Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Apply CAD principles to solve complex problems.
• Create accurate orthographic projections, including sections.
• Solve isometric and perspective drawing problems.
• Analyse civil and mechanical engineering drawings.
• Manage time effectively and apply drawing standards.

Lesson summary

This week marks a crucial phase in your Grade 12 Engineering Graphics and Design (EGD) journey: intensive revision and targeted examination preparation. As the final examinations loom, a structured approach to revisiting key concepts and practicing exam-style questions is paramount. Mastering EGD not only equips you with valuable technical drawing skills, but also fosters spatial reasoning, problem-solving abilities, and meticulous attention to detail – all highly sought-after attributes in various fields such as engineering, architecture, construction, and design, which are vital for South Africa's developing infrastructure and economy.

Lesson notes

This week's revision encompasses key areas within the Grade 12 EGD curriculum. We will revisit CAD commands, orthographic projections, isometric and perspective drawings, civil engineering drawings, and mechanical engineering drawings.

Let's delve into each of these areas:

A. CAD (Computer-Aided Design): CAD software (like AutoCAD, SolidWorks, or similar) is an indispensable tool for modern engineering drawing. It allows for precise and efficient creation, modification, and analysis of designs.

Key Commands & Concepts: Line, Circle, Arc: Basic drawing elements. Understanding how to specify coordinates (absolute, relative, polar) is crucial.

Offset: Creates parallel lines or curves at a specified distance. Useful for drawing walls, pipes, etc.

Trim & Extend: Essential for cleaning up drawings and creating precise intersections.

Fillet & Chamfer: Create rounded or angled corners. Used for stress reduction or aesthetics.

Array: Creates multiple copies of an object in a rectangular or circular pattern.

Mirror: Creates a symmetrical copy of an object.

Hatch: Fills an area with a pattern (e.g., to indicate a material in a section view). Ensure the correct hatch pattern is used according to drawing conventions.

Layers: Organize drawing elements onto different layers for visibility control and editing.

Dimensioning: Accurately annotating drawings with sizes and tolerances. Use correct dimensioning standards.

Blocks: Grouping multiple objects into a single entity for easy insertion and modification.

Worked example

Draw a rectangle with dimensions 100mm x 50mm, and then create a circular array of 6 circles (radius 5mm) around the centre of the rectangle.

Step 1: Use the RECTANGLE command. Specify the first corner at (0,0). Specify the other corner at (100,50).

Step 2: Locate the centre of the rectangle (50,25).

Step 3: Use the CIRCLE command. Specify the centre at (50,25) and the radius as

5. Step 4: Use the ARRAY command. Select the circular array option. Select the circle as the object to array. Specify the centre point as (50,25). Specify the number of items as 6 and the angle to fill as 360 degrees.

B. Orthographic Projections:

Orthographic projections are a fundamental method of representing 3D objects in 2D. They involve projecting the object onto multiple planes (usually front, top, and side views).

Key Concepts:

First-angle vs.

Third-angle Projection: Understanding the difference and using the correct projection symbol. South Africa uses First-angle projection.

Placement of Views: The top view is above the front view, and the side view (usually left-side view) is to the right of the front view.

Hidden Detail: Use dashed lines to represent features that are hidden from view.

Sectional Views: Cutting through the object to reveal internal features. Correct hatch patterns are essential for indicating the material.

Auxiliary Views: Projecting a view onto a plane that is not parallel to any of the principal planes. Used to show true shapes of inclined surfaces.

Worked

Example:

Imagine a triangular prism sitting on its rectangular base. The base is 60mm wide and 40mm deep. The height of the prism is 50mm. The triangular end faces are equilateral triangles. Draw the front, top, and left-side orthographic views in first-angle projection.

Front View: A rectangle 60mm wide and 50mm high.

Top View: A rectangle 60mm wide and 40mm deep. The apex of the triangle will be represented as a hidden line in the middle of the rectangle, representing the top view of the apex.

Left-Side View: An equilateral triangle with sides of 40mm and a hidden rectangle underneath representing the 60mm length.

C. Isometric and Perspective Drawings: