Transcript 02-3 shielded metal arc welding (smaw) - file-012009-09

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Shielded Metal Arc Welding
Shielded Metal Arc Welding
(SMAW)
 Manual arc welding
 Heat for welding generated by electric arc established
between flux-covered consumable metal rod (electrode) and
work
 Called stick electrode welding
 Combustion and decomposition of electrode creases
gaseous shield
 Protects electrode tip, weld puddle, arc, and highly heated
work from atmospheric contamination
• Additional shielding provided by covering of molten slag
(flux)
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SMAW
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SMAW Advantages
 Equipment less complex, more portable and less
costly
 Can be done indoors or outdoors, in any location and
any position
 Electrodes available to match properties and strength
of most base metals
 Not used for welding softer metals
 Not as efficient in deposition
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SMAW Operating Principles
 Sets up electric circuit
 Includes welding machine, work, electric cables,
electrode holder and electrodes, and a work clamp
 Heat of electric arc brings work to be welded
and consumable electrode to molten state
 Heat intense: as high at 9,000ºF at center
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Welding Process
 Electric arc started by striking work with electrode
 Heat of arc melts electrode and surface of base metal
 Tiny globules of molten metal form on tip of
electrode and transferred by arc into molten pool on
work surface
 After weld started, arc moved along work
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SMAW Operating Principle
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AC Welding Machine
 Most common type
found in homes, farms,
etc.
 Good for farm repairs,
light jobs.
 Low cost
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DC Welding Machine
 Often generator type
machines
 Diesel or gasoline engine
driven
 Portable
 Expensive
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AC/DC Welding Machine
 Can weld in AC or DC
polarity
 Less expensive than DC
machine
 Quieter than DC
machine
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Welding Power Sources
 Each type of power source has fundamental electrical
differences that best suit particular processes
 Welding machine
 Must meet changing arc load and environmental
conditions instantly
 Must deliver exact amount of electric current precisely
at right time to welding arc
 Available in wide variety of types and sizes
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Welding Power Sources
 Also known as power supplies and welding machines
 Two classifications
 Output slope
• Whether constant current or constant voltage
 Power source type
• Transformer
• Transformer-rectifier
• Inverter
• Generator
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Type of Output Slope
 Two basic types
 Constant current
• Referred to as variable voltage
 Constant voltage
• Referred to as constant potential
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Output Slope
 Relationship between output voltage and output
current (amperage) of machine as current increased
or decreased
 Also called volt-ampere characteristic or curve
 Largely determines how much welding current will
change for given change in load voltage
 Permits welding machine to control welding heat and
maintain stable arc
 Indicates type and amount of electric current
designed to produce
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Typical Output Slopes
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Four Types of Power Source
 Engine-driven generators
 Powered by gas or diesel combustion engine
 Can be found with a.c. or d.c. electric motor
• No longer being manufactured and rarely found
 Transformer-rectifiers
 Use basic electrical transformer to step down a.c. line
power voltage to a.c. welding voltage
 Welding voltage then passed through rectifier to
convert a.c. output to d.c. welding current
 May be either d.c. or a.c.-d.c. machines
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Four Types of Power Source
 A.C. transformers
 Used to step down a.c. line power voltage to a.c.
welding voltage
 Inverters
 Increases frequency of incoming primary power
 Constant current, constant voltage, or both
 Produce a.c. or d.c. welding current
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Constant Current
Welding Machines
 Used for shielded metal arc welding
 Current remains fairly constant regardless of changes
in arc length
 Called drooping voltage, variable voltage, or droopers
• Load voltage decreases as welding current
increases
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Constant Current Output Slope
 Constant current welding machines
 Steep output slope
 Available in both d.c. and a.c. welding current
 Steeper the slope, the smaller current change
 Enables welder to control welding current in specific
range by changing length of arc
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Constant Current Output Slope
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Open Circuit and Arc Voltage
 Open circuit voltage runs between 50-100
volts
 Drops to arc voltage when arc struck
 Arc voltages
 Range: 36 volts (long arc) to 18 volts (short arc)
 Determined by arc length held by welder and type of
electrode used
 Arc lengthened, arc voltage increases and current
decreases
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Open Circuit and Arc Voltage
 Open circuit voltage on constant current machines
higher than on most constant voltage machines
 Arc voltage depends on physical arc length at point
of welding and controlled by welder
 Shielded metal arc welding
 Arc voltage much lower than open circuit voltage
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Polarity Switch
 Electrode negative and electrode positive used in d.c.
