Plasma Cutter questions

I recently found out that a friend of mine has had a Miller Spectrum 300 plasma cutter (25 amp) sitting in his basement for a few years --
he bought it to have an employee cut some holes in some small stainless steel boxes to mount electronic components for a job for his factory automation business. Unfortunately, the employee could not get good results cutting the stainless, so the plasma cutter has been sitting in his basement ever since. (He sent the boxes out to a place that used CNC laser cutting.)
Needless to say, I tried to think of a tactful way of saying "I'LL TAKE IT!!!" but before I could think of one, he asked me if I could help him learn how to use it. Specifically, he wanted to bring it over to my house and leave it with me for a while so that I could figure out how to use it, and then show him. (Naturally, being a friend, I reluctantly agreed to take in this poor underused plasma cutter!)
Okay, now for the questions: I've not used a plasma cutter before, and can't wait to try -- but is there anything special I need to know before I do? For example, are there special precautions that I should take above and beyond what I do for stick welding (hood, leathers, watch out for flammables, etc.)? What do any of you usually cut over -- do you have a special cutting rack, or do you put the steel across some sawhorses and start cutting? I know I need clean, dry air ... anything else?
A couple of other questions: This unit can be switched for 110v or 220v. (It can only output up to 20 amps on a 110v, 20 amp circuit.) It is wired with a standard 20-amp 110v plug. To change it over to 220v, should I change the plug, or is it possible just to put together an extension cord with the appropriate plug for my existing 220v welder on one end, and a socket to accept the existing plug from the plasma cutter on the other end? Hmm, that didn't come out very clearly -- I guess my question is whether the existing plug on the plasma cutter can handle 220v.
Also, any thoughts about why my friend's employee had trouble getting good results on the stainless? The stainless he was cutting is around 18 gauge; my friend said that when he cut the holes, it left too much dross, and it was taking too long to clean it up. I've read enough here to know (or to think I know!) that dross can be caused either by going too fast, or by going too slow. Would stainless be more susceptible to this problem than mild steel?
Many thanks for any answers!
Andy
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snip

Good ventilation. At the very least some dust masks, but a good respirator is a good idea. All it needs is a HEPA filter.
Different metal have different effects. When Cutting Steel you will get a lot of vaporized metal. If you cut over concrete the concrete will end up absorbing some of this fine iron oxide dust and will start to rust. No big deal if you don't care about looks.
Aluminum tends to stay as a liquid and sprays all over the floor under the cut as does copper.
So if you plan on cutting over a floor you want to keep nice, then do so over a sacrificial steel sheet or use a 55 gallon barrel with some grating on top.

Change the plug to a standard 110 volt plug and make an adapter to plug it into a 220 volt plug. Mark the adapter quite visibly so you don't fry a 110 volt power tool.

Stainless steel always leaves a nasty cut edge with a plasma cutter. Steel dross just shatters off, but the nickle and chrome in the Stainless steel prevents it from completely oxiding so it has to be ground off.
It is just a limitaion of the materiel and the process. He would have been better off to invest in some Greenlee chassis punches.
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Can second that, as much as Ernie's comments need seconding. With Hypertherm 380, get beautiful beautiful cuts on mild steel. Two strokes of a file or one wipe with an angle-grinder and they're ready for welding. Not so stainless. 304 s'less, 0.9mm (about 35thou-of-inch) thick - best result is if turn down current to near minimum - but can only minimise "dripping" metallic dross / burr hanging from lower edges - not totally avoid it.
OK prototyping, but for a production run, the grinding needed to clean-up would be too much.
This experience when making things like this:
http://www.weldsmith.co.uk/vq_sless200_10mc.jpg
Was thinking of having blanks for successful tested designs laser-cut.
Richard Smith
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wrote:

Hmmm ... what would happen if you put a thin sheet of mild steel under the stainless, and cut through both together -- would that let the dross accumulate on the mild steel, and leave the stainless with a clean cut? Or would you just wind up with the two sheets welded together?
I'm thinking about my woodworking experience here -- to prevent tear out when drilling or routing, put a piece of scrap under/behind the wood. Of course, even if this idea worked for plasma cutting, it would quickly get un-economical ... but would it work? Or perhaps instead of mild steel, putting a thin sheet of aluminum?
Just letting the wild hares out ...
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snipped-for-privacy@duke.edu says...

Well, how about giving it a try! I actually have cut multiple sheets of thin steel in order to get identical parts cut on a single pass at home and it worked great! The pieces were barely stuck togeather, pried apart by hand.
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says...

