Technical Graphics - Projection Systems - Orthographic
The Glass Box
"The Glass Box" is the name sometimes given to a theoretical see-through box that any object can fit inside. It is a very useful tool when it comes to trying to explain how Orthographic Projection works. Below you can see the box and the object we are going to use to show you how Orthographic Projection works.
1st Angle Projection is the type of Orthographic
Projection that is mostly used in this part of the
world, that is Europe. You should take time to read this
section carefully as it will make understanding 3rd Angle Projection much easier.
Now that our object is in the 1st Angle section we can start to see how Orthographic Projection works.
If we remove the exterior panes of "glass" that are slightly obstructing our view we will be left with 3 panes of glass, one underneath, one behind, and one to the side of the object. The surfaces that we are going to project the image of the object onto are at the far side of the object from where you are viewing it.
You may notice in the drawing on the right that there is
a yellow line. This is not a mistake but a very important
part of Orthographic Projection. Imagine you were looking at
a L-shaped house from the front. You would just see a
rectangle and from the front you would not be able to tell
that there is a bit of the house sticking out at one side or
the other. You have the same problem in Orthographic
Projection with complicated objects. You can show these
hidden pieces with what is called Hidden Detail,
funnily enough !! Hidden Detail lines are different from the
other lines in the drawing in that they are dashed. Later
you will see where and how this Hidden Detail is used.
What we end up with are the 3 projection that we made on a flat surface, and this is how a drawing in Orthographic Projection looks.
The drawing of the front of the object is called the Elevation, the drawing of the top of the object is called the Plan, and the drawing of the side of the object is called the End Elevation.
You should also write the names of the views beside the images, just to avoid confusion.
The main thing to remember now is that when you are working in 1st Angle Projection, the surfaces that you are looking at should be drawn on the opposite side of the object. This is easy to see with the Plan, which is the drawing of the top of the object but is drawn on the bottom of the page. Similarly the right side of the object is drawn on the left hand side of the page.
Section 1 : This Lesson shows you the imaginary Glass Box and then goes onto First Angle Projection.
Section 2 : This Lesson shows Third Angle Projection.
Section 3 : This Lesson shows you how to determine wheather you are working in First or Third Angle Projection.
Section 4 : In this Lesson you get to put all of your newly acquired knowledge to the test with a wide variety of problems and question relating to Orthographic Projection.