Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Sections and Sectional Views
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Sections and Sectional Views
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Subject: Technical Drawings
Class: Senior Secondary 3
Term: 2nd Term
Week: 2
Theme: Building And Engineering Drawing
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Identify different types of sections in engineering drawing. Draw sections of simple engines and as semblies. Draw the sectional views of a building through the foundation and roof.
from the main view and placed in a separate location on the drawing sheet. It can be drawn to a larger scale for clarity.
Application: When a revolved section would clutter the main view or when a larger, more detailed view of the cross-section is required.
Example (Nigerian Context): Showing the detailed cross-section of a specifically shaped hand-tool handle or a structural member in a bridge design.
5. Offset Section: Description: The cutting plane changes direction (offsets) through the object to pass through several non-aligned internal features. The changes in direction are shown by short, perpendicular lines on the cutting plane line. No lines are drawn in the section view where the cutting plane changes direction (i.e., the offsets are ignored in the sectional view).
Application: Useful for complex objects with various internal features that are not on a single straight plane.
Example (Nigerian Context): An offset section through an engine cylinder block, revealing multiple internal passages for water, oil, and valves.
6. Broken-out Section: Description: A small portion of an object is broken away using a freehand break line (a jagged line) to reveal the internal features beneath, without using a full cutting plane line.
Application: When only a small internal detail needs to be shown without creating a full or half section.
Example (Nigerian Context): Showing the wall thickness of a pipe or a casing for a local borehole pump, or the internal structure of a brick wall to show mortar joints.
Rules for Sectioning Thin Features: Ribs, Webs, Spokes, Shafts: When the cutting plane passes longitudinally through thin features like ribs, webs, spokes, or shafts, these features are generally not sectioned (i.e., no hatching). This convention prevents a misleading impression of solidity that would arise if they were hatched, making them appear as thick, solid masses rather than thin connecting elements. The cutting plane line should pass through them, but the sectional view will show them in their un-sectioned form. Drawing Sections of Simple Engine Parts and Assemblies: Procedure:
1. Understand the object: Analyze the given orthographic views (plan, front, end views) to visualize the 3D shape and internal features.
2. Define Cutting Plane: Determine the most appropriate cutting plane position and orientation to reveal the desired internal details. Draw the cutting plane line with arrowheads and labels.
3. Project External Features: Draw the visible external outlines of the object as they would appear after the imaginary cut.
4. Project Internal Features: Project all internal features that are cut by the plane, and those visible behind the plane, onto the sectional view.
5. Apply Section Lines: Hatch the surfaces that have been physically "cut" by the imaginary plane. Ensure lines are thin, evenly spaced, and at 45 degrees. Vary hatching for adjacent parts in an assembly.
6. Remove Hidden Lines: Generally, hidden lines are omitted in sectional views unless absolutely necessary for clarity and cannot be shown otherwise.
7. Add Dimensions/Labels: Include necessary dimensions and labels for clarity, where appropriate.
Example: A simple bearing block assembly (showing a bush inserted into a housing). Teacher demonstrates drawing the orthographic views. Teacher then draws a full sectional view showing the cutting plane passing through the axis of the bush and housing.
Key points: Hatching on the housing, different hatching for the bush, no hatching on the shaft if it passes through longitudinally. Drawing Sectional Views of a Building (Foundation and Roof): Purpose: Building sections are crucial for communicating structural details, material specifications, and construction methods for different parts of a building, enabling artisans and engineers to execute construction accurately. Procedure for a Foundation Section (e.g., Strip Foundation for a typical Nigerian bungalow):
1. Establish Ground Level: Draw a horizontal line representing the natural ground level.
2. Excavation: Draw the excavated trench for the foundation below ground level.
3. Concrete Strip Footing: Draw the concrete strip footing at the base of the trench, often wider than the wall above for load distribution.
4. Concrete Block Wall: Draw the concrete block (or sandcrete block) wall rising from the footing. Show mortar joints.
5. Damp Proof Course (DPC): engineers to execute construction accurately. Procedure for a Foundation Section (e.g., Strip Foundation for a typical Nigerian bungalow):
1. Establish Ground Level: Draw a horizontal line representing the natural ground level.
2. Excavation: Draw the excavated trench for the foundation below ground level.
3. Concrete Strip Footing: Draw the concrete strip footing at the base of the trench, often wider than the wall above for load distribution.
4. Concrete Block Wall: Draw the concrete block (or sandcrete block) wall rising from the footing. Show mortar joints.
5. Damp Proof Course (DPC): Indicate the DPC (typically polythene sheeting or a layer of rich cement mortar) above ground level to prevent moisture rising into the walls.
