Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Differentiate between first-angle and third-angle projection.
• Draw orthographic views using both methods.
• Identify the correct projection method from a drawing.
• Apply the correct symbols for each method.
• Explain the positioning of views in both projections.

Lesson summary

Orthographic projection is a fundamental skill in Engineering Graphics and Design (EGD). It's a method of representing a 3D object using 2D views. Understanding first-angle and third-angle projection is crucial because they dictate the arrangement of these views, ensuring clear and unambiguous communication of design ideas. Misinterpreting these projections can lead to costly errors in manufacturing and construction. In South Africa, this skill is essential for anyone pursuing a career in engineering, architecture, construction, or design, all of which are vital for our nation's infrastructure development and economic growth.

Lesson notes

Orthographic projection involves projecting the features of an object onto mutually perpendicular planes. These planes are typically the horizontal plane (HP), the vertical plane (VP), and the profile plane (PP). The resulting views are the top view, front view, and side view, respectively. The crucial difference between first-angle and third-angle projection lies in the assumed position of the object relative to the observer and the projection plane. 2.1 First-Angle Projection: Concept: The object is placed in front of the projection plane (between the observer and the plane). Imagine placing a small toy car between you and a glass wall. The car's shadow on the wall represents the projection.

View Arrangement: The front view is projected onto the vertical plane (VP). The top view is projected onto the horizontal plane (HP) and is placed below the front view. The right side view is projected onto the profile plane (PP) and is placed to the left of the front view.

Symbol: The symbol is a truncated cone with the small end facing the observer. The full circle representing the base is positioned behind the smaller, truncated circle.

Why it matters: First-angle projection is commonly used in Europe and many parts of the world, including some industries in South Africa. Understanding it is necessary for interpreting drawings from these regions. 2.2 Third-Angle Projection: Concept: The projection plane is placed between the observer and the object. Imagine placing a glass wall between you and a small toy car. The car's shadow on the wall represents the projection.

View Arrangement: The front view is projected onto the vertical plane (VP). The top view is projected onto the horizontal plane (HP) and is placed above the front view. The right side view is projected onto the profile plane (PP) and is placed to the right of the front view.

Symbol: The symbol is a truncated cone with the small end facing the observer. The full circle representing the base is positioned in front of the smaller, truncated circle.

Why it matters: Third-angle projection is the standard in the United States and is increasingly used in South Africa, especially in industries influenced by international standards. 2.3 Visual Analogy: Think of a stamp. In first-angle projection, you are stamping onto the back of the paper. The resulting image shows what is behind the object. In third-angle projection, you are stamping onto the front of the paper. The resulting image shows what is in front of the object. 2.4 Worked

Examples: Example 1: Simple Block (First-Angle) Imagine a rectangular block with dimensions 50mm x 30mm x 20mm (length x height x width).

Front View: A rectangle 50mm wide and 30mm high.

Top View: A rectangle 50mm wide and 20mm deep. This is placed below the front view.

Right Side View: A rectangle 20mm wide and 30mm high. This is placed to the left of the front view.

Projection Lines: Project lines vertically downwards from the front view to establish the width of the top view. Project lines horizontally from the front view to establish the height of the side view. Project lines from the top view across at 45 degrees to the side view.

Symbol: Draw the first-angle projection symbol in the bottom-right corner of the drawing.

Example 2: Simple Block (Third-Angle) Using the same rectangular block as above (50mm x 30mm x 20mm): Front View: A rectangle 50mm wide and 30mm high.

Top View: A rectangle 50mm wide and 20mm deep. This is placed above the front view.

Right Side View: A rectangle 20mm wide and 30mm high. This is placed to the right of the front view.

Projection Lines: Same as in the First-Angle example.

Symbol: Draw the third-angle projection symbol in the bottom-right corner of the drawing.

Example 3: Object with a Hole (First-Angle) Consider a cube (40mm sides) with a cylindrical hole (diameter 20mm) drilled through the centre from front to back.

Front View: A square (40mm x 40mm) with a hidden circle (diameter 20mm) represented by dashed lines. The circle is centered within the square.

Top View: A square (40mm x 40mm) with a rectangle (40mm x 20mm) centered in the middle.

Right Side View: A square (40mm x 40mm) with a hidden circle (diameter 20mm) represented by dashed lines. This view looks identical to the front view.

Projection Lines: Same as in the First-Angle example.

Symbol: Draw the first-angle projection symbol in the bottom-right corner of the drawing. Guided Practice (With Solutions)

Question 1: Sketch the front view and top view in first-angle projection of a triangular prism lying on one of its rectangular faces. The triangular face is an equilateral triangle with sides of 40mm. The length of the prism is 60mm.

Solution: Front View: A rectangle representing the rectangular face (60mm x 40mm).

Top View: An equilateral triangle (40mm sides) positioned below the front view.

Commentary: The key here is understanding that in first-angle, the top view goes below the front view.