Lesson Notes By Weeks and Term v3 - Junior Secondary 3
Orthographic Projection
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Orthographic Projection
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Subject: Basic Technology
Class: Junior Secondary 3
Term: 2nd Term
Week: 7
Theme: Drawing Practice
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explain the meaning of or thographic projection; identify components of or thographic projections; describe the principal views in quadrants; draw the or thographic views of simple objects.
A complete orthographic drawing consists of several standard lines and elements that convey specific information about the object.
Views: Front View (Elevation): Shows the object as seen from the front. This is usually the most informative view and often chosen to display the longest or most descriptive side.
Top View (Plan): Shows the object as seen from directly above. It is projected either above or below the Front View, depending on the projection method.
Side View (End Elevation): Shows the object as seen from either the left or the right side. It is projected either to the left or right of the Front View. The Right Side View shows the object as seen from the right, and the Left Side View shows it as seen from the left.
Types of Lines: Visible Lines (Object Lines): Thick, continuous lines used to represent edges and outlines of the object that are visible from the chosen viewing direction. (e.g., `-`)
Hidden Lines (Dashed Lines): Thin, dashed lines used to represent edges and features of the object that are not visible from the chosen viewing direction, but exist. (e.g., `-----`)
Centre Lines: Thin, long-short-long dashed lines used to indicate axes of symmetry, centres of circles, and the centres of cylindrical features. (e.g., `--- - --- - ---`)
Projection Lines (Construction Lines): Very thin, continuous lines used to project features from one view to another. These lines are erased or made very faint after the main drawing is complete.
Dimension Lines: Thin lines with arrowheads at their ends, used to indicate the extent of a dimension. They are usually broken in the middle to insert the dimension value.
Extension Lines: Thin, continuous lines extending from the object's features to the dimension lines. They do not touch the object's outline.
Folding Lines/Reference Lines: Imaginary lines (often drawn very faintly) that represent the intersection of projection planes and help in transferring dimensions between views. The arrangement of views in orthographic projection depends on the quadrant in which the object is assumed to be placed, relative to the projection planes and the viewer.
There are two primary methods: First Angle Projection and Third Angle Projection. In Nigeria, First Angle Projection is the internationally recognised standard.
Concept of Quadrants: Imagine two perpendicular planes: a Horizontal Plane (HP) and a Vertical Plane (VP). These planes divide space into four quadrants. First Angle Projection (Standard in Nigeria): Concept: The object is imagined to be in the first quadrant. The viewer looks at the object, and the projection plane is placed behind the object (between the object and the viewer for the Front View, and below the object for the Top View, etc.).
Arrangement of Views: The Front View is drawn centrally. The Top View is placed below the Front View. (Looking down at the object, the view projects onto a plane below it). The Right Side View is placed to the left of the Front View. (Looking from the right, the view projects onto a plane to its left). The Left Side View is placed to the right of the Front View.
Symbol: A truncated cone with the smaller end pointing to the right, showing the front view (small circle) and the side view (large circle) as if looking through it.
Third Angle Projection: Concept: The object is imagined to be in the third quadrant. The viewer looks at the object, and the projection plane is placed between the viewer and the object.
Arrangement of Views: The Front View is drawn centrally. The Top View is placed above the Front View. (Looking down at the object, the view projects onto a plane above it). The Right Side View is placed to the right of the Front View. (Looking from the right, the view projects onto a plane to its right). The Left Side View is placed to the left of the Front View.
Symbol: A truncated cone with the smaller end pointing to the right, showing the front view (large circle) and the side view (small circle) as if looking at it.
Emphasis for JSS3: The teacher must stress that First Angle Projection is the primary method to be learned and applied in Nigeria. The process involves careful observation, understanding of projection principles, and neat drawing techniques. Step-by-Step Procedure (Using First Angle Projection): Analyze the Object: Carefully observe the 3D object or isometric drawing provided. Identify its overall dimensions (length, width, height) and all its features (holes, cut-outs, sloped surfaces).
Select the Front View: Choose the view that provides the most information about the object's shape, usually the view showing the longest side or most distinctive features. Mentally orient the object.
Determine Drawing Space: Estimate the space required for all three views, leaving adequate spacing between them for dimensions and clarity. Draw a faint horizontal and vertical reference line (folding lines) to define the boundaries of your views.
Draw the Front View: Begin by drawing the visible outline of the Front View using visible lines. Add any visible internal features. Add hidden details (edges, holes) using hidden lines. Add centre lines for symmetrical features or circles.
Project to the Top View: Draw thin, faint projection lines vertically downwards from every significant point (corners, ends of lines, centres of circles) of the Front View. Draw the faint horizontal folding line for the Top View. From these projection lines, construct the Top View, showing its visible and hidden features. The width of the Top View must align with the width of the Front View. Project to the Side View (e.g., Right Side View): Draw thin, faint projection lines horizontally from every significant point of the Front View to the left. Draw a 45-degree mitre line (or use a compass) from the intersection of the Top View and Side View folding lines to transfer dimensions from the Top View to the Side View plane. Draw vertical projection lines from the Top View (after transferring width dimensions across the 45-degree line). From these projections, construct the Right Side View, showing its visible and hidden features. The height of the Side View must align with the height of the Front View.
