Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Sketch basic geometric shapes freehand.
• Draw simple 3D objects in isometric and oblique views.
• Apply standard dimensioning techniques to sketches.
• Use correct dimensioning terminology.
• Produce a fully dimensioned freehand sketch of an object.

Lesson summary

This week, we delve into the essential skills of freehand drawing and dimensioning in Engineering Graphics and Design (EGD). These skills are the foundation upon which all technical drawings are built. Freehand sketching allows us to quickly communicate ideas and visualize designs before committing to precise, computer-aided drawings. Dimensioning provides crucial information about the size and shape of an object, ensuring accurate manufacturing and construction.

Lesson notes

2.1 Freehand Sketching Techniques Freehand sketching is the process of creating drawings without the aid of instruments such as rulers, compasses, or set squares. It relies on visual perception, hand-eye coordination, and practice.

Line Types: Learn to differentiate between thin lines for construction and dimensioning and thick lines for outlines. Practice consistent line weight.

Proportion: Develop your ability to estimate relative sizes and distances accurately. Use light construction lines to block out shapes before defining the final outline.

Ellipses (Circles in Isometric): When sketching circles in isometric projection, remember they appear as ellipses. Practice drawing ellipses by lightly sketching a square and then fitting the ellipse within the square, tangent to all four sides. Isometric vs.

Oblique: Isometric Projection: All three axes are equally foreshortened at 120 degrees to each other. It provides a good overall view of the object.

Oblique Projection: One face of the object is drawn in its true shape, while the other faces are projected at an angle (typically 45 degrees). It is useful for showing the internal features of an object.

Example 1: Sketching a Cube in Isometric Start with three axes: Draw three lines intersecting at a point, one vertical and two at 30 degrees to the horizontal. These are your isometric axes.

Determine the edge length: Decide on the size of the cube. Let's say 50mm.

Draw the sides: From the intersection point, measure 50mm along each of the three axes and mark these points.

Complete the cube: Draw lines parallel to the axes from each of the marked points. The lines should intersect to form the remaining corners of the cube.

Outline: Darken the visible edges of the cube to make them stand out.

Example 2: Sketching a Cylinder in Oblique Draw the circular face: Draw a circle (or a good approximation of one) to represent the front face of the cylinder.

Determine the length: Decide on the length of the cylinder.

Draw receding lines: From the circumference of the circle, draw lines at 45 degrees to the horizontal (or another chosen angle) and parallel to each other. The length of these lines should represent the length of the cylinder.

Draw the back face: Draw another circle (or ellipse if you want to represent foreshortening) at the end of the receding lines, matching the shape of the front face.

Complete the outline: Darken the visible edges of the cylinder. 2.2 Dimensioning Techniques Dimensioning involves adding measurements to a drawing to specify the size and location of features. Accurate dimensioning is crucial for manufacturing and construction.

Dimension Lines: Thin, continuous lines with arrowheads at each end, indicating the extent of the dimension. They are placed outside the outline of the object.

Extension Lines: Thin, continuous lines extending from the feature being dimensioned to the dimension line. They should extend slightly beyond the dimension line.

Arrowheads: Placed at the ends of dimension lines, pointing to the extension lines. They should be neat and consistent in size.

Leader Lines: Thin lines used to connect a dimension or note to a specific feature on the drawing. They usually have an arrowhead pointing to the feature.

Dimension Text: The numerical value of the dimension, placed above or within the dimension line.

Dimensioning Rules: Avoid dimensioning hidden lines. Place dimensions outside the object whenever possible. Group dimensions together for clarity. Use a consistent unit of measurement (e.g., mm). Dimensions should be clear and easy to read. Avoid crossing dimension lines or extension lines whenever possible.

Example 3: Dimensioning a Rectangle Imagine a rectangular block representing a brick. Its length is 220mm and width is 110mm. Draw the rectangle freehand.

Add extension lines: Draw thin lines extending from the sides of the rectangle, slightly beyond where the dimension lines will be.

Add dimension lines: Draw thin lines parallel to the sides of the rectangle, outside the rectangle.

Add arrowheads: Place arrowheads at the ends of the dimension lines, touching the extension lines.

Add dimension text: Write "220" above the longer dimension line and "110" above the shorter dimension line.

Example 4: Dimensioning a Circle Imagine you are drawing a freehand sketch of a steel pipe (circular cross section). Draw the circle freehand.

Add a center mark: Draw a small "+" at the center of the circle.

Add a leader line: Draw a thin line from a point on the circumference of the circle to a point outside the circle.

Add an arrowhead: Place an arrowhead at the end of the leader line, touching the circumference of the circle.

Add dimension text: Write "Ø 50" (where Ø represents diameter) next to the leader line. If you are dimensioning the radius, write "R25". Guided Practice (With Solutions)

Question 1: Sketch a square prism (length = 60mm, width = 60mm, height = 80mm) in isometric projection and add dimensions for length, width, and height.