Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Measuring and Marking Out
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Measuring and Marking Out
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Subject: Metal Work
Class: Senior Secondary 1
Term: 3rd Term
Week: 4
Theme: Handtools, Equipment And Bench Work
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Identify different measuring to ols Select and use measuring to ols correctly Identify marking out to ols and equipment Select appropriate marking out to ols. Mark out using the correct techniques
a conical point, typically ground to an angle of 90 degrees. It has a knurled body for grip.
Uses: To create small, deep indentations (punch marks) on a workpiece. Primarily used to locate the centre of holes to be drilled, preventing the drill bit from "walking" or wandering.
Technique: Position the point precisely on the marked centre, hold vertically, and strike the head sharply with a hammer. A smaller, lighter tap first can help confirm position, then a heavier blow.
Care: Keep the point sharp and properly ground, avoid hitting with excessive force that could mushroom the head.
Example Application: Punching the centre point on a metal plate where a bolt hole needs to be drilled for mounting a signpost in Abuja.
Dot Punch: Description: Similar to a centre punch but with a much sharper conical point, typically ground to an angle of 60 degrees.
Uses: To make small, shallow marks along a line that has been scribed, making the line more visible and permanent. Also used for general marking on layout lines or for indicating areas.
Technique: Follow a scribed line, making a series of evenly spaced dots along its length by striking with a hammer.
Care: Keep the point sharp, avoid using for drilling centres as the sharp point can damage the drill bit.
Example Application: Marking a series of dots along a curved line on a decorative metal gate component to guide the plasma cutter.
Engineer's Square (Precision Square): Description: A precision tool similar to a try square but made entirely of hardened and ground steel, with greater accuracy. The blade and stock are usually permanently fixed.
Uses: Highly accurate checking of 90-degree angles. Marking precise 90-degree lines on critical components. Checking the squareness of machined faces.
Care: Treat as a precision instrument; protect from drops, impacts, and corrosion. Store in a protective case.
Example Application: Verifying the squareness of a machined jig used in a medium-scale fabrication shop in Ogun state.
Surface Plate: Description: A very flat, smooth, and rigid reference surface, usually made of cast iron or granite. It provides a precise datum (reference plane) for marking out, inspection, and measurement.
Uses: Providing a stable and level base for accurate marking out. For checking the flatness of workpieces. As a reference for height gauges and surface gauges.
Care: Keep clean, cover when not in use, avoid placing heavy or sharp objects on it, do not use as an anvil.
Example Application: Providing a stable and accurate base for marking out complex patterns on a stainless steel sheet for a commercial kitchen sink in Kano.
Surface Gauge (Scribing Block): Description: Consists of a heavy base, a vertical pillar (spindle), and an adjustable scriber that can be clamped at any height. A fine adjustment screw is often included for precise settings.
Uses: Marking parallel lines at a specific height from a datum surface (usually a surface plate). Transferring measurements from a master component. Checking the height of a component. Finding the centre of round bars when used with a V-block.
Technique: Set the scriber to the desired height using a steel rule or height gauge on the surface plate. Lock the scriber and gently slide the gauge along the surface plate, allowing the scriber to mark the workpiece.
Care: Keep base clean, protect scriber point.
Example Application: Marking a series of parallel lines at a consistent height on a metal component that needs multiple machining operations, using a surface plate as the reference.
Dividers: Description: A two-legged tool, with both legs sharpened to a point, joined by a pivot. Types include firm-joint and spring-joint dividers.
Uses: Transferring measurements from a rule to a workpiece. Scribing circles and arcs (especially small ones). Stepping off equal distances along a line or curve. * Technique: Set the desired radius using a steel rule, then place one leg at the centre point and rotate the other leg to scribe the arc This section provides in-depth explanations of essential terms, tools, and techniques related to measuring and marking out. 2.
1. Measuring Measuring is the process of determining the dimensions (length, width, height, thickness, diameter) of an object or the distance between two points. Accurate measurement is crucial for producing components that fit together correctly and function as intended. 2.1.
