Transcript Fastening Manufacturing Processes

Fastening
Manufacturing
Processes
Outline
Brazing
Filler Metals and Fluxes
Brazing Methods
Soldering
Adhesive Bonding
Adhesives
Design Consideration
Threaded Fasteners
Standard Screw Threads
Thread Classes
Rivets
Brazing
Brazing
A joining process in which filler
metal is melted between the
surfaces to be joined; the base
metal is not melted
Brazing
Filler Metals and Fluxes
Filler metals
Melted between the surfaces to
form the join
Brazing fluxes
Used to dissolve, combine
with, or inhibit formation of
oxides or other by-products; it
is displaced by the filler metal
and removed after the
operation
Filler Metals
Brazing Methods
Torch brazing
A flame is used to heat the joint, then
filler is added
Furnace brazing
The parts and filler are heated in a
furnace; flux may not be needed
Induction brazing
Uses heat from electrical resistance to an
induced current in the work
Resistance brazing
A current is supplied directly to the part to
cause resistance heating
Brazing Methods
Dip brazing
Parts are immersed in a molten salt
or 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 base
metal does not melt
Soldering
Soldering
A filler metal is melted and
distributed between the
surfaces to be joined; the base
metal does not melt, but the
filler metal combines with it to
form a metallurgical bond
Soldering
Soldering a wire: soldering iron (heat source), left,
solder rod, right
Examples of Soldered
Parts
Examples of Soldered
Parts
Soldering Materials
Soldering Fluxes
Soldering fluxes should:
- be molten at soldering
temperatures
- remove oxide films and tarnish
- prevent oxidization
- promote wetting of the
surfaces
- be displaced readily by the
filler metal
- leave residue that is not
corrosive or conductive
Adhesive Bonding
Courtesy of Air Products and Chemicals, Inc
Adhesive Bonding
Types of Adhesives
1. Natural adhesives
starch
animal products
2. Inorganic adhesives
sodium silicate
magnesium oxychloride
3. Synthetic organic adhesives
thermoplastics (soften when heated,
harden when cooled)
thermosetting (harden permanently
when heated)
Adhesive Bonding
Primers
Primers used as metal surface
conditioners perform these
functions:
1. protect corrosion sensitive
surfaces
2. modify the properties of the
bond
3. tacky resins help hold or
position bonded parts
Theories of Adhesion
Chemical bond theory
Adhesion between adhesive
and adherend is related to the
forces holding atoms and
molecules together
The predicted strength of the
attractive force is far higher
than in reality; however, the
theory is valid because
chemically active primers
improve adhesion
Theories of Adhesion
Weak boundary layer theory
Poor adhesion or adhesive
failure is caused by an
inclusion of low-molecular
weight liquid or an
unpolymerized liquid; the liquid
forms a weak boundary layer
that prevents the proper
bonding
Theories of Adhesion
Surface energy theory
A liquid should have a lower
surface tension than the critical
surface tension of a solid in order to
spread on the solid surface
Surface contamination of the solid
could lower the critical surface
tension below the surface tension
of the liquid, causing the bond to be
ineffective
Theories of Adhesion
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
Theories of Adhesion
Polar theory
Polar adhesives bond to polar
adherends and nonpolar
adhesives bond to nonpolar
adherends
Liquids wet and spread more
effectively on solid surfaces
with similar properties
Common Adhesives
1. Epoxies
thermosetting
metal, glass, ceramic, wood
two-component epoxies
resin
curing agent
application time: 8-12 hours
2. Cyanoacrylates
liquid monomers
application time: 2 seconds
Common Adhesives
3. Anaerobics
thermosetting polyester
acrylics
almost any material
application time:6-24 hours; 5
min. with proper additives and
heat
4. Acrylics
thermoplastic
plastics, metals, ceramics,
composites, even with dirty
surfaces
Common Adhesives
5. Urethanes
one-part thermoplastic or
two-part thermosetting
application time: solidifies in
2 min, cures fully in 24
hours
6. Silicones
silicone thermosets
metal, glass, paper,
plastics, rubber
cures slowly
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 as
heated liquids and bond as
they solidify
Application Methods
Brushing
Performed manually; coating is often
uneven
Manual rollers
Similar to paint rollers
Silk screening
Brushing through open areas of a screen
to coat selected areas
Flowing
Using manually operated flow guns; more
consistent control than brushing
Application Methods
Spraying
Using a spray gun for fast application
over large areas
Automatic applicators
Automatic dispensers and nozzles for
medium and high production rates
Roll coating
A roller is partially immersed in the
adhesive and transfers it to the part
Pull-tab sheets
The adhesive sheet comes prepared
covered by a protective layer which is
removed before use
Design Consideration
Material
porosity, hardness, surface
properties
thermal expansion
Assembly
bond area
stresses
Temperature
during use of parts
temperature extremes, rate of
temperature change
Design Consideration
Exposure
to solvents, water, fuels, light,
ultraviolet, acid, weathering
Flexibility/stiffness
Stability
required length of time
load
Appearance
Adhesive application
Cost
Design Consideration
Limitations
1. Proper selection is required
2. Most are unstable above
350°F (180°C)
3. High-strength adhesives are
often brittle
4. Long term durability and life
expectancy are difficult to
predict
5. Surface condition, adhesive
preparation, and curing can
be critical
Limitations
6. Assembly times may be
greater than other methods
7. Joint quality is difficult to
evaluate
8. Some adhesives contain or
produce unfavorable
chemicals
9. Many adhesives deteriorate
under certain conditions
10. Joints cannot be easily
disassembled
Threaded Fasteners
Screw
An externally threaded fastener,
generally assembled into a blind
threaded hole
Bolt
An externally threaded fastener
inserted through holes in parts and
screwed into a nut on the opposite
side
Types of Screw Heads
Types of Screws
Threaded Fastener
Units
U.S. Standard
measured in inches and fractions of
inches: ¼, 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 the
other system, but they will not fit
together properly
Types of Screw
Threads
1. Coarse-thread series
general use where not
subjected to vibration
2. Fine-thread series
automotive and aircraft
3. Extra-fine thread series
thin-walled material
4. Eight-thread series
pipe flanges, cylinder-head
studs
5. Twelve-thread series
not used extensively
Types of Screw
Threads
6.
Sixteen-thread series
where fine threads are required
7.
American Acme thread
transmitting power and motion
8.
9.
10.
11.
Buttress thread
Square thread
29° Worm thread
American standard pipe
thread
pipe joints
Standard Screw
Threads
60°
d
r
p
Where p = thread pitch, d = thread depth,
and r = fillet radius,
d = .54127p
r = .14434p
This applies to U.S. Standard and Metric
bolts, but does not necessarily apply to
screws
Standard Screw
Threads
Standard Screw
Threads
Standard Screw
Threads
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
Thread Classes
ISO system
e – large tolerance
g – small tolerance
H – no allowance
3-9 – tolerance grade
Lower = fine quality; higher =
coarse quality
Grade 6 is roughly equivalent to
U.S. class 2A and B
Rivets
Rivets
Rivet
An unthreaded, headed pin used to
join parts by passing the pin
through holes in the parts and then
forming a second head on the
opposite side
Rivets
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 form
a bond
Screws, bolts, nuts, and rivets
are applied and hold material
together by physical means