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


Figure 3-34 Duty cycle of a typical shielded metal arc welding machine. © Cengage Learning 2012


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

Documents

Ch 3 Shielded Metal Arc Welding Setup and Operation