mprocesses notes 17

8/11/2019 MProcesses Notes 17

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Fastening

Manufacturing

Processes


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Outline

Brazing

Filler Metals and Fluxes

Brazing Methods

Soldering

Adhesive Bonding

AdhesivesDesign Consideration

Threaded Fasteners

Standard Screw ThreadsThread Classes

Rivets


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Brazing

Brazing

A joining process in which filler

metal is melted between thesurfaces to be joined; the base

metal is not melted


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Brazing


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Filler Metals and Fluxes

Filler metals

Melted between the surfaces to

form the join

Brazing fluxes

Used to dissolve, combinewith, or inhibit formation of

oxides or other by-products; it

is displaced by the filler metaland removed after the

operation


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Filler Metals


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Brazing Methods

Torch brazing

A flame is used to heat the joint, thenfiller is added

Furnace brazing

The parts and filler are heated in afurnace; flux may not be needed

Induction brazing

Uses heat from electrical resistance to aninduced current in the work

Resistance brazing

A current is supplied directly to the part tocause resistance heating


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Brazing Methods

Dip brazing

Parts are immersed in a molten saltor metal bath to heat them

Infrared brazing

Uses heat from a high-intensity

infrared lamp

Braze welding

Applies filler metal to a

conventional weld joint; the basemetal does not melt


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Soldering

Soldering

A filler metal is melted and

distributed between thesurfaces to be joined; the base

metal does not melt, but the

filler metal combines with it toform a metallurgical bond


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Soldering

Soldering a wire: soldering iron (heat source), left,solder rod, right


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Examples of Soldered

Parts


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Examples of Soldered

Parts


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Soldering Materials


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Soldering Fluxes

Soldering fluxes should:

- be molten at soldering

temperatures

- remove oxide films and tarnish

- prevent oxidization

- promote wetting of thesurfaces

- be displaced readily by the

filler metal- leave residue that is not

corrosive or conductive


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Adhesive Bonding

Courtesy of Air Products and Chemicals, Inc
http://www.airproducts.com/pressroom/photogallery/Adhesive.jpg


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Adhesive Bonding

Types of Adhesives

1. Natural adhesives

starch

animal products

2. Inorganic adhesives

sodium silicatemagnesium oxychloride

3. Synthetic organic adhesives

thermoplastics (soften when heated,harden when cooled)

thermosetting (harden permanentlywhen heated)


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Adhesive Bonding


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Primers

Primers used as metal surface

conditioners perform these

functions:

1. protect corrosion sensitive

surfaces2. modify the properties of the

bond

3. tacky resins help hold orposition bonded parts


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Theories of Adhesion

Chemical bond theory

Adhesion between adhesive

and adherend is related to theforces holding atoms andmolecules together

The predicted strength of theattractive force is far higherthan in reality; however, the

theory is valid becausechemically active primersimprove adhesion


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Weak boundary layer theory

Poor adhesion or adhesive

failure is caused by aninclusion of low-molecular

weight liquid or an

unpolymerized liquid; the liquidforms a weak boundary layer

that prevents the proper

bonding


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Surface energy theory

A liquid should have a lower

surface tension than the criticalsurface tension of a solid in order to

spread on the solid surface

Surface contamination of the solidcould lower the critical surface

tension below the surface tension

of the liquid, causing the bond to be

ineffective


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Inherent roughness theory

Once the adhesive has wet and

spread over the adherend surface,the mechanical strength of the resin

coupled to the rough surface is the

basic reason for the strength of the

bond

Roughness would increase the

surface area in which electrostatic

forces could operate


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Polar theory

Polar adhesives bond to polar

adherends and nonpolaradhesives bond to nonpolar

adherends

Liquids wet and spread more

effectively on solid surfaces

with similar properties


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Common Adhesives

1. Epoxies

thermosetting

metal, glass, ceramic, woodtwo-component epoxies

resin

curing agentapplication time: 8-12 hours

2. Cyanoacrylatesliquid monomers

application time: 2 seconds


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Common Adhesives

3. Anaerobics

thermosetting polyesteracrylics

almost any material

application time:6-24 hours; 5min. with proper additives andheat

4. Acrylics

thermoplastic

plastics, metals, ceramics,composites, even with dirtysurfaces


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Common Adhesives

5. Urethanes

one-part thermoplastic or

two-part thermosettingapplication time: solidifies in2 min, cures fully in 24hours

