View Full Version : Plasma cutting fault diagnoses....

21-03-2014, 07:10 PM
Having finally got my table up and running, I need assistance with poor cuts, could anyone give some suggestions as to what I can check, these are the results I am getting....


There is no visible slop in my axes movements, speed seems to make little difference. Cutting height was 1.5mm it was worse at 2mm but no better at 1mm.

this is 1.5mm steel at 30A, rear view.

the dross chips off pretty easy, but there is a lot of bevel in the edges which varies as the torch goes round, the edges show a swirly cut type effect in many places, not smooth at all.

in a straight line it cuts far far better.

any suggestions?

21-03-2014, 11:30 PM
I noticed you'd asked about look ahead on the Mach forum, which may help.

Have you tried asking over on the CNCzone plasma forum?
I'd guess you're more likely to find an answer/suggestions over there, as it's the biggest cnc plasma forum I'm aware of.

It could be related to speed/power, due to the fact the plasma/flame lags behind the actual torch position, however I'm only guessing.

22-03-2014, 08:22 AM
Suggestions are coming in that I have the wrong torch for 1.5mm cuts, it's a 100A torch on a 50A plasma with a 1.1mm tip. It seems the flame is far too hot/wide but the torch does not have a smaller tip.

i need a 50A torch with possibly 0.9 or 1mm tips for this thin steel.

i will get some thick steel to test, this should prove the theory if it cuts well.

22-03-2014, 08:36 AM
How does the plasma cut manually? If you're able to get good cuts by hand you can measure distances and estimate speeds to get close to a starting set of figures to work from.
I have a plasma cutter that's currently manual but has remote switching for automation, the torch it came with has cut 1mm through to 10mm neatly without the need for nozzle swaps, are you delivering enough air pressure at the machine while it's cutting to provide sufficient flow? Low air delivery will give you wide, wandering and messy cuts,

- Nick

22-03-2014, 08:47 AM
It has a hand torch torch too which has smaller tips, I will try this Monday, air pressure is good, dry and stable.

What size tips and what current were your cuts made at?

22-03-2014, 09:24 AM
I'll have to dig out the torch to check tip size but if memory serves it looked less than 1mm, I just dialled in the recommended current from the manual (in the box with the leads) and it cuts nice.
I did make my own bent wire height guide to make it easier and quicker when using a length of timber as a rule for cutting 2m x 3m x 3mm plates into manageable 2m x 1m sections for storage.
The cutter is rated to 12mm with the supplied torch and there's no mention of using bigger tips, i think swapping to the standard tip size supplied with your machine's OEM torch will solve your issues,

22-03-2014, 04:07 PM
Thanks, swapping tips is out as this is the smallest listed for the machine torch, I will have to fit the hand torch to the machine, that is a smaller torch and has a better range of tips.

i will certainly be trying the hand torch Monday on this metal and have a feeling I will also be talking to the suppliers, I am guessing they presume everyone will be cutting thick steel so put a nice big torch in, but when it comes to thin sheet it's like using a sledge to crack a nut, works but not very well.

22-03-2014, 04:41 PM
Just found this tip, can any experienced users verify it's correctness?

"There is a rule of thumb that I have come up with when using a plasma cutter to 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 ” 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."

24-03-2014, 08:44 AM
Results are in...

30A Machine torch cut on 3mm steel...

30A Hand torch cut on 3mm steel (top line only)...

20A Hand toch cut on 1.5mm steel at a good speed...

The same cut but showing the reverse side (top)...

The back dross knocked off easy, there was none on the front, one side of cut was bevelled, other was straight, i think this is normal behaviour.

SO I will be ringing the suppliers for a smaller torch more suited to 20-50A cuts than the 50-100A torch they supplied

I have also found another user who was sent a massive torch and is now just using the hand torch.

This seems to be the answer.

24-03-2014, 07:05 PM
Tried it on 3mm sheet and much better...



Both at 35A and 1500mm/min, the dross is easily knocked off, the "good" side of the cut is nice and square and within 0.1mm of dimension too.

It seems it just does not like thin steel??

I have some 1/4" scrap somewhere, will try that too, but its looking more promising.

I have a feeling that thin stuff is going to be tougher than thick stuff.

25-03-2014, 07:09 PM
Keeping the speed fixed, I varied the power this morning and got this set of results.





The 32A cut is terrible, there is severe bevelling on the X axis cuts but oddly not on the Y axis cuts, no idea what's at play there?

I think the 35A cut is best, the dross knocked off pretty easy and it's on size.

The 40A cut is not much different than the 35 one so best speed is likely 35-37A?

Overall, I'm not impressed and am slowly losing faith. it seems this machine needs way more power than it should and does not give results to match, yet.

Do any of these cuts look respectable?

25-03-2014, 09:14 PM
What Machine is it.?

25-03-2014, 09:16 PM
Which the CNC or the plasma?

the plasma is an r-tech p50hf and the machine was built by myself, basic belt drive, stepper motors, mach3 & sheetcam software.

25-03-2014, 09:28 PM
Which the CNC or the plasma?

the plasma is an r-tech p50hf and the machine was built by myself, basic belt drive, stepper motors, mach3 & sheetcam software.

I was meaning Plasma because I thought with this statement you meant your not happy with Plasma machine.? . . . Which did you mean.?

I'm not impressed and am slowly losing faith. it seems this machine needs way more power than it should and does not give results to match, yet.

If you mean Plasma then i'm surprised because R-tech are great Company to deal with and I've yet to know of any who use there Stuff and complain.

My Tig welder is awsome and I was just considering buying a Plasma from them.!

25-03-2014, 09:44 PM
Yes, I have their I-mig160 and it's great, it just seems odd that I've been really struggling with this beast when it should be easier I'm sure.

It will not cut 1.5 mm at all and prefers to just vaporise the metal and have the arc wandering about looking for fresh meat, this I'm told indicates running too slow but increasing it causes it to stop cutting which means increase power!

speaking to r-tech yesterday, who usually are great, left me with the impression that they have run out if ideas and it must be something I'm doing and it just needs dialling in, but with conflicting results, I'm not going to get anywhere.

They have been very helpful so far but I've hit a brick wall.

i wanted to cut artistic stuff but with some of the results I'm getting, I can only just about manage a simple circle in a square!

01-04-2014, 07:04 PM
Ok, I'm making progress now, anyone recommend a THC controller.

the Proma units are affordable and seem ok?