Technical drawing
A technical drawing shows an object from the front, from above and from the side, with fixed line types, dimensions in millimetres and a scale.
Why
How does a joiner know exactly which cupboard you want, if all they get is a drawing?
A photo shows an object from one side, and you cannot see how big it is. A sketch on a napkin often leaves out half. Yet a joiner in Ghent can build a cupboard that a designer in Hasselt thought up, without the two of them ever talking.
That works because a technical drawing is a language with fixed conventions. You show the object straight from the front, from above and from the side. Every line type means something, every dimension is written in millimetres, and a scale says how much smaller or bigger everything is drawn.
See it
Pick a shape: L block, step block or block with a slot. You see the block in 3D and its three views in the European method (first-angle projection). Tap a view to see which side you are looking from.
dashed: a hidden edge
Description of the picture
A block in perspective, with its three views in the European method next to it: the front view, the top view below it and the left side view to the right of the front view. When you tap a view, an arrow on the 3D block shows which side you are looking from. You choose between an L block, a step block and a block with a slot.
Get it
A view is what you see when you look straight at one side of an object. The front view shows the side that tells you the most about the shape. The top view shows the object from above, the side view from the side. Each view is flat and shows only two of the three dimensions: the front view the width and the height, the top view the width and the depth, the side view the depth and the height.
At school you draw in the European method, also called first-angle projection. The top view sits straight below the front view, just as wide. The left side view, what you see when you look from the left, sits to the right of the front view, just as tall. Remember it like this: what you see from above goes below, and what you see from the left goes to the right.
Every line type has a meaning. A visible edge is drawn with a thick continuous line. An edge you cannot see, because it is behind or inside the object, is drawn with a dashed line. The centre of a hole or a symmetrical shape is marked with a centre line: a thin chain line of dashes and dots. Dimension lines are thin continuous lines with arrowheads, and the dimensions are written next to them in millimetres, without a unit.
A scale says how big you have drawn something compared to reality: drawing to reality. At 1:1 you draw full size. For a reduction such as 1:2, 1:5, 1:10, 1:20, 1:50 or 1:100 you draw everything smaller, for an enlargement such as 2:1 or 5:1 bigger. At scale 1:20, 1 mm on paper is 20 mm in reality. On the dimension lines you always write the real size, never the drawn size.
For a reduction 1:n the real size is n times the drawn size: w=
Worked example
On a building plan at scale 1:50 a room is 8 cm long. How long is the room in reality, in metres?
Scale 1:50 means everything is drawn 50 times smaller. So the real size is 50 times the drawn size.
Work it out in centimetres.
w=
8× 50= 400 400 cm is 4 m, because 1 m is 100 cm.
- Answer
The room is 4 m long in reality.
Do it
Pick a level. You get a new exercise every time, checked straight away.
Make it
Draw a small box in three views35 minutes
What you need
- a small empty box, for example from tea or toothpaste
- squared paper or a sheet of paper
- a pencil and an eraser
- a ruler or a set square
- two or three other boxes for the test
How to do it
- Choose which side of the box will be the front view. If you like, cut or draw an opening in the back, so you have a hidden edge.
- Measure the width, the height and the depth in millimetres.
- Draw the front view full size (1:1). If the box is big, draw at scale 1:2 and divide every measurement by 2.
- Draw thin guide lines straight down and draw the top view below the front view, just as wide.
- Draw guide lines to the right and draw the left side view to the right of the front view, just as tall.
- Draw the opening at the back as a dashed line in the front view and write the real sizes in millimetres on the dimension lines. Write the scale at the bottom.
Safely: This task is calm and safe. Use an empty, clean box, and carry pencils with the point down when you walk around.
Test it
Lay your drawing next to the box: every drawn size times the scale number must give the real size. Then put your box among the other boxes and let someone pick the right one using only your drawing. If they pick it straight away, your drawing is clear.
Pitfalls
Wrong: I draw the top view above the front view.
In the European method, which you use at school, what you see from above goes below the front view.
Right: Put the top view straight below the front view, just as wide, and the left side view to its right, just as tall.
Wrong: On a drawing at scale 1:10 I write the drawn size on the dimension line.
Whoever reads the drawing wants to know how big the object really is. Otherwise they make everything ten times too small.
Right: Always write the real size in millimetres on the dimension line, whatever scale you use.
Wrong: An edge I cannot see, I just leave out.
Then the reader does not know there is a slot at the back or a hole in the middle.
Right: Draw a hidden edge as a dashed line, so the whole shape is still on the drawing.
Wrong: Scale 2:1 is a reduction.
The first number belongs to the drawing. At 2:1 the drawing is twice as big as the real object.
Right: 2:1 and 5:1 are enlargements, handy for small parts. 1:2, 1:5 and smaller are reductions.
In real life
- An architect usually draws a house at scale 1:50 or 1:100. Plans like these go with an application for planning permission for a renovation.
- A walking map often has a scale such as 1:25 000. A distance of 1 cm on the map is then 250 m in reality.
- A flat-pack cupboard comes with instructions full of views, so you can see which panel goes where.
- In a metal workshop people work with working drawings on which every dimension is in millimetres.
History
Around 1800 the French mathematician Gaspard Monge described how to draw an object precisely on a flat sheet, with views that line up neatly with each other. His work is the basis of technical drawing as we know it today.
More in Ruit
Working with a scale means calculating with ratios. You practise that more in Ruit.
To RuitAt the table
Look together for a map, a plan or a set of instructions at home. Which scale or which views do you find on it?
A question to talk about together, for parents and children. There is no wrong answer.