Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Pattern Development and Cutting
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Pattern Development and Cutting
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Subject: Metal Work
Class: Senior Secondary 2
Term: 1st Term
Week: 3
Theme: Sheet Metal Work
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Develop patterns of rectangular, cylindrical and conical for ms. Transfer pattern to sheet metal and cut.
A. Pattern Development (Sheet Metal Layout) Pattern development is the systematic process of creating a flat, two-dimensional shape (the pattern or 'net') that, when folded and joined, will form a desired three-dimensional object.
It is crucial for: Material Economy: Minimizing waste by accurately planning cuts.
Accuracy: Ensuring the final product has the correct dimensions and shape.
Efficiency: Streamlining the fabrication process. There are three primary methods of pattern development:
1. Parallel Line Development: Used for objects with parallel sides, such as prisms (rectangular, square) and cylinders. All elements or lines on the surface are parallel to each other and to the axis or bases.
2. Radial Line Development: Used for objects with a single apex, such as cones and pyramids. All elements radiate from the apex.
3. Triangulation: Used for complex or irregular shapes where neither parallel nor radial line methods apply. It involves dividing the surface into a series of triangles. (Focus for this lesson will be on Parallel and Radial Line for simple shapes). B. Development of Patterns for Specific Forms
1. Rectangular Forms (e.g., Rectangular Box, Tray) This typically involves parallel line development. A rectangular box consists of a base, four sides, and optionally a lid.
Principle: Imagine unfolding the box along its edges. The flat pattern will show the base with the four sides attached to its respective edges. Steps for a Simple Open-Top Rectangular Tray (Length L, Width W, Height H):
1. Draw the Base: Draw a rectangle representing the bottom of the tray with dimensions L x W.
2. Draw the Sides: From each side of the base rectangle, extend lines outwards to a distance equal to the height (H) of the tray. This forms the four side panels.
3. Add Seam Allowances/Laps: For joining, extra material (laps) must be added. These are typically small strips (e.g., 5-10mm) added to one or more edges of the side panels. These laps will be folded and joined (e.g., by riveting, soldering, or welding) to form the corners. Laps are usually added to the shorter sides of the side panels for easy folding.
4. Notches/Relief Cuts: At the corners where the side panels meet, small V-shaped or square notches are often cut out to prevent material overlap and ensure clean folds.
Example: A small rectangular planter box, 300mm long, 150mm wide, and 100mm high.
Base: 300mm x 150mm.
Sides: Two panels of 300mm x 100mm, two panels of 150mm x 100mm.
Arrangement: Draw the 300x150 base. Attach a 300x100 panel to each 300mm side. Attach a 150x100 panel to each 150mm side. Add 10mm laps to one 100mm edge of each 150x100 panel and to both 100mm edges of the 300x100 panels (or as needed for desired joining method). Cut relief at corners.
2. Cylindrical Forms (e.g., Pipe, Tank Section) This also uses parallel line development. A cylinder's curved surface, when unrolled, forms a rectangle.
Principle: Imagine cutting a cylinder vertically along its length and unrolling it flat. The width of this rectangle will be the circumference of the cylinder's base, and its length will be the height of the cylinder. Steps for a Simple Open Cylinder (Diameter D, Height H):
1. Calculate Circumference: The width of the rectangular pattern will be the circumference of the cylinder's base: C = πD (where π ≈ 3.142 or 22/7).
2. Determine Length: The length of the rectangular pattern will be the height (H) of the cylinder.
3. Draw the Rectangle: Draw a rectangle with dimensions (πD) x H.
4. Add Seam Allowance: Add a longitudinal seam allowance (e.g., 10-20mm) to one of the longer sides (height H) for joining the ends to form the cylinder. If ends are to be capped, add an allowance for flanging or folding at the top and bottom edges as well.
Example: A cylindrical pipe section with a diameter of 100mm and a height of 500mm. Circumference = π 100mm = 314.2mm (approx.).
Pattern dimensions: 314.2mm (width) x 500mm (height). Add a 15mm longitudinal seam allowance along one 500mm side.
3. Conical Forms (e.g., Funnel, Conical Roof) This uses radial line development to one of the longer sides (height H) for joining the ends to form the cylinder. If ends are to be capped, add an allowance for flanging or folding at the top and bottom edges as well.
Example: A cylindrical pipe section with a diameter of 100mm and a height of 500mm. Circumference = π 100mm = 314.2mm (approx.).
