Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define orthographic projection and its principles.
• Draw the views of a simple object in both first-angle and third-angle.
• Identify the projection method used in a drawing.
• Convert a 3D object into 2D orthographic projections.
• Explain the importance of the projection symbol.

Lesson summary

This week, we delve into the crucial skill of orthographic projection, focusing on the difference between first-angle and third-angle projection. Orthographic projection is the foundation of technical drawing, allowing engineers, architects, and designers to accurately represent 3D objects in 2D. Understanding these projections is vital in South Africa, as it is applied everywhere from designing RDP houses to manufacturing parts for the automotive industry in places like Gqeberha (formerly Port Elizabeth) or creating infrastructure for rural electrification projects. A clear understanding avoids costly mistakes and ensures projects are built to specification.

Lesson notes

What is Orthographic Projection? Orthographic projection is a method of representing a three-dimensional object using two or more two-dimensional views. These views are projected onto planes perpendicular to each other. Imagine shining a light directly onto each face of an object. The shadow created on a screen placed perpendicular to the light beam is an orthographic projection. The most common views are the front view (also called the elevation), the top view (plan), and the side view. First-Angle vs. Third-Angle Projection The key difference between first-angle and third-angle projection lies in the position of the object relative to the observer and the projection plane.

First-Angle Projection: In first-angle projection (commonly used in Europe and South Africa), the object is placed between the observer and the projection plane.

Think of it this way: the object is projected onto the plane that is behind it. The front view is placed above the plan view, and the left side view is placed to the right of the front view.

Third-Angle Projection: In third-angle projection (commonly used in North America), the projection plane is between the observer and the object.

Think of it this way: the object is projected onto the plane in front of it. The front view is placed below the plan view, and the left side view is placed to the left of the front view.

Visualizing the Difference: Imagine holding a coffee mug.

First-Angle: If you place the mug behind a transparent screen, and you look at the mug through the screen, the image of the mug you see on the screen is the first-angle projection. The top view (plan) would be below the front view on your drawing.

Third-Angle: If you place the mug in front of a transparent screen, and you look at the mug through the screen, the image of the mug you see on the screen is the third-angle projection. The top view (plan) would be above the front view on your drawing. The Projection Symbol The projection symbol is a crucial indicator of whether a drawing uses first-angle or third-angle projection. It is usually found in the title block of the drawing.

First-Angle Symbol: The first-angle symbol looks like a truncated cone seen from the side. The larger diameter circle is closer to the line representing the projection plane.

Third-Angle Symbol: The third-angle symbol also looks like a truncated cone seen from the side, but the smaller diameter circle is closer to the line representing the projection plane. This is the opposite of the first-angle symbol.