shielded metal arc welding principles

Shielded Metal Arc Welding Principles

Chapter 11

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Objectives

1. List the percentage of usage of SMAW in the industry. 2. Name the components that make up the

schematic representation of the shielded metal arc. 3. Know the maximum arc temperature of an

SMAW electrode. 4. List the four constant current welding machines.

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Objectives

5. List the common type and uses of constant current welding machines. 6. Name the power supply

ratings. 7. Name the characteristics of the four basic types of welding machines. 8. Choose the

correct cable size based on the application. 9. List the welder’s safety equipment.

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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)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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SMAW

American Welding Society


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Process Capability

• Shielded metal arc welding one of most used of various electric arc welding processes

42%

34%

13%

9% 2%

SMAW GMAW/FC GTAW SAW Others

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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

American Welding Society


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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


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Output Slope

• Indicates type and amount of electric current designed to produce • Each arc welding process has

characteristic output slope – SMAW and GTAW require steep output slope from constant current

welding machine – GMAW and FCAW require relatively flat output slope from constant voltage

power source – Submerged arc welding adaptable to either slop

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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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Power Sources

• Important to select right power source for each job • Table 11-1 "Common types and uses of arc

welding machines" should be studied • Study of job indicates whether a.c. or d.c. – Shielded metal

arc welding and gas tungsten arc welding must use constant current machine – Gas metal arc prefers

constant voltage machine

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Constant Current Welding Machines

• Used for shielded metal arc welding and gas tungsten 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


Some jobs require steep volt-ampere curve Other jobs use less steep volt-ampere curve

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Open Circuit Voltage

• Voltage generated by welding machine when no welding being done – Machine running idle • Arc

voltage – Voltage generated between electrode and work during welding • Load voltage – Voltage

at output terminals of welding machine when arc is going – Combination of arc voltage plus voltage

drop in welding circuit

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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 – Gas Tungsten arc welding • Arc voltage much lower than open circuit voltage

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Motor Generator Welding Machines

• Usually supply only direct current – Can be made to supply a.c. • Most constant current type •

Used chiefly for shielded metal arc welding and gas tungsten arc welding

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Motor-Generator Welding Machine

Hobart Brothers Co.


Sturdy steel lifting eye Large voltmeter Calibrated dial for easy, fine adjustment of heat and relation

between voltage and current Outer wheel and dial for selecting desired welding range Stop button

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Hobart Brothers Co.


Ground cable connector Optional steel guard

Heavy-duty ball bearings Pressed steel bearing cap Arc welded copper squirrel cage rotor

Motor stator Heavy steel fan

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Hobart Brothers Co.


Steel frame Fourpole "Multi-Range" generator

Large commutator

Heavy-duty metallic graphite brushes

Heavy-duty ball bearings Pressed steel bearing cap Arc welded steel frame

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Hobart Brothers Co.


Removable steel covers Heavy-duty single unit steel shaft

Polarity switch Welding cable connector Large ammeter Steel turret top with removable cover

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Motor Generator Welding Machine for SMAW

The Lincoln Co.

Variable voltage control

Instruction on the Name-plate

Polarity switch

Current control


Toggle switch

Constant voltage

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External Construction of d.c. Motor Generator Welding Machine

The Lincoln Electric Co.


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Internal Construction of a D.C. Motor Generator Welding Machine



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Motor Generator Desirable Characteristics

• Have forceful penetrating arc • Versatile – Can be used to weld all metals that are weldable by arc

process • Flexible – With proper electrode, can be used in all positions • Durable and have long

machine life

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Generators

• Classified by type of motor that drives generator • Consists of a.c. motor, d.c. generator, and

exciter built on single shaft


Miller Electric Mfg. Co. The Lincoln Electric Co.

Generators used in the field

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Sizes

• Determined on basis of amperage • Range – 100-ampere rated for home – 1,500 ampere rated

for use with automatic submerged arc welding equipment • Manual welding machine idle

sometimes • Automatic power supply units 100% duty cycle – Should not be used at or beyond max

over extended period

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Maintenance

• Contacts of starter switch and control rheostat should be inspected, cleaned frequently and

replaced when necessary • Brushes need frequent inspection for wear – Check commutator for wear

or burning • Rewound and turned on lathe • Main bearings on shaft inspected and greased at each

6-month period – Clean old grease out of bearing housings

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Start and Stop Buttons

• Purpose of starting and stopping the motor • Start button black • Stop button red • Important

buttons be engaged firmly

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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 Tapped-Current Control

• Coarse adjustment dial selects current range – Called steps or taps – Impossible to secure current

value between two steps by setting dial between them • Fine adjustment dial trims current

between steps – Whether set high or low depends on type and size of electrode, thickness of metal,

soft or digging arc required, arc starting, restricting characteristics, and Position of welding

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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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Engine Driven Generator

Miller Electric Mfg. Co.



