Scales

in Engineer’s language scale means the proportion or ratio between the dimensions adopted for the drawing and the corresponding dimensions of the object.

1. Engineering Graphics Scales Introduction Representative Fraction Types of Scales

2. Introduction Usually the word scale is used for an instrument used for drawing straight lines. But actually in Engineer’s language scale means the proportion or ratio between the dimensions adopted for the drawing and the corresponding dimensions of the object .

3. Introduction I.S. have recommended the following standard scales. Full Scale (1:1) Reduced Scale 1 : 2 1 : 2.5 1 : 5 1 : 10 1 : 20 1 : 50 1 : 100 1 : 200 Enlarged Scale 10 : 1 5 : 1 2 : 1

4. Introduction

5. Introduction

6. Representative Fraction (R.F.) The ratio of the size of the element in the drawing to the size of the same element in the object is called the Representative Fraction (R.F.).

7. Representative Fraction (R.F.) Example 1 If 1 cm length of drawing represents 5 m length of the object, then in engineering scale it is written as 1 cm = 5 m and in graphical scale it is denoted by

8. Representative Fraction (R.F.) Example 2 If a 5 cm long line in the drawing represents 3 km length of a road, then in engineering scale it is written as 1 cm = 600 m and in graphical scale it is denoted by

9. Representative Fraction (R.F.) Example 3 If a gear with a 15 cm diameter in the drawing represents an actual gear of 6 mm diameter in graphical scale, it is expressed by

10. Types Of Scales Mechanical Engineers’ scale Architects’ scale Civil Engineers’ scale 2. Plain scale Diagonal scale Comparative scale Vernier scale Scale of Chords Isometric scale Scales are classified in two different manner as under:

11. Types Of Scales These scales are 300 mm long and each unit is sub-divided. Mechanical Engineers generally use following scales. 1:1 1:2 1:2.5 1:5 2:1 5:1 Mechanical Engineers’ scale

12. Types Of Scales Architects are required to take very small R.F. since buildings are comparatively very big as compared to drawing paper size. Only the first main division of the architects’ scale is sub-divided. Architects’ scale

13. Types Of Scales Civil Engineers dealing with road maps and survey maps are required to take very very small R.F.. These scales are sub-divided on their entire lengths. Civil Engineers’ scale

14. Plain Scales Plain scales read or measure upto two units or a unit and its sub-division, for example centimeters (cm) and millimeters (mm). When measurements are required upto first decimal, for example 2.3 m or 4.6 cm etc. It consists of a line divided into number of equal main parts and the first main part is sub-divided into smaller parts .

15. Plain Scales Example A 3 cm long line represents a length of 4.5 meters. Extend this line to measure upto 30 meters and show on it units of meter and 5 meter. Show the length of 22 meters on this line.

16. Construction: Draw a straight line of 20cm length and divide into 6 equal parts. Divide again first part into 5 equal parts. Give numbers as shown. To represent 22 meters, take 4 main parts to represent 20 meters and 2 small parts to represent 2meters. Give names as A and B so that the distance between A and B is 22 meters as shown. Note: Assume height of the plain scale as 1 cm .

17. Diagonal Scales Diagonal scales are used to read or measure upto  three units . For example: decimeters (dm), centimeters (cm) and millimeters (mm) or miles, furlongs and yards etc. This scale is used when very small distances such as 0.1 mm are to be accurately measured or when measurements are required upto second decimal. For example: 2.35dm or 4.68km etc. Small divisions of short lines are obtained by the principle of diagonal division, as explained below: Principle of diagonal scale: To divide a given line AB into small divisions in multiples of 1/10 its length for example 0.1AB; 0.2AB etc.

18. Diagonal Scales Example An area of 144 sq. cm on a map represents an area of 9 sq. km on the field. Find the R.F . of the scale for this map and draw a diagonal scale to show kilometers, hectometers and decameters and to measure upto 5 kilometers. Indicate on the scale a distance of 3 kilometers, 5 hectometers and 6 decameters or 3.56km. 

19. Construction: Draw a line AB of 20 cm and construct a rectangle on it, by taking AD 5cm as shown. Divide AB into 5 equal parts and number them from second part starting with 0 to 4 towards right side to indicate kilometers (km). Divide 0A into 10 equal parts, each part represents a hectometer (hm). Divide AD into 10 equal parts, each part represents one decameter (dam). Join diagonals as shown. To mark 3.56km, take it as sum of 3.50km and 0.06km. On the plain scale take 3.5km and on the diagonal at 5 upto 6 parts diagonally which is equal to 0.06km, giving a total of 3.56km as shown by MN. Note : Assume the height of the diagonal scale AD as 5cm for dividing it into 10 equal parts conveniently.

20. Isometric Scales A scale is now constructed by stepping off true measurements along line 'AB 1 ' which is a true length line. The measurements are then transferred back to line 'AB' to get a smaller scale, in this case an isometric scale. Lines drawn using the isometric scale are approximately 80% of true size. This scale is usually marked off on a piece of paper and used to step off the foreshortened measurements along the projection of axes lines and lines parallel to them. Lines parallel to the projection of axes are known as isometric lines. Lines which are not parallel to theses axes are known as non-isometric lines. It is important to note that you can only use the scales on isometric lines.