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Visualization and Sketching

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3.23 Oblique Sketches

Oblique drawing is an easy method for creating quick pictorials (Figure 3.55). In most oblique sketches, circles and angles parallel to the projection plane are true size and shape and are therefore easy to construct. Although circular shapes are easy to sketch in the front oblique plane, they would appear elliptical in the top or sides. Oblique views are primarily a sketching technique used when the majority of circular shapes appear in the front view or when the object can be rotated to position circles in the front view.

CAD is not typically used to create oblique views because better-appearing isometric or perspective drawings can be created easily from 3D CAD models.

Appearance of Oblique Drawings

Three things affect the appearance of your oblique sketch:

1. The surface of the object you choose to show parallel to the projection plane

2. The angle and orientation you choose for the receding lines that depict the object’s depth

3. The scale you choose for the receding lines depicting the object’s depth (Figure 3.56)

Meat Cereals. Oblique sketching shows the front surface parallel to the view, making these meat cereal boxes easy to draw. (Courtesy of Randall Munroe.)

Choosing the Front Surface

Think about which surface of the object would be the best one to think of as parallel to the plane of projection. For example, a cube has six different surfaces. As you are creating your sketch, any of those six surfaces could be oriented as the “front” of the part. Of course, with a cube it wouldn’t matter which one you chose. But a cube with a hole through it will make a much better oblique sketch when the round hole is oriented parallel to the projection plane.

Angle of Receding Lines

An angle of 45° is often chosen for the angle of the receding lines because it makes oblique sketches quick and easy. You can use graph paper and draw the angled lines through the diagonals of the grid boxes. An angle of 30° is also a popular choice. It can look more realistic at times. Any angle can be used, but 45° is typical. As shown in Figure 3.57, you can produce different oblique drawings by choosing different directions for the receding lines.