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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.
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.
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.
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UNITS TO REMEMBER
1.
2.
3.
4.
5.
6.
1 KM
1 Hecto
1 Deca
1 Meter
1 Deci
1 Centi
=
=
=
=
=
=
10 Hectometer
10 Decameter
10 Meter
10 Decimeter
10 Centimeter
10 Milimeters
Engineering Drawing with an Introduction to AutoCAD”, Dhananjay A Jolhe, Tata McGraw Hill, Ed. 2008
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Classification of Scales:1. 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:a)
b)
c)
d)
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.
e) The scale or its R.F. should be mentioned along with the figure.
f) The name of main unit and its sub-units should be mentioned either below or
at the respective ends of the scale.
Engineering Drawing with an Introduction to AutoCAD”, Dhananjay A Jolhe, Tata McGraw Hill, Ed. 2008
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Engineering Drawing with an Introduction to AutoCAD”, Dhananjay A Jolhe, Tata McGraw Hill, Ed. 2008
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@ McGraw-Hill Education
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.
Engineering Drawing with an Introduction to AutoCAD”, Dhananjay A Jolhe, Tata McGraw Hill, Ed. 2008
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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
Similarly
B–5 = ½(BC ), 5–5’ = ½(AB )
1–1’ = 0.1(AB ), 2–2’ = 0.2(AB), 3–3’ = 0.3(AB ), and so on.
Engineering Drawing with an Introduction to AutoCAD”, Dhananjay A Jolhe, Tata McGraw Hill, Ed. 2008
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@ McGraw-Hill Education
Engineering Drawing with an Introduction to AutoCAD”, Dhananjay A Jolhe, Tata McGraw Hill, Ed. 2008