Transcript The Design Core - Oxford Materials

The Design Core
Market
Assessment
Specification
DETAIL
DESIGN
Concept
Design
A vast subject. We will concentrate on:
Materials Selection
Detail
Design
Process Selection
Cost Breakdown
Manufacture
Sell
Systematic Process Selection
All Processes
Screening: apply attribute limits
(eliminate processes that cannot do the job)
Ranking: order by relative cost
(find processes that can do the job economically)
Subset of Processes
Supporting Information: handbooks, suppliers data sheets, databases, WWW
(Search “family history” of candidates)
Prime Candidates
Local Conditions
(does the choice match local needs, expertise etc.?)
Final Process Choice
Categories of Component Shape
SLENDERNESS
Ratio of section thickness to the
square root of section area:
s
t
A
Similar to aspect ratio in 2-d
COMPLEXITY
Relates to the number of specified
dimensions of the component and
the precision required:
 L 
C  n log2 


L


But life is more complicated, e.g.
spheres have low complexity, but
are difficult to make compared
with cylinders of higher
complexity. We should also
consider other attributes such as
symmetry.
Process for a Vacuum Cleaner Fan
Fans for vacuum cleaners are designed to be
cheap, quiet and efficient. Nylon and Al alloys
have been identified as candidate materials.
Net shape processing is preferred for low cost.
Complexity is classified as 3-D solid.
Constraint
Materials
Value
-Nylon
-Al alloys
Complexity
Minimum section
Surface area
Volume
Weight
Mean precision
Roughness
Process
Tm = 550 - 573 K, H = 150 - 270 MPa
ρ = 1080 kg/m3
Tm = 860 - 933 K, H = 150 - 1500 MPa
ρ = 2700 kg/m3
2-3
1.5 - 6 mm
0.01 - 0.04 m2
1.5x10-5 - 2.4x10-4 m3
0.03 - 0.5 kg
0.5 mm
<1 µm
Net shape preferred
Process for a Vacuum Cleaner Fan
SLENDERNESS
Process choice is often limited by the
capacity to make long, thin sections
(slenderness S of a component), where
S
t
A
Define a search region
that has limits a
factor of 2 on either side of the target values.
The fan can be shaped in a large number of
ways including die-casting for Al alloys and
injection moulding for polymers.
The hot working processes for metals
cannot be chosen.
Process for a Vacuum Cleaner Fan
COMPLEXITY
Define a search region
that has
limits on either side of the target values.
The micro-electronic fabrication methods and
sheet working processes for metals
are eliminated.
The search region falls in a regime in which
many alternative processes are possible.
Hence, in this case, we learn nothing new.
Process for a Vacuum Cleaner Fan
HARDNESS / MELTING POINT
Define search regions
that have
limits on either side of the target values.
In this case almost all processes for polymers
and metals are viable.
Only electron beam casting
is eliminated.
Hence, in this case, we again learn nothing new.
Process for a Vacuum Cleaner Fan
SURFACE ROUGHNESS
In the designer’s view, it is the surface
finish is the discriminating requirement. It
(and the geometry) determines the fan’s
pumping efficiency of and influences the
noise it makes.
The design constraints, R < 1 µm and
T < 0.5 mm, define the search region
on the tolerance/roughness process
selection map.
A significant number of processes
are
eliminated.
A number of polymer moulding processes,
including injection moulding are acceptable.
Machining from solid meets the specifications,
but is not net-shape.
Many casting processes are eliminated, but
pressure die-casting, squeeze casting and
investment casting are acceptable.
Process for a Vacuum Cleaner Fan
Process
Nylon and Al-alloys
Machine from solid
Electro-form
Al-alloy only
Cold deformation
Investment casting
Pressure die casting
Squeeze casting
Nylon only
Injection moulding
Resin transfer moulding
Comment
Expensive, not a net-shape process.
Slow, and thus expensive.
Cold forging meets the design constraints.
Accurate, but slow.
Meets all the design constraints.
Meets all the design constraints.
Meets all the design constraints.
Meets all the design constraints.
N.B. The charts can only narrow the choice. There are other considerations
of course: capital investment, batch size and rate, supply, local skills etc.
A cost analysis is now required to establish the best choice.
Forming Ceramic Tap Valves
Vitreous alumina is commonly used in hot eater valves, but
it may not be the best due to thermal shock. The materials
