Not sure just what you are talking about cutting and/or welding.
https://www.harborfreight.com/catalogse ... tering+jig
This is a centering device for a drill press to center a hole in the tube. While this is HF, everyone seems to sell the same or a similar device. I made and used a similar method to mark and join front beam tubes when I was installing adjusters during the beam build.
I had some length of angle iron lying around so I took a length of it, marked a center across both flanges of the angle iron then made a notch in it, wider than the weld would be, it down to a point lower than the "to be cut" tube would sit at. I then put the tube in the angle iron spun it to mark the cut line making sure the line being drawn joined exactly with the start of the line.
For welding: the tube sections were then clamped to the angle iron with the joins at the notch then I welded it using the notch to keep the weld from interfering with the spinning of the tube. It worked OK, not a pro job but it worked OK and kept me from wandering all over as I welded.
IMG_1543 copy.jpg
These are two short versions of what I was talking about. These I've used them for drilling and even had them in the shop press to bend metal with. What I was talking about would be longer to support both sections of tube when joining them while keeping the tubes from drooping and giving a bend to the tube.
One is two pieces of angle butt fitted together then welded and the other was the same thing but with a third piece of angle iron facing up welded to them.
Lee