Eliminating dross on 1" plate

[thenewcraft]

Hey hoping someone can help me out. I need to cut ALOT of 1" plate and am not looking forward to cleaning the parts. It seems no matter what I do (speeds, heights, air pressure) I can't seem to change my amount of dross. Any suggestions? Here are the settings I've been working around:
Amps: 200
Cut height: .25
speed: 25ipm
shield pressure: 60 psi
cut pressure: 45 psi
Have a safe new year.
Neil

[jimcolt]

Plasma systems are not all the same. What plasma system (brand and model) do you have? What gases are you using (Air, Oxygen?). What cutting machine and torch height control are your using?


There are a least a dozen plasma systems on the market with 200 Amp cutting capability! Some will cut 1" dross free....some will not.

Jim Colt Hypertherm

[thenewcraft]

Jim,
I have a Hypertherm Max 200 on a Retrosystems 5 x 10 table. I am using air/air now but could hook up oxygen if a six pack would have enough volume or nitrogen if I can use a single bottle.
Neil

[jimcolt]

Neil,

The Max200 can cut 1" steel very well using oxygen as the plasma gas and air as the shield gas. Nitrogen will not help you for cutting steel.

Follow the oxygen cutting specs in your manual exactly......120 psi inlet pressure on oxygen and 90 to 120 on the air....the manual tells you how to replumb the inlet gases for use with oxygen. You need to use oxygen consumables as well....they are quite different in design from the air consumables....so again, check your manual for the correct part numbers.

You should be able to get a mostly dross free cut on 1" steel with the Max200......should be completely dross free on 3/4". I was hoping you had our HT2000 system...which is also 200 Amps.....and will cut 1" dross free....at 50 ipm.......your 200 will cut the 1" at 35 ipm.

Oxygen as the plasma gas provides the high temperature thermal plasma process, as well as an exothermic (chemical oxidation) reaction for higher speeds and cleaner cuts.

Jim Colt

Jim,
I have a Hypertherm Max 200 on a Retrosystems 5 x 10 table. I am using air/air now but could hook up oxygen if a six pack would have enough volume or nitrogen if I can use a single bottle.
Neil

[Loyd]

Hey Jim,
I noticed on the PlasmaCam (Hollywood Production) dvd that they were cutting 1" with a hypertherm 1250. How were they doing that?
Loyd

[jimcolt]

Very easily....you edge start and cut it.....at about 12 inches per minute. Max pierce thickness rating for a 1250 is 5/8"....many pierce thicker, but they also change the shield and nozzle often! I have edge started and cut 2" with the 1250. Then you chip or grind the dross! The edge, with a air plasma, will be extremely hardened due to nitriding from the nitrogen content in air. Cut quality will be nice, it is just slow, and no where near as good as with an Oxygen plasma.

We have oxygen plasma systems that can cut 1" steel at 85 inches per minute, no dross, and a very clean, soft, square edge.....and can make holes in 1" material that are perfectly round and with virtually no taper....

Hey Jim,
I noticed on the PlasmaCam (Hollywood Production) dvd that they were cutting 1" with a hypertherm 1250. How were they doing that?
Loyd

[Loyd]

Jim,
As they say in China.....Velly interlesting!
Thanks,
Loyd

[thenewcraft]

Jim
I have hooked up the oxygen on my system. I am getting better results but not on every piece. I will have one or two that are virtually dross free then a couple that have a horrible amount. I have everything set as the manual says and even went as far as grinding and wire wheeling my plate clean. Whats the deal?
Neil

[jimcolt]

If there is a thin web between two parts...or you are cutting close to an edge.....the resulting heat build up can form dross. Try nesting the parts a little further apart. As I said in my previous post....you can cut mostly dross free on 1"....not completely. If you had a Hypertherm HT2000 you could use our HySpeed consumables for dross free cutting at 200 Amps on 1"....your system is advertised to cut dross free on 3/4". Regardless.....try these tricks:

1. If you are doing multiple parts.....try increasing the cut speed 2" per minute consecutively......so book speed on the first part, book speed plus 2 ipm on the second, book plus 4 ipm on the third and so on.....look carefully at the cuts......by increasing speed you may get to the dfz (dross free zone) if there is one for 1" plate with a Max200.
2. If your plate has a dirty or oily or primer side....put that side down. Dross has more trouble sticking to this type of surface. If one side is rough as in shot blasted, put the rough side up. try spraying MIG welding anti spatter spray on the bottom side of the plate. Grinding your plate clean...especially on the bottom.....may actually cause dross.
3.O consecutive cuts adjust the arc voltage (torch height) 2 volts higher, 2 more volts higher.....look at the dross. Also try 2 volts lower than book specs.