1. Measuring Tools: Steel Rule (Meter Rule): Description: A straight-edged measuring instrument, typically made of hardened and tempered steel. It usually has imperial markings (inches) on one edge and metric markings (millimetres, centimetres) on the other. Common lengths are 150mm (6 inches), 300mm (12 inches), 600mm (24 inches), and 1000mm (1 meter).
Uses: Measuring linear dimensions up to its length. Checking the straightness of a surface. Marking short distances.
Care: Keep clean, avoid bending or dropping, store in a protective casing. Rust prevention is crucial in humid Nigerian environments.
Example Application: Measuring the length of a piece of angle iron before cutting it to fabricate a window frame for a building project in Lagos.
Measuring Tape: Description: A flexible ruler made of metal (steel or fibreglass) or cloth, housed in a casing with a winding mechanism. Available in longer lengths (e.g., 3m, 5m, 10m, 30m, 50m).
Uses: Measuring long distances (e.g., dimensions of a room, length of sheet metal). Measuring curved or irregular surfaces. Used widely in construction and large-scale fabrication.
Care: Avoid kinking or twisting the blade, retract carefully, keep dry to prevent rust (especially metal tapes).
Example Application: Measuring the perimeter of a workshop floor in Aba to determine the length of material needed for a new metal partition.
Try Square: Description: Consists of a thick, flat stock (handle) and a thinner blade, usually fixed at a precise 90-degree angle. Made of steel or a combination of wood and steel.
Uses: Checking the squareness (90-degree angle) of an edge or corner of a workpiece. Marking lines at 90 degrees to an edge. Checking flatness over short distances.
Care: Protect the edges, avoid dropping, clean regularly. Accuracy can be lost if dropped.
Example Application: Checking if the corners of a newly welded mild steel frame for a local market stall are perfectly square before applying finish.
Combination Square: Description: A versatile tool consisting of a steel rule (blade) and three interchangeable heads: a stock head (or square head) for 90° and 45° angles, a protractor head for any angle from 0° to 180°, and a centre head for finding the centre of round stock.
Uses: Checking and marking 90° and 45° angles. Measuring and marking specific angles. Finding the centre of cylindrical workpieces. Measuring depth and height.
Care: Keep the blade and heads clean, protect the edges, store assembled or with parts safely.
Example Application: Finding the centre of a metal pipe before drilling a hole for a connection in a plumbing installation in Port Harcourt.
Micrometer Screw Gauge: Description: A precision measuring instrument used for measuring small external dimensions with high accuracy, typically to 0.01mm or 0.001 inch. It operates on the principle of a screw thread.
Parts: Anvil, spindle, frame, thimble, sleeve (barrel), ratchet stop, lock nut.
Reading Principle (Metric): Main Scale (Sleeve/Barrel): Readings are taken from the main line on the sleeve. Each full millimetre is marked, and half-millimetre marks are below the line.
Thimble Scale: Readings are taken from the rotating thimble, which is divided into 50 or 100 divisions. Each division represents 0.01mm (if 50 divisions). Total Reading = Main Scale Reading + (Thimble Scale Reading × Smallest Division)
Worked Example (Metric): Problem: A micrometer shows 15.5 mm on the main scale (15 mm visible, plus the 0.5 mm mark visible below the line). The 23rd division on the thimble scale aligns with the main line. What is the reading?
Solution: Main Scale Reading = 15.50 mm Thimble Scale Reading = 23 divisions Smallest Division (Accuracy) = 0.01 mm * Thimble Value = 23 Each division represents 0.01mm (if 50 divisions). Total Reading = Main Scale Reading + (Thimble Scale Reading × Smallest Division)
Worked Example (Metric): Problem: A micrometer shows 15.5 mm on the main scale (15 mm visible, plus the 0.5 mm mark visible below the line). The 23rd division on the thimble scale aligns with the main line. What is the reading?
