Spot welding /certified fixed orthodontic courses by Indian dental academy

INDIAN DENTAL ACADEMY
Development of a controller to control the
strength of spot welds

Leader in continuing dental education
www.indiandentalacademy.com

Project Synopsis


Spot welding is a metal joining process which joins
two or more metal sheets by applying heat and
pressure. The main parameters of a spot welding
process are current, force and time.



Current has been the only control parameter for
weld nugget growth for years while the other
parameters were maintained constant throughout the
welding period. The project looks into introducing
force as an additional welding parameter.


Project Synopsis ( cont…. 1 )


The project will investigate the ability to control weld
growth at real time and maintain consistency in weld
strength by integrating both the current and force
control parameters.



The spot welding machine that will be used in this
project will be converted from the usual pneumatic
based to servo based to achieve a better control of
the welding force. Electrode wear and expulsion are
two phenomenon which will lead to inconsistency in
weld strength.

Project Synopsis ( cont…. 2 )


Both these phenomena were found to occur due to
inability to control force during the welding process.



The controller that will be developed in this project,
which will be integrating both current and force
control, will be used to study these phenomenon.



Finally the developed controller will also be used to
study the ability to produced stronger welds when
welding dissimilar metals and metals of different
thickness.


Spot Welding
(Introduction)


Introduction


Spot welding is a process in which contacting metal surfaces
are joined by the heat obtained from resistance to electric
current flow.



Work-pieces are held together under pressure exerted by
electrodes.




Typically the sheets are in the 0.5-3.0 mm thickness range.
The process uses two shaped copper alloy electrodes to
concentrate welding current into a small "spot" and to
simultaneously clamp the sheets together.



Forcing a large current through the spot will melt the metal and
form the weld.

Introduction ( cont…. 1 )


Attractive feature of spot welding is a lot of energy can be
delivered to the spot in a very short time (ten to one hundred
milliseconds).



Permits the welding to occur without excessive heating to the
rest of the sheet.



Amount of heat (energy) delivered to the spot is determined by
the resistance between the electrodes and the amplitude and
duration of the current.



Amount of energy is chosen to match the sheet's material
properties, its thickness, and type of electrodes.


Introduction ( cont…. 2 )


Applying too little energy won't melt the metal or will make a
poor weld.



Applying too much energy will melt too much metal and make
a hole rather than a weld.



Another attractive feature of spot welding is the energy
delivered to the spot can be controlled to produce reliable
welds.



Much higher thermal conductivity and electrical conductivity
mean that up to three times higher welding currents are
needed.


Spot Welding
(Processing)


Processing


Spot welding involves three stages; the first of which involves
the electrodes applying being brought to the surface of the
metal and applying a slight amount of pressure.



The current from the electrodes is then applied briefly after
which the current is removed but the electrodes remain in place
in order for the material to cool.



Weld times range from 0.01 sec to 0.63 sec depending on the
thickness of the metal, the electrode force and the diameter of
the electrodes themselves.


Spot Welding
(Equipment)


Equipment


Equipment


The equipment used in the spot welding process consists of
tool holders and electrodes.



The tool holders function as a mechanism to hold the
electrodes firmly in place and also support water hoses which
cool the electrodes during welding.



Tool holding methods include a paddle-type, light duty,
universal, and regular offset. The electrodes generally are
made of a low resistance alloy, usually copper, and are
designed in many different shapes and sizes depending on the
application needed.


Equipment ( cont…. 1 )


The two materials being welded together are known as the
work-pieces and must conduct electricity.



The width of the work-pieces is limited by the throat length of
the welding apparatus and ranges typically from 5 to 50 inches.




Work-piece thickness can range from 0.008in. to 1.25in.
After the current is removed from the work-piece, it is cooled
via the coolant holes in the center of the electrodes.



Both water and a brine solution may be used as coolants in
spot welding mechanisms.