6. Silicones

silicone thermosets

metal, glass, paper,plastics, rubber

cures slowly


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Common Adhesives

7. High-temperature adhesives

epoxy phenolics, modified

silicones or phenolics,polyamides, ceramics

cures slowly

8. Hot melts

thermoplastic resins

not true structural

adhesives; applied asheated liquids and bond asthey solidify


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Application Methods

Brushing

Performed manually; coating is oftenuneven

Manual rollers

Similar to paint rollers

Silk screeningBrushing through open areas of a screento coat selected areas

Flowing

Using manually operated flow guns; moreconsistent control than brushing


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Application Methods

Spraying

Using a spray gun for fast applicationover large areas

Automatic applicators

Automatic dispensers and nozzles formedium and high production rates

Roll coatingA roller is partially immersed in theadhesive and transfers it to the part

Pull-tab sheets

The adhesive sheet comes preparedcovered by a protective layer which isremoved before use


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Design Consideration

Material

porosity, hardness, surface

propertiesthermal expansion

Assembly

bond areastresses

Temperature

during use of parts

temperature extremes, rate oftemperature change


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Exposure

to solvents, water, fuels, light,

ultraviolet, acid, weathering

Flexibility/stiffness

Stability

required length of time

load

Appearance

Adhesive application

Cost


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Limitations

1. Proper selection is required

2. Most are unstable above

350F (180C)

3. High-strength adhesives are

often brittle

4. Long term durability and lifeexpectancy are difficult to

predict

5. Surface condition, adhesivepreparation, and curing can

be critical


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Limitations

6. Assembly times may be

greater than other methods

7. Joint quality is difficult toevaluate

8. Some adhesives contain or

produce unfavorablechemicals

9. Many adhesives deteriorate

under certain conditions10.Joints cannot be easily

disassembled


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Threaded Fasteners

Screw

An externally threaded fastener,generally assembled into a blindthreaded hole

Bolt

An externally threaded fastenerinserted through holes in parts andscrewed into a nut on the oppositeside


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Types of Screw Heads


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Types of Screws


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Threaded Fastener

UnitsU.S. Standard

measured in inches and fractions of

inches: , 5/8,1-1/8 etc.

Metric (SI)

measured in millimeters

A fastener specified to one

measurement system may have

similar sizes to a fastener of theother system, but they will not fit

together properly


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Types of Screw

Threads1. Coarse-thread series

general use where not

subjected to vibration2. Fine-thread series

automotive and aircraft

3. Extra-fine thread series

thin-walled material

4. Eight-thread series

pipe flanges, cylinder-head

studs5. Twelve-thread series

not used extensively


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Types of Screw

Threads6. Sixteen-thread series

where fine threads are required

7. American Acme threadtransmitting power and motion

8. Buttress thread

9. Square thread

10. 29 Worm thread

11. American standard pipe

threadpipe joints


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Standard Screw

Threads

Where p = thread pitch, d = thread depth,and r = fillet radius,

d = .54127p

r = .14434p

This applies to U.S. Standard and Metricbolts, but does not necessarily apply toscrews

r

p

d

60


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Standard Screw

Threads


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Standard Screw

Threads


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Standard Screw

Threads


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Thread Classes

Unified system

Class 1

special applications

Class 2 normal production

grade

Class 3

minimum tolerances,tight fit required

A external thread

B

internal thread


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Rivets


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Rivets

Rivet

An unthreaded, headed pin used to

join parts by passing the pinthrough holes in the parts and then

forming a second head on the

opposite side


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Rivets


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Summary

Brazing and soldering melt only

filler material to form a join

Adhesive bonding uses a viriety

of chemicals that are applied

as a liquid and solidify to forma bond

Screws, bolts, nuts, and rivetsare applied and hold material

together by physical means


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mprocesses notes 17

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