Download - Lecture 32
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Lecture 32: Process Selection
Jayant Jain Assistant Professor,
Department of Applied Mechanics, IIT Delhi, Hauz Khas, 110016
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Recap
Classification of processes Important process attributes Process selection flow chart Classification of shapes Process charts Material-process chart, Shape-process chart, Process-mass chart, Process-section thickness chart Physical limits to size: casting
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Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Metal shaping processes such as rolling, forging, or extrusion involve flow – solid metals flow by plastic
deformation or by creep
The minimum thickness that can be achieved from these processes is limited by plastic flow – very thin sections cause substantial friction forces and stick to the tools, even at very large pressures
Very thin objects: Forging
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Tolerance and Roughness
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Tolerance Dimension y specified
as y = 100 ± 0.1 mm
Roughness Measure of the irregularities of a surface specified as an
upper limit such as R < 100 μm
No process can shape a part exactly to a specified dimension. Manufacturing processes vary in the levels of tolerance and roughness they can achieve economically.
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Process-Tolerance
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Real process allow tolerances between a range of 10R and 1000R – R being surface roughness See the colored band
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Process-Roughness
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Processing cost for precision and surface finish increase exponentially as the requirements are made more severe
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The next step is to rank the survivors by economic criteria. To do this we need to examine process cost.
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Ranking: process cost
The manufacture of a component consumes resources, each of which has an associated cost – the final cost is the sum of those of the resources it consumes Trying to build the cost model: Already in CES, it will help you in ranking on the basis of cost comparison
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Cost model
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Shaping Cost Per Unit
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
Material: material cost and mass per unit; f is the scrap fraction for unused material
Tooling: cost is dedicated and written off for production of n units; nt represents life of tool in case replacement is necessary
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Capital: non-dedicated cost; written off against time for a given hourly rate; two is the write-off time, L is the load factor, and ń is the production rate Overhead: gross overhead costs divided by the production rate
Shaping Cost Per Unit
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Material cost independent of batch size and production rate n = Batch size/production volume ṅ = Production rate
Total cost have three components