Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Screw Threads, Fasteners and Devices
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Screw Threads, Fasteners and Devices
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Subject: Technical Drawings
Class: Senior Secondary 3
Term: 3rd Term
Week: 1
Theme: Building And Engineering Drawing
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Identify screw threads, fasteners and locking devices. Draw screw threads, fasteners and locking devices.
Introduction (15 minutes): Teacher Activity: Display a variety of actual screw threads, fasteners (bolts, nuts, screws, washers), and locking devices. Ask students to identify them and discuss their common uses in everyday objects they encounter in Nigeria (e.g., bicycle parts, furniture, generator engines, car parts).
Student Activity: Students identify the physical samples, discuss where they have seen them, and brainstorm their functions.
Exploration and Explanation (40 minutes): Teacher Activity: Using a projector or whiteboard, systematically explain the key concepts: Definition and terminology of screw threads (Major/Minor diameter, Pitch, Lead, Crest, Root, Flank, Thread Angle). Illustrate with clear diagrams. Discuss various thread forms (ISO Metric, Square, Acme, Buttress) and their applications, relating them to industries found in Nigeria (e.g., square threads in car jacks, ISO Metric in general machinery). Explain conventional representation rules for external and internal threads in both side and end views, demonstrating step-by-step drawing on the board. Introduce different types of fasteners (bolts, nuts, screws, washers) with visual examples (actual samples or clear images). Explain proportional drawing methods for standard fasteners (e.g., hexagonal bolt head and nut) and demonstrate the calculations and drawing process. Discuss different locking devices (lock washers, lock nuts, pins, wire locking) and their mechanisms, emphasizing their importance in preventing loosening, particularly relevant in applications like vehicle maintenance or industrial machinery in Nigeria.
Student Activity: Students take notes, ask clarifying questions, and participate in discussions about the applications of different thread forms and fasteners. They observe the drawing demonstrations closely.
Practical Application (40 minutes): Teacher Activity: Provide students with unlabelled diagrams of screw threads for identification practice. Distribute drawing paper and instruments.
Guide students through drawing exercises: Drawing conventional symbols for M20 external and internal threads. Drawing a hexagonal bolt head and nut using proportional methods for a given diameter (e.g., D=16mm). Demonstrate drawing an assembly of a bolt, two plates, a plain washer, a spring washer, and a nut in sectional view.
Student Activity: Students attempt to identify thread parts from diagrams. They practice drawing the conventional representations of external and internal threads, and the proportional drawing of bolt heads and nuts, seeking assistance where needed.
Class Discussion and Wrap-up (15 minutes): Teacher Activity: Review common mistakes in drawing conventions. Facilitate a discussion on the importance of selecting the correct fastener and locking device for specific applications, linking back to Nigerian industrial practices (e.g., why a mechanic might use a spring washer on a car engine bolt, or a cotter pin on a wheel hub).
Student Activity: Students share their drawing challenges and solutions. They contribute to the discussion on real-world applications and the significance of correct fastener choice. The teacher should provide these questions and guide students through the solutions, emphasizing correct drawing conventions and proportional methods.
Question 1: Identify the following thread forms and state one typical application for each: a)
Thread A: Profile with 60° angle, rounded crest/root. b)
Thread B: Profile with 90° angle, square shape. c)
Thread C: Profile with 29° angle, trapezoidal shape.
Solution 1: a)
Thread A: ISO Metric Thread (or Unified National Thread).
Application: General purpose fastening in mechanical assemblies, e.g., securing components in generator engines, bicycle parts, furniture. b)
Thread B: Square Thread.
Application: Power transmission systems requiring high efficiency, e.g., screw jacks used in local auto repair workshops, vices. c)
Thread C: Acme Thread.
Application: Lead screws for machine tools (lathes, milling machines), valve stems, screw presses, often found in fabrication workshops.