Capable of SMAW, GTAW, SMAW, FCAW and PAC.

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Remote Control

• Welding machines may be installed in remote part of plant • Welder may adjust current without

leaving job • Timesaving on work – Welder does not have to leave fabrication to readjust current

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Air Filters

• Wear in arc welding machines costly – Cost of replacement parts and labor – Loss of production

due to nonuse of machine • Bearing wear critical – May be reduced through use of air filter fitted on

suction end of motor generator machine • Filter cleaned regularly – High pressure air, commercial

solvent or steam

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D.C. Transformer-Rectifier Welding Machines




• Have many designs and purposes • Flexibility one reason for wide acceptance – Deliver either

DCEN or DCEP • May be used for: – Stick electrode welding – Gas tungsten arc welding – Submerged

arc welding – Multi-operator systems – Stud welding

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Transformer-Rectifier Machines

• Have two basic parts – Transformer for producing and regulating alternating current that enters

machine – rectifier that converts a.c. to d.c. • Third important part is ventilating fan – Keeps rectifier

from overheating • Design improves arc stability and makes it easy to hold short arc which is soft

and steady • No major rotating parts so consume little power

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A.C.-D.C. Transformer-Rectifier Welding Machines

• Permit welder to select either a.c. or d.c. and electrode negative or electrode positive • Switch –

Permits welder to use only transformer part of machine for a.c. welding – Flipping switch then

output current directed through rectifier which converts it to d.c. welding • High frequency arc-

starting devices, water/gas flow controls, balance controls for a.c. operation, remote control often

built into machine

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A.C.-D.C. Transformer-Rectifier Welding Machines




Miller Electric Mfg. Co. A 300 amp a.c.-d.c. GTAW/SMAW machine.

Portable SMAW/GTAW welding machine

A 300 amp model shown in use

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A.C. Transformer Welding Machines

• Most popular a.c. welding machine • Function of transformer – Step down high voltage of input

current to high amperage, low voltage current required for welding • Especially suited for heavy

work



Miller Electric Mfg. Co

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Advantages of a.c. Power Sources

• Reduces tendency to arc blow • Can use larger electrodes – Resulting in faster speeds on heavy

materials • Lower cost • Decreased power consumption • High overall electrical efficiency •

Noiseless operation • Reduced maintenance

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D.C. and A.C.-D.C. Inverter Welding Machines

• Portable, lightweight, and versatile • May be either constant current, constant voltage or both •

Can perform several different processes


Miller electric Mfg. Co.


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Cost Comparisons: Arc Power Sources

• Three main areas – Cost of purchasing equipment (nearly equal) – Operating efficiency • Motor

generator machine: 52-65% • Transformer-rectifiers: 64-72% • Inverters: 85% – Maintenance •

Motor generator machine: replacing parts, lubrication • Transformer-rectifiers and inverters have no

moving parts See Table 11-2 for more comparisons

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Multiple-Operator Systems

• Can be installed away from work site and be connected to control panels close to welding operator

• When using direct current, all welders must weld with same polarity



• Most installations are d.c. • Power: 600-2,500 amperes • Cost less, saves space and cable, lowers

operating cost

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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 – Efficiency of output – Power factor

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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 – Over 2,000 amperes for submerged arc welding

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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 and GTAW

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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 See Table

11-3 to help choose the right size of welding cable

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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.

Lenco dba NLC. Inc. Spring loaded C-clamp

Rotary type

Lenco dba NLC. Inc.

This clamp stops twisting and turning of welding cable where work rotates. Clamp welded to part,

thus permits work clamp to be attached in seconds. It is generally used in fabricating tanks,

pressure vessels, and on weld positioners.

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Quick Connector

• Attached to ends of different lengths of cables • Allow quick and easy attachment to any length

cable


Lenco dba NLC. Inc.

This connector has cam-type action that ensures positive stop and lock and cannot come loose or

accidentally fall apart.

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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


Lenco dba NLC. Inc.

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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

Lenco dba NLC. Inc.


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Short-stub Electrode Holder


Cable connections Special extruded copper-alloy body High welding current conductance One-ton

pressure contact

Lifeguard insulation

Entirely insulated Bernard Welding Equipment Co. Holds electrodes burned to very short stub. Twist-

type locking device permits electrode- gripping power in excess of 2,000 pounds.

