Assembly and Joining

1. 1
1
Assembly and Joining
2
Manufacture
Unit
Manufacturing
Processes
Unit
Manufacturing
Processes
Assembly and
Joining
Assembly and
Joining
Market
Research
Factory,
Systems &
Enterprise
•Welding
•Bolting
•Bonding
•Soldering
•Machining
•Injection molding
•Rapid prototyping
•Stamping
•Chemical vapor
deposition
Conceptual
Design
Design for
Manufacture
3
Impact of Design on Product Cost
The majority of cost commitment is made
early in the product design cycle.
Cost in
Percent
Conception
Validation
Development
Production
Operation
Time
Source:British Aerospace
100
0
10
20
30
40
50
60
70
80
90
85%
95%
5%
20%
Committed
Spent
4
Cost of Design Changes
CONCEPT
DESIGN
X
10X
TOOLING
TESTING
100X
1000X
RELEASE
10,000X

2. 2
5
“The best design is the
simplest one that works”
Albert Einstein
6
Assembly and Joining
• Assembly as a mfg process
– Process components
– Cost, Quality, Rate and Flexibility
• Specific joining processes
– Mechanical
– Adhesives
– Solder and brazing
– Cold welding
– Fusion welding
7
Assembly statistics
8
Justifying the need/type of
assembly
•For assembly
– Can’t be made in
one piece
– Functional
(bushings, bearings)
– Manufacturability
– Repair and
maintenance
– Transport: Does it fit
in a plane?
•Against assembly
– Defects that occur at
interfaces
– Loss of stiffness
– Time & effort
– Labor costs
– Size

3. 3
9
Rough assembly process diagram
Material / parts
Feeding Orientation
Storage
Present
&
fixture
Manipulation
Transport
Joining
Curing
time
Post
treatment
Joining
process
Present
&
fixture
Inspection
Material / parts
10
Manual vs Automatic Assembly
ƒManual assembly
• Dexterity
• Time (small run, urgent)
• Complexity
ƒAutomatic assembly
• Precision
• Load
• Time
ƒWhat is important
• Minimize parts
• Ergonomics
• Cost
• Training
• Reliability
11
PART TRANSPORT,
ORIENATION AND
FEEDING
12
Part handling and feeding
ƒ Moving parts between processing stations
ƒ Control amount delivered
ƒ Control spacing and frequency of delivery
ƒ Robust with respect to tangling and
jamming

4. 4
13
Part transport: Overhead
http://www.tegopi.pt/pt/novidades/images/25-10-2000.jpg http://accosystems.com/autoclient%20(2).jpg
14
Part transport: Conveyor
http://www.ssiconveyors.com/pics/ http://www.ssiconveyors.com/pics/
15
Part transport: Transfer line
16
AGV
Pallet load /
unload
mechanism
Workpiece
Pallet
Safeguard
s against
collision

5. 5
17
Part feeding and orientation
18
Part feeding and orientation
19
Part feeding and orientation
20
Part feeding and orientation

6. 6
21
Guidelines to
Good Design-for-Assembly
ƒ Minimize the number of parts
ƒ Design assembly process in a layered fashion
ƒ Consider ease of part handling
ƒ Utilize optimum attachment methods
ƒ Consider ease of alignment and insertion
ƒ Avoid design features that require
adjustments
22
DFA - Example
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
24 8 4 2
Annual production volumes :
robotic - 200,000
automatic - 2,400,000
Assembly
cost
($)
Number of parts
Minimize the number of parts
23
Minimize the number of parts
DFA - Example
24
Design assembly process in a layered fashion
DFA - Example

7. 7
25
Consider ease of part handling
DFA - Example
26
Consider ease of part handling
DFA - Example
27
Utilize optimum attachment methods
DFA - Example
28
Utilize optimum attachment methods
10 20 30 40 50 100 200
0
2
4
6
8
box-end wrench
open-end
wrench
Socket ratchet wrench
Nut driver
Clearance, mm
Run
down
time
/
rev,
s
Clearance
Clearance
Hex-nut driver
Socket ratchet wrench
Box-end wrench
Open-end wrench
DFA - Example

