Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Marking out of Primary shapes on metal
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Marking out of Primary shapes on metal
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Subject: Welding & Fabrication
Class: Senior Secondary 1
Term: 2nd Term
Week: 2
Theme: Practical Works / Projects
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This lesson focuses on the fundamental skill of marking out in welding and fabrication. Marking out is the process of transferring dimensions and outlines from a drawing or design onto a workpiece (typically metal) using various tools, prior to cutting, drilling, bending, or other fabrication processes. This skill is critical for achieving accuracy, minimizing material waste, and ensuring the final product meets specifications.
This will be the first side of the rectangle.
4. Measure Width: Along this perpendicular line, measure 70mm from point A and mark point B.
5. Measure Length: Along the initial reference line, measure 120mm from point A and mark point C.
6. Draw Parallel Side: Place the try square with its stock on the datum edge and its blade aligned with point
C. Scribe a line perpendicular to the reference line from point C.
7. Complete the Rectangle: From point B, using the steel rule and try square, scribe a line parallel to the initial reference line until it intersects the perpendicular line from
C. Mark the intersection as point D.
8. Punch Marks: Use a centre punch and hammer to make small, clear indentations at each corner (A, B, C, D) to preserve the points for future operations. B. Marking Out a Square (e.g., 90mm x 90mm) A square is a special type of rectangle where all four sides are equal in length and all internal angles are 90 degrees.
Steps:
1. Follow steps for a rectangle: Apply the same method as for a rectangle.
2. Equal Sides: Ensure that when measuring both length and width, the dimension is the same (e.g., 90mm for both sides).
3. Punch Marks: Centre punch the four corners. C. Marking Out a Triangle There are various types of triangles. We will focus on two common types relevant to fabrication: Equilateral and Right-angled. C.
1. Marking Out an Equilateral Triangle (e.g., 80mm side length) An equilateral triangle has all three sides equal in length and all three internal angles equal to 60 degrees.
Steps:
1. Draw Base Line: Scribe a straight line on the prepared metal surface.
2. Mark Base Length: Along this line, measure 80mm and mark two points, say P and
Q. This forms the base of the triangle.
3. Set Dividers: Open the dividers to the length of the side (80mm) using a steel rule.
4. Scribe Arcs: Place one leg of the dividers on point P and scribe an arc above the line. Then, without changing the divider setting, place the other leg on point Q and scribe another arc that intersects the first arc.
5. Identify Third Vertex: The point where the two arcs intersect is the third vertex of the triangle (say, point R).
6. Join Vertices: Use the scriber and steel rule to connect point P to R and point Q to R.
7. Punch Marks: Centre punch points P, Q, and R. C.
2. Marking Out a Right-angled Triangle (e.g., Base 100mm, Height 75mm) A right-angled triangle has one internal angle that is exactly 90 degrees.
Steps:
1. Draw Base Line: Scribe a straight line on the prepared metal surface.
2. Mark Base Length: Along this line, measure 100mm and mark two points, say X and
Y. This forms the base of the triangle.
3. Draw Perpendicular (Height): Place the try square firmly with its stock against the base line (XY) and its blade aligned with point X (or Y). Scribe a line perpendicular to the base line.
4. Measure Height: Along this perpendicular line, measure 75mm from point X and mark point Z.
5. Complete Triangle: Use the scriber and steel rule to connect point Y to point Z.
6. Punch Marks: Centre punch points X, Y, and
Z. Worked Example (Nigerian Context): A local metal fabricator in Aba needs to mark out a rectangular frame component for a window grille. The component is to be 150mm long and 80mm wide.
