Brief Contents
PART I: ENGINEERING METROLOGY METROLOGY
1. Basic Principles of Engineering Metrology
2. Standards of Measurement
3. Limits, Fits, and Tolerances
4. Linear Measurement
5. Angular Measurement
6. Comparators
7. Optical Measurement and Interferometry
8. Metrology of Gears and Screw Threads
9. Metrology of Surface Finish
10. Miscellaneous Metrology
11. Inspection and Quality Control
PART II: MECHANICAL MEASUREMENTS
12. Measurement Systems
13. Transducers
14. Measurement of Force, Torque, and Strain
15. Measurement of Temperature
16. Pressure Measurements
PART III: NANO IMPACT ON METROLOGY
17. Nanometrology
Appendix A: Universal Measuring Machine
Appendix B: Flow Measurement
Appendix C: Laboratory Experiments
Appendix D: Control Chart Factors
ABOUT THE BOOK
Engineering Metrology and Measurements is a core subject for mechanical, production, and allied disciplines in all the major universities in India. Although there are a few good books available on metrology, the coverage of topics on mechanical measurements is either scanty or superficial, necessitating students to refer to different books on mechanical measurements. This book provides a comprehensive coverage of both metrology and mechanical measurements.
Divided into three parts, the first part of the book comprising Chapters 1–11, begins with a comprehensive outline of the field of engineering metrology and its importance in mechanical engineering design and manufacturing. The basic concepts of engineering standards, limits, fits, and tolerances, for ensuring interchangeability of machine components are then discussed.
Some details about Metrology
METROLOGY
Metrology literally means science of measurements. In practical applications, it is the enforcement, verification, and validation of predefined standards. Although metrology, for engineering purposes, is constrained to measurements of length, angles, and other quantities that are expressed in linear and angular terms, in a broader sense, it is also concerned with industrial inspection and its various techniques. Metrology also deals with establishing the units of measurements and their reproduction in the form of standards, ascertaining the uniformity of measurements, developing methods of measurement, analysing the accuracy of methods of measurement, establishing uncertainty of measurement, and investigating the causes of measuring errors and subsequently eliminating them.
The word metrology is derived from the Greek word ‘metrologia, which means measure.
Metrology has existed in some form or other since ancient times. In the earliest forms of metrology, standards used were either arbitrary or subjective, which were set up by regional or local authorities, often based on practical measures like the length of an arm.
It is pertinent to mention here the classic statement made by Lord Kelvin (1824–1907), an eminent scientist, highlighting the importance of metrology: ‘When you can measure what you are speaking about and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge of it is of a meagre and unsatisfactory kind. It may be the beginning of knowledge, but you have scarcely in your thought advanced to the stage of science.’
FITS
Manufactured parts are required to mate with one another during assembly. The relationship between the two mating parts that are to be assembled, that is, the hole and the shaft, with respect to the difference in their dimensions before assembly is called a fit. An ideal fit is required for proper functioning of the mating parts. Three basic types of fits can be identified, depending on the actual limits of the hole or shaft:
1. Clearance fit
2. Interference fit
3. Transition fit
1. Clearance fit : The largest permissible diameter of the shaft is smaller than the diameter of the smallest hole. This type of fit always provides clearance. Small clearances are provided for a precise fit that can easily be assembled without the assistance of tools. When relative motions are required, large clearances can be provided, for example, a shaft rotating in a bush. In case of clearance fit, the difference between the sizes is always positive. The clearance fit is described in Fig. 3.9.
2. Interference fit : The minimum permissible diameter of the shaft exceeds the maximum allowable diameter of the hole. This type of fit always provides interference. Interference fit is a form of a tight fit. Tools are required for the precise assembly of two parts with an interference fit. When two mating parts are assembled with an interference fit, it will be an almost permanent assembly, that is, the parts will not come apart or move during use. To assemble the parts with interference, heating or cooling may be required. In an interference fit, the difference between the sizes is always negative. Interference fits are used when accurate location is of utmost importance and also where such location relative to another part is critical, for example, alignment of dowel pins. The interference fit is illustrated in Fig. 3.10.
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