[snaggletto]

Hello,
I'm needing to build a water tank to put under my plasma table to catch slag and hopefully reduce smoke etc...

1. How far below the surface of the material should the water be?

2. How deep or how shallow can the water be so that the tank won't be so heavy?

3. Any specific design recomendations or plans/drawings available?

Thanks for any input.

[svenakela]

Hi,

As far as i know, plasma cutters are moist sensitive.
I know a simple second solution (I'm not saying yours is bad, it's just a second way ), make a dish out of thin metal sheet with 5-10cm (2-4") high sides and put a couple of buckets of sand in it. When you cut a lot you can spray some water on the sand.

Cheers,
Sveb

[whiteriver]

I'm not so sure about moister sensitive. Where I buy most my steel they have a 8'x20' plasma cutter that if I recall correctly has a 2" cut capacity. Anyway to keep noise fumes and the light down they cut under water. Have a large tank with a pump on it. Lay the metal down on the finger table then hit the switch. Tank fills with water. It produces a few bubbles and works great. The operaters only problem is you can't see the cut so you don't know if you have a bad tip till your done. I have watched it work a few times. Just bough a buch of plased circles in 3/8" thick for a current project and they look good.

Donny

[Al_The_Man]

The Plasma water tables I have come across cut around 1/2" under the water , I don't think the depth under the plate matters, I believe they use the same method that submarines use for quick ballast dumping. i.e. compressed air tanks to refill quickly between jobs.
Al

[metlmunchr]

The moisture sensitivity is with the air being supplied to the plasma cutter from the compressed air system. They're very sensitive to moisture in that respect, but not sensitive to moisture in their surroundings.

[snaggletto]

Hey,
Maybe I'm mistaken. I was told to put the water tank under the material. In other words, the material sits 3-5" (or whatever) ABOVE the surface of the water at all times. I know some of the big/professional tables cut under water, but I'm talking about cutting above the water. Will this be of any benefit? Maybe I just misunderstood.

[Al_The_Man]

Yes, your right, moisture in the gas line, especially if you are using compressed air shortens the life of consumables , and they are not usually cheap!
Al

[metlmunchr]

Having water under the material should help a lot. I know it does on my oxy-fuel burning table. Most commercial plasma tables in the lighter capacities are set up with a draft table to suck away the dust and fumes. Most folks i've seen mention using a setup like this on a home system are trying to do it with too little fan, so it ends up ineffective. There's a definite threshold for perimeter capture velocity for a device like this (similar to a commercial range hood in that respect) and if you're much under that velocity the stuff just goes wherever it wants to even if the fan is running.

[Ries]

I have been running a water table on my plasma cutter for over 10 years now, and I highly recommend it. I built a steel tank about 6" deep, out of 1/8" plate. I took two pieces of 3" angle, the length of the tank, and plasma cut slightly angled slots in them to recieve 1/4" x 3" flat bars the width of the tank, about 2" apart. Slight angle off of straight up and down allows slag to fall off, and makes it easier to drag sheet across. Fill up the tank with water to just about level with the top of the slats. Yes, you can run the metal you are cutting slightly below water level, but I usually try to run it a 1/4" or so above water level. The water catches all the sparks, which are actually gritty slag. And it catches all the red hot drops from interior cuts, which otherwise you would step on, or might catch your floor on fire. It cuts down greatly on smoke.
Make sure you put levelling feet on your water table, especially if you dont have auto torch height, but even if you do. You want the table nice and flat in relationship to your torch. Also make sure you put a hose bib- a faucet, if you will, at the low point underneath the table, so you can hook up a hose and drain the table once or twice a year. With a wire mesh screen over the drain, so it doesnt fill up with mud. The water will be horribly cold when you have to reach in and fish out a piece that you wanted that fell in. And it will fill up with nasty mud like slag stuff, which is one of the dirtiest substances on earth, when you have to clean it out. But all of that nasty stuff would otherwise be on your floor and in the air in your shop, so it is definitely worth building one. Water is heavy- a pint is a pound, the world around. No two ways about it. But a water table will do 75% as well as a 5000 dollar fume extraction system, and only cost a couple of hundred bucks to build.

[Al_The_Man]

- a pint is a pound, the world around. .

Only in the US. Most everywhere else its a pound and a quarter (20ozs).
Al

[Txfatboy]

Would the table work as well if it were only 3 " deep and also does it miniumize warping even if you are cutting above the water?

[thielert]

Asking what Txfatboy just did, only different..