I don't have any stainless sheet on hand at the moment, only mild steel ... but if & when I get some I will give it a try and see what happens. Your experience indicates that it might not be totally hare-brained!
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As always, Ernie is correct, but let me emphasize a couple of things he mentioned.
First of all, if you cut much mild steel in your garage there will be a load of fine plasma dust in and on everything...and I do mean everything!. I moved my plasma cutting outside after everything was covered with this fine black powder and won't make that mistake again. And don't forget to clean your shoes before going into the house..as it will track in nicely...!
The other is don't cut over the concrete unless you want a picture like this:
http://www.texasmetalcraft.com/TMC/images/other/SprayRust.JPG
get a sheet of thin steel and use it as a barrier:
http://www.texasmetalcraft.com/TMC/images/other/PlasmaSheet.JPG
Other than that, have a great time. Randy Ray www.texasmetalcraft.com
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Ernie, very helpful as always. I didn't know about the vaporized metal, though I suppose that should have been a "duh!" -- I did a little bit of trial today without any mask or respirator, cutting mild steel -- have I just ruined my health?
As far as the punches -- what would something like that cost? (I have learned a lot about welding on this NG over the last year, but I'm just starting to learn a little about other metalworking!) It sounds like he doesn't need a lot of boxes made at any one time; maybe a dozen here, a dozen there, from time to time. Of course, the plasma cutter was not exactly a cheap investment (around $1200 at the time?) but how would that compare to a system to punch the metal? I assume he would have to have the punches made to the exact shape he needs ... or am I totally off-base here?
Thanks again for the help!
wrote:

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Iron is a very benign metal. We have iron based blood and iron particles are easily broken down by our bodie as oxides and absorbed.
Stainless steel on the otherhand has enough nickle and chrome in it to no be oxidized so it will stay in your lungs until you hok it up as phlegm. Aluminum oxide does the same thing.

I spent about $300 2 years ago on eBay and ended up with a very complete set of chassis punches from 1/2" to 3-1/2" diameter rounds and some square punches too.
Check ebay under metalworking tools with the search words,
greenlee chassis punch
You will soon get an idea of what they cost used. Check with a local electrical contractor supply for prices on new.
They can be purchased in sizes from 1/2" to 10" and will usualy handle up to 1/8" material. They do make special ones for stainless steel with harder edges.
For large holes invest in the Greenlee 760 hydraulic puller for the chassis punches.

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On 30 Aug 2004 18:46:01 -0700, snipped-for-privacy@duke.edu (Andy Wakefield) wrote:

Mild steel is no big deal. Neither is _most_ stainless. But I once cut some weird high-chrome stainless and gave myself a 24 hour dose of chrome poisoning. Unpleasant, so now I mask up for other than plain steel. Galvanised steel is another one to watch out for.

"Q-Max" is the UK brand. They were infamously expensive, but a recent thread in uk.d-i-y turned up this site http://www.lawson-his.co.uk/scripts/products.php?cat=q-max%20cutters
If you use a big one, use a thrust bearing too or they're impossible to turn.
As to the "floor damage" issue, then don't do what a friend of mine did and use a PVC picnic table as a trestle - without checking where the cut line would go ! Plasma won't cut through PVC, but it does make a mess and a stick.
--
Smert' spamionam

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I've bought a number of Q-max cutters from farnell electronics and never considered them expensive. The prices are about the same as the site you listed. IIRC they do get expensive above 55mm.
Andy Dingley wrote:

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<snip>

So, is this why O/A can't cut stainless? I've always wondered about that. For that matter, why can't O/A cut aluminum -- why doesn't the oxygen cause the aluminum to burn just the same way that it causes the steel to burn?
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snipped-for-privacy@duke.edu (Andy Wakefield) writes:

Better way to put it is - why will oxy-acet. only cut "ordinary" steel and nothing else?
Because, almost uniquely for an engineering material, its oxide melts at a lower temperature than the metal. Iron (and steel) melts at about 1700C (3092F). Iron oxide melts at around 1300C (2372F). So the oxide forms flows away down the kerf - you can damn' well see that happening - it trickles away, the way it does so being another thing to control a regular cut on.
Aluminium melts at 600C; its oxide at over 2000C (very refractory!).
Stainless steel forms refractory oxides from the Cr (and Ni?).
If you put an oxy-acet. torch to stainless, the metal seems to heat-up OK, but when you squeeze the oxygen valve, to oxygen flows but nothing happens.
The fact that the oxide of iron melts at a lower temperature than iron itself is also why steel is so easy to weld. It's why it's used so much. It is joint-top reason why steel is so cheap compared to other metals (also it is abundantly available as a richly-concentrated ore) - the oxide is easily reduced and the metal easily refined under a fluid slag - and easily recycled (electric-arc steelmaking is "recycling").
Iron has a defective crystal structure, making it susceptible to brittleness, but that can be minimised and lived-with by steel microstructural design and by engineering macrostructural design - and it makes steel very free-machining! - another huge huge cost advantage.
Then iron has a crystal-phase tranformation at 900C (down to 723C when carbon added), which is the whole basis of heat-treatment - you can "turn on and off" at will the hardness of carbon steel. Very useful during parts manufacture.
Richard Smith
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Nice post, Richard. Thanks.
Jeff Dantzler
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Great information Richard. Thanks. I'm sure many of us will benefit from your knowledge.

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