Chapter 8 Plasma Arc Cutting

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

Introduction Plasma process Early experiments 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

Plasma Plasma meanings Plasma created by an arc is an ionized gas 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

Arc Plasma Arc plasma Plasma arc 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

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

FIGURE 8-7 Replaceable torch parts. Hobart Brothers Company

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

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

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

Nozzle and Water Shroud 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

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

FIGURE 8-11 Typical manual plasma arc cutting setup. © Cengage Learning 2012

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

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

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

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

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

Figure 8-14 Ohm’s Law. © Cengage Learning 2012

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

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

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

FIGURE 8-17 A smaller heat-affected zone will result in less hardness or brittleness along the cut edge. © Cengage Learning 2012

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

Cutting Speed High cutting speeds are possible New machines 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

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

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

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

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

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

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

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

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

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

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

Manual Cutting Most versatile PAC process Setup 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

Safety PAC has many safety concerns Electrical shock Moisture Noise Light Fumes Gases Sparks Operator check out

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

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

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

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