8. 8
29
Utilize optimum attachment methods
DFA - Example
30
Utilize optimum attachment methods
DFA - Example
31
Consider ease of alignment and insertion
ƒ Design parts to maintain location
DFA - Example
32
Consider ease of alignment and insertion
ƒ Mating parts should be easy to align and insert
DFA - Example

9. 9
33
Consider ease of alignment and insertion
DFA - Example
34
Avoid design features that require adjustments
DFA - Example
35
Avoid restricted vision
DFA - Example
36
Avoid obstructed access
DFA - Example

10. 10
37
Joining
ƒ Assembly cost often account for
more than 50%
ƒ Joining processes
ƒ High scrap cost
ƒ Failure spot
38
Joining processes/types
39
Selection Criteria
ƒ Geometry
ƒ Material type
ƒ Value of end product
ƒ Size of product run
ƒ Availability of joining method
40
MECHANICAL
JOINING

11. 11
41
Mechanical Fastening
ƒ Any shape and material
ƒ Semi-permanent
ƒ Least expensive for low volume
(standardized)
ƒ Problems: strength, seal, extra part,
assembly, loosen
42
Fasteners
• Bolts/screws
– Good for disassembly and reassembly
– Cross threading can be a problem
• Rivets
– Used when disassembly is not required
• Dowel pins
• Cotter pins
• Snap fits
43
Elastic Averaging
Non-Deterministic
Pinned Joints
No Unique Position
Exact constraint
Kinematic Constraint
Common alignment mechanisms
Contaminants: Sensitive
Load capacity: High
Wear in: Long
Exact constraint: No
Cost: $$$
Complexity: Simple
Common: Yes
Repeatability ~ 5 micron
after wear in
Contaminants: Insensitive
Load capacity: Low - High
Wear in: No
Exact constraint: No
Cost: $ <-> $$$
Complexity: Simple
Common: Yes
Repeatability ~ 25 micron
at best
Contaminants: Insensitive
Load capacity: Low - High
Wear in: Short
Exact constraint: Yes
Cost: $$
Complexity: Moderate
Common: No
Repeatability ~ ¼ micron
is common
Relative cost:
$ = low cost
$$ = moderate cost
$$$ = expensive
44
Embossing and hemming
• Embossed protrusions
– Plastic deformation / interference
– Appliances
– Automotive
– Furniture
• Hemming
– Bend edge of one component over another
– Automobile doors

12. 12
45
Press fits
( ) ( )
( ) ⎥
⎦
⎤
⎢
⎣
⎡
−
⋅
⋅
−
⋅
−
⋅
⋅
=
i
o
i
o
r
r
R
r
R
R
r
R
E
p 2
2
2
2
2
2
2
2
δ
ro
R
ri
insert
insert x
R
p
F ⋅
⋅
⋅
⋅
⋅
= π
µ 2
T
R
thermal ∆
⋅
⋅
=α
δ
δ = radial interference α = Coefficient of thermal expansion
E = Young’s modulus ∆T = Temperature change
p = interface pressure
Insertion force for press fit
0
500
1000
1500
2000
2500
3000
0.00 0.05 0.10 0.15 0.20 0.25
Press Depth, in
Press
Force,
lbf
46
ADHESIVE
JOINING
47
Example – Adhesive Bonding
48
Adhesive joining
• Advantages
– Different materials, Easily automated, Damp /seal
• Limitations
– Time, Preparation/curing, Loading
• Important properties/characteristics of
adhesives
– Strength, Chemically inert, Compatibility

