Pictorial Drawing (Isometric, Oblique, Perspective)
This domain tests three-dimensional representation: the rules of each system, correct axis angles, correct treatment of circles, and choosing the right pictorial form for the purpose.
Isometric
Three axes at 120° to each other, drawn with the two horizontal axes at 30° to the horizontal and the vertical axis upright. An isometric drawing is set out at full scale along the axes; a true isometric projection foreshortens every axis to 0.816 of true length. Non-isometric lines are never measured along the axes - locate their endpoints by coordinates on the isometric axes first. A circle on any face becomes an ellipse whose major axis is perpendicular to the axis normal to that face. Approximate it by the four-centre method, or use the exact dimensions: major axis 1.22D, minor axis 0.71D.
Oblique
The front face is drawn true shape in the plane of the paper; the third dimension recedes at an angle, usually 45°. Cavalier oblique uses full depth, cabinet oblique half depth to reduce distortion. Oblique suits objects with one important flat face, but it distorts badly when the receding dimension is long.
Perspective
Vertical lines stay vertical, and receding parallel lines converge on vanishing points on the horizon. One-point perspective suits a face-on view; two-point perspective sets the object corner nearest the eye with two vanishing points. Heights are measured on the picture plane. Keep vanishing points well apart, otherwise the drawing looks violently distorted.
Worked example
A 40 × 40 × 10 mm plate with a Ø20 hole through the top face. Draw the two base axes at 30°, mark 40 mm along each, and 10 mm up the vertical for thickness. To place the hole centre, measure 20 mm along both isometric axes and draw parallels. The circle's ellipse then has a major axis of 1.22 × 20 = 24.4 mm, perpendicular to the vertical axis, and a minor axis of 14.2 mm.
Traps
Measuring non-isometric lines at full length; drawing the isometric ellipse as an egg or a circle; wrong orientation of the ellipse's major axis; using full depth in a cabinet drawing; placing vanishing points too close together.