Is the need for an auto height torch as great when cutting above a water table?
Is an auto height torch needed at all when cutting below water?

Will your common Hypertherm 600 plasma cutter work under water or is this discussion specific to the industrial machines?

Thanks, I love this discussion, so little info on plasm cnc out there. I'm considering adding plasma to my 4'x4' table. Just need to run a 220v circuit to the garage.

TT

[Ries]

3" deep should work just fine- you just have to clean it out more often, as you have less space for slag and interior drops.

As far as warping goes- when you are cutting below the water, with the material actually underwater, then it should have some affect on cutting down warpage. But the kind of table we are discussing is mostly to catch sparks and hot drops, and cut down on smoke and mess in the shop. It will not affect warpage at all.

I have auto torch height on my machine when I cut just above the water, but Auto torch height is just as handy when cutting below water- 50,000 degrees will still warp steel a little. So unless your sheet is perfectly flat, or say, 2" thick, I think you still want auto torch height. As I have said before, I find it essential for gauge thicknesses up to at least 3/8" plate- it prevents crashes, ruined parts, and interrupted cuts.

My plasma machine uses a Thermal Dynamics Pak 75 with a machine torch- not the cheapest, but certainly not a big industrial machine. I have seen cnc machines which just clamp in the hand torch from a smaller power supply- it works, and all of the things we have been discussing apply equally to an 800 dollar machine as they would to a 30k Hi-def unit.

[Johnuk]

I have been reading quite a lot recently about high power plasma systems used in ship yards and such lately - The 1000A kind of system.

These systems have a cutting ability of up to 150mm in plate. Cutting with water injection, to define the beam, cool the torch and the work, reduces their capacity to 75mm. Cutting underwater reduces it futher.

Remember, water has a massive energy absorption capacity when it turns from a liquid to a gas.

If there is steam coming off the torch, it's dropping a substantial amount of power into boiling the water that's coming into contact with the plasma and work.

Cooling the work will help prevent warping, but it also turns the work into an excellent heatsink. The torch needs heat to cut the work.

This begins to raise the same questions people have with soldering electronics. It would seem sensible to use a lower wattage soldering iron on SMD components, but in reality, the lower wattage means it takes longer to heat the parts to be joined to the correct temperature - if they ever reach it! During this phase, the parts are being saturated with heat; causing extended heat exposure and increasing the risk of damage.

It's much safer to use a higher wattage soldering iron, touch the parts for a second, heat them to the right temperature and solder them. In comparision, you put less heat into the work because the parts that need the heat get it quickly and then the heat can be removed; instead of the heat having enough time to be conducted away almost as fast as it's added.

In a similar way, running the torch at a higher wattage with higher feed rates will likely cause less warping than running it at a lower wattage and lower feed rates. The work only needs to reach a certain tempeature before it melts, but it takes time for the heat to conduct through the work. If you can heat the work to the same temperature quickly and then move on, you give the heat less time to be conducted away from the cut in progress.

Materials handling can be an substantial cause of warping. Steel yards often handle materials in such a way that they sag in their middle during movement.

In ship yards using higher power systems, it was estimated that for every kilogram of material cut away, around 14 grams or more of metal vapour became airborne.

Not a problem if you're cutting some sheets in your workshop. If you're cutting away kilos over a single cycle, that vapour is a serious health concern. With stainless steel containing heavy metal alloying elements like Chromium and Vanadium, the vapour is accumulatively toxic.

Saying that, plasma cutting is still much safer in terms of vapour emission compared to gas cutting using natural gas or other gases like MAPP. Gouging very sharply increases emissions. Carbon arc gouging is the most concerning with well over 40 grams of emission per kilo removed.

The easiest way to clean the vapours from the machine's output is to submerge the work. In fact, I would expect it to become almost mandatory in industrial work places soon. If the work is not submerged, the table will likely require some form of HVAC system equipped with a particle filter on it's output.

A few tens of grams sounds like nothing, but imagine how much damage those grams would do if they were uniformly lining the insides of your lungs.

Aside from that, the water blocks a lot of the UV and sound emission from the torch. From what I've read, a 1000A system produces quite a lot of these. So much so that ear protection is needed.

I think using a subemerged table to lower warping in a home enviroment is perhaps a bit extreme. Experimenting with feed rates and power ratings might be a better place to start.

I think Miller actually publish a graph with at least one of their plasma cutters showing the maximum feed rate they recommend for a given material thickness.

