Chapter 10 Plasma Arc Cutting Objectives List and

Chapter 10 Plasma Arc Cutting

Objectives • List and identify the equipment required to cut with PAC. • Assemble the equipment for PAC. • Select and set values for cutting. • Perform an acceptable cut using the PAC process. • Recognize PAC safety and fire hazards. • Know the safety equipment required when PAC. © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Cutting Principles • Plasma is an ionized superheated gas • The plasma arc cutting (PAC) process uses a constricting nozzle to create, concentrate, and direct high-velocity plasma • Plasma gas is always used in plasma arc cutting • When a shielding gas is also used, the process is called dual flow plasma arc cutting © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Cutting Principles • Some advantages of PAC include – – – PAC has the ability to cut all metals Cutting speeds are faster than for oxyfuel cutting No preheating is required Distortion and heat-affected zones are minimized No hazardous or explosive gases are used © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Cutting Principles • Some disadvantages of PAC include – The cost of PAC equipment can be more than for other processes – A source of electricity must be available – The presence of an arc can cause safety hazards – Resulting metal fumes can be a health hazard © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Cutting Equipment and Supplies • The main parts of a plasma arc cutting system – – A power supply A torch A supply of gas or gases and a regulator Proper safety equipment for the operator • The torch design must create the plasma gas and properly direct it to perform the cutting operation © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Cutting Equipment and Supplies • This illustration shows plasma arc cutting equipment (Miller Electric Mfg. Co. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Power Supplies • A power supply used for plasma arc cutting is a constant-current machine • High-amperage power supplies use a large transformer and are very large machines • Smaller units are inverter power supplies • Inverter power supplies change the frequency of supplied power going to the transformer • Hertz (Hz) is a unit of measure for frequency © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Power Supplies • A portable inverter power supply for plasma arc cutting is shown in this illustration (Thermal Dynamics Corp. , a division of Thermadyne Industries, Inc. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Torches and Cables • The torch holds the electrode, provides an electrical connection to the electrode, and provides a path for the plasma • The plasma and the arc pass through the constricting nozzle (American Torch Tip Co. ) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Torches and Cables • A second nozzle may be used to direct shielding gas to the cutting area • The shape of the swirl ring causes the plasma to swirl, creating a tornado-like column • Some cutting torches have a heat shield to protect the torch from the high heat of the cutting process • Usually, a single combination cable assembly is used to connect the torch to the power supply © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma and Shielding Gases • The same gas may be used for shielding gas and plasma gas, or different gases may be used • Air is used as both plasma and shielding gas • Compressed air is supplied from a gas cylinder or a shop manifold system © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma and Shielding Gases • When plasma arc • Nitrogen is a good cutting is done properly, choice as a plasma gas the amount of nitriding when cutting stainless and oxidation are steel or aluminum minimal • Plasma gas composed of • Oxygen is another gas 65% argon and 35% that can be used to cut hydrogen is often used carbon steel for cutting stainless steel or aluminum © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Setting Up the PAC Station • Setting up a PAC station for the first time requires – Connecting the power supply, shielding gas, and cable assembly – Assembling the torch • Future setup may require setting the correct gas flow and assembling the torch © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety Equipment • Plasma arc cutting dangers include intense light, heat, and radiation; flying particles; and fire hazards • Wear earplugs or earmuffs to prevent damage to the ears from loud noise • Wear appropriate clothing, gloves, and a welding helmet • Cut only in well-ventilated areas © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Safety Equipment • Maintain and replace equipment as needed • Check that hoses and connections are tight and without leaks • Inspect the cable assembly from the power supply to the cutting torch • Inspect the power cable connecting the power supply to the electrical source © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Preparing to Cut • Plasma arc cutting requires few settings – Turn on the power supply – Open the gas valve(s) – Press the gas test or purge button to cause the gas to flow – Adjust the regulator to the desired pressure – Set the desired current on the power supply © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Cutting Procedure • The cutting process begins by pressing a trigger switch on the torch • Plasma gas flows and the arc is initiated • The base metal will start to melt immediately • Initially, the arc starts as a pilot arc • The pilot arc is nontransferred—not between the electrode and the work © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Cutting Procedure • When the arc is between the electrode and metal to be cut, it is called a transferred arc • Cutting can be done in any direction • Cutting can be started from the edge or center of the part to be cut • Plasma arc cutting can be done in any position and cuts any metal © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

• To stop the cutting process, release the switch or pull the torch away from the base metal • This illustration shows a plasma arc cut in progress © Goodheart-Willcox Co. , Inc. (Thermal Dynamics Corp. , a division of Thermadyne Industries, Inc. ) Cutting Procedure Permission granted to reproduce for educational use only.

Cutting Speeds • Plasma arc cutting speeds are fast, especially on thin materials • Proper cutting speeds can be judged by watching the sparks from the cut • If the travel speed is too slow or too fast, dross is found on the bottom side of the workpiece • Adjust the travel speed to eliminate dross © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Quality of Cut • A properly made arc cut has – A kerf width about two times the diameter of the hole in the constricting nozzle when cutting thin base metals – Little or no dross – Cut surfaces that are fairly smooth with a slight taper on the cut surface © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

• An automated PAC can produce a very good cut with no dross © Goodheart-Willcox Co. , Inc. (Thermal Dynamics Corp. , a division of Thermadyne Industries, Inc. ) Quality of Cut Permission granted to reproduce for educational use only.

Quality of Cut • Often one side of the part to be cut remains and one side is scrap • Cut in the direction that produces the better cut edge on the piece that will be kept • A good cut has a slight taper on the cut edges • If the taper is too great, adjust the torch-to-work distance © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Quality of Cut • If the quality of the cut is not good, check the electrode and nozzle • If the nozzle opening is blocked by slag, remove the slag without enlarging the orifice or hole diameter • The main reason for wear on the electrode and nozzle is the pilot arc • Keep the pilot arc time to a minimum © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Gouging • Plasma arc gouging is similar in application to plasma arc cutting • In gouging, the goal is to make a wide gouge not a narrow cut • To perform a gouge, the torch is held at an angle to the base metal (Hypertherm) © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Gouging • Changes to the power supply, torch, constricting nozzle, and shielding gas are necessary to perform a plasma arc gouge • When gouging, the nozzle can be over 1/2″ from the metal • A power supply with a higher voltage is required to jump this large distance © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Gouging • The addition of a shielding gas and shielding gas nozzle or heat shield works to protect the torch • The constricting nozzle for plasma arc gouging has a bigger hole • Plasma gas used for gouging steel can be air or an inert gas, such as argon © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Plasma Arc Gouging • Factors that determine the quality of a plasma arc gouging operation include – – – Shielding gas Travel angle Torch-to-work distance Travel speed Current setting © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Shutting Down a PAC Station • To shut down the station – Close the valve on the shielding gas cylinder or manifold and turn out the regulator adjusting screw – Set the gas flowmeter to zero – Press the purge switch to relieve pressure in the line – Turn off the power supply – Place the torch and leads where they belong – Remove the ground clamp – Coil the ground lead and place it where it belongs © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.

Shutting Down a PAC Station • If using gas, close the valve and turn out the regulator adjusting screw • Set the shielding gas flowmeter to zero • Press the purge switch to clear the gas line • Turn the power supply off • Place the torch and leads where they belong • Remove the ground clamp • Place the ground lead where it belongs © Goodheart-Willcox Co. , Inc. Permission granted to reproduce for educational use only.