How do I cut a long groove in square tubing?

[Sberry]

I would skip the fabcy features you wont use and concentrating on basics you will. We see a lot of benches on garage journal and a lot of wasted effort. I wouldn't bother with adjustable legs, one of these days I am going to build a new one and its not on the list.

[AchMed]

I understand you want a approx. 3.5" inner tube and a 4" outer tube and that's a good choice, very beefy, but you might consider a trailer hitch size 2.5" (0.188" wall) tube with the inner weld knocked down, and a 2" tube on the inside. No grinding, no mishap in funky machining work. Understandably, this outer tube is more expensive, but a safe way to proceed. It's not the beef of a 4", granted, but you'd be surprised what vertical tubing (as legs), with a diagonal brace this way and that, can provide especially if you're not parking your truck on it.

[Nautilus]

A lot of good ideas, though I admit I had not considered a uni-strut (I assume by uni-strut you mean a single big pedestool in the middle - if I am wrong, please advise). After having read the replies here, I think the solution is to get the DOM square tube. Trying to cut a slot is going to be more work than it is worth. Also, I am waiting for a Metabo burnishing machine to arrive as I was going to clean up all the metal before welding together. I figured this might be a bit easier than trying to do it later. Has anyone had experience with this machine? I am hoping it is good.

I agree that I am unlikely to adjust the bench much, but I plan on doing a lot of tig-welding at some point (making a dune buggy frame) and I think that sitting down might be easier to keep things steady.

[All-about-design]

I realize this is an older post and I am sure the problem has been solved. That said I will offer my two cents.

Receiver hitches typically use 2.5" square tube, .238 wall, and the key is "flash controlled" or "flash removed" weld. It is a tight fit yet still moves.

Flash removed is available in other sizes but I haven't searched for them much to see what all is out there.