Introduction to Metrology

MetrologyThe Science of

Measurement

By,

Afaqahmed M J

AIKTC

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.

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

Let’s take a trip back in

time…

Prehistoric people

didn’t have time to

measure

But over time….

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

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

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

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

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 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. Ex: International

Prototype meter, Imperial Standard yard.

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.

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)

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

TERMINOLOGY• NOMINAL SIZE: It is the size of

a part specified in the drawing.

• BASIC SIZE: It is the size of apart to which all limits of variationare determined. Or It is thetheoretical size from which limitsof size are derived by theapplication of allowances andtolerances.

• ACTUAL SIZE: It is the actualmeasured dimension of a part.Nominal and basic size are oftenthe 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.

LIMIT OF SIZES

• There are twoextreme possiblesizes of acomponent.

• The largestpermissible size fora component iscalled upper limitand smallest size iscalled lower limit.

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.

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.

ZERO LINE

• It is the straight line

corresponding to the basic size.

The deviations are measured

from this line.

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 sideof basic size, it is calledunilateral and if it is both inplus and minus then it is knownas 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

Cumulative Tolerances

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.

`

IS919 ACT ( REVISED)

Fits Between Mating PartsFit is the general term used to signify the

range of tightness or looseness that mayresult from the application of a specificcombination of allowances andtolerances in mating parts.

There are four types of fits between parts

1. Clearance Fit: an internal member fits inan external member (as a shaft in a hole)and always leaves a space or clearancebetween the parts.

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. 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.

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.

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.

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.

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.

1. Plug Gauge

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:

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.

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.

Filler Gauges L, WUse 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

REFERANCES-

For Video

https://www.youtube.com/watch?v=X5xEE6YTpqI

Introduction to Metrology

Engineering

Transcript of Introduction to Metrology