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Module 2.pptx

  2. COURSE CONTENT S/N TOPIC/FOCUS 1. General Introductory Class 2. Theory and practice of high precision 3. Mechanical measurements under strict control conditions 4. Super micro-metry, 5. Comparator Profilometry TEST 1 6. Collimators application in machine installations, etc. 7. Tolerances and quality. 8. Fits: Clearance, transition and interference fits TEST 2
  3. Recommended Books 1. Metrology and Measurement by Anand K Bewoor and Vinay A Kulkarni, McGraw Hill Education(India) Ltd 2013 seventh reprint, 2. Engineering Metrology and Measurements by N V Raghavendra and L Krishnamurthy, Oxford University Press 2013 3. Precision Engineering by V C Venkatesh and Sudin Izman, McGraw Hill Education(India) Ltd 2007,
  4. INTRODUCTION TO METROLOGY i. Measurement in our everyday life ii. Definitions of Metrology iii. Types of Metrology iv. Need for Inspection v. Principal Aspects of Measurement vi. Methods of Measurements vii. Factors affecting Accuracy of Measuring Instruments viii. Errors in Measurement ix. Metric Units in Industry
  5. Measurement in Our Everyday Life •Greek word of Metrology – Metro(Measurement) + Logy(Science) •Metrology is the Science of measurement •Measurement is defined as the set of operations having the objective of determining the value of a quantity
  6. Definitions of Metrology a. Metrology is the science of measurement b. Metrology is the field of knowledge concerned with measurements and includes both theoretical and practical problems with reference to measurement, whatever their level of accuracy and in whatever fields of science and technology they occur c. Metrology is the process of making extremely precise measurements of the relative positions and orientations of different optical and mechanical components d. Metrology is the science concerned with the establishment, reproduction, conversion, and transfer of units of measurement and their standards
  7. Types of Metrology a) Scientific Metrology(standards) b) Industrial Metrology c) Legal Metrology d) Fundamental Metrology
  8. Need for Inspection 1. To ensure that the part, material, or component conforms to the established standard. 2. To meet the interchangeability of manufacture. 3. To maintain customer relations by ensuring that no faulty product reaches the customers. 4. It also helps to purchase good quality raw materials, tools, and equipment which governs the quality of the finished products.
  9. 5. Provide the means of finding out shortcomings in manufacturing. The results of the inspection are not only recorded but forwarded to the manufacturing department for taking necessary steps, so as to produce acceptable parts and reduce scrap. 6. It also helps to coordinate the functions of quality control, production, purchasing, and other departments of the organization. 7. To take a decision on the defective parts i.e., to judge the possibility of making some of these parts acceptable after minor repairs. Need for Inspection
  10. Objectives of Metrology 1. Thorough evaluation of newly developed products, to ensure that the components designed are within the process and measuring instrument capabilities available in the plant. 2. To determine the process capabilities and ensure that these are better than the relevant component tolerance. 3. To determine the measuring instrument capabilities and ensure that these are adequate for their respective measurements. 4. Maintenance of the accuracies of measurement. This is achieved by periodical calibration of the metrological instruments used in the plant.
  11. Objectives of Metrology 5. To minimize the cost of inspection by effective and efficient use of available facilities and to reduce the cost of rejects and rework through the application of Statistical Quality Control Techniques. 6. Standardization of measuring methods. This is achieved by laying down inspection methods for any product right at the time when production technology is prepared. 7. Arbitration and solution of problems arising on the shop floor regarding methods of measurement.
  12. Measuring system element A measuring system is made of five basic elements. These are: 1. Standard 2. Workpiece 3. Instrument 4. Person 5. Environment. The most basic element of measurement is a standard without which no measurement is possible.
  13. Methods of Measurement The methods of measurement can be classified as: 1. Direct method 2. Indirect method 3. Absolute/Fundamental method 4. Comparative method 5. Transposition method 6. Coincidence method 7. Deflection method 8. Complementary method 9. Contact method 10. Contactless method etc.
