CNC Plasma help...

[Davek0974]

Having difficulties with a plasma table I've almost finished building, looking for some help.

Having finally beaten the electrical gremlins today I ran a pen-test and the results were good all the way up to 5000mm/min so it appears the transport is reasonable with a pen fitted.

The torch cut still seems poor though, these were the results after some fooling around (1.5mm steel)...

The dross does knock off easy as can be seen in the first part of this one...

There are lots of issues here that even my minimal knowledge can see.

A fair amount of dross,
Wobbly cuts that do not appear to mechanical,
Severe edge bevel that varies as the cut goes round,
When stacked up, the parts are different sizes, slightly distorted in places,
The poor cut seems unaffected by cut speed from 3300 to 4000mm/min,
The cut was worse at 2mm cut height,
The best cut seems to be at 1.5mm height, 1mm gave no improvement at all.

So, I need the voice of CNC plasma experience here to guide me in a direction for tests/checks etc.

Can anyone offer advice?

Thanks

Dave

 

[dan.taylor.1]

Is that zintec sheet? Might be an issue bit I don't know. Have you got the air pressure set correct?

 

[Lawnmowerboy]

Try another 5 psi of air. How far away are you setting the nozzle from the sheet and what condition are your consumables in. is your air dry

 

[Davek0974]

Yes it's zintec, it's all I've got at the minute.

Air pressure is 75psi as recommended by supplier, it's very dry as I'm running it at work and we have a large air plant with dryer etc.

The best results, if you can call them that, are at 1.5mm, it was worse at 2mm but no better at 1mm.

The consumables have likely had 10 minutes runtime, I've only just managed to sort out my electrical issues.

I have no idea of what they look like when knackered?

 

[Lawnmowerboy]

Sort of bits burned and gouged out of them. What amperage are you rrunning at. Try less and more pressire

 

[snowcat]

Im guessing your not running a THC, keeping a constant arc voltage will make huge improvements to the cut, The other problem is your very unlikely to cut a constant speed on a shape like that, curves need a different cut speed to lines in truth its just too time consuming to start changing travel speeds mist a shape so experience will start to tell you what the happy medium will be.

That shape is too complex for a test piece, start with some lines and get them to cut and pierce clean, (correct delays and plunge). then cut some circles, your cut alot of scrap before you get the table working nicely.

Your have to take some time doing this with every thickness of material your going to use, once your there you can save them as a tool set in your CAM software.

Took me six months to really get to grips with machine settings, much more to it than meets the eye.

 

[Welderpaul]

One other thing to think about is the earth connection to the table. When i bought ours he told us to earth each corner otherwise the arc could jump about as it travels.
No idea how much truth is in this but i would have no reason to doubt him TBH.

 

[snowcat]

Id agree earth issues will cause a wander also you can get false a deep starts or a doped arc, not likely though on 1.5mm though, nearly all plasmas will end up with some beveling on curves and rads. Also 1.5mm is a hard task to get a table is going to struggle with 5m a minute, 3mm would be ideal to test on.

 

[Davek0974]

Interesting, the cuts were all made at 30A, straight lines always seem to work nicely, yes it gets lively at 5m/min, I would prefer to cut slower if possible.

The torch is a cebora 100A unit, but the plasma only goes to 50A, don't know if that matters, they came together from the supplier so I presume they work together. The nozzle is a 1.1mm item. I have the hand torch as well, this is a 50A unit with a 1mm nozzle. I could possibly rig that up if it proves anything?

I also cut some 20mm circles, these were equally ugly.

I will try and find some 3mm scrap too

 

[Davek0974]

The earth was to the table bed but it's just the same if I clamp direct.

No THC but I do have a floating z axis so the height is probed at the start of every cut.

 

[Shox Dr]

Don't you need to run the smallest size tip you can use, for the given sheet thickness

 

[Davek0974]

Well that is what I need to know. This is the smallest nozzle for this torch it seems.

I will get some thicker metal under it Monday, maybe I'm just trying something that won't work because it's just wrong - nozzle too big for 1.5mm sheet.

