Plasma cutter

How it works

Miller describes the plasma arc as the result of electrically heating a gas, typically air, to a very high temperature, which ionizes its atoms and lets the gas conduct electricity. A swirl ring spins the gas around an electrode, the gas is heated in the chamber between the electrode and the torch tip, and the small opening of the tip constricts the plasma and accelerates it toward the work. Miller gives speeds up to 20,000 feet per second and temperatures up to 30,000 degrees Fahrenheit. The jet melts a small area and its force pushes through the workpiece and removes the molten metal.

Gage’s 1957 patent, the earliest source read here, describes the same principle in an argon torch: a cooled metal nozzle laterally confines a portion of the arc, and the arc stream and the gas jet that leave the nozzle are one current-carrying effluent. A smaller orifice raises the arc voltage and total power at a given current, and the flow of gas through the nozzle adds to both.

In the hand-held machine the operator presses the trigger, which starts the preflow air, and a pilot arc lights inside the torch. When the torch is near the work the cutting arc follows. Miller says the process uses relatively high voltage and low amperage, the opposite of welding, and that the machine starts the pilot arc either by high frequency or by contact start. Air continues to flow for a period after the trigger is released to cool the torch.

Anatomy

Power supply: Miller notes that the machine’s size is set by its amperage and voltage, and that total output power in watts is amperage times voltage, so amperage alone is an incomplete measure.
Torch: A hand-held or machine-mounted body that carries the consumables and the trigger or start signal.
Electrode: Carries the arc inside the torch. Miller advises discarding an electrode when its tip develops a pit.
Swirl ring: Spins the gas around the electrode (Miller).
Tip (nozzle): The small orifice that constricts and accelerates the plasma. Miller says a lower-amperage tip has a smaller orifice and that a worn or spattered tip hole should be discarded.
Retaining cup and drag shield: Holds the consumables. Miller says that on units above 40 amperes a built-in drag shield holds the tip about 1/8 inch off the work.
Work cable and ground clamp: Completes the circuit through the workpiece; the clamp goes on clean metal as close to the cut as possible (Miller).
Air or gas supply: Miller says most manufacturers of hand-held cutters recommend ordinary air, and that contractors in mobile work sometimes use bottled nitrogen.

Variants

Hand-held air plasma cutter: The common shop and field machine, run on compressed air. Miller says the average hand-held system cuts a maximum of about 1 inch of metal.
Contact-start and high-frequency-start machines: Miller says high frequency can interfere with nearby telephones, computers and CNC equipment, and that its own plasma cutters use contact start, which also gives a visible pilot arc.
Water-cooled argon torch: The form in Gage’s 1957 patent, with cooling water circulated around the nozzle and argon as the gas.
Water-injection torch: Couch and Dean’s 1972 patent describes a radially inward laminar jet of liquid, practically water, that constricts the plasma. Hypertherm dates the invention of water-injection plasma cutting to 1968.
Extended tip: Protrudes 1/2 to 1 inch past the retaining cup for beveling, corners and a better view of the arc (Miller).
Gouging tip: Miller says its hole is three to four times wider than a regular tip and that it removes old or imperfect welds.
Machine torch: The same torch type mounted on a cutting table. See the CNC plasma table entry.

Specifications

Arc temperature · °F: Miller: up to 30,000 °F at the tip. Gage’s 1957 patent estimates arc gas temperatures of 8,000 to 20,000 K for argon.
Plasma jet velocity · ft/s: Miller: 20,000 feet per second.
Standoff · in: 0.125Miller’s guide gives typically 1/8 inch; its tips article says to hold 1/16 to 1/8 inch.
Rated cut speed · in/min: 15Miller’s “rated cut” is the thickness of mild steel an operator can manually cut at 15 inches per minute, which it calls the minimum speed for a smooth, steady cut by hand. Miller says there is no universal standard for stating capacity.
Open-circuit and cutting voltage · V dc: Miller says plasma cutting needs higher voltage than welding to start and maintain the arc, typically 110 to 400 V dc.
Post-flow air · s: Miller: 20 to 30 seconds after the trigger is released; pressing the trigger during post-flow restarts the arc.
Maximum thickness, hand-held · in: 1Miller: about 1 inch for the average hand-held system. For a 55 ampere unit it states a severing capacity of 1-1/4 inches, with slow speeds and significant clean-up.
Minimum protective lens shade · shade number: OSHA 1910.133: shade 8 below 300 A, shade 9 for 300 to 400 A, shade 10 for 400 to 800 A. Miller refers to ANSI Z49.1 for shade numbers for plasma cutting above 80 amperes.

