I will ask my tech guys to post up some settings that they work with here. Remember, there is no hard and fast rule.
With Cut Quality being the overall goal with adjusting your speed and amperage, I will explain a few things below about what it is, and how to achieve your nominal cut.
When talking to a new or potential customer I explain your potential cut quality as a combination of Speed, Height, and Amperage. Each one of these values will have an effect on the quality of cut in every type of material. When I'm talking about quality, I would define that as Bevel (angle of the face of the cut) and Dross (slag building on top and bottom of the material). Kerf, or Cut width is a factor as well, but because most machines have the ability to offset for this width, it doesn't tend to matter nearly as much as the Bevel or Dross. We run our Line Speed test at a particular set height and amperage. Most plasma cuts on .1" material and thicker we usually recommend running at or above 40 amps. Our Thermal Dynamics and Hypertherm cutters recommend a cutting height of approximately .125" to .1875" for these amperages. This is set value number one for our line speed test. The Amperage is a matter of installing the correct tip into the torch head, and setting the amperage value on the face of the plasma cutter to match this tip, +/- 5%. With the second part of this equation is then set. At these values we run our line speed test. As the pictures illustrate above, we basically run from too fast to too slow, 130 to 10 inches per minute (ipm) at 10 ipm intervals. With this coupon of material cut out, we can easily look at the Bevel, Dross, and even measure kerf width from it at our best cut speed. Say none of the cuts look good, we can make an adjustment to the height or amperage and run the same test again. This is an easy way to determine appropriate values for your Speed Height and Amperage.
Speed: If your speed is too fast, you will usually experience a wider cut at the top of the material, to a skinnier cut at the bottom with a small bead of dross that is difficult to remove. If the speed is too slow you will have a wide kerf, with good bevel but large dross accumulation. While the bevel may be good, and the dross easy to remove, cutting too slow will result in longer cuts for paths, quickly degrading consumable life, and excessive warpage of the material. A correct speed results in a uniform dross accumulation of manageable proportions and good bevel defined as 3-8 degrees of bevel by the plasma cutter manufactures.
Height: If you height is too far away from the material, you will have a wide bevel at the top of the material, and the torch may have trouble extending the arc that far resulting in missed cuts, and bad consumable life. If the height is too low, you will have a wide bevel at the bottom of the cut, and may have trouble piercing the material or keeping the torch going if it comes into contact with the material during a cut. On both Thermal Dynamics and Hypertherm plasma cutters, if the tip comes into direct contact with the material at an amperage higher then 40, it will automatically reduce it's amperage output to 40 amps. For example, cutting at 80 amps it would reduce your cut capacity be half, and you will more then likely lose your pierce at this point. With correct height, you should have appropriate bevel, and it won't be stressing the torch by being to far away or possible damage the cut or consumables by touching it. This is easier explained by a simple chart out of our basic troubleshooting guide.
Amperage: This one is a little trickier, and when running a line speed test comes in handy. You can cut .125" plate with 100 amps, and you could probably run it at a rapid speed that would make the cut look decent. Just because you can, doesn't mean you should. You can make the same cut at a reasonable speed with much less heat distortion running 40 amps. I always recommend running the lowest amperage possible that the line speed test determines gives you the best cut. This will reduce duty cycling and air consumption.
The little known/talked about facts:
Cut direction matters, when dealing with Plasma, the right hand side of the torch moving away from you will give you less degrees of bevel, and less dross. This is due to the circulation of plasma coming out of the tip. This means Clockwise external (male) cuts, and Counter-Clockwise internal (female) cuts. In our Torchmate CAD software this is the "Climbing" option under the basic cut tab of the tool-pathing window.
Quality of air also matters. Having moisture in the air lines not only decreases consumable life, but it can also have an effect on your cut quality. The results may be negligible, but I have heard of reports of less dross on the cuts from people who have changed their air system to run a dedicated air dryer for their Plasma cutter.
Volume of air has it's consequences as well. While purging air, at the regulator at the plasma cutter supple, you should have at least 75psi to 85 psi. The longer the torch lead the higher this should be.
With all of this being said, at our Reno NV Facility with clean dry air, we typically cut .125" mild steel plate at 40 amps and 85 ipm. .1875" mild steel plate is cut at 60 amps, 80 ipm. .25" mild steel plate 60 amps at 55 ipm.
Aluminum being a less dense material, usually results in a faster cut then the same material thickness then steel. Also high speed dross on aluminum is almost non existent, but when you go too slow you get a really large accumulation of dross that is difficult to remove. (Think melted shredded aluminum)
Stainless steel usually results in a lower speed cut, and a cut face that is rougher then mild steel cuts. The dross is a carbide accumulation that unlike mild steel isn't easy to remove, and requires some mechanical assistance. I would recommend cutting this with the idea being as little cleanup work as possible.
When cutting like that. Does it change the line speed needs as the material heats up?
Not that I have ever noticed on long cuts. What does change is with an Automatic Arc Voltage Height control is that the electrode wears over time. When the electrode wears it gets shorter. Because the height is based off of the arc voltage reading between the electrode and the material, this can cause the tip to slowly progress closer to the material. If you stick with the new electrode every two tips, this usually won't amount to much height difference. Now if you are working tight tolerance work where the torch doubles back on itself and puts a lot of heat into one sliver of material, this can cause the material to become red hot, and this accumulates dross more then a cooler piece of material.
-Mike