Meterology # 1

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INTRODUCTION

WHAT IS Metrology? It is the science of measurement. It deals with instruments and their sensitivity. It is well known saying that the knowledge about anything is complete only when it can be expressed in number and something is known about it thus of every kind of quantity measured, there must be a unit to measure it and express it in numbers of that unit. Most important parameter in metrology is the length which can be measured in several forms and in several ways. In metrology, which laterally is the science measurements we have to go one step ahead and bother our selves about the correctness of measurement also. We have to see whether the result is given with the sufficient correctness and accuracy for the particular need or not. Metrology is therefore also concerned with the methods, execution and estimation of accuracy of measurements. In the broader sense, metrology is not limited to length measurement but is also concerned with the industrial inspection and its various techniques. Due to big industrial resolution and great advancement, industrial inspection does not simply mean the fulfilling of the specification laid down by the manufacturers. Rather inspections in real sense is concerned with the checking of a product at various stages from the raw material form to finished products and even assemble parts in the form of machine also.

Need of Inspection In order to determine the fitness of anything made man has always used inspection. But industrial inspection is of recent origin and has scientific approach behind it. It came into being because of mass production which involved interchangeability of parts. The various parts to be assemble together in assembly shop come from various shops. Rather some parts are manufactured in other factories also and then assemble at one place. So it is very essential that parts must be so fabricated that the satisfactory mating of any pair chosen at random is possible. In order that this many be possible, the dimensions of the component part must be confined with in the prescribed limits which are such as to permit the assembly with a predetermined fit.

Objective of Metrology: The basic objective of a measurement is to provide the required accuracy at minimum cost, metrology would have further objective modern engineering plant with different shops like tool Room, Machine 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 objective would be as follows: 1- Thorough evaluation of newly developed products, to ensure that components designed is with in the process and measuring instrument capabilities available in the plant.

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2- TO determine the process capabilities and ensure that these are better than the relevant component tolerances. 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 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 on and solution of problems arising on the shop floor regarding methods of measurement. 8- Preparation of designs for all gauges and special inspection fixtures.

Precision and Accuracy Both these terms are accounted with the measuring process precision is defined as the repeatability of a measuring process, While accuracy is the agreement of the result of measuring with the true value of the measured quantity. In most measurements it is the precision which is of greater important. If a carpenter had to cut a board to fit the shelf into two projections in the wall, It does not matter, whether his scale is accurate or not, provided he uses the some scale for the measurement of board and the distance between projections in the wall. in the fig.

a) PRECISION BUT NOT ACCURATE

b) ACCURATE BUT NOT PRECISION

c) ACCURATE AND PRECISION

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The illustrating distinction between accuracy and precision by measuring a component several times and plotting the readings, by three instruments.. 1. Precision size but not accurate. 2. Accurate but not precision. 3. Accurate and precision.

Both use the accurate scale in accordance with a standard scales. In this case accuracy of the scale in important and it should be manufactured such that its units are in accordance with the standard units set. The distinction between the precision and accuracy will be come clear from example, shown in fig. In which several measuring are made in a components by different types of instrument and result recorded.

Accuracy: In mechanical inspection, the accuracy of the measurement is must important aspect. The accuracy measurement to some extend is also depended upon the sense of hearing or sense of touch or sense. for example In certain instrument the proportional of sub division have to be estimated by the sense of sight.