Refine and Darken: Once all views are drawn, check for accuracy. Darken all final visible lines, hidden lines, and centre lines with appropriate thickness. Erase all faint construction/projection lines.
Dimensioning (Briefly): Add dimensions clearly, using dimension lines, extension lines, and arrowheads, ensuring they are placed outside the views and do not overlap. Worked
Example: A Rectangular Block Object: A simple rectangular prism (cuboid) with Length (L) = 60mm, Width (W) = 30mm, Height (H) = 40mm.
Step 1 & 2: Assume the 60mm x 40mm face is the Front View.
Step 3: Draw faint vertical and horizontal reference lines.
Step 4: Front View: Draw a rectangle 60mm wide and 40mm high.
Step 5: Top View (Below Front View, First Angle): Project vertical lines down from the 60mm width of the Front View. Measure 30mm (width of the block) downwards from the bottom edge of the Front View to define the depth of the Top View. Draw a rectangle 60mm wide and 30mm deep.
Step 6: Right Side View (Left of Front View, First Angle): Project horizontal lines left from the 40mm height of the Front View. Use a 45-degree mitre line or compass from the corner of the Top View to transfer the 30mm depth. Project vertical lines upwards from the transferred 30mm depth. Draw a rectangle 30mm wide and 40mm high.
Step 7 & 8: Darken lines and add dimensions (e.g., 60, 40, 30mm). --- Teacher Activities: Introduction (5 mins): Review previous related topics (e.g., isometric drawing, types of lines). Show physical 3D objects (e.g., a matchbox, a small wooden block, a bottle, an eraser, a simple mechanical part like a bolt). Ask learners how they would accurately describe these objects to someone who cannot see them, leading to the need for multiple views. Introduce Orthographic Projection as the standard method.
Explanation and Demonstration (20 mins): Using a large chart, projector, or blackboard, clearly explain the definition of orthographic projection and its components (views, lines). Emphasize First Angle Projection as the Nigerian standard. Using a simple physical object (e.g., a rectangular block, L-shaped block) or a pre-drawn isometric sketch on the board, demonstrate step-by-step how to derive and draw the Front, Top, and Right Side Views in First Angle Projection. Show how to select the Front View. Demonstrate the use of projection lines (faint). Illustrate how to transfer dimensions accurately. Show how to represent hidden features and centre lines. Use different coloured chalks or markers to distinguish between object lines, hidden lines, and projection lines during the demonstration.
Guided Practice Facilitation (15 mins): Provide a new, slightly more complex object (e.g., a simple block with a hole or a step) as an isometric sketch. Guide learners through drawing its orthographic views on their drawing sheets, providing instructions step-by-step. Circulate around the classroom, checking progress, identifying common errors, and offering individual assistance.
Review and Reinforcement (5 mins): Summarize the key concepts learned: definition, components, and the First Angle Projection method. Address any remaining questions or difficulties observed during guided practice.
Student Activities: Participation and Observation: Actively participate in class discussions and answer questions. Observe the teacher's demonstration keenly, noting the techniques for drawing different views and lines.
Sketching and Drawing: Sketch observed objects from different angles to understand the concept of views. Follow the teacher's guided instructions to draw orthographic views of simple objects on their drawing sheets, using appropriate drawing instruments (pencil, ruler, set squares). Practice identifying visible, hidden, and centre lines in given objects.
Questioning: Ask clarifying questions when concepts or drawing steps are unclear.
Collaboration (Optional): Discuss and compare their drawings with peers, helping each other to identify and correct errors under teacher supervision. ---
Building and Construction Industry: Orthographic projection is the language of architects and builders. When constructing a house, a classroom block, or a market stall in Nigeria, workers must be able to read and understand orthographic plans (floor plans, elevations, sections) to correctly interpret the dimensions, layout, and structural details. Without this skill, construction errors are frequent, leading to wasted resources and unsafe structures.
Manufacturing and Artisan Trades: In workshops across Nigeria (e.g., metal fabrication, carpentry, tailoring), orthographic drawings are used to design and produce items. A local furniture maker uses these principles to sketch out the different parts of a chair or table before cutting the wood. A welder fabricating a gate or a water tank stand uses orthographic views to ensure components fit together precisely. This skill enhances the quality and precision of locally manufactured goods.
Mechanical and Electrical Engineering: Orthographic projection is crucial for understanding and designing machinery, vehicle components, and electrical layouts. For example, a technician repairing a generator engine uses orthographic diagrams to identify and correctly assemble various engine parts. Engineers designing new agricultural equipment (like cassava processing machines) use orthographic views to communicate designs to fabricators. ---