Assuming it is a Spectrum 875, I see charts in the manual, for maximum and optimum cutting speed of various thicknesses at 25, 40, and 60 A, for mild steel, stainless, and aluminum. I'd take that at a starting point.
I've never seen a chart associating thickness with a specific current like you would see with any welding process, and as far as I can tell, it can vary greatly between manufacturers. I know that my current plasma cutter can cut thicker sheet faster at 20A (its lowest setting), than my previous cutter which was 25A all the time (the power level was fixed). I've seen more modern plasma cutters that still max out at 40A just like my machine, but are rated for over double the severance capacity.
Nothing against Miller (I love my Miller welders), but some other manufacturers have better charts in their manuals on this.
One thing that especially bothers me about the Miller manual I just looked up, is that it isn't clear about standoff.
With my machine, it works best drag cutting thinner sheet, with lower power, but I need about 1/16" standoff at higher power cutting thicker things.
In general you will get better consumable life with less power, BUT, the big things that eat consumables are the pilot arc (so have your torch fully positioned before pushing the button), and piercing (or cutting too fast, which also splashes sparks back at the torch). Also, you typically don't want to drag cut above 25A. If you're used to O/A cutting, setting your speed by watching the sparks deflect at an angle works the same way.
As a general guideline on steel, I use amps times ten to represent thousandths of an inch thickness that can be cut. 100 amps for 1", 50 amps for .500" (1/2"), 25 for .250" (1/4") and so on.
I am new at this Myself, but I think some of the disparity between amperage and cutting speed on various machines has to do with arc voltage. The bigger machines are up about 135 volts, and some of the smaller ones are about 100 or even a little less.
Power = amps X voltage.
Of coure improved torch design probably has a lot to do with differences between machines as well.
Sounds reasonable, but I'd think that the nozzle diameter and the shape of the plasma stream can make a difference.
I'm not sure why, but a lot of newer inverter machines are rated to sever more per amp than older transformer machines.
Maybe that has to do with the difference between true DC, and a rectified AC waveform, again, I've got no clue why, and the literature I've seen is lacking in details on this.
My unit uses different nozzle orfice sizes for diferent amperage ranges, so it must have an effect on the performance.
Other than the Centuyr/Dayton/Snap-On plasma cutter, I have not seen any transformer machines [at least higher amperage machines] made since the early '90s. I expect there have been a lot of improvements since then. Inverters have much greater electrical efficiency, and probably do have a smoother DC output, at least smoother than a rectified single phase transformer output. I don't know if that makes a measurable difference in cut speed or not.
Severance capacity I think may be an abused advertizing issue, as they never state how long You have to gnaw away at the piece to get through it.
I wonder if severance ratings are creeping up like compressor or lawnmower or shop-vac horsepower ratings. It wouldn't surprise me, and would explain the disparity in severance ratings I've seen between machines with the same output amperage.
I know I've severed things much more than my old machine was rated for, but it was more like gouging for much of the cutting. Maybe that would be the severance rating now. I've got no idea. I'd have to play with some more newer machines to compare to my Century machine from the early '90's. :) Actually, it's got a Matco label, but it's the same thing. At least the aftermarket torch consumables make plasma cutting significantly cheaper than any other way I have of cutting metal that won't fit in my shears.
You're correct about the pilot arc and yet, another thing that really shortens the life of the torch components is the type of filtration being used to filter out the moisture in the air supply or the grade of other cutting gases being used on thicker cutting also...
For instance, if someone is using one of those air plasma cutters that include a pilot arc, the best way to ensure long torch component life is to make darn sure one has an excellent moisture filter connected before the pressure gauge attached to the machine because if one only rely's on the filtration coming off the compressor, then they run the risk of condensation forming downstream of those filters which allow moisture to reach the pressure gauge which usually on the smaller air powered machines do not come with a separate cartridge type moisture filter connected to the pressure gauge... Thus allowing moisture to enter into the machine and really causing a whole lot of chaos inside and to the torch components as well.
So if you don't have a separate moisture filter attached to your pressure gauge, then I recommend that you install one since they are relatively inexpensive compared to the costs of either constantly replacing parts prematurely or needing a very expensive repair or replacement of the torch. It also depends where you're at in deciding what the relative humidity is for you area in order to determine how crucial it is to install one of these puppies on your machine. ;) Regardless, it's still something that cannot hurt and only help.
Respectfully,
Henry
Good point. I complete forgot about that. Every plasma cutter I've owned has come with a coalescing filter attached inline with it's air inlet, so I don't think about it much, but if yours didn't come with one, it's not something to forget.
Yes, a filter installed right at a compressor's outlet will not really work too well. It'll remove particles and oil from the air, but water needs 20 feet or so of plumbing before it really starts to condense. Some people run a bunch of pipe out of the regulator and around the shop before installing the coalescing filter, and then finally splitting off to taps for your tools. That can help, but while a little water blown through a properly lubricated air tool won't do too much, just a little water or oil blown through your plasma cutter is really bad, so having an additional filter right on the machine is a very good idea.
If you have a refrigerated air dryer (like one used for a paint booth), then you're probably good to go. I don't, so I actually installed a desiccant canister between the coalescing filter and my machine, just to be sure. That reduces the humidity of the air too (which a coalescing filter cannot do, but a refrigerated dryer can do).
If I owned a machine with expensive consumables, or one I was more proud of, I'd probably have a motorguard sub-micron filter in there too, but my plasma cutter is a beater I got from a body shop, and I treat it as such.
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