Spot Welding
(Tool Style)


Tool Style


Electrodes used in spot welding can vary greatly with different
applications.




Each tool style has a different purpose.
Radius style electrodes are used for high heat applications,
electrodes with a truncated tip for high pressure, eccentric
electrodes for welding corners, offset eccentric tips for reaching
into corners and small spaces, and finally offset truncated for
reaching into the work-piece itself.


Spot Welding
(Effects)


Effects


The spot welding process tends to harden the material, cause
it to warp, reduce the materials fatigue strength, and may
stretch the material as well as anneal it.



The physical effects of spot welding include internal cracking,
surface cracks and a bad appearance.



While the chemical properties effected include the metals
internal resistance and its corrosive properties.


Spot Welding
( Time Calculation )


Time Calculation


Spot welding setup can be calculated using the following
equation:



Total weld time = (L + W + U) x N



Where L is the load time in seconds, N is the number of welds,
U is the unload time in seconds, and W is the weld time in
seconds.


Spot Welding
( Electrical notes )


Electrical notes


The basic spot welder consists of a power supply, an energy
storage unit (e.g., a capacitor bank), a switch, a welding
transformer, and the welding electrodes. The energy storage
element allows the welder to deliver high instantaneous power
levels. If the power demands are not high, then the energy
storage element isn't needed. The switch causes the stored
energy to be dumped into the welding transformer. The welding
transformer steps down the voltage and steps up the current.


Electrical notes ( cont…. 1 )


An important feature of the transformer is it reduces the current
level that the switch must handle. The welding electrodes are
part of the transformer's secondary circuit. There is also a
control box that manages the switch and may monitor the
welding electrode voltage or current.



The resistance presented to the welder is complicated. There
is the resistance of secondary winding, the cables, and the
welding electrodes. There is also the contact resistance
between the welding electrodes and the work-piece. There is
the resistance of the work-pieces, and the contact resistance
between the work-pieces.




At the beginning of the weld, the contact resistances are
usually high, so most of the initial energy will be dissipated
there. That heat and the clamping force will soften and smooth
out the material at the electrode-material interface and make
better contact (that is, lower the contact resistance).
Consequently, more electrical energy will go into the workpiece and the junction resistance of the two work-pieces. As
electrical energy is delivered to the weld and causes the
temperature to rise, the electrodes and the work-piece are
conducting that heat away. The goal is to apply enough energy
so that a portion of material within the spot melts without having
the entire spot melt.




The perimeter of the spot will conduct away a lot of heat and

keep the perimeter at a lower temperature. The interior of the
spot has less heat conducted away, so it melts first. If the
welding current is applied too long, the entire spot melts, the
material runs out or otherwise fails, and the "weld" becomes a
hole.


The voltage needed for welding depends on the resistance of

the material to be welded, the sheet thickness and desired size
of the nugget. When welding a common combination like 1.0 +
1.0 mm sheet steel, the voltage between the electrodes is only
about 1.5 V at the start of the weld but can fall as low as 1 V at
the end of the weld.




This decrease in voltage results from the reduction in
resistance caused by the work-piece melting. The open circuit
voltage from the transformer is higher than this, typically in the
5-10 V range, but there is a large voltage drop in the electrodes
and secondary side of the transformer when the circuit is
closed.



The resistance of the weld spot changes as it flows and
liquefies. Modern welding equipment can monitor and adjust
the weld in real-time to ensure a consistent weld. The
equipment may seek to control different variables during the
weld, such as current, voltage, power, or energy.




Projection welding is a modification of spot welding. In this
process the weld is localized by means of raised sections, or
projections, on one or both of the work-pieces to be joined. heat
is concentrated at the projections, which permits the welding of
heavier sections or the closer spacing of welds. The projections
can also serve as a means of positioning the work-pieces.
Projection welding is often used to weld studs, nuts, and other
screw machine parts to metal plate. It's also frequently used to
join crossed wires and bars. This is another high-production
process, and multiple projection welds can be arranged by
suitable designing and jigging.