Made to

IEC 60974-1
IEC 60974-7
ISO 9013
AWS Z49.1
ANSI Z87.1
NFPA 70
OSHA 29-CFR-1910-133

Safety and care

Safety

The arc needs 110 to 400 V dc to start and run, and touching live parts can cause fatal shocks; inspect cables daily and replace rather than repair worn ones (Miller).
Wear eye protection with the correct lens shade for the amperage; OSHA 1910.133 lists shade 8 below 300 A, 9 for 300 to 400 A and 10 for 400 to 800 A (light, medium and heavy cutting), and notes that lighter filters may be used where the arc is hidden by the workpiece.
Move flammables at least 35 feet from the cutting area or cover them with flameproof covers; sparks pass through small cracks and openings (Miller; OSHA 1910.252).
Do not cut on closed or pressurized containers, or on containers that may have held combustible, toxic or reactive materials unless a qualified person has tested them (Miller).
Do not grip material near the cutting path, and keep clear of the torch tip when pressing the trigger, because the arc and the pilot arc can burn through gloves and skin (Miller).
Remove coatings from galvanized, lead-plated or cadmium-plated steel where possible and ventilate or use an exhaust hood, because cutting them gives off toxic fumes (Miller).
Chain gas cylinders upright, keep the threaded cap on a cylinder that is moved or stored, and never splice a gas hose with tape (Miller).

Care

Inspect the tip and electrode regularly and replace them together; Miller says to discard a tip whose hole is irregular or covered with spatter, and an electrode whose tip has a pit.
Moisture in the air supply, cutting excessively thick material and poor technique shorten consumable life (Miller).
Finger tighten the retaining cup only, because the parts inside must move apart to create the arc (Miller).
Hold the standoff and avoid touching the tip to the work; Miller says touching an extended tip to the work three or four times can enlarge its hole until it no longer constricts the arc.

Tips

Trace the cut path with the trigger released
Miller advises tracing the path without firing the torch, practicing long cuts to be sure there is freedom of movement for one continuous cut, and making a sample cut on the same material. It says stopping and restarting in the same place is hard and usually leaves irregularities in the edge.1
Steady the torch hand with the other hand and pull the torch toward the body
Miller says the second hand helps hold a constant 1/16 to 1/8 inch standoff, that most people find it easier to pull the torch toward them than to push it, and that the right standoff extends consumable life and raises the capacity of smaller machines.1
Read the sparks and the arc angle for travel speed
Miller says the cutting sparks should emerge from the bottom of the metal, and that at the right speed the arc leaves the material at 15 to 20 degrees behind the direction of travel. An arc going straight down means the travel is too slow, and one that sprays back means it is too fast.1 2
Set amperage by test cuts, then lower it
Miller advises making a few practice cuts with amperage high and then reducing it to suit the travel speed. Too much amperage or too slow a travel speed overheats the work and builds dross. On thin material it recommends a lower-amperage tip to keep the kerf narrow.1
Use the drag shield against a guide
On units above 40 amperes the built-in drag shield holds a 1/8 inch standoff, so its edge can be braced on a ruler, a piece of wood or a pattern. Roller and circle guides keep the tip from touching the work and make straight lines and circles easier.1
Finish the cut by angling or pausing
Miller says to angle the torch slightly toward the end of the cut or pause briefly before releasing the trigger so the last of the metal is severed. Air keeps flowing for 20 to 30 seconds afterward, and pressing the trigger in that period restarts the arc immediately.1 2
Fit a gouging tip to remove welds
Miller says a gouging tip has a hole three to four times wider than a cutting tip and a cone shape that pushes out the arc, and that plasma gouging gives much less smoke and noise than carbon arc gouging, although carbon arc removes metal faster.1
Clamp the work cable to clean bare metal near the cut
Miller advises grinding off rust or paint where the ground clamp attaches, because they inhibit the flow of electricity, and placing the clamp as close to the cut as possible or on the workpiece itself.1 2

History

Earliest documented

1955 (application); 1957 (patent)

Buffalo, New York, United States

US Patent 2,806,124, “Arc Torch and Process,” was applied for on 26 July 1955 by Robert M. Gage of Buffalo, assignor to Union Carbide Corporation, and issued on 10 September 1957. It describes a torch in which a cooled metal nozzle wall-stabilizes and constricts an arc and says the effluent can sever plate of aluminum and other metals; it states that the invention applies to cutting and welding arcs at currents of hundreds of amperes.1

The plasma cutter is a constricted arc. Gage’s 1957 patent describes passing an arc through a water-cooled metal nozzle that laterally confines it, so that the arc stream and the gas jet leaving the nozzle are one current-carrying effluent. It says a smaller orifice raises arc voltage and power at a given current, estimates arc gas temperatures of 8,000 to 20,000 K for argon, and gives severing examples on several metals at a few hundred amperes. That torch used argon and circulating cooling water.