Your process 200 Amps, Max200.....is right on the edge of being able to cut dross free on 1" steel. By doing the above you are attempting to find the sweet spot on the particular steel that you have.....if steel has a high silicone content it has a higher propensity of dross formation.....same goes for carbon content. The other trick is to use step 1 process above....and slow down your cut speed...this will get you to "low speed dross".....which is more consistent and very easy to remove....also has the tendency to square up the edge a bit.

Let me know if any of this works.

Jim Colt

[zzzzzzzz]

Hello Jim,

Is it possible to plumb oxygen into a Hypertherm 1250 and configure it with oxygen compatible parts?

[jimcolt]

No.....any plasma system that is not specifically designed for oxygen cutting should not have oxygen connected. To do so will likely cause a major fire in the torch and leads....that the oxygen will support! The materials and design of oxygen plasma torches are designed to allow a high teperature plasma arc in an oxygen rich environment....without supporting combustion!

Air is roughly 20% oxygen and 80% nitrogen......that is about the highest oxygen content that is safe in an air plasma torch.

Good question.....but please don't try it!

Best regards, Jim

Hello Jim,

Is it possible to plumb oxygen into a Hypertherm 1250 and configure it with oxygen compatible parts?

[zzzzzzzz]

Hello Jim,

Some materials become sensitive to oxygen when it is applied at high pressure, such as thousands of psi versus hundreds of psi. Titanium comes to mind.

It isn't very convincing that changing the oxygen partial pressure from ~25 psi to ~100 psi is going to have a dramatic effect on torch components. Particularly since the service temperature of the torch is already high in light of a plasma application.

Industrial compressors that are oil-lubricated and operate at several thousands of PSI will tolerate up to 40% oxygen. This represents an oxygen partial pressure of several thousands of PSI.

In short, if the torch was sensitive to oxygen that changing the oxygen partial pressure from 25 to 100 psi was sufficient to cause a meltdown, more torches would likely burn up at current oxygen partial pressures.

If it is so dangerous, you wouldn't have any video of such a meltdown? It stands to reason that somebody would have to do safety testing to determine the limits of combustibility.

A minimum requirement would be to ensure the system components are O2 cleaned.

[jimcolt]

Oxygen compatibility in a plasma torch has nothing to do with the pressure of the oxygen....it has to do with the ionized, high temperature plasma that we are creating.....and the fact that there is electrical insulating materials inside the torch (composites) that under high temperature conditions in an oxygen rich atmosphere will burn. Once the fire starts...and the oxygen is still on.....the fire will continue. I have seen a few cases where the fire pretty much destroyed the torch and worked all the way back to the plasma power supply before the user was able to shut the oxygen source off.

Plasma systems that are compatible....have totally different torch designs....all of which are liquid cooled, and use different insulating materials inside the torch. As an example....air plasma torches use a composite material called Vespel as the insulator that separates cathode and anode DC voltages.....all of Hypertherm's oxygen capable torches use volcanic lava (machineable ceramics are very costly) as the insulator between cathode and anode. The oxygen arc is hotter, there are no known (affordable, machineable) composites that have adequate properties for use with oxygen under the conditions in a plasma torch.

I don't have any videos....but I'll see if I can't dig uo some pics of oxygen fires that have occured in plasma systems. I won't argue this point.....but again I will say...do not use pure oxygen in a plasma system that was not designed for it!

Jim

[zzzzzzzz]

Hello Jim,

The great part about this being a torch material issue is that one could adapt a torch from an oxygen plasma system.

