1. Shielded Metal Arc Welding, Setup, and Operation
Chapter 3
Shielded Metal Arc Welding, Setup, and Operation

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

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

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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. Figure 3-13 Magnetic forces concentrate around bends in wires.
© Cengage Learning 2012

17. 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

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

19. 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

20. 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

21. Figure 3-27 Schematic diagram of an alternator.
© Cengage Learning 2012

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

23. 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

24. 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

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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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