 6
Sampling Techniques
Sampling inspection and basic concepts, OC curves,
consumer & producer risk, single & double sampling plans
and use of sampling tables.

Let‟s take a trip back in
time…

Prehistoric people
didn‟t have time to measure

People started growing
food

And developing permanent
settlements

So, they desired a
system of
measurement…

Or there would be
pandemonium (chaos) !

History of Measurement
Wall and Leather Painting

Fast-forward 5000 years
to current measurement
systems

METROLOGY?????
Metrology
Mechanical
Measurements

METROLOGY????
Greek Word
Metro – Measurement
Logy - Science
Standardization

METROLOGY and
CONCERN
Establishing
Units of
Measurement
Developing
Methods of
Measurement
Analyzing
Accuracy

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

Accuracy
Cost of
Accuracy
Precision
Reliability Calibration
Response
time
Metrological Terms

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

Industrial Metrology
– Adequate functioning of measurement
instruments used in industry as well as
production and testing processes

Pharmaceutical
Industry
• Metrology laboratories test weights and
volume standards for pharmaceutical
companies
• Products include medicines like aspirin,
antibiotics, vaccines, insulin, & vitamins

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 aretop secret

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

STANDARDS ???
SI (Act 89, 1956)
BIS
ISO For Engineering
matters

BIS ???
1
• Formulation, Publication,
Promotion, Inspection for
Certification
2
• Establishment, Formulation,
coordination and
Maintenance
3
• Information, Documentation
and recognition

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”.

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.
 At the highest level, a primary reference standard is
assigned a value by direct comparison with the reference
base.
 Ex: International Prototype meter, Imperial Standard
yard.

2. 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.

3. 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.

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)

STANDARDS OF MEASUREMENT
LINE
END
WAVELENGTH

LINE STANDARD
According to it, yard or metre is defined
as the distance between scribed lines on a
bar of metal under certain conditions of
temperature and support.
Yard  The Imperial Standard Yard is a
bronze bar of one inch square cross-section
and 38 inches long.
METRE  This is the distance between the
centre portions of two lines engraved on the
polished surface of a bar of pure platinum-
iridium alloy (90% platinum and 10% iridium).

END STANDARDS
Is expressed as the distance between
two surfaces, this is referred to as end
standard.
For all the important works in the shop,
the users prefer end standards e.g. slip
gauges, length bars, the ends of
micrometre anvils, gap gauges, and so
on.
Dimensional tolerance as small as
0.0005 mm can be obtained.
They are not subjected to parallax effect
as their use depends on “feel”.
Well suited to measurements of close
tolerance.
Lethargic

WAVELENGTH
A meter is defined as equal to
1670763.73 wavelength of red-
orange radiations of Krypton
isotope 86 gas .
Length of path travelled by light in
vacuum in 1/299792458 second.
Higher Accuracy
Environment-friendly
Reproducible
No Wear and Tear

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)

• 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

• 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

• 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)

ACCURACY ACCURACY with PRECISION
PRECISION without ACCURACY No ACCURACY, No PRECISION

LIMITS, FITS &
TOLERANCES

LIMITS
FITS
&
Toleranc
es
Based on Standards,
Sufficient in sizes
Both Uni and Bi lateral
Tolerances shd b accptbl
Carefully designed

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.

DEVIATION
• LOWER
DEVIATION: It is
the algebraic
difference between
the minimum limit of
size and the basic
size.
• UPPER
DEVIATION: It is
the algebraic
difference between
the maximum limit
and the basic size.

LIMITS OF SIZE
Considerations for Deciding LIMITS
Functional
Req
Interchan
geability
Economi
cs

LIMITS OF SIZE
 20 grades of Fundamental Tolerances and 28 types of
Std. Deviations

Basic size and Zero line

• 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.
LIMITS OF SIZE

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.
 HOLE BASIS SYSTEM:
In this system, the hole is kept as
a constant member and
different fits are obtained by
varying the shaft size.

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.

Tolerances
The Tolerance is 0.001” for the Hole as
well as for the Shaft

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.

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

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.
`

Numerical on
Tolerances

Diameters specified by ISI
1-3, 3-6, 6-10, 10-18, 18-30, 30-50, 50-80, 80-120, 120-180,
180-250….. 2500-3000mm
D is Avg. Size
Question
Calculate the limits of tolerance and allowance for 25 H8d9.

FITS
Range of tightness or looseness

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 Three 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. (Largest
Shaft, Smallest Hole)
Minimum air space is 0.002”. This is the
allowance and is always positive in a
clearance fit.

2. Interference Fit: The internal member is
larger than the external member such
that there is always an actual interference
of material. Min Dia of shaft > Max
allowable diameter of hole.
Appln- Bearing Bush, Small end of Conn Rod

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. +ve and –ve clearance is
employable

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.

ADVANTAGES
Free from Errors
Portable and Independent on
Power
No auxiliary setup required
Check Combination in
Dimensions
Not expensive

FEELER RADIUS
DRILL N
WIRE
PLATE STANDARD LIMIT
TYPES OF GAUGES

FEELER GAUGE
Use for precise spacing inspection
by inserting into the gap between two
flat surfaces
 Narrow strip of sheet (steel made)
Work on sense of feel
It starts from 0.04 – 1mm
Each blade is marked with thickness

1.
2.
3.
1.Radius Gauge
2.Drill / Wire Gauge
3.Plate Gauge

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)

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.

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.

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.

Taylor’s Principle of Gauge Design

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.

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.

Introduction to Metrology

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