An auxiliary view is an ortographic view taken in such a manner that the lines of sight are not parallel to the principal projection planes (frontal, horizontal, or profile). There are an infinite number of possible auxiliary views of any given object.
When creating engineering drawings, it is often neccessary to show features in a view where they appear true size so that they can be dimensioned. The object is normally positioned such that the major surfaces and features are either parallel or perpendicular to the principal planes. Views are normally selected so that most of the features will be visible in the three principal views. The front, top, and left or right side views are most commonly drawn.
Many objects are quite complex, and the three principal views may not best present the geometry of the part. Certain features may not appear true size and shape in those views, or may be hidden. In this case one or more auxiliary views typically are drawn. Primary auxiliary views are projected off one of the principal views. Secondary auxiliary views are projected off a primary auxiliary view.
Imagine an object in a fixed position. As an observer, you are free to move around the object. You can look at the object so that certain features are visible and also true size and shape. The number of possible views is infinite. Determining which views to provide is a criticial decision. Too many views can make a drawing difficult to read and more expensive to create. Omitting views that provide true size views of features will make it impossible to correctly dimension the drawing. Realize that the person producing the part will have to turn it as it is being made. One should provide views that will aid in production and give a representation of the geometry of the object.
Despite the fact that auxiliary views are projected onto planes which are inclined to the principal projection planes, they are still classified as orthographic views. The lines of sight are still parallel to each other and perpendicular to the plane of projection. Therefore, when reading lines on the object in an auxiliary view adjacent to a principal view, the same rules apply to reading lines in adjacent principal views.
To utilize an auxiliary view to show a surface true size (TS), a view must exist or be drawn where that surface appears as a line. It is not possible to show an oblique surface TS in a primary auxiliary view. Once a given or constructed view showing the surface as a line is identified, projecting that surface onto a plane parallel to it will show it TS.
Complete auxiliary views are not commonly drawn in industry. It is much more common to see partial auxiliary views that show only the TS features. Since most of the other surfaces will be foreshortened, a complete auxiliary view becomes more difficult to read. For the sake of learning, we will use for the most part examples showing complete auxiliary views, and have you draw complete auxiliary views in your assignments.