Lecture 32: Process Selection Jayant Jain Assistant Professor, Department of Applied Mechanics, IIT Delhi, Hauz Khas, 110016
Jan 15, 2016
Lecture 32: Process Selection
Jayant Jain Assistant Professor,
Department of Applied Mechanics, IIT Delhi, Hauz Khas, 110016
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
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
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.
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
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
The next step is to rank the survivors by economic criteria. To do this we need to examine process cost.
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
Cost model
Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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
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
Material cost independent of batch size and production rate n = Batch size/production volume ṅ = Production rate
Total cost have three components