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ME 350 – Lecture 7 – Chapter 24 ECONOMIC AND PRODUCT DESIGN CONSIDERATIONS IN MACHINING Tolerances and Surface Finish Selection of Cutting Conditions Product Design Considerations in Machining

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Machinability Criteria in Production Tool life – longer tool life for the given work material means better machinability Forces and power – lower forces and power mean better machinability Surface finish – better finish means better machinability Ease of chip disposal – easier chip disposal means better machinability

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Tolerances and Surface Finish Tolerances –Machining provides high accuracy relative to most other shape-making processes –Closer tolerances usually mean higher costs Surface roughness in machining determined by: 1.Geometric factors of the operation 2.Work material factors 3.Vibration and machine tool factors

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Effect of Cutting Conditions: End Cutting Nose Radius Feed Edge Angle

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Ideal Surface Roughness where R i = theoretical arithmetic average surface roughness; f = feed; NR = nose radius

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Work Material Factors Built up edge effects Damage to surface caused by chip Tearing of surface when machining ductile materials Cracks in surface when machining brittle materials Friction between tool flank and new work surface

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Effect of Work Material Factors To predict actual surface roughness, first compute ideal surface roughness, then multiply by the ratio from the graph

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Vibration and Machine Tool Factors Related to machine tool, tooling, and setup: –Chatter (vibration) in machine tool or cutting tool –Deflections of fixtures –Backlash in feed mechanism If chatter can be eliminated, then surface roughness is determined by geometric and work material factors

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How To Avoid Chatter Add stiffness and/or damping to setup Operate at speeds that avoid frequencies close to natural frequency of machine tool system Reduce feed (and sometimes depth) Change cutter design Use a cutting fluid

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Determining Feed Select Determining feed rate depends on: –Tooling – harder tool materials require lower feeds –Is the operations roughing or finishing? –Constraints on feed in roughing Limits imposed by forces, setup rigidity, and sometimes horsepower –Surface finish requirements in finishing Select feed to produce desired finish

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Optimizing Cutting Speed Select speed to achieve a balance between high metal removal rate and suitably long tool life Mathematical formulas available to determine optimal speed Two alternative objectives in these formulas: 1.Maximum 2.Minimum

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Maximum Production Rate Maximizing production rate = minimizing cutting time per unit In turning, total production cycle time for one part consists of: 1.Part handling time per part = T h 2.Machining time per part = T m 3.Tool change time per part = T t /n p, where n p = number of pieces cut in one tool life (round down) Total time per unit product for operation: T c =

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Cycle Time vs. Cutting Speed

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Minimizing Cost per Unit In turning, total production cycle cost for one part consists of: 1.Cost of part handling time = C o T h, where C o = cost rate for operator and machine 2.Cost of machining time = C o T m 3.Cost of tool change time = C o T t /n p 4.Tooling cost = C t /n p, where C t = cost per cutting edge Total cost per unit product for operation: C c =

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Unit Cost vs. Cutting Speed

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Comments on Machining Economics As C and n increase in Taylor tool life equation, optimum cutting speed –Cemented carbides and ceramic tools, compared to HSS, should be used at speeds: v max is always than v min – Reason: C t /n p term in unit cost equation pushes optimum speed to left in the plot As tool change time T t and/or tooling cost C t increase, cutting speed should be –Disposable inserts have an advantage over regrindable tools if tool change time is significant

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Product Design Guidelines Design parts that need no machining –Use processes such as precision casting, closed die forging, or plastic molding Minimize amount of machining required –Use near net shape processes Avoid difficult to machine features: –sharp internal corners, and external points (breakage) should be avoided

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Better designs? A B C D EFEF

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