This document discusses metrology, which is the science of measurement. Precise and accurate measurements are needed for quality inspection during manufacturing. There are two types of errors in measurement: systematic errors, which are constant due to issues like faulty instruments, and random errors, which are unpredictable due to external factors. Systematic errors cannot be eliminated by repeated measurements but can be reduced by calibration, while random errors can be minimized through repetition and require statistical analysis to characterize. The objectives of metrology are to ensure components meet specifications and identify sources of error.

1. INTRODUCTION
• METROLOGY
• WHY WE NEED INSPECTION?
• PRECISION AND ACCURACY
• OBJECTIVES OF METROLOGY AND MEASUREMENTS
• ERRORS
• SYSTEMATIC AND RANDOM ERRORS
• DIFFERENCE BETWEEN SYSTEMATIC AND RANDOM
ERRORS

2. METROLOGY
 The word metrology is derived from Greek word ‘metrologia. It literally
means science of measurements. Measurements are generally done for
industrial inspections.
 Field of metrology also deals with establishing units for measuring,
analysing the accuracy of methods of measurements, determining most
accurate methods to measure, investigating causes of measuring errors and
eliminating them.

3. WHY WE NEED INSPECTION?
 Inspection is required to conform the manufactured product is according to
the design specification. It helps to evaluate quality or specific design
attribute of component or product.
 As now a days importance of inspection is significant because of mass
production, which involves interchangeability of parts. This various
components are made at different locations and then assembled at another
location. So it is essential that parts must be assembled that satisfy mating of
any pair chosen randomly.
 In order to achieve this, parts must be produced withing there tolerance limit.
To ensure that continuous inspection is to be done of parts produced in
mass.

4. PRECISION AND ACCURACY
 Precision can be said as degree of repetitiveness of the measuring process. It is a degree of agreement
of repeated inspection of parts produced with same method under similar condition to ensure quality.
 Accuracy can be said as degree of agreement of measured dimension within its tolerance. It can also
be expressed in terms of result difference between actual value and measured value, of the expressed
in percentage.
 a) Impression of dots are precise but not accurate.
 b) Impression of dots are accurate but not precise.
 c) Impression of dots are accurate and precise.
 d) Impression of dots are nether accurate nor precise.
Fig1

5. OBJECTIVES OF MEASUREMENTS AND
METROLOGY
 To ensure and evaluate whether newly manufactured components are
according to the design and within tolerance limit.
 To design gauges and fixtures for rapid and accurate inspection.
 To identify and eliminate sources if errors.
 To adopt manufacturing methods with minimum scrap rate and rework.
 To maintain accuracy in measurement by calibrating measuring instruments
at regular interval.
 To manufacture components with better approval rate in terms of tolerance
limit.

6. ERRORS
 Its appropriate to assume there will be various uncertainty associated while
performing measurements, and it is important to note that measured values
are not accurate. To understand nature of error it is essential to analyse
measured data. Two broad categories of errors in measurement are:
 Systematic Errors
 Random Errors

7. SYSTEMATIC AND RANDOM
ERRORS
 Systematic Errors – It is a type of error which are constant/random in nature and are caused due to
faulty measuring device or faulty design. This type of error can be controlled by its magnitude and
direction by analysing all the sources of error. Following are the reasons due to which systematic error
occurs:
 Calibration error
 Ambient conditions
 Faulty instrument
 Deformed workpiece
 Random Errors – It is a type of error which are unpredictable in nature and are not constant. Following
external factors are likely sources of random errors:
 Inability of person to note readings.
 Factors such as temperature, noise, ambience etc also generates random errors.
 Person’s ability to judge scale.
 User interface of measuring device.

8. DIFFERENCE BETWEEN SYSTEMATIC
AND RANDOM ERRORS
Systematic Errors Random Errors
Cannot be eliminated by repeated
inspection.
Can be minimized by repeated
inspection.
Can be analysed and assessed easily. Require statistical analysation.
Difficult to detect source of error. Source of error can be detected easily.
Can be reduced by calibrating
measuring instrument
No effect of calibration
Characterization is not necessary Errors need to characterized by mean,
standard deviation and variance

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