Plasma-arc-cutting

speedy and economic

Plasma-arc-cutting is a widely used process.

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It is found useful in many different industries

It is capable to provide clean cuts in ferrous and nonferrous metals.

Plasma-arc-cutting has definite advantages over competitive processes, like versatility as to materials cut, and higher cutting speed.

When used within its range of applications it is more economic.
It does not need preheat, due to instant startup.

Plasma-arc-cutting produces high quality cuts and narrow heat affected zone.

Its limitations refer to the maximum thickness it can cut, a wider kerf or gap produced, and possibly non square cut.

Also new spare torch parts may be needed frequently, making the equipment more expensive.

And of course electrical dangers must be taken care of properly.

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In the past we dealt with some aspects of Plasma-arc-cutting in several issues of our monthly Practical Welding Letter, available at no cost to subscribers.
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Interested readers may find pertinent information in the following issues:
PWL#003, PWL#043, PWL#063, PWL#076, PWL#103, PWL#108.

The Article that was published in the Welding Journal, March 2005 was reported in PWL#20.
The Great Debate: Plasma Cutter or Oxyfuel Torch?
Its original link is broken, it is now downloadable from:
millerwelds.

Plasma refers to an ionized gas conducting current.

Plasma arc is constricted, meaning that it is produced in torches with a constricting orifice.

Plasma gas passing through the electric arc is heated instantly to high temperature, sufficient for melting locally most metals.

Different methods of arc initiation are in use.

A pilot nontransferred arc, between electrode and torch, provides a conductive path to the workpiece through which the cutting current flows.

Alternatively a high frequency spark between electrode and workpiece establishes the ionized path.

A third method uses a special torch with movable electrode or nozzle which can be brought to temporary contact or shorted and then separated to establish the arc.

Torches for Plasma-arc-cutting

Torches can be light and handy for manual use.
Torches for mechanized use are be more sturdy and use water cooling, for operation at over 100 amperes.
The orifice diameter limits the maximum current usable.

Different applications require torches with suitable characteristics.
In general higher power levels can cut thicker metal at higher speeds.

Due to the high temperatures, various parts of the torch wear rapidly and need frequent replacement to maintain quality cuts.

Power sources are the most demanding and costly element of every Plasma-arc-cutting setup.

If portability is a necessary consideration special switch mode power sources are available offering high efficiency and small size.

The process of Plasma-arc-cutting operates with direct current with electrode negative polarity (DCEN).

Gas flow, water supply and power level can be controlled for each application to provide the most acceptable results.

Interlocks automatically stop operation when necessary to avoid damage to the system.

By comparison with oxyfuel gas cutting (OFC)(see our Cutting-torch page) the following indications can be summarized.

Up to 3 in. (75 mm) of carbon steel plate thickness Plasma-arc-cutting (PAC) is faster.
 
Up to 1 in. (25mm) PAC speed can be up to 5 times that for OFC.

Over 1.5 in (38mm) PAC selection may depend on other factors like cost of equipment and utilization for other thinner plates if that is the case.

As the speed advantage of PAC decreases with thickness, Plasma-arc-cutting stations are usually reserved for work up to 1 in (25 mm), while thicker plates are cut by OFC.

Detailed Tables showing operating data and cutting speeds are offered by equipment manufacturers as a service to customers.

For more information on a few derivative variants of the basic Plasma-arc-cutting process, see the following article:

Cutting processes - plasma arc cutting - process and equipment considerations
TWI.

An Article on Smart Plasma Arc Cutter was published (3) in Issue 165 of Practical Welding Letter for May 2017.
Click on PWL#165.

Watch the following VIDEO (no recommendation intended):

CNC Plasma Cutting Machine Ride Along

from:
https://www.youtube.com/watch?v=f5TwzRW_DtY


Torches for Plasma arc gouging

Positive gouging refers to the removal of any unwanted feature protruding from the surface of the part. Negative gouging refers to removal of material from the surface to produce a depression.

Standard and best practices for plasma gouging are only lately starting to develop.
This process offers a safe, cost-effective, efficient option for when there’s a need for gouging.

One of the claimed advantages of plasma gouging vs. other processes stems from its low-fume characteristics, appreciated with increasingly tightened environmental requirements.

When plasma arc gouging is done with an inert gas in the plasma gas mixture the fume reduction is very significant if compared with other gouging processes.

According to the Welding Handbook, the recommended plasma gas for all gouging is a mixture of argon with 35% to 40% hydrogen from a gas mixing device.

The secondary gas is argon, nitrogen or air, depending on cost and environmental requirements (fumes) and on the required finish of the gouge.

The reasons for the slow adoption of plasma gouging in the industry are attributed to a lack of understanding of both how the technology works and how it can be adapted to applications-based solutions.

The quality of a gouge can be evaluated by determining if an operator has removed the correct amount of material in a controlled manner.

It is recommended that the width of a gouge be greater than its depth and that the profile be symmetric.

Furthermore slag, if any, should be easily removed, as well an any occasional surface hardening due to the process. Surface texture should be smooth.

Determining if a certain gouge geometry is suitable in any given case depends on the application itself. Welding specifications may provide examples of typical dimensions.

Plasma is essentially a superheated, electrically conductive, ionized gas, generated within a torch when a gas stream flows through an electric discharge (non transferred arc) between a nonconsumable electrode and a constriction nozzle producing hugh current densities and high energy concentration.

Like other fluid systems, plasma constricted in its crosssectional area increases its velocity and resultant energy density.

Typical velocities in the cutting nozzle bore reach supersonic speeds, though this parameter can vary significantly depending on the bore design.

There is a fundamental difference between nozzles designed for plasma cutting vs. those for gouging: suitable nozzles must be selected for the use required.

In theory, as the plasma stream becomes more constricted and velocity increases, the higher its cutting capabilities are in terms of material thickness, speed, and kerf minimization.

In plasma arc cutting the heat is directed on the workpiece to cut and the molten material is thrown away: therefore the torch is not spattered nor affected by the radiated heat.

For gouging however, the arc is on top of the plate and much of the heat is radiated back into the torch head. The best material for a gouging torch head is a special high temperature glass filled polymer, often with the auxiliary protection of fiberglas-silicon insulators.

As for the bore shape, one looks for less constriction, resulting in slower gas velocity but higher volumetric flow rate.

In gouging, a diffuse plasma stream coupled with a high shield gas flow rate provides the correct combination to produce partial melting and its removal.

The plasma gouging nozzle bore diameter will be greater than twice the diameter of a cutting nozzle bore, but much shorter in length.

Additionally, there will be a sizable counterbore in the nozzle to provide relaxation of the flow.

A shielding gas at high flow rates is necessary to remove the molten material and create the gouge.

Inert gas torches are required for aluminum and stainless steel gouging. Dual gas torch construction is best because the secondary shield further protects the gouging zone from atmospheric contamination.


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