1. Chapter 8
Plasma Arc Cutting

2. 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

3. 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

4. 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

5. 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

6. 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

7. FIGURE 8-7 Replaceable torch parts.
Hobart Brothers Company

8. 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

9. 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

10. 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

11. 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

12. 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

13. FIGURE 8-11 Typical manual plasma arc cutting setup.
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14. 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

15. 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

16. 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

17. 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

18. 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

19. Figure 8-14 Ohm’s Law.
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20. 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

21. 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

22. 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

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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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

34. 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

35. 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

36. 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

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

38. 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

39. 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

40. 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

41. 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