Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Wood abrasives
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Wood abrasives
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Subject: Wood-Work
Class: Senior Secondary 2
Term: 1st Term
Week: 8
Theme: Design And Construction
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Produce abrasives using local materials Identify abrasive grades available in the market Correctly use abrasives
This section provides a detailed explanation of wood abrasives, their characteristics, grading, usage, and local production. 2.
1. Definition of Abrasives Abrasives are hard materials used to rub, grind, or polish other materials (in this case, wood) to remove unwanted surface material, create a smooth finish, or prepare a surface for subsequent treatments like staining, varnishing, or painting. The action of an abrasive involves the cutting or scraping away of minute particles from the workpiece surface. 2.
2. Types of Abrasives Abrasives can be broadly categorized into natural and artificial (manufactured) types.
Natural Abrasives: These are materials found in nature that possess abrasive qualities. Examples include sand, pumice, natural stones (e.g., fine-grained river pebbles), and even certain plant leaves or barks (though less common for modern woodworking, the concept of natural abrasion is important).
Local Application: In many rural Nigerian settings, sharp, clean river sand or fine-grained stones might have been historically used, or even rough leaves to achieve a basic smooth finish on softer woods. For this lesson, the focus on 'producing abrasives using local materials' will draw primarily from the principle of applying sand to a backing.
Artificial (Manufactured)
Abrasives: These are produced industrially and are the most common type used in modern woodworking.
Sandpaper/Abrasive Paper: This is the most widely used artificial abrasive for wood.
It consists of three main components: Abrasive Grains: These are the actual cutting particles.
Common types include: Garnet: A natural mineral, reddish-brown, often used for hand sanding wood. It is tough and fractures to expose new sharp edges, providing a consistent cut.
Aluminum Oxide: A synthetic, very tough, and durable abrasive, typically brown or reddish-brown. It is excellent for sanding wood and is widely used for both hand and machine sanding due to its longevity.
Silicon Carbide: A synthetic, very hard and sharp abrasive, typically black or grey. It cuts aggressively and is often used for sanding between coats of finish (wet sanding), or on very hard woods or non-wood materials.
Backing Material: This is the support material onto which the grains are bonded. Common backings include paper (various weights, e.g., A-weight for light sanding, D-weight for heavier use), and cloth (more flexible and durable for machine sanding belts and discs).
Adhesive/Bond: This is the glue that holds the abrasive grains to the backing material. Resins and glues are commonly used, designed to withstand heat and pressure during sanding. 2.
3. Grades of Abrasives (Grit Sizes) Abrasives are graded by their "grit size," which refers to the size of the abrasive particles. The grit number system is typically inverse: a lower number indicates coarser particles, while a higher number indicates finer particles. Coarse Grit (e.g., 40-80 grit): Purpose: Used for aggressive material removal, shaping wood, removing deep scratches, old paint, or varnish, and leveling very rough surfaces.
Appearance: The grains are large and feel very rough to the touch.
Application: Initial sanding on severely damaged or very rough timber. Medium Grit (e.g., 100-150 grit): Purpose: For general sanding, smoothing slightly rough surfaces, removing marks left by coarse grits, and preparing the surface for finer sanding.
Appearance: The grains are smaller than coarse grit but still distinct.
Application: Most common starting point for new, moderately smooth timber. Fine Grit (e.g., 180-220 grit): Purpose: For final smoothing of bare wood before applying finishes like stain, varnish, or paint. It removes swirl marks or minor imperfections left by medium grits.
Appearance: The grains are small, and the paper feels relatively smooth.
Application: Essential for achieving a high-quality finish. Very Fine Grit (e.g., 240-400+ grit): Purpose: For extremely smooth surfaces, sanding between coats of finish (to remove dust nibs and smooth the previous coat), and for delicate polishing.
Appearance: The paper feels very smooth, almost like regular paper.
Application: Advanced finishing techniques, often used by professional furniture makers. 2.
4. Methods of Using Abrasives Hand Sanding: This is the most common and accessible method in Nigerian classrooms.
With a Sanding Block: A sanding block (a piece of wood, cork, or rubber) ensures achieving a high-quality finish. Very Fine Grit (e.g., 240-400+ grit): Purpose: For extremely smooth surfaces, sanding between coats of finish (to remove dust nibs and smooth the previous coat), and for delicate polishing.
Appearance: The paper feels very smooth, almost like regular paper.