13. 13
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Adhesive Bonding
ƒ Quick, non-invasive, inexpensive
ƒ Low stress
ƒ Variety of materials
ƒ Seal
ƒ Low part count
ƒ Temperature-limited
ƒ Long set time
ƒ Surface preparation
ƒ Reliability/disassembly
ƒ Best for components with high
surface to volume ratio
50
Stefan Equation
u
y
r
a
h/2
h/2
F
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
−
= 2
2
4
1
1
4
3
i
f h
h
a
Ft
µπ
51
What does Stephan equation
mean?
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅
⋅
⋅
⋅
≈
⋅ 2
4
1
4
3
final
final
h
a
t
F
π
µ
Viscosity: Start with low µ & finish with high µ
Gap: Small assembled gap
52
DFM with adhesives
• What is important
– Matching type of loading to the design
(shear/tension vs. peeling)
– Maximizing contact area
Poor
Good
Poor
Good
Poor
Good

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53
BRAZING
&
SOLDERING
54
Example – Brazing / Soldering
55
Soldering/Brazing
ƒ Capillary
ƒ Shear strength
ƒ Temperature limit
• Soldering : < 450 degree : Lead, Tin
• Brazing : > 450 degree: Silver, Brass, Bronze
ƒ Substrate phase stays solid : can be
used on dissimilar metals
ƒ Used in electronics, plumbing, jewelry,
and recently, as a structural joining
56
Soldering & brazing
• Brazing: T > 450o C
– Brazes: Silver, Brass, Bronze
• Soldering: T < 450o C
– Low temps require fluxes
– Solders: Lead, Tin, Bismuth
• Processes
– Torch: Oxyacetylene torch and filler metal
– Furnace: Lay filler around joint and send into furnace
– Induction: Heat part by induction
– Resistance: Heat part by contact/current

15. 15
57
Anatomy of solder/brazing
process
•Purpose of fluxes
– Remove oxides
– Protect from oxidization
– Improve surface wetting
•Process
– A. Flux on top of oxidized metal
– B. Boiling flux removes oxide
– C. Base metal in contact with molten flux
– D. Molten solder displaces molten flux
– E. Solder adheres to base metal
– F. Solder solidifies
58
DFA for Soldering and Brazing
ƒ Provide large surface area contact
ƒ Filler metal should have good wetting characteristics
(capillary action)
ƒ Joined surfaces should not be smooth
59
COLD WELDING
(Interatomic)
PROCESSES
60
Ultrasonic and friction welding
• Ultrasonic welding (shear @ 10 to 75kHz vibration)
– Contamination is redistributed, not displaced
– Example: Wires/connectors in electronics components
• Friction welding
– Material surfaces sheared against each other
– Some metal extrudes out of weld zone, removes
contaminates

16. 16
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Diffusion bonding
• Diffusion bonding is often divided into three stages
– Asperities + pressure = decreased interface porosity
– Continued heating causes the porosities to shrink
– Crystals grow across interface, some porosity may be trapped
• Time: Important variables/parameters:
– Pressure = (500 to 5,000 psi) Temperature (2/3 Tmelt)
– Surface finish Surface cleanliness
62
FUSION
WELDING
PHYSICS
63
Example – Spot Welding
64
• Process of fusing two materials to join them
• Challenges in fusion welding
– Disrupting oxide layers and contamination
– Achieving high % surface area contact
Fusion welding

17. 17
65
Welding
ƒ Heat source
ƒ Heat intensity
• Control (overmelting/evaporation)
• Heat affected zone (HAZ)
• Efficiency
• Depth/width ratio
66
Heat Intensity
ƒ A measure of radiation intensity, W/cm2
ƒ Obviously the more intense the source, the
faster the melting
ƒ Very difficult to prevent overmelting,
therefore automation!
102 103 104 105 106 107
Air/Fuel
Gas Flame
Electroslag
Oxyacetylene
Thermit
Friction
Arc
Welding
Resistance Welding
(Oxygen Cutting)
Electron Beam,
Laser Beam
67
Heat intensity and interaction time
• Heat intensity (in W/cm2)
– HI = Power per unit area directed into the welding zone
– HI ~103 melting in < 25 seconds
– HI ~106 vaporizes metal in µseconds
• Propagation of heat in solids: x ~ (α·t)0.5
– x = distance thermal disturbance travels into thick slab
– t = elapsed time
ρ k cp α Tmelt
g/cm3 W/m/K J/g/oC cm^2/s oC oF
Aluminum 2.7 200 0.890 0.832 660 1220
Copper 8.9 400 0.385 1.167 1085 1985
1020 steel 7.9 50 0.448 0.141 1500 2732
Delrin 1.4 0.36 1.464 0.002 175 347
68
Weld pool from 2D simplification
t
J
s
c
k
h
T
T
c
J
a
p
fs
initial
melt
p
a
α
ρ
α
2
)
(
=
=
−
=
:
as
moves
front
melt
The
.
by
given
is
y
diffusivit
thermal
The
.
number,
Jacob
The
s

