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MAE486_Fall11_L21_S - MAE 486 Design of Mechanical Systems...

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Unformatted text preview: MAE 486 Design of Mechanical Systems Lecture 21 Fall 2011 Today s topics: Manufacturing consideration of design (Chapter 13); 1 Find the π (Pi) term for the static pressure P by using repeating variables of average velocity V and fluid density ρ. 2 Convert the following partial differential equation into the finite difference equation. ∂2T ∂2T ∂2T 2 2+ + = 3T (x, y ) 2 ∂x ∂ x∂ y ∂y 3 Find the force-displacement equation for the three-node 1-D FEM model when the force is applied at the third node and the first node is fixed. Review Manufacturing Func4ons What are the different aspects involved in manufacturing? Types of Manufacturing Processes Classes of Manufacturing Processes Types of Manufacturing Systems Manufacturing Process Selec4on Quan4ty of Parts Required Example 1 •  Problem statement: Automobile plas4c bumpers –  What are the requirements for a good bumper? –  Four polymeric materials were chosen: Review Required Quality of the Part •  How to characterize the quality of the part? –  No external and internal defects, –  Surface finish, –  Dimensional accuracy and tolerance. •  Defects –  Internal defects: voids, porosity, cracks, or regions of different chemical ­composi4on (segrega4on) –  Surface defects: surface cracks, roll ­in oxide, extreme roughness, or surface discolora4on or corrosion. Required Quality of the Part (cont’d) •  Surface finish –  The appearance, –  Affects the assembly of the part with other parts, –  Influence its resistance to corrosion and wear. •  Surface roughness: –  Fa4gue failure, fric4on and wear, and assembly with other parts. –  Measured with a profilometer. •  Parameters to describe the state of surface roughness. –  Rt : –  Ra: Required Quality of the Part (cont’d) –  Rq is the root ­mean square of the devia4on from the mean surface. –  Rq is some4mes given as an alterna4ve to Ra because it gives more weight to the higher peaks in the surface roughness. –  Lay: direc4onal scratches –  Waviness: long range distance than the peaks and valleys of roughness Required Quality of the Part (cont’d) The surface are defined in words and by the preferred values, N, given by ISO surface roughness standard. Dimensional Accuracy and Tolerances Approximate values of surface roughness and tolerance on dimensions typically obtained with different manufacturing processes. Cost to Manufacturing What should be included in the manufacturing cost? •  •  •  •    Cost of material CM Labor cost CL Capitalized cost of the equipment CC Cost of tooling CT Cost of overhead COH –  Lumps together many necessary costs, like plant maintenance, general engineering, accounDng etc. Cost to Manufacturing (cont’d) •  How to determine unit cost CU, of a part with weight m? –  Material cost (f is a frac4on of material scrap) –  Labor cost: labor cost per unit 4me (  ­ ­ produc4on rate) –  Tool cost: (  ­ ­ number of 4mes the tooling must be replaced) Cost to Manufacturing (cont’d) –  Capital cost of equipment: (  ­  ­ ­ capital write ­off 4me; L – load factor, frac4on 4me the equipment is produc4ve; q – apprecia4on factor due to produc4on share with mul4ple products) –  Overhead cost: •  Tool unit cost: Example •  Problem Statements: –  Cost for making 500000 units of the fan. –  Specifica4ons: •  Radius of the blade: 9 in.; •  Hub: 0.5 in thick; Diameter of 4 in. •  12 blades, each with 1 in. wide at the root and 2.3 in. wide at the 4p. •  Each blade is 0.4 in. thick. Example (cont’d) •  The volume of the cas4ng is about 89 cu. In. If cas4ng in aluminum, it weighs 8.6 lb (3.9 kg) •  Only cas4ng or molding processes are considered (For an integral hub and blade process). –  Low ­pressure permanent mold casDng –  Squeeze casDng: Example •  Could be achieved by several cas4ng processes •  Die cas4ng •  Investment cas4ng – Lost ­ wax cas4ng •  Injec4on molding •  Squeeze fas4ng Example (cont’d) •  The surface finish on the blades must be at least N8 to minimize fa4gue failure. Tolerance must be +/ ­0.020 in. Example (cont’d) Investment casDng is eliminated due to the small economic batch size Example (cont’d) Design for Manufacturing (DFM) •  Why DFM important? •  What is the key in DFM? •  DFA: •  During what stage of design should DFMA be applied? DFM Guidelines •  What are DFM guidelines? –  Years of experience  good design prac4ces; –  Rule 1: Best way to Minimize total number of parts. –  Rule 2: –  Rule 3: DFM Guidelines (cont’d) –  Rule 4: –  Rule 5: •  Fewest parts; •  Least intricate shapes; •  Few precision adjustments, etc. –  Rule 6: DFM Guidelines (cont’d) –  Rule 7: : •  Trade ­off between higher strength and poor workability or fabricability; •  Minimizing the manufacturing opera4ons. –  Rule 8: Avoid excessively Dght tolerance –  Rule 9: Minimize secondary and finishing operaDons: –  Rule 10: UDlize special characterisDcs of processes: DFM: More Specific Design Rules •  Space holes in machined, cast, molded, or stamped parts: –  Made in one opera4on without tooling weakness. •  Avoid generalized statements on drawing; Be specific and unambiguous. –  Example: “polishing this surface”. •  Dimensions should be made from –  Specific surfaces or points on the part, not from points in space. –  A single datum surface rather than from a variety of points •  Avoid overlap of tolerance. •  Minimize weight consistent with strength and s4ffness requirements. •  Use general ­purpose tooling rather than special dies, form culers, etc. •  Use generous fillets and radii on cas4ngs and on molded, formed, and machined parts. •  Parts to allow as many opera4ons as possible without reposi4oning. Design for Assembly (DFA) •  Assembly process: –  Handling  inser4ng  fastening. •  What are the different types of assembly (by the level of automa4on): •  What are the factors in the cost of assembly? Some DFA Examples DFA Guidelines •  General guidelines –  G ­Rule 1: Minimize the total number of parts: •  What will make parts to become essen$al parts, or theore$cal parts? –  Must exhibit mo4on rela4ve to another part that is declared essen4al –  Exist a fundamental reason that the part be made from a material different from all other parts –  Impossible to assemble/disassemble the other parts unless this part is separate –  Maintenance of the product may require disassemble and replacement of a part –  Parts used only for fastening or connec4ng other parts are prime candidates for elimina4on DFA Guidelines (cont’d) –  Rule 2: Minimize the assembly surfaces –  Rule 3: Use subassemblies –  Rule 4: Mistake ­proof the design and assembly: e.g., orienta4on notches, asymmetrical holes, etc. •  Guidelines for handling –  Avoid separate fasteners or minimize fastener costs: –  Minimize handling in assembly DFA Guidelines (cont’d) •  Guidelines for inser4on –  Minimize assembly direcDon: –  Provide unobstructed access for parts and tools: examples? –  Maximize compliance in assembly: Standardiza4on in DFMA •  Benefits of standardiza4on –  Cost reducDon –  Quality improvement Standardiza4on in DFMA (cont’d) –  ProducDon flexibility –  Manufacturing responsiveness Achieving Part Standardiza4on •  With the importance of standardiza4on in mind, what we should avoid? –  Not free to make arbitrary decisions when sizing parts. •  A common misconcep4on: –  Minimum ­cost design minimum ­weight design. Why is it not always right? •  How to be aware of the existence of an iden4cal part? ...
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