welding
 DCEN (d.c. electrode negative)
 Electrode connected to negative terminal of power
source and work connected to positive terminal
 DCEP (d.c. electrode positive)
 Electrode connected to positive terminal of power
source and work connected to negative terminal
 Switch changes to either electrode positive or
electrode negative
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Volt-Ampere Meters
 Sometimes serve dual purpose
 Can indicate polarity as well as current
 Others, individual meters for volts and amperes
 Some, single meter that indicates both volts and
amperes
• Button engaged to get individual readings
• Would need second person for monitoring
 Increased demand for additional devices so meters
have been discontinued by some manufacturers
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Current Controls
 Amperage
 Quantity of current
 Determines amount of heat produced at weld
 Voltage
 Measure of force of current
 Determines ability to strike an arc and maintain its
consistency
 Two types of dual control generators
 Tapped-step current control
 Continuously variable current control
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Dual Continuous Control
 Coarse adjustment dial continuously adjusts current
 Fine dial adjusts both current (amperage) and open
circuit voltage
 Operator adjusts output slope for given current setting
by manipulating both coarse and fine adjustment dials
together
 Wheel or knob on both amperage and voltage setting
devices gives welder continuous control of both
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Power Supply Ratings
 Standards set
 The National Electrical manufacturers Association
 Occupational Safety and Health Administration
 Provide guidelines for manufacture and performance
of power sources
 Rated by
 Current output
 Open circuit voltage
 Duty cycle
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Current Output
 Rated on basis of current output in amperes
 Amperage range
 200 amperes or less for light or medium work
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Open Circuit Voltage
 Maximum allowable used for manual welding
 80 volts for a.c. or a.c.-d.c. machines
 100 volts for d.c. machines
• Very smooth output (less than 2% ripple)
 Automatic machine welding
 Some constant current machines rated up to 125
 Constant voltage types normally rated from
15 to 50
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Duty Cycle
 Percentage of any given 10-minute period that
machine can operate at rated current without
overheating or breaking down
 Rating of 100% means machine can be used at rated
amperage on continuous basis
• Required by continuous, automatic machine
welding
 Rating of 60% means machine can be used at its
capacity 6 out of every 10 minutes without damage
• Satisfactory for heavy SMAW
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Efficiency
 Relationship of secondary power output to primary
power input
 Indicated in percent
 Determined by losses through machine when actually
welding at rated current and voltage
 Average efficiencies
 Motor generator welding machines: 50%
 Transformer-rectifier: 70%
 Inverter: 85%
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Power Factor
 Measure of how effectively welding machine makes
use of a.c. primary line power
 Primary power used divided by amount total drawn
 Expressed in percent
 Three-phase d.c. transformer-rectifiers: 75%
 Single-phase a.c. power units: 55%
 Welding machines can be purchased with power
factor correction
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Power Cable
 Conductors of ample capacity and adequately
insulated for voltage transmit power
 Necessary to ground frame of welding machine
 Portable cable with extra conductor fastened to
machine frame on one end and solid ground on other
 Important cable adequately insulated with tough
abrasion-resisting insulation
 Stand up under rough usage in welding shops
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Electrode and Work Cable
 Required to complete electric circuit between
welding machine and work
 Electrode cable (welding cable) attached to electrode
holder
 Work cable attached to work
 Rubber-covered multistrand
copper cable generally used
 Must have high flexibility
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Cable Core Construction
 Woven of thousands of very fine copper wire
 Greater the number of strands the more flexible
 Components
A.Wires stranded for extra flexibility
B.Paper wrapping around wires
allows conductor to slip within
robber covering when bent
C.Extra strength from open-braided
reinforcement of extra cotton cords
D.Special composition and curing of
heavy rubber covering
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Work Cable
 Not necessary to have flexibility of electrode cable
 Usually same cable used
 Important considerations
 Amperage of welding machine
 Distance from work
 Larger cable
 Greater the amperage and greater the distance
 Resistance increases as diameter of cable decreases
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Cable Lugs
 Required on both electrode cable and work cable
 Soldered or fastened mechanically
 Connections MUST be tight and secure
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Work Clamps
 Variety of structures to be welded; many types of
clamps may be used
 Copper hook, heavy metal weight, C-clamp
 Specialized work clamps
Lenco dba NLC. Inc.
Spring loaded
C-clamp
Lenco dba NLC. Inc.
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Metal Electrode Holders
 Device used for holding electrode mechanically
 Conveys electric current
from welding cable to electrode
 Insulated handle protects hand from heat
 Jaws of holder grip electrode at any angle
 Made of metal with high electrical conductivity and
ability to withstand high temperatures
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Metal Electrode Holders
 Jaws can be replaced with new ones
 Should be light in weight, well-balanced and have
comfortable grip
 Size of holder must be in line with size of welding
machine
 Fully insulated so stays cool even with high duty
cycles
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Spring Type Electrode Holder
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Short-stub Electrode Holder
One-ton pressure
contact
High welding current conductance
Special extruded copper-alloy body
Cable connections
Bernard Welding Equipment Co.
Entirely insulated
Holds electrodes burned to very short stub.
Lifeguard insulation Twist-type locking device permits electrodegripping power in excess of 2,000 pounds.
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Knowledge is the key to success
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