Application: Advanced finishing techniques, often used by professional furniture makers. 2.
4. Methods of Using Abrasives Hand Sanding: This is the most common and accessible method in Nigerian classrooms.
With a Sanding Block: A sanding block (a piece of wood, cork, or rubber) ensures even pressure distribution and prevents "dishing" (creating concave areas). The abrasive paper is wrapped around the block.
Without a Sanding Block: For irregular shapes, curves, or intricate carvings, direct hand sanding is necessary, using fingers to conform the abrasive to the surface.
Machine Sanding: (Brief mention for awareness, as machinery might not be available in all schools).
Orbital Sanders: Use circular or random orbital motion for general surface sanding.
Belt Sanders: For aggressive stock removal on flat surfaces.
Detail Sanders: For tight corners and intricate areas. 2.
5. Producing Abrasives using Local Materials (Practical Demonstration of Principle) This activity aims to demonstrate the fundamental principle of abrasive paper using materials that are locally available and inexpensive. The resulting product will not match commercial sandpaper quality but serves as an excellent illustrative tool.
Materials Required:
1. Sharp Sand: Clean, dry construction sand (often called 'plaster sand' or 'sharp sand' in Nigeria). Ensure it is free from dirt, dust, or organic matter. Sieve if necessary to ensure uniform particle size for a particular 'grit'.
2. Sturdy Backing Material: Thick paper (e.g., old carton/cardboard box material, exercise book covers), sturdy cloth (e.g., old denim, jute bag material, thick cotton fabric).
3. Local Adhesive: Option 1 (Traditional): Cassava starch paste (made by boiling cassava starch/flour with water to a thick consistency).
Option 2 (Natural Gum): Gum arabic or other natural tree gums (if available and known to be adhesive). Option 3 (Commercial, if available): Strong wood glue (PVA glue) or contact adhesive.
4. Brush or spreader.
5. A flat surface for drying.
Step-by-Step Procedure:
1. Prepare Backing Material: Cut the chosen backing material (paper or cloth) into manageable sizes, e.g., 15 cm x 10 cm rectangles.
2. Prepare Adhesive: If using cassava paste, ensure it is thick and smooth. If using commercial glue, prepare it according to instructions.
3. Apply Adhesive: Using the brush or spreader, apply a thin, even layer of adhesive to one side of the backing material. Ensure full coverage without excessive thickness.
4. Apply Sand: Immediately and generously sprinkle the sharp sand over the adhesive-coated surface. Ensure the sand particles are densely packed and cover the entire adhesive layer. Gently press down on the sand to ensure good adhesion.
5. Remove Excess Sand: Once the surface is fully covered, gently shake off any loose, unadhered sand.
6. Dry: Place the newly formed abrasive sheet on a flat, clean surface and allow it to dry completely. This may take several hours or even a full day, depending on the adhesive and humidity. Ensure it is fully dry before handling or testing.
7. Test: Once dry, test the abrasive sheet on a small piece of scrap wood to assess its effectiveness. 2.
6. Precautions When Using Wood Abrasives Safety is paramount in any woodworking activity. The following precautions must be observed:
1. Wear Personal Protective Equipment (PPE): Always wear a dust mask (to protect against fine wood dust inhalation) and safety glasses or goggles (to protect eyes from flying dust particles).
2. Ensure Good Ventilation: Work in a well-ventilated area to minimize dust accumulation and inhalation. If working indoors, open windows and doors.
3. Secure the Workpiece: Always secure the wood being sanded in a vice or with clamps to prevent it from moving during sanding, which can lead to injury or uneven results.
4. Sand with the Grain: Always sand in the direction of the wood grain. Sanding across the grain will create noticeable scratches that are difficult to remove and will be visible after finishing.
5. Progress Through Grits: Start with a Ensure Good Ventilation: Work in a well-ventilated area to minimize dust accumulation and inhalation. If working indoors, open windows and doors.
3. Secure the Workpiece: Always secure the wood being sanded in a vice or with clamps to prevent it from moving during sanding, which can lead to injury or uneven results.
4. Sand with the Grain: Always sand in the direction of the wood grain. Sanding across the grain will create noticeable scratches that are difficult to remove and will be visible after finishing.
5. Progress Through Grits: Start with a coarser grit and gradually move to finer grits. Never skip grits (e.g., jump from 80-grit to 220-grit) as the coarser scratches will not be adequately removed by the much finer grit.