Solution: Main Scale Reading = 15.50 mm Thimble Scale Reading = 23 divisions Smallest Division (Accuracy) = 0.01 mm Thimble Value = 23 × 0.01 mm = 0.23 mm Total Reading = 15.50 mm + 0.23 mm = 15.73 mm Uses: Measuring the thickness of sheet metal, diameter of wires, small shaft diameters, checking engine parts during vehicle repair.
Care: Handle with extreme care, avoid dropping, clean jaws before use, do not overtighten, apply light oil to prevent rust.
Vernier Caliper: Description: A precision measuring instrument used for internal, external, and depth measurements, typically accurate to 0.02mm or 0.001 inch. It consists of a main scale and a sliding vernier scale.
Parts: External jaws, internal jaws, depth rod, main scale, vernier scale, thumb screw, lock screw.
Reading Principle (Metric): Main Scale Reading: Read the value on the main scale immediately to the left of the '0' mark on the vernier scale.
Vernier Scale Reading: Find the line on the vernier scale that perfectly aligns or coincides with any line on the main scale. Multiply this vernier division number by the least count (smallest division) of the vernier caliper (e.g., 0.02mm). Total Reading = Main Scale Reading + (Coinciding Vernier Division × Least Count)
Worked Example (Metric): Problem: A vernier caliper shows the '0' mark of the vernier scale past the 48 mm mark on the main scale, but not yet reaching 49 mm. The 13th division on the vernier scale coincides exactly with a main scale division. The least count is 0.02 mm. What is the reading?
Solution: Main Scale Reading = 48.00 mm Coinciding Vernier Division = 13 Least Count = 0.02 mm Vernier Value = 13 × 0.02 mm = 0.26 mm Total Reading = 48.00 mm + 0.26 mm = 48.26 mm Uses: Measuring the outside diameter of shafts, inside diameter of pipes, depth of holes, thickness of various parts.
Care: Handle gently, keep jaws clean and free of burrs, avoid dropping, store in its case. 2.
2. Marking Out Marking out is the process of transferring the dimensions and shape of a design onto a workpiece (usually metal) using lines, points, and centres. These markings serve as guides for subsequent operations like cutting, drilling, bending, or machining. 2.2.
1. Marking Out Tools and Equipment: Scribers: Description: A slender rod, typically made of hardened steel, with one or both ends ground to a sharp, hardened point. Some have a bent end for reaching difficult areas.
Uses: To scratch fine, visible lines onto the surface of metal workpieces, indicating cut lines, bend lines, or centres.
Technique: Hold like a pencil, tilted at an angle (approx. 30-45 degrees) to the surface, and draw along a straightedge or template with firm, even pressure. Only one line should be drawn.
Care: Keep the point sharp, protect from damage, avoid using as a punch or pry tool.
Example Application: Marking the cut line on a sheet metal piece for a custom exhaust pipe in a Nigerian auto repair workshop.
Centre Punch: Description: A hardened steel tool with a conical point, typically ground to an angle of 90 degrees. It has a knurled body for grip.
Uses: To create small, deep indentations (punch marks) on a workpiece. Primarily used to locate the centre of holes to be drilled, preventing the drill bit from "walking" or wandering.
Technique: Position the point precisely on the marked centre, hold vertically, and strike the head sharply with a hammer. A smaller, lighter tap first can help confirm position, then a heavier blow. * Care: Keep the point sharp and properly ground, avoid that needs multiple machining operations, using a surface plate as the reference.
Dividers: Description: A two-legged tool, with both legs sharpened to a point, joined by a pivot. Types include firm-joint and spring-joint dividers.
Uses: Transferring measurements from a rule to a workpiece. Scribing circles and arcs (especially small ones). Stepping off equal distances along a line or curve.
Technique: Set the desired radius using a steel rule, then place one leg at the centre point and rotate the other leg to scribe the arc or circle.
Care: Keep points sharp, avoid using for prying.
Example Application: Laying out the pattern for a series of bolt holes on a circular metal flange for a pump connection.