6. Hardcore: Show the compacted layer of broken stones or rubble (hardcore) beneath the floor slab.
7. Blinding: A thin layer of weak concrete (blinding) over the hardcore for a smooth surface.
8. Floor Slab: Draw the reinforced concrete floor slab on top of the blinding.
9. Finishing: Indicate floor finishes (e.g., screed, tiles).
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0. Hatching: Hatch cut materials (concrete, blockwork).
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1. Labels: Label all components clearly (e.g., "Natural Ground Level," "Strip Footing," "225mm Block Wall," "D.P.C.", "150mm Concrete Slab"). Procedure for a Roof Section (e.g., Pitched Roof with Timber Truss and Metal Roofing Sheets):
1. Wall Top and Wall Plate: Draw the top of the block wall and the timber wall plate resting on it, which distributes the roof load.
2. Rafters: Draw the rafters, typically timber members sloping from the ridge (apex) to the eaves.
3. Purlins: Show the purlins, horizontal timber members fixed to the rafters, supporting the roofing sheets.
4. Eaves Detail: Carefully draw the eaves, which is the part of the roof that overhangs the wall. Show the fascia board (covering the rafter ends) and a gutter (for rainwater collection, common in Nigeria).
5. Roofing Material: Indicate the roofing material, e.g., long-span aluminium sheets or corrugated iron sheets, resting on the purlins.
6. Truss Members (if applicable): If showing a full truss section (e.g., king post truss), draw the tie beam, principal rafters, king post, struts, etc.
7. Hatching: Hatch cut timber members.
8. Labels: Label components like "Rafter," "Purlin," "Wall Plate," "Fascia Board," "Gutter," "Long Span Roofing Sheet." * Worked Example (Conceptual - Teacher demonstration): Teacher sketches a section of a typical Nigerian market stall foundation (simple strip foundation) and a simple pitched roof structure, highlighting the local materials (concrete blocks, timber, corrugated iron sheets).
Definition of Sectioning: Sectioning is an imaginary cutting process where a plane is passed through an object to expose its internal features. The resulting view, known as a sectional view, shows the cut surface and the visible parts behind it, as if a portion of the object has been removed.
Purpose of Sectional Views: The primary purpose of sectional views is to: Clearly reveal internal details that would be obscured by hidden lines in conventional orthographic projections. Avoid the ambiguity and clutter associated with numerous hidden lines, thus simplifying complex drawings. Provide a clearer understanding of an object's construction, material distribution, and assembly.
Cutting Plane Line: A cutting plane line represents the imaginary plane used to cut the object. It is drawn as a thick, continuous chain line (long dash, short dash, long dash). Arrowheads are added at the ends of the line to indicate the direction of sight for the sectional view, and capital letters (e.g., A-A, B-B) are used to label the cutting plane and the corresponding sectional view.
Section Lines (Hatching): Section lines (or hatching) are thin, evenly spaced parallel lines drawn on the surfaces that have been cut by the imaginary cutting plane.
Standard Practice: Lines are typically drawn at 45 degrees to the main outlines or axes of the object.
Spacing: Spacing depends on the size of the drawing; usually 2-3 mm apart.
Material Representation: While specific patterns exist for different materials (e.g., steel, cast iron, concrete), for general purposes in this level, a simple 45-degree hatching is sufficient. For adjacent parts in an assembly, the hatching direction or spacing should be varied to distinguish them.
Types of Sections:
1. Full Section: Description: The cutting plane passes entirely through the object, usually along its axis of symmetry, dividing it into two halves. One half is then conceptually removed, and the remaining half is viewed.
Application: Ideal for objects with symmetrical internal features or when all internal details along the cutting plane need to be shown.
Example (Nigerian Context): A full section of a simple pump casing for an irrigation system often used by Nigerian farmers, showing the internal impeller housing and water channels.
2. Half Section: Description: The cutting plane passes through one-quarter of a symmetrical object. This allows for both the internal features (on the sectioned side) and the external features (on the unsectioned side) to be shown in a single view. The boundary between the sectioned and unsectioned part is typically a centre line or a solid line.
Application: Exclusively used for symmetrical objects where both interior and exterior details are important, such as a pulley, a bearing block, or a bush.
Example (Nigerian Context): A half section of a bearing housing from a locally fabricated cassava grating machine, showing the bearing seat internally and the mounting flanges externally.
3. Revolved (Rotated)
Section: Description: A section taken perpendicular to the main axis of a long, thin feature (like a spoke, rib, arm, or shaft) and then rotated (revolved) into the view, usually directly on the orthographic view. The object is not actually "cut" but rather the cross-section is shown.
Application: Used to show the cross-sectional shape of features like spokes of a wheel, ribs of a bracket, or shaft keyways without needing a separate full view.
Example (Nigerian Context): Showing the cross-section of a spoke on a hand-drawn water pump wheel, or the cross-section of a structural beam in a market stall.
