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.
• Create orthographic projections using both methods.
• Identify the correct arrangement of views.
• Convert a 3D object into 2D orthographic projections.
• Apply the correct international symbols for each method.

Lesson summary

Orthographic projection is a fundamental skill in Engineering Graphics and Design (EGD). It allows us to represent three-dimensional objects accurately and unambiguously in two dimensions. This is crucial for communication between designers, engineers, and manufacturers. Understanding first-angle and third-angle projection methods is essential as these are the globally recognised standards for creating technical drawings. Imagine designing a new taxi rank in your community – you need orthographic projections to communicate its design and construction details clearly to everyone involved. Or consider designing a more efficient water tank for a rural area.

Lesson notes

What is Orthographic Projection? Orthographic projection is a method of representing a 3D object by projecting its features onto a 2D plane using parallel lines that are perpendicular (orthogonal) to the projection plane. In simpler terms, it's like shining a light directly onto the object and tracing its shadow on a piece of paper. We create multiple views of the object from different viewpoints (usually front, top, and side) to fully define its shape.

The Importance of Views: Front View (Elevation): Shows the object as seen from the front.

Top View (Plan): Shows the object as seen from above.

Side View (End Elevation/Profile View): Shows the object as seen from the side (either left or right). These three views are usually sufficient to describe a relatively simple object fully. More complex objects might need additional auxiliary views. First-Angle vs.

Third-Angle Projection: The Key Difference The key difference between these two methods lies in the relative position of the object, the projection plane, and the observer. Imagine you are drawing on a transparent piece of glass (the projection plane) between you (the observer) and the object.

First-Angle Projection: In first-angle projection, the object is placed in front of the projection plane (between the observer and the plane). Think of it as the object being the "first" thing you encounter between you and the plane. The view is then projected onto the plane behind the object.

Therefore, the views are arranged as follows: Top View is below the Front View. Right Side View is to the left of the Front View. Left Side View is to the right of the Front View. Rear view is to the right of the left side view.

Third-Angle Projection: In third-angle projection, the object is placed behind the projection plane (the observer is between the object and the plane). Imagine it as the object being in the "third" position (observer - plane - object). The view is then projected through the plane onto the plane.

The views are arranged as follows: Top View is above the Front View. Right Side View is to the right of the Front View. Left Side View is to the left of the Front View. Rear view is to the left of the left side view. The Projection Symbol It is essential to indicate which projection method is being used on a technical drawing. This is done using a standardized symbol. The symbol represents a frustum of a cone, viewed in both front and side elevation. The smaller circle always faces the direction of viewing.

First-Angle Projection Symbol: The larger circle is on the left, and the smaller circle is on the right, in the side view component of the symbol.

Third-Angle Projection Symbol: The smaller circle is on the left, and the larger circle is on the right, in the side view component of the symbol.