Note for Mechanical Measurement and Control - MMC by Ayush Agrawal

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1 Mechanical Measurement & Metrology Course Contents 1.1 Introduction 1.2 Need Of Inspection 1.3 Objectives of Metrology 1.4 Precision And Accuracy 1.5 Errors in Measurement 1.6 General Care Of Metrological Instrument 1.7 Standardization and Standardizing Organization Department of Mechanical Engineering Darshan Institute of Engineering & Technology, Rajkot Page 1.1

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1. Introduction Mechanical Measurement & Metrology (2141901) 1.1 Introduction  Metrology is a science of measurement. Metrology may be divided depending upon the quantity under consideration into: metrology of length, metrology of time etc. Depending upon the field of application it is divided into industrial metrology, medical metrology etc.  Engineering metrology is restricted to the measurement of length, angles and other quantities which are expressed in linear or angular terms.  For every kind of quantity measured, there must be a unit to measure it. This will enable the quantity to be measured in number of that unit. Further, in order that this unit is followed by all; there must be a universal standard and the various units for various parameters of importance must be standardized.  It is also necessary to see whether the result is given with sufficient correctness and accuracy for a particular need or not. This will depend on the method of measurement, measuring devices used etc.  Thus, in a broader sense metrology is not limited to length and angle measurement but also concerned with numerous problems theoretical as well as practical related with measurement such as: 1. Units of measurement and their standards, which is concerned with the establishment, reproduction, conservation and transfer of units of measurement and their standards. 2. Methods of measurement based on agreed units and standards. 3. Errors of measurement. 4. Measuring instruments and devices. 5. Accuracy of measuring instruments and their care. 6. Industrial inspection and its various techniques. 7. Design, manufacturing and testing of gauges of all kinds. 1.2 Need of Inspection  Inspection means checking of all materials, products or component parts at various stages during manufacturing. It is the act of comparing materials, products or components with some established standard. Page 1.2 Department of Mechanical Engineering Darshan Institute of Engineering & Technology, Rajkot

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Mechanical Measurement & Metrology (2141901) 1. Introduction  In old days the production was on a small scale, different component parts were made and assembled by the same craftsman. If the parts did not fit properly at the time of assembly, he used to make the necessary adjustments in either of the mating parts so that each assembly functioned properly.  Therefore, it was not necessary to make similar parts exactly alike or with same accuracy as there was no need of inspection.  Due to technological development new production techniques have been developed. The products are being manufactured on a large scale due to low cost methods of mass production. So, hand fit method cannot serve the purpose any more. The modern industrial mass production system is based on interchangeable manufacture, when the articles are to be produced on a large scale.  In mass production the production of complete article is broken up into various component parts. Thus the production of each component part becomes an independent process. The different component parts are made in large quantities in different shops. Some parts are purchased from other factories also and then assembled together at one place. Therefore, it becomes essential that any part chosen at random should fit properly with any other mating parts that too selected at random. This is possible only when the dimensions of the component parts are made with close dimensional tolerances. This is only possible when the parts are inspected at various stages during manufacturing.  When large number of identical parts are manufactured on the basis of interchangeability if their dimensions are actually measured every time lot of time will be required. Hence, to save the time gauges are used, which can tell whether the part manufactured is within the prescribed limits or not. Thus, the need of inspection can be summarized as: 1. To ensure that the part, material or a component conforms to the established standard. 2. To meet the interchangeability of manufacture. 3. To maintain customer relation by ensuring that no faulty product reaches the customers. Department of Mechanical Engineering Darshan Institute of Engineering & Technology, Rajkot Page 1.3

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1. Introduction Mechanical Measurement & Metrology (2141901) 4. Provide the means of finding out shortcomings in manufacture. The results of inspection are not only recorded but forwarded to the manufacturing department for taking necessary steps, so as to produce acceptable parts and reduce scrap. 5. It also helps to purchase good quality of raw materials, tools, equipment which governs the quality of the finished products. 6. It also helps to co-ordinate the functions of quality control, production, purchasing and other departments of the organization. To take decision on the defective parts i.e., to judge the possibility of making some of these parts acceptable after minor repairs. 1.3 Objectives of Metrology While the basic objective of a measurement is to provide the required accuracy at minimum cost, metrology would have further objective in a modern engineering plant with different shops like Tool Room, Machine Shop, Press Shop, Plastic Shop, Pressure Die Casting Shop, Electroplating and Painting Shop, and Assembly Shop; as also Research, Development and Engineering Department. In such an engineering organization, the further objectives would be as follows: 1. Thorough evaluation of newly developed products, to ensure that components designed is 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. To minimize the cost of inspection by effective and efficient use of available facilities and to reduce the cost of rejects and rework through application of Statistical Quality Control Techniques 5. Standardization of measuring methods. This is achieved by laying down inspection methods for any product right at the time when production technology is prepared. 6. Maintenance of the accuracies of measurement. This is achieved by periodical calibration of the metrological instruments used in the plant. 7. Arbitration and solution of problems arising on the shop floor regarding methods of measurement. Page 1.4 Department of Mechanical Engineering Darshan Institute of Engineering & Technology, Rajkot