Chapter 3 Shielded Metal Arc Welding Setup and

Chapter 3 Shielded Metal Arc Welding, Setup, and Operation 2 © 2012 Delmar, Cengage Learning

Objectives • Describe the process of shielded metal arc welding (SMAW) • List and define 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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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

FIGURE 3 -3 Ohm’s law. © Cengage Learning 2012 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

Figure 3 -13 Magnetic forces concentrate around bends in wires. © Cengage Learning 2012 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

Figure 3 -17 Diagram of a step-down transformer. © Cengage Learning 2012 View Welding Video Click here to view closed captioning 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

Figure 3 -27 Schematic diagram of an alternator. © Cengage Learning 2012 2 © 2012 Delmar, Cengage Learning

Figure 3 -28 Diagram of a generator. © Cengage Learning 2012 2 © 2012 Delmar, Cengage Learning

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 ACDC transformer rectifier welder. © Cengage Learning 2012 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

Figure 3 -34 Duty cycle of a typical shielded metal arc welding machine. © Cengage Learning 2012 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 2 © 2012 Delmar, Cengage Learning

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 – Properation and maintenance • Keeping work area clean and orderly – Helps prevent accidents 2 © 2012 Delmar, Cengage Learning