[InventIt]

Johnuk, Very informative post! Always wondered how harmfull that airborn vapor was.

[traveler]

This is all good stuff to learn. I have wanted to better control the dust, smoke and fumes from my plasma table for a long time. I did however talk to my welding supply store about underwater plasma cutting and they called Hypertherm and had me talk to one of Hypertherm's tech's. The tech told me to keep my Hypertherm 900 with machine torch as far from water as I could. Sounds to me like he suffers from paranoia if so many actual users are having sucess and not damaging their power sources.

never set a pace that you can't maintain

[Johnuk]

This is all good stuff to learn. I have wanted to better control the dust, smoke and fumes from my plasma table for a long time. I did however talk to my welding supply store about underwater plasma cutting and they called Hypertherm and had me talk to one of Hypertherm's tech's. The tech told me to keep my Hypertherm 900 with machine torch as far from water as I could. Sounds to me like he suffers from paranoia if so many actual users are having sucess and not damaging their power sources.

never set a pace that you can't maintain

It's probably a case of company talk like you say. If they advised their customers that it was okay to use their gear around water, and a customer then electrocuted themselves, they'd almost certainly be liable for legal action.

There'll be a million and one safety regulations that a piece of high power consumer electronics needs to pass before it can be used around water.

Something I've been thinking about recently was the work suspension method. I know a lot of guys go for slates on a DIY table. Industrially, these can end up getting damaged too quickly, so I've seen them using other ideas.

One was rollers. Essentially, a circle of plate held up vertically through it's centre, allowing it to rotate - like a wheel dressing tool. This way, contact is minimal, and you can just rotate the rollers to get a new contact point.

Another, for underwater cutting, looked kind of strange - almost like a T slotted table made out of green plastic or ceramic. They didn't provide any details on it, unfortunately.

I was thinking, although plasmas are very hot, the way in which they transfer their heat is much more controllable than the way a gas flame would.

For the plasma to conduct energy to the work, it must be electrically referenced to the plasma it's self. If the table is metal, the table them becomes bound and so it will become damaged when the plasma makes contact with it.

However, if the table it's self could be electrically isolated from the work piece, it should last a great deal longer.

I've been spending some time thinking of ways you could practically implement the idea.

One would be by resting the work on stone. You can have stone cut into slab like strips. I figured that was probably a bad idea since the heat might cause the stone to burst.

Another would be to use a castable refractory. Theoretically, you should be able to get a very accurate table with an accurate mold. High aluminium oxide content refractories can last well over a thousand degrees Celsius for prolonged times - which you don't find on plasma tables, the longest being a dwell time of a few seconds perhaps.

If you didn't want to use messy refractories, science labs use rigid heat proof tiles for bunsens and hot equipment. The ones we used in my secondary school a few years were made from a fiberous, crystalline like material. They were also quite strong. I can't recall ever seeing one broken. Which, for a public boys school is a seriously impressive feat!

The tiles could be cut into small sections to act as supports or used as they come - they're about A5 sheet length but square. I doubt they're very expensive if they let kids touch them.

The last idea I had was to use a form of ceramic based paint, glaze or wash on a normal table. I did a bit of quick searching and found a paint made for motor bike exhausts that seems interesting.

http://www.bpmsports.com/hpc/

Although, I'm not sure about what it actually decomposes to when heated, or what it's based on. It may contain metal particles so you'd need to contact BPM.

A ceramic glaze or wash seems like an interesting idea because it should be very easy to do accurately and make it tidy looking. Artists who work with pottery will be able to help you learn more about them. Given the character of most artists, they'll probably help you for free and then colour code the table.

A glaze could be as easy as taking the slates off the table, painting them with the glaze and then putting them in a pottery kiln. You could do it a few times to build up a thick layer on them.

The only thing to think about with these ideas is that when the torch cuts a part away it shouldn't be able to fall onto any metal surfaces exposed on the table. Also, if the torch or work damages the table, it should damage it in such a way that the rest of the table can still keep the work off any metal substructure within the table.

I am not fully aware of these ideas being used at the moment, but I know that a plasma transmits many times more of it's energy to metal that is electrically referenced to it.

If you're building a table, or can modify you current table, you might like to give one of them a go.

A quick way of trying the idea out would be to use some of those small easy mosaic effect tiles girls love, the kind that come preattached to a flexible mesh - you can cut them to shape or remove individual tiles. These things -

http://www.mosaicsupply.com/advendrkblue.jpg

Their small size should help reduce the chance of them cracking due to point heating.

