Technical Drawing Training Lesson 10: Pictorial Projection, Isometric Drawing and Sketching

Everything so far has been orthographic projection: several flat views that together describe a part. That method is perfect for manufacturing, but it has one weakness — you cannot see the part. You have to assemble it in your head from two or three views.

Pictorial projection solves that. It shows three faces of a part in one drawing, so the shape is understood at a glance. It is not a substitute for orthographic views — it is a companion to them, used on assembly instructions, catalogues, patent drawings and, above all, on sketches.

Types of pictorial projection

Axonometric

The part is tilted relative to the projection plane; the line of sight stays perpendicular to it. Three variants:

Oblique

Here the front face stays parallel to the paper and is drawn in its true shape; only the depth runs off at an angle, usually 45°.

• Cavalier: depth drawn at full scale. Simple, but the part looks unnaturally deep.
• Cabinet: depth drawn at half scale. Looks far more natural and is the usual choice.

Oblique projection is the easiest method when the front face contains circles — they stay circles instead of becoming ellipses.

Perspective

True perspective, with vanishing points, is how the eye actually sees. It is used in architecture and illustration but not in technical drawing, because parallel edges converge and nothing can be measured off the drawing.

Isometric drawing: the box method

1. Draw the box. Take the overall length, width and height of the part and build that box in isometric — two axes at 30° to the horizontal, one vertical.
2. Mark the sizes. Set out the real dimensions along the box edges. In isometric all three axes are at full scale, so no calculation is needed.
3. Remove the surplus. Cut away everything that is not part of the component, and the shape appears.

Freehand sketching

A sketch is a drawing made freehand, without instruments and without a scale. It is not a lesser drawing — it is the fastest way to record an idea, a broken part, or a measurement taken on the shop floor. Almost every finished drawing begins life as one.

• No scale, but proportions must be right — a part twice as long is drawn twice as long.
• Dimensions are always written on, and they are the real dimensions.
• The same line types apply: thick for visible, dashed for hidden, chain for centres.
• Draw long lines with the whole arm, short ones with the wrist; rotate the paper rather than twisting your hand.
• Sketch light construction lines first, then firm up the outline.

To sketch a circle, draw a square of the right size, mark the midpoints of the sides and the diagonals, and join the eight resulting points with a smooth curve. For an ellipse on an isometric face, sketch the rhombus first and fit four arcs inside it.

Worked Example: The Isometric Axis Set and a Cube

The whole secret of isometric drawing lies in the axis set: one vertical axis and two axes at 30° to the horizontal. Every dimension travels along its own axis, and the same scale is used in all three directions (isometric = equal measure). The one trap is circles: in isometric a circle becomes an ellipse:

Test yourself

1. What are the axis angles in isometric projection?
   Two axes at 30° to the horizontal, one vertical.
2. What is the difference between cavalier and cabinet oblique?
   Cavalier draws the depth full size; cabinet draws it at half size, which looks more natural.
3. Why is true perspective not used in technical drawing?
   Parallel edges converge, so nothing on the drawing can be measured.
4. How does a circle appear on an isometric face?
   As an ellipse, fitted inside the isometric square (a rhombus).
5. Is a sketch drawn to scale?
   No — but its proportions must be right and its written dimensions are real.
6. Where are dimensions measured in an isometric drawing?
   Only along the three axes, never along a diagonal.