TIG welding plasma cut edges

On Stainless steel, with or without a nitrogen cutting gas, you will still have a skin of iron and chromium carbides on the cut edge.
In theory you can weld through this if you have a decent pulser and use an argon/helium gas mix, but it is far better to take a hand grinder with a flapper wheel and simply dust off the edge to remove this very thin skin.
On Aluminum you will have no choice but to sand the edges before welding. The plasma cut leaves a thick layer of aluminum oxide on the cut edge and it must be removed before welding.
It is for this reason that water jet cutting is so popular with large aluminum boat manufacturers. Water jet leaves a truly clean weldable edge.

Hello Ernie I know helium can be used with SST, but I have always used 2% hydrogen. Is there any reason you would choose helium instead?
Richard

Honestly I have never used a hydrogen gas mix. I would be curious as to how it compares to a helium gas mix.

Ernie Leimkuhler wrote in news: snipped-for-privacy@news.west.earthlink.net:
Ernie,
Thank you very much for your answer. I went ahead and bought the Powermax 1000 and have been playing with it a bit. Results are not spectacular so far using air which I expected and I will try some other gas combinations in the next week. I have helium, argon, oxygen and air around for diving and will pick up a tank of nitrogen. I'll try blending up some combinations in small tanks and see where I get to. The Powermax torch is only rated for air or nitrogen. I don't know if using other gases would be a problem and will telephone Hypertherm and see what they have to say.
With regards to using small percentages of hydrogen I would theorize that it might be there to help prevent oxidation by combining with any oxygen present before the oxygen can combine whith the metal being cut.
I have also been doing a bit more research and stumbled onto a group on plasma cutting at snipped-for-privacy@yahoogroups.com where I found the info attached below which details plasma cutting using exotic gas mixtures most of which get used in a higher end torch setup than the one that comes with the Powermax.
Another apparently common (although not recomended by the manufacturer) technique that can be used with the Powermax type torches is to cut with the torch and part submerged in a water bath. Obviously I won't try this with a hand torch but will keep it in mind if I build a CNC table for the torch.
Dave
The stuff below is quoted from postings by Jim Colt to the Yahoo group on plasmacutting.
When cutting non-ferrous material, the best cut face will result from using gas combinations that minimize oxidation - usually inert gases. Plasma gas combinations such as Nitrogen or H35 (Ar 65% and 35% H2) are common. Also, H35 mixed with N2 is used as a plasma with nitrogen shield on non-ferrous under 1/2".
Low current torches (under 100A) are often the single gas design. This means the plasma and shield are delivered to the torch with a single hose. The gases are split inside the torch. Some benefit is derived from using inert gases with single gas torches. However, single gas torches are generally not used with expensive inert gases.
The best results will come from a torch that is designed with two flow paths - referred to as Dual Flow or Dual Gas. This type torch will have a plasma flow path and another separate shield flow. The more expensive inert gases for the lower flow plasma and typically a less expensive gas for the much higher flow shield. Also, gas cooled torches are generally single gas design. Liquid cooled torches, which deliver much longer consumable life, are generally the Dual Gas design.
Some Dual Gas torches also permit Nitrogen plasma and normal tap water to be used as the shield. The water flow is a very low 2 - 6 gallons per hour - like a garden hose set to mist. N2 / water is the least expensive choice of all choices in terms of gas cost and consumable life. The plasma arc causes the water shield to reduce into O2 and H2. The H2 burns using up the available O2 in the cut area. Thus oxidation is decreased on the cut face. This reducing action also occurs when using air plasma and air mixed with methane for the shield gas.
Air or nitrogen plasma and air shield are good choices for thin sheet metal aluminum because the cut face is very small. The effect from any gas is minimal. As the metal being cut becomes thicker the negative effects of oxidation increase. The thicker the metal, the greater the need to address oxidation.
In general here are some good choices: Under 1/8 Air plasma / air shield
1/8 - 1/4" Air plasma / air shield mixed with methane shield - low cost, reduced oxidation Nitrogen plasma / air shield - low cost, some oxidation Nitrogen plasma / water shield - lowest cost, very low oxidation N2 95% - 5% H2 plasma / N2 shield - higher gas cost, excellent surface finish
1/4" - 1" Nitrogen plasma / air shield - low cost, some oxidation Nitrogen plasma / water shield - lowest cost, very low oxidation H35-N2 plasma / N2 shield - higher gas cost, excellent surface finish
Nitrogen plasma / air shield - low cost, some oxidation H35 plasma / N2 shield - higher gas cost, excellent surface finish

