Shielded Metal Arc Welding, Setup, and Operation 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

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

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

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 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 Will vary with: Also called welding or closed circuit voltage Will vary with: Arc length Type of electrode Type of current Polarity

Arc Blow Electrons flow Arc blow 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 Welding transformers Electric motors or internal combustion engines Step-down transformers Welding transformers Use high-voltage AC to produce low-voltage welding power Takes high voltage, low amperage current and turns it into low voltage, high amperage current

Figure 3-17 Diagram of a step-down transformer. © Cengage Learning 2012

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. © Cengage Learning 2012

Figure 3-28 Diagram of a generator. © Cengage Learning 2012

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 Duty cycle 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