1

1

Assembly and Joining

2

Manufacture

Unit Manufacturing

Processes

Unit Manufacturing

ProcessesAssembly and

JoiningAssembly and

Joining

MarketResearch

Factory, Systems & Enterprise

•Welding•Bolting•Bonding•Soldering

•Machining•Injection molding•Rapid prototyping•Stamping•Chemical vapor deposition

ConceptualDesign

Design forManufacture

3

Impact of Design on Product Cost

The majority of cost commitment is made early in the product design cycle.

Cost inPercent

ConceptionValidation

DevelopmentProduction

Operation

TimeSource: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

5

“The best design is thesimplest 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

9

Rough assembly process diagram

Material / parts

Feeding OrientationStoragePresent

&fixture

Man

ipul

atio

nTr

ansp

ort

Join

ing

Curingtime

Posttreatment

Joiningprocess

Present&

fixture

Insp

ectio

n

Material / parts

10

Manual vs Automatic AssemblyManual assembly

• Dexterity• Time (small run, urgent)• Complexity

Automatic assembly• Precision• Load• Time

What is important• Minimize parts• Ergonomics• Cost• Training• Reliability

11

PART TRANSPORT, ORIENATION AND

FEEDING

12

Part handling and feeding

Moving parts between processing stationsControl amount deliveredControl spacing and frequency of deliveryRobust with respect to tangling and jamming

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

WorkpiecePallet

Safeguards against collision

5

17

Part feeding and orientation

18

Part feeding and orientation

19

Part feeding and orientation

20

Part feeding and orientation

6

21

Guidelines to Good Design-for-Assembly

Minimize the number of partsDesign assembly process in a layered fashionConsider ease of part handlingUtilize optimum attachment methodsConsider ease of alignment and insertionAvoid design features that require adjustments

22

DFA - Example

0.10.20.30.40.50.60.70.80.91.0

024 8 4 2

Annual production volumes :robotic - 200,000automatic - 2,400,000

Ass

embl

y co

st ($

)

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

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 2000

2

4

6

8 box-end wrench

open-end wrench

Socket ratchet wrenchNut driver

Clearance, mm

Run

dow

n tim

e / r

ev, s

Clearance

Clearance

Hex-nut driver

Socket ratchet wrench

Box-end wrench

Open-end wrench

DFA - Example

8

29

Utilize optimum attachment methodsDFA - Example

30

Utilize optimum attachment methodsDFA - Example

31

Consider ease of alignment and insertion

Design parts to maintain location

DFA - Example

32

Consider ease of alignment and insertion

Mating parts should be easy to align and insert

DFA - Example

9

33

Consider ease of alignment and insertionDFA - Example

34

Avoid design features that require adjustmentsDFA - Example

35

Avoid restricted vision

DFA - Example

36

Avoid obstructed accessDFA - Example

10

37

Joining

Assembly cost often account for more than 50%Joining processes

High scrap costFailure spot

38

Joining processes/types

39

Selection Criteria

GeometryMaterial typeValue of end productSize of product runAvailability of joining method

40

MECHANICAL JOINING

11

41

Mechanical Fastening

Any shape and materialSemi-permanentLeast expensive for low volume (standardized)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 AveragingNon-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

45

Press fits

( ) ( )( ) ⎥

⎦

⎤⎢⎣

⎡−⋅⋅−⋅−

⋅⋅=io

io

rrRrRRr

REp 222

2222

2δ

ro

R

ri

insertinsert xRpF ⋅⋅⋅⋅⋅= πµ 2

TRthermal ∆⋅⋅=αδ

δ = radial interference α = Coefficient of thermal expansionE = Young’s modulus ∆T = Temperature changep = 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

Pres

s Fo

rce,

lbf

46

ADHESIVEJOINING

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

49

Adhesive BondingQuick, non-invasive, inexpensiveLow stressVariety of materialsSealLow part countTemperature-limitedLong set timeSurface preparationReliability/disassemblyBest for components with high surface to volume ratio

50

Stefan Equation

u

y

r

a

h/2h/2

F

⎥⎥⎦

⎤

⎢⎢⎣

⎡−= 22

4 114

3

if hhaFt µπ

51

What does Stephan equation mean?

