Orthographic Projection

Subject: Basic Technology
Class: JSS 3
Term: First Term
Week: 12
Age: 12–15 years
Topic: Orthographic Projection
Sub-topic: Definition, Types (1st and 3rd Angle), Drawing Orthographic Views, and Dimension Techniques
Duration: 60 minutes

Behavioural Objectives

By the end of this lesson, students should be able to:

1. Define orthographic projection.
2. Differentiate between 1st-angle and 3rd-angle projections.
3. Draw orthographic views of simple objects.
4. Apply correct dimensioning techniques in orthographic drawings.

Keywords

• Orthographic Projection
• 1st-Angle Projection
• 3rd-Angle Projection
• Plan View
• Front View
• Side View
• Dimensioning

Set Induction

The teacher shows students a 3D model of a cube and rotates it to ask, “If I lay this flat, what will it look like from the top, front, and side?” This introduces the concept of representing 3D objects in 2D views through orthographic projection.

Entry Behaviour

Students have prior knowledge of basic shapes and can draw simple objects in isometric views.

Learning Resources and Materials

• Whiteboard and markers
• Graph paper, pencils, rulers, and erasers
• 3D models (e.g., a cube or rectangular block)
• Orthographic projection templates

Building Background/Connection to Prior Knowledge

In previous lessons, students learned about isometric and oblique drawings, which represent objects in 3D. Orthographic projection, on the other hand, breaks an object down into multiple 2D views for better clarity.

Embedded Core Skills

• Critical thinking
• Problem-solving skills
• Visualization skills

Learning Materials

• Graph paper, pencils, and rulers
• 3D object models
• Projection charts

Reference Books

• Lagos State Scheme of Work
• Basic Technology Textbook for Junior Secondary Schools
• “Technical Drawing for Beginners” by J. O. Oloruntola

Instructional Materials

• Models of simple objects (e.g., cube, prism)
• Orthographic projection charts

Content

I. Definition of Orthographic Projection

• Orthographic projection is a method of drawing a 3D object from multiple views at right angles to each other.
• It involves projecting the object onto a 2D plane to display its front view, side view, and plan view (top view).

II. Types of Orthographic Projection

1st-Angle Projection

• The object is placed between the viewer and the plane of projection.
• The views appear as follows:
  • Plan view (Top) is placed below the front view.
  • Side view is placed opposite the side shown.

3rd-Angle Projection

• The plane of projection is placed between the object and the viewer.
• The views appear as follows:
  • Plan view is placed above the front view.
  • Side view is placed on the same side as the view shown.

III. Drawing Orthographic Views of Simple Objects

1. Step 1: Draw the front view of the object on the graph paper.
2. Step 2: Draw the plan view (top view) directly above or below the front view, depending on the projection type.
3. Step 3: Draw the side view (right or left) beside the front view.
4. Step 4: Label all views properly (Front, Plan, Side).
5. Step 5: Use appropriate dimensioning techniques.

IV. Dimension Technique

• Use a continuous thin line for dimension lines.
• Place dimensions outside the object to avoid cluttering the drawing.
• Use arrowheads to mark the ends of dimension lines.
• Label the dimension values in millimeters (mm) or as required.

Examples of Simple Objects to Draw in Orthographic Views

1. Cube
2. Rectangular block
3. L-shaped block
4. Cylinder
5. Prism

Evaluation (Fill-in-the-Blank Questions)

1. Orthographic projection represents a 3D object using multiple __________ views.
   (a) artistic (b) 2D (c) isometric (d) oblique
2. In __________-angle projection, the plan view is placed above the front view.
   (a) 1st (b) 3rd (c) 2nd (d) 4th
3. A __________ view shows the object from the top.
   (a) side (b) front (c) plan (d) rear
4. The __________ view shows the object from the side.
   (a) plan (b) top (c) side (d) bottom
5. Orthographic projection uses a __________ to mark dimensions.
   (a) thick line (b) broken line (c) thin line (d) dotted line
6. The lines meeting at the ends of a dimension line are called __________.
   (a) arcs (b) arrowheads (c) ticks (d) angles
7. In orthographic projection, each view is drawn at a __________ degree angle to the others.
   (a) 30 (b) 60 (c) 90 (d) 120
8. The side view of an object is placed beside the __________ view.
   (a) plan (b) bottom (c) front (d) top
9. Which drawing method uses continuous thin lines for measurements?
   (a) Isometric (b) Orthographic (c) Oblique (d) Artistic
10. Dimension values are written in __________ units.
    (a) meters (b) kilometers (c) millimeters (d) inches

Class Activity Discussion (FAQs)

1. What is orthographic projection?
   It is a method of drawing 3D objects using multiple 2D views.
2. What are the common views used in orthographic projection?
   Front view, plan view, and side view.
3. How is 1st-angle projection different from 3rd-angle projection?
   In 1st-angle, the plan view is below the front view, while in 3rd-angle, it is above.
4. Why is orthographic projection important?
   It helps engineers and designers create detailed 2D plans for 3D objects.
5. What type of projection is used in technical drawings?
   Orthographic projection is used to create precise technical drawings.
6. What is a plan view?
   It is the top view of an object.
7. What tools are needed for orthographic drawing?
   Pencils, rulers, graph paper, and erasers.
8. Why are thin lines used for dimensions?
   Thin lines prevent clutter and maintain clarity in the drawing.
9. What is the correct unit for dimensions in technical drawings?
   Millimeters (mm).
10. Can complex shapes be drawn using orthographic projection?
    Yes, but it requires practice to break them into simpler parts.

Teacher’s Activities

1. Demonstrate the differences between 1st-angle and 3rd-angle projections using charts.
2. Guide students in drawing the orthographic views of a cube.
3. Explain and demonstrate dimensioning techniques.

Learners’ Activities

1. Draw the orthographic views of an L-shaped block.
2. Identify and label the plan, front, and side views in their drawings.
3. Practice using correct dimensioning techniques.

Evaluation Questions

1. Define orthographic projection.
2. What is the difference between 1st-angle and 3rd-angle projections?
3. List three views used in orthographic projection.
4. Draw the front, plan, and side views of a cube.
5. Why are thin lines used for dimensions?
6. Where is the plan view placed in 3rd-angle projection?
7. Name two tools used for orthographic drawing.
8. How many degrees separate the views in orthographic projection?
9. What is a side view?
10. Draw the orthographic views of a rectangular block.

Conclusion

The teacher moves around to check students’ work, provides feedback, and makes corrections where necessary.