A spot welding machine normally consists of tool and
electrodes, which are mechanisms for making and holding
contact at the weld. Tool holders have two functions: to hold the
electrode firmly in place and to support water hoses that
provide cooling of the electrodes. Weld currents can range from
4000 to 24,000 amps for different types of 1010 mild steel.


Circuit Diagram of Spot Welding
Machine (TC 15/20)


Spot Welding
( Safety Factors )


Safety Factors


Spot welding can be extremely dangerous with the use of large
amounts of current and heat. Always be sure to clamp the
electrodes tightly, protect your eyes from the intense light given
off in the welding process and protect your hands because the
work-piece can get very hot during welding. Take all safety
precautions necessary before, during and after spot welding.


Spot Welding
( Precise Measurement )


Signature Image Processing


Signature Image Processing (SIP) is a technology for analyzing
electrical data collected from welding processes—usually
automated, robotic welding. In developed countries, some form
of welding is used in more than 50% of manufactured products.
Acceptable welding requires fine tuning and exact conditions;
even minute variations in conditions and control can cause a
weld to be unacceptable. Thus, there has been a need for a
robust, reliable, real-time welding fault detection, especially in
safety-critical applications in automotive manufacture. SIP
allows welding faults to be identified in real time, measures the
stability of welding processes and enables welding processes
to be optimized.


( 1 )


The advance was possible only through the advent of more
powerful PCs. The quality monitoring of automatic welding can
save production downtime, reduce the need for product
reworking and recall, and is becoming increasingly important as
car makers increase the number of robots and reduce the
number of human operators in their plants. The technology is
used by eight auto component manufacturers, including leading
companies in Australia and the EU, in which it has transformed
welding practices, significantly improving the industrial process
and the quality of the products.


( 2 )


Resistance Welding
( An example )


Resistance Welding










Resistance welding is the science of joining two or more metal
parts together in a localized area by the application of heat and
pressure.
The heat is produced by the resistance of the material to carry
a high amperage current. The greater the path of resistance is,
the higher the heat intensity.
This heat is controlled via time application and level of current
applied
The pressure is applied to forge the joint and consolidate the
nugget to provide the weld strength.
No extraneous materials such as rods, fluxes, inert gasses,
oxygen, or acetylene are required

Welding Machine


Design philosophy of machinery






Interchangeable top and bottom electrode systems providing
infinitely variable adjustment for all welding configurations,
both spot and projection welding with wide gap adjustment
Rigid main frame of fabricated steel construction, for
minimum on-load deflection
Precision double or triple acting pneumatic cylinders for low
inertia fast follow-up
Efficient secondary loop and welding transformer for high
welding currents provide low mains demand


Standard Machine Ranges




The standard machines have common features across the
range and are manufactured in four different frame sizes.
Within these frame sizes, the options of spot, projection, seam,
and special application are also available.
Dependant upon application, the guards for the machinery are
chosen. Standard units include: gravity drop safety unit,
cantilever pull-down, total enclosure or light guard protection.
Seam welders come in both circumferential and longitudinal
configurations and will be selected dependant on an
application.


Catalog (Spot Welding Machine)











DELAX CAT CATALOG
RESISTANCE WELDING and PHYSICS OF SPOT WELDING
SPOT WELDER FOR CAR
SPOT WELDING ANALYSIS
DEVELOPMENT OF DIGITAL DC-ARC WELDING MACHINE
FUSION WELDING PROCESS
LASER WELDING OPTICAL SYSTEM
THREE SPECIFIC TYPES OF WELDING MODULES
UNDER WATER WELDING
WELDING TERMS GLOSSARY


Literature Survey
 Servo

Mechanism
 Design Consideration
 Technical Aspect


Thank you
Leader in continuing dental education


Spot welding /certified fixed orthodontic courses by Indian dental academy

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