CHAPTER I

Metrology and Control

មាត្រសាស្រ្ត និង ការត្រួ រនិនិរ

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I. Metrology
➢ Metrology is the science of measurements. It deals with
establishment, reproduction, conservation, and transfer of units of
measurements and their standards.

➢ Metrology consists of comprehensive study of different measuring

instruments for defining precision, accuracy, possible sources of
errors, and methods for elimination of errors to improve accuracy.

➢ Raw material received by a manufacturing industry from an external

source (supplier) is processed by number of operations carried out
on various machines to give a final finished product, which can be
sold to customers in market.

➢ During entire process of transformation of raw material into finished

product, inspection is carried out for inspecting the product being
processed at various stages of manufacturing.
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II. Scope of Metrology
➢ Manufacturing: Metrology is concerned with the manufacturing of
various instruments.
➢ Range and capabilities: Metrology is used to find the ranges and
capabilities of various instruments used for measurement.
➢ Calibration: Metrology is used to calibrate the measuring
instruments according to the prescribed standards with a high
degree of accuracy.
➢ Methods of measurement: Metrology is concerned with the
different methods of measurements, essential to obtain precise
measurements.
➢ Preparation of design and drawings: Metrology is concerned with
the preparation of design and drawings for different types of gauges.
➢ Maintaining and defining the standards: For accurate
measurements, metrology is concerned with defining and
maintaining the standards.

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III. Notion of measure
All most of the work pieces have neither exactly dimension
value nor perfect surface. The dimension values are
function with tolerances, correction geometric of the
surface, surface condition and so on.

➢The purposes of metrology are measurement and

verification or comparison work pieces for assure the
condition function.

➢Metrology is used with all the mechanical work, optic,

pneumatic, or electric that can be assure the control.

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IV. Quality of instruments in measurement

➢ Control is the range of the biggest measure for

work piece.

➢ It indicates in measurement instrument that

doesn’t have high error and maximum tolerance.

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IV.1. Sensitivity of instrument in measure
➢ Sensitivity is defined as the ability of a measuring device
to detect small variations in a quantity being measured.
Higher the ability of such detection of an instrument,
more sensitive it is.

➢ The external parameters include temperature of

surrounding atmosphere, vibrations and so on.

➢ Too much sensitivity leads to affect the working of

instrument. It will adversely affect the accuracy and
precision of measurements.

Ex. A wooden yardstick has far less sensitivity than a finely

graduated steel rule.
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 IV.2. Fidelity
The quality of apparatus is the accurate value of the
measurement. However, the exact value dimensions
can not be get. There is fidelity errors which are sum
of many errors such as error parallel and a long
position.

e = vmax - vmin

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 IV.3. Justice
➢ It is sum of error systematic on instrument measurement (ex.
Error zero, graduation …).

Where, vm is value measure c = vm - v

v is the real value
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IV.4. Accuracy
➢ Accuracy compares the measured value of a
manufactured component with its true value.
➢ True valve means the specified value of a component
before start of production
➢ Measured value is the reading obtained after inspecting
the actually produced component.
➢ Accuracy is defined as, the closeness of measured value
of component produced with its true value.
➢ The act of measurement should indicate true dimensions
of a part. But, in actual practice no measurement can be
absolutely accurate.
➢ There is always some error and the amount of error or
inaccuracy depends upon various factors.
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IV.5. Factors Affecting the Accuracy
➢ Calibration standards: accuracy may be affected by ambient
(atmospheric) conditions such as expansion of metal used in
measuring instruments due to rise in temperature, elastic properties
of metals.
➢ Workpiece being measured: accuracy may be affected by the
condition of workplace such as cleanliness, surface condition,
proper arrangement.
➢ Measuring instruments: measuring instruments and hence
accuracy may be affected by sources of errors such as backlash,
friction, zero drift error, use of heavy workpieces.
➢ Human error: the person or inspector carrying out the
measurement may not have the required skill.
➢ Environment influences: changes in atmospheric conditions such
as temperature, vibrations, lightening, pressure.

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IV.6. Precision
➢ Precision has no meaning for only one measurement, but exists only
when number of measurements are carried out for the same
quantity under identical conditions.

➢ Precision is defined as the repeatability of a measuring process.

Example: Let us take a component having true length of 100 mm. The
inspector/worker takes number of readings to measure length of this
component, with the same instrument, in same environmental
conditions and within short intervals of time. If the reading obtained are
100.001, 100.002 and 100.003 mm, then we say that, readings are not
close to true value. Therefore readings are said to be in-accurate.
But, if we calculate the average value or arithmetical mean value of
these readings, we get mean value = 100.002 mm

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IV.7. Difference Between Accuracy and Precision

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IV.7. Difference Between Accuracy and Precision

➢ The centre circle represents true value. The marks X

indicate results/shoot points achieved by the striker after
shooting 6 bullets on the target.

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 V. Errors of Measurement

➢ It is the different result from different readers that is

different from the exact value of measure.

➢ The smallest error, the greatest precision measure.

Ex. The real dimension of the work piece is 50mm and the
value from reader is 50.02mm. The precision of this measure
is 0.02mm. We also call error absolute. Other error call
error relative which is error in percentage.
[(50.02-50)/50]×100 = 0.04%.

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VI. Causes of Errors Metrology

The causes of those errors are originally from:

➢Apparatus

➢Operators

➢ Atmosphere

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 VI.1. Errors of Apparatus

➢ The constant fault of origin position on the

apparatus.

Ex. Caliper, zero position is not at the close position.

➢ Because of:
• Unequal of the intervals graduations.
• Slip of guiding.
• Surface of the contact incorrect.

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VI.1. Errors of Apparatus

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I.4.2. Errors of Operators

➢ Fault appreciation of the correct mark. For

prevent this fault, the operators need to see in
the position of perpendicular to the mark point.

➢ Fault in manipulation:
✓Use to much force on the contact apparatus.
✓Fault of presentation of the apparatus.

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VI.2. Errors of Operators

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VI.3. Errors of Varies Temperatures

➢ Environment temperature in the room measure

is important for the measurement.

➢ In general, the temperature using of the

apparatus is the same as work piece
temperature.

➢ In opposite, dilating of equipment with the work

piece creates fault in measure.

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 VII. Units of Length and Angle in SI System

The units of the length in SI system is metre (m) [mètre].

In fabrication, unit measure is mm.

Table of Length Unites

Symbols nm μm mm cm dm m km

1nm 1 10-3 10-6 10-7 10-8 10-9 10-12

1μm 103 1 10-3 10-4 10-5 10-6 10-9
1mm 106 103 1 10-1 10-2 10-3 10-6
1cm 107 104 10 1 10-1 10-2 10-5
1dm 108 105 102 10 1 10-1 10-4
1m 109 106 103 102 10 1 10-3
1km 1012 109 106 105 104 103 1
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VII. Units of Length and Angle in SI System
Unit of measure angles in system SI is radian (rad).
Ex. Angle of 900 = π/2 rad, angle of 1800 = π rad.
In fabrication, the unit measure is degree ( 0 ).

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VIII. Class of the Control Way

➢ In metrology, the dimensions of the control are obtained

by two ways, measurement and calibration.

By Measure
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VIII. Class of the Control Way

By Plug Gages (gauges)