Trammels: Description: Similar to dividers but designed for scribing larger circles and arcs. It consists of a long beam with two sliding heads, each holding a scriber point, which can be locked at any position along the beam.
Uses: Scribing large circles and arcs that are beyond the capacity of dividers.
Care: Keep scribers sharp, handle the beam carefully to avoid bending.
Example Application: Marking out a large diameter circular opening on a sheet metal panel for a ventilation duct in an industrial building.
Protractor (Bevel Protractor): Description: A tool used to measure and mark angles. Simple protractors have a fixed baseline and a rotating arm. Universal bevel protractors are more precise and allow for acute angle measurements.
Uses: Measuring existing angles on a workpiece or marking specific angles for cutting or bending.
Care: Protect the edges, keep clean, avoid bending.
Example Application: Measuring the angle of a pre-fabricated steel truss component to ensure it matches the design specifications for a roof structure.
V-Blocks: Description: A block, usually made of hardened steel, with one or two 'V'-shaped grooves machined into its faces.
Uses: To hold cylindrical workpieces securely for marking out, drilling, or machining operations. Often used in conjunction with a surface gauge to find the centre of round stock.
Care: Keep clean, protect from damage.
Example Application: Holding a round bar securely on a surface plate while marking its centre line with a surface gauge before drilling a pilot hole.
Layout Dye (Engineer's Blue): Description: A fast-drying liquid coating, usually blue, purple, or red, applied to the surface of metal before marking out.
Uses: To provide a contrasting background against which scribed lines become highly visible, improving accuracy and reducing eye strain.
Application: Clean the metal surface, then apply a thin, even coat of layout dye using a brush or aerosol spray. Allow to dry completely before marking.
Care: Store in a sealed container, avoid skin contact if possible.
Example Application: Coating a piece of aluminium sheet metal before marking out complex patterns for a custom laptop stand, ensuring clear visibility of all scribed lines.
Understanding measuring and marking out is fundamental to various aspects of Nigerian life and industry.
Local Artisanry and Entrepreneurship: Community: In virtually every Nigerian community, local welders, fabricators, and carpenters (e.g., those fabricating gates, security doors, metal furniture, cooking stoves, or vehicle parts in "mechanic villages" like Ladipo Market in Lagos or spare parts markets in Onitsha) rely daily on precise measurement and marking. Their livelihood depends on producing items that are dimensionally accurate, functional, and aesthetically pleasing. Students can observe these practices and potentially intern with local artisans, seeing the direct application of their classroom learning.
Economic Impact: Accurate measuring and marking reduce material waste (e.g., less scrap metal from incorrect cuts), saving costs for small businesses. This efficiency enhances profitability and competitiveness in a bustling market where resources can be expensive. Students mastering these skills are better positioned for successful entrepreneurship in the metal fabrication sector. Infrastructure Development and Maintenance: Environment & Economy: In Nigeria's ongoing infrastructure development (roads, bridges, buildings, oil and gas facilities), metal structures are ubiquitous. Engineers and technicians use sophisticated measuring and marking tools to ensure components fit perfectly and structures are sound. For instance, during the construction of a new bridge over the Niger River, the steel girders and beams must be precisely measured and marked before fabrication and assembly to guarantee structural integrity and safety. Similarly, in the maintenance of oil pipelines or refinery equipment, accurate marking out is essential for repairs and modifications, directly impacting the safety and efficiency of critical national assets.
Agriculture and Rural Development: Community & Economy: Many locally manufactured agricultural implements (e.g., ploughs, hoes, cassava processing machines, crop dryers) require metalwork. Farmers and local fabricators in rural areas benefit from knowing how to accurately measure and mark out parts for repairing existing machinery or designing new, more efficient tools. For example, a village mechanic needing to replace a worn-out component in a locally fabricated palm oil press would use these skills to measure the old part and mark out the new one from available scrap metal, reducing downtime and costs for the farmer. This promotes self-sufficiency and local innovation in a key economic sector.