Oblique projections and drawings are used when a designer wishes to clearly depict a part's irregular face. The face is placed parallel to the picture plane with the depth lines obliquely projected from the part at either 30°, 45° or 60°. Although any angle can be used, these angles are generally recommended as they are the common angles available on Set Squares. Of the three angles, 45° is the most commonly used.
Oblique drawing is believed to be the early versions of perspective drawing. They have been found on Greek vase decorations from the fourth century BC and on Chinese paintings up until the eighteenth century AD.
Selecting the Orrientation
The rules for orrientating oblique views are:
- Irregular surfaces of features (angles, radii, circles, elippses etc} should be orrientated perpendicular to the view angle.
- The longest dimension should be aligned perpendicular to the viewing angle.
- When there is a conflict in rules 1 and 2, rule 1 will take precendence.
Style of Oblique Projection
There are three forms of oblique projection:
- Cavalier Oblique
- Cabinet Oblique
- General Oblique
Cavalier drawings are projected in full scale despite this causing the drawing to be unnaturally elongated and make the depth lines appear to diverge.
Cabinet drawings have the depth of the object scaled by 0.5. The foreshortening of the depth provides a realistic illustration of the part being pictured.
General Oblique drawings have the depth of the object scaled by any factor other than 1 or 0.5. The foreshortening of the depth provides a realistic illustration of the part being pictured.