Pattern dimensions: 314.2mm (width) x 500mm (height). Add a 15mm longitudinal seam allowance along one 500mm side.
3. Conical Forms (e.g., Funnel, Conical Roof) This uses radial line development because all elements radiate from an apex. The developed pattern for a cone is a sector of a circle.
Principle: Imagine cutting a cone along its slant height from the apex to the base, then unrolling it. The result is a sector of a circle whose radius is the slant height of the cone, and whose arc length is the circumference of the cone's base.
Key Terms: True Length (L): The actual length of the slant edge from the apex to the base circumference. For a right cone, L = √(H2 + R2), where H is the vertical height and R is the base radius.
Base Radius (r): The radius of the cone's circular base.
Circumference of Base (C): 2πr. Development Radius (R_dev): This is the true length (L) of the cone.
Arc Length of Sector: This will be equal to the circumference of the cone's base (2πr). Angle of Sector (θ): The angle of the developed sector is calculated using the formula: θ = (r / L) 360°, where r is the base radius and L is the true length (development radius). Steps for a Simple Open Cone (Base Radius r, Vertical Height H):
1. Calculate True Length (L): Use Pythagoras theorem: L = √(H2 + r2). This L will be the radius of the development sector.
2. Calculate Arc Length: Circumference of the base, C = 2πr.
3. Calculate Sector Angle (θ): Use the formula: θ = (r / L) 360°.
4. Draw the Pattern: Mark a point (the apex of the pattern). This will be the center of your development circle. Using the true length (L) as the radius, draw an arc from the apex. From the apex, draw two radial lines separated by the calculated angle θ. This forms the sector.
5. Add Seam Allowance: Add a seam allowance (e.g., 10-15mm) along one of the radial lines for joining. If the base needs to be closed or flanged, additional material needs to be accounted for in the base circle development (not covered here for simple open cone).
Example: A funnel with a base diameter of 100mm (r = 50mm) and a vertical height of 120mm. True Length (L) = √(1202 + 502) = √(14400 + 2500) = √16900 = 130mm. (This is the radius of the development sector). Sector Angle (θ) = (50mm / 130mm) 360° = (5/13) 360° ≈ 138.46°.
Pattern: A sector of a circle with a radius of 130mm and an angle of approximately 138.46°. Add a seam allowance along one straight edge. C. Transferring Pattern to Sheet Metal Once a pattern is developed on paper or cardboard, it needs to be transferred to the actual sheet metal.
Tools: Scriber, steel rule, try square, centre punch, hammer. * Steps:
1. Prepare Sheet Metal: Ensure the sheet metal surface is clean and free of burrs or rust. Sometimes, a thin layer of chalk or layout dye is applied to enhance visibility of scribed lines.
2. Position Pattern: Place the paper/cardboard pattern carefully on the sheet metal, considering grain direction (if critical) and material economy. Use weights or tape to hold it securely.
3. Scribing: Use a sharp scriber to trace all the lines of the pattern onto the sheet metal. Apply firm, consistent pressure.
4. Punching (Optional but Recommended): Use a centre punch and hammer to mark the corners, centres of holes, and critical intersections. This provides permanent marks even if scribed lines are obscured during processing.
5. Remove Pattern: Carefully lift the paper scribed lines.
2. Position Pattern: Place the paper/cardboard pattern carefully on the sheet metal, considering grain direction (if critical) and material economy. Use weights or tape to hold it securely.
3. Scribing: Use a sharp scriber to trace all the lines of the pattern onto the sheet metal. Apply firm, consistent pressure.
4. Punching (Optional but Recommended): Use a centre punch and hammer to mark the corners, centres of holes, and critical intersections. This provides permanent marks even if scribed lines are obscured during processing.
5. Remove Pattern: Carefully lift the paper pattern. The scribed lines and punch marks should be clearly visible on the sheet metal. D. Cutting Sheet Metal After transferring the pattern, the sheet metal is cut along the marked lines.
Tools: Hand Shears/Snips: Straight snips for straight cuts, aviation snips (right, left, straight cut) for curves and intricate shapes.
Bench Shears/Guillotine: For larger, heavier gauge sheets and long, straight cuts.
Hacksaw: For thicker sections or specific cuts.
Nibblers: For internal cuts and irregular shapes (manual or power).
Grinders/Angle Grinders: For removing excess material or smoothing edges (with appropriate safety).
