Ch 3 shielded metal arc welding setup and operation

2 © 2012 Delmar, Cengage Learning

Chapter 3

Shielded Metal Arc Welding, Setup, and Operation


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


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



• 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



• 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


Introduction

• Shielded metal arc welding (SMAW) – A flux-covered metal electrode carries current

• SMAW is a widely used welding process– Low cost– Flexible – Portable – Versatile


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


FIGURE 3-2 Electrons traveling along a conductor. © Cengage Learning 2012


FIGURE 3-3 Ohm’s law. © Cengage Learning 2012


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


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


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


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


Operating Voltage

• Voltage at the arc during welding– Also called welding or closed circuit voltage

• Will vary with:– Arc length– Type of electrode– Type of current– Polarity


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


Figure 3-13 Magnetic forces concentrate around bends in wires.

© Cengage Learning 2012


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


Figure 3-17 Diagram of a step-down transformer.



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


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


Figure 3-27 Schematic diagram of an alternator.



Figure 3-28 Diagram of a generator.



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. © Cengage Learning 2012


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


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


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


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


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


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


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

Ch 3 shielded metal arc welding setup and operation

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