process Manufacturing selection
School of Mechanical Engineering The University of Western Australia Professor Brett Kirk Room 116/118 Phone [phone] email: [e-mail]
Process selection
The selection of process is a critical factor that affects manufacturing costs both directly and indirectly. Not only do different processes have different costs, their different characteristics will dictate the design of components, the materials that can be used, the quantities for economical production, the batch sizes to be used .... The processes selected and the limitations they place on design and materials selection must therefore be carefully considered.
Materials selection: Mechanical Design
Motion Wear & lubrication Impact loading Cyclic loading Fatigue (wear and failure) Loosening Dampening Elasticity Vibrations Energy dissipation Design for displacement, not strength Dimensional rigidity is often critical
Non-functional design factors
Aesthetics Form Feel Texture Colour Decoration ie. We sometimes need some 'art' as well as science in our design !
Design processes
Original design - "Invention" Open mind as widely as possible Process selection can often be the key to invention (eg: silicon depositrion for transistors, glass drawing for optical fibres). Adaptive design - "Innovation" New processes can offer enable very logical innovations (eg: polymers substituted for metals in household appiances, carbon fibre replacing wood in sporting goods) Variant design - change of size or shape Often a change in size or shape can necessitate a change in materials and process.
Process selection
We need an objective way by which to select manufacturing processes, similarly to materials Increasingly, this is done using computer software We will be using the Cambridge Engineering Selector in laboratories. This approach can also be done manually using charts. The charts and the approach are outlined in detail in Materials selection in Mechanical Design, M F Ashby, Pergamon Press, 1992. Samples of these charts will be available for class use.
Shape Factors
Elastic Bending:
B
e
e
Elastic Twisting: T
Failure in Bending: B
f f
Failure in Twisting: T
The shape factors are set to 1 for circular cross sections.
Practical limitations of shape factors
Shape factors are limited by manufacturing constraints. Shape factors are limited by material constraints (eg: local buckling). Steel Wood Aluminium
> 30 < 30 < 25
Shape factors must therefore be considered along with performance indices, and with manufacturing process.
Manufacturing process and design
Material properties and design specifications dictate the manufacturing process. For example: Ductile forged rolled drawn Brittle casting (low melting point) machined Slender shapes rolled drawn not casting High precision machining not forging
Manufacturing other factors
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Process selection selection charts
Process charts exist for the following properties: Size-Shape (Charts P1, P1(a), P1(b)) Information content-Size (Chart P2) Size-Melting point (Chart P3) Hardness-Melting point (Chart P4) Tolerance-Surface Finish (Chart P5)
Size-Shape (Charts P1, P1(a), P1(b))
T h i s c h a r t p lo ts th e s u r f a c e a r e a a g a i n s t th e m in im u m s e c t i o n t h i c k n e s s . T h is a l lo w s e x a m in a t io n o f th e s u r f a c e t o v o l u m e r a t i o , a n d t h e s l e n d e r n e s s o r a s p e c t r a ti o . T h e s l e n d e r n e s s r a t io is ty p i c a ll y i n th e r a n g e ( t , t ) .
1 2
A2
1
A
C h a rts P 1 (a ) a n d P 1 (b ) fo c u s o n re g io n s r e l e v a n t t o c a s t in g a n d p o l y m e r f o r m in g re s p e c tiv e ly .
Information content Size (Chart P2)
The com plexity of shape, or its inform ation content, can be m easured. This can be done using the m inim um num ber of independent variables required to describe a shape or its sym etry, but it can also be m m easured in bits (C: inform ation content):
l l l l 1 C = n log 2 , S = log 2 1 2 3 .... n l l l ln S 1 2 3 where S is the success probability ln is the specified precision of dimension ln
This can be approxim ated by:
l C = n log 2 l
Size-Melting point (Chart P3)
The materials themselves impose a limit on manufacturing process through their melting point. As the melting point rises, the primary forming options decrease.
Hardness-Melting point (Chart P4)
The hardness of a material limits the ability to deform and machine. Forging and rolling pressures, and tool loading, is limited by the hardness of a material. The strongest superalloys therefore cannot be forged or machined, and ceramics are very difficult to shape.
Tolerance-Surface Finish (Chart P5)
Each manufacturing process has a limitation on surface finish and dimensional accuracy. The surface finish is measured by the RMS amplitude of the irregularities on the surface. Each material and process thus has fundamental limitations and it is the tolerance and surface finish specified that is likely to have the greatest impact on manufacturing process, and therefore cost.
Practical issues - Join...