The hand-held machine is a later arrangement. Miller says most manufacturers of hand-held cutters recommend ordinary compressed air as the gas, and Hypertherm says it entered the air-plasma market in 1985. Miller adds that small affordable machines under 50 pounds arrived in the mid-1990s, which it says led to growth among artists, ranchers, repair shops and others who had not used the process before.

A second line of development concerned narrowing the arc further. Couch and Dean’s 1972 patent says earlier proposals for concentrating heat on the work concerned the shaping of the gas flow or the cooling of the plasma boundary layer, citing Gage’s patent as an example. It proposes instead a radially inward laminar jet of liquid, which it says apparently constricts the plasma by virtue of its greater density, and it states that water was found practical. Hypertherm dates its founders’ invention of water-injection plasma cutting to 1968. The patent was assigned to Chemetron Corporation; the Hypertherm history read does not mention Chemetron.

What the plasma cutter displaced, and what it did not, is stated by Miller. Granjon and Rosemberg wrote in 1913 that only iron and steel can be cut by a jet of oxygen and that copper, brass, bronze and aluminum do not burn in oxygen in that manner. Miller says plasma cuts any electrically conductive metal, needs no preheat, makes a narrower kerf and a smaller heat-affected zone than oxy-fuel, and is the standard process for stainless steel and aluminum. In the same articles Miller says an average hand-held oxy-fuel system can cut steel 6 to 12 inches thick against about 1 inch for hand-held plasma, and that above 1 inch oxy-fuel torches are faster than a typical 100 ampere plasma cutter.

Stated capacities differ between makers. Miller says there is no universal standard for rating a plasma cutter, that cutting capacity of a given size varies greatly by manufacturer, and that it qualifies its own machines with a rated cut, the thickness of mild steel cut by hand at 15 inches per minute, and a sever cut. No independent test of plasma cutter performance was read, so this entry makes no ranking.

Timeline

1955
Robert M. Gage of Buffalo, New York, applies on 26 July for a patent on an arc torch and process in which a cooled nozzle constricts the arc.1
1957
US Patent 2,806,124 issues to Gage on 10 September, assigned to Union Carbide Corporation; its examples sever plate of several metals using argon at currents of a few hundred amperes. Miller’s history of welding dates Gage’s invention to 1957 and describes it as plasma arc welding that is also used for gouging and cutting.1 2
1968
Hypertherm says Dick Couch and Bob Dean invent water-injection plasma-arc cutting in a two-car garage in Hanover, New Hampshire.1
1970
Couch and Dean file the application for US Patent 3,641,308 on 29 June, as a continuation-in-part of an application of 8 April 1969 that was abandoned. In the same year Hypertherm records its first product sale, two PAC-400 plasma systems in Rochester, New York.1 2
1972
US Patent 3,641,308 issues on 8 February to Richard W. Couch, Jr. and Robert C. Dean, Jr., assigned to Chemetron Corporation of Chicago, for a torch in which a radially inward laminar liquid jet constricts the plasma.1
1985
Hypertherm says it sells its 1,000th system and enters the air-plasma market.1
mid-1990s
Miller states that the introduction of small, portable machines under 50 pounds at affordable prices produced rapid growth in the use of plasma cutters by artists, ranchers, repair shops and others.1
2017
Hypertherm says it introduces the first X-Definition class plasma system; in 2021 it says it introduced a smart plasma system with a single-piece cartridge consumable.1

Where the sources disagree

Who invented plasma arc cutting, and when?

A constricted, wall-stabilized arc torch capable of severing metal was patented by Robert M. Gage of Union Carbide, applied for in 1955 and issued in 1957.1
US Patent 2,806,124
Gage invented plasma arc welding in 1957, a constricted arc process that is also used for metal spraying, gouging and cutting.1
Miller Electric, history of welding
Dick Couch and Bob Dean invented water-injection plasma-arc cutting in 1968; the corresponding patent was filed in 1970 and assigned to Chemetron Corporation.1 2
Hypertherm Associates; US Patent 3,641,308

Gaps in the record (7)

The first commercial plasma cutting torch and the date Union Carbide or any other company put one on the market have not been found.
Chemetron Corporation’s relationship to the founders of Hypertherm is not explained in the sources read.
The patent that introduced air as the plasma gas has not been located, and the 1985 date for air plasma is Hypertherm’s own statement.
Other prior arc-constriction patents cited in Couch and Dean’s patent, such as the Arata patent of 1963 (US 3,082,314), were not read.
No independent test of cut quality, consumable life or rated capacity was found.
Miller’s lens shade figure for plasma cutting was not captured; ANSI Z49.1 and the IEC and ISO standards listed have not been read.
Care has only manufacturer guidance from Miller.