Drafting Teacher blog: Oblique Surfaces

Oblique Surfaces and Edges - Definition Surfaces or lines that are positioned at an angle with respect to all the principal planes of projection are oblique. An oblique surface will never appear as an line or as a true size area in any of the principal views. It appears as a foreshortened area in each principal view. An oblique line will NEVER appear true length or as a point in a principal view. It will ALWAYS appear foreshortened.

To show an oblique surface true size, you would first have to draw an auxiliary view to show the edge view of the surface. A second auxiliary view that projects the edge view of that surface onto a plane parallel to it will show that surface true size.

Reading Oblique Surfaces To identify oblique surfaces in given orthographic views, realize that the oblique surface will appear as an area in all principle views. The area must contain the same number of vertices in each view. Unlike the foreshortened views of inclined surfaces, the general shape of oblique surfaces need not maintain similarity of shape in the principal orthographic views.
When given two views of an object, it is usually easy to identify surfaces which match up vertex for vertex. If a surface is identified and the points are labeled, project it into the remaining view to see if in fact all vertices of the surfaces match up. Once the two views of the surface are identified and labeled, the points can easily be projected into the missing view.
If you look at the handout Orthographic Reading of Plane Surfaces, you will notice that inclined surfaces and oblique surfaces can appear as areas in two principal views. When reading a drawing, if you locate a surface that appears as an area in two views,you may not be able to determine immediately whether the surface is inclined or oblique. To make the determination, label the vertices of the surface in both given the two views then project into the missing view. Once this is accomplished, the type of surface will be obvious. If the surface is inclined, it will appear as a line in the missing view. If the surface is oblique, it will appear as an area.

Follow this link to a site on Oblique surface

The following group of pictures show how an oblique surface is different from an inclined surface in that there is not a view where the "slant" is represented by a line. The angle between the top, front and right or profile projection planes IS LESS THAN 90 degrees, so it is not possible to appear as a line in any view.

Top view of Oblique surface

Right view of Oblique surface


Front view of Oblique surface

The following pictures show an isometric view of a solid block in addition to the front, top and right (or profile) views. Notice how in all three views the oblique surface NEVER appears as a line, but ALWAYS appears as a "foreshortened" area. I have added dimensions so you can keep track of the faces in the various views.