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Understanding Scales in Technical Drawings

Explore scales, RF, enlargement, and reduction scales, with examples of plain and diagonal scales in technical drawings. Learn the principles and applications of various scales in engineering designs.

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Understanding Scales in Technical Drawings

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  1. Chapter 5 SCALES The proportion by which the drawing of a given object is enlarged or reduced is called the scale of the drawing. The scale of a drawing is indicated by a ratio, called the Representative Fraction (RF) or Scale Factor. RF is a ratio of the length of an object on a drawing to the actual length of the object. i.e., RF = (Length of line on drawing)/(Actual length of the line on the object) The terms ‘scale’ and ‘RF’ are synonymous. The scale is most commonly expressed in the format X :Y while RF is expressed in the format X/Y.

  2. Enlarging or Enlargement Scales When smaller objects are to be drawn, they often need to be enlarged. The scales used in such cases are called enlarging scales. Obviously, the length of an object on the drawing is more than the corresponding actual length of the object. Enlarging scales are mentioned in the format X : 1, where X is greater than 1. Clearly, RF > 1. For eg: - 2:1 means drawing made to twice the actual size Enlarging scales are used for objects like screws and gears used in small electronic gadgets, wristwatch parts, resistors, transistors, ICs. Reducing or Reduction Scales When huge objects are to be drawn, they are reduced in size on the drawing. The scales used for these objects are called reducing scales. It is clear that the length of the object on the drawing is less than the actual length of the object. Reducing scales are mentioned in the format 1 :Y, where Y is greater than 1. Hence, RF < 1. For eg:- 1:2 means drawing made to one HALF of the actual size. Objects like multistoreyed buildings, bridges, boilers, huge machinery, ships, aeroplanes, etc., are drawn to reducing scales.

  3. Full Scale When an object is drawn on the sheet to its actual size, it is said to be drawn to full scale. As the length on the drawing is equal to the actual length of the object, the full scale is expressed as 1:1. Obviously, for full scale, RF = 1.

  4. UNITS TO REMEMBER • 1 KM = 10 Hectometer • 1 Hecto = 10 Decameter • 1 Deca = 10 Meter • 1 Meter = 10 Decimeter • 1 Deci = 10 Centimeter • 1 Centi = 10 Milimeters

  5. Classification of Scales:- • PLAIN SCALE:- • PLAIN SCALES ( FOR DIMENSIONS UP TO SINGLE DECIMAL) • A line which is divided into suitable no. of equal parts or units, the first part of which is further sub-divided into small parts or sub-units of main unit is known as Plain Scale. • The Plain scales are used to represent either two units (such as Kilometers, Decimeters ) OR one unit and its fraction (Meters and 1/10 th of meter) • POINTS TO DRAW PLAIN SCALE:- • Find the R.F. if not given. • Find the length of the scale = R.F. X Actual Length of Object. • The mark zero (0) should be placed at the end of the first main division. • The main units should be numbered to the right and its sub-units to the left from the zero (0) mark. • The scale or its R.F. should be mentioned along with the figure. • The name of main unit and its sub-units should be mentioned either below or at the respective ends of the scale.

  6. 2. DIAGONAL SCALE:- DIAGONAL SCALES ( FOR DIMENSIONS UP TO TWO DECIMALS) The scale in which small divisions of short lines are obtained by following the principle of diagonal divisions is known as diagonal scale. Diagonal scales are used to represent either 3 units of measurement (such as meters, decimeters and centimeters) OR two units and a fraction of its second unit.

  7. DIAGONAL SCALES Diagonal scale is used to indicate the distances in a unit and its immediate two subdivisions. The diagonal scales are better than vernier scales—any distance can be measured easily on them. A diagonal scale consists of a plain scale and a diagonal construction. Principle of Diagonal Scale The construction of a diagonal scale is based on the principle of similarity of triangles. Let line AB represent any length, say 1 cm, Fig. 5.8. To divide line AB into 10 equal parts, draw a line BC, of any length, perpendicular to AB and complete the rectangle ABCD. Draw diagonal BD. Now divide BC into 10 equal parts. Through 1, 2, 3, …, 9, draw lines parallel to AB intersecting BD at 1’, 2’, 3’, …, 9’ respectively. From the geometry of the figure, it is clear that triangles B–1–1’, B–2–2’, B–3–3’, …, BCD are similar triangles. As B–5 = ½(BC ), 5–5’ = ½(AB ) Similarly 1–1’ = 0.1(AB ), 2–2’ = 0.2(AB), 3–3’ = 0.3(AB ), and so on.

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