4. Removed (Detail)
Section: Description: Similar to a revolved section, but the cross-section is removed from the main view and placed in a separate location on the drawing sheet. It can be drawn to a larger scale for clarity.
Application: When a revolved section would clutter the main view or when a larger, more detailed view of the cross-section is required.
Example (Nigerian Context): Showing the detailed cross-section of a specifically shaped hand-tool handle or a structural member in a bridge design.
5. Offset Section: * Description: The cutting plane changes direction (offsets) through the object to pass through several non-aligned internal features.
The changes Teacher Activities: Introduction and Review: Begin by reviewing orthographic projection and the challenges of representing internal features using hidden lines. Engage students in a discussion about why understanding the inside of an object is important in design and construction (e.g., for engine repair, building plumbing).
Concept Presentation: Introduce sectioning as a technique to reveal internal features. Explain the cutting plane line, its representation, and direction of sight. Explain section lines (hatching), their purpose, and general rules (45 degrees, even spacing, varying for adjacent parts). Systematically introduce and explain each type of section: Full, Half, Revolved, Removed, Offset, Broken-out, providing visual examples (on chalkboard, projector, or through physical models if available). Emphasize the specific rule about not sectioning thin features (ribs, webs, spokes, shafts) when the cutting plane passes longitudinally through them, explaining the rationale.
Demonstration (Machine Parts): Using the chalkboard or a projector, demonstrate step-by-step how to draw a full or half sectional view of a simple machine part (e.g., a bearing block or a pulley). Highlight the process of defining the cutting plane, projecting features, and correctly applying hatching. Show how to draw a revolved section for a simple object like a shaft with a keyway.
Demonstration (Building Sections): Demonstrate drawing a section through a typical Nigerian strip foundation, detailing the footing, block wall, DPC, hardcore, blinding, and floor slab. Emphasize standard dimensions and materials common in Nigeria. Demonstrate drawing a section through a typical pitched roof eaves detail, showing the wall plate, rafters, purlins, fascia, gutter, and roofing sheets.
Guided Practice: Lead students through sketching exercises for different section types on their drawing sheets, providing immediate feedback.
Supervision and Feedback: Circulate among students during practical drawing sessions, offering individual guidance, correcting errors, and reinforcing correct techniques.
Student Activities: Active Listening and Participation: Listen attentively to explanations and participate in discussions about the importance and types of sections.
Sketching and Identification: Sketch examples of different cutting plane lines and section lines. Identify various types of sections from sample drawings provided by the teacher.
Drawing Exercises: Practice drawing full and half sections of simple symmetrical and asymmetrical objects provided as orthographic views. Attempt to draw revolved sections for thin features. Sketch basic broken-out sections.
Building Section Practice: Practice drawing the sectional details of a typical Nigerian strip foundation and a pitched roof eaves, paying attention to material representation and labelling.
Collaborative Learning: Work in pairs or small groups to discuss appropriate section types for given scenarios or to review each other's drawings.
Questioning: Ask clarifying questions to deepen understanding of concepts and drawing techniques.
Building and Construction Industry in Nigeria: Sectional views are fundamental in Nigerian architecture and civil engineering. Architects use them to detail the internal structure of buildings, from the foundation design of a local mosque or church to the roof framing of a commercial complex in Lagos. Builders rely on these drawings to understand critical elements like the placement of reinforcement bars in concrete slabs, the layering of floor finishes, the layout of plumbing and electrical conduits within walls, and the construction of staircases and roof trusses. This directly impacts the safety, durability, and functionality of structures across the country. Local Manufacturing and Fabrication (e.g., Small and Medium-scale Enterprises - SMEs): In Nigeria's growing manufacturing sector, particularly among SMEs involved in fabricating agricultural machinery (e.g., cassava processing machines, palm oil presses, irrigation pumps) or spare parts for vehicles and industrial equipment, sectional drawings are indispensable. Engineers and skilled artisans use sections to understand the internal mechanisms of gears, bearings, fluid passages, and housing designs. This knowledge helps in precise fabrication, assembly, troubleshooting, and maintenance of locally produced goods, contributing to indigenous technological development and economic growth. Maintenance, Repair, and Overhaul (MRO)
Services: Technicians and mechanics across various sectors in Nigeria, including automotive repair, generator servicing, and industrial equipment maintenance, regularly consult sectional diagrams found in service manuals. For instance, an auto mechanic troubleshooting an engine issue will refer to a sectional view of the engine block to understand the arrangement of pistons, valves, and crankshaft without disassembling the entire engine. This aids in accurate diagnosis, efficient repair, and prolonging the lifespan of machinery, from farm tractors in rural areas to power generators in urban homes and offices.