Building a chain driven Plasma table.

Ohmic sensing has been working reliably for the past few days. I would not want to be without it. Makes probing so much faster and more reliable.

So my fifth stepper motor driver arrived and I will soon take the plunge to add a rotational axis.

So I read linuxcnc can only handle 3 axis trajectory planning.

But I need a fourth axis to place the torch above the center of the pipe.
I guess this can be done during homing.

Should I make a new configuration with the rotation axis being the X axis. Change the current X axis to the A axis in order to move the torch to the center of the rotation table?

Yes, that is the easiest (and probably the best) solution.
Edit:
Make configs for each tube diameter and set the scaling so they do a full rotation for each diameter.

So now I need to figure out the hardware side. I do have a dividing table with a chuck on it. Unfortunately it seems the gearing is not suitable 1:90 4 degrees per turn. 
so I guess I will be better off removing tha chuck and make a way to fit it on a shaft with bearings. The cam belt gears have a ratio of 1:4 which seems a bit small to me. 
will it work with a 100mm chuck and pipes up to 150mm 


 

150*3.1415=471.225mm of surface times material thickness times 7850kg/m3
That gives you the weight, and at 6 meter length, even 1mm thick walls would be 22KG!
At 4mm, that is 88Kg, that have to be turned plus the friction and the chuck weight.
You can make it work with 1:4 reduction, but it would have to be at miserably low acceleration.
Do you have a 3D printer? Print some pulleys, find a HTD5 belt, at least 15mm wide, use a lot of walls to enforce the mount holes on pulleys.

I think I will go max to 3 meter
I don’t have a 3D printer.
I could put some pulleys in series to get a better gear ratio.
What ratio would you recommend?

Minimum would probably be 1:10 for a Nema 23 or 34 motor, should be usable up to 1:30.
I would reference from the other side of the equation, 2-3 meters per minute of cut feed rate for 40A to 60A plasma, so for 150mm tube that would make 4 turns per minute roughly, easily done, but for 50mm tube that would be roughly 15 turns per minute.
The good thing with steppers in this case is more torque at lower speed, so start form 3 to 5 turns per minute and see what you can get on a high side.