Lesson Notes By Weeks and Term v3 - Senior Secondary 1

Lesson Video

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Performance objectives

• Explain what oblique drawing is.
• Draw simple shaped blocks using oblique projection.
• Draw more complex shaped blocks using oblique projection.

Lesson summary

Explain oblique drawing Produce oblique drawing of shaped blocks. Produce oblique drawing of shaped blocks.

Lesson notes

True Shape of Front Face: The front view or the chosen principal face of the object is drawn in its true shape and size, without any distortion or foreshortening.

Receding Lines: Lines that represent the depth or receding axis are drawn at a specific angle (the oblique angle) to the horizontal axis. Common angles include 30°, 45°, and 60°. The 45° angle is most frequently used due to its ease of execution with a 45° set square.

Axes: An oblique drawing uses three axes: A horizontal axis (for width). A vertical axis (for height). A receding axis (for depth), drawn at the chosen oblique angle.

No Parallelism Distortion: Lines that are parallel in the object remain parallel in the oblique drawing. There are two main types of oblique drawing, distinguished by the length of the receding lines: Cavalier Oblique Projection: In Cavalier oblique, the receding lines (representing depth) are drawn to their true length or full scale. This type of projection makes objects appear somewhat elongated or distorted in depth, as there is no visual foreshortening. It is often used when the true depth dimensions are critical or when the object's depth is not excessively large. The receding angle is typically 30°, 45°, or 60°.

Cabinet Oblique Projection: In Cabinet oblique, the receding lines (representing depth) are drawn to half their true length (i.e., scaled down by 1:2). This foreshortening of the depth dimension makes the drawing appear more realistic and less distorted than Cavalier oblique, mimicking natural perspective. It is generally preferred for presenting objects with significant depth, as it reduces the visual elongation. The receding angle is typically 30°, 45°, or 60°.

Example Comparison: Drawing a Cube 50mm x 50mm x 50mm Cavalier Oblique (45° receding angle): The front face (50mm x 50mm) is drawn. The receding lines are drawn at 45° to the horizontal and are 50mm long. Cabinet Oblique (45° receding angle): The front face (50mm x 50mm) is drawn. The receding lines are drawn at 45° to the horizontal and are 25mm long (half of 50mm). | Feature | Isometric Drawing | Oblique Drawing | | :----------------- | :------------------------------------------------------ | :---------------------------------------------------------- | | Front Face | Not true shape; foreshortened. | True shape and size. | | Axes | Three axes at 120° to each other; vertical axis is truly vertical. Horizontal axes are at 30° to the horizontal. | One vertical axis, one horizontal axis, one receding axis (e.g., 45° to horizontal). | | Foreshortening | All lines parallel to the isometric axes are drawn to true length (or uniformly scaled). Foreshortening is inherent in the projection. | Lines parallel to the principal (front) plane are true length. Receding lines are either true length (Cavalier) or half length (Cabinet). | | Angles | Axes are at 30° to the horizontal. | Receding angle typically 30°, 45°, or 60° to the horizontal. | | Appearance | Appears more natural and less distorted for overall view. | Can appear distorted (Cavalier) or more realistic (Cabinet), especially for the receding dimension. | | Ease of Drawing| Requires isometric grid or precise use of 30° set square. | Often easier for objects with a dominant front face, especially using 45° set square. | Step 3: Draw the full box envelope. Connect the ends of the 60mm receding lines to form the back face.

Step 4: Incorporate the L-shape cut-out.

On the front face: Measure 40mm from the top right corner horizontally inwards. Measure 30mm from the top right corner vertically downwards. Connect these points to form the 'notch' of the L-shape. Now, from the new corners of this notch on the front face, draw new receding lines parallel to the main 30° receding axis. These new receding lines will also be 60mm long. Connect these points at the back to complete the internal shape.

Step 5: Darken Visible Lines and Erase Construction Lines. Carefully go over all the visible edges of the L-block. * Erase all faint construction lines to leave a clean final drawing. (Self-Correction during thought process: I need to ensure the worked example is clear and descriptive for the teacher, without drawing it directly.) ---

Real-life applications

Architectural Visualization of Nigerian Buildings: Oblique drawings are excellent for quickly sketching the front view of a building in a Nigerian urban or rural setting. For instance, an architect can draw the true front elevation of a local bungalow or a commercial shop and then add depth using an oblique projection to give a 3D feel. This helps clients visualize the building's appearance, including details like windows, doors, and roof overhangs, before expensive construction begins. Local builders and masons often interpret such sketches on site. Product Design for Local Artisans and Industries: Furniture Making (e.g., in Aba, Nnewi, Lagos workshops): A carpenter designing a new reading table, a set of chairs, or a wardrobe can use oblique drawing to present the design. The front face of the table (e.g., showing drawers or decorative carvings) can be drawn in true shape, and then depth added, allowing for clear communication with clients or workshop colleagues. Metal Fabrication (e.g., gates, security doors): A welder creating a new gate design for a compound in Port Harcourt or Kaduna can draw the intricate patterns on the gate's front panel in true shape and then use oblique projection to add depth, showing how the gate would stand in 3

D. Instructional Diagrams for Assembly in Local Context: Oblique drawings are valuable for creating step-by-step assembly instructions for products locally manufactured or assembled in Nigeria. For example, a furniture company producing flat-pack items (e.g., office desks, kitchen cabinets) can use oblique views to illustrate how components fit together. This is more intuitive than orthographic views for non-technical users and more realistic than isometric in some cases for specific details. ---

Teacher activity

• Explains oblique drawing.
• Demonstrates how to draw shaped blocks and geometrical solids in oblique.

Evaluation guide

• The evaluation will focus on assessing students' conceptual understanding and practical drawing skills related to oblique projection.
• Formative Assessment Strategies:
• Classroom Observation: Teacher monitors students' progress during guided practice, noting common errors, instrument handling, and neatness.
• Question & Answer (Q&A): During explanation and demonstration, teacher asks questions to gauge understanding (e.g., "What is the key difference between Cavalier and Cabinet oblique?", "Why do we draw the front face true size?").
• Drawing Progress Checks: Periodically check students' drawings during practice sessions, providing immediate feedback on axis setup, line lengths, and angle accuracy.

Reference guide

• Drawing in struments and materials