How Does A Plasma Cutter Really Work

Hi Mike,

Thanks for the reply. My posting went from "fresh post" to not being posted on any page except for "see all forum threads" so fast that I was assuming that no one would be responding. I guess I should have made it look like a homework question so it would get a little more time out in public.

Your link is helpful but it still leave a lot of questions and issues. We do not touch the cutting head to the material to be cut to start the arc and our 1.1 SCFM air flow at 40 PSI is not likely to produce a 20,000 feet per second air flow.

I guess my main question is why does the small hole in the nozzle not get eroded? I'm leaning toward my guess that the spark only travels from the electrode when starting the arc and it flows from the insert in the nozzle assembly after the arc is established. This is something that I have never heard or seen, but the rapid flow of cold air through the nozzle would cool the tip of the nozzle. It is probably wrong, but so far it makes the most sense.

Hopefully someone that knows the answer can find my lost thread.

Thanks,

Bruce

The quick answer is the arc attaches to the middle of the electrode that has a small insert of material in it, the swirl ring does exactly that spins the gas that will be ionzed as it comes out of the nozzle and the shield (thing with the two holes in it)shapes and restricts the arc so it doesnt eat into the nozzle. the electrode is negative and the workpiece is positive. This is a cutdown explenation but gives you a general idea

Interesting aspect of the cutting process gleaned from an instruction manual:

"Travel in the Direction that will Give You the Best Finished Work
If you are making a circular cut and plan to keep the round piece as your finished work, move in a clockwise direction. If you plan to keep the piece from which the circle was cut, move in a counterclockwise direction.

As you push the torch away from you, the better cut will appear on the metal that is on the right hand side, since it will tend to have a better, squarer edge."

Probably has to do with the swirl of the cutting gas and arc, the swirl being in a counterclockwise direction.

Interesting. That is a detail I have not heard before and I would never have guessed.

Thanks,

Bruce

1) From manufacturer ... typically 5k to 10k volts AC

2) DC electrode negative cutting ... can have ac starting

3) no ... generally chrome and tungsten

4) depends on which starting type you have ... one is dc+ tip (nozzle or funnel) ... one is high frequency start, one is capacitive start (rectified) ... initial pilot spark goes to plasma arc when electric potential is sensed on the surface to be cut.

5) Once the pilot arc touches the cutting surface the resistance is lower, so it stops the pilot and works off the less resistant path through the material being cut. or a dc+ tip touch start

6) Machines are different ... see manufacturer ... depends on your amperage mostly.

8) Consumables are the tip and the electrode ... tip is not 100% protected ... it will wear ... just not that fast due to negligible arc wander (as long as electrode is in good shape). Tip WILL wear as will the electrode ... depends on your technique ... the closer you are to the surface of the cut, the faster it will wear the tip ... and if you run it with an electrode that needs to be replaced, it will basically make crap out of a torch.

9) Some go to neutral until the arc is dropped and then switches back ... better machines better sense this potential change ... For instance, cutting expanded metal the machine will cycle the pilot arc on and off.

10) No I have never heard one say anything ... sorry ... seen the opening on that one ... had to take it.

11) The spark is from the electrode to the tip then it transfers to the cutting surface.

7 is my unlucky number ... try this link for pics of how the starting process works ... http://www.hypertherm.com/en/Training_and_Education/What_is_Plasma/what_is_plasma.jsp

This vid shows how tip and electrode gets used up (consumable) ... and some other info. http://lib.store.yahoo.net/lib/weldingdirect/Plasma-video.mov

Wow, a huge answer to a huge question. Thank you very much.

Bruce

first let's talk about how an oxy-acetylene cutting torch works, then we'll talk about how a plasma cutter works. an oxy-acetylene torch first uses the neutral (blue) flame to heat the metal red hot, then the HP Oxygen lever is pressed injecting high pressure oxygen into the flame. the excess oxygen combined with the higher pressure, literally burns and blasts the iron in the steel now that it is hot enough to burn in an oxygen rich atmosphere. What doesn't burn gets hot enough to be blown out of the cut by the pressure.

A plasma cutter works on the same basic principle. The initial arc heats a spot on the workpiece hot enough that the ionized air (ozone which is O3 and quite reactive is formed in ionized air.) begins to burn the iron in the steel. once the process starts the arc intensity can drop to a maintenance level to maintain the ozone production. the small diameter outlet of the funnel combined with the shape accelerates the burning air jet much the way a rocket nozzle does. The swirl helps to keep the ionized channel in the middle of the vortex isolated from the surrounding air.

Thank you, that makes sense. It also reinforces my belief that all the wonderful things said about plasma, while still true, might not be 100% responsible for the cutting. It would seem that the plasma is 100% responsible for the ease with which the arc is maintained, probably 100% responsible for cleaning out the cut, but probably sharing credit with the hot arc for melting the item being cut. To some extent it doesn't matter since the arc and the plasma will always be together.

