Lesson Notes By Weeks and Term v5 - Grade 10
Orthographic projections: first-angle and third-angle – Week 2 focus
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Orthographic projections: first-angle and third-angle – Week 2 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: 2
Theme: General lesson support
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Welcome to Week 2 of Engineering Graphics and Design! This week, we're diving deep into the crucial topic of orthographic projections, focusing specifically on first-angle and third-angle projection methods. This is a fundamental skill for anyone involved in design, engineering, architecture, or manufacturing. Imagine designing a new RDP house or a piece of furniture manufactured in a local workshop. Orthographic projection is the language used to communicate the precise shape and dimensions of these objects to builders and manufacturers. Being able to accurately create and interpret orthographic drawings is vital for effective communication in these fields.
What is Orthographic Projection? Orthographic projection is a method of representing a 3D object in 2D by projecting its features onto a series of planes that are perpendicular to each other. Think of it like shining a light onto an object from different angles. The shadows cast by the light onto the planes give you different views of the object. These views are then drawn as 2D representations. The main views used in orthographic projection are the Front View (or Elevation), Top View (or Plan), and a Side View (either Left Side View or Right Side View). Why Orthographic Projection? Orthographic projection provides a clear and unambiguous representation of an object's shape and dimensions. Unlike pictorial drawings (isometric, oblique, perspective), orthographic projections allow for accurate measurements and are essential for manufacturing and construction. First-Angle Projection vs.
Third-Angle Projection: The Key Difference The critical difference between first-angle and third-angle projection lies in the placement of the views relative to the object. First-Angle Projection (Used in South Africa): Imagine the object is between the observer and the projection plane. The view is then projected behind the object, onto the plane. This is the standard used in South Africa and many European countries. In first-angle projection, the Front View is above the Top View and the Left Side View is to the right of the Front View. Think of it as 'behind the object'.
Third-Angle Projection (Used in the USA): Imagine the object is behind the projection plane. The view is projected in front of the object, onto the plane. In third-angle projection, the Top View is above the Front View and the Right Side View is to the right of the Front View. Think of it as 'in front of the object'.
Mnemonic Devices: First-Angle: Front view F*irst. Front over Top.
Third-Angle: Top view T*hird. Top over Front.
The Symbol: A truncated cone is used to indicate which projection method is being used. The larger circle represents the front of the cone, and the smaller circle represents the back.
First Angle: The smaller circle is inside the larger circle when the shape has been projected, therefore the smaller circle goes on the left.
Third Angle: The larger circle is inside the smaller circle when the shape has been projected, therefore the larger circle goes on the left.
Creating an Orthographic Projection: Visualize the Object: Understand the 3D shape of the object.
Choose the Views: Decide which views (Front, Top, Side) are necessary to fully describe the object.
Establish the Projection Plane: Mentally position the projection planes. Decide if you are using First-Angle or Third-Angle.
Project the Views: Project the visible edges and features of the object onto the planes. Remember hidden details are shown using dashed lines.
Draw the Views: Draw the views accurately, using appropriate scales and dimensions.
Add Dimensions and Annotations: Include all necessary dimensions, notes, and the projection symbol.
Example 1: First-Angle Projection Let's consider a simple rectangular block with a square hole drilled through it. The block is 60mm long, 40mm wide, and 30mm high. The hole is 20mm x 20mm and is centered on the block.
Visualize: We have a basic rectangular prism with a square hole.
Choose Views: Front, Top, and Left Side View will show all the features.
Projection Plane: We will project the views onto the planes using First-Angle projection.
Project & Draw: Front View: A rectangle (60mm x 30mm) with dashed lines indicating the hole (20mm x 20mm centered).
Top View: A rectangle (60mm x 40mm) with a square representing the hole (20mm x 20mm centered). The Top View is below the Front View.
Left Side View: A rectangle (40mm x 30mm) with dashed lines representing the hole (20mm x 20mm centered). The Left Side View is to the right of the Front View.
Dimensions and Annotations: Add dimensions to show the overall size of the block and the size and location of the hole. Draw the First-Angle symbol.
Example 2: Third-Angle Projection Let's use the same rectangular block with the square hole.
Visualize: Same as before.
Choose Views: Same as before.
Projection Plane: We will project the views onto the planes using Third-Angle projection.
Project & Draw: Front View: A rectangle (60mm x 30mm) with dashed lines indicating the hole (20mm x 20mm centered).
Top View: A rectangle (60mm x 40mm) with a square representing the hole (20mm x 20mm centered). The Top View is above the Front View.
Right Side View: A rectangle (40mm x 30mm) with dashed lines representing the hole (20mm x 20mm centered). The Right Side View is to the right of the Front View.
Dimensions and Annotations: Add dimensions to show the overall size of the block and the size and location of the hole. Draw the Third-Angle symbol.
Example 3: A more complex example showing hidden detail.