I tried straight nitrogen with mild steel in a Pakmaster 38XL and the results were unimpressive. It did cut, but the cuts were dirtier and slower than cuts made with air and nitrogen costs a good bit more than air.
Good luck, Bob

304 stainless cut with a "380" (?) plasma cutter using air blast from a compressor were drossy and needed grinding before TIG. I believe that is the general finding.
If nitrogen were to solve that, it would be good. But you would be using a lot of nitrogen...
Richard Smith

I cut 304 with my 600 plasma at full current and max speed - very little dross. I suspect you are going to slow. I cut 14 ga. and typically don't do anything but run a glove down the edge for spider webs.
Maybe you are cutting to slow or limiting your current.
Martin
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member
Richard Smith wrote:

Martin, I have never seen a plasma cutter at work, and would like to know, just how accurate edges do you get after a plasma cut?
i

By hand - looks like the gravel in the driveway :-) By CNC - perfect cut lines - curves and I just 'hacked' up with my hand torch a 38" ellipse that I did on top of some words in my company sign.
Depending on thickness, the grain of cut is either verticle lines, smooth blur, back wave in an arc and the worse - back wave and to a side at the same time. Therefore a bottom flare is generated..
I can keep to sub mill on my servo drives.
I just finished some AR400 1/2" thick steel - and it was out of spec for my 1/2" standard torch. So I put on my machine the hand torch I got - RT-60 put in a 60 amp nozzle and kept the current at a max (45 amps IIRC). (can't remember the setting - but max for my powermax 600 . The RT-60 went through it but not as consistent as I wished.
From what I saw in the FA report I did - the three items that didn't cut clean were closest to my return lead. (sounds crazy right ? ) - The plate of steel was rusty, and had a heavy scale on it due to the heat treating it just undertook. I had ground the top surface and attached the 300 amp return clamp on the ground off area. That worked just fine for the items 3 and 4' away but within 2 feet the beam current was sneaking the return leg and was to close to just go to the table. It tended to swing hard to the side and making a longer cut it didn't cut through. Hand cutting of these last few was painful since it was difficult to see down the slot and cut the small areas still holding on. A unique technique (naturally :-) ) was simply to turn over the slab and cut from behind. Generally the line was there - and the small areas holding on were exposed and easily cut. Simply a mathematical issue - point of view or the like.
Martin
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member
Ignoramus22091 wrote:

I put a gel pen in the plasma cutter holder and it will draw lines thinner than the gel will draw. Specs are sub mil.
When I put in a fine cut - I want to say it is - have a doc finecut (r) Hypertherm does in copper - a .025" kerf or a slot that wide in cutting. Normal kerf is .055".
The beam is a mixture of electrons and atoms of Hf that is the source of the electron shower of high bursts into a very very hot stream of plasma.
One time I did an experiment (last month) - just for fun. I put a scrap tile of Granite on the table. I had to support it on a thin sheet for support. My plasma started - and began a line when it halted the stream. Needless to say - a melted path deep into the now glass like line and a strand of glass in the splatter direction. It was kinda cool - I used a just about gone source just in case...
Hum - have to take a picture of the blast through and under the table while cutting some of this armor plate. Boy do I get pretty sparklers under the table! Like high allow steel on a grinder...
Martin
Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member
Ignoramus22091 wrote: