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.

You will notice that the glass box is broken up into 4 equal sections. It is enough for you to know now that each of these sections represents different ways of drawing in Orthographic Projection, and you will learn more about this in the next section.	Here you can see the object that we are going to draw orthographically. We will put it into one of the 4 different sections of the glass box. Notice that the faces of the object are coloured differently. The faces to the right are green, the faces to the left are blue and the top faces are red. You will see how useful this is later.

1^st Angle Projection

1^st 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 3^rd Angle Projection much easier.

Right let's take this in stages.

If you read the section on the Glass Box you will remember that we said we were going to put the object into one of the 4 sections of the Glass Box. Well for 1^st Angle Projection the section of the box we use is always the top left corner, which seems to make sense.

Now that our object is in the 1^st 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.

The plane underneath the object is called the Horizontal Plane (HP).	The planes that are behind and beside the object are called Vertical Planes (VP). They have slightly different names but there will be more about that later.

First of all we are going to look from the right at the green portions of the object. We project the corners of the green portions onto the Vertical Plane on the left as you can see in the diagram opposite. Our lines of projection are parallel to the Horizontal Plane. What we get is the exact shape of the green sections on the object.
Secondly we will look from the left at the object and project what we see onto the Vertical Plane at the back. We can only see the blue sections and again we project parallel to the Horizontal Plane.
Finally we are going to look at the top of the object where we can see al of the red sections. This time we project the corners perpendicular to the Horizontal Plane, and our resulting image lies on it. Opposite you can see the resulting panes after the object has been removed.

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.

How does all this information relate to Technical Graphics I hear you ask… well lets fold out our "Glass Box" so that it looks more like a page.

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 1^st 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.