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Why cut under water with Plasma?

March 25, 2013

That’s Plasma?

At the 2012 IMTS show I was again surprised to hear so many people as this question: “That’s plasma? So what’s the reason for cutting under water?” And we’re not talking about just cutting on a table that has water below the plate. Under water CNC plasma cutting actually submerges the plate below 2 to 4” of water, so the torch tip and the entire arc are submerged, just like we were doing at IMTS. It’s nothing new, we’ve been cutting under water for 40 years, which is why I’m always a little shocked when someone has never even heard of it, let alone doesn’t understand the reasons why. So here’s a quick run-down of the benefits, and disadvantages, of plasma cutting while completely submerged.

Noise Levels

The first thing you notice when cutting under water is that it dramatically reduces the noise emitted by the plasma arc. Noise levels from dry plasma cutting can be as high as 120 decibels, requiring hearing protection for the operator and anyone else working near the machine. By submerging the plate, the noise level can be reduced by as much as 40 dB, and for most cutting will be well below the 85 dB level allowable for continuous exposure. In a small shop this can affect the working environment in the entire building, and make for a much nicer working environment.

Plasma noise levels dry and under water - Click for larger view

Arc Glare

Next, you will notice that under water plasma cutting significantly reduces the brightness of the arc. When cutting on a dry table, the arc is so bright that anyone in the area should be wearing dark safety glasses of an appropriate shade, or a welding helmet. Looking at the plasma arc, either directly or at an angle, can cause serious eye damage. But when that same arc is under a few inches of water, all you see is a bubbling reddish-purple glow. Operators should still wear eye protection, but dark shades are no longer needed, making the entire working area safer and more pleasant.

Smoke

When plasma cutting under water, the water will absorb the vast majority of the plasma smoke. Instead of billowing clouds of smoke that have to be captured by an expensive dust collector, a water table will usually emit an occasion little puff that quickly dissipates.

Emission of fume when plasma cutting mild and stainless steel
Material Thickness Dry Cutting (lb/hr) Under Water Cutting (lb/hr)
Mild Steel 5/16" 2.65 - 3.44 .01 - .05
Stainless Steel 5/16" 3.97 - 5.29 .03 - .07
Stainless Steel 1-3/8" .24 - .45 0.003

Heat

Cutting under water keeps the entire plate cooler, which reduces warping of the cut parts and the skeleton, or scrap plate. Heat distortion occurs when you heat up one part of a plate and then let it cool, causing uneven expansion and contraction. Most people want their cut parts to remain flat, and when the scrap curls up, it can cause problems by interfering with the machine. The cooling effect of the water also means that you can immediately handle the parts after cutting. When cutting on a dry table, the parts can remain hot for hours, making them more difficult to handle, and also posing a safety risk.

Table Cost

Also consider the cost of the initial investment. If buying a large CNC plasma shape cutting machine, the cutting table can be a significant percentage of the entire system cost. A water table is a simple design, easily installed, with little need for maintenance. By comparison, downdraft tables usually cost 40% to 60% more than a water table, and also require an expensive dust collector to capture the smoke. The table is much more complex because it needs a system of dampers to pull smoke from one small area at a time. It also requires installing custom made ductwork to connect the table to the dust collector, which also takes up more valuable floor space.

Altogether, you could easily wind up paying $50,000 to $100,000 more for a dry table, depending on the size of the system.

The Down Side

There are trade-offs with every decision. Water tables are not the perfect solution, or else everyone would be using them. Here are some things to consider about a water table that might put you off a little.

Cleaning – water tables will accumulate slag and sludge at the bottom of the table, and require periodic clean out. So do dry tables, but it’s a little messier with a water table. A “tilt-frame” table from JB Technical Services is more easily cleaned by lifting out the slag buckets. But after a number of years of operation, even those tables will need to be completely emptied and thoroughly cleaned out in order to maintain proper operation.

Water Disposal – if you do empty the water table, you might have a “toxic waste” on your hands. Depending on what you cut and where you are located, you might have to pay someone to pump out the table and haul away the waste water and sludge.

Shop Mess – pulling wet cut parts out of a table all day is going to spill some dirty water on your floor. White carpeting is not recommended!

Dirty Parts – The fine particles suspended in the water will settle on top of the plate while cutting. Most shops with a water table also install a hose for the operator to use to spray off the parts after he lowers the water.

Water filtration systems, like the Ebbco GRS can keep the water pretty clean and eliminate some of these hassles, and may be worth looking into if you decide to go with a water table for your CNC plasma system. 

 

Posted in Cutting Systems , Tagged with Plasma, Safety, Water Table

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