selection procedure offered Zirconia as a possible
alternative. How should the valve discs be shaped?
Constraint
Material
Value
-Zirconia
Complexity
Minimum section
Surface area
Volume
Weight
Mean precision
Roughness
Tm = 2820 K, H = 15000 MPa
ρ = 3000 kg/m3
1-2
5 mm
10-3 m2
1.5x10-6 m3
4.5x10-3 kg
0.02 mm
<0.11 µm
Forming Ceramic Tap Valves
SLENDERNESS
Process choice is often limited by the
capacity to make long, thin sections
(slenderness S of a component), where
S
t
A
Define a search region
that has limits a
factor of 2 on either side of the target values.
The ceramic discs are not particularly slender.
Some metal forming and polymer moulding
processes
are eliminated, but we would not
expect to use those processes for ceramics in
any case.
Hence, we do not learn much.
Forming Ceramic Tap Valves
COMPLEXITY
Define a search region
that has
limits on either side of the target values.
The micro-electronic fabrication methods and
ceramic moulding processes
are
eliminated.
Powder routes, machining and molecular
methods are viable alternatives based on
complexity.
Forming Ceramic Tap Valves
HARDNESS / MELTING POINT
Define search regions
that have
limits on either side of the target values.
High melting point and hardness are restrictive.
Machining
is now eliminated.
Electron beam casting, electroforming, and CVD
and evaporation methods are possibilities.
Powder routes emerge as the practical
alternative, but can these methods adhere to the
tolerance and surface finish required?
Forming Ceramic Tap Valves
SURFACE ROUGHNESS
The surface of the discs must be flat and
smooth to ensure a good seal between
the mating faces.
The design constraints, R < 0.1 µm and
T < 0.02 mm, define the search region
on the tolerance/roughness process
selection map.
Powder routes
are now eliminated as
they cannot give the required tolerance and
surface finish.
Mechanical polishing is possible.
Forming Ceramic Tap Valves
Process
Powder methods
CVD and evaporation methods
Electron beam casting
Electro-forming
Machining
Comment
Capable of shaping the disc, but not
desired precision.
No CVD route available. Other gas-phase
methods possible for thin sections.
Difficult with a non-conductor.
Not practical for an oxide.
Material too hard, but polishing is
possible.
No single process is ideal for producing the ceramic valve discs from zirconia.
A combination of processes emerges. Powder methods can be used to form
the discs. The mating faces could then be polished to the desired tolerance
and surface finish.
Process Selection: Cost
Three rules for minimizing cost
1.
Keep things standard: It is cheaper to buy a standard part than make it in house.
If nobody makes the part you want, then design it to be made from standard stock
materials, and use as few of them as possible.
2.
Keep things simple: If a part requires machining then it will need to be clamped.
Keep it simple so that the number of times it has to be re-jigged is minimized. If a
part requires casting the minimize re-entrant angles which require complicated and
expensive dies.
3.
Do not over-specify performance: Higher performance increases cost. Higher
strength alloys are more heavily alloyed with expensive elements. Higher strength
materials require more energy to form. Increased tolerance leads to higher
machining or finishing costs.
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MATERIALS COST ($/kg)
Materials Costs
100
10
1
0.1
Process Selection: Cost
Economic Criteria for Process Selection
Cost Modelling
The producing a component consumes resources (see below).
All processes consume these resources to some extent and
thus a resource based approach is useful at the broad level we
are dealing with.
Resource
Materials:
Capital:
Time:
Energy:
Space:
Information:
inc. consumables
cost of equipment
cost of tooling
basic overhead rate
power
cost of energy
area
cost of space
R&D, royalty payments
Symbol
Unit
Cm
Cc
Ct
$/kg
$
$
$/hr
kW
$/kWh
m2
$/m2h
$/yr
C Lo
P
Ce
A
C
s
Ci
Cost Modelling
Materials
Cost:
C  mCm  
Tooling
Time Capital Energy Space
1
Ct   1 C Lo  Cc  PCe  AC s 
n
n 
Ltc

where m is the mass of material used, n is the batch size (no. units),n is the batch rate (no.
units per hour), tc is the capital write-off time, and L is the capital load factor (the fraction of
time over which the equipment is used productively)
Materials
This reduces to: C  mCm 
Dedicated cost/unit Gross overhead/unit

