Mostly Plasma CNC

Hi everyone, like a lot of folks I thought a plasma cutter might be fun. But first things first let’s see some hot plasma and sparks:

My initial motivation was that sheet metal parts are “quick” and “easy”. Also, a cheap plasma cutter is like 250$CAD. If the EMI blows everything up, at least I’ll still have a plasma cutter. This was enough reasoning to begin 4 months of anguish.

I think most people are aware that there’s a lot of EMI with a plasma cutter. What isn’t explained is that EMI is a problem that is very difficult to track down and that you will go paranoid and mad trying to reduce it. I kept a log of all the cut I tried and it is in the 40s of attempts before I got anything to cut.

What did eventually result in cuts working was:
Physical Shielding:

  • Shielded cable for all steppers
  • Star connected grounds
  • Shielded control box with shielded internal boxes for the arduino (grbl) and rpi.
  • Double shielded USB cable between rpi and arduino
  • Ferrites on anything that looked suspiciously conductive.
  • Ziptied the plasma and ground lines together to reduce antenna loop size
  • Separate the plasma from the controller as much as possible. I’m doing plasma on the left, controller on the right.

Software (GRBL + cnc.js):

  • Run the job with 0 z-offset.
  • Get the metal relatively flat and then auto-level (great cncjs extension) to get the torch snug to the surface.
  • Added a 250ms delay after the M03 (torch trigger) command

What I considered doing but luckily it started working before I had to :

  • Use a completely different (ethernet/etherCAT) controller
  • Organize the cables in the control box to separate power and signal, 90 degree crossings, maximize separation etc.
  • Buy a plasma torch that definitely is blowback

Some unanswered questions:

  • What type of plasma torch do I have? I really do not know. It is certainly a cheap Chinese import plasma by the name: Reboot RBC-4500 aka CUT45. It claims to be a “touchless start”, there is no external wire I’ve seen on HF systems. If you look into the air tube there is a little bit of copper so that’s definitely used for some sort of starting arc. Hypertherm says they invented blowback by using a moving electrode. There appear to be no spring elements in my torch but I have no real clue. Is it a blowback in disguise? Couldn’t say.

Hopefully this inspires some more people do mostly plasma CNC setups. Plasma is fun and cool!

Just some general tips:

  • Do not plug in your plasma machine until you are absolutely ready to hit start and run the job. There is a lot of water and electricity going on and you are made of water.
  • I redid all my connectors while I was adding the shielded wires. I think as long as you’re careful with the crimps dupont connectors are fine but I did a bad job with the crimping and replaced everything with JST-SM or XH and haven’t had connector trouble for a while. For tiny wires under 16ish gauge I like to fully fold the conductor under the insulator now.

Man that’s some great video! I’ll have to wait until my new shop gets built, but I’m headed this way, too.

Sounds like you missed dui’s thread. He went to great lengths to describe what a pita it is to track down and shield everything from EMI. If anyone else is considering this, please go find it.

Bet it feels pretty good to cut steel parts, though, now that you’re there.