⎟⎟⎠

⎞⎜⎜⎝

⎛⋅

⋅⋅⋅≈⋅ 2

4 14

3

finalfinal h

atF πµ

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

14

53

BRAZING&

SOLDERING

54

Example – Brazing / Soldering

55

Soldering/Brazing

CapillaryShear strengthTemperature limit

• Soldering : < 450 degree : Lead, Tin• Brazing : > 450 degree: Silver, Brass, Bronze

Substrate phase stays solid : can be used on dissimilar metalsUsed 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

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 solidifies58

DFA for Soldering and BrazingProvide large surface area contactFiller metal should have good wetting characteristics (capillary action)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

61

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

FUSIONWELDINGPHYSICS

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

65

Welding

Heat sourceHeat intensity

• Control (overmelting/evaporation)• Heat affected zone (HAZ)• Efficiency• Depth/width ratio

66

Heat IntensityA measure of radiation intensity, W/cm2

Obviously the more intense the source, the faster the meltingVery difficult to prevent overmelting, therefore automation!

102 103 104 105 106 107

Air/FuelGas Flame

ElectroslagOxyacetylene

ThermitFriction

ArcWelding

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 Tmeltg/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 34768

Weld pool from 2D simplification

tJs

ck

hTTcJ

a

p

fs

initialmeltpa

α

ρα

2

)(

=

=

−=

: as moves front melt The

. by given is ydiffusivit thermal The

. number, Jacob The

s

18

69

Welding Rate

The rate at which the welding device must be moved is governed by:

The Heat Intensity - the greater the intensity, the faster the motion must be to keep the weld pool size, smax, constantThe product, α Ja, The greater it is, the faster the melt front moves

70

Welding Rate (cont.)In fact, the time at a spot, tmax is given by s2

max/ 2α Ja. Anymore, and you over-melt!

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

pck⋅

=ρ

αfs

initialmeltp h

TTcJ −⋅=

max2 tJs ⋅⋅⋅= α

maxtdVweld >

72

Welding Rate (cont.)

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.

Clearly the greater the H.I. The faster the metal melts.

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)

Inte

ract

ion

Rat

e (S

econ

ds-1

)

Inte

ract

ion

Tim

e (S

econ

ds)

Copper,Aluminum

Steels,Nickels

Uranium,Ceramics

Hig

her 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)

Hea

t Sou

rce

Spot

Dia

met

er, d

n(c

m)

Max

imum

Wel

d Tr

avel

Vel

ocity

(cm

/s)

)(maxmI

IH

ttdV •=

75

Heat Affected Zone

Region near the weld pool is affected by heat. Microstructure changes.

The size of the heat affected zone is controlled by the thermal diffusivity, α.

s

76

Heat LocalizationWhy does alpha affect the size of the HAZ?

Lower values of alpha, more localized heating, which is better for welding?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

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

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

FUSIONWELDINGDEFECTS

84

Typical defects

Source: American Welding Society

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

89

FUSIONWELDING

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

93

Cost - JoiningMetal arc welding

Low tooling costs, moderate equipment costsHigh direct labor costsEconomical for low production runs

Resistance weldingLow tooling costs, high equipment costsLow direct labor costsFull automation can be easily formed

Soldering / BrazingLow tooling costs, various equipment costs depending on the automation levelLow to moderate direct labor costs

Adhesive bondingLow tooling costs, moderate equipment costsLow direct labor costs

94

Quality - JoiningMetal arc welding

Relatively moderate HAZ existsGood surface finish

Resistance weldingClean, high quality welding with low distortionSmall HAZHigh strength welds are produced by flash welding

Soldering / BrazingVirtually stress and distortion free jointsExcellent surface finish

Adhesive bondingExcellent quality joints with virtually no distortionJoint strength may deteriorate with time and sever environment conditions

95

Rate - JoiningMetal arc welding

Economical for low production runs – manual weldingWell suited to traversing automated and robotic systems

Resistance weldingHigh production rate is possible due to short weld timesEasy full automation

Soldering / BrazingHigh production rates are possible for dip soldering ex>Printed Circuit Boards

Adhesive bondingLow production runs

96

Flexibility - joining

Metal arc weldingGenerally high flexibility but depends on the automation level

Resistance weldingLow flexibility due to high automation level

Soldering / BrazingVarious level of automation is possible

Adhesive bondingVery flexible processCan aid weight minimization in critical applications