Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Marking Tools
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Marking Tools
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Subject: Welding & Fabrication
Class: Senior Secondary 2
Term: 1st Term
Week: 1
Theme: Tools / Equipment And Maintenance
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This topic introduces students to the fundamental concept of marking out in welding and fabrication. Marking out is a crucial initial step in any fabrication process, involving transferring dimensions and patterns from a drawing to the workpiece. Mastery of marking tools ensures accuracy, reduces material waste, and contributes to the overall quality and safety of fabricated products. In Nigeria, proficiency in marking out is essential for artisans and technicians in various sectors, including construction, furniture making, metal fabrication workshops (e.g., gates, security doors, vehicle parts), and vocational training centres.
Tools / Equipment And Maintenance and slide it along the rule to the desired position, then tighten the locking nut. Use as a try square, centre finder, or angle setter.
5. Protractor: Description: A semi-circular tool, usually made of steel, marked with degrees from 0° to 180°. Some have a movable arm.
Use: For measuring or marking angles on a flat surface when the combination square's protractor head is insufficient or unavailable.
Correct Usage: Align the base of the protractor with a reference line, and its centre point with the vertex of the desired angle. Mark the angle using the scale.
6. Dividers: Description: Two pointed legs joined by a pivot, similar to a compass but with both legs pointed.
Use: Transferring measurements from a rule to a workpiece. Scribing circles and arcs (especially small ones). Dividing a line or circumference into equal parts.
Correct Usage: Set the desired radius by opening the legs against a steel rule. Place one leg at the centre point and rotate the other leg to scribe the arc or circle.
7. Calipers (Inside and Outside): Description: Inside calipers have outward-curving legs for measuring internal dimensions, while outside calipers have inward-curving legs for external dimensions. They do not have their own scale.
Use: For transferring or comparing dimensions. Not for direct reading. They take a measurement, which is then read against a steel rule.
Correct Usage: Adjust the legs to fit the workpiece dimension. Carefully remove and read the dimension using a steel rule.
8. Surface Gauge (Scribing Block): Description: A stable base with an upright pillar holding an adjustable scriber arm and fine adjustment screw.
Use: Marking parallel lines at a given height from a datum surface (e.g., a surface plate). Checking alignment of surfaces. Locating centres of round bars (when used with V-blocks).
Correct Usage: Place on a flat surface plate. Adjust the scriber to the desired height using a steel rule, then slide the gauge along the workpiece or surface plate to scribe the line.
B. Scribing Tools:
1. Scriber (Engineers' Scriber): Description: A sharp, pointed hardened steel tool, often with one end straight and the other bent.
Use: For drawing fine, clear lines on metal surfaces. Used in conjunction with rules, squares, and straight edges.
Correct Usage: Hold the scriber at a slight angle (around 45 degrees) to the surface, leaning it against the edge of the guide (rule or square). Apply light, consistent pressure and draw a single, continuous line.
2. Chalk/Soapstone: Description: Soft, white mineral (soapstone) or compressed chalk.
Use: For marking on rough or dark surfaces where a fine line is not required, or as a preliminary marking before more precise scribing. Common for marking large plates or structural steel.
Correct Usage: Draw directly onto the surface. The marks are temporary and easily wiped off.
3. Marking Pen/Marker: Description: Industrial markers with permanent ink.
Use: For temporary marking, identification, or when a broad, visible line is sufficient and precision is not paramount. Not suitable for critical marking out.
Correct Usage: Draw directly onto the surface.
C. Punching Tools:
1. Dot Punch: Description: Has a sharp, conical point, usually ground to an angle of 60 degrees.
Use: For making small indentations along a scribed line to make it more permanent and visible after subsequent operations (e.g., cutting, bending). These dots serve as a guide.
Correct Usage: Place the point of the punch precisely on the scribed line, then strike the head gently but firmly with a ball-peen hammer. Space dots evenly (e.g., 5-10mm apart).
2. Centre Punch: Description: Similar to a dot punch but with a point ground to a wider angle (typically 90 degrees).
Use: To create a larger, deeper indentation (a "centre mark") for drilling. The wider angle helps the drill bit locate accurately without wandering.
Correct Usage: Place the point of the punch precisely on the centre mark for drilling, then strike the head firmly with a ball-peen hammer.
D. Striking Tools:
1. Ball-peen Hammer: Description: A hammer with the other, for repetitive identification practice.
2. Repetitive Practice on Scrap Material: Provide ample scrap material (e.g., thin aluminum sheets, thick cardboard) for repetitive practice of basic marking actions (e.g., drawing a straight line, making a dot punch mark) without fear of wasting expensive materials.