Solution:
1. Obtain a piece of mild steel sheet and clean its surface. Apply Engineer's blue and allow it to dry.
2. Establish a datum edge on the steel sheet. Scribe a reference line parallel to this edge.
3. Mark point A on the reference line.
4. Using a try square, draw a perpendicular line from point A.
5. Measure 80mm along the perpendicular line from A and mark point B (width).
6. Measure 150mm along the reference line from A and mark point C (length).
7. Using the try square, draw another perpendicular line from point C.
8. From point B, draw a line parallel to Definition of Marking Out: Marking out is the process of accurately transferring the dimensions and geometric patterns from an engineering drawing or design onto a raw workpiece (metal, in this context) using specialized tools. It essentially involves drawing lines, circles, arcs, and points on the material to guide subsequent operations like cutting, drilling, or bending.
Importance of Marking Out: Accuracy: Ensures that the final fabricated part matches the required dimensions and specifications, preventing errors and rework.
Material Economy: Allows for optimal placement of patterns on the raw material, reducing scrap and saving costs, which is particularly vital in resource-conscious Nigerian workshops.
Guidance for Fabrication: Provides clear visual guides for cutting, drilling, and other operations, making the fabrication process more efficient and precise.
Quality Control: Serves as the initial step in quality assurance, ensuring that components are fabricated to standard.
Tools for Marking Out: Teachers should present and demonstrate the correct handling and usage of these tools:
1. Steel Rule: Used for measuring lengths and distances. Available in various lengths (e.g., 150mm, 300mm, 1 meter).
2. Scriber: A sharp, pointed hardened steel tool used to scratch fine, visible lines onto the metal surface. Available with straight or bent ends.
3. Try Square: Used for checking squareness (90-degree angles) and for marking lines perpendicular to an edge. It consists of a blade fixed at right angles to a stock.
4. Engineer's Square: Similar to a try square but often larger and more precise, used for checking and marking 90-degree angles on larger workpieces.
5. Centre Punch: Used with a hammer to make small indentations (punch marks) on scribe lines, especially at corners or where holes are to be drilled. This prevents the drill bit from wandering.
6. Ball Peen Hammer: Used to strike the centre punch to create punch marks.
7. Dividers: Used for marking circles, arcs, and for transferring distances. They have two pointed legs joined by a pivot.
8. Protractor: Used for marking and measuring angles other than 90 degrees.
9. Marking Medium (Engineer's Blue or Chalk): Applied to the metal surface to provide a contrasting background, making the scribed lines more visible. Engineer's blue is a quick-drying dye; chalk can be rubbed on rougher surfaces.
1
0. Surface Plate/Table: A precisely flat, rigid surface used as a reference base for accurate marking out.
Surface Preparation: Before marking out, the metal surface must be properly prepared:
1. Cleaning: Remove dirt, grease, rust, or scale using a wire brush, sandpaper, or degreaser. A clean surface ensures good adhesion of the marking medium and clear lines.
2. Applying Marking Medium: Apply a thin, even coat of engineer's blue (for smooth, finished surfaces) or rub chalk (for rougher, scaled surfaces) to the area to be marked. Allow engineer's blue to dry completely.
Marking Out Techniques for Primary Shapes: A. Marking Out a Rectangle (e.g., 120mm x 70mm) A rectangle has four straight sides with opposite sides equal in length and all internal angles at 90 degrees.
Steps:
1. Establish a Datum Edge and Line: Select one long, straight edge of the metal plate as the datum (reference) edge. Scribe a reference line parallel to and a few millimetres from this datum edge using a steel rule and scriber.
2. Mark the First Corner: Along the reference line, measure and mark the starting point (e.g., point A).
3. Draw Perpendicular Side (Width): Place the stock of the try square firmly against the datum edge (or the reference line if working purely from it) and align the blade with point A. Scribe a line perpendicular to the reference line from point
A. This will be the first side of the rectangle.
4. Measure Width: Along this perpendicular line, measure 70mm from point A and mark point B.
5. Measure Length: Along the initial reference line, measure 120mm from point A and mark point C.
6. Draw Parallel Side: Place the try square with its stock on the datum edge and its blade aligned with point
C. Scribe a line perpendicular to the reference line from point C.
7. Complete the Rectangle: From point B, using the steel rule and try square, scribe a line blue and allow it to dry.
2. Establish a datum edge on the steel sheet. Scribe a reference line parallel to this edge.
3. Mark point A on the reference line.
4. Using a try square, draw a perpendicular line from point A.
5. Measure 80mm along the perpendicular line from A and mark point B (width).
6. Measure 150mm along the reference line from A and mark point C (length).
7. Using the try square, draw another perpendicular line from point C.
8. From point B, draw a line parallel to the reference line to intersect the line from C. Mark this intersection as point D.
