Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Produce neat and accurate freehand sketches.
• Apply the principles of good dimensioning practice.
• Use appropriate units and symbols (SANS standards).
• Use aligned and unidirectional dimensioning systems.
• Identify and correct common errors in sketching.

Lesson summary

Freehand sketching and dimensioning are fundamental skills in Engineering Graphics and Design (EGD). While CAD software is widely used in industry, the ability to quickly and accurately communicate ideas through freehand sketches remains invaluable. Imagine a construction worker on a building site in Johannesburg needing to adapt a design slightly to overcome an unforeseen problem. A clear, freehand sketch with accurate dimensions can save time, prevent costly errors, and ensure everyone is on the same page. Similarly, a mechanic in a rural workshop in Limpopo might need to quickly sketch a damaged part to order a replacement.

Lesson notes

2.1 Freehand Sketching Techniques: Line Quality: Practice drawing straight lines, curved lines, and circles/arcs freehand. Use light construction lines initially, then darken the final lines. Avoid thick, wobbly lines. Aim for consistent line weight.

Proportion: Train your eye to estimate proportions accurately. Use simple shapes (squares, rectangles, triangles) as building blocks. Imagine the object enclosed in a box to help maintain overall proportions.

Perspective: Understand basic perspective principles (vanishing points, horizon line) even for freehand sketches. This adds depth and realism to your drawings. For example, parallel lines appear to converge in the distance.

Sketching Circles and Arcs: Mastering freehand circles is crucial. A good technique is to lightly sketch a square first, then inscribe the circle within the square, touching the sides at their midpoints. Refine the circle by gradually adjusting its shape. For arcs, use the same principle but only sketch the relevant portion of the square.

Sketching Angles: Use known angles (45 degrees, 90 degrees) as reference points. Practice estimating angles relative to these reference angles.

Example 1: Sketching a Cube Lightly sketch a square. This will be the front face of the cube. From each corner of the square, draw lines receding into the distance at approximately 45 degrees. These lines should be roughly the same length. Connect the ends of these receding lines to form the back face of the cube. Ensure the back face is also a square (or close to it). Darken the visible edges of the cube.

Example 2: Sketching a Cylinder Lightly sketch two parallel lines representing the top and bottom edges of the cylinder. Between these lines, lightly sketch two ellipses (elongated circles) representing the circular ends of the cylinder. The ellipses should be symmetrical about the central axis of the cylinder. Connect the ends of the ellipses with vertical lines to complete the cylinder. Darken the visible edges of the cylinder. 2.2 Dimensioning Principles: Dimensioning is the process of adding measurements to a drawing to define the size and location of features. Accurate and clear dimensioning is essential for manufacturing or construction purposes.

Dimension Lines: Thin, continuous lines with arrowheads at each end. The dimension value is placed above the center of the dimension line (or inline with it, depending on the standard).

Extension Lines: Thin, continuous lines that extend from the feature being dimensioned to the dimension line. They should start slightly away from the object outline.

Leader Lines: Thin, continuous lines used to dimension circles, arcs, or other features that are not easily dimensioned with standard dimension lines. They end with an arrowhead touching the feature being dimensioned.

Placement: Dimensions should be placed outside the object outline whenever possible. Avoid crossing dimension lines or extension lines.

Units: Use appropriate units (millimeters are standard in South Africa). Do not include the unit symbol unless it is necessary to avoid confusion.

Scale: All dimensions on a drawing should be to the same scale, even if the drawing itself is not to scale. This is crucial for accurate manufacturing.

Clarity: Dimensioning should be clear, concise, and unambiguous. Avoid unnecessary dimensions.

SANS Standards: South African National Standards (SANS) provide guidelines for dimensioning practices. While we won't delve into the full SANS document, we'll adhere to its fundamental principles. 2.3 Dimensioning Systems: Aligned Dimensioning: The dimension value is placed parallel to the dimension line, and it should be read from the bottom or right-hand side of the drawing.

Unidirectional Dimensioning: The dimension value is placed horizontally, regardless of the orientation of the dimension line. This system is easier to read but may require more space.

Example 3: Dimensioning a Rectangle (Aligned System) Sketch a rectangle freehand. Draw extension lines from the sides you want to dimension. Draw dimension lines between the extension lines, parallel to the sides. Place arrowheads at the ends of the dimension lines, touching the extension lines. Write the dimension values above the center of the dimension lines, aligned with the lines themselves.

Example 4: Dimensioning a Circle (Unidirectional System) Sketch a circle freehand. Draw a leader line from the center of the circle to a point on the circumference. Place an arrowhead at the end of the leader line, touching the circumference. Write the diameter symbol (Ø) followed by the diameter value horizontally above the leader line. 2.4 Common Errors in Freehand Sketching and Dimensioning: Poor Line Quality: Wobbly or inconsistent lines.

Incorrect Proportions: Features that are not drawn to scale.

Overlapping Lines: Lines that obscure other features.

Incorrect Dimension Placement: Dimensions that are placed inside the object outline or that cross other lines.