Oblique Drawings

Oblique projection is a simple form of parallel graphical projection used mainly for producing pictorial, two-dimensional images of three-dimensional objects. Oblique drawings are similar to isometrics except that the front view is shown in its true shape on the horizontal line—i.e., when drawing an object in oblique, the front view is drawn flat (Figure 5.12). Thus, it projects an image by intersecting parallel rays from the three-dimensional source object with the drawing surface. In oblique projection (as in orthographic projection), parallel lines from the source object produce parallel lines in the projected image. The projectors intersect the projection plane at an oblique angle to produce the projected image, as opposed to the perpendicular angle used in orthographic projection.

A 45-degree angle is the most commonly used to draw the receding lines from the front view, but other angles are acceptable. In an oblique sketch, circular lines that are parallel to the frontal plane of projection are drawn at their true size and shape. Hence, circular features appear as circles and not as ellipses. This is the main advantage of the oblique sketch. The three axes of the oblique sketch are drawn at the horizontal, vertical, and a receding angle that can vary from 30 to 60 degrees.

Whereas an orthographic projection is a parallel projection in which the projectors are perpendicular to the plane of projection, an oblique projection is one in which the projectors are not perpendicular to the plane of projection. With oblique projection all three dimensions of an object can be shown in a single view.

Oblique drawing is a primitive form of 3D drawing and the easiest to master. It is not a true 3D system but a two-dimensional view of an object with contrived depth. Instead of drawing the sides full size, they are only drawn at half the depth, creating a suggested depth that adds an element of realism to the object. Even with this contrived depth, oblique drawings look very unconvincing to the eye. In Figure 5.13 the side views are drawn at a 45-degree angle. In oblique projection the side views are typically fore-

Figure 5.12 Technique for producing an oblique drawing. The front view is shown in its true form on the horizontal line and should be drawn flat.

shortened to provide a more realistic view of the object. To foreshorten the side views, the object's side measurements are normally halved. In this case, the sides are 50 mm (2 inches) long, but they have been drawn at 25 mm (1 inch). Because the oblique drawing is not realistic, it is rarely used by professional architects and engineers.

Occasionally we find surfaces or features on drawings that are oblique to the principal planes of projection and still are shown in their true shape. Other features in modern construction are also designed at various angles to the principal planes of projection. To show these features or surfaces in true shape for accurate description, auxiliary views are used. Auxiliary views are appropriate to obtain a true size view; similar techniques to standard views unfolding about an axis are used. Auxiliary views are usually partial views and show only the inclined surface of an object. In Figure 5.14 the true size and shape of the object are shown in the auxiliary views of the angular surface. An auxiliary view is similar to an orthographic view except that it is projected to a plane parallel to the auxiliary surface and not to the customary orthographic planes. It is thus drawn at an angle to best view an object but not one of the primary or-

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