Plasma Arc Accuracy

In CNC plasma cutting, the limiting factor in accuracy is the plasma arc process itself. Any industrial quality CNC machine is capable of several times more accuracy than the plasma torch it controls. An example is shown here.

Using a spring-loaded ball point pen mounted on our Z axis equipped Torchmate 2, we drew the Harley Davidson logo on the back of a business card. The drawing is 1 1/2" long and is perfect, including the tiny lead-ins and lead-outs. Detail doesn't get any better.

Please don't misunderstand us. We're not saying that all CNC machines are equally accurate. But given the inherent limitations of the plasma process, it isn't necessary to build this amount of accuracy into a machine.

The line the machine drew with the pen is approximately .002" wide. Imagine using a magic marker with a line 30 times that wide (.060"), which is the gap, or kerf, a plasma torch makes when it slices through 1/4" plate. Obviously, far less detail would be possible.

The plasma cut logo, shown above is approximately 16 1/2" long. However, with the plasma torch producing a .060" gap at 55 amps in 1/4" steel, the letters start to fall off if the piece is made less than about a foot long. We could probably reduce the shape size to 8" or so in 1/8" steel with a 40 amp nozzle. Making the legs on the letters wider would also help.

A CNC machine would have to either be very poorly designed, or worn out, to be less accurate than the plasma process. Any manufacturer who claims that their CNC machine will produce better detail using plasma than other machines is either being misleading, or is on the sales side, with no plasma cutting experience.

More information about the plasma process, and what it can and can't do, is presented below. We hope it is helpful.

Plasma arc is a marvelous process, allowing you to cut non-ferrous metals as well as mild steel. However, we believe that you should be aware of its limitations regardless of whose CNC machine you purchase. Plasma will produce good results for perhaps 95% of the applications out there, but you're not going to cut watch parts with it.

Plasma produces a cut edge that is as smooth as laser, but it leaves a slight (4 to 5 degree) bevel in the cut face. The bevel is caused by the swirling action of the air in which the cutting arc travels when it exits the torch. The thicker the material, the more pronounced the bevel is, because it has further to travel. It isn't particularly noticeable in material 1/8" or thinner. The bevel is most noticeable when cutting 3/8" or smaller holes in 1/4" or thicker material.

Generally, the top surface edges of a shape cut with plasma will be slightly better than the bottom. This difference increases with the thickness of the material.

There is a warping effect from the 5,000 degree plasma arc on thin material, particularly aluminum. A fully automatic torch height control is necessary to react to material that is bowing and moving while being cut. It is best to use a fast cutting speed, and not spend much time in a single spot. If necessary, come back to finish up an area after it has cooled a bit. The slower a material conducts heat, the better a job plasma will do on it. Stainless steel cuts the best, and brass the worst.

Also, as mentioned above, there is a gap (approximately .040" to .080" wide, depending on the nozzle) produced as the torch slices a channel out of the material.

Plasma arc is a thin material process. It is faster than oxy-fuel when cutting thicknesses up to about 1/2", and then it loses the speed advantage. Unlike plasma, oxy-fuel does not produce a bevel in the cut face. For that reason, most people prefer to use that process when cutting mild steel 1/2" and thicker. The downside -- oxy-fuel will not cut stainless, aluminum, or other non-ferrous metals.

We get calls each week from people who have little or no familiarity with the plasma arc process, but are considering the purchase of a CNC machine. Many have seen the finely finished and painted shapes on some of the other web sites, accompanied by glowing claims of accuracy. The inherent accuracy limitations of plasma are never mentioned. As a result, these callers often have an unrealistic impression of what is going to fall out of their plate when they complete a cut.

We believe that all of us in the metal working industry are a fraternity, and that to an extent, we all depend on one another. We want to be totally "up front" with our customers. Give us a call on (775) 673-2200 to discuss your needs. We will give you our honest opinion as to the suitability of CNC plasma for your particular application.

Call Toll Free (866) 571-1066
International Calls (775)673-2200
or Email: sales@torchmate.com

Our NEW 2009 Online Catalog is Now
Available for Download!