When reading a Engineering drawing, one needs to communicate how the different views was taken from a three dimensional (3D) object. I have found a easy way to show students how projection is done and never make them forget it again!
There are two different types of projection methods in Engineering drawings, namely First Angled Projection and Third Angled Projection. The difference between first angle and third angle projection is the way that a 3D object is rotated on a drawing to show the different sides and views.
It is very important that the projection method that was used is indicated on the drawing, as it will eliminate any confusion when interpreting how a component or part should be assembled or manufactured.
First Angled Projection (watch the video above)
Imagine yourself as looking at a box with the drawing paper in the background. We will divide what you see into 3 different layers. The box, the ink on the paper and then the paper. If the paper is fixed and cannot move and you want to rotate the box, the box will pivot on the paper at the end where it is rotated to.
Eg. If you want to see the top of the box, the bottom edge of the box (that is touching the paper) will be the pivot point. After the box was rotated, you will now see the top of the box, but the box is situated at the bottom of the original view. The same goes for the bottom view. When (having the box in the original starting position) you want to draw the bottom of the box, you would rotate the box upwards with the top edge (that is against the paper) as the pivot point. That will show you the bottom view above the original view.
The left and right views works in a similar way. When you want to see the left side of the box, you would pivot the box to the right with the right edge (that is on the paper) as the pivot point, showing the left view on the right hand side of the original view. The same rule applies to the right view.
Third Angled Projection (watch the video above)
We will now be working on the same principle as with the first angled projection, but now the box will not be on the first layer, but on the third. So in your view, you will see the ink first, then the paper and THIRD, you will see the box. (imagine you can see through the paper)
Where the box and paper meets will still be the plane where the box pivots, but you will see the difference it makes in the projection!
When wanting to see the top of the box, the box will pivot on the top edge that is on the paper. This will show the top of the box on the top of the original position (and not below it as in first angled projection)
The exact same principle works for the bottom, right and left views.
Indicating the Projection method on the Engineering Drawing
You should indicate the type of projection that was used to create the engineering drawing on every drawing you make. Indicating this on the drawing is done by means of a small projection symbol. This should preferably placed at the left bottom of the drawing where it is visible.
First Angled Projection Symbol
Third Angled Projection Symbol
Both types of projections should NEVER be used simultaneously on a drawing or set of drawings. Most companies has a strict rule (and is normally noted in a specification) that only one of the projection types may be used.
Always be very attentive when projecting different views on a drawing, as an incorrect use of the projection type can cause a wrong interpretation of the part that needs to be manufactured, which in turn can lead to re-work.
I found it very useful to sit with something and rotate it in different ways and naming the sides to fully understand the different types of projection.
