How are you planning to accomplish that? Isn't plasma cutting a high frequency electric arc traveling from a torch's electrode to a piece of metal which then melts and is ejected by blasting it with a high velocity gas?

Yeah that's my understanding. Yet as I observed, it shoots a plasma arc into the air with the ground clamp dangling 10ft away. I don't know why.

 

How are you planning to accomplish that? Isn't plasma cutting a high frequency electric arc traveling from a torch's electrode to a piece of metal which then melts and is ejected by blasting it with a high velocity gas?

Plasma cutting is creating a jet of hot plasma by blowing a suitable gas through an arc. the target may not be part of the circuit, or not. At least much of the time the target does not matter, and the ground clamp helps direct the stream but mostly reduces the shock hazard.

 

Plasma cutting is creating a jet of hot plasma by blowing a suitable gas through an arc. the target may not be part of the circuit, or not. At least much of the time the target does not matter, and the ground clamp helps direct the stream but mostly reduces the shock hazard.

Well, there ya go! Thanks for the explanation. The shock hazard is real; I can confirm.

 

Apparently the idea that only conductive materials can be plasma cut is an extremely pervasive misconception. Two "authorities" on the topic spouting that narrative in the same screenshot as that of a guy plasma cutting glass, wood, PVC, water bottles, rocks, etc.

 

Apparently the idea that only conductive materials can be plasma cut is an extremely pervasive misconception. Two "authorities" on the topic spouting that narrative in the same screenshot as that of a guy plasma cutting glass, wood, PVC, water bottles, rocks, etc.


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Very, very interesting... I didn't know that plasma could be used on non-conductive materials too... I wonder if its performance would improve if one were to use a thin sheet of metal as a backer of sorts (that would be sacrificed) placed below a sheet of glass that one wanted to cut. Maybe that way the plasma arc would be better focused and a better finish would be attained.

 

Sorry, I couldn't let this go.

Something in the back of my head kept suggesting "HF start" but the front of my head said "no, dummy. that's for TIG" Turns out Plasma cutters have it too. I went back and watched the "plasma vs. stuff" video again and noticed that every time he cuts into something non-conductive, he only cuts for about a second; whereas when he cuts metal he cuts for longer.

So my take on it is that what @MisterBill2 said is true, only for a second, only at the start, and then from there the plasma arc is transferred from [within the torch head], to [between torch head and workpiece]. Once the transfer has happened, all (or most) of the energy from the arc is then dumped into the workpiece instead of remaining in the torch head. The ground clamp does carry cutting current.

Then another thought; if you could rig the plasma cutter to stay in "HF start" mode for longer than a second, you could conceivably cut non-conductive things. But then, with all the energy of the arc being contained in the torch head, you would probably burn up the torch head. Went back and watched the video again, noticed his torch head severely burnt and cracked from all these shenanigans. He's overusing the crap out of his HF start and the torch is suffering greatly from it.

I found the link below that gives a good explanation that gives a few nods in the direction of the theory I just delivered.

https://weldingweb.com/showthread.php?25405-Plasma-Cutter-Start-Functions

 

I wonder if its performance would improve if one were to use a thin sheet of metal as a backer of sorts (that would be sacrificed) placed below a sheet of glass that one wanted to cut. Maybe that way the plasma arc would be better focused and a better finish would be attained.

This absolutely works, according to random people on the internet.

EDIT: I wonder if a layer of aluminum foil or foil tape would be sufficient? You would need to go over your CAM paths with a fine tooth comb, making sure to work from the inside out, cutting out voids first, to prevent creating islands of electrically isolated material that still need to be cut.

 

The plasma jet is about 10,000 degrees and so it will cut through a lot of things, some better than others and some faster than others.

 

I used to service CNC plasma M/C's they used oxygen or compressed air fed to the torch, the HF igniter is the very simple LC oscillator with spark gap generator.
The air is started through the torch and the HV DC is connected and the HF spark ionizes the air and conduction to the conductive material starts, as soon as the plasma transfer has taken place, the HF pilot arc is turned off. .
For non conductive material, a Laser torch is generally used.
If the transfer does not take place within a couple of seconds, the HF is turned off.
P.S. The table/Material is the +ve side of the H.V DC . supply.
Max.

 

i skimmed over the thread and if i understand correctly you are looking at positional feedback for your home-made CNC. if so, can you post some numbers? for example what volume, what accuracy and why not use encoders? did you consider mounting the torch on the robot arm? this gives you great range and small footprint, easy to level or get parallel to any surface (product), can do repetitions etc.

 

Incidentally the Spark gap oscillator typically used is based on Marconi's transmitter principle!
Max.

 

My plasma cutter is kind of old tech. It is a "scratch start" this was what was available at the time. Now those are the bottom of the heap, the good ones are HF start. But both of them have yet another higher newer one, pilot arc. Both scratch and HF start will go out and need to be restarted when hitting paint or even rust, but the pilot arc will burn through those and keep cutting.

All but the top of the line industrial ones for thick metal use dry clean air for the plasma, T he industrial one sometimes use argon or an argon mix. The one they had at work in the fab shop could clean cut 1" and messily cut 1.5" and it used argon.

 

With the CNC style, the pilot arc is across the torch shield to centre, so the material is not involved, it normally is only on for 1 to 2 seconds.
Max.

 

i skimmed over the thread and if i understand correctly you are looking at positional feedback for your home-made CNC. if so, can you post some numbers? for example what volume, what accuracy and why not use encoders? did you consider mounting the torch on the robot arm? this gives you great range and small footprint, easy to level or get parallel to any surface (product), can do repetitions etc.

I don't have a robot. I want to make a 5x10 plasma/routing table. I will use encoders on the axes. My question related to using non-standard materials for the linear guides. Materials with no straightness spec, like ordinary mild steel square or round tube, which are by machine tool standards "not straight." A system to correct for irregularities in the axes that encoders wouldn't be able to catch.