Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Identify and rectify common errors in orthographic projections.
• Apply surface development techniques with accuracy.
• Solve complex isometric drawings, including sectional views.
• Demonstrate a thorough understanding of CAD principles.
• Manage time efficiently during exam simulations.

Lesson summary

This week is dedicated to intensive revision and examination preparation, specifically focusing on areas where students commonly struggle. EGD skills are crucial not just for academic success, but also for various careers in South Africa, including engineering, architecture, construction, and design. Mastering these skills allows you to contribute to the development of infrastructure, innovative products, and sustainable solutions that benefit our communities. We'll address common pitfalls and practice problem-solving strategies to improve your confidence and performance. This week's focus is on a holistic review of all topics covered thus far and applying problem-solving techniques.

Lesson notes

This section reviews the core concepts you need to master for the EGD exam. 2.

1. Orthographic Projections Definition: Orthographic projection is a method of representing a 3D object using 2D views, typically Top, Front, and Right/Left Side views. These views are mutually perpendicular.

Key Principles: Projection Lines: Imaginary lines projecting from the object to the projection planes. These lines are perpendicular to the projection plane.

Line Types: Correct use of visible outlines (thick, continuous), hidden detail lines (dashed, thin), center lines (long-short dash), and construction lines (very thin and light).

Alignment: All views must be correctly aligned, both horizontally (Front & Top views) and vertically (Front & Side views).

Dimensions: Accurately and clearly dimension all features of the object, following dimensioning rules (extension lines, dimension lines, arrowheads, and numerical values).

Scale: Ensure the drawing is to the correct scale, clearly stated on the drawing sheet.

Common Errors: Missing hidden detail, incorrect line types, misaligned views, incorrect or missing dimensions, and drawing the wrong view.

Example: Consider a simple rectangular prism with a hole drilled through it. Draw the orthographic projections (Top, Front, Right Side) and identify where hidden detail would be needed. Assume the prism is 50mm x 30mm x 20mm and the hole is 10mm in diameter, centered.

Solution: Front View: Show the rectangle 50mm x 20mm. Draw the hidden detail lines for the hole, representing the circle, as dashed lines.

Top View: Show the rectangle 50mm x 30mm. Draw the circle representing the hole in the center using hidden detail lines.

Right Side View: Show the rectangle 30mm x 20mm. Draw the hidden detail lines for the hole, representing the circle, as dashed lines. 2.

2. Surface Development Definition: Surface development involves unfolding a 3D object's surfaces onto a 2D plane. This is crucial for manufacturing objects from sheet metal or other flat materials.

Types: Prisms: Unfold the rectangular faces and add the top and bottom faces.

Pyramids: Unfold the triangular faces and add the base. Use true length diagrams to find the true length of sloping edges.

Cylinders: Unfold the curved surface into a rectangle (length = circumference, height = height of cylinder) and add the top and bottom circles.

Cones: Unfold the curved surface into a sector of a circle. The radius of the sector is the slant height of the cone, and the arc length is the circumference of the base. Use true length diagrams to find the true length of sloping edges.

Key Steps: Identify all surfaces. Determine the true shape and size of each surface. Arrange the surfaces in the correct order, ensuring they are connected. Add any necessary seam allowances or tabs.

Example: Develop the surface of a square pyramid with a base of 40mm and a height of 60mm.

Solution: Draw the base square (40mm x 40mm). Determine the true length of the sloping edges.

Use the Pythagorean theorem: Slant Height = sqrt((Base/2)^2 + Height^2) = sqrt(20^2 + 60^2) = 63.25mm. Draw four isosceles triangles, each with a base of 40mm and sides of 63.25mm, connected to the sides of the base square. 2.

3. Isometric Drawings Definition: Isometric drawing is a type of pictorial drawing where all three axes are equally foreshortened, creating a 3D representation. Lines are drawn at 30 degrees to the horizontal.

Key Principles: Isometric Axes: Three axes intersecting at 120 degrees.

Isometric Lines: Lines parallel to the isometric axes.

Non-Isometric Lines: Lines that are not parallel to the isometric axes. Use the box method to draw these lines accurately.

Circles: Drawn as ellipses using isometric axes. Use the four-center method or an ellipse template.

Sectional Views: Important to show internal features. Cut the object with an imaginary cutting plane and remove the front portion, then draw the cut surface with hatching.

Example: Draw an isometric view of a cube with a side length of 50mm. Show a circular hole of 20mm diameter centered on the top face.

Solution: Draw the isometric axes. Construct the cube using isometric lines. Locate the center of the top face. Use the four-center method or an ellipse template to draw the ellipse representing the circular hole. 2.

4. CAD Principles Interface: Familiarity with the CAD software's interface (e.g., AutoCAD, Fusion 360, SolidWorks). Understanding menus, toolbars, command line, and status bar.

Basic Commands: Line, Circle, Arc, Rectangle, Polyline, Erase, Move, Copy, Rotate, Mirror, Offset, Trim, Extend, Fillet, Chamfer.

Layers: Organizing drawing elements into layers to control visibility and properties.

Dimensioning: Accurate and standardized dimensioning using CAD tools.

Blocks: Creating reusable components to save time and ensure consistency.

Hatching: Applying hatching patterns to represent materials in sectional views.

Example: Using CAD, draw a rectangle 100mm x 50mm.