  14. Methods of Measurement •Direct Method - In this method the value of a quantity is obtained directly by comparing the unknown with the standard. - It involves, no mathematical calculations to arrive at the results, for example, measurement of length by a graduated scale. - The method is not very accurate because it depends on human insensitiveness in making judgement.
  15. • Indirect Method: - In this method several parameters (to which the quantity to be measured is linked with) are measured directly and then the value is determined by mathematical relationship. - For example, measurement of density by measuring mass and geometrical dimensions. Methods of Measurement
  16. Precision The terms precision and accuracy are used in connection with the performance of the instrument. Precision is the repeatability of the measuring process. It indicates to what extent the identically performed measurements agree with each other. If the instrument is not precise it will give different (widely varying) results for the same dimension when measured again and again. The set of observations will scatter about the mean. The scatter of these measurements is designated as 0, the standard deviation. It is used as an index of precision. The less the scattering more precise is the instrument. Thus, lower, the value of 0, the more precise is the instrument Precision and Accuracy
  17. Accuracy  Accuracy is the degree to which the measured value of the quality characteristic agrees with the true value. The difference between the true value and the measured value is known as the error of measurement. It is practically difficult to measure exactly the true value and therefore a set of observations is made whose mean value is taken as the true value of the quality measured. Precision and Accuracy
  18. Distinction between Precision and Accuracy Accuracy is very often confused with precision though much different. The distinction between precision and accuracy will become clear in the following example. Several measurements are made on a component by different types of instruments (A, B, and C respectively) and the results are plotted. In any set of measurements, the individual measurements are scattered about the mean, and the precision signifies how well the various measurements performed by the same instrument on the same quality characteristic agree with each other.
  19. The difference between the mean of a set of readings on the same quality characteristic and the true value is called error. The fewer the error more accurate the instrument. Figure 1(a) shows that instrument A is precise since the results of several measurements are close to the average value. However, there is a large difference (error) between the true value and the average value hence it is not accurate. For Fig 1(b), readings taken by the instruments are scattered much from the average value and hence it is not precise but accurate as there is a small difference between the average value and true value. Fig. 1 (c) shows that the instrument is accurate as well as precise. Distinction between Precision and Accuracy
  20. Factors Affecting Accuracy of Measurements i. Standards of Calibration for Setting Accuracy ii. Workpiece Control During Measurement iii. Inherent Characteristics of Measuring Instrument iv. Human Factor v. Environmental Conditions
  21. Errors in Measurement It is never possible to measure the true value of a dimension, there is always some error. The error in measurement is the difference between the measured value and the true value of the measured dimension. Error in measurement = Measured value - True value. The error in measurement may be expressed or evaluated either as an absolute error or as a relative error.
  22. The accuracy of measurement, and hence the error depends upon so many factors, such as: - Calibration standard - Work piece - Instrument - Person - Environment etc. as already described. Errors in Measurement
  23. No matter, how modern is the measuring instrument, how skillful is the operator, and how accurate the measurement process, there would always be some errors. It is therefore attempted to minimize the error. To minimize the error, usually, a number of observations are made and their average is taken as the value of that measurement. Errors in Measurement
  24. If these observations are made under identical conditions i.e., same observer, same instrument, and similar working conditions except for time, then, it is called a Single Sample Test’. If however, repeated measurements of a given property using alternate test conditions, such as different observers and/or different instruments are made, the procedure is called a `Multi-Sample Test’. The multi-sample test avoids many controllable errors e.g., personal error, instrument zero error, etc. The multi-sample test is costlier than the single-sample test and hence the latter is in wide use. In practice, good numbers of observations are made under single-sample tests, and statistical techniques are applied to get results that could be approximate to those obtainable from the multi-sample test. Errors in Measurement
  25. During measurement several types of error may arise, these are 1. Static errors which includes - Reading errors - Characteristic errors - Environmental errors. 2. Instrument loading errors. 3. Dynamic errors. Errors in Measurement
