Machine Mechanic Exam Note Easy To Score 100

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1. Q1. Casting Process (a) Define the manufacturing technologically . Manufacturing refers to the process of converting raw materials into finished products through the use of tools, machinery, and various processes. It encompasses a wide range of activities from design and production to quality control and distribution. (b) i. Open mold : An open mold is a simple type of mold where the molten metal is poured directly into an open cavity. The shape of the cast part is determined by the shape of this cavity. ii. Closed mold : A closed mold has a fully enclosed cavity into which molten metal is poured through a gating system. This mold type is typically used for more complex shapes and to prevent oxidation of the molten metal. iii. Expandable mold : An expandable mold is a mold that can be broken apart or disassembled to remove the cast part. Common materials for expandable molds include sand and plaster. iv. Permanent mold : A permanent mold is made from durable materials such as metal and is designed to be reused multiple times. The cast part is removed by opening the mold without destroying it. Q2. Expandable mold casting process is usually done for sand casting. Explain: 1. Solid pattern: A solid pattern is a single, solid piece that includes the entire geometry of the part to be cast. 2. Split pattern: A split pattern is divided into two or more parts that are joined together to form the complete shape. This allows for easier removal from the sand mold . 3. Match plate pattern: A match plate pattern consists of two patterns mounted on either side of a plate. This helps in aligning the mold halves and includes gating systems for molten metal flow. 4. Cope and drag pattern: Cope and drag patterns are used for creating the upper (cope) and lower (drag) halves of a sand mold separately, which are then assembled for casting. (b) Expandable mold casting process: i. Expanded polystyrene casting process: This process involves making a pattern from polystyrene foam, which is then surrounded by sand in a mold . When molten metal is poured into the mold , the foam pattern vaporizes, leaving behind the cast part. ii. Investment casting process: Also known as lost-wax casting, this process involves creating a wax pattern, coating it with a refractory ceramic material to form a mold , and then melting away the wax, leaving a cavity into which molten metal is poured.

2. (c) Seven examples of defects related to the use of sand molds : 1. Gas porosity : Formation of gas bubbles within the metal. 2. Shrinkage : Contraction of metal during solidification causing cavities. 3. Mold shift : Misalignment of the two halves of the mold . 4. Mold crack : Cracks in the mold material that can lead to defects. 5. Inclusions : Foreign particles trapped in the cast part. 6. Fusion : Improper fusion of different parts of the mold leading to weak spots. 7. Run-out : Loss of molten metal from the mold cavity due to cracks or gaps. Q5 . Welding (a) Describe each of the welding types: i. Arc Welding : Uses an electric arc to melt the metals at the welding point. Common types include Shielded Metal Arc Welding (SMAW) and Gas Tungsten Arc Welding (GTAW). ii. Resistance Welding : Uses electrical resistance to generate heat and melt the metals. Common types include Spot Welding and Seam Welding. iii. Oxygen Gas Welding : Uses a flame produced by burning oxygen and a fuel gas (such as acetylene) to melt the metals. Common types include Oxy-Acetylene Welding (OAW). (b) Some of the welding process doesn’t need to use heat from a flame or other sources; instead, it achieves its temperature from the movement of the part itself. Explain what are: i. Friction welding : Joins metals by generating heat through mechanical friction between the workpieces, followed by the application of axial force to forge the pieces together. ii. Friction stir welding : A rotating tool is used to generate frictional heat and plastic deformation at the joint line, producing a solid-state weld. (c) Issues in welding quality: Porosity : Gas pockets trapped in the weld. Cracking : Formation of cracks in the weld or heat-affected zone. Distortion : Warping or deformation due to uneven heating and cooling. Inclusions : Foreign materials trapped in the weld, such as slag or tungsten.

3. Question 4: Machining (a) Differences between turning and milling: Turning : The workpiece rotates while the cutting tool moves linearly to remove material. Milling : The cutting tool rotates and moves over the stationary workpiece to remove material. (c) Formation of the chip depends on the type of material being machined and the cutting conditions of the operation. Explain the formation of: i. Discontinuous chip : Formed when brittle materials are machined or at low cutting speeds. The chips break into small segments. ii. Continuous chip : Formed when ductile materials are machined at high cutting speeds and low feed rates, resulting in long, continuous chips. iii. Continuous chip with built-up edge (BUE) : Formed when a portion of the material being cut adheres to the cutting edge, altering the tool geometry and surface finish. iv. Serrated chip : Formed in materials with low thermal conductivity or during high-speed machining, resulting in a saw-tooth-like chip structure.