4-11 Oblique Sketches
Oblique sketches are based on an axis system that contains one perpendicular set of axis lines and one receding line. See Figure 4-20. The front plane of an oblique axis is perpendicular, so the front face of a cube will appear as a square and the front face of a cylinder as a circle. The receding lines can be at any angle, but 30° is most common.
The receding lines of oblique sketches are parallel. As with isometric sketches, this causes some visual distortions, but unless the object is very large, these distortions are acceptable.
Holes in the front plane of an oblique sketch may be sketched as circles, but holes in the other two planes are sketched as ellipses. See Figure 4-21. The axis lines for the ellipse are parallel to the edge lines of the plane. The proportions of the ellipses are determined by points equidistant from the center point along the axis.
Figure 4-22 shows an example of a circular object sketched as an oblique sketch. Oblique sketches are particularly useful in sketching circular objects because they allow circles in the frontal planes to be sketched as circles rather than the elliptical shapes required by isometric sketches.