Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Apply Orthographic Projection principles (SABS 0111).
• Construct Isometric and Oblique drawings.
• Solve Descriptive Geometry problems (true lengths & shapes).
• Interpret Mechanical and Civil drawings.
• Apply sketching and CAD skills effectively.

Lesson summary

This week is dedicated to intensive revision and examination preparation for Grade 11 Engineering Graphics and Design (EGD). We will consolidate our understanding of key concepts covered throughout the year, focusing on common problem areas and exam techniques. Effective exam preparation is crucial not only for achieving a good grade but also for building a solid foundation for future studies and careers in engineering, architecture, and design. In South Africa, with its growing infrastructure demands, skilled EGD professionals are highly sought after, playing a vital role in building sustainable and innovative solutions.

Lesson notes

This revision session focuses on integrating various concepts, particularly Orthographic Projection, Isometric and Oblique Projection, Descriptive Geometry, and CA

D. We will also briefly touch on Mechanical and Civil drawing conventions. 2.1 Orthographic Projection: Orthographic projection is the process of representing a 3D object in 2D using multiple views (front, top, and side) projected onto planes perpendicular to each other. In South Africa, the preferred standard is SABS 0111, which largely aligns with ISO standards.

There are two main methods: First Angle and Third Angle Projection.

First Angle Projection: The object is placed in front of the projection planes. The view on the drawing is the view as seen from behind the object. Think of it as projecting a shadow of the object onto the plane. The top view is above the front view, and the right-side view is to the left of the front view.

Third Angle Projection: The object is placed behind the projection planes. The view on the drawing is the view as seen from in front of the object. The top view is below the front view, and the right-side view is to the right of the front view. This is more common in countries like the US

A. Key considerations for Orthographic Projection: Accuracy: Dimensions must be precise. Use scales carefully.

Neatness: Lines should be crisp and clean. Use correct line types (visible, hidden, center lines).

Layout: The views should be arranged logically and proportionally on the drawing sheet.

Hidden Detail: Use hidden lines to show features not visible from the outside.

Center Lines: Use center lines to indicate axes of symmetry and circles.

Example: Imagine a simple rectangular prism. In orthographic projection (First Angle), you would draw the front view as a rectangle. The top view, also a rectangle, would be positioned above the front view. The right-side view, another rectangle, would be to the left of the front view. If the prism has a hole drilled through it, the hole would be represented by hidden lines in the views where it's not directly visible. 2.2 Isometric and Oblique Projection: These are pictorial projections that show a 3D view of the object in a single drawing. They are useful for visualizing the overall shape of the object.

Isometric Projection: All three axes are equally inclined to the plane of projection (120 degrees apart). Lines parallel to these axes are measured at full scale. It's quick to draw, but it doesn't show true proportions perfectly.

Oblique Projection: One face of the object is drawn true size and shape (parallel to the projection plane), while the receding lines are drawn at an angle (usually 45 degrees). Two common types are Cavalier (receding lines are drawn at full scale) and Cabinet (receding lines are drawn at half scale, to reduce distortion). Converting Orthographic to Isometric/Oblique: Analyze the orthographic views to understand the object's shape and dimensions. Establish the isometric axes or the oblique projection plane. Transfer dimensions from the orthographic views to the isometric/oblique drawing, ensuring accurate placement of features. Use light construction lines to guide the drawing process and erase them after completion.

Example: If you have orthographic views of a chair, you can create an isometric drawing of the chair. You'd start by drawing the isometric axes. Then, using the dimensions from the orthographic views, you would construct the legs, seat, back, and any other features of the chair, making sure to maintain the isometric angles. 2.3 Descriptive Geometry: Descriptive Geometry deals with solving spatial problems using orthographic projections. It allows you to find true lengths of lines, true shapes of planes, and angles between lines and planes.

True Length of a Line: Project the line onto a plane parallel to it. If the line is not parallel to either the front or top plane, you need to perform auxiliary views to find its true length.

True Shape of a Plane: Project the plane onto a plane parallel to it. This often requires two auxiliary views. First, you project the plane as a line (edge view), then you project the edge view onto a plane parallel to it to obtain the true shape.

Example: Determining the angle of a roof truss. In South Africa, roof truss angles are crucial for proper water runoff and structural stability. Descriptive geometry allows you to accurately calculate the angle between the roof surface (a plane) and the horizontal (another plane), using orthographic projections and auxiliary views. 2.4 CAD (Computer-Aided Design): CAD software like AutoCAD, SolidWorks, or similar allows you to create accurate and detailed drawings electronically. Proficiency in CAD is a valuable skill for EGD students.

Key functionalities include: Drawing tools: Lines, circles, arcs, polygons, etc.

Modification tools: Trim, extend, fillet, chamfer, offset, etc.

Dimensioning tools: Linear, angular, radius, diameter, etc.

Layering: Organizing drawing elements for clarity.