Too bad this isn't an available option for air plasma systems, especially if it would boost cutting capability on steel.

[jimcolt]

There are some other issues with switching to pure oxygen cutting that are addressed with gas flow systems, plasma power supplies and torch cooling systems....while you can make it work in a simple (air or nitrogen style torch)....the combustion issue is just one problem. Oxygen has different properties.....and affects the erosion of the hafnium electrode emitter, causing oxygen electrode life to be much shorter than when used with air. A process that Hypertherm calls Long-Life technology was developed to combat this issue....it uses a microprocessor controlled gas flow that changes gas mix, gas flow and gas pressures during the starting ramp up from pilot arc current to cutting current......this ramp up is tied as well to the current up ramp.....consequently at the end of each cut cycle gas pressure is maintained and accurately ramped down concurrently with the cutting current. Further, gas swirl rates in the torch are modified.....with a dual swirl pattern, one to control cut squareness.....another swirl to create a centrifuge effect that slings cool, unionized gas molecules to the outside of the nozzle orifice...creating a cool, unionized boundary layer of gas that protects the nozzle from erosion. Further, oxygen electrodes are manufactured with proprietary processes (different from air electrodes) that improve both the thermal and electrical conductivity between the hafnium element and the copper body....this improves heat transfer to the liquid cooling system....and minimizes galvanic corrosion between the emmiter and the copper body of the electrode.

This is just the start of the defferences between an air plasma and an oxygen plasma. Granted, early oxygen plasma systems did not have this type of technology available.....but they also were not very popular due to extremely short consumable life. Todays systems, like Hypertherms HPR (high definition) class oxygen systems cam provide more than 3000 starts on a set of consumables.....compared to our first oxygen system (The HT400, circa. 1983) which provided roughly 100 starts per set of consumables.

Hypertherm has over 85 engineers that develop our systems and processes.....14 of which have pHD's......we have 11 cutting machines that are used simply to develop processes and to do benchmark testing of our production consumables and systems to ensure our quality is maintained. Trust me...if it was as easy as just plumbing oxygen in the back of an air plasma system to improve quality...there would be many of these systems on the market today!

Jim Colt

[zzzzzzzz]

Hello Jim,

Certainly, there is always more than meets the eye. Thank you for an informative post. This helps frame the discussion.

Adding pure oxygen is not a materials problem within the torch from an ignition issue since there is no combustion within the torch. Rather, it seems to be from the heat generated from the workpiece since this is where combustion takes places and impinges on the torch through convection. Once ignition is achieved on torch components, the added oxygen doesn't help. Cooling the torch using liquid cooling aims simply to keep the torch within a safe temperature envelope to allow using materials effectively and economically.

The electrode concerns appear to be solved using silver plating, another element that is easily achieved since silver plating and soldering have been around for some time. Silver oxides have the advantageous property of being conductive, unlike copper oxides that act as insulators. In addition, conductivity, both thermal and electrical are highest for silver. Since temperature control become dominating factors due to the induced heat of combustion from the work, it is normal that a cascading effect occurs on the components. The properties of silver have long been used in relays and contact switches to extend their service life.

The gas dynamics are another matter with arc constriction as a goal. The corollary is that substantial research has gone into extending cutting capability. Likewise with the varying flow rates and pressure.

It is surprising that liquid cooling is not applied directly to the nozzle.

Thank you again for an informative post. It is still very much a pity that an intermediate solution is not within the realm of the feasible.

ATB

[jimcolt]

On high end plasmas....again, the Hypertherm HPR systems...liquid cooling is directly on the nozzle, and through conduction on the shield. Silver plating per se does not help the electrode, we have even tested gold plating with no noticeable effects. We do have what we call silver-plus electrodes that use pure silver to bond the hafnium emmitter to the copper electrode body...this produces a better thermal bond, further increasing electrode life....but at a higher manufacturing cost.

The smaller air systems fit the market needs, determined by the users buying them. Portability, reliability, ease of use, and affordable price are most important for a succesful product. That us why they use compressed air, and do not use oxygen or liquid cooling.

Jim