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