Pictorial Drawing: Isometric and Oblique
Pictorial Drawing: Isometric and Oblique
This domain tests three-dimensional representation: isometric axes and ellipses, and cavalier or cabinet oblique projection. Pictorial views communicate shape quickly but are not working drawings.
Core rules
- Isometric axes are 120° apart; the two receding axes are 30° above the horizontal and vertical edges remain vertical. All three axes use the same scale.
- No perspective: parallel edges stay parallel.
- Non-isometric lines are not measured directly; locate their endpoints on isometric axes first.
- An isometric circle is an ellipse. Its major axis is perpendicular to the face normal; for a top face the major axis is horizontal. Approximate major = 1.22D and minor = 0.71D, or use the four-centre method.
- Oblique: the front face is drawn true shape. Cavalier uses full depth; cabinet uses half depth, usually on a 45° receding axis.
Worked example
Draw a 40 mm cube isometrically. Draw a vertical 40 mm edge. From its lower end draw two 30° lines 40 mm long. Complete the top and side faces with parallel lines. For a 24 mm circle on the top face, draw the isometric square, find its centre and four centres, then draw the ellipse. Its major axis is about 29.3 mm horizontally and its minor about 17.0 mm. Reasoning: the circle lies in a horizontal plane, so it is foreshortened into an ellipse aligned with that face.
Traps
- Using 45° for isometric receding edges.
- Measuring along a non-isometric edge.
- Drawing isometric circles as true circles.
- Using full depth in a cabinet drawing.
- Forgetting that hidden edges are usually omitted in pictorials.