ENGINEERING DRAWING

Lesson 2. TYPES OF SCALES

Module1. Lines, lettering and scales

Lesson 2
TYPES OF SCALES

The types of scale normally used are
1 . Plain scales
2 . Diagonal

1. Plain scales

A plain scale is simply a line which is divided into a suitable number of equal parts, the first of which part is further sub-divided into small parts. It is used to represent either two units.
Example: On a survey map the distance between two places 1 km apart is 5 cm. Construct the scale to read 4.6 km

Fig. 2.1 Plain scale

2. Diagonal scales

Plain scales are used to read lengths in two units such as meters and decimeters, centimeter and millimeters etc., or to read to the accuracy correct to first decimal.

Diagonal scales are used to represent either three units of measurement such as metres, decimeters, centimeters or to read to the accuracy correct to two decimals.

Principle of diagonal scale:

1 . Draw a line AB and erect a perpendicular at B.
2 . Mark 10 equal distant points (1, 2, 3 etc.) of any suitable length along this perpendicular and mark C.
3 . Complete the rectangle ABCD.
4 . Draw a diagonal BD.
5 . Draw a horizontal through the division points to meet BD at 1’, 2’, 3, etc.

The line 1-1’, 2-2’ etc. measure 0.1CD, 0.2CD, etc. respectively. The line CD is divided into 1/10 the divisions by the diagonal BD, i.e. each horizontal lines is a multiple of 1/10CD.

Fig. 2.2 Principle of diagonal scale

Problem:

On a plan, al line of 22 cm long represents a distance of 440 metres. Draw a diagonal scale for the plan to read up to a single metre. Measure and mark a distance of 187 m on the scale.

Fig. 2.3 Diagonal scale