The notion of plasma cutting underwater might surprise newbies, but they quickly learn that it’s nothing new.  In fact, they may discover that they prefer performing plasma cutting underwater to conventional methods.  Several reasons suggest that underwater plasma cutting truly is the way to go whenever possible.

Why perform plasma cutting underwater?  You perform plasma cutting underwater for these and other reasons:

• You have to!
• Less noisy.
• Reduced arc glare.
• Smoke and fumes reduced.
• Reduced heat.

However, several downside factors exist that would suggest open-air plasma cutting.  Those include:

• Rougher cuts.
• No ohmic sensing.
• Limited aluminum cutting.
• Water table difficult to transport.
• Creates a large mess.

Getting Started With Underwater Plasma Cutting

Simply stated, underwater plasma cutting consists of the same elements as open-air plasma cutting, except that the work proceeds underwater.  The base metal, the arc and the torch operate in a pool of water, usually three to four inches under the surface.  Techniques vary and safety precautions must be followed meticulously.

The Water Table

You (or the business) will first need to purchase a water table.  Professional grade water tables vary in size and price, running from $3000 to over $8000.  Think of a simple water table as a shallow swimming pool with tall metal ridges in it.  The ridges – called burn bars – suspend the work metal in the water, therefore allowing easy access to it.  A steel tray underneath moves up or down with the water level.

More sophisticated water tables include a computerized torch guidance system, USB ports for connection to Windows-based laptops and thumb drives and an anti-rust solution.  These more expensive models also have cable management ports, guide rails and the ability to perform repetitive tasks automatically.  Generally speaking, you can cut metal plates up to a half-inch thick (and perhaps a bit more) on a water table.

You adjust the water level by introducing air into a chamber that has holes only at the bottom.  As air collects in the chamber, the steel tray rises, pushing the water level higher. Letting water out has the opposite effect.

Home hobbyists and infrequent users may want to create a water table cheaply with a galvanized tub and a network of steel bars as burn bars.  Cut-up bed frames can serve as a handy source of burn bar metal, and you can purchase ready-made tubs with grills over the top. However, unless your project is a once-in-a-blue moon kind of thing, you will quickly tire of the trouble you have to go through to use your homemade or on-the-cheap water table.

The Torch

A few fancy-schmancy water tables have a built-in cutting torch that rides on rails and may or may not be removable.  For the most part, you can use the torch you already have, or buy one.  Nearly all plasma torches will work underwater, but ones specifically manufactured for use underwater offer the best value.

Experts can always be counted on for a lively discussion about the pros and cons of a product or technique, and this is no exception.  Some advise to never attempt underwater cutting with a conventional torch, while others say to go ahead, but with the knowledge that consumables – primarily the electrode – may burn up faster.  Safety switches within the torch head may also fail prematurely.

Your number one consideration when buying a plasma cutting torch is the thickness of the steel you intend to cut.

Oxy-fuel Cutter

Some folks prefer oxy-fuel cutters for use with a water table.  This differs from plasma cutting in several ways, primarily in that it uses a flame to cut rather than an electric arc.  This process requires quite a bit in terms of equipment, such as:

• An air compressor to provide a bubble around the work zone.
• A tank for the fuel (most common – acetylene).
• A tank for oxygen.

Oxy-fuel cutting and welding uses a very hot, fuel-fed flame to raise the edge temperature of the workpiece to approximately 1800 degrees, causing the edge to oxidize and crumble away.  Some experts call it “controlled rusting.”

Using an oxy-fuel cutter underwater makes sense if you already have everything you need for the process.  Any advantages of this technique over plasma cutting would not justify the expense of ordering the peripherals outright.

CNC

No, it’s not a news network.  CNC stands for Computer Numerical Control, a system that plots coordinates to design patterns in digital (numerical) form.  This provides automated control of machining tools by means of a computer.  CNC plasma cutters follow code supplied by a laptop computer to cut intricate patterns of steel – usually thin sheet metal.

Upscale sewing machines have this kind of technology, which allows them to embroider decorative designs onto fabric.

In an industrial application, CNC machines can perform as high-tech Rembrandts, creating magnificent works of art, or as menial servants in the tedious cut-separate-repeat humdrum of standardized production runs.  For either purpose, a good CNC machine is worth its weight in gold.

What About the Mess?

Water tables have a nasty habit of accumulating slag.  As the metal piece loses mass during the cutting process, the bits of molten metal don’t just evaporate into thin air (or water).  No, they fall through the water to the bottom of the tank.  While you get the same thing with dry tables, it’s worse with a water table.  Not only do solid chunks of metal sink to the bottom, so do (eventually) the fine particles of metal loosened by the cutting process.

Good work habits will require that you periodically clean your water table.  Here, the concept of performing frequent small jobs ranks better than performing less frequent big jobs.  Before buying a water table, consider how well it facilitates cleaning.  Most water tables have lift-out trays whose contents can be dealt with away from the work area, which is a nice feature.  Some of the larger, more industrial-sized tables have the ability to pivot the burn bar grid so that the user can scrape debris into slag buckets.

