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© 2012 Delmar, Cengage Learning Chapter 8 Plasma Arc Cutting
© 2012 Delmar, Cengage Learning Objectives Describe plasma and describe a plasma torch Explain how a plasma cutting torch works List the advantages and disadvantages of using a plasma cutting torch Demonstrate an ability to set up and use a plasma cutting torch
© 2012 Delmar, Cengage Learning Introduction Plasma process –Developed in the mid-1950s Early experiments –Found that restricting the arc in a fast-flowing column of argon formed a plasma Plasma was hot enough to melt any metal Introduced as a cutting process Invention of the gas lens –Allowed plasma to be used for welding
© 2012 Delmar, Cengage Learning Plasma Plasma meanings –Fluid portion of blood –State of matter found in region of an electrical discharge (arc) Plasma created by an arc is an ionized gas –Has both electrons and positive ions Temperature of the concentrated arc –About 43,000 degrees
© 2012 Delmar, Cengage Learning Arc Plasma Arc plasma –Gas that has been heated to at least a partially ionized condition Plasma arc –Arc plasma used in welding and cutting processes –Produces high temperatures and intense light associated with arc cutting processes
© 2012 Delmar, Cengage Learning Plasma Torch Characteristics –Creates and controls plasma for welding or cutting –Torch body is made of a special plastic –Torch head is where cables and hoses attach to electrode tip, nozzle tip, and nozzle –Power switch is a thumb switch located on the torch body –Electrode tip, nozzle insulator, nozzle tip, nozzle guide, and nozzle must be replaced periodically
© 2012 Delmar, Cengage Learning FIGURE 8-7 Replaceable torch parts. Hobart Brothers Company
© 2012 Delmar, Cengage Learning Electrode Tip Often made of copper with imbedded tungsten tip –Heat at tip can be conducted away faster Keeping tip as cool as possible –Lengthens its life Earlier torches –Required welder to accurately grind tungsten electrode
© 2012 Delmar, Cengage Learning Nozzle Insulator Between electrode tip and nozzle tip –Provides critical gap spacing and electrode separation of parts Electrode setback –Spacing between electrode tip and nozzle tip –Critical to proper operation of the system
© 2012 Delmar, Cengage Learning Nozzle Tip Has a has a small, cone-shaped, constricting orifice in the center –Plasma is formed between electrode tip and nozzle tip (i.e., electrode setback) Major factors in torch operation –Diameter of constricting orifice –Electrode setback
© 2012 Delmar, Cengage Learning Nozzle and Water Shroud Nozzle –Sometime called the cup –Made of high-temperature-resistant substance Prevents internal electrical parts from shorting Controls shielding gas or water injection Water shroud nozzle –Attached to some torches Water surrounding tip controls hazards of light, fumes, noise, and other pollutants Both are designed to be replaceable
© 2012 Delmar, Cengage Learning Power and Gas Cables Usually covered –Provide some protection to the cables and hoses inside –Makes handling the cable easier –Covering is heat resistant Will not prevent damage to cables and hoses inside if it comes in contact with hot metal or is exposed directly to cutting sparks
© 2012 Delmar, Cengage Learning FIGURE 8-11 Typical manual plasma arc cutting setup. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Power Cable Characteristics –Must have a high-voltage-rated insulation –Insulation is made of finely stranded copper wire –As equipment capacity increases: cable must be larger Larger cable are less flexible –Water-cooled torches: cable is run inside the water return line
© 2012 Delmar, Cengage Learning Gas Hoses Two gas hoses run to the torch –One carries gas to produce plasma Other provides a shielding gas coverage Some small-amperage cutting torches have only one gas line –Gas line is made of a special heat-resistant, ultraviolet-light-resistant plastic Be sure to replace the tubing with tubing provided by manufacturer or welding supplier
© 2012 Delmar, Cengage Learning Control Wire Two-conductor, low-voltage, stranded copper wire –Connects power switch to power supply –Allows welder to start and stop plasma power and gas as needed
© 2012 Delmar, Cengage Learning Water Tubing Medium- and high-amperage torches may be water cooled –Early model torches use deionized water Refer to the manufacturer's manual –Cooling water must be turned on and off at the same time as the plasma power
© 2012 Delmar, Cengage Learning Power Requirements Plasma requires a DC, high-voltage, constant- current power supply –Amperage is lower than most welding processes –Plasma process uses same amount of wattage as a similar nonplasma process
© 2012 Delmar, Cengage Learning Figure 8-14 Ohm’s Law. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Compressed Air Characteristics –Used by most small shop plasma arc cutting torches –Must be clean and dry –Used filter dryer to prevent contaminants –Supplied by an external or internal compressor Many PA cutting machines have air compressors built into the power supply
© 2012 Delmar, Cengage Learning Heat Input Very high temperatures allow high traveling rates –Same amount of heat is spread over a larger area –Lowers joules per inch of heat the weld will receive High travel speed –Results in a lower heat input than the OFC process
© 2012 Delmar, Cengage Learning Distortion Metal heated in a localized zone –Expands in that area –After metal cools, it is no longer straight or flat –Distortion is a greater problem with thin metals Preheating plates before cutting using oxyfuel reduce the heat-affected zone