6. Avoid Excessive Pressure: Allow the abrasive to do the work. Excessive pressure does not make sanding faster; it only wears out the abrasive quicker, generates more heat, and can create uneven surfaces or gouges.
7. Check for Embedded Grit: After using a coarse grit, meticulously brush or wipe away all dust from the workpiece before moving to a finer grit. Residual coarse grit particles can get embedded in the wood and cause scratches when using the finer paper.
8. Proper Disposal of Dust: Collect wood dust for proper disposal. Some wood dusts can be allergenic or even hazardous. 3.
1. Teacher Activities Introduction (5-10 minutes): Initiate a discussion by asking students about the difference between a rough piece of wood and a finished furniture surface. Elicit responses related to smoothness and appearance. Introduce the term "abrasives" and explain their role in achieving desired finishes in woodworking. Display various samples of commercial sandpaper with different grit sizes (e.g., 80, 120, 180, 220 grit).
Concept Explanation (15-20 minutes): Define abrasives and explain the different types (natural vs. artificial), focusing on the components of sandpaper (grains, backing, adhesive). Explain the grit numbering system, demonstrating with the physical samples how coarser grits have larger particles and finer grits have smaller particles. Discuss the appropriate uses for each grade of abrasive. Demonstration of Local Abrasive Production (20-25 minutes): Showcase the materials required for producing abrasives using local materials (sharp sand, backing, adhesive). Perform a step-by-step live demonstration of how to apply adhesive and sprinkle sand onto a backing material to create a local abrasive sheet. Emphasize even application and drying.
Explain the purpose of this activity: to understand the principle of abrasive action. Demonstration of Abrasive Usage (15-20 minutes): Demonstrate correct hand sanding techniques using a sanding block on a piece of scrap wood. Emphasize sanding with the grain, starting coarse and progressing to fine grits, and applying even pressure. Highlight the importance of dust removal between grits. Discuss and demonstrate the necessary safety precautions (PPE, ventilation, securing workpiece). Facilitation of Practical Activities (30-40 minutes): Guide students as they produce their own local abrasive sheets. Supervise students as they practice sanding on individual pieces of scrap wood, providing feedback on technique and safety.
Wrap-up and Assessment (10 minutes): Summarize key learning points. Conduct a brief Q&A session to check for understanding. Assign independent practice questions. 3.
2. Student Activities Participate in Discussion: Engage in the initial discussion about wood surface finishes.
Observe Demonstrations: Pay close attention to the teacher's demonstrations of abrasive types, grading, local production, and usage techniques.
Identify Abrasive Grades: Handle and examine samples of commercial sandpaper, identifying and distinguishing between various grit sizes based on their feel and appearance.
Produce Local Abrasives: In small groups or individually, students will follow the teacher's instructions to produce their own abrasive sheets using local materials.
Practice Sanding: Each student will practice hand sanding on a provided piece of scrap wood, applying different grit sizes (e.g., 80-grit, then 150-grit, then 220-grit) to achieve a smooth finish. They will practice sanding with the grain and securing the workpiece.
Discuss Precautions: Participate in discussions about the importance of safety measures when using abrasives.
Ask Questions: Seek clarification on any aspect of the lesson from the teacher.
Furniture Manufacturing and Joinery: In bustling woodworking centres across Nigeria (e.g., in Lagos, Aba, Nnewi), skilled artisans and industrial manufacturers rely heavily on various grades of abrasives to prepare wood for high-quality furniture, doors, windows, and cabinets. Proper sanding ensures a smooth surface that enhances the aesthetic appeal and longevity of finishes (varnishes, paints, lacquers). This lesson directly equips students with foundational skills for employment or entrepreneurship in this vital sector.
Local Craft and Art Production: Nigerian carvers and sculptors often use abrasives, both traditional (like specific leaves or fine stones) and modern sandpaper, to finish their wooden artworks, masks, and decorative items. The ability to produce simple abrasives using local materials can empower small-scale artisans who may not have ready access to commercial products, enabling them to refine their crafts and potentially increase their market value.
Home Renovation and DIY Projects: Many Nigerians undertake personal home improvement projects, such as refurbishing old furniture, preparing wooden surfaces for repainting, or sanding down new installations. The knowledge of identifying abrasive grades and using them correctly allows individuals to achieve professional-looking results, saving costs and enhancing the lifespan of their wooden assets. Understanding precautions ensures safety during these common household tasks.