Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Apply principles of orthographic projection (first- and third-angle).
• Draw accurate sectional views (full and half sections).
• Use descriptive geometry to find true lengths and shapes.
• Create surface development drawings for various solids.
• Apply SANS conventions and symbols correctly.

Lesson summary

Welcome, Grade 11 EGD learners! This week is dedicated to focused revision and examination preparation. EGD is not just about drawing; it's about visual communication, problem-solving, and spatial reasoning – skills highly valued in various fields in South Africa, from architecture and engineering to manufacturing and design. Think about how you see construction plans for new RDP houses, designs for safer minibus taxis, or even the layout of a new shopping mall. All of these started with EGD principles. Mastering EGD empowers you to understand and contribute to the built environment around you.

Lesson notes

Let’s dive into the core concepts we'll be revising this week. 2.1 Orthographic Projection (First- and Third-Angle): Orthographic projection is a method of representing 3D objects in 2D by projecting different views (front, top, side) onto mutually perpendicular planes.

First-Angle Projection: The object is placed in front of the projection planes. The observer is looking through the object onto the plane. This is the European standard. Think of shining a light through the object to create a shadow on the wall behind it. The Front View is above the Top View. The Left View is on the right of the Front View.

Third-Angle Projection: The object is placed behind the projection planes. The observer is looking directly at the object. This is the American standard (and increasingly used globally). Think of placing the object behind a pane of glass and drawing what you see on the glass. The Front View is below the Top View. The Left View is on the left of the Front View.

Example: Imagine a simple rectangular block. In first-angle projection, if the front view is drawn at the top of the page, the top view will be drawn below it. In third-angle, the top view would be drawn above the front view. Remember to always label which projection method you are using. 2.2 Sectional Views (Full and Half Sections): Sectional views are used to reveal the internal details of an object that would otherwise be hidden. A cutting plane is used to imagine slicing through the object. The sectioned surface is then hatched using standard hatching symbols (usually at 45 degrees).

Full Section: The cutting plane passes completely through the object.

Half Section: The cutting plane cuts only halfway through the object. This is useful for symmetrical objects, showing both the internal and external features in a single view.

Example: Think of a pipe. If you draw a regular orthographic view, you only see the outside. But if you draw a sectional view, you can see the thickness of the pipe wall, any internal features, and so on. Remember to indicate the cutting plane line clearly with appropriate arrowheads. If the cutting plane is a broken plane, indicate it clearly with chain lines and arrows. 2.3 Descriptive Geometry (True Lengths, True Shapes, Angles): Descriptive geometry deals with finding the true shape and size of objects and the angles between lines and planes.

True Length (TL): The actual length of a line. A line appears at its true length when it is parallel to the projection plane.

True Shape (TS): The actual shape of a plane surface. A plane appears at its true shape when the line of sight is perpendicular to the plane.

Finding True Length: Rotate the line until it is parallel to the reference plane. Project the rotated view of the line to the adjacent plane. This is the true length.

Finding True Shape: Project an auxiliary view where the edge view of the inclined plane is seen. Project from the auxiliary view to an adjacent plane to get the true shape.

Example: Consider an inclined line connecting two points. To find its true length, you can rotate it in either the front or top view until it's parallel to the horizontal or vertical plane, respectively. Project that rotated line into the other view, and the resulting projection will show the true length. 2.4 Surface Development: Surface development involves unfolding a 3D object onto a 2D plane. This is crucial for sheet metal work, packaging design, and other manufacturing processes.

Prisms: Develop by unfolding the rectangular sides and adding the top and bottom faces.

Pyramids: Develop by drawing the triangular faces connected at the apex. Calculate the true length of the slant edges.

Cylinders: Develop into a rectangle. The length of the rectangle equals the circumference of the base circle (2πr).

Cones: Develop into a sector of a circle. The radius of the sector equals the slant height of the cone. The arc length of the sector equals the circumference of the base circle.

Example: For a square prism, you'd draw four rectangles representing the sides and then add the top and bottom squares to complete the development. Remember to consider seam allowances when designing the development. 2.5 SANS Conventions and Symbols: The South African National Standards (SANS) dictate the conventions and symbols used in engineering drawings. Understanding these is crucial for interpreting and creating technical drawings that comply with industry standards.

Line Types: Continuous thick lines (visible outlines), continuous thin lines (dimension lines, projection lines), dashed lines (hidden outlines), chain lines (centre lines, cutting plane lines).

Hatching Symbols: Different materials are represented by different hatching patterns.

Dimensioning: Rules for placing dimensions, using extension lines, and arrowheads.

Tolerance Symbols: Indicating acceptable variations in dimensions.

Surface Finish Symbols: Indicating the required surface roughness.