Most water tables come with drains, but if you procrastinate on cleaning, your handy drain might not be so handy.  It can become clogged with slag and metal particles.  Some users just let the mess go on for many months, adding water if necessary.  You could adopt this practice, but the first time you drop an important part into the murky sludge and cut your finger on a jagged edge, you might change your mind.

The parts you intend to keep even get messy during the process, accumulating metal flakes and tiny particles, not only from the process at hand but from earlier projects.  The smaller the particles, the more easily they can get stirred up from the bottom and live to float again.

Water + Steel = Rust

Water plus steel equals rust.  You can’t escape this reality when it comes to underwater plasma cutting.  Granted, the parts that you keep get yanked out of the water and dried off quickly, but the metal left in the water remains at risk.  The tank and the burn bars never leave the water environment and run the risk of rust and corrosion over a surprisingly short period of time.

Treating the water with an anti-rust agent helps.  A number of water treatment products boast of great results, but a host of mitigating factors may skew the claims.  A compound that seems to make everyone’s list, however, has one or both of these ingredients – borax and sodium nitrite.

Some users simply add 20 Mule Team Borax laundry detergent to their water tables.  This may be effective in controlling rust, but everything gets a coating of white grunge.  Sodium nitrite alone needs a little help, so combining the two seems to be the way to go.

This recipe comes from a plasma cutting user’s forum, based on a 190-gallon water table:

• 4 pounds sodium nitrite
• 3 pounds of 20 Mule Team Borax
• Blue water coloring*

*The water coloring is entirely optional.  This prevents anyone from thinking the water is drinkable.  Any color will do, as long as the water looks unnatural.

Smoking is Bad for You

Plasma cutting in the atmosphere creates massive amounts of smoke.  Water absorbs a substantial amount of smoke during the cutting process, but some of it breaches the surface and rises into the air around the worker.  Some users employ a downdraft table and a water table to eliminate nearly all the smoke.

The variants that determine how much smoke is generated during a cut include:

• The material being cut.
• The cutting amperage.
• Water level.

The deeper the work sits in the water, the better the smoke containment, but you should always submerge the work at the right depth for the cutting process, not so you have better smoke capture.

You Still Need Eye Protection

Plasma cutting generates intense light as well as a powerful rush of electromagnetic radiation.  This includes a wide range of frequencies, requiring the use of a welding helmet.  Arc brightness is dulled considerably when the work is submerged under two to three inches of water, greatly reducing the hazard.

Despite this bit of reassuring information, you should still wear eye protection, as a guard against splashes and mishaps with the cutting process.  You’re still dealing with a lot of heat, and a lot of light and a lot of tiny metallic particles.

 . . . But Maybe not Ear Protection (Maybe)

With atmospheric plasma cutting, noise levels in excess of 115 decibels are common. Move this process underwater, and you get maximum noise levels of 85 decibels, safe for a worker to endure for an eight-hour shift without ear protection, according to OHSA.

But please note that in factory or worksite settings, other components may contribute to the overall noise level of the work environment.  Reducing the noise in the adjacent work area certainly makes things better on a small scale, but doesn’t help if the machinery on the other side of the forklift chargers pounds away at 100+ decibels.

Less Heat, but More Mess

Cutting underwater reduces heat in all phases of the process.  For the most part, that’s a good thing.  Less heat means less warping and a much shorter cool-down period for finished pieces.

But that also means rougher cuts and more slag.  In some cases, the metal cools down so quickly, molten metal solidifies into little knots that may or may not fall off the work metal.  Depending on the designed purpose of the finished metal piece, that may require more post-cut finishing.  Whatever doesn’t stick to the metal falls into the water and settles to the bottom.

The thinner the work metal, the sloppier the cut when performed underwater.  Metal less than a quarter-inch thick is not suitable for underwater plasma cutting.

Ohmic Sensing

Ohmic sensing uses resistance to determine the thickness of metal.  With CNC automation, determining the thickness of the work metal helps guide the torch head to the surface so that it can begin the arc at the right spot.  Without specialized equipment, ohmic sensing cannot be used underwater.

Aluminum

Aluminum poses a challenge for plasma cutting, regardless of the method. As difficult as it is to safely cut aluminum with atmospheric plasma cutting, it’s even more frustrating underwater. In some circles, experts consider it unsuitable for use with a water table.

When submerged in water, aluminum releases hydrogen gas in tiny bubbles you can see.  The larger the piece of aluminum, and the longer it remains in the water, the greater the release of hydrogen.  But the real trouble erupts (sometimes literally) when aluminum waste pieces, slag and dust accumulate in the water tank.  As long as they’re swimming around down there, they’re releasing those pesky little bubbles.

For safe aluminum cutting underwater, follow these tips:

• Never cut aluminum that has been alloyed with lithium.
• Raise the water level.
• Cut just below the surface.
• Remove all work as quickly as possible.
• Remove waste pieces as quickly as possible.
• If you work with aluminum frequently, clean the water table frequently.

Commercially-sold water filtration systems can help remove hydrogen gas from the water.  They keep the water in motion with multiple small jets and capture fine particles by means of a centrifugal separator.