GRADE 9 Pre-technical COMMUNICATION IN PRE-TECHNICAL STUDIES – OBLIQUE PROJECTION Notes

COMMUNICATION IN PRE-TECHNICAL STUDIES

Subtopic: OBLIQUE PROJECTION

Oblique projection is a simple way to draw 3D objects on paper so they look three dimensional. It is useful in pre-technical studies because it is quick and easy to sketch by hand and helps people understand shape, size and depth.

1. Key ideas (simple)

• Front face (or principal face) is drawn true size (no distortion).
• Receding lines show depth. They are drawn at an angle (often 45°) from each corner of the front face.
• Depth scale: Cavalier projection uses full depth (scale 1.0). Cabinet projection uses half depth (scale 0.5) — it looks more natural.
• All receding lines are parallel (this is a parallel projection).

2. Two common types

3. Simple drawing steps (use mm)

1. Draw the front face in true size (for example a rectangle 60 mm × 40 mm).
2. Choose an angle for receding lines (commonly 45° to the horizontal) and draw light guide lines from the corners.
3. Measure the depth on the guide lines:
   • For cavalier: use full depth (e.g., 30 mm).
   • For cabinet: use half depth (e.g., 15 mm).
4. Connect the corresponding depth end points to form the back edges.
5. Darken the visible edges, use dashed lines for hidden edges if necessary, and label dimensions in mm.

4. Visual examples (simple)

5. Example: Draw a block (step-by-step)

Given: block 80 mm (width) × 40 mm (height) × 30 mm (depth). Draw a cabinet oblique at 45°.

1. Draw front face rectangle 80 mm by 40 mm (width horizontal, height vertical).
2. From each corner draw light 45° guide lines.
3. Measure depth 30 mm, but for cabinet use half → 15 mm along each guide line. Mark points.
4. Join marked points to form the back edges. Darken visible edges. Label: 80 mm, 40 mm, depth 30 mm (note: drawn as 15 mm).

6. Uses and why we learn it (Kenyan pre-technical context)

• Makes it easy to show how a design will look before building it (carpentry, metalwork).
• Quick communication in workshops and classroom practicals — teachers, learners and craftsmen understand sketches fast.
• Simple to teach and draw with a ruler, protractor and set square — suitable for Form 2/3 (age 14) pre-technical lessons.

7. Tips for neat drawings

• Use light lines for guides and receding lines; darken final outlines.
• Always write units (mm) beside dimensions.
• Use cabinet projection for hand sketches — it usually looks better.
• Keep receding lines parallel and at the chosen angle (45° is common and easy to measure with a protractor).

8. Practice questions

1. Draw a cavalier oblique of a book 150 mm × 100 mm × 20 mm (use 45° angle). Show all dimensions.
2. Draw a cabinet oblique of a wooden box 120 mm × 60 mm × 40 mm. Label the sides and show depth measured as half.

Ask your teacher to check your practice drawings. Keep copies of good sketches in your notebook — they help when you make models in the workshop.