9. Centre punch points A, B, C, and
D. The fabricator now has the accurate outline for the window grille component.
Teacher Activities: Introduction and Motivation: Begin by discussing the importance of marking out in everyday fabrication, showing examples of products (e.g., metal gates, furniture, car parts) that require precise marking. Relate it to local artisanal work in Nigeria.
Tool Identification and Function: Display and explain each marking out tool (steel rule, scriber, try square, dividers, centre punch, hammer, marking medium). Demonstrate how to hold and use each tool correctly and safely. Emphasize the need for sharp scribers and clean, flat surfaces.
Surface Preparation Demonstration: Demonstrate cleaning a metal surface and applying marking blue or chalk.
Step-by-Step Demonstration of Marking Out: On a piece of prepared mild steel sheet or a large whiteboard/chalkboard (using appropriate drawing tools like large squares and rulers for visibility), demonstrate the marking out process for a rectangle, a square, an equilateral triangle, and a right-angled triangle, following the steps outlined in section
2. Emphasize establishing datum lines and datum edges. Stress accuracy in measurement and neatness of scribed lines. Show how to use the centre punch at corners and important points.
Safety Precautions: Remind students of safety measures during marking out (e.g., handling sharp tools carefully, wearing appropriate PPE if necessary, ensuring stable workpieces).
Supervision and Guidance: Circulate among students during practical sessions, observe their techniques, provide individualized feedback, and correct any misconceptions or errors in tool usage or procedure.
Question and Answer: Encourage questions and facilitate discussions to clarify doubts.
Student Activities: Observation and Participation: Actively observe the teacher's demonstrations of tool usage and marking out techniques. Ask clarifying questions.
Tool Familiarisation: Identify and handle each marking out tool, practicing correct gripping and positioning.
Surface Preparation Practice: Practice cleaning and applying marking medium to small metal scraps.
Practical Application (Guided Practice): Under supervision, students will individually attempt to mark out the following shapes on prepared metal offcuts: A rectangle (e.g., 100mm x 60mm) A square (e.g., 75mm x 75mm) An equilateral triangle (e.g., 50mm side) A right-angled triangle (e.g., base 80mm, height 60mm)
Peer Review: Students can informally review each other's marked-out pieces for accuracy and neatness, fostering collaborative learning.
Documentation: Sketch the steps for marking out each shape in their notebooks.
Metal Fabrication Industry (Gates, Windows, Furniture): In Nigeria, the demand for metal gates, window frames, security doors, and metal furniture (e.g., office tables, school desks) is high. Welders and fabricators in workshops across cities like Lagos, Kano, and Port Harcourt rely heavily on accurate marking out to produce components that fit together precisely, ensuring strength, aesthetic appeal, and customer satisfaction. This directly contributes to local economies and job creation.
Automotive Bodywork and Repair: Panel beaters and auto mechanics in Nigerian workshops frequently mark out sections of sheet metal for patching rusted areas, fabricating custom body parts, or repairing accident-damaged vehicles. Precise marking ensures that new panels or patches fit seamlessly, restoring the vehicle's structural integrity and appearance, thus supporting the transport sector.
Agricultural Tool Manufacturing/Repair: Many small-scale farmers in Nigeria rely on locally fabricated or repaired agricultural tools and implements (e.g., hoes, ploughs, farm gates, animal pens). Marking out is crucial for cutting and shaping metal components for these tools, ensuring they are functional, durable, and cost-effective, directly impacting food security and agricultural productivity.