All the stories of plasma in labs and outer space just didn't quite "ring true" when looking at a low end cutter from a discount tool store. I'm not saying any of the theory or facts were wrong, but I am suggesting that reality might force some adjustments when shifting your discussion from an advanced high energy lab to a back yard in Florida.

Thank you,

Bruce

You have a lot of questions but I only have time to answer your main concern.

There are two voltages involved , the first is the high voltage initiation , this flashes over to the tip from the electrode and is what you see sparking from the tip when you press the trigger. The tip has a return circuit through a resistance to the same polarity as the base metal you wish to cut, so when you approach the base metal the arc flashes over to it. This current flow to the base metal actuates the main plasma cutting voltage which naturally flows to the base metal not the tip then the high voltage cycles off until the next initiation.

Damage to the tip hole is usually from arc deflection and bad cutting practices, the brown insulator is a swirl ring which causes the air to spiral as it leaves the tip this gives a more stable plasma stream which is why a nice round hole is important.

Nice clean answers. I didn't previously know that the ring with a hole in it was a swirl ring. That gives me something new to think about.

Having the tip the same positive potential as the item being cut but connected through a large resistance goes a long way toward helping me understand why the tip is not rapidly cut like the workpiece.

The swirl and resistor help, but I still feel like I am lacking at least one detail. I assume that there is plasma all along the arc. This would include (based upon my current understanding/guess) the full length of the arc inside the tip. Thus, I am once again back at "how does the tip survive"? I'm not banging my head against the wall nearly as hard as I was, but I am back to it.

I'd love to get corrected/confirmed by someone who knows, but I still think that the "plasma is 30,000F and can cut anything" facts might be overplayed a little for a discount tool store plasma cutter. I know the plasma is hot but I'm still guessing that the arc is doing most of the cutting. If the plasma played less of a "can cut anything" role then it is easier for me to accept that the swirl of air prevents all but slow wear to the tip. The plasma is necessary to maintain a conduction path and I'm sure it helps to sweep the cut clean.

Several times I have been tempted to suggest that the arc striking the steel workpiece creates more heat and more plasma than the arc leaving the electrode and traveling through the tip. Several times I have talked myself out of making this suggestion. I guess I'll just ask if the plasma is more intense at the point where the arc strikes the workpiece than at mid-arc or the electrode end of the arc? The arc at the workpiece is very hot and violent, but if a plasma cutter is generating plasma so that the plasma can do the cutting then one would expect the plasma to come from the plasma torch.

Thank you,

Bruce

...

Trying to wrap my head around plasma physics on a Friday evening. There is probably room in there for comments about engineers and negative polarity. This isn't exactly a chick magnet activity.

You keep talking as if the arc and the plasma are separate things. They aren't. the arc IS plasma. the arc starts against the torch head, but as it is moved closer to the workpiece the arc switches to the workpiece for two reasons, it is a lower resistance path, AND the airflow "blows" the arc towards the workpiece. the arc is following an ionized plasma channel in the air which because it is ionized, it is more conductive than the surrounding air. As an example of how air currents can affect the arc path ever seen the old evil mad scientist horror movies where there is a device called a "jacob's ladder" which is two high voltage electrodes angled apart in a V shape? Notice how the arc travels upwards even though the electrodes are getting farther apart? the reason is because the ionized air is hotter than the surrounding air (obviously) and it rises as hot air is wont to do, but the ionized air is such a low resistance path that the arc continues to flow as the electrodes get farther apart.

This is one that I will have to chew on for a while. I am still thinking that they are not the same. The arc won't be there without the plasma and the plasma won't be there without the arc. I'm trying to talk myself into thinking that the plasma is a higher energy gas (ionized air) and the arc is an even higher energy level flow. I guess in some respects I accept it but it still seems strange.

Thanks,

Bruce

You didn't read the article and look at the video that I posted the links to did you ... It tells you all of this with pictures. And you can see that the plasma arc goes from the tip of the electrode to the cutting surface.

The reason the tip is not immediately used up is because the Ionized gas is created by the AC starter current. Initially only ionized gas is in the tip. Ionized gas is conductive. When the ionized gas flows OUT the tip and touches the cutting surface, it completes the circuit to the high DC current. Then BOOM - the plasma arc initiates from the surface of the cutting material. The shape of the arc is like a skinny football due to the swirling cutting gas and shape of the tip. The tip, ideally, is located far enough from the cutting surface that the plasma arc does not get wider. This is why poor technique ruins the tip faster.

If for some reason the gas flow slows down quickly (dirt or you crimp a line or you just run out of gas in a poorly designed machine) you will get a pulse jet situation, and need a new tip ... instant big whole in tip. ... If you ride the tip on the cutting surface too shallow from perp, the arc gets nice and fat at high IPMs, get a new tip.

Follow the links in my previous post ...

Oops, as I was responding I saw the link but never went to it. It is a VERY GOOD reference. I'm still bothered a bit by the tip not receiving much damage, but I suspect that I will just have to accept that good engineering, knowledge and a little trial and error allow machines to be built that will provide good service life.

Thank you,

Bruce