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Angle-head Electrode Holder


Jackson Products Co.

Available in sizes of 400 and 600 amperes and takes electrodes from 1/16 through 5/16 inch in

diameter.

Shorter and lighter than other holders of comparable capacity.

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Other Electric Arc processes

• Generates heat for several major welding processes – Gas tungsten arc welding (GTAW) – Gas

metal arc welding (GMAW) • Arc cutting utilizes much of same equipment as arc welding

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Carbon Arc Welding

• Welding heat comes from arc formed between base metal and carbon electrode or arc formed

between two carbon electrodes – With or without addition of filler rod • Carbon electrodes

available: 1/4, 5/16, 3/8 inch • Alternating current: 30-125 amperes • Metal electrode holders not

suitable – Carbon electrode hotter than metal electrode

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Twin Carbon Electrode Holder



Two leads are required because the arc is created between the two electrodes.

Metal shield to protect welder's hand from intense heat.

Larger than the metal electrode holder Holder is water cooled

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Atomic-Hydrogen Arc Welding

• Process in which electric arc surrounded by atmosphere of hydrogen – Gas shields molten metal

from oxidation and contamination from the air – Transfers heat from electrode to work – Arc formed

between two electrodes • Temperature produced by arc: 7,500ºF • Current supplied by a.c. welding

transformer • Hydrogen supplied in cylinders

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Atomic-Hydrogen Electrode Holder


General Electric Co.

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Atomic-Hydrogen Arc Welding

• Metal of same analysis as being welded can be deposited • Welds may be heat treated – Unusually

smooth, ductile, nonporous and free from impurities – Surface free from scale • May weld hard-to-

weld metals • Advantages: increased production, low operating cost, and low maintenance cost

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Hand and Head Shields

• Brilliant light caused by electric arc contains two kinds of invisible rays which injure eyes and skin –

Ultraviolet – Infrared • Rays affect eyes within 50 feet; and skin any distance within 20 feet


Fibre-Metal Products Co. Hand shield so person may hold shield in front of face

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• Also called hood or helmet – Attached to adjustable headband • Allows it to be moved up or down

as wearer desires • Dependable protection – Both hands free to grasp electrode holder • Partial

protection – Must also wear leather or nonflammable cap for adequate protection


Fibre-Metal Products Co.


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• Constructed of heat-resisting, pressed-fiber insulating material • Shields fully molded at top and

bottom to protect head and neck • Usually black to reduce reflection with window frame to hole

protective lens


Wilson Products

Flip-front welding helmet permits welder to inspect and brush weld without lifting the hood.

Chrome leather helmets ideal for hard-to-get-into areas.


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Head Shield Lens

• Sizes: 2 x 4.5 or 4.5 x 5.25 • Colored to screen out ultraviolet, infrared rays, and most of visible rays

• Variety of shades of color – Density depends on brilliance of arc – Varies with size of electrode and

volume of current • Side exposed to weld pool protected by clear polycarbonate plastic cover lens –

Protect costly filter lens from molten metal spatter and breakage (replaced when pitted and

clouded)

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Auto-darkening Electronic Filter Helmet • Useful when working close quarters, doing high

production work, or avoiding inadvertent arc strikes • Single shade or variable shades • Filter can

switch from light to dark in less than 1/10,000 of a second • Battery powered – May have solar

battery booster Jackson Products Co.


.

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Safety Glasses

• Should be worn – Also behind hood to prevent severe arc flash reaching eyes • Can Absorb more

than 99.9% harmful ultraviolet rays • Worn by others who work with welders • Light in weight, well

ventilated, and comfortable • Lenses have light tint (Not dark!) and tented side shields


© RobCrandall/The Image Works

11 - 86

Protective Clothing

• Gloves to protect hands – Made of leather or some other type of fire-resistant material • Leather

capes, sleeves, shoulder garments with detachable bibs, aprons • Split-type apron if sitting down –

No place to collect hot particles • High-top shoes – Leggings and spats


Department of Labor

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Protective Clothing

• 100% cotton or wool • Thick enough to prevent injurious untraviolet rays from penetrating to skin

• Long sleeves • Shirts buttoned to neck • Shirttails tucked • Cuffless pants long enough to cover top

of leather boots Miller Electric Mfg. Co.


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Protective Clothing


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Further Protection

• Ear Protection – Full ear muffs that cover entire ear or ear plugs – Dangers • Noise • Hot weld

spatter or slag entering ear canal • Flume Protection – Always use proper ventilation to keep head

out of fume plume

shielded metal arc welding principles

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