# Selecting Scale for Drawing

Rural Technology

### a)Engineering drawing

To draw engineering drawing or map on a small piece of paper, we need scale. By selecting appropriate scale we can accommodate all details of the object on a small piece of paper. It helps is studying a large area.

Scale is selected by the maximum length available for drawing on the paper divided by maximum actual length to be plotted. For example if 25cm size drawing paper is available for drawing and we want to draw engineering drawing of sketch given in the fig.10-1 Maximum dimension to be plot is 300 meter on the drawing paper. Therefore, you can select scale as — Scale = length available on the paper / Maximum length of the object to be draw: = 25 cm / 300 m = 1 cm / 12 m Therefore, scale will be 1 cm = 12 m Please remember all dimensions on the plot has to be reduced to this scale. For make it simple, try to select the scale in integers. i.e. 1, 2, 3 … As far as possible do not select a scale in fractions 1.2, 1.4, 4.5 etc.

### b) For Geographical maps & contours –

Distances plotted on the maps are very high. They can be hundreds of Kms. Consider maximum actual distance in the area to be mapped (A in KM) and maximum possible length available on the paper (B in cms), then, Scale = B in cm / A in KM

For example if maximum distance in the area to be mapped is 70 KM (A) and maximum possible length available on a wide A3 size (11.693" ´ 16.535") is say, 35 cm (B); then, Scale = 35 cm / 70 KM = 1 : 2 KM This means every 1 cm on the paper represents 2 KM. Suppose, distance between two places on the map is 6 cm then the actual distance between those two places would be 12 KM.

### c) For Electrical Diagrams –

To communicate electric connections, position of switches and load, polarities etc electric circuit diagram is very necessary. We have seen various symbols used in electric circuits. Scale for electric diagram is selected as in engineering drawing. For e.g. Maximum actual distance in the wiring diagram is A = 100 m and the maximum possible length available on the paper to be B = 35 cm. Then the scale would be: B / A = 35 / 100 = 1 / 2.85 For simplicity of calculation, we can make it 1cm : 2 m That means, every 1 cm of the figure on the paper represents 2 meter of actual wiring.

### d) Graphs

generally have grid of 1cm ´ 1cm. Therefore considering number to be plotted on graphs , we can calculate the scale as shown above.

### e) Flow charts -

Flow Charts represent only flow of any procedure, so they don’t need any scale.

### Intext Questions

01. Select appropriate scale for plotting on the maximum drawing sheet of size 40 cm.

02. Write down size of all dimensions for drawing on the paper. Scale = __________

Rural Technology