[Ries]

Sounds good in theory, but when the rubber meets the road, your ideas are a little impractical. For the plasma cutter to work, the metal must be grounded. All your glazes, ceramics, and refractories dont conduct electricity, which means that you have to clamp your ground lead on the metal every time you cut out a piece.
This is annoying, time consuming, and hard to get little pieces of metal to sit flat. What about after the interior cut piece comes loose, and it is no longer connected to the piece of metal with the ground clamp on it? What about when the weight of the ground clamp wire itself pulls the little piece of scrap right off the table, in the middle of the cut, and ruins the last piece of that material you have, and a full sheet costs $500?
Every commercial table I have ever seen was made of steel- for a reason- it conducts electricity. That way, you can throw a piece of metal down on it anywhere you like, and begin cutting. Much more efficient and effective. The slats on my table last years and years- yes, they get slag on em, but it doesnt affect anything. About every year or two, we pull the slats out, and throw em up in the air and let em fall on the concrete floor, and slag falls in every direction. Cleans em up pretty good. I havent replaced the slats in over 5 years, and even when I have to, its only maybe a hundred bucks worth of material.
Now my waterjet cutting subcontractor that I use- they replace their slats every few months- but they are routinely holding tolerances in the thousandths of an inch, for aerospace and other critical applications. But unless you have a 30 thousand dollar Hidefinition plasma power supply, your plasma cuts are never going to be accurate enough to justify that degree of worrying. I can hold a 1/16", probably 1/32" with my optical trace unit- and everything that comes off the plasma cutter is going to have to have the slag chipped off, and probably be sanded or ground anyway.
Trust me, metal slats made from 3/16" flat bar is the way to go.

[Ries]

Oh, and one more reason why steel slats are where its at- lots of times, you need some special work holding jig, if you are cutting things besides just flat sheet. With a steel table, you just tack weld down your jig, and your piece is held in place securely. When done, you grind it off. I had a job a few years ago where I needed to cut lengthwise slots in 1/2" square tubing- a welded steel jig, and vise grips, enabled me to cut hundreds of lineal feet of it with very little rejects- But without a firm jig, the square tubing would have danced all around, and the slots would have been a joke.
Same thing for mounting a rotary welding postioner on your plasma table, and doing cuts around moving pipe and the like- you need to be able to fasten to the table, for registration, and nothing is easier than just migging the jig down.
The water below still catches 90% of the sparks and slag.

[Johnuk]

Oh, and one more reason why steel slats are where its at- lots of times, you need some special work holding jig, if you are cutting things besides just flat sheet. With a steel table, you just tack weld down your jig, and your piece is held in place securely. When done, you grind it off. I had a job a few years ago where I needed to cut lengthwise slots in 1/2" square tubing- a welded steel jig, and vise grips, enabled me to cut hundreds of lineal feet of it with very little rejects- But without a firm jig, the square tubing would have danced all around, and the slots would have been a joke.
Same thing for mounting a rotary welding postioner on your plasma table, and doing cuts around moving pipe and the like- you need to be able to fasten to the table, for registration, and nothing is easier than just migging the jig down.
The water below still catches 90% of the sparks and slag.

Sorry Ries, I think I perhaps implied a bit more complexity than I was hoping. Rather, I only really wanted to present a number of different ideas.

As to the table not being conductive, a series of points on it could be made so. For example, patches on the slates could be exposed for conduction. The idea behind my thoughts was that if the majority of the table isn't electrically conductive it's wear should be substantially lower. Usually, the last cycle a code will do is to cut the part free of the plate. So, once the part disconnects from the rest of the conducting sheet, it is finished anyway.

The big heavy clamps that are usually supplied with a welder or plasma torch could be replaced with sacrificial clamps that are designed not to pull the work off the table.

Again, I see what you're saying, I just thought I'd suggest the ideas as a possibility.

It's all kind of 'in theory' stuff for people to think about. I imagine that as the power of the torch begins to rise, the problems get more pronounced, as dwell times and such would begin cutting much deeper into the supports.

I guess I was trying to isolate conductivity from support so that the two don't effect each other.

Do you know what it is about your table that limits it's accuracy? I mean, is it the positioning system, the table, the torch it's self or so on?

I know that the guys who rebuild machine tools or CNC'erise Sherlines can quite often get 0.02mm accuracy, so I'm guessing it's something to do with the torch rather than the positioning system?

Best wishes,
John