|Carpentry for Vocational Schools - A Teachers Handbook|
TOPIC: 2. SCALES
INTRODUCTION: Drawing and reading in scale is very difficult and needs a lot of time to make the students familiar with it.
Therefore it depends much on the emphasis put on this topic if the students are to be able to use scales in practical drawing and work.
- Students should be able to distinguish between a drawing in the real measurements and drawing in scale.
- Students should be able to describe in words and numbers the relationship between two objects and/or numbers.
- Students should be able to read and interprete a scale drawing of a simple piece of furniture.
- Students should be able to do a neat and accurate scale drawings of top, front and side views of simple objects.
- Explain why scale drawing is necessary.
- Explain what a ratio is.
- Prepare worksheets with figures drawn in scales 1:10, 1:20, 1:50 and 1:100. Write the scale to it and let students work out the actual measurements and write them to the figures.
- Prepare another worksheet with numbers of actual measurements which must be converted for scale drawing by ratios of 1:10, 1:20, 1:50 and 1:100.
- Prepare a scale drawing of a house plan in the scale of 1:50 without given measurements. Students have to find out the actual measurements of the house.
- Finally students can do their own scale drawing. Give each student a piece of paper and draw a plan of a simple house with the actual measurements on the black board. Students have to draw the house to a scale of 1:100 on their paper.
NOTE: After this topic prepare a worksheet for the students with questions about converting actual measurements into scale measurements and assess it later.
On maps, plans and drawings the size of things are reduced to make them small enough to fit on the paper or in the case of small parts increased to show detail.
A SCALE can be expressed in the form of a RATIO where the unit of both numbers must be the same, eg. 1:50 means that 1 mm on the drawing represents 50 mm on the building and 1 on the drawing represents 50 cm on the building.
On a detail drawing of small parts (eg. joint) which has been enlarged for clarity the scale may read 5:1.
REMEMBER: The first number in the ratio represents the measurement on the drawing and the last number represents the actual measurement on the building, part or ground.
A scale can also be a ruler like a draughtmans SCALE RULER or a ruler drawn on a map. In both cases each division on the ruler represents an ACTUAL measurement.
Scale rulers are made in different scales: 1:20, 1:25, 1:50, 1:75, 1:100 and 1:125.
When drawing a plan in a scale you must write down in which scale the drawing is made in order to enable anybody to read the plan.
If you measure the same distance with a straight ruler, you will measure 50 mm on the paper.
In case no ruler is available the actual measurement of 1 m = 1000 mm is divided by the number of the scale = 20.
The result, 50 mm, is the measurement on the drawing.
If you measure the same distance with a straight ruler, you will measure 20 mm on the paper.
In case no scale is available the actual measurement of 1 m = 1000 mm is divided by the number of the scale = 50.
The result, 20 mm, is the measurement on the drawing.
For enlarging very small things in a scale eg. 5:1, you have to multiply the actual measurement = 1 cm with the number of the scale = 5.
The result, 5 cm, will be the measurement on the drawing.