1
Ct 
n


1 
CL,gross

n

So, Cost has 3 terms
1. Materials costs: independent of batch size and rate.
2. Dedicated capital investment (tooling, jigs, dies etc.): varies with the reciprocal of batch size.
3. Time dependent (operators, space, power etc.): varies with the reciprocal of batch rate.
Cost Modelling: A Cast Connector Rod
Cost parameter
Material, mCm
.
Basic overhead, CLo(h-1)
Capital write-off time, tc (yrs)
Dedicated tool cost, Ct
Capital cost, Cc
.
Batch rate, n (h-1)
Sand
Casting
Die
Casting
1
20
5
210
10000
5
1
20
5
16 000
300 000
200
All costs are normalized to the material cost
The cost of both processes is dominated by
capital and tooling costs for small batch sizes;
and dominated by materials and labour costs for
large batch sizes.
For very large batch sizes the cost of die casting
is dominated by material costs.
10000
Relative Cost per Component
The materials and process selection processes
have identified the sand casting and die casting
processes for a connector rod. Which process is
economical?
Die
Casting
1000
Sand
Casting
100
10
Labour (sand)
Labour
(die)
1
Material Cost
0.1
1
10
100
1000
10000 100000 1E+06
Number of Components
For batch sizes < 4000, sand casting is most economical.
For batch sizes > 4000, die casting is most economical.
UNIT COST
Process Selection: Cost
103
102
10
1
1
10
102
103
104
105
106
107
ANNUAL PRODUCTION
Fixed Costs
Setup:
Variable Costs
Material:
570g of grey cast iron
$0.50 each
Tooling:
$1.8k
8 impressions/pattern
no cores
Processing:
Setup:
Material:
2.6kg of grey cast iron
$2.30 each
120 pcs/hr at $44/hr
Tooling:
$2.4k
2 impressions/pattern
1 core
Processing:
Setup:
Material:
260g of yellow brass
$0.713 each
Tooling:
$1.5k
no cores
Setup:
30 pcs/hr at $44/hr
Processing:
4 pcs/hr at $50/hr
Material:
180g of 712 aluminium
$0.395 each
Tooling:
$7k
3 cores
Processing:
1 pc/hr at $50/hr
Volume
Total Unit Cost
10
$180.87
100
$18.87
1000
$2.67
10
$243.77
100
$27.77
1000
$6.17
10
$163.21
100
$28.21
1000
$14.71
10
$750.40
100
$120.40
1000
$57.40
CASTINGS: Sand (top), Investment (bottom)
Fixed Costs
Setup:
0.75hr at $60/hr
Variable Costs
Material:
1.11kg of 6061 aluminium
$9 each
Tooling:
Programming
0.25hr at $60/hr
Processing:
Setup:
Material:
1.96kg of 6061 aluminium
$16 each
1.75hr at $60/hr
6min/unit at $60/hr
Tooling:
Prog’g 1hr at $60/hr
Fixtures: $150
Processing:
Setup:
Material:
4.6kg ultra-high Mw PE
$25 each
5.5hr at $60/hr
55min/unit at £60/hr
Tooling:
Programming
2hr at $60/hr
Processing:
Setup:
Material:
1.5kg of 6061 aluminium
$12 each
2hr at $60/hr
Tooling:
Programming
2hr at $60/hr
2.85hr/unit at $60/hr
Processing:
6hr/unit at $60/hr
Volume
Total Unit Cost
1
$75.00
10
$21.00
100
$15.50
1
$386.00
10
$102.50
100
$74.15
1
$646.00
10
$241.00
100
$200.50
1
$612.00
10
$396.00
100
$374.40
CNC MACHINING
Part Data
Assembly Times
(s)
No. Parts:
16
Slowest Part:
9.7
No. Unique Parts:
12
Fastest Part:
2.9
No. Fasteners:
0
Total:
No. Parts:
34
Slowest Part:
10.7
No. Unique Parts:
25
Fastest Part:
2.6
No. Fasteners:
5
Total:
No. Parts:
49
Slowest Part:
14.0
No. Unique Parts:
43
Fastest Part:
3.5
No. Fasteners:
5
Total:
No. Parts:*
56/17
Slowest Part:*
8.0/8.0
No. Unique Parts:*
44/12
Fastest Part:*
0.75/3.0
No. Fasteners:*
0
Total:*
277.0/138.0
*electronic/mechanical
Assembly Cost
at $15/hr
$0.52
125.7
$0.78
186.5
$1.11
266.0
$1.73
ASSEMBLY
Product Life Cycle
SALES
Maturity
Decline
Growth
Introduction
to market
Development
TIME
log UNIT COST
UNIT COST
Cost Experience Curves
No. CUMULATIVE UNITS PRODUCED
log No. UNITS PRODUCED
log £
log £
Pricing
log TIME
Umbrella Pricing
log TIME
Price pegged to manufacturing cost
The Design Core
Market
Assessment
Specification
MANUFACTURE
Concept
Design
Detail
Design
Manufacture
Sell
The Design Core
Market
Assessment
Specification
SELL
Concept
Design
Detail
Design
Manufacture
Sell