Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Apply First and Third-Angle Orthographic Projection.
• Construct Isometric and Oblique drawings.
• Use SANS conventions for lines, lettering, and dimensioning.
• Use geometric constructions to solve problems.
• Understand basic CAD commands and their use.

Lesson summary

This week focuses on consolidating our understanding of key EGD concepts covered in Term 1 and Term 2, specifically those frequently tested in Grade 10 examinations. Mastering these concepts is crucial not just for exam success but also for developing a strong foundation for future EGD studies and potentially careers in fields like architecture, engineering, and design. In South Africa, with its rapidly developing infrastructure and growing need for skilled professionals in technical fields, a solid understanding of EGD principles is increasingly valuable. This revision period will address areas where students commonly struggle, providing focused practice and clarification.

Lesson notes

This week's revision covers several crucial EGD concepts.

Let's explore them in detail: 2.

1. Orthographic Projection: Orthographic projection is a method of representing a three-dimensional object using two-dimensional views. The views are created by projecting lines perpendicular to the planes of projection.

There are two main systems: First-Angle Projection: The object is placed in the first quadrant, behind the projection planes. The view is drawn as if you are looking through the object onto the plane. This system is commonly used in Europe and Asia.

Third-Angle Projection: The object is placed in the third quadrant, in front of the projection planes. The view is drawn as if you are looking directly at the object. This system is predominantly used in North America and, importantly, South Africa according to SANS standards. Important

Note: In South Africa, we MUST use Third-Angle Projection unless specifically instructed otherwise. The symbol for Third-Angle Projection must be included on your drawings. This symbol consists of a truncated cone viewed from the front and side.

Example: Imagine a rectangular block. In orthographic projection, you'd typically draw the Front View (Elevation), Top View (Plan), and Right-Hand Side View. Each view shows the object from a different perspective, revealing its dimensions and features. 2.

2. Isometric and Oblique Projection: These are pictorial projections, meaning they attempt to represent the object in a single view that gives a more realistic impression.

Isometric Projection: All three axes are equally inclined to the plane of projection (at 120 degrees to each other), resulting in equal foreshortening along the axes. Lines parallel to the isometric axes are measured at their true length.

Oblique Projection: One face of the object is drawn parallel to the plane of projection, and the receding lines are drawn at an angle (usually 30°, 45°, or 60°) to the horizontal. Depth is usually drawn at half or full scale, depending on the type of oblique projection (Cavalier or Cabinet).

Example: Think of a shoebox. In an isometric drawing, all the edges are angled equally. In an oblique drawing, the front face would look like a regular rectangle, while the sides and top recede at an angle. 2.

3. SANS Conventions: Adhering to SANS (South African National Standards) is critical in EG

D. Key conventions include: Line Types: Visible Outlines: Thick, continuous lines.

Hidden Detail: Dashed lines.

Center Lines: Long-short dashed lines.

Dimension Lines: Thin, continuous lines with arrowheads.

Extension Lines: Thin, continuous lines extending from the object to indicate the dimension.

Cutting Plane Lines: Thick, dashed lines with arrowheads indicating the viewing direction for a section view.

Lettering: Use neat, legible lettering (either freehand or using a stencil) with consistent height and spacing. Guidelines are crucial for maintaining consistency.

Dimensioning: Place dimensions clearly and accurately. Avoid dimensioning hidden lines. Use leader lines to point to circles and arcs.

Sectioning: Use appropriate hatching to indicate the material that has been cut through in a section view. Different materials require different hatching patterns. 2.

4. Geometric Constructions: Mastering geometric constructions allows you to create accurate drawings using only a compass and straightedge.

Common constructions include: Bisecting a line or angle. Constructing perpendicular lines. Constructing parallel lines. Dividing a line into equal parts. Constructing polygons (e.g., equilateral triangle, square, pentagon, hexagon). Constructing tangents to circles.

Example: Constructing a hexagon inside a circle. First, draw the circle. Then, with the compass set to the radius of the circle, place the point on the circle and mark an arc. Repeat this process around the circle. Join the six points to form the hexagon. 2.

5. Introduction to CAD: Computer-Aided Design (CAD) software allows you to create drawings electronically. Familiarize yourself with basic commands such as: Line: Draws straight lines.

Circle: Draws circles (by center and radius or diameter).

Arc: Draws arcs.

Rectangle: Draws rectangles.

Polygon: Draws polygons.

Trim: Trims lines and curves.

Extend: Extends lines and curves.

Offset: Creates parallel lines or curves.

Copy: Copies objects.

Move: Moves objects.

Rotate: Rotates objects.

Mirror: Mirrors objects.

Fillet: Creates rounded corners.

Chamfer: Creates angled corners.

Dimension: Adds dimensions to drawings.

Example: To draw a square using CAD, you could use the "Line" command to draw four lines connecting at right angles or use the "Rectangle" command, specifying the length and width. Guided Practice (With Solutions)

Question 1: Draw the third-angle orthographic projection of a rectangular prism with dimensions: Length = 80mm, Width = 40mm, Height = 50mm.

Solution: Front View (Elevation): Draw a rectangle with a width of 80mm and a height of 50mm. Use thick, continuous lines.