The ellipse is without a doubt the most important element in Technical Illustration. The following article contains some background knowledge that will help you choose the right ellipse and arrange it correctly.

In Technical Illustration, ellipses are used to depict drill holes and cylinders. Depending on the position of the drill hole or the cylinder, the ellipses are drawn differently.

There are four values that are relevant in this context:

• The diameter (A)
• The groove angle (B)
• The angle (C)
• The thrust-axis (D)

The groove angle is defined by the angle through which the viewer looks at the ellipse. Drag your mouse pointer over the illustration below to see how the groove angle changes when the circle rotates :

Therefore, an ellipse (or a drill hole) is more or less open or closed, depending on how it is being looked at.

In the past, ellipses were drawn with ellipse stencils that offered various groove angles. A typical ellipse stencil looked like this:

A technical illustrator who wanted to place an ellipse into an illustration had to first construct the thrust-axis of the ellipse. For this, there were special perspective protractors. These were applied to the drawn object to find the thrust-axis. The thrust-axis on the protractor also displayed the ellipse with the appropriate apex angle:

 

The ellipse stencil with the displayed groove angle was then applied right-angled to this thrust-axis. Then, the ellipse could be drawn.


 
You apply the same procedure with planes that are not lying in a right angle to the isometric grid. However, you must then choose an ellipse stencil with an appropriate groove angle for the ellipse.

You start with the thrust-axis:

Then choose the right ellipse stencil and draw the ellipse:

 

This technique can also be performed on a computer. In the early days of graphics programs, technical illustrators - mainly in England - created templates and libraries in their graphics programs that allowed them to go on working in the habitual manner.

In IsoDraw, the ellipses are created automatically. You just have to choose the thrust-axis and an ellipse that corresponds to the form of the goniometer and execute the "Find ellipse" function.

 

These techniques allow you to place the correct ellipses in the proper positions within an illustration.