Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Create accurate orthographic projections (SANS 10111).
• Apply principles of isometric and oblique drawing.
• Solve complex descriptive geometry problems.
• Understand and create developments of geometric figures.
• Apply CAD principles for professional drawings.

Lesson summary

This week focuses on intensive revision and examination preparation for Grade 12 Engineering Graphics and Design (EGD). EGD provides crucial skills in spatial reasoning, problem-solving, and communication, which are highly valuable in various fields, from engineering and architecture to design and manufacturing. Proficiency in EGD not only ensures academic success but also equips you with the ability to visualize and interpret technical drawings, a skill demanded in many South African industries like construction, automotive, and infrastructure development, all contributing to our nation’s economic growth.

Lesson notes

This week's revision covers several key areas critical for exam success. Let’s delve into each area with detailed explanations and examples. 2.1 Orthographic Projections: Orthographic projection is a method of representing a three-dimensional object using two or more two-dimensional views. The most common views are the front view, top view, and side view. Remember SANS 10111 dictates the projection system. In South Africa, we predominantly use First-Angle Projection. This means the object is conceptually placed "behind" the viewing plane.

Key Considerations: Placement of Views: Front view typically goes below the top view and to the left of the side view.

Hidden Detail: Use dashed lines to represent edges and features hidden from view.

Center Lines: Use chain lines (long dash, short dash, long dash) to indicate axes of symmetry and centers of circles/holes.

Cutting Plane Lines: Thick chain lines with arrowheads indicate the location of a cutting plane in a sectional view.

Sectional Views: Essential for revealing internal details. Hatching lines indicate the material that has been "cut" by the cutting plane.

Example: Imagine a simple L-shaped bracket. The front view will show the height and width. The top view will show the width and depth. The side view shows the height and depth. Carefully transfer dimensions between views to maintain accuracy. 2.2 Isometric and Oblique Drawings: These are pictorial representations of 3D objects.

Isometric Projection: All three axes are equally inclined (120 degrees apart). Lines parallel to these axes are measured at true length.

Isometric Scale: Isometric scale is a crucial concept. While objects appear 3D, their true sizes are slightly reduced. You can create an isometric scale graphically, or use an approximate scale of 0.

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6. However, for most exam purposes, drawing lines at true length along the isometric axes is accepted unless explicitly instructed otherwise.

Oblique Projection: One face of the object is drawn parallel to the projection plane, and the other faces are projected at an angle (usually 45 degrees). One axis is at 90 degrees to the horizontal. Lines along the receding axis are often drawn at half-scale (cavalier oblique) or full scale (cabinet oblique). Cavalier oblique projections result in a less distorted appearance.

Circles and Curves: Circles in isometric views become ellipses. You can approximate these ellipses using the four-center method or use an ellipse template. In oblique views, circles on the front face remain true circles.

Example: Think about a rectangular box. In isometric, you'd draw its three visible edges at 120-degree angles from each other. In oblique, you'd draw the front face as a rectangle and then project the other faces at an angle. 2.3 Descriptive Geometry: This involves finding the true shape of a plane surface or the true length of a line. It often requires auxiliary views.

True Length of a Line: Project the line onto a plane that is perpendicular to the line itself.

True Shape of a Plane Surface: Find the edge view of the plane (project the plane onto a line). Then, project the edge view onto a plane perpendicular to the edge view. This projection will show the true shape.

Intersection of Lines and Planes: This often involves drawing auxiliary views to find the point where a line pierces a plane.

Example: Consider finding the true shape of an inclined surface on a machine component. You'd first project the object to get an edge view of the surface. Then, you'd project from the edge view to get the true shape. 2.4 Developments: Development refers to unfolding a 3D object onto a 2D plane. This is used extensively in sheet metal work.

Parallel Line Development: Used for prisms and cylinders. The development consists of a series of parallel lines representing the edges of the object.

Radial Line Development: Used for pyramids and cones. The development consists of sectors of circles, with the apex of the pyramid/cone as the center.

Triangulation: Used for complex shapes that cannot be developed using parallel or radial line methods. The surface is divided into a series of triangles, and each triangle is developed individually.

Allowances: Adding material for seams, bends, and overlaps.

Stretchout: The process of creating a flat pattern with necessary deductions for material thickness and fabrication processes.

Example: Imagine developing a rectangular duct. You'd unfold the duct to create a flat pattern showing the length of each side and the location of the seams. 2.5 CAD: CAD (Computer-Aided Design) is an essential tool in modern engineering. Proficiency in CAD software like AutoCAD is expected.

Key Commands: Line, Circle, Arc, Polygon, Rectangle, Trim, Extend, Offset, Fillet, Chamfer, Mirror, Array, Move, Copy, Rotate, Scale, Hatch, Dimension.

Layers: Use layers to organize your drawing and control the visibility of different elements.

Dimensioning: Accurate and clear dimensioning is crucial.