24 ENGINEERING TOLERANCE This is the permissible limit of variation in the

24 engineering tolerance this is the permissible

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2.4 ENGINEERING TOLERANCE This is the permissible limit of variation in the physical dimension, a measured value or the physical property of a material, manufactured object, system or service other measured values such as temperature humidity e.t.c. dimension properties or conditions may vary within a certain practical limits without significantly affecting functionality of a given machine or process. Note: this would not occur if the scale was perfectly thin.
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15 Tolerances are specified to allow reasonable leeway for imperfection and inherent variability without compromising performance. Consideration when setting tolerances A primary concern is to determine how wide the tolerance may be without affecting other factors such as functionality of a part or process. This can be done by the use of scientific principles, engineering knowledge and profession experience. Experimental investigation is useful for determination of the effects of tolerance on a part. Process controls must be in place and effective quality management system such as total quality management control needs to ensure that actual produced products are within required tolerances. Exercise a) Differentiate Systematic errors from Random errors b) State the sources of errors for systematic and random errors c) Explain how these errors can be avoided d) Why is it important to know about errors in measurement?
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16 UNIT THREE 3.0 BASIC MEASURING INSTRUMENTS Measuring instruments and gauges are used to measure various parameters such as clearance, diameter, depth, ovality, trueness etc. These are important engineering parameters which describe the condition of the working machinery or tell whether the manufactured part is made to the required accuracy. 3.1 Rulers They are used to measure lengths and other geometrical parameters. They can be single steel plate or flexible tape type tool shown below. These are used for making line measurement of length. A flexible rule (e.g. measuring tape) is a flexible form of ruler. It consists of a ribbon of cloth, plastic, fiber glass, or metal strip with linear-measurement markings. It is a common measuring tool. Its design allows for a measure of great length to be easily carried in pocket or toolkit and permits one to measure around curves or corners. Fig 3.1a Steel Rule Fig 3.1b measuring tape 3.2 Calipers They are normally of two types- inside and outside caliper. They are used to measure internal and external size (for e.g. diameter) of an object. It requires external scale to compare the measured value. Some calipers are provided with measuring scale. Other types are odd leg and divider caliper. Calipers are also used in situations where the other known measuring instruments cannot be used because of lack of space.
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17 TYPES OF CALIPERS 3.2.1 Inside Caliper Fig.3.2.1 Two inside calipers The inside calipers are used to measure the internal size of an object.
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