18. 18
69
Welding Rate
The rate at which the welding device must be
moved is governed by:
ƒ The Heat Intensity - the greater the intensity, the
faster the motion must be to keep the weld pool size,
smax, constant
ƒ The product, α Ja, The greater it is, the faster the melt
front moves
70
Welding Rate (cont.)
ƒ In fact, the time at a spot, tmax is given by s2
max/ 2α Ja.
Anymore, and you over-melt!
ƒ If the weld pool size is d in length, then you must feed
at a rate that exceeds d/tmax. Similarly a lower limit.
Weld pool size d
Vmin = d / tmax
71
• Definitions of Jacob # and thermal diffusivity:
• Solidification front moves in “s” direction as:
• To maintain constant weld pool depth at s
Melting front and welding velocity
p
c
k
⋅
=
ρ
α
fs
initial
melt
p
h
T
T
c
J
−
⋅
=
max
2 t
J
s ⋅
⋅
⋅
= α
max
t
d
Vweld >
72
Welding Rate (cont.)
ƒ For a planar heat source on steel,
tm= (5000/H.I.)2
: where H.I. Is the heat intensity in W/cm2. The
number 5000 includes the material constant
related terms.
ƒ Clearly the greater the H.I. The faster the
metal melts.

19. 19
73
Weld Pool – Heat Source
Interaction Time
10-3
10-2
10-1
100
101
102
103
104
105
106
107
108
103 104 105 106 107 108
103
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
Heat Intensity (Watts/cm2)
Interaction
Rate
(Seconds
-1
)
Interaction
Time
(Seconds)
Copper,
Aluminum
Steels,
Nickels
Uranium,
Ceramics
Higher
α,
Ja
Main factor : Thermal diffusivity 74
Weld Travel Velocity
102
101
100
10-1
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
103 104 105 106 107 108
109
108
107
106
105
104
103
102
101
100
10-1
10-2
Heat Intensity (Watts/cm2)
Heat
Source
Spot
Diameter,
dn
(cm)
Maximum
Weld
Travel
Velocity
(cm/s)
)
(
max
m
I
I
H
t
t
d
V •
=
75
Heat Affected Zone
ƒ Region near the weld pool is affected by heat.
Microstructure changes.
ƒ The size of the heat affected zone is
controlled by the thermal diffusivity, α.
s
76
Heat Localization
ƒ Why does alpha affect the size of the HAZ?
ƒ Lower values of alpha, more localized heating, which
is better for welding?
ƒ Have you tried taking a soldering iron to plastic? You
get small weld pool. But on metal, you heat the
whole piece!
Heat applied
Temperature
Increasing α or decreasing α?

20. 20
77
FUSION
WELDING
PROCESSES
78
Oxyfuel gas welding
• Air/fuel welding
– Low-cost, portable and flexible
– Oxidizing and reducing flames
79
General arc welding
• Voltage difference between electrode & work piece
– Voltage ~ 100 - 500 V
– Current ~ 50 – 300 A
• Electrode may be consumed
80
Fillers and shielding in arc
welding
• Filler metals
– Adds metal to weld zone
– Flux on/in filler cleans/prevents oxidization
– Slag protects molten puddle from oxidation
• Shielding
– Pellets or grains
– Gaseous

