The Physics of Plasma Cutters

Does anyone have a resource to explain the physics of plasma cutters? I’m having a bit of trouble getting the arc to spark with my table and want to understand it better.
How do air pressure, current, material thickness, nozzle diameter, tip offset and any machine logic affect cutting and whether the arc sparks? I’d like to understand the blowback part with respect to these too.

Separately, I set my Primeweld Cut60 up last night. I don’t have z axis adjustment and I needed a tiny gap to get the arc started on the table, but when hand held the gap could be larger. Also, I wasn’t sure if there was a initial contact needed or I was imagining it.

Thanks!

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It should not have to touch the metal to start. The pilot arc should fire into the air. It won’t transfer to a cutting arc, without the work clamp attached, but the pilot arc will fire into the air.

I’m sure someone else could provide a more technical explanation, but I’ll try to explain what should happen. The electrode is spring mounted and air pressure blows the electrode back, creating a spark that initiates the pilot arc. The pilot arc is low amperage until it completes the circuit with the work clamp that is attached to the work pierce. Then it transfers to the set cutting amperage.

If your torch requires contact to initiate an arc, there are a number of things to check.

• make sure the electrode moves freely in and out.
• set air pressure at around 65psi when air is flowing
• don’t clamp the torch in the lower 3 inches on a machine torch. You could be crushing the moving parts.
• don’t over tighten the cap on the torch.
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Hey Alex,

Good question! Put simply, plasma cutters work by initiating an electric arc, then pushing gas (typically compressed air) through it. This ionizes the gas, turning it into plasma; the continued flow of compressed air pushes the plasma through the torch’s nozzle towards the workpiece - thicker materials require either higher gas flow or lower feed rate for the arc to cut all the way through. The nozzle of the torch constrains and focuses the plasma arc, increasing its velocity/pressure.

By “tip offset,” I assume you’re referring to cut height. The voltage of the arc can vary based on the distance between the torch and the workpiece; if the torch is too high, voltage will increase, and vice versa. The nominal voltage varies based on the material. This is why using Automatic Torch Height Control generally yields better cut quality.

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in other words…

flame…burn metal…hot…
fire…cook meat…like fingers…

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Ouch Phillip!

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[/quote]

You need to check your table, that hole you were trying to cut in your hand was not even close to being round. Loose couplers can cause that, in case you didn’t know

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Yeah found a new way to spell dumb​:joy:

That was my very thought. I knew something wasn’t right. I guess I can hand that one to you

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Knick what amperage setting would pick for that gauge of skin thickness?

Perhaps he should smooth out some of the 'skin dross" on the topside with beeswax.

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Not sure how you did that with a Plasma Torch, but having done something very similar on my CO2 Laser (where the beam is invisible), I can certainly identify and sympathize with the OUCH factor. Glad your hand survived. That was potentially tragic!

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Have you ever heard these words. Hey y’all watch this!!

That was definitely almost a Darwin moment. I was reluctant to share but it is a lesson. Don’t hold a plasma torch by the nozzle.

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Ditto I drilled a hole into my thumb trying to focus a K40. Couple of years later I was being careful to keep my thumb away from the invisible beam and set my shirt sleeve on fire instead

Having a noisy very visible flame has its safety advantages.

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You would think someone would know better huh?

I told my wife it doesn’t take long to look at a plasma flame.

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No joking I am glad it was not any worse! I could write a book on all the stupid stuff I have done, its a wonder I am alive.

I will share one of the most stupidest things I have ever done.

Back when I was in early 20’s I was repairing a rusted out frame on some car at my dads shop.
Car was on the hoist and I was trying to weld a patch on the rusty weak section. So I had everything clamped in place and was trying to strike an arc. I was having trouble as I could not see anything of course with the hood down. So I tried once or twice and the rod stuck so I pulled it off and lifted the helmet to see where I was trying to strike the arc and I see a burn mark on the gas line. Not sure what I did after that but I hope for everyone’s sake I covered that gas line with some protection.

I like to think I am safer now. Remember this is only one of the many stupid things I did.

PS Phillip I sent this privately so not everyone in the Forum would know, so lets just keep this between us.

Thanks

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No worries. We will erase from our memories.

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OK. I figured out a few factors in my struggle to cut thanks to this thread.
Cutting lots of small test squares is a good way to hone in on optimal (adequate in my case) settings.
I increased feed rate, it was much too low and tweaked the current up a touch, I increased the supply pressure and then used the primeweld internal regulator to set the final pressure. It also seems the cut height needs to be teeny tiny.

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Normal cut height is .060", which is basically the thickness of a piece of 16ga metal. Your table should have come with shims to set the torch height.

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I have an old generation table. The shims seemed to be too high and wouldn’t arc unless very very close, but perhaps it was a combination of other factors. Now that I have corrected the feed rate, I’ll try again.

Again, if it won’t arc without being very close to the table, something is wrong with the torch.

It should pilot arc with no metal around it.

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