today i will be eating mostly Y axis

[routercnc]

6kg at the centre of 2off dia 25mm rails of 1500mm length is about 0.5mm sag (at the centre of the span). Fairly easy to calculate.

The question then is how important is this amount of Z sag for a plasma? How critical is the variation of distance to the workpiece when cutting?

Also, if the load is offset then the rails will twist. This will rotate the plasma away from 90deg to the material. Again, how important would it be if the cut angle was a few degrees out? I suspect the problem would be a slight chamfer on the edge of the workpiece.

I'm sure some Plasma expert can help you out with these further questions.