© 2012 Delmar, Cengage Learning Chapter 8 Plasma Arc Cutting

© 2012 Delmar, Cengage Learning

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 – 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– 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 – 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

• 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


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– 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– 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 cutsif 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

Documents

© 2012 Delmar, Cengage Learning Chapter 8 Plasma Arc Cutting