© 2012 Delmar, Cengage Learning FIGURE 8-17 A smaller heat-affected zone will result in less hardness or brittleness along the cut edge. © Cengage Learning 2012
© 2012 Delmar, Cengage Learning Applications Early plasma arc cutting systems –Required helium or argon gas –Nitrogen reduced operating cost of a plasma system Development of process improved –Less expensive gases and dry compressed air could be used –By the 1980s: PAC used for most mild steel
© 2012 Delmar, Cengage Learning Cutting Speed High cutting speeds are possible –25 feet per minute –0.25 mile per hour New machines –Operate at upper limits of plasma torch capacity –Automatically maintain optimum torch standoff distance –Some systems will follow irregular surfaces of preformed part blanks
© 2012 Delmar, Cengage Learning Metals Any electrically conductive material can be cut using PAC –Most popular materials Carbon steel up to one inch Stainless steel up to four inches Aluminum up to six inches –Other materials commonly cut using PAC Copper and nickel alloys High strength, low alloy steels Clad materials
© 2012 Delmar, Cengage Learning Standoff Distance Distance from nozzle to the work –Critical to producing quality plasma arc cuts –Distance increases: arc force is diminished and tends to spread out On some torches, it is possible to drag the nozzle up along the surface of the work –Refer to the owner's manual
© 2012 Delmar, Cengage Learning Starting Methods First method: high-frequency alternating current carried through the conductor –Ionizes gas and carries current to pilot arc –Pilot arc: arc between the electrode tip and nozzle tip within torch head Non-transfer arc with low current Second method: short together electrode and nozzle tip –Automatically move them together and immediately separate them again
© 2012 Delmar, Cengage Learning Kerf Space left in the workpiece as metal is removed during a cut –Width of a PAC kerf: often wider than an oxyfuel cut –Many factors affect kerf width Standoff distance Orifice diameter Power setting Travel speed Gas Electrode and nozzle tip Swirling of the plasma gas Water injection
© 2012 Delmar, Cengage Learning Gases Almost any gas or mixture can be used Effects of changing the gas –Force –Central concentration –Heat content –Kerf width –Dross formation –Top edge rounding –Metal type
© 2012 Delmar, Cengage Learning FIGURE 8-27 Controlling the pressure is one way of controlling gas flow. Some portable plasma arc cutting machines have their own air pressure regulator and dryer. Air must be dried to provide a stable plasma arc. Larry Jeffus
© 2012 Delmar, Cengage Learning Stack Cutting Thin sheets can be stacked and cut efficiently –Oxyfuel stack cutting of sheets Important there are no air gaps Often necessary to weld along side of the stack –PAC does not have these limitations Recommended that sheets be held together for cutting Can be accomplished by using standard C-clamps
© 2012 Delmar, Cengage Learning Dross Metal that resolidifies and attaches to bottom of cut –Made of unoxidized metal, metal oxides, nitrides –Much harder to remove than slag –Stainless steel and aluminum are easily cut dross free Carbon steel, copper, and nickel-copper alloys are much more difficult
© 2012 Delmar, Cengage Learning Machine Cutting Almost any plasma torch can be attached to a semiautomatic or automatic device –Simplest devices are oxyfuel portable flame cutting machines on tracks Good for mostly straight or circular cuts –High-powered PAC machines must be used with some semiautomatic or automatic system Hazards make them unsafe for manual operations
© 2012 Delmar, Cengage Learning Water Tables Machine cutting lends itself to the use of water cutting tables –Can be used with most hand torches Advantages –Reduces noise level –Controls plasma light –Traps sparks –Eliminates most of the fume hazard –Reduces distortion
© 2012 Delmar, Cengage Learning Manual Cutting Most versatile PAC process –Used in all positions –Used on almost any surface –Used on most metals –Limited to low-power plasma machines Setup –Wear all of required personal protection equipment –Follow all of manufacturer’s safety rules
© 2012 Delmar, Cengage Learning Safety PAC has many safety concerns –Electrical shock –Moisture –Noise –Light –Fumes –Gases –Sparks –Operator check out
© 2012 Delmar, Cengage Learning Straight Cuts Most common type of cuts made with PAC torches FIGURE 8-32 It is easier to make straight, smooth cuts if you can brace the torch closer to the tip, as in cut B. American Welding Society
© 2012 Delmar, Cengage Learning Plasma Arc Gouging Similar to air carbon arc gouging –U-groove can be cut into metal's surface –Torch is set up with a less-concentrated plasma stream –Effective on most materials –Do not remove too much metal in one pass
© 2012 Delmar, Cengage Learning Cutting Round Stock Often it is necessary to PA cut a round piece of metal –Challenge Cut starts out like a gouged groove and transitions to something like piercing a hole –Important to keep plasma stream straight and in line with line being cut
© 2012 Delmar, Cengage Learning Summary Plasma arc cutting –Quickly becoming one of the most popularly used cutting processes Used by almost every segment of the industry –High rate of cutting speed One of the biggest challenges for beginning students Developing an eye and ear for sights and sounds will aid in skill development
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