3. Focus on One Tool at a Time: Dedicate a short session to mastering one tool (e.g., just the scriber and steel rule) before introducing others.
4. Checklist Guidance: Provide a step-by-step checklist for each marking task to ensure all stages are followed correctly.
C. Extension/Enrichment for High-Achieving Learners:
1. Complex Layout Design: Challenge students to design and mark out a more complex component, such as a simple bracket with multiple holes, angled cuts, and radii, requiring the use of various tools and geometric principles.
2. Research Advanced Marking Techniques: Encourage research into more advanced marking out methods or tools, such as height gauges used with V-blocks for marking cylindrical workpieces, scribing machines, or even the basics of CNC (Computer Numerical Control) marking systems used in modern industries.
3. Problem-Solving Scenario: Present a scenario where a given workpiece needs to be marked for a specific fabrication task, but some conventional tools are unavailable. Students must devise alternative, accurate marking methods using the available tools.
4. Maintenance and Calibration: Task them with investigating how marking tools are maintained (e.g., sharpening scribers, checking squareness of try squares) and how their accuracy is ensured through calibration. the workshop. Students should be encouraged to describe the steps and tools used, or physically perform the tasks if materials are available.
1. Describe the process of marking out a square of 70mm x 70mm on a metal sheet, ensuring all angles are 90 degrees. List all tools required.
2. You need to drill 3 holes of 10mm diameter, equally spaced along a straight line of 120mm on a flat bar. Explain how you would mark the exact positions for drilling each hole.
3. A workshop supervisor asks you to check if a newly cut metal plate has truly square corners. Which marking tool(s) would you use, and how would you perform the check?
4. Explain the difference in use between a dot punch and a centre punch. When would you use each?
5. You have a piece of round bar stock and need to find its centre for machining. Outline the steps using a combination square.
6. Describe why layout dye (engineer's blue) is often used before scribing lines on metal.
7. Identify three safety precautions that must be observed when using marking tools like scribers and punches.
8. You need to mark a line parallel to an existing edge of a large metal plate, 250mm away. Which tool would be most suitable for this task if extreme precision is required and you have a surface plate? Describe its application.
9. A welder needs to cut out a circular blank of 150mm diameter from a larger metal sheet. Describe the marking process for this task.
1
0. Explain how dividers can be used to transfer a measurement of 35mm from a steel rule to a workpiece without directly measuring on the workpiece.
6. Evaluation and Assessment Evaluation Guide (as provided): Students to use marking out tools accurately.
A. Formative Assessment:
1. Observation of Practical Work: Strategy: During guided practice and independent practical sessions, the teacher will circulate and observe students' tool handling, technique, and adherence to safety.
Focus: Correct grip and angle of scriber. Firm placement of rules and squares. Accuracy of measurements and lines. Proper use of punches (location, force). Safe handling and storage of tools.
Feedback: Provide immediate verbal feedback and correction.
2. Question and Answer: Strategy: Ask direct questions during the lesson to assess understanding of tool names, functions, and proper procedures.
Example Questions: "Name this tool and its primary use." "Why is it important to use a centre punch before drilling?" "What is a datum line?"
3. Workpiece Check: Strategy: Collect marked workpieces after practical sessions and quickly review the quality of lines, accuracy of dimensions, and presence of punch marks.
B. Summative Assessment:
1. Practical Skill Test (Aligned to Evaluation Guide): Task: Students will be given a mild steel plate (e.g., 200mm x 150mm) and asked to perform a specific marking task within a time limit (e.g., 20-30 minutes).
Example Task: "On the provided mild steel plate, mark out a rectangle 120mm x 80mm. Then, mark the exact centre of this rectangle and apply a centre punch mark. Finally, mark two drill points, 15mm from each of the shorter sides, on the top longer side." Marking Scheme/Rubric: Accuracy of 120mm x 80mm Rectangle: Within +/- 0.5mm: 4 marks Within +/- 1.0mm: 2 marks Greater than +/- 1.0mm: 0 marks Squareness of Rectangle Corners (90°): Visibly square, no gaps with try square: 3 marks Slight deviation: 1 mark Significant deviation: 0 marks Clarity and Consistency of Scribed Lines: Clear, continuous, single lines: 3 marks Faint or multiple lines: 1 mark Poor or illegible lines: 0 marks Accuracy of Centre Mark (for rectangle): Within +/- 0.5mm of true centre: 3 marks Within +/- 1.0mm: 1 mark Greater than +/- 1.0mm: 0 marks Correct Use of Centre Punch: Clean, well-defined punch mark: 2 marks Faint or off-centre punch: 1 mark No punch mark or badly done: 0 marks Accuracy of Two Drill Points (15mm from short sides): Both points within +/- 0.5mm: 3 marks