Pictorial drawing and 3D representation
This domain tests pictorial methods that give a single, three-dimensional reading: axonometric (isometric, dimetric, trimetric), oblique, and perspective. Questions ask for an isometric view from orthographic views, or for identification of the method used.
Core rules
- Isometric: three axes at 120 degrees to each other, the vertical axis upright and the other two at 30 degrees to the horizontal. Lengths are true only along the axes; any line not parallel to an axis must have its endpoints located by coordinates and then joined.
- Isometric scale is 0.816 of true length if a true-length drawing is required, but standard practice draws full lengths along the axes.
- Isometric circles are ellipses: major axis 1.22d perpendicular to the axis normal to the face, minor axis 0.71d. A four-centre approximation uses 60 degree construction lines.
- Oblique: the front face is drawn true shape; the receding axis is usually 45, 30 or 60 degrees. Cavalier uses full depth, cabinet uses half depth.
- Perspective: one-point and two-point, with a horizon line, station point, picture plane and vanishing points. Objects diminish toward the vanishing points.
Worked example
A 40 mm cube with a 20 mm diameter hole centred in its top face. Draw verticals 40 mm, and 30 degree lines 40 mm for the two receding directions. For the top-face circle, the major axis is 24.4 mm and lies horizontal, parallel to the top face, and the minor axis is 14.2 mm vertical. Place the ellipse centre at the face centre before drawing it.
Traps
Measuring along a 30 degree axis with the true-length scale when the question asks for isometric scale; drawing the ellipse major axis along the wrong direction; calling half-depth oblique cavalier; omitting hidden edges in a pictorial view.