Safety Precautions: Wear appropriate Personal Protective Equipment (PPE): safety goggles, gloves (cut-resistant), sturdy footwear. Ensure cutting tools are sharp and in good condition. Secure the workpiece properly to prevent movement during cutting. Be aware of sharp edges and burrs created during cutting. Use a file or deburring tool to remove them. Never force a cut; select the right tool for the job.
Teacher Activities: Introduction (10 minutes): Begin by showing examples of common sheet metal objects (e.g., a funnel, a small metal box, a section of pipe or gutter) and ask students how they think these shapes are made from flat sheets. Briefly introduce pattern development as the "unfolding" of 3D objects into 2D templates. State the lesson objectives clearly. Key Concepts Explanation and Demonstration (30 minutes): Explain the concept of pattern development, its importance, and the main methods (Parallel Line, Radial Line) using whiteboard/projector.
Demonstrate Pattern Development: Rectangular Form: Using cardboard or thick paper, demonstrate step-by-step how to develop the pattern for a simple rectangular box or tray. Include drawing the base, sides, and adding seam allowances and relief cuts. Physically fold the cardboard to show how the pattern forms the 3D shape.
Cylindrical Form: Demonstrate how to develop a pattern for a simple open cylinder. Use a physical cylinder (e.g., a tin can with top/bottom removed) to illustrate how it unrolls into a rectangle. Calculate circumference and draw the pattern on paper, including seam allowances.
Conical Form: Demonstrate how to develop a pattern for a simple open cone (e.g., a funnel). Explain True Length, base radius, and the formula for the sector angle. Draw the sector on paper and show how it forms a cone when rolled. Emphasize calculation steps for cylindrical and conical forms. Transferring and Cutting Demonstration (20 minutes): Using a prepared paper pattern and a small piece of scrap sheet metal, demonstrate the process of transferring the pattern: Cleaning the metal surface. Positioning the pattern. Scribing accurately along the lines. Punching key points. Removing the pattern. Demonstrate safe cutting techniques using hand shears/snips: Correct grip. Safe positioning of hands. Cutting along the scribed lines. Deburring sharp edges with a file. Reinforce safety precautions.
Guided Practice Setup (5 minutes): Distribute drawing tools (pencils, rulers, compasses, protractors), paper, and pre-cut cardboard/scrap metal pieces. Assign the first guided practice question.
Circulate and Support: Move around the classroom, observe students' work, provide individual assistance, and answer questions. Check for correct understanding of concepts and proper use of tools.
Student Activities: Active Listening and Note-Taking: Students listen attentively to explanations and take comprehensive notes on definitions, formulas, and steps for pattern development.
Observation and Engagement: Observe teacher demonstrations closely, asking clarifying questions. Participate in discussions about the importance and applications of pattern development. Practical Application (Paper/Cardboard Practice): Under teacher guidance, students will individually practice developing patterns for rectangular, cylindrical, and conical shapes on paper or cardboard. They will perform necessary calculations for cylindrical (circumference) and conical (true length, sector angle) patterns. Students will practice accurately drawing and marking seam allowances. Transferring and Cutting Practice (Optional/If materials available): If materials and time permit, students will transfer one of their developed paper patterns onto a small piece of scrap sheet metal. Under strict supervision and with appropriate PPE, students will practice cutting out the pattern using hand shears/snips. Students will deburr the cut edges.
Building and Construction Sector: Sheet metal workers in Nigeria use pattern development extensively for fabricating roofing components (gutters, downspouts, flashings), ventilation ducts for air conditioning systems in offices and homes, water tanks, and industrial piping. Students can understand how these skills contribute to the infrastructure development in their communities.
Automotive Repair and Fabrication: Panel beaters and auto body repair technicians frequently develop patterns for repairing or fabricating custom body parts, exhaust systems, and fuel tanks for vehicles in local workshops (e.g., in Ladipo market, Mushin, Lagos, or similar clusters across Nigeria). This directly links to entrepreneurial opportunities in vehicle maintenance and custom fabrication.
Local Craft and Entrepreneurship: Artisans in Nigeria develop patterns for making various household items and craft products like metal dustbins, buckets, funnels for pouring beverages or palm oil, metal lampshades, and even parts for local farming equipment. Students can be encouraged to see how their acquired skills can translate into small-scale businesses, providing locally manufactured goods and services.