MassiveD

https://www.youtube.com/watch?v=85HR...P1znpJ_0M3Q5iA

Not sure whether they are selling that, or bragging about it. What I liked was that the slides and angle adjust are parts easily found on craigs, or for around 50 from China. The power head looks very easy to fab, or repurpose from some machine parts. No serious Z axis would be one limit.

It is similar to a horizontal milling machine except the shaft is aligned with the x axis, rather than the y (I hope I have that right). This seems to make the parts fit together better than they do on a horizontal, which often requires an offset plate. And the whole machine looks sturdy enough, without weighing 500 pounds.

I would probably give one a shot, if this was 10 or 15 years ago, but I have what I need for now. And I need the versatility to do stuff other than tubes.

Smudgemo

Vee blocks on their sides would accommodate a variety of tube sizes making the Z axis a non-issue, but you'd need taller vise jaws (probably) to hold things firmly. I'm sure I'll never do it, but I still like the idea of building an abrasive mitering machine because it would be so easy to shorten a tube just that little bit.

JohnDThompson 

When I worked with Mike Appel in Madison back in the 80s, he had an abrasive mitering machine that worked quite well. You just needed to apply some judicious downward pressure to the other end of the tube, to counteract the tendency for the abrasive belt to pull the the tube off-center on the miter.

MassiveD

I think the great advantage to abrasive machines is that it is probably the one type you don't need all the gear for. In a metal shop, an abrasive machine is useful for a ton of stuff, and mitres can be easily shot without any cross slides or vises. It takes a little practice, but say 5% compared to learning to weld. If one is going to go to all the hassle of all that gear, I think one should have a cutter based machine, since it does not have the problem of dust. Quality abrasives release a lot less dust than the cheap ones, but there is still that. I use mine out of doors, it is just on casters. I have the option of using a milling machine to do miters, but it takes so long to set up, I prefer to run the abrasive machine.

However, most forums jump all over abrasive machines on the grounds of the dust, or the usual arguments that it is easy to make mitres by hand, which is also true.