Lesson Notes By Weeks and Term v4 - SHS 3
Manufacturing Processes
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Manufacturing Processes
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Subject: Manufacturing Engineering
Class: SHS 3
Term: 1st Term
Week: 10
Grade code: 1.3.2.LI.2
Strand code: 3
Sub-strand code: 2
Content standard code: 1.3.2.CS.2
Indicator code: 1.3.2.LI.2
Theme: Manufacturing tools, equipment and processes
Subtheme: Manufacturing Processes
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This lesson introduces three fundamental sheet metal working operations: cutting, bending, and drawing. Sheet metal is all around us in Ghana – from the roofing sheets on our houses and schools, to the "chop boxes" we use for storage, the cooking pots (dadesen) in our kitchens, and the body panels of tro-tros and cars. Understanding how a flat, thin sheet of metal is transformed into these useful products is a core skill in manufacturing. This knowledge opens up opportunities in fabrication, construction, automotive repair, and entrepreneurship for our local artisans and engineers. We will explore the principles, tools, and applications of these processes both theoretically and practically.
Introduction to Sheet Metal Working Sheet metal working involves forming and shaping parts from thin sheets of metal, typically less than 6mm thick. The processes are usually performed at room temperature (cold working), which increases the strength and hardness of the metal. The three main categories of operations we will study are cutting, bending, and drawing. A. Cutting Operations Cutting is the process of separating a piece of metal from a larger sheet by subjecting it to shear stress. The force is applied by two opposing cutting edges, much like a pair of scissors. Key Process: Shearing Principle: A punch (upper blade) and a die (lower blade) are used. The sheet metal is placed between them. As the punch descends, it pushes the metal into the die. The stress exceeds the metal's shear strength, causing a fracture, and the metal separates. Tools: Hand shears (snips), bench shears (guillotines), and power shears. Clearance: The small gap between the punch and the die is critical. If the clearance is too small, it requires excessive force. If it's too large, the cut edge will be rough and burred. Two Specific Types of Shearing: Blanking: Definition: In blanking, the piece that is cut out from the sheet is the desired product. The surrounding material is scrap. Example: Imagine using a special press to cut out a circular disc from a large aluminium sheet to make the base of a cooking pot. The circular disc is the "blank," and it is the useful part. The large sheet with a hole in it is the scrap. Punching (or Piercing): Definition: In punching, the material that is cut out is the scrap (called a slug). The remaining sheet, now with a hole in it, is the desired product. Example: Making a hole in a metal ruler for hanging. The small circle of metal that is removed is scrap. The ruler with the hole is the final product.
How to Remember the Difference: Blanking -> The Blank (cut-out part) is the good part. Punching -> You Punch a hole in the good part.
[Teacher's Sketch on the Board] *Draw a simple diagram showing a punch and die. For blanking, label the cut-out piece "PRODUCT (Blank)". For punching, label the sheet with the hole "PRODUCT" and the cut-out piece "SCRAP (Slug)".* B. Bending Operations Bending involves straining a sheet of metal around a straight axis to create a permanent change in its shape, such as an L-shape or V-shape. Principle: During bending, the material on the outside of the bend is stretched (in tension), while the material on the inside is compressed. There is a line within the metal's thickness, called the neutral axis, where no stretching or compression occurs. Common Bending Methods: V-Bending: Process: The sheet metal is placed over a V-shaped die, and a V-shaped punch forces the sheet down into the die cavity, causing it to bend. Application: This is the most common method for creating simple bends and is used to make brackets, channels, and enclosures like a computer system unit casing. Edge Bending (Wiping): Process: The sheet is held down by a pressure pad, and a punch pushes on the overhanging portion of the sheet to bend it over the edge of a die. Application: Used for making flanges, seams, and hems, like the folded edge on a metal trunk ("chop box") to make it smooth and strong. Key Concept: Springback Definition: After the bending pressure is removed, the metal has a tendency to partially return to its original flat shape due to elastic recovery. This is called springback. Implication: To get a perfect 90° bend, you might have to bend the metal slightly more, for example, to 92°. This is called overbending. Experienced fabricators develop a feel for how much to overbend for different materials and thicknesses. C. Drawing Operations Drawing is a forming process used to transform a flat sheet metal blank into a hollow or concave shape, like a cup, box, or can. Principle: The process involves forcing a flat sheet metal blank into a die cavity with a punch. The Drawing Process (Step-by-Step): A flat, circular blank (created using blanking) is placed over the opening of a die. A blank holder presses down on the edges of the blank, holding it firmly against the die. The punch, which has the shape of the final product, moves downwards and pushes the metal into the die cavity. The metal is forced to flow and stretch, taking the shape of the punch and die. The punch is then withdrawn, and the finished cup is ejected. Example: Manufacturing an aluminium cooking pot ("dadesen"). A circular blank is cut (Blanking). The blank is placed in a large press. The punch pushes the blank into a die, forming the pot's shape (Drawing). Key Elements & Potential Defects: Blank Holder: This is crucial. It applies just enough pressure to allow the metal to flow into the die without creasing. If blank holder force is too low -> Wrinkling occurs on the walls of the cup. If blank holder force is too high -> It restricts metal flow, causing the sheet to stretch too much and tear or fracture. Drawing Ratio: The ratio of the blank diameter to the punch diameter. If this ratio is too large, the operation will likely fail.
Guided Practice (With Solutions)