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MIG WELDINGArc welding as used in this unit refers to shielded metal arc welding(SMAW)
An electric welder and flux covered electrodes are used
Temperature of the electric arc is about 9, 000 degrees F
Welding area should be equipped with metal benches
These serve both for fire protection and to electrically ground the work
Fire extinguishers suitable for class A, B and C fires should be available
Safety equipment and a first-aid kit should be within easy reach at all times
Wool fire blanket in another piece of fire control equipment
Water spilled on the floor of the work area must be removed seeing that it will create an electric shock hazard
All grease, oil, paper, rags and other flammable materials must be removed from areas where welding is completed
The number 10 shade lens must always be worn when welding, however is too dark to see through except when the arc is burning
Wear safety glasses under helmetAll skin areas must be covered“Sun-burning” will occur from the rays of
the arcThe method described is called the
“scratch method” of striking an arcA bead is called a “stringer bead” if it is
made without weavingWeaving means moving the electrode
back and forth sideways to create a bead that is wider than normal
Beginners should lean the electrode slightly in the direction of travel
A 75 to 80 degree angle should be usedThe arc length should sound like bacon
fryingThe best method to control movement is
to watch the welded metal solidify behind the puddle
When the speed is correct, evenly spaced semicircles form
The first pass is called a “root pass”The “root pass” is the most important
weld in the processWelds made by moving horizontally
across a vertical piece of metal is called “horizontal welding”
Low amperage results in narrow, stringy beads
Other symptoms are:-electrode covering turns brown-the bead does not have clear marks
-the puddle burns through the plate-the arc is noisy
Welding Joints:When welding one bead or a layer of
beads this process is called a “pass”Welds made by moving downward
across a vertical piece of metal are called “vertical down” welds
Welds made by moving upward across metal are called “vertical up”
Metal Inert Gas (MIG) welding became popular when manufacturers began using thin-gauge, high strength, low alloy steels
In addition, it is possible to weld aluminum castings with MIG
The MIG welding method uses a welding wire that is fed automatically at a constant speed as an electrode
Since the wire is fed automatically at a constant rate, this method is also called semi-automatic arc welding
When the heat is sufficient to cause the metal to melt, the process is called “spot welding”
The device used to spot weld is called a “spot welder”
During the welding process, either inert or active gas shields the weld from the atmosphere and prevents oxidation of the base metal
For most steel welds, carbon dioxide (CO2) is used
With aluminum, either pure argon or a mixture of argon and helium is used
It is also possible to weld stainless steelMIG welding is sometimes called carbon
dioxide arc weldingThe term MIG is used to describe all gas
metal arc welding processes
In fact, many welders in the industry can use CO2 (active gas) or argon (inert gas) by simply changing the gas cylinder and the regulator
MIG welding uses the short circuit arc method which is a unique method of depositing molten drops of metal onto the base metal
The short circuit arc method used very thin welding rods, low current and low voltage
Heating and cooling occurs on a average of 100 times a second
The nozzle works as a constricting force in the direction of the center of the cylinder
This action is known as the “pinch effect” and the size of the force is called the “pinch force”
Before starting to weld, the operator sets:
- wire speed
- arc voltage- gas flow rate- then presses the power button
The wire is positive and the workplace is negative
Weld penetration is greatest using this connection
Weld penetration is also greatest using CO2 gas, however CO2 provides a harsher more unstable arc
In MIG welding, the polarity of the power source is important in determining the penetration to the work piece
Direct Current (DC) power sources used for MIG welding typically use DC reverse polarity which leads to increased spatter
It is preferable to use argon/CO2
PLASMA ARC CUTTING
Plasma- fluid portion of blood found in region of electrical charge-discharge (arc)
Ionized Gas- plasma created by an arcBoth electrons and positive ions whose charges are nearly equal
to each otherPlasma results when a gas is heated to a
high enough temperature to convert into positive and negative ions-11, 000 to 23,000 degrees F
Arc Plasma- defined as a gas that has been heated to at least a partially ionized condition
Plasma arc- used to refer to the arc plasma used in welding
and cutting processes
* Produces both high temperature and intense light
* Arc welding like cutting
Plasma torch- creation & control of the plasma for welding or
cuttingTorch body- special plastic-resistant to
high temperatures-ultraviolet light and impact
*available in variety of lengths and sizes
Generally the longer, larger torches are used for the higher capacity machines
Body is called a “barrel”Torch Head- angles 75-90-180
cooling for low power torches may either be by air or water
Power Switch- variety of optionsElectrode Setback- spacing between
electrode tip and nozzle tip critical for proper operation
Nozzle tip- small, cone shaped, constricting orifice in the center
Space between the electrode tip and nozzle tip is where the electric current forms the plasma
Nozzle- referred to as the “cup”* made of ceramic* high temperature resistant substance
Common Torch Parts:-electric tip -nozzle insulator-nozzle tip -nozzle guide-nozzle
The torch parts are made of copperElectrode tip- made of copper with
tungsten tip-keeping tip cools, lengthens the life of the tip and allows better quality cuts for longer period of time
Nozzle insulator- between electrode tip and nozzle tip
Gas Hose- 2 types of hoses-1 hose carries gas used to produce plasma and one produces a shielding gas coverage
Central wire- 2 conductors-low voltage stranded copper wire connects power switch to power supply
Water tubing- medium/high amperage torches may use water-failure to use de-ionized water will result in the torch arching out internally
Water Shroud- water surrounding the tip is used to control the hazards of light, fumes and noise
Power and Gas cables- power cable-high voltage rated insulation made of finely stranded copper wire to allow for maximum flexibilityPower Conductors- positive & negative-size and current carrying capacity of this cable is a controlling factor to the power range of the torch
Voltage- production of plasma requires a drooping arc voltage or constant current
direct current high voltage power supply-this allows for a rapid start of the plasma arc at the high open voltage
Amperage- flow is much lower than with most welding processes-PAC torches operate on 10 amps-the higher the amperage capacity the faster & thicker they will cut
Stack Cutting- sheets of steel
Dross- metal compound that re-solidifies & attaches itself to the bottom of a cut-made up of un-oxidized metal
Water table- used to reduce the noise level, control the light, trap sparks and eliminate fumes
Plasma arc gouging- similar to air carbon arc gouging
Safety Hazards: light-fumes-sparks-electrical shock-moisture-noise
Heat Input- Jouls-per inch of heat the cut will receive-work per linear foot
Distortion- metal is heated and moves (bends) up towards the heat, while the metal is cooled moves
( bends) away from the heated areaHeat Affected Zone- area heated for
workPAC- Plasma Arc CuttingCutting Speed- high-25 feet per
minute-1/4 mile per hour
Standoff distance- distance from the nozzle tip to the work-distance is crucial for quality cuts
Starting Methods- most common type is high frequency, alternating current
Pilot air- arc between the electrode tip & the nozzle tip within the torch head
Kerf- space left in the metal as the metal is removed during a cut
Factors for Success:-standoff distance -gas used-orifice diameter -travel speed-power setting -water injection-swirling of gas -electrode tip
Factors that affect the cut by changing the gas:
-force -heat content-central concentration
-kerf width -dross formation-top edge rounding -metal type
HAPPY WELDING & CUTTING