21. 21
81
General resistance welding
• High current through weld area (3k – 40k
Amps)
– Important: contact pressure & weld current & weld time
– Simple, reliable fusion processes
– Welds often not easy to inspect ( i.e. spot welds )
– Energy in: i2 R t
– Energy req’d: ?
– Melting requires ~ 1400 J / g for steel
82
Electron beam and laser
welding
• Electron beam
– Uses electrons to transfer energy
• Laser welding
– Uses photons to transfer energy
• Pros and cons
– Advantages: Small HAZ
– Disadvantages: $$$!, deep welds require careful fixturing
83
FUSION
WELDING
DEFECTS
84
Typical defects
Source: American Welding Society

22. 22
85
Porosity and inclusions
• Common causes
– Trapped gases
– Contaminants
– Flux particles
• Prevention
– Pre-heat
– Pre-cleaning
– Improved shielding
86
Cracks in welds
• Usually result from one or combo of:
– Temperature gradients and inability of metal to
contract during cooling
• Prevention
– Preheat components or design to minimize stress
from thermal shrinking
87
Residual stresses
• Residual stresses cause problems
– Distortion on welding
– Reduce fatigue life
• Prevention
– Preheating
– Stress relieving in furnace (post heating)
A B 88
Heat affected zone (HAZ)
• Material is not melted, but is heated
• Properties in HAZ differ and depend on:
– Temperature
– Time
– Original properties
– HI

23. 23
89
FUSION
WELDING
DFM
90
DFM for Welding
One-half the
weld metal
same strength Twice the
weld metal -
same strength
Previous Design
21 Pieces Improved
3 Pieces
Design simplicity can save much
welding and assembly time
91
DFM for Welding (conti.)
Electrode must be held close to
45° when making these fillers
Easy to draw, but the 2nd weld
will be hard to make
Very difficult
Easy
Easy to specify
“weld all
around” but …..
Try to avoid placing pipe joints near
wall so that one or two sides are
inaccesible. These welds must be
made with bent electrodes and mirror
Too close to side to allow
proper electrode positioning.
May be OK for average work but
bad for leakproof welding
Pipe
Wall
92
• Cracks open in tension
• Most welds are hard, not easily machined
• Part preparation
DFM for welding

24. 24
93
Cost - Joining
ƒ Metal arc welding
ƒ Low tooling costs, moderate equipment costs
ƒ High direct labor costs
ƒ Economical for low production runs
ƒ Resistance welding
ƒ Low tooling costs, high equipment costs
ƒ Low direct labor costs
ƒ Full automation can be easily formed
ƒ Soldering / Brazing
ƒ Low tooling costs, various equipment costs depending
on the automation level
ƒ Low to moderate direct labor costs
ƒ Adhesive bonding
ƒ Low tooling costs, moderate equipment costs
ƒ Low direct labor costs
94
Quality - Joining
ƒ Metal arc welding
ƒ Relatively moderate HAZ exists
ƒ Good surface finish
ƒ Resistance welding
ƒ Clean, high quality welding with low distortion
ƒ Small HAZ
ƒ High strength welds are produced by flash welding
ƒ Soldering / Brazing
ƒ Virtually stress and distortion free joints
ƒ Excellent surface finish
ƒ Adhesive bonding
ƒ Excellent quality joints with virtually no distortion
ƒ Joint strength may deteriorate with time and sever
environment conditions
95
Rate - Joining
ƒ Metal arc welding
ƒ Economical for low production runs – manual welding
ƒ Well suited to traversing automated and robotic systems
ƒ Resistance welding
ƒ High production rate is possible due to short weld times
ƒ Easy full automation
ƒ Soldering / Brazing
ƒ High production rates are possible for dip soldering
ex>Printed Circuit Boards
ƒ Adhesive bonding
ƒ Low production runs
96
Flexibility - joining
ƒ Metal arc welding
ƒ Generally high flexibility but depends on the
automation level
ƒ Resistance welding
ƒ Low flexibility due to high automation level
ƒ Soldering / Brazing
ƒ Various level of automation is possible
ƒ Adhesive bonding
ƒ Very flexible process
ƒ Can aid weight minimization in critical applications