Pantograph: help against EMI from plasma cutter or arc welder?

o_lampe

Hi gents,
I just got me an MIG MAG arc welder and a plasma cutter. (damn black friday offers )
I read about crashing controllers from their EMC if one would try to DIY a CNC-plasma table or cladding welder.

Now I saw a video about a pantograph, especially made for plasma cutters to copy patterns.
IMHO it would be a good way to CNC the cutting process and at the same time put some distance between arc and controller.

I reckon, the controller would still have to be in a shielded housing but what else could I do? ( No WiFi controller I guess)
I could put some distance between controller and steppers, too. But that would probably be counterproductive?
What about the power grid? Is an isolating transformer mandatory?

Any input is welcome, thanks
Olaf

droftarts

@o_lampe All the toys! Funny thing... what set me on my RepRap journey was wanting a CNC plasma cutter table. I had a business idea back in 2010 to use one for custom motorbike parts. I looked around, but machines were really expensive. Maybe a CNC router? Too expensive. What can I afford? Eventually I found RepRap, and got into automation that way!

From memory, you can get Plasma controller boards that do most of the Plasma cutter control; striking the arc, moving the plasma cutter to the correct distance from the work, reading the voltage and modulating height, all of which helps for longevity of the torch, and better cut quality. I haven't looked at it for a long time, but I remember LinuxCNC, Plasmac (plug in for LinuxCNC), Mesa controllers and Mesa THCAD boards being mentioned. The CNC forums have a lot of info on this, and plasma cutters do seem to be well supported in LinuxCNC. The THCAD board (which measures the voltage at the torch, so provides the feedback to the controller to change the height) is the part that would need to be interfaced with a Duet, no idea if that is possible!

Ian

o_lampe

@droftarts Actually I want to use a pilot-arc torch, so I might get away with constant height. The replacement parts are also cheap.

All I ask from Duet is to start the arc with a certain dwell time for the initial cut and then cut with steady speed.
The on/off commands and the slicing SW will be the same as for laser cutting I guess. E. g. Inkscape has some good features to add offsets around shapes.
The compressor is independent and controlled by the plasma inverter.

droftarts

@o_lampe Usually when using laser cutter mode (M452), there's no dwell time, and the laser is only activated when moving (G1 S# moves). M3/5 are not supported any more in laser mode. It's possible to control it with M42 - generally a macro is used to switch the control pin and allow M42 to control it, and fire a short burst, for focussing and adjusting mirrors.

You could use CNC mode, and treat it like a spindle, and control it with M3/5. Again, there's no default way of setting dwell time, but it's easy to insert a G4 dwell command after M3. Depending on how the torch is controlled, you could use PWM to analogue (0-10V) converter board, which might work better if you want to modulate the plasma cutter: https://docs.duet3d.com/User_manual/Machine_configuration/Configuration_CNC#connecting-a-spindle

If you don't want to modulate the plasma cutter, it's then a case of making sure your feed speed stays the same, and there's no slow downs.

As for shielding against EMC... I don't know!

Ian

o_lampe

@droftarts I mentioned laser mode because Inkscape has some useful plugins for path finding and gcode generation. I can define which gcode is used for On/Off control.
I'm not sure if my plasma cutter can handle PWM on it's control input. In the end I'd be happy to cut out shapes without controller crashes.

I found EMI filters for mains voltage, then use shielded cable and probably snap-on ferrit cores.
My hopes are that my plasma cutters "B" classification (allowed to be used in residential areas) means it has lower EMI emission.