Square and Rectangular Plasma Tube

hello everyone here, i have used linuxcnc for some time and i find it an amazing tool, now i see a new project in which i need some help, i want to make a square or rectangular tube Plasma, in this case i am a bit lost i have used plasmaC with table cards before only in 3 axes but I have read about it and I have my doubts: 
1. THC and it is not clear to me whether to use it or not?
2. Use of SheetCAM and the 4-axis module for square tube?
 3. Is there any free post processor?
 4. Need for 3 or 4 axes for the machine, in view of the final results? 
5. Any example of kinematic models for 4 or 5 axes

 Somewhere I have read about Freecad and the possibility of a 4 rotary axis although I am not completely sure.
 I think I'm asking a lot but I trust the masters of this forum who have created this powerful project called linuxcnc, thanks in advance

THC is required for square tube to lift the torch as it rotates
Sheetcam has a 4 axis addon at extra cost
One way to do it is to treat it as an xyz machine and unwrap the tube.
With Solidworks, or fusion 360, you can create your tube, apply an extruded 0.01mm cut along its length and convert to sheet metal
Then flatten the part and cut it as if it was being cut as a normal sheetmetal part
You need to set the scaling on your x axis so the steps per mm are calibrated to the circumference of the part.

hello rodw, first of all thank you for your answer and for your very interesting videos on your youtube channel, I understand everything you tell me about treating the machine as 3 xyz axes, I think it would be the easiest way, although I have seen some videos in which the torch does not rotate but in my opinion they use 4 axes, in this video the above is evidenced:



I imagine that this is done to mitigate the problem of acute angles between the arc and the surface when rotating the tube, perhaps you have more experience and can shed some light on the subject.

For me it would be:
Z: torch height
Y: torch lateral movement
X: tube displacement
A: Rotation about X axis
Although I'm still wrong, which is very possible haha, thanks to this community, I highly respect everyone's contribution

I've never done this but I think the Z axis/torch height would be managed by the THC when cutting non-round tube. The rest of the machine is just a normal XY machine. a 4 jaw chuck with rollers that allow it to move could be mounted near the torch so it can roll along the tube while cutting.

Hello, I have had some time again and I have spent some time creating a Vismach model of the plasma for a square tube, I have attached a photo of how I think it should be.

Rodw, you're talking about a normal XY machine, I can't understand you because in the video it can be seen that when the tube rotates, the X axis also rotates just when the torch reaches where the radius begins in the corner of the tube, in the case of dispensing with the X axis and leave everything to the Z axis height control (THC) like a plasma for a cylindrical tube we would have angles from the beginning of the rotation, which is very bad.
I see the thing more as a xyzb-trt configuration, only in the corner because otherwise the faces would be like an xyz configuration, I imagine a change from the gcode between both configurations, I just think I don't even know if this configuration exists.
I think I would need some help with the kinematics if I had to develop it, although I don't know if I could use the trivkins module directly.

Lets called this a wrapped rotary. That means the tube is flattened around its circumference. Google coping calculator to see a crude example where a piece od paper is actually wrapped around the tube  and the cut line is traced onto the tube. You can achieve this in CAD by applying a narrow slit to the tube, converting to sheet metal and flattening the part.

So then in your example, X is the circumference of the tube and there is no B axis
There is no need to worry about the Z as it should be under THC control provided it can accelerate fast enough to keep up with a square tube as it lifts going around the corners