Question 2: Draw the conventional representation of an M24 external thread (a bolt shank) showing a threaded length of 60mm and a 45° chamfer at the end. Show both the side view and the end view.
Solution 2: Side View:
1. Draw two parallel thick lines, 24mm apart (major diameter).
2. At one end, draw a 45° chamfer.
3. From the chamfer, measure 60mm for the threaded length.
4. Inside the thick lines, draw two parallel thin lines (representing the minor diameter) extending 60mm from the chamfer. These thin lines should be approximately 0.85 24mm (around 20.4mm) apart, but for conventional drawing, the exact minor diameter is less critical than clearly distinguishing thick and thin lines.
5. The thin lines for the minor diameter stop at the end of the chamfer.
End View:
1. Draw a thick circle with a diameter of 24mm (major diameter).
2. Draw a thin arc (approximately 3/4 of a circle) concentric with the thick circle, representing the minor diameter. The gap in the arc is usually at the top or bottom.
Question 3: Draw, using proportional methods, a hexagonal bolt head and a hexagonal nut for an M20 bolt. Show them in elevation (front view) and plan (top view).
Solution 3: Given: Major Diameter (D) = 20mm.
Calculations: Across Flats (AF) = 1.5D + 3mm = (1.5 20) + 3 = 30 + 3 = 33mm. Thickness (H) = 0.8D = 0.8 20 = 16mm.
Drawing Steps: Hexagonal Bolt Head (Elevation): Draw a rectangle 33mm wide and 16mm high. Add the chamfer curves on the top face using construction lines or an approximation. The shank below the head would be 20mm diameter.
Hexagonal Bolt Head (Plan): Draw a hexagon with 33mm across flats. Draw a thick circle (20mm diameter) for the bolt shank and a thin arc (3/4 circle) for its minor diameter within.
Hexagonal Nut (Elevation): Similar rectangle, 33mm wide and 16mm high. Add chamfer curves on both top and bottom faces. Show the internal thread with thin and thick parallel lines (representing major and minor internal diameters).
Hexagonal Nut (Plan): Draw a hexagon with 33mm across flats. Draw a thick inner circle (representing the minor diameter of the internal thread) and a thin outer arc (3/4 circle, representing the major diameter of the internal thread).
Question 4: Identify the specific type of fastener shown in each description: a) An externally threaded fastener with a rounded head and a square neck underneath, commonly used in wood. b) An internally threaded fastener with wings designed for hand tightening. c) A thin, flat ring placed under a bolt head to distribute load, sometimes having small teeth around its circumference.
Solution 4: a) Carriage Bolt. b) Wing Nut. c) Washer (specifically, if teeth are present, a Toothed Lock Washer; otherwise, a Plain Washer).
Question 5: Draw a bolted joint in sectional elevation, showing two plates joined by an M12 bolt, a hexagonal nut, and a split spring washer placed under the nut. The plates are each 10mm thick.
Solution 5: Drawing Steps:
1. Draw two rectangles, each 10mm high, representing the plates, stacked one above the other.
2. Draw a vertical centreline through the plates.
3. Draw a central hole (diameter slightly larger than 12mm, e.g., 13mm) through Nut. c) Washer (specifically, if teeth are present, a Toothed Lock Washer; otherwise, a Plain Washer).
Question 5: Draw a bolted joint in sectional elevation, showing two plates joined by an M12 bolt, a hexagonal nut, and a split spring washer placed under the nut. The plates are each 10mm thick.
Solution 5: Drawing Steps:**
1. Draw two rectangles, each 10mm high, representing the plates, stacked one above the other.
2. Draw a vertical centreline through the plates.
3. Draw a central hole (diameter slightly larger than 12mm, e.g., 13mm) through both plates.
4. Draw an M12 bolt passing through the holes. The bolt head (hexagonal) should be drawn proportionally (D=12mm, AF=1.512+3=21mm, H=0.8*12=9.6mm).
