Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Produce neat, proportionally accurate freehand sketches.
• Apply standard dimensioning techniques (SANS).
• Interpret dimensioned freehand sketches.
• Sketch orthographic views of 3D objects.
• Use construction lines correctly.

Lesson summary

This week, we delve into the essential skills of freehand drawing and dimensioning. These are foundational skills in Engineering Graphics and Design (EGD). Freehand sketching allows you to quickly visualize and communicate design ideas without relying on instruments. Dimensioning provides the precise information needed to manufacture and construct those ideas. Mastering these skills is crucial for interpreting technical drawings, creating your own designs, and effectively communicating with engineers, architects, and other professionals.

Lesson notes

2.1 Freehand Sketching: Freehand sketching is the process of creating drawings without the aid of instruments such as rulers, compasses, or protractors. It's about capturing the essence of a design quickly and accurately using only your hand and eye.

Importance of Proportion: A good freehand sketch maintains the correct proportions of the object being represented. This means that the relative sizes of different parts of the object are accurately depicted. Practice estimating relative lengths and heights using your pencil as a measuring tool.

Line Quality: Use light, thin lines for initial construction and layout. These lines can be easily erased or darkened later. Once you are satisfied with the proportions, use darker, thicker lines for the final outline of the object. Remember to use continuous lines, not sketchy or broken lines for outlines.

Sketching Techniques: Blocking-in: Start by sketching a rough outline of the object using straight lines to define its overall shape and size. This helps establish the proportions.

Circle/Ellipse Construction: Sketching circles and ellipses requires practice. Imagine the circle or ellipse enclosed within a square or rectangle. Sketch the square or rectangle first, then sketch the circle/ellipse tangent to the midpoints of the sides. Rotate your paper to improve circularity.

Parallel Lines: Practice sketching parallel lines accurately. Use your eye to judge the distance between the lines, and keep your wrist relaxed.

Construction Lines: Use very light construction lines to help you build up your drawing. These lines should be barely visible and erased once the drawing is complete. 2.2 Dimensioning: Dimensioning is the process of adding measurements to a drawing to indicate the size and location of features. Proper dimensioning is essential for manufacturing or constructing the object accurately. Dimensioning must adhere to the SANS (South African National Standards) codes.

Dimension Lines: Thin, continuous lines with arrowheads at each end, indicating the extent of the dimension. The dimension line should be placed outside the object outline whenever possible. There should be a gap between the object and the start of the dimension line extension line.

Extension Lines: Thin, continuous lines that extend from the object to the dimension line. They should extend slightly beyond the dimension line.

Dimension Text: The numerical value of the dimension, placed above the dimension line, preferably in the middle. Use a consistent font and size. The text must be clear and legible.

Leader Lines: Thin, continuous lines used to point to a feature when the dimension text cannot be placed directly above a dimension line.

Units: Unless otherwise specified, all dimensions are assumed to be in millimeters (mm) in South Africa.

General Rules: Dimensions should be placed outside the object whenever possible. Avoid crossing dimension lines or extension lines. Dimension the object in its most descriptive view. Use chain dimensioning (dimensions placed in a line) only when necessary. Use overall dimensioning (showing the total length, width, and height) in addition to detail dimensions.

SANS Standards: Adhere to the South African National Standards (SANS) for technical drawings. These standards govern line types, lettering, dimensioning practices, and other aspects of technical drawing. While we won't memorize specific standard numbers, understand the importance of these standards for clear communication and manufacturing accuracy in South African engineering contexts. 2.3 Worked Examples Example 1: Freehand Sketch of a Cylinder with Dimensioning Sketching: Block-in the cylinder by sketching two circles (or ellipses) for the top and bottom faces. Connect the circles with straight lines to form the sides of the cylinder. Refine the shape, ensuring the circles are smooth and the sides are parallel. Erase construction lines.

Dimensioning: Add a dimension line to indicate the diameter of the cylinder. The dimension line should pass through the center of the top circle. Add extension lines extending from the circle to the dimension line. Place the dimension text (e.g., "Ø50") above the dimension line. The "Ø" symbol indicates diameter. Add a dimension line to indicate the height of the cylinder. Add extension lines extending from the bottom and top edges to the dimension line. Place the dimension text (e.g., "80") above the dimension line.

Example 2: Freehand Sketch of a Rectangular Prism with Dimensioning Sketching: Block-in the rectangular prism by sketching a rectangle. Add lines to represent the receding edges, creating the illusion of depth. Refine the shape, ensuring the lines are parallel and the angles are correct. Erase construction lines.

Dimensioning: Add dimension lines to indicate the length, width, and height of the prism. Add extension lines extending from the edges to the dimension lines. Place the dimension text (e.g., "100", "60", "40") above the dimension lines.