Isometric and Oblique Projections
What the exam tests
Isometric and oblique projections test pictorial drawing from orthographic views, correct axes and scales, and the difference between isometric drawing and isometric projection. Oblique cavalier and cabinet projections are also examinable.
Core rules
- Isometric axes are 120° apart; the three visible axes are drawn at 30° to the horizontal, with verticals vertical.
- Isometric projection uses an isometric scale of about 0.816 to reduce true lengths. Isometric drawing is made full size along the axes for convenience; state which method you are using.
- In oblique projection, the front face is drawn true shape; the receding axis is usually 45°. Cavalier uses full depth, cabinet uses half depth.
- Non-isometric lines are not drawn directly to scale; locate their endpoints on the axes and join them.
- Circles in isometric become ellipses; use the four-centre approximate method or an isometric ellipse template.
Worked example
Draw a 40 mm cube isometrically. From a corner O, draw a vertical axis and two axes at 30° to the horizontal. Mark 40 mm along each axis. At each marked point draw lines parallel to the other two axes to complete the three visible faces. For isometric projection, multiply each true length by 0.816, so 40 mm becomes 32.6 mm before setting out. Do not measure the sloping face edges directly; they are isometric lines only because they are parallel to an axis.
Traps
Using 45° for isometric axes, or drawing a cube with equal 90° corners, is wrong. Do not apply 0.816 to an isometric drawing unless the question asks for projection. In oblique cabinet, halving only the depth is often forgotten. Hidden edges are usually omitted in pictorial views unless asked.