Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Accurately apply geometric construction principles.
• Correctly apply first-angle orthographic projection.
• Confidently apply and interpret standard dimensioning practices.
• Demonstrate proficiency in freehand sketching techniques.
• Understand and differentiate between various line types.

Lesson summary

This week is dedicated to laying a strong foundation for your Grade 12 EGD examination preparation. Engineering Graphics and Design is a crucial subject, not just for academic success, but also for developing valuable spatial reasoning, problem-solving, and technical communication skills. These skills are highly sought after in various fields, including architecture, engineering, manufacturing, and construction - all industries with significant presence and growth potential in South Africa. Understanding EGD empowers you to interpret, create, and communicate technical information effectively, preparing you for further studies and diverse career paths.

Lesson notes

This section revisits the core principles that form the bedrock of EGD. A solid understanding of these basics is essential for tackling more complex topics later. 2.1 Geometric Constructions: Geometric constructions are the foundation of accurate drawing. They allow us to create precise shapes and features without relying solely on measuring. Remember, accuracy is paramount in EG

D. Bisecting a Line: To bisect a line AB, use a compass with a radius greater than half the length of AB. Place the compass point at A and draw arcs above and below the line. Repeat with the compass point at B. The intersection of the arcs creates two points. Join these points with a line; this line bisects A

B. Bisecting an Angle: To bisect an angle BAC, place the compass point at A and draw an arc intersecting both AB and AC. Label these intersection points D and E. With the compass point at D (using a radius greater than half the distance between D and E), draw an arc. Repeat with the compass point at E. The intersection of these two arcs is point

F. Draw a line from A to F; this line bisects angle BA

C. Drawing a Tangent to a Circle: A tangent is a line that touches a circle at only one point. To draw a tangent at point P on circle O, first draw a radius from the centre O to point P. Then, construct a line perpendicular to the radius at point

P. This line is the tangent.

Drawing a Normal to a Curve: A normal is a line perpendicular to a tangent.

Therefore, to draw a normal, first determine the tangent at the desired point. Then construct a perpendicular line to the tangent at that point.

Constructing a Hexagon: To construct a regular hexagon inscribed in a circle, set the compass radius to the radius of the circle. Place the compass point on the circle and mark off arcs along the circumference, maintaining the same radius. Join these points to form the hexagon. 2.2 Orthographic Projection (First Angle): In first-angle orthographic projection, the object is placed in front of the projection planes. The view is then projected onto the plane behind the object. This is the standard used in South Africa.

Top View (Plan): Shows the object as seen from directly above.

Front View (Elevation): Shows the object as seen from directly in front.

Right-Side View (End Elevation): Shows the object as seen from the right.

Key considerations: Hidden Detail: Use dashed lines (thin, evenly spaced dashes) to represent edges and features that are hidden from view in that particular projection.

Sectioning: Used to reveal internal features. Imagine cutting through the object with a cutting plane. Section lines (hatching) are drawn on the cut surface to indicate the material. The direction and spacing of the section lines should be consistent within a single component. Different materials are indicated with different section line patterns (although this is usually simplified at the Grade 12 level). The cutting plane line is a thick chain line with thick ends pointing in the direction of view.