Plasma over water

[Wayne]

I have heard that cutting over a tank of water will give cleaner cuts and I have som questions.
1 How close should the water be to the cutting table?
2 How deep should the water be?
3 How much does it help?
Thanks for your help.
Wayne

[Sberry]

It makes less mess when cutting. I think thats the point, blow the dross into a pan of water.

[Bob Sigmon]

I was told tonight by my welding instructor that plasma cutting over water makes the water a hazadous material. They are putting in a plasma cutter that has to cut over water and the state requires them to put the water into a HazMat barrel to dispose of the water. It's going to cost over $400 a barrel to dispose of. I'm not sure how often you need to change the water but I'd make it last a long time. He did say that you shouldn't dispose of the water in a waste drain, due to the HazMat, and I certainly wouldn't want to put it into my septic field.

Not sure what causes the problem. He siad that it was the plasma vaporizing the metal, or some such thing. Anyways, just some food for thought.

Any Scientist/welders out there?

[HAWK]

Wayne,

Plasma over water is great for automatic CNC cutting tables. It can be a pain to do manually and is probably more trouble than it is worth unless the water is several inches below the cutting table and is used just to catch the sparks and slag. On the automatic table the water is often times nearly toucing the base metal during cutting so when it drops it hit the water quiclkly for cooling. This picture is with an O/A torch, but the plasma is also used here on thinner material.

[timw]

Page 3 of my Spectrum 625 owners manual-
"EXPLODING HYDROGEN hazard. When cutting aluminum under water or with the water touching the underside of the aluminum, free hydrogen gas may collect under the workpiece.
See your cutting engineer and water table instructions for help."

[Pile Buck]

Hi Wayne, I built a tank for my cnc table, mostly because I had nothing better to do, but really doubted it would work as well as I had heard. I poor boyed it! Big mistake, it works fantastic. I wish now I would have done a better job. I have my water level 7 to 8-inches from the top of grates. My table is 4-feet wide, but the tank is only 3-feet wide, so when the torch cuts in an area that is not over water I can really tell the difference!

[Stick rod]

timw is right.Was helping out a buddy who has a CNC plasma burning table with water.We were cutting some aluminum and there was a build up of gas to the point of causing an "explosion" there was water and junk blown as high as the ceiling in his shop.No one was hurt but there was one big mess.

[Scott V]

Some more info from the Yahoo plasma cutting site. Jim has worked for Hypertherm for a while and owns two hobby type tables. Plus he was been around at least a couple more.


Water tables were originally designed to be used for plasma and oxyfuel cutting to capture the small, very hot particles of metal that are ejected during cutting. The smaller particles are lighter than air.....therefore rise as smoke........however, during cutting they are forced downward by the plasma gas flow.......and if they come in contact with water, they will cool, become heavier, and will sink. Many users keep the water level far enough (maybe 6 to 8 inches ) below the plate...which can minimize splashing and wetting of the plate. If the water is exactly at the bottom edge of the plate....smoke is almost 100% eliminated as none escapes from the sides......and if you cut completely underwater (top of the plate 2 or more inches submerged) additional benefits of noise reduction and ultr-violet glare reduction can be realized. In som high power underwater applications dye is added to the table water to almost completely eliminate ultra-violet.

Here are some things to consider:

Water 6" under the plate (so no water splashes the cut edge).....or no water at all will produce the best cut quality (smooth finish, minimal dross, softer edge) on most materials.

When water splashes the cut face....or the cut face is submerged a lot of effects can take place.Water, when introduced to the electrical process and the heat of the process can produce small amounts of hydrogen through disassociation and electrolysis......the hydrogen immediately pops (like a small firecracker) and nudges the arc.....creating a rough edge. The hydrogen is usually completely consumed...and does not present a danger.

When cutting aluminum, much of the slag from the aluminum is in the form of aluminum oxide. When particles of aluminum oxide fall into the water in the water table...they absorb oxygen from the water.....which leaves hydrogen. If you look at a water table hours after cutting aluminum you will notice tiny bubbles rising to the surface.....these are hydrogen! Generally....this hydrogen just disipates into the air.....however if there is a large plate sitting on the water table with an upward bow in it.....the hdrogen bubbles can create a large pocket of gas under the plate......which could certainly be dangerous the next time the plasma torch fires!

It is often suggested that an aerator made from submerged pipes with many holes drilled in them.....and compressed air applied...is an adequate remedy to dilute hydrogen produced by aluminum oxide particles.

Most small plasma torches will work under water......but most manufacturers do not recommend using them for underwater cutting. Follow the manufacturers recomendations. Underwater cutting is usually reserved for industrial plasma torches that are designed specifically for underwater cutting.

There are a few cases where submerging the metal can improve quality. If you want to eliminate the discoloration (oxide layer) present when cutting stainless steel.....you can submerge the stainless.....and use nitrogen instead of air as the plasma gas. The edge of the stainless should be shiny if this is done....as oxygen is completely eliminated from contacting the cut face.

Also....I have worked with a few users that were cutting very long, thin parts and were experiencing a "camber" (part was bending to the left or the right due to heat induced stresses)....and the cooling effect of the water seemed to eliminate this issue.

Personally, I like to have water 8 or more inches below the plate, and also have a downdraft system with a filter device. The water takes care of the majority of particles and the smoke eater takes care of the rest. With the water that far below the plate I do not worry about trapping hydrogen when aluminum is cut.

Best regards, Jim Colt

[Pile Buck]

Hey Scott, according to this Jim Colt, I guess I can set back and say I lucked out this time by being a cheap a$$, not wanting to notch the legs of my table, and also not wanting to use more than a full 4’ x 8’ sheet of 16Ga.

[pejenkins]

Question for Pile Buck

What brand CNC table did you get ?
Can you share some of the particulars about it ?
What do you cut with it, mainly ?

Would appreciate the info...have often thought it may not
be a bad thing for a retired guy to have (along with my welders). Can a fellow make money with those things ?

Thanks!!

PJ

[Pile Buck]

Hi PJ, my cnc shape cutter is a “Tracker”. Made in London Ontario, Canada.
Ken Glendenning is the owner. One of the nicest guys you will ever meet! I flew back to London to see it work before I bought it. Goes against my nature to spend that much money sight un seen! Whenever I would get into trouble with it, Ken would walk me through it time and time again, with no attitude! I bought the 4’x 8’ X 3-inch model. I bought it for entertainment only! Never intended to make a dime with it. I had to quite working at a fairly young age do to an industrial accident, I never have been much of a drinker, so no need to hang out in the bars, LOL! 99.8 % of our cutting is artwork. I can play with the numbers and get it to cut with in .007. Not to bad for what I would call a Low-end unit. About a year ago a guy calls me, said he got my phone number from Ken. He is thinking about buying the 4’ x 4’ model. Paul lives about 2-hours north west of me. He wanted to see mine work. After comparing the different units on the market he bought a Tracker. I ended up teaching Paul the cutting software, Corel Draw, and Auto Cad. Paul is retired, and wanted something to bring in a little extra money and entertain himself.
I talked to Paul 3 or 4 months ago, he said he is averaging $15.00- hour profit. If you have any more questions, fire away, I got no secrets!
http://www.trackercnc.com/

[pejenkins]

Thanks, Man.
I'll go the website and if any questions come up I'll give you a hollar.

Thanks,

PJ

Miller AC/DC Thunderbolt[
MM 210 and spoolmate