Pictorial Drawing: Isometric and Oblique Projection
This domain tests your ability to build a single pictorial view from orthographic information, and to handle the two things that defeat most candidates: non-isometric lines and circles.
Isometric
Three axes at 30° to the horizontal, with verticals remaining vertical. In an isometric drawing full sizes are stepped off along the three axes; in an isometric projection true lengths are multiplied by 0.816 (or the work is set out on an isometric grid). Only lines parallel to the axes can be measured directly along the paper. Any sloping or oblique edge must be located by its two endpoints, each found by measuring along two axes and projecting.
Circles in isometric
A circle on any isometric face becomes an ellipse. For a circle of diameter D the isometric ellipse has a major axis of about 1.22D and a minor axis of about 0.71D, with the major axis always perpendicular to the axis of the cylinder. Use the four-centre approximation, an isometric square with tangent points, or a template.
Oblique
Front face drawn true shape and true size; receding axis at 45° (sometimes 30°). Cavalier uses full depth, cabinet uses half depth. Circles on the front face stay circles; circles on receding faces become ellipses.
Worked example
A block 50 long × 30 deep × 25 high has a 20 mm step cut from the top front edge along its full length. Set out the 50 × 30 base on the two 30° axes, erect 25 mm on the verticals, then measure 20 mm down the front vertical and 20 mm back along the depth axis at each end. Join those two points to form the step; the line joining them is parallel to an axis, so it can be checked by measurement.
Traps
Setting axes at 45° instead of 30°; measuring along a sloping edge as if it were an axis; drawing the ellipse with its major axis along the cylinder axis; mixing cabinet half-depth with cavalier full-depth in one drawing.