Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Interpret complex engineering drawings (SANS 10111).
• Apply geometric construction principles.
• Use CAD software effectively (if applicable).
• Analyze mechanical assemblies.
• Demonstrate effective exam time management.

Lesson summary

This week focuses on intensive revision and examination preparation, concentrating on common challenges and high-weightage topics within the Grade 12 EGD curriculum. Mastering these concepts is crucial not only for achieving a good grade in the final exam but also for developing essential skills needed in various engineering and design fields, which are vital for infrastructure development and technological advancement in South Africa. This revision period allows you to consolidate your knowledge, identify areas requiring further attention, and refine your problem-solving strategies under exam conditions. It also helps you manage your time effectively during the actual examination.

Lesson notes

This week's revision covers crucial areas: a)

Orthographic Projections: This forms the bedrock of EG

D. Remember the principle: project from the object onto three mutually perpendicular planes (Front View, Top View, Right/Left View). Understanding hidden detail (indicated by dashed lines) is vital. Always adhere to SANS 10111 for line types and conventions.

Example: Consider a simple rectangular prism with a hole drilled through its center. The Front View will show the rectangle and the hole as hidden detail. The Top View will show the rectangle and the circular shape of the hole. The Side View will similarly show the rectangle and the hidden hole. Practice projecting complex shapes. b)

Isometric and Perspective Drawings: Isometric drawings show an object in 3D with all three axes equally foreshortened. Use isometric axes (120 degrees apart). Perspective drawings create a more realistic visual representation, with lines converging at a vanishing point (one-point, two-point, or three-point perspective). Remember that isometric drawings preserve measurements, while perspective drawings do not.

Example: Imagine drawing a typical South African shack. In Isometric, you'd show the basic rectangular shape, maintaining proportions along the isometric axes. In One-point perspective, you'd draw the front face as a rectangle and converge the receding edges to a single vanishing point on the horizon. Perspective drawing is often used in architectural design and urban planning. c)

Sectional Views: Sectional views are used to reveal internal details of an object that would otherwise be hidden. A cutting plane line indicates where the object is "cut." Hatching (section lining) indicates the surfaces that are cut by the cutting plane. Different materials have different hatching symbols (consult SANS standards).

Example: Consider a pipe fitting. The external features are visible in an orthographic view.

However, a sectional view will reveal the internal diameter, wall thickness, and any internal threads. Hatching would be applied to the cut surfaces. d)

Geometric Constructions: This involves using only a compass and straightedge to create geometric shapes (circles, arcs, tangents, ellipses, polygons). Understanding the underlying geometric principles is essential.

Example: Constructing a tangent to a circle from a point outside the circle. The steps involve drawing a line from the point to the circle's center, bisecting that line, and then drawing a circle with the bisected point as the center. Where this circle intersects the original circle, draw lines to the external point - these are the tangents. This skill is fundamental to drawing gears, cams, and other machine elements. e)

CAD Software (If Applicable): Proficiency in CAD software (like AutoCAD, Inventor, or similar) is increasingly important. Familiarize yourself with the interface, drawing commands, editing tools, and dimensioning techniques. Ensure you can create accurate drawings and modify existing ones according to specifications. Remember to use layers effectively for organization and to adhere to SANS standards for drawing setup and annotation. f)

Mechanical Assemblies: Understand how different mechanical components (gears, bearings, shafts, etc.) fit together and function as a system. Be able to identify different types of fits (clearance, interference, transition) and their applications.

Example: Consider a bicycle crank assembly. You should be able to identify the crank arm, pedal, bottom bracket, and their respective functions. Understanding how these components interact allows you to analyze the assembly and identify potential problems (e.g., a loose bearing). g)

SANS Standards: Throughout all aspects of EGD, adherence to SANS standards (especially SANS 10111) is paramount. These standards specify line types, dimensioning conventions, hatching symbols, and other drawing practices. Familiarize yourself with these standards to ensure your drawings are accurate, clear, and compliant.

Example: When dimensioning a drawing, SANS standards dictate the placement of dimension lines, extension lines, and arrowheads. It also specifies the units of measurement (typically millimeters in South Africa) and the format for expressing tolerances. Using SANS standard for dimensioning allows your drawing to be interpreted by other professionals in engineering sectors. Guided Practice (With Solutions)

Question 1: Draw the front view, top view, and right view of a hexagonal prism, with one face resting on the Horizontal Plane. The prism has a side length of 30mm and a height of 70mm.

Solution: Front View: Draw a rectangle 70mm high. The width will be determined by the hexagonal shape as viewed from the front. Since one face rests on the HP, the front view will show a hexagon with two parallel sides vertical. Calculate the height of this hexagon using trigonometry (30mm sin(60) 2) = 51.96mm.