Metrology and Quality Control

1. Measurement Standards
By
Prof N D Sadaphal
Assistant Professor
Sanjivani College of Engineering,
Kopargaon (Maharashtra State) 423601
Mechanical Engineering

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Mechanical
Engineering
COE, Kopargaon
Prof N D Sadaphal
Assistant Professor
Engineering Metrology and Quality Control
Measurement Standards
UNIT- I
METROLOGY
AND MEASUREMENTS

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Metrology.
Metrology defines as the Science of pure
measurement. But in engineering purposes,
it in restricted to measurements of length
and angles and other qualities which are
expressed in linear or angular terms.
Definition of Standards:
• 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”.
• Rule which is universally accepted.

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Line and End standard measurements
• Line standard
Length is expressed as the distance between two lines.
• End standard
Length is expressed as the distance between two flat
parallel faces
• Wavelength standard
Wavelength of monochromatic light is used to measure
length.
• Precision  Degree of repetitiveness. If an instrument
is not precise it will give different results for the same
dimension for the repeated readings.
• Accuracy  The maximum amount by which the result
differ from true value(ie) Closeness to true value
Terminology in Measurment

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• Accuracy
Accuracy is how close a measured value is to the actual
(true) value.
• Precision
Precision is how close the measured values are to each
other.
Examples of
Precision and
Accuracy:
Characteristics of Measuring Instruments
• Accuracy
• Precision
• Sensitivity-
The sensitivity denotes the smallest change in the measured variable
to which the instrument responds
• Resolution-
The least count of any instrument is taken as the resolution of the instrument.
• Stability-
It is the ability of an instrument to retain its performance throughout is
specified operating life.
• Range or span-
The minimum & maximum values of a quantity for which an instrument is
designed to measure is called its range or span.

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Gauge R&R
Repeatability and Reproducibility in measurement systems
Repeatability-
The ability of an operator to consistently repeat the same measurement of the same part, using the
same gage, under the same conditions.
Operator 1 measures a single part with Gage A 20 times, and then measures the same part with Gage B.
The solid line is the measurements from Gage A. The dashed line is the measurements from Gage B. Gage
A has less variation, so it is more repeatable than Gage B.
Gauge R&R
Repeatability and Reproducibility in measurement systems
Reproducibility
The ability of a gage, used by multiple operators, to consistently reproduce the same measurement of the
same part, under the same conditions.
Operators 1, 2, and 3 measure the same part 20 times with the same gage.
The three lines are the measurements from Operator 1, 2, and 3. The variation in average measurements
between Appraisers 1 and 2 is much less than the variation between Appraisers 1 and 3. Therefore, the
gauge's reproducibility is too low.

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What/why is a gage R&R study?
• A gage R&R study helps you investigate:
 Whether your measurement system variability is small compared with the
process variability.
 How much variability in the measurement system is caused by differences
between operators.
 Whether your measurement system is capable of discriminating (good
judgement) between different parts.
 For example, several operators measure the diameter of screws to ensure that
they meet specifications. A gage R&R study indicates whether the inspectors
are consistent in their measurements of the same part (repeatability) and
whether the variation between inspectors is consistent (reproducibility).
. Classification of measuring Instruments.
According to the functions:
• Length measuring instrument
• Angle measuring instrument
• Instrument for checking deviation from geometrical
forms
• Instrument for determining the quality of surface
finish.

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Linear measuring instruments
• Straight edge (Steel rule)
• Outside caliper
• Inside caliper
• Vernier caliper
• Outside micrometer
• Inside micrometer
• Vernier height gauge
• Vernier depth gauge
• Dial gauges
Angular measurements
• Measuring the angle of Taper.
1. Bevel Protractor
2. Tool Makers microscope
3. Sine bar
4. Auto Collimator
5. Sine Centre

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Measuring tools and instruments
Direct (contact) measurement
(e.g. micrometer or caliper)
Indirect (non-contact) measurement
(advanced methods such as optical,
ultrasonic, laser, etc.)
h Calipers
h Gauges and Gauge Blocks
h Sine Bar
h Special-purpose tools
h Rules
h Vernier Calipers
h Vernier Gauges
h Micrometers
h Protractors
h Dial Indicators
1
Measuring tools and instruments
Graduated
(either linear or angular
graduations incorporated
into measuring system of
the tool)
Non-graduated
(gauges or adjustable
tools which compare
the measurements)
Imperial steel rule with various lengths
having graduations on each side
Same rule with relatively larger
graduations
Metric steel rule with various lengths
having graduations on each side
resolution?
How to read a rule:
h A = 12 mm (12th graduation)
h B = 22 mm (22nd graduation)
hC = 31.5 mm (between
hD = 40.5 mm (between
31st
40th
and 32nd)
and 41st)
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Graduated Linear Measurement - Rules

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Graduated Linear Measurement - Vernier Calipers
Direct reading of an internal length
using digital Vernier caliper Direct reading of an external length
using digital vernier caliper
Vernier caliper with a dial indicator 4
Graduated Linear Measurement - Vernier Calipers

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Designed for use in toolrooms, workshops, inspection departments to measure or mark off vertical
heights and locating center distances.
Standard Height gauge Dial Height Gauge Digital Height Gauge
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Graduated Linear Measurement - Vernier Height Gauges
Designed for use in toolrooms, workshops, inspection departments to measure depths of holes, slots,
recesses, and so on.
Standard Depth Gauge Dial Depth Gauge Digital Depth Gauge
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Graduated Linear Measurement - Vernier Depth Gauges

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THIMBLE READINGVERNIER READING
Metric Micrometer
SLEEVE (BARREL) READING
Metric Vernier
Micrometer
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Graduated Linear Measurement - Outside Micrometers
V-anvil Micrometer (measuring odd-fluted taps, milling
cutters, reamers, and checking out of roundness)
Dial-indicating Micrometer
Screw Thread Micrometer
(measuring pitch diameter
of screw threads)
Direct-reading
Micrometer
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Graduated Linear Measurement - Outside Micrometers

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Standard Inside Micrometers Digital Inside Micrometers
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Graduated Linear Measurement - Inside Micrometers
h Standard calipers have a fine adjustment screw and a quick-adjusting spring nut.
h Accuracy obtained with these tools depends mostly on the inherent skill of users.
h The measurements are carefully transferred to a graduated measuring tool.
Caliper for inside
measurement
Caliper for outside
measurement
Caliper used
as a divider
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Non-Graduated Linear Measurement - Calipers

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Screw Pitch Gauges (consisting of a metal case containing
many separate leaves. Each leaf has teeth corresponding to
a definite pitch. By matching the teeth with the thread on
work, the correct pitch can be read directly from the leaf)
Tap and Drill Gauges (consisting
of a flat rectangular steel plate with
holes accurately drilled and
identified according to their size)
Radius Gauges (available as individual leaves and each
leaf is marked with its radius. They are designed to check
both convex and concave radii)
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Non-Graduated Linear Measurement - Special Purpose Gauges
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Non-Graduated Linear Measurement - Rectangular Gauge Blocks
Slip Gauge Box

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(a)
(b)Simple
Protractor
(measuring
angles from
0 to 180º) Universal Bevel Protractor (main
scale consists of 4 portions of 90º)
Measuring acute (a) and
obtuse (b) angles
How to read an angle on a bevel protractor:
Main div. = 1º = 60´
Vernier div. = 1/12th of main div. ≈ 0.0833º = 5´
h The highest figure: 50 * (main div.) = 50º
h The matching figure: 4 * (vernier div.) ≈ 0.333º = 20´
h The final reading is: ≈ 50.333º or 50º 20´
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Graduated Angular Measurement - Protractors
*
Limitations of Sine Bar:
Maximum angle 45°
Sine Center - 60° 18
Non-Graduated Angular Measurement - Sine Bar

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Calibration
• Calibration is one of the primary processes used to maintain
instrument accuracy.
• Calibration is the process of configuring an instrument to provide a
result for a sample within an acceptable range.
• Measurement of Accuracy.
• Establishment the relation of an instrument’s accuracy to the
international standard.
Success of Calibration
• Consistency of results obtained
Need of Calibration
• Quality control & quality assurance in production.
• To meet requirement of ISO
• To comply with requirement of global market.
• To promote international recognition.
Benefits of Calibration
• Fulfils requirement of ISO 9000, ISO 14000.
• As a proof that the instrument is working properly.
• Confidence in using instrument.
• Reduce rejection, failure rate.
• Improved product & service quality leading to satisfied
customer.
• Cost saving, safety.

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Types and sources of ERRORS
Systematic Errors
• Systematic errors are regularly repetitive and can be
eliminated.
• They results from improper condition or procedure of
experiment .
• These error can be controlled & reduced if properly
analyze, so called as Controllable errors.
Errors may be of four kinds;
1. Instrumental : For example, a poorly calibrated
instrument such as a thermometer that reads 102° C
when immersed in boiling water and 2°C when
immersed in ice water at atmospheric pressure. Such a
thermometer would result in measured values that are
consistently too high.
2. Observational : For example, parallax in reading a
meter scale.
3. Environmental : Variation in atmospheric
condition i.e. temperature, pressure etc. at place of
measurement.
4. Stylus pressure :
Variation in Force applied by anvils of micrometer
on component to be measured results in different
reading.

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Random Errors
• Random errors in experimental measurements are caused by unknown
and unpredictable changes in the experiment. These changes may occur in
the measuring instruments or in the environmental conditions.
• Sources of random errors cannot always be identified. Possible sources of
random errors are small variations in the position of setting standards and
work piece, slight displacement of lever joints in the measuring joints in
the measuring instrument.
• Examples of causes of random errors are:
1. electronic noise in the circuit of an electrical instrument,
2. Irregular changes in the heat loss rate from a solar collector due to
changes in the wind.
• These error cannot be eliminated.
1. Observational : For example, errors in judgment of an observer when
reading the scale of a measuring device to the smallest division.
2. Environmental : For example, unpredictable fluctuations in line voltage,
temperature, or mechanical vibrations of equipment.
Parallax Error :
• Parallax is a displacement
or difference in the apparent
position of an object viewed
along two different lines of sight,
and is measured by the angle or
semi-angle of inclination
between those two lines.

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Comparators
• Classification of comparators
1. Mechanical
2. Electrical and Electronics comparators
3. Optical comparators
4. Pneumatic comparators
5. Electro-Mech. Comparators.