5. Below the bolt head, a plain washer (standard thickness, e.g., 2mm-3mm, diameter approx. 2.5D) can be optionally shown.
6. Below the plates, draw a split spring washer (conventional symbol) then an M12 hexagonal nut (proportional drawing, same dimensions as the bolt head).
7. Use conventional sectional hatching for the plates and appropriate thread representation for the bolt and nut. The spring washer will be drawn with its characteristic gap and twisted ends in section.
Differentiation (for struggling learners): Simplified Visuals: Provide pre-drawn outlines or partially completed diagrams of common fasteners and threads for tracing and completing, reducing the initial complexity of drawing from scratch.
Physical Samples: Ensure constant access to actual bolts, nuts, washers, and cotter pins. Allow students to handle, measure (if safe), and visually inspect the components before attempting to draw them.
Step-by-Step Guides: Offer laminated step-by-step drawing instructions with clear diagrams for each type of thread and fastener, breaking down complex drawings into smaller, manageable stages.
Peer Support: Pair struggling learners with more capable students for collaborative drawing tasks, encouraging peer teaching and mutual learning. Remediation (for learners needing extra support): Focused Review: Revisit the absolute basics of thread terminology and the simplest conventional symbols (e.g., a simple M10 external thread side view) through one-on-one or small-group tutoring.
Reinforcement through Repetition: Provide additional, simpler practice sheets focusing on a single type of fastener or thread representation at a time until mastery is achieved. Use larger scale drawings to reduce precision demands.
Manipulatives: Use physical models or 3D printed examples of threaded parts and fasteners to help conceptualize the components before drawing them in 2
D. Visual Aids with Labels: Provide fully labelled diagrams of conventional representations that students can refer to during practice, rather than expecting recall initially.
Extension (for high-achieving learners): Advanced Thread Forms: Task students with researching and drawing more specialized thread forms like Pipe Threads (NPT, BSPT) or Ball Screws, understanding their unique applications and drawing conventions.
Assembly Design Challenge: Challenge students to design and draw a simple mechanical assembly (e.g., a clamp, a simple bracket) that requires different types of fasteners and locking devices, justifying their choices based on load, vibration, and ease of assembly/disassembly.
Failure Analysis: Research common modes of fastener failure (e.g., shear failure, fatigue, thread stripping) and how proper selection, tightening, and locking devices mitigate these risks. Students could present their findings.
International Standards Comparison: Investigate and compare different international standards for fasteners (e.g., DIN, JIS, ISO) and how they relate to the manufacturing and import/export of machinery in Nigeria.
Automotive Maintenance and Repair (e.g., in mechanic workshops across Nigeria): Vehicle mechanics constantly deal with various types of bolts, nuts, and screws in engine components, chassis, and suspension systems. Understanding thread types (metric vs. imperial) and selecting appropriate fasteners and locking devices (e.g., Nyloc nuts for critical connections, cotter pins for ball joints) is crucial for vehicle safety and performance. This knowledge directly translates to effective diagnosis and repair of common Nigerian vehicles (e.g., Toyota, Mercedes, Peugeot).
Building and Construction Industry: Fasteners are extensively used in steel frame construction (e.g., bolting beams and columns), roofing (e.g., securing corrugated iron sheets with self-tapping screws and washers), and timber work. Knowledge of different bolt grades, their strengths, and the necessity of locking devices (e.g., for vibrating structures or external installations) is vital for ensuring structural integrity and safety in Nigerian buildings and infrastructure projects. Local Fabrication and Manufacturing (e.g., "Weldon" shops, small-scale industries): Artisans and small enterprises involved in fabricating metal gates, window frames, furniture, agricultural tools, or repairing machinery (e.g., grinding machines, cassava processing equipment) frequently use screw threads and fasteners. The ability to correctly specify, purchase, and install appropriate fasteners, or even to cut new threads, ensures the durability and functionality of locally manufactured goods and repaired items.