  26. Static errors These errors result from the physical nature of the various components of the measuring system. The three components are listed above. i. Reading errors ii. Reading errors apply exclusively to the read-out device. These do not have any direct relationship with other types of errors within the measuring system. Reading errors include: Parallax error, Interpolation error. Errors in Measurement
  27. ii. Characteristic Errors It is defined as the deviation of the output of the measuring system from the theoretically predicted performance or from nominal performance specifications. Linearity errors, repeatability, hysteresis, and resolution errors are part of characteristic errors if the theoretical output is a straight line. Calibration error is also included in characteristic error. Errors in Measurement
  28. iii. Loading Errors Loading errors result from the change in measurand itself when it is being measured, (i.e. after the measuring system or instrument is connected for measurement). Instrument loading error is the difference between the value of the measurand before and after the measuring system is connected/contacted for measurement. For example, soft or delicate components are subjected to deformation during measurement due to the contact pressure of the instrument and causing a loading error. Errors in Measurement
  29. Iv Environmental Errors These errors result from the effect of surrounding such as temperature, pressure, humidity, etc. on the measuring system. External influences like magnetic or electric fields, nuclear radiations, vibrations or shocks, etc. also lead to environmental errors. V Dynamic Errors A dynamic error is an error caused by time variations in the measurand. It results from the inability of the system to respond faithfully to a time- varying measurement. It is caused by Inertia, damping, friction, or other physical constraints in the sensing or readout, or display System. Errors in Measurement
  30. For statistical study and the study of the accumulation of errors, these errors can be broadly classified into two categories 1. Systematic or controllable errors, and 2. Random errors. Errors in Measurement
  31. Comparison between Systematic Errors and Random Errors Systematic or controllable error Random error These errors are repetitive in nature and are of constant and similar form These are non-consistent. The sources giving rise to such errors are random. These errors result from improper conditions or procedures that are consistent in action. Such errors are inherent in the measuring system or measuring instruments. Except for personal errors, all other systematic errors can be controlled in magnitude and sense. Specific causes, magnitudes and sense of these errors cannot be determined from the knowledge of measuring system or condition. If properly analyzed these can be determined and reduced or eliminated. These errors cannot be eliminated, but the results obtained can be corrected. These include calibration errors, variation in contact pressure, variation in atmospheric conditions, parallax errors, misalignment errors etc. These include errors caused due to variation in position of setting standard and work-piece, errors due to displacement of lever joints of instruments, errors resulting from backlash, friction etc.
  32. General Care of Metrological Equipment The equipment (apparatus) used for precision measurements is designed to fine limits of accuracy and is easily liable to be damaged by even-slight mishandling and such damage may not be noticeable. A great deal of careful handling is, therefore, required. As far as possible, the highly finished surfaces should not be touched by hand because the natural acids on the skin are likely to corrode the finished surface and also the temperature of the body may upset the dimensions of the precision instruments.
  33. In order to overcome this many standard metrology laboratories recommend washing of hands thoroughly and coating them with a thin film; of pure petroleum jelly before handling the instruments. Further very precise equipment like slip gauges is allowed to be handled only by using a piece of chamois leather or tongs made from a strip of "Perspex". This mixture spreads much more easily and is applied with cloth or with fingers. Brushing is not recommended as it is liable to ti air which, with the moisture, it contains, may cause rusting. General Care of Metrological Equipment
  34. General Care of Metrological Equipment When the equipment is not in use, it should be protected from atmospheric corrosion. For this purpose the highly finished surfaces are first wiped with a solvent to remove any finger mark and then coated with mixture of heated petroleum jelly and petrol. As the standard temperature for measurement is 20°C, for very precise measurement the instruments and work pieces should be allowed to attain this temperature before use and the handling should be as little as possible.
  35. Metric Units in Industry • SI Base Units Length, Mass, Time, Electric Current, Thermodynamic Temperature, Amount of Substance, Luminous Intensity • SI Derived Units Plane angle, Solid Angle, Angular Velocity, Angular Acceleration, Frequency, Speed and Velocity, Acceleration, Force, Pressure and stress, Energy, Work and Heat, Power, Power Flux Density, Linear Momentum, electric Charge, Celcius Temperature