Transcript Chapter 3

Chapter 3
Shielded Metal Arc Welding,
Setup, and Operation
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Objectives
• Describe the process of shielded metal arc
welding (SMAW)
• List and define the three units used to measure a
welding current
• Tell how adding chemicals to the coverings of the
electrodes affects the arc
• Discuss the three different types of current used
for welding
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Objectives (cont'd.)
• Explain the types of welding power supplies and
which type the shielded metal arc welding process
requires
• Define open circuit voltage and operating voltage
• Explain arc blow, what causes it, and how to
control it
• Tell what the purpose of a welding transformer is
and what kind of change occurs to the voltage and
amperage with a step-down transformer
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Objectives (cont'd.)
• Compare generators and alternators
• Tell the purpose of a rectifier
• Read a welding machine duty cycle chart and
explain its significance
• Demonstrate how to determine the proper welding
cable size
• Demonstrate how to service and repair electrode
holders
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Objectives (cont'd.)
• Discuss the problems that can occur as a result of
poor work lead clamping
• Describe the factors that should be considered
when placing an arc welding machine in a welding
area
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Introduction
• Shielded metal arc welding (SMAW)
– A flux-covered metal electrode carries current
• SMAW is a widely used welding process
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Low cost
Flexible
Portable
Versatile
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Welding Current and Electrical
Measurement
• Electric current
– Source of heat
– Flow of electrons
• Units used to describe electrical current
– Voltage: measurement of electrical pressure
– Amperage: total number of electrons flowing
– Wattage: measurement of electrical energy or
power in the arc
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FIGURE 3-2 Electrons traveling along a conductor.
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FIGURE 3-3 Ohm’s law.
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SMA Welding Arc Temperature and Heat
• Temperature
– Degree or level of energy in a material
– Measured in degrees with a thermometer
• Heat
– Quantity of energy in a material
– Cannot easily be measured
– Determined by knowing temperature and object
mass
• Temperature of a welding arc
– Dependent on voltage, arc length, and atmosphere
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Types of Welding Currents
• Three types of current used for arc welding
– Alternating Current (AC)
– Direct-current electrode negative (DCEN)
– Direct-current electrode positive (DCEP)
• Some electrodes can used with only one type of
current
– Others can be used with two or more
• Each current has a different effect on the weld
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Types of Welding Power
• Welding power can be supplied as:
– Constant voltage (CV)
• Arc voltage remains constant
– Rising arc voltage (RAV)
• Arc voltage increases as amperage increases
– Constant Current (CC)
• Total welding current remains the same
• Shielded metal arc welding
– Requires a constant current arc voltage
characteristic
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Open Circuit Voltage
• Voltage at the electrode before striking an arc
– Usually between 50 V and 80 V
• Higher open circuit voltage
– Easier to strike an arc
– Maximum safe open circuit voltage for welders is 80
volts
• High voltage increases chance of electrical shock
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Operating Voltage
• Voltage at the arc during welding
– Also called welding or closed circuit voltage
• Will vary with:
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Arc length
Type of electrode
Type of current
Polarity
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Arc Blow
• Electrons flow
– Create lines of magnetic force that circle around the
path of flow called magnetic flux lines
• These lines space themselves evenly along a
current-carrying wire
• Arc blow
– Movement of the arc
– Makes arc drift like a string would drift in the wind
– More of a problem when magnetic fields are the
most uneven
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Figure 3-13 Magnetic forces concentrate around bends in wires.
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Types of Power Sources
• Electrical devices used
– Electric motors or internal combustion engines
– Step-down transformers
• Welding transformers
– Use high-voltage AC to produce low-voltage
welding power
• Step-down transformers
– Takes high voltage, low amperage current and
turns it into low voltage, high amperage current
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Figure 3-17 Diagram of a step-down transformer.
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Types of Power Sources (cont'd.)
• Multiple-coil machine
– Allows the selection of different current setting
• Movable coil or core
– Has high and low current
– Handwheel moves internal parts
– Closer coils: greater current
• Inverter welding machines
– Smaller, but with same amperage range
– Power changed to thousands of cycles per second
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Generator and Alternator Type Welders
• Both produce welding electricity
– Alternator: magnetic lines of force rotate inside a
coil or wire
• Produces AC only
– Generators: welding current is produced on the
armature and is picked up with brushes
• Produces DC
– Portable engine-driven welders
• Require more maintenance
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Figure 3-27 Schematic diagram of an alternator.
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Figure 3-28 Diagram of a generator.
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Converting AC to DC
• Alternating current can
be converted to direct
current by using a series
of rectifiers
– Current flows in one
direction only
• Rectifiers become hot as
they change AC to DC
– Heat reduces power
efficiency
FIGURE 3-33 Typical dial on an AC-DC
transformer rectifier welder.
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Duty Cycle
• Welding machines
– Produce internal heat as they produce the welding
current
• Duty cycle
– Percentage of time a welding machine can be used
continuously
• 60% duty cycle: machine can operate six minutes out
of every ten at maximum rated current
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Figure 3-34 Duty cycle of a typical shielded metal arc welding machine.
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Welding Cables
• Characteristics
– Must be flexible, well insulated, and the correct size
– Most are made of standard copper wire
– Only specially manufactured insulation should be
used for welding cable
– Electrode cable and work cable must be the correct
size
– A whip-end cable must not be over ten feet long
– Splice in a cable should not be within ten feet of the
electrode
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Electrode Holders
• Characteristics
– Should be of proper amperage rating and in good
repair
– Designed to be used at their maximum amperage
rating or less
– Holder overheats and burns at higher amperage
values
– Large holders are hard to manipulate
– Never dip a hot electrode holder in water to cool
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Work Clamps
• Characteristics
– Must be the correct size for the current
– Must clamp tightly to the material
– Clamp should be carefully touched occasionally to
find out if it is getting hot
– A loose clamp may cause arcing
• May damage a part
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Equipment Setup
• Arc welding machines
– Should be located near the welding site
• Far enough to avoid spark showers
– Machines can be stacked to save space
– Ensure each machine has sufficient air circulation
– Keep away from cleaning tanks and corrosive
fumes
– Water leaks must be fixed and puddles cleaned up
before a machine is used
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Equipment Setup (cont'd.)
– Power shutoff must be easy to reach in an
emergency
– Machine case or frame must be grounded
– Cables should not be placed on the floor
– Work station must be free of combustible materials
– Cable should never be wrapped around the body or
tied to scaffolding or ladders
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Summary
• Understanding electricity and magnetism
– Aids in understanding welding currents
• Failure to control arc blow
– Can result in weld failures
• Check equipment manufacturer's safety guidelines
– Proper operation and maintenance
• Keeping work area clean and orderly
– Helps prevent accidents
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