If it is that simple, I'll send the torch back because I really need to cut stuff from 1-6mm thick not 20mm!

My last plasma was only a little 30A hand only unit so only had one tip.

 

[scotty41]

Dave, can i see a picture close up of the edge of the the cut, the drag lines.
Cheers
Scot

 

[Normsthename]

Lack of THC is the first problem on thin material.
I used a Hypertherm plasma with my home built CNC table and they had support for 'Finecut' Consumables
If I had tried to cut anything less than 3mm without finecut consumables I bet I would have got similar results to yours.
With the finecut consumables I used to cut a lot of 1.5mm Stainless with good results. (With a THC)

Andy

 

[gerrymoore]

My twopence worth..... I use my home built CNC for mainly 2mm and 3mm mild steel. I use a Hypertherm powermax 30 usually set at 20 amps or less. My cut speeds are MUCH slower at around 900mm/min. I do not have THC but use a floating head without problems. Sheetcam and Mach3 main programs. I get perfect cuts with some dross that comes off fairly easily. Check out some of my work in the 'sculptures' section. It took me a long time getting the electronics right and also had terrible problems when using an RTech plasma cutter. Since buying the Hypertherm I've never looked back. Love it to bits!!!

 

[Davek0974]

Yes, yes, yes, hypertherm is the king, I have been told many, many times. But I can't afford it and that's the end of it, this is a home use thing so it's never likely to make money and I can't drop £2k on a torch.

Can you elaborate on the r-tech problems you had please?

The metal is at work so I can put pictures up on Monday.

I am getting suggestions that the flame is way too hot/wide for 1.5mm steel because I am using such a big torch, I will likely have go back to r-tech and get a smaller 50A torch and 0.9-1mm tips which should allow me to go down to 20A or so.

I have no idea about tip size, current and material ranges, is there a chart somewhere?

I will get some thicker steel to try, that should back up the theory of using the wrong tip etc.

 

[gerrymoore]

Don't want to knock the RTech too much as I'm sure it's fine non-cnc. Suffice to say the machine torch they sell is way to big for anything like 6mm or less so you will need to adapt the hand torch...I have had to do this with the Hypertherm anyway. It also seems to be very electrically noisy. If you shop around you can get the Powermax 30 for about a grand...with a 3 year warranty....money very well spent. As others have said, you will need to do a lot of experimenting with speed, height and current settings..no easy way unfortunately. Took me a LONG time to get it all right.

 

[Davek0974]

I will agree there, I really don't know why they ship a 100A torch with a 50A machine! It just seems wrong and the torch is massive.

I am going to try the hand torch on the same metal and see what results I get.

I have tamed the electrical issues and getting reliable runs now so that's one issue solved.

 

[Davek0974]

Just found this tip on a site, can any experienced users comment on its correctness?

"There is a rule of thumb that I have come up with when using a plasma cutterto achieve the best cut. You won’t find it in any plasma cutting manuals though. I have found that 20 amps is truly the lowest practical limit for a plasma cutter. That amperage is good up to 1/8” of an inch. After 1/8” you can typically add 10 amps for every additional 1/8” of cut. While this isn’t perfect for all conditions, it works well as a general guide. For 3/8” cut, 40 amps works out well for most brands of plasma cutters. For ½” cut, 50 amps works out well, etc. Keep in mind this isn’t the maximum thickness of cut that a plasma cutter is capable of cutting but rather, it is a good rule to follow for decent cutting speed and quality. For most torches, various tip sizes are available, each with its optimum cut range."

Despite hours of searching, I can't find one mention of the correlation of tip size to current to material thickness?

 

[gerrymoore]

I use .8mm tip at 20 amps for 2mm and get perfect cuts......for 3mm I use same .8mm tip at about 23 amps...perfect cuts too. For 1.5mm I only use about 17amps and still get perfect cuts. Check out my plasma horse...very fine, quality cuts in 2mm all at 20 amps too.