Lesson Notes By Weeks and Term v5 - Grade 10
Orthographic projections: first-angle and third-angle – Week 1 focus
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Orthographic projections: first-angle and third-angle – Week 1 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Engineering Graphics and Design
Class: Grade 10
Term: 2nd Term
Week: 1
Theme: General lesson support
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Orthographic projection is a fundamental skill in Engineering Graphics and Design (EGD). It's a method of representing a 3D object using 2D views, specifically the front, top, and side views. These views are crucial for understanding the object's shape, size, and features. Imagine a builder trying to construct a new house in Soweto without proper plans – orthographic projections are the plans, ensuring everyone understands the design and minimizing costly mistakes.
What is Orthographic Projection? Orthographic projection is a method of representing a 3D object using multiple 2D views. Each view shows the object as seen from a particular direction, usually perpendicular to one of the object's principal faces. These views are typically the front, top, and right-side views. We use parallel projectors (imaginary lines) to project the object onto a plane of projection. These projectors are perpendicular to the projection plane, hence the term "orthographic" (meaning "right-angled"). First-Angle vs.
Third-Angle Projection: The Core Difference The key difference lies in the relative positions of the observer, the object, and the projection plane.
Think of it like this: First-Angle Projection: The object is placed between the observer and the projection plane. Imagine you are looking at an object, and a transparent screen is placed behind the object. You draw the object as you see it projected onto that screen from your viewpoint. This is the standard used in South Africa, most of Europe, and many other countries.
Third-Angle Projection: The projection plane is placed between the observer and the object. Imagine you are looking at an object, and a transparent screen is placed in front of the object, between you and the object. You draw the object as you see it projected onto that screen from your viewpoint. This is primarily used in North America. Visualizing the Difference It's easier to understand with a simple object like a cube.
First-Angle: Front View: Shows the front face of the cube.
Top View: Placed below the Front View (imagine folding the top projection plane down).
Right-Side View: Placed to the left of the Front View (imagine folding the right-side projection plane to the left).
Third-Angle: Front View: Shows the front face of the cube.
Top View: Placed above the Front View (imagine folding the top projection plane up).
Right-Side View: Placed to the right of the Front View (imagine folding the right-side projection plane to the right). The Importance of Symbols To clearly indicate which projection method is used, a standard symbol is used on engineering drawings. These symbols are defined by SANS (South African National Standards) and ISO (International Organization for Standardization) standards. You must learn these symbols.
First-Angle Projection Symbol: A truncated cone, with the smaller circle closer to the observer.
Third-Angle Projection Symbol: A truncated cone, with the larger circle closer to the observer.
Example 1: First-Angle Projection of a Simple L-Block
Let's say we have an L-shaped block with the following dimensions:
Total Height: 50mm
Total Width: 60mm
Base Width: 40mm
Base Height: 20mm
Depth (into the page): 30mm
Front View: Draw the L-shape as seen from the front. The height is 50mm, base width is 40mm, and the remaining width is 20mm. The base height is 20mm, and the remaining height is 30mm.
Top View: This view is placed below the front view in first-angle projection. It shows the top surface of the L-shape. The width is 60mm, and the depth is 30mm. You'll see a rectangle representing the top of the base (40mm x 30mm) and another rectangle representing the top of the remaining section (20mm x 30mm), adjacent to each other.
Right-Side View: This view is placed to the left of the front view. It shows the side of the L-shape. The height is 50mm, and the depth is 30mm. You'll see a rectangle representing the side of the base (20mm x 30mm) and another rectangle representing the side of the remaining section (30mm x 30mm) sitting on top of each other.
Example 2: Third-Angle Projection of the Same L-Block
Using the same L-block dimensions as above:
Front View: The Front View remains the same as in the First-Angle projection.
Top View: This view is placed above the front view in third-angle projection. It is identical to the Top View in Example
1. Right-Side View